^aiWeeCONTENTS

A PERIODICAL OF PARTICLE PHYSICS

VOL. 21, NUMBER 3

EditorsRENE DONALDSON, BILL KIRK

Contributing EditorMICHAEL RIORDAN

Editorial Advisory BoardJAMES BJORKEN, ROBERT N. CAHN, DAVID HITLIN,

STEWART C. LOKEN, JOHN REES, RONALD RUTH

MARVIN WEINSTEIN

Photographic ServicesTOM NAKASHIMA

BETTE REED

IllustrationsTERRY ANDERSON, KEVIN JOHNSTON

SYLVIA MACBRIDE, JIM WAHL

DistributionCRYSTAL TILGHMAN

FEATURES

WHITE DWARFS

Going Gentle Into that Good Night

A review of what we know and

don't know about these planet-sized stars.

Virginia Trimble

7 PARTICLE PHYSICISTS, TEACHERS

& SCIENCE EDUCATION REFORM

Particle physicists are teaming with teachers

to improve the quality of pre-college

science teaching in the United States.

Marjorie Bardeen

12 WORKSHOPS TARGET THE NEXT

LINEAR COLLIDER

A review of recent linear collider conferences

by two participants.

David Burke and Ewan Paterson

The Beam Line is published quarterly by theStanford Linear Accelerator Center,P.O. Box 4349, Stanford, CA 94309.Telephone: (415) 926-2585BITNET: BEAMLINE@SLACVMFAX: (415) 926-4500SLAC is operated by Stanford University undercontract with the U.S. Department of Energy.The opinions of the authors do not necessarilyreflect the policy of the Stanford LinearAccelerator Center.

Cover: A photograph ofa planetarynebula in Aquarius,taken with the 200-in. telescope on Mt. Palomar.Planetary nebulae are the ejected outer layers of dyingstars of relatively low mass. The relic cores becomethe white dwarfs discussed in Virginia Trimble's articlein this issue. Photo courtesy of California Institute ofTechnology.

DEPARTMENTS

17 DATES TO REMEMBER

18 TOWARD THE NEXT LINEAR COLLIDER:

THE ACCELERATOR TEST FACILITY AT KEK

Koji Takata

21 FROM THE EDITORS' DESK

22 LETTERS TO THE EDITORS

23 CONTRIBUTORS

FALL 1991

»^4

WE HAVE MADE several attempts during the past two years to securea spectrum of the companion of Sirius. .. .The great mass of the star, equal to

of Sirius, and its low luminosity, one one-hundredth part of that of the Sun andone ten-thousandth part of that of Sirius,make the character of its spectrum amatter of exceptional interest....

The line spectrum of the companionis identical with that of Sirius in allrespects so far as can be judged from aclose comparison of the spectra....

Direct photographs taken byDr. van Maanen with and withoutthe use of a yellow color screen agreewith the spectrographic resultsin indicating that the companionof Sirius has a color index notappreciably different from that of theprincipal star.

valter Adams"The Spectrum of the Companion of Sirius," Walter S. Adams, Publications of theAstronomical Society of the Pacific 27, 236-237 (1915) reproduced in A Source Bookin Astronomy and Astrophysics, 1900-1975, Lang and Gingerich, Editors, Harvard, 1979.

Only a very few stars that begintheir lives with more than six toeight times the mass of our sun willend spectacularly. In other words, ifyou allowed every star a vote , * thenwhite dwarfs would win, handsdown, but supernovae seem to havemuch better public relations.Admittedly, they are easier to makesound exciting, being responsible forthe production of heavy elementslike uranium and thorium (and thedistribution of most of the rest), foraccelerating cosmic rays, leavingpulsars behind, and (perhaps) trig-gering new generations of star for-mation. Still, numbers should countfor something, and white dwarfs havethe additional, parochial attractionof showing us what our sun willeventually become-unfortunatelynot in time to prevent the next roundof presidential primaries, but onlyafter five or six billion years.

Briefly, a white dwarf containsthe mass of a star, crammed into avolume the size of a planet, and hastherefore a density larger than that ofterrestrial substances by a factor of amillion. The force of gravity holdingstars together is balanced by ordinary

(PV = nRT) thermal-gas pressure instars like the sun, but by electrondegeneracy pressure in white dwarfs.

HISTORYFriedrich Bessel (as in functions)

advocated massive, dark companionsas the cause of wiggles in the motionof Sirius and Procyon that he hadseen in data from a couple of decadesup to 1844. The companion of Siriusadvanced from dark to dim in 1862through the intervention of AlvanG. Clark, arguably the last opticalastronomer to make a fundamentaldiscovery with a telescope he haddesigned and built himself. * * Turn-ing a newly-ground 18.5-inch lenstoward the brightest star in the sky,he saw separately the 0.01% of thephotons that come from the com-panion, 10 arc-sec away from theglare of Sirius A. Procyon B turnedup as a separate point of light in1894.

White dwarfs counted as aproblem, however, only after the on-set of serious classification of stellarspectra and the recognition thatabsorption line patterns, stellar colors

(hence temperatures), and lumi-nosities were nearly always stronglycorrelated. The properties of SiriusB, reported in 1914 by Walter S.Adams at Mt. Wilson Observatory,defied the correlations. The star wasblue (hot) and yet faint. A handful ofadditional white dwarfs joined in thedefiance over the next decades.

Theoretical understanding beganwith R.H. Fowler's development of(non-relativistic) quantum statisticsin 1926 and was essentially com-plete when S. Chandrasekhar pre-sented the exact equation of state forrelativistically degenerate electronsand the properties of stars dominatedby that degeneracy between 1930and 1935. His contemporaries wereslow to understand the correctnessand importance of what he had done,but I think this belongs more to the

"Of course, if stars are allowed to vote,then they should also pay taxes. Onedollar per star in our Milky Way galaxywould just about take care of this year'sbudget deficit (which I decline abso-lutely to describe as "astronomical.")

" A point which may not be totallyirrelevant to recently revealed problemsin optical telescope design.

2 FALL 1991

Alvan Clark

I

realm of sociology than to history ofscience. Incidentally, 20 years be-tween recognition of an astronomi-cal problem and arrival at a solutionthat then holds up for much longer isa fairly typical time scale.

THINGS WE UNDERSTAND

About one white dwarf per yearforms somewhere in our galaxy, anumber arrived at independently bycounting the young (hot, relativelybright) ones, by counting prospectiveprogenitors, and by counting thenebulae ejected in the intermediate"thin shell" phase (called planetarynebulae, and if you want to knowwhy, you must read a real astronomybook). Ten percent or more of acomplete stellar inventory consistsof white dwarfs, just sitting there,radiating away the thermal (kinetic)energy of their carbon and oxygennuclei from underneath very thinskins of hydrogen and helium. Theywill continue this uneventful courseuntil the universe recontracts, theirbaryons decay, or they collapse toblack holes by barrier penetration. *

Half or so of all stars havecompanions close enough for gas toflow from one to the other. Materialtransferred to a white dwarf falls intoa deep potential well, hence radiatingcopiously as it goes, and explodingfrom time to time, when it piles upto fusion temperatures and densities.Collectively such systems are calledcataclysmic variables, botanicallydivided among novae, dwarf novae,

*Likely time scales for these three outcomesare 1014, 1033, and 101076(years for the firsttwo and for the third one it doesn't matter).

and a good many others. Enoughastronomers earn a living studyingCVs to permit gatherings of a hundredor two every few years.

Very occasionally, mass transferor merging in white dwarf pairs maytrigger explosive burning of the car-bon and oxygen core material up toiron. The stars are blown tosmithereens called Type I superno-vae. (Massive stars make type II, com-moner but less bright.)

00)

C)

gdt-

Subrahmanhan Chandrasekhar

BEAM LINE 3

The Great Scheme of Things

AT LEAST BY THE STANDARDS OF ASTROPHYSICS, stellar structure andevolution are in pretty good shape-meaning we can marshall a set of coupled,non-linear differential equations and their solutions to support what is said in thenext few paragraphs.

Stars are, by definition, stable objects deriving their energy from nuclearreactions. A gas cloud of standard composition (about 3/4 hydrogen, 1/4 helium,and 2% everything else) and mass = 0.085-100 solar masses [one solar mass(M0 ) = 2x10 33g] will contract to form such a star. Smaller masses do not get hotenough for any fusion reactions; bigger ones are unstable. All stars spend 90percent or more of their lives fusing hydrogen to helium. The time to exhausthydrogen fuel is, very crudely, t = [10 yr/(M/M) 2], that is, millions of years forbig stars, billions for little ones, including the sun.

Masses less than about 0.4 Me leave their helium cinders so degenerate thatno further contraction, heating, or fusion can occur. The universe is not yet oldenough for this to have happened to any isolated star. But stripping of outer layerscan occur among close stellar pairs, leaving low-mass helium cores in shortertimes. There exist low-mass white dwarf binaries where the stars are tearing eachother apart viciously enough that the layers accessible to spectroscopy confirm themostly-helium composition predicted for these cores.

Larger masses all heat to at least 108 K at their centers and so fuse helium tocarbon and oxygen, in roughly equal amounts, first in their cores and then in thinshells. A combination of radiation pressure and pulsational instability in this "thin-shell" phase ejects much of the hydrogen-rich envelope. Thus initial masses up to6 or 8 M0 are stripped down to carbon-oxygen cores of 0.4-1.4 M0 . Thesedegenerate C-O cores are much the commonest kind of stellar relic.

Only higher masses still go on to fuse carbon and oxygen to heavier elementsand eventually make supernovae. Afew on the ragged edge may burn out en route,leaving degenerate cores of oxygen, neon, and magnesium, a third category ofwhite dwarf. The C-0 and O-Ne-Mg compositions are confirmed spectroscopicallyin nova explosions that eject some material from white dwarfs in more massiveclose pairs.

3600 4600

Observed spectrum and line identificationsin the magnetic white dwarf PG 1031+234(greatly simplified from G.D. Schmidt 1987,in A.G.D. Philip et al., Eds. IAU Colloq. 95,the Second Conference on Faint Blue Stars,Schenectady: L. Davis Press, p. 377). Thewiggly line is observed flux vs. wavelength.Long vertical lines are placed at the wave-lengths of the indicated transitions formagnetic fields of 200-300 MG; arrows andshorter vertical lines indicate the range ofwavelengths of absorption in thosetransitions for somewhat stronger fields. Thelines you expect to see are those that reachturning points or asymptotes, so thatwavelength changes only slowly with fieldstrength in the 100 MG range. Rememberthat, for hydrogen in the absence of amagnetic field, all components of 2-3(called H a or Balmer a) are at 6562.8 A,and all components of 2-4 (called H f orBalmer f) are at 4861.3 A. The strong fieldbreaks the degeneracy of the transitions aswell as shifting absorption wavelengths.

5600

(A)

AND THINGS WE DONITWhy are white dwarfs such slow

rotators? Why do a few have enor-mously strong magnetic fields andmost very little? Why are there sofew faint white dwarfs? Where arethe Type I supernova progenitors?There are lots of others, but these aremy favorites and will do to get onwith.

ROTATION. A spinning white dwarfbegins to break up only for rotationperiods shorter than a few seconds.Conservation of angular momentumin layers as stars evolve from initialhydrogen burners to degenerate coresshould lead to rotation periods ofminutes to hours. In most cases, weknow only limits to rotation speeds(from absence of Doppler blurring ofspectral lines) and periods (from ab-sence of changing surface magneticfield patterns). The limits range fromhours upward to years.

Neutron stars left by supernovaeexhibit the same phenomenon. Mostof them are not born spinning nearlyas rapidly as stability permits or asangular momentum conservation inlayers predicts. Evidently stars aregood at transporting angular momen-tum outward, through turbulence,magnetic fields, or some other form

.2

.FI (

n(1:2

C/)w

c66600 <

Rudolph Minkowski

of coupling. But we cannot currentlydescribe the process well in words,let alone in differential equations.

MAGNETIC FIELDS. For the majorityof white dwarfs, absence of quadraticZeeman shift tells us that surfacefields are ' 105 G (10 Tesla*). Conser-vation of flux from a 1-10 G solardipole through the contraction pro-cess brings us within this limit. Buta few percent of single degeneratedwarfs and 10-20% of the close bi-nary ones have fields between 1 and1000 MG. Wavelengths of atomictransitions are so distorted by suchfields that absorption lines in mag-netic white dwarfs remained uni-dentified for some 40 years afterRudolph Minkowski recorded thefirst spectrogram of one in 1939. Earlyfield determinations thus came frommeasurements of circular polariza-tion. Recognition that the random-looking features were just the shiftedBalmer lines of hydrogen came gradu-ally and from close interplay oftheory, observations, and laboratoryexperiments. The largest stellar field

*White dwarfs seem to be the only objectsin the universe for which the Tesla is areasonable-size unit, which isundoubtedly why we never use it.

4 FALL 1991

2s0 -3pl

I I

: i~~~~~~I I

JL . .

-2

-3

Z0 -4

-5

-5

Jesse Greenstein

for which deconvolution has beenachieved approaches 300 MG (workin progress by Jesse L. Greenstein atCaltech on G 240-72).

It is permitted to attribute therange of fields to exact flux conserva-tion from main sequence progeni-tors with l-3x104 G dipole fields.This does not explain the excess ofstrong fields in binary white dwarfsor the difference from neutron stars,all of which start with 1012-13 Gfields, though their progenitors pre-sumably have a wide range. In anycase, the genesis of stellar magneticfields in general is not understood, soone is merely pushing the problemback a step in time.

FAINT WHITE DWARFS. Start with theplausible assumptions that whitedwarfs have formed at a constantrate since the galactic disk has beenaround and cooled as black bodyradiators ever since, drawing on theirinternal kinetic energy supply. Then,after a lot of hard work (occasionedby crystalization of the core, radiativetransfer through partly degenerategas, and so forth) you can predictwhat the current distribution ofwhite dwarf brightnesses should be.You generally get the wrong answer.We see far fewer faint white dwarfs

-o0 -1 -

than expected unless (a) some seriousphysics has been left out of theparentheses a couple of sentencesback or (b) the galactic disk is notvery old. Notice that if more starsformed in the past, the problem getsworse.

Blaming everything on the sec-ond cause yields a disk much youngerthan the galactic halo stars (globularclusters), and we have learned some-thing about galaxy formation andevolution. Active researchers areabout equally divided on whether ornot a young disk is required by whitedwarf statistics; the rest of us aresitting on the side lines, attemptingto make encouraging noises.

PROGENITORS OF TYPE 1 SUPERNOVAE.

Having confidently told you that weknow these nuclear explosions comefrom binary white dwarfs, I mustnow back off and confess that thereare no known examples of the mostpromising sort of system. To get therequisite explosion, pairs must be (a)close enough for angular momentumloss in gravitational radiation to bringthe stars together in the age of theuniverse and (b) massive enough thatthe merged pair will comfortablyexceed the Chandrasekhar limit tomasses supported by electron

2 -3 -4 -5 -6

Log L/L o

Observed vs. expected numbers ofwhite dwarfs in the solar neighborhoodas a function of luminosity in solar units.Dots are observed numbers per cubicparsec per decade in L (from J. Liebert,C. Dahn and D.G. Monet, 1989, Astro-physical Journal 332, 891). Dashedcurve is the simplest possible model,with constant birthrate and constantratio of surface to central temperature,roughly normalized to the intermediatepoints. Solid curve is a model withconstant birthrate but much bettertreatment of radiative transfer and otherphysics (from F. D'Antona andI. Mazzitelli 1989, Astrophysical Journal347, 934). Both clearly predict too manyfaint stars. The observed cutoff belowLog L /Lo = -4 can be achieved byhaving the faint stars cool very suddenlyor by imposing an upper limit to ages of6-8x109 yr (vs. 12-17x109 yr for the halostars in our galaxy).

BEAM LINE 5

.E

ca

Ic0-

a

|cc/

c.E

Oc

cf

degeneracy (about 1.4 Mo). But theknown close pairs are all low mass,and the known massive ones are toowide to merge. There the problemsits, and has sat through about fiveyears of increasingly serious searchesfor massive close pairs.

F YOU HAVE STAYED with us thisfar, I will try your patience a mo-

ment longer and explain my owninvolvement with white dwarfs overthe years. All Caltech graduate stu-dents in astronomy were (and are)required to complete a test researchproject before settling on a PhD dis-sertation topic. The one assigned tome in 1965 was measurement of ra-dial velocities of about 60 whitedwarfs, based on 200-in. telescopeprime focus spectrograms obtainedby Jesse L. Greenstein in the 50s and60s. In retrospect, I think this mayhave been partly because the taskwas thought to be impossible, andfemale graduate students were new,at least to Caltech, in those days.

The task turned out to be difficult(pressure broadening makes the spec-tral lines of white dwarfs remark-ably broad) but not impossible. Themeasurements led to an estimate ofthe amount of gravitational redshiftand so to values for masses of singlewhite dwarfs, not otherwise avail-able. Our average value, 0.75 Mo,was slightly larger than the modernaverage, 0.6 Mo, as found from pres-sure effects on the shape of the stars'spectral continua. Deep down, I stillbelieve the larger value, and, afterall, every scientist is entitled to becranky about one or two points.

These same radial velocity dataproduced the 105 G limit on magneticfields for typical white dwarfsmentioned above and were the sourceof the first list of candidate massivedouble degenerate stars, none ofwhich have actually turned out to belikely SN Ia progenitors. In April1991, I submitted a proposal to lookfor coronal x rays from single, coolmagnetic white dwarfs using theROSAT satellite in collaboration withKeith Arnaud of Goddard Space FlightCenter.

o

6 FALL 1991

s, Teachersi i: L :

;cience Edt

by MARJORIE BAfi: .11...

Particle

Ition Reform

EN

are teamingwith teP, nprove the

*e science

teachil ite

ought scientiether to revan

,,BSHOCKED AMERICAN,ii ed"ueators and t....teachethie wag ci ence and math'Ihe ahabtft soutp" igcurriid Chem Study at the high (

SCIS a

F. support U.ed univ ersity -b ased A t e4ac he e n h an e n tt ied some of the best teachers in

s assrooms. In 1983, A Nation at Risk, the first inseries of reports critical of contemporary Americanecollege education, produced similar results. Againsearch scientists joined with educators to give teachersd(theirstutdfents an opportunity to discover the true

tof the wor;ld of science.

:MLINE

vei

1S:0 <:000v0040:~ :-:and

n

Particle physicists have been lead-ers in this effort since 1979 whenLeon M. Lederman, director ofFermilab, started Saturday MorningPhysics for high school students. Atthe time Fermilab funds could not bespent on precollege education pro-grams, so Lederman encouraged theestablishment of Friends of Fermilab,a not-for-profit corporation dedicatedto supporting Fermilab precollegeprograms. Since its incorporation in1983, Friends of Fermilab has raisedover $2,600,000 from public and pri-vate sources including DOE, NSF,the state of Illinois, and various pri-vate foundations.

The Conference on the Teachingof Modern Physics was held atFermilab in 1986. Organized by theAmerican Association of PhysicsTeachers and Fermilab, the Confer-ence brought high school teachersand college professors together todevelop ways of translating theexcitement of modern physics tointroductory physics classrooms. Animportant collaboration to emergefrom the Conference was the

Contemporary Physics EducationProject based at LBL and SLAC. Thisgroup of 20 educators and scientistshas created the first physicsequivalent of the Periodic Table ofthe Elements. The Particle Chart isthe centerpiece of a package ofteaching materials (above) thatincludes interactive HyperCardsoftware for the Macintosh com-puter. A student book and a teacher'spacket are being developed underthe leadership of R. Michael Barnett,Lawrence Berkeley Laboratory, andHelen R. Quinn, SLAC. Workshopsfor teachers are an essential aspect ofthe project-SLAC held a one-weekworkshop this summer, and TheModern Physics Institute, held atWestminster College, New Wil-mington, Pennsylvania, has close tiesto the Project. Workshops have alsobeen presented in conjunction withprofessional meetings.

INCE 1989 OTHER PARTICLEphysics laboratories funded by

DOE have initiated precollege

education programs in response toSecretary of Energy James D.Watkins' aggressive education policyopening laboratory doors to schools.For example, at Bates LinearAccelerator in Massachusetts, ErnestJ. Moniz has inaugurated a programto augment high school teachers'knowledge about nuclear and particlephysics. Three high school teachersworked with local staff mentors toproduce a draft resource manual thatwill be used in a fall lecture series forhigh school teachers. The course willbe team taught by the teachers andfaculty members from MIT, BostonUniversity, and Northeastern Uni-versity. Under the leadership ofBeverly Hartline, a partnership be-tween the Continuous Electron BeamFacility (CEBAF) in Virginia and theNewport News Public Schools offersa program for fifth and sixth graderscalled Becoming Enthusiastic AboutMath and Science (BEAMS). BEAMSbrings regular classes and theirteachers to the Laboratory for week-long visits. The classes follow theirnormal curriculum augmented by

8 FALL 1991

special activities conductedby CEBAFscientists, engineers and technicians.A parent open house is held oneevening during the week.The Superconducting Super ColliderLaboratory (SSCL) has developed aprogram, Adopt a Magnet, to helpstudents in kindergarten throughfifth grade understand the purposeand function of the SSC. The youngerstudents are introduced to the SSCthrough songs and games. Olderstudents do various electronicprojects. Upon completion of theprogram, participating schools"adopt" one of the more than 10,000SSC magnets. Schools receive anadoption certificate, and the school'sname is placed on the adoptedmagnet.

Particle physicists based atuniversities also participate in schoolreform. For example, HowardS. Goldberg leads a team of scientistsand mathematicians from theUniversity of Illinois at Chicagowhich is developing a K-8 curri-culum, Teaching IntegratedMathematics and Science (TIMS).This initiative, based on under-standing fundamental concepts andprocesses, integrates the teaching ofscience and mathematics whileremaining faithful to the spirit ofboth disciplines. The conceptualframework for TIMS experimentsincludes a four-step format that con-tains the essence of the scientificmethod allowing students to definethe two primary variables, measurethe responding variable after havingset up an appropriate experiment andanalyze the results.

Kenneth G. Wilson participatesin The Ohio Mathematics/ScienceDiscovery Project, led jointly by theOhio board of Regents, the Ohio StateUniversity and Miami University.Initially, the project is aimed atimproving the way science andmathematics are taught to fifth-through ninth-graders by enablingthem to recreate some of the mostfundamental discoveries in science

through their own hands-on exper-iments. Resource centers for teacherswill be established across the state.The new extended community ofteachers, faculty, and researcherscreated by this program will ensurethat every classroom teacher hasstate-of-the-art advice and assistanceavailable every day.

Other particle physicists are in-volved with hands-on science muse-ums modeled after the Exploratoriumin San Francisco, California, foundedby Frank Oppenheimer. MICRO-COSM is a new science museum atCERN. Hannu I. Miettinen is afounder and former director ofHeureka, the Finnish Science Cen-tre. SciTech, a science museum inAurora, Illinois, was founded byErnest Malamud.

AT FERMILAB THE OLDESTcontinuing collaboration be-

tween particle physicists andeducators is responsible for the 35precollege education programsoffered in 1990 for over 10,000students and 2100 teachers. Thefollowing description of thoseFermilab programs focused directly

Physicist Jim Strait listens intentlyto Zelda Tetebaum during the SummerScience Project at Fermi NationalAccelerator Laboratory.

BEAM LINE 9

Middle: California high school physicsteachers Richard Taylor, left, and EarlBoynton review material during SLAC'sweek-long Teacher Workshop. Above.Visitors to SciTech in Aurora, Illinois,enjoy the three-armed Chaotic Pendu-lum, one of over 130 hands-on exhibits.Scitech, an interactive science andtechnology center, had over 62,000visitors its first year.

on particle physics serves as anexample of what a research facilitycan offer.

HIGH SCHOOL PROGRAMS. Fermilab'shigh school programs include re-search participation, institutes andworkshops, classes, support net-works, instructional materials, andPublic Information Office guidedtours.

Fermilab has offered summer re-search appointments to teacherssince 1983. A first-hand experiencein the laboratory can translate intoenhanced teaching strategies and newinstructional materials for teacherswho may never have worked in aresearch environment. Current ap-pointments are supported in part bythe DOE Teacher Research Associ-ates Program (TRAC).

Institutes and workshops offeropportunities to enhance teachers'backgrounds in basic science, modelsuccessful teaching strategies, andpresent new instructional materials.Fermilab has offered the SummerInstitute for Science and Mathe-matics Teachers (Summer Institute)since 1983. Today, the Summer Insti-tute is sponsored jointly by Fermilaband Chicago State University and ishosted at the university primarilyfor Chicago public high schoolteachers. The program combines the

Physicist Steven Delchamps, left,explains the process of superconduct-ing magnet making to teachers duringthe Summer Science Project at FermiNational Accelerator Laboratory.

expertise of scientists and mathe-maticians who give seminars on cur-rent research topics with theexpertise of master teachers who leadlaboratory sessions that present thetopics at a high school level.

One of the most effective spin-offs of the Summer Institute isPhysics West, a teacher supportnetwork. Teachers meet on a month-ly basis during the school year toshare successful teaching strategiesand instructional materials. A corpsof dedicated teachers keeps thenetwork alive, and Fermilab supportsthe newsletter.

Topics in Modern Physics is acollection of instructional materialsdeveloped from Saturday MorningPhysics, the Summer Institute andthe 1986 Conference. Included are athree-volume teacher resourcemanual, a series of 15 videotapesfrom the Fermilab archives and sev-eral posters. Fermilab provides thematerials at cost, and the six physicsteachers who prepared the collec-tion give presentations and work-shops at professional meetings andon request.

Three sessions of SaturdayMorn-ing Physics are offered annually forhigh school students. The class,which meets for three hours on tenSaturdays, includes a lecture by asenior scientist and small discussiongroups and tours of the Laboratorywith postdoctoral students.

Research experiences are offeredto high school students who havethe background to join either anexperiment or a technical group. Twoprograms, the DOE High SchoolHonors Research program andTarget: Science and Engineering,enhance the research experience with

10 FALL 1991

seminars or classroom work on arelated science project.

MIDDLE/ELEMENTARY SCHOOL PROGRAMS.

Fermilab makes research appoint-ments to middle school teachers inthe TRAC Program, offers institutesand workshops, supports a middleschool teacher network, and providesinstructional materials. Studentsmay visit the Laboratory and partici-pate in a variety of science experi-ences. Two particle physics programsare described below.

Beauty and Charm: an Introduc-tion to Particle Physics includes aninstructional unit, a tour, and ateacher training workshop. The pur-pose of the unit is to provide anexperience in science to broaden andenrich attitudes and to develop anappreciation for physics. Studentswho study the unit at school maytour the Laboratory and meet with aphysicist.

The Summer Science Project is asummer institute for middle schoolteachers who receive training as leadteachers. During the institute par-ticipants develop a disseminationproject to implement during the fol-lowing school year. The curriculumincludes seminars on basic science,training sessions on effective scienceinstruction and inservice techniques,laboratory sessions where partici-pants try out hands-on classroomactivities, and tours.

In 1983 it was not at all clear thata research facility was an appropri-ate setting for a major science reforminitiative. Teachers and studentshave given us the answer: "Yes!"Precollege education programs workat DOE national laboratories becauseit is not business as usual. Teacherscome to a world class high energyphysics research laboratory for aunique opportunity to witness sci-ence conducted at the frontiers ofhuman understanding and to learnfrom leading research scientists. Stu-dents have an experience in sciencethat broadens and enriches their at-titudes and develops their apprecia-tion for science. Students see, per-haps for the first time, what the worldof science is really like, and they likewhat they see.

Above left. High school teachers fromMassachusetts engage ProfessorJerome I. Friedman, right, one of lastyear's Physics Nobel Laureates, indiscussion after his talk to the group.The program is supported by BatesLinear Accelerator Center and theNational Science Foundation. The"mentors" helping the teachers aresupported by the Department of Energy,an example of collaboration amongfunding agencies to improve the qualityof pre-college science education. Aboveright. Students in CEBAF's BEAMSprogram compete in a "slow" bicyclerace, computing their average velocityby measuring their time and distance.

0

BEAM LINE 11

1V„~~~~or~~~~~ 1<s~~~~~~~~ ,ops~ TargetH. Breuil, Four Hundred Centuries of Cave Art

Workshops TargetTHE NEXT LINEAR COLLIDERA review of recent linear collider conferencesin Lapland and Protvino by two participants.

by DAVID BURKE and EWAN PATERSON

WO WORKSHOPS CONVERGED THIS FALL to focus and

sharpen the goals and development of future electron-positron linear colliders. These two meetings, onecentered around the particle physics of e +e - collisions,and the other around the accelerator physics andtechnology of linear colliders, were attended by a totalof nearly 250 physicists from all parts of the globe.Together they marked the first opportunity for such abroad representation of particle and accelerator physi-cists to meet for serious discussions aimed at therealization of the next high-energy electron-positronlinear collider.

12 FALL 1991

The First International Workshopon Physics and Experiments withLinear Colliders was held atSaariselka in Finnish Lapland thesecond week in September and wasfollowed a week later by the gather-ing of accelerator physicists inProtvino, USSR, for the Third Inter-national Workshop on the Accelera-tor Physics and Technology of Lin-ear Colliders. These meetings werespawned by recommendations fromthe International Committee on Fu-ture Accelerators (ICFA) and wereorganized by the Finnish Institutefor High Energy Physics (SEFT) andby the Branch of the Institute forNuclear Physics at Protvino (BINP).The tightly coupled sequence of twomeetings created a most effectiveformat.

Burton Richter (SLAC) opened theSaariselka meeting with a report onthe recent performance of the SLCand an overview of progress in theadvancement from the SLC to thenext linear collider. Gus Voss (DESY)introduced the audience to the vari-ety of work and problems being ad-dressed by accelerator physicistsaround the world, and MichaelPeskin (SLAC) outlined the particlephysics issues to be confronted bythe workshop participants.

The opening session was followedby extensive parallel meetings inwhich various studies and resultswere presented and critically com-pared. A unique and compelling pro-gram of physics to be addressed by acollider capable of reaching 300-500GeV emerged in the synthesis of workreported by European, Japanese, So-viet, and American scientists. De-tailed studies of the production anddecay of top quarks, studies of theinteractions of electroweak gaugebosons, and exhaustive searches forscalar particles comprise the heart ofthe physics program for the next lin-ear collider. These investigations willdirectly and decisively probe the mostpuzzling of the outstanding prob-lems of the Standard Model of

electroweak interactions. Displayedthroughout the plenary talks thatended the conference, this consen-sus of the participants was succinctlyput forth by Bj6rn Wiik (DESY) in hisfinal summary: "The physicsprogramme of a 300-500 GeV e+e-collider with luminosity 10 fb-1 peryear has unique features and iscomplementary to the physicsprogramme at the hadron collidersLHC and SSC."

It is remarkable that the mass ofthe top quark is (apparently) greaterthan that of the electroweak gaugebosons, and furthermore, approachesthe vacuum expectation value of theHiggs field. Understanding the roleof this particle in the hierarchy ofnature is an important goal for thefuture. The analysis techniques andmachine parameters required for ex-perimental investigation of the topquark were extensively discussed andconsiderably sharpened at Saariselka[Peter Zerwas (DESY) and KeisukeFujii (KEK)]. Helicity analyses of theproduction and decay of top quarkswill provide constraints of a few percent on the static moments of thetop quantum state and similar con-straints on unexpected componentsin its charged electroweak coupling(e.g., a right-handed current). Theseanalyses are significantly enhancedby the ability to control the spin ofthe incident electrons to project outdifferent polarization states of theproduced tt pairs.

The top quark may also be a sourceof new particles. It will be particularlyimportant to look for decays such ast -9 bH+ or t - ty. Some of these mayoccur with significant branchingfractions, and are easily isolated atan electron-positron collider. Withsufficient luminosity (30-50 fb-1) itwill be possible to measure the crosssection for production of the ZOttfinal state, which allows a deter-mination of the Higgs Yukawacoupling of the top quark. This firstprobe of the Yukawa piece of theStandard Model Lagrangian will

constitute a major test of ourunderstanding of the origin of mass.

Scan of the tt threshold regionwill allow measurement of the massof the top quark to better than0.5 GeV. This is an interesting mea-surement in itself, and, when com-bined with expected improvementsin measurements of the W mass atCDF and LEP II, will constrain theHiggs mass with accuracies of 20-30%. This would comprise the ulti-mate test of radiative corrections tothe Standard Model. These same datawill pin down the strong couplingconstant as(2mt) to within 5%-comparable to measurements madeat the ZO with LEP, and with studiesof QCD at center of mass energiesabove the tt threshold [SiegfriedBethke (Heidelberg) and Valery Khoze(St. Petersburg)].

Many new and beautiful studiesof the physics of gauge bosoninteractions were reported at theworkshop [Kaoru Hagiwara (KEK) andTimothy Barklow (SLAC)]. Thefundamental structure of the massiveW and ZO boson states and their selfinteractions remain an important un-tested sector of the Standard Model.The static magnetic dipole andelectric quadrupole moments of thecharged W bosons can be measuredwith accuracies of 1-2% by carefulanalysis of the helicity structure ofthe WW final state produced at 400-500 GeV center of mass. This eleganttechnique is fully able to resolve thediffering couplings of the photon andZO boson to the W from a single dataset; the improvement in accuracy ofmore than an order of magnitudeover similar measurements to bemade at LEP II is attributable to therapid growth with center-of-massenergy of the production of longi-tudinal gauge bosons-the remnantsof the Higgs fields that are essentialto the Standard Model. Confirmationthat the interactions of these newmassive polarization states aredescribed correctly by the StandardModel is essential, and no other

BEAM LINE 13

technique will provide the precisionand discrimination offered by anelectron-positron collider.

Production of multiple gaugebosons will allow further investi-gation of the three-boson couplingprescribed by the Standard Model,and samples of tagged W-pairs aslarge as 105 can be accumulated andused to search for anomalous W-decays. Beam energies of 200-250GeV yield the largest cross sectionsand best detection efficiencies forthese studies. At considerably higherenergies and luminosities it willbegin to be possible to study W-Wscattering processes [Ken-Ichi Hikasa(KEK)]. Unfortunately, in the absenceof TeV-scale resonant structures, theevent rates for such process areextremely low, and it will requiremachines with TeV center-of-massenergies and luminosities approach-ing 1035cm-2s - 1 to resolve differingmodels of strong WL interactions.This clearly awaits a future genera-tion collider.

A centerpiece of experimental par-ticle physics for the foreseeable fu-ture is the systematic search for sca-lar particles that can play the role ofHiggs fields. It is essential that nopossible avenue of escape be left forthese states and that we fully under-stand the spectrum of such particlesand their interactions. Extensiveanalyses reported at the conference[Howard Haber (Santa Cruz) andSachio Komamiya (Tokyo)] madeclear that data samples well belowthe performance expectations of vari-ous accelerator designs will be suffi-cient to illuminate even the mostcomplex Higgs spectra. As with LEP,these studies are limited only by theenergy of the accelerator; masses upto 70% or so of the nominal center ofmass are accessible. Of particularinterest is the fact that recent calcu-lations of loop corrections increasethe upper limit on Higgs masses thatcan be tolerated within the param-eter space of the Minimal Supersym-metric Standard Model (MSSM).

These limits exceed the grasp ofLEP II but are well within the reachof the next linear collider. The SUSYtheory working group concluded[Fabio Zwirner (CERN)] that a linearcollider with center-of-mass energyof 500 GeV is guaranteed either tofind at least one Higgs (and mostprobably three neutral states and thecharged states), or the MinimalSupersymmetric Standard Model isruled out. These new states gener-ally appear in the intermediate massregion where discovery and investi-gation at hadron colliders are diffi-cult tasks.

Initial studies of more subtle, butequally important, aspects of Higgsphysics were also reported- searchesfor non-dominant decay modes wereamong the most interesting. Theexciting prospect of measuring thecoupling of the Higgs to two photonswas explored in some detail. Photon-photon collisions, created by back-scattering laser beams from the in-coming electrons and positrons justprior to the interaction point, pro-vide a rich variety of final states forstudy [Valery Telnov (Novosibirsk)].Production of Higgs particles by thismechanism occurs with good rate inmost cases and is easily detected.The H°y coupling is attributable totriangle graphs and receives contri-butions from all particles that carryelectric charge and whose mass isgenerated by the Higgs mechanism.This is true regardless of the mass ofthe particle in the loop (!), and itendows this measurement with aspectacularly large discovery reach.

Historically electron-positroncolliders have been ideal huntinggrounds for new particle states, and awide variety of search goals and strat-egies were presented at the work-shop. These ranged from exotic lep-tonic states like heavy Majorana neu-trinos [Wilfred Buchmiiller (DESY)]to the familiar spectra of particlespredicted to exist in models of super-symmetry [Jean-Francois Grivaz(LAL)]. While colored objects such as

squarks and gluinos are copiouslyproduced at hadron colliders, it willrequire successful experiments withe+e- colliders to find and sort thespectra of colorless sleptons, charg-inos, and Higgsinos.

T HE PARTICIPANTS LEFT theautumn colors of Saariselka (300

kilometers north of the Arctic Circle)with a crisp view of the future ofelectron-positron physics and look-ing forward to the next workshop inthis series to be held on the balmyshores of Hawaii in the spring of1993. For many, however, the road toHawaii passed through the SovietUnion, where the Third InternationalWorkshop on Linear Colliders (LC9 1)was held in Protvino, 100 kilometerssouth of Moscow. Nearly 100 scien-tists from Europe, Japan, the U.S.,and the USSR met for ten days ofdiscussion and review of current is-sues in the development of futurelinear colliders.

A report from the Saariselka work-shop [David Burke (SLAC)] set the

stage for two days of plenary sessionsin which updates were heard fromeach of the laboratories working todevelop linear colliders. The encour-aging performance of the SLC duringthe past year, and the lessons thathave been learned from it, were re-viewed by Stan Ecklund (SLAC). Af-

ter considerable upgrading of the in-frastructure of the accelerator andwith maturation of the control andfeedback systems, the luminosity andefficiency of accelerator operationshave shown dramatic improvement;the SLC met all of its luminositygoals for this past year's run. TheSLD detector was rolled onto thebeam line, and was able to success-fully complete an engineering testrun. Further improvements are ex-pected in the next year, and with thepolarized gun also being installedand debugged, the SLD group is an-ticipating a fruitful run for particlephysics in 1992 and 1993.

14 FALL 1991

Linear colliders can be broadlycategorized by the method used togenerate and utilize rf power and thestructure of the main accelerator.The DESY/Darmstadt Collaboration[Thomas Weiland (THD)] has madeconsiderable progress toward under-standing the limits of machine de-signs based on existing S-band(2.8 GHz) klystrons for rf generation.Meanwhile KEK [Koji Takata] andSLAC [Ron Ruth] are pressing klys-tron technologies toward higher fre-quency (X-band at 11.4 GHz) wherehigher accelerating gradients are ex-pected to be achievable. The SLCprovides the experimental tool re-quired for further development ofS-band accelerators, but stand-alonefacilities are needed to test newX-band technologies. Both SLAC andKEK are planning such test beds.Klystrons suitable for incorporationinto these facilities are under devel-opment at SLAC and are expected tobe available soon. High-power testsof rf pulse-compression techniquesmodeled after the SLED cavities usedat SLAC will be completed withinthe year. Success with these testswill allow completion of the neces-sary engineering and the start ofconstruction of full-scale sectionsof X-band accelerators with gradi-ents of 50-100 MeV/m.

A slightly higher frequency(14 GHz) is being pursued by thegroup from Protvino working on theVLEPP Project [Vladimir Balakin(BINP)]. The VLEPP rf system uses anovel coaxial long pulse charge-stor-age network that is discharged todrive 14 GHz klystron tubes. Keyelements in the klystron are amulticell gridded cathode usedto control the current and a per-manent magnet focusing system.One of the highlights of the work-shop was a tour of the 20-meter-long test facility, presently un-der construction in the labora-tory at Protvino, that will be usedto test these systems later nextyear.

The advent of superconductinglinacs offers another approach tolinear collider design that is beingstudied by the TESLA Collaboration[Dieter Proch (DESY)]. Very long(msec) low-frequency (1.5 GHz) rfpulses are generated by conventionalklystrons, stored in superconductingstructures, and used to accelerate alarge number (-1000) of electron orpositron bunches. Spot sizes of a fewtenths of a micron can then be usedto reach the required luminosity.The large spacing between bunchesand the small luminosity per bunchcrossing that are inherent to thistechnique eliminate many of theproblems created by the high-brightness beam-beam interactionsincorporated into other designphilosophies. Unfortunately, thesemachines require extensive refriger-ation plants, and much of the lengthof the linac is consumed by cryostatsand other plumbing. The challengeto this collaboration is to arrive at acost-effective design.

Progress is continuing at CERNon development of a two-beam linacfor CLIC [Wolfgang Schnell (CERN)].

Robert Palmer's original Guide to LinearCollider Design (in white). As noted onpage 17, the three topics shown at thetop of the figure are recent additions tothe design matrix.

BEAM LINE 15

These accelerators differ ratherfundamentally from existing accel-erators in that generation of rf poweris not done in klystron tubes but iscreated by a low-energy high-currentbeam of electrons that is transportedalongside the high-energy low-current beam that is being accel-erated. Radio-frequency power isperiodically extracted from thissecondary beam and fed to theaccelerating structure of the mainlinac. The energy is replaced usinglow frequency (353 MHz) supercon-ducting cavities. The secondary beamand its transport structure are inessence a single multi-port klystronthat operates at very high frequency(30 GHz in the CLIC design). Tests ofstructures are underway, and a testfacility is expected to be operating inabout a year.

The opening plenary sessions wereconcluded with summary talks onthe recent improvements in themodeling of the interactions of high-energy, high-intensity beams [KaoruYokoya (KEK)], and a status report

from the Final Focus Test Beam Col-laboration (FFTB) [Alexander

Mikhailichenko (Novosibirsk andBINP)]. The FFTB is well underway.

Approximately one half of the mag-nets fabricated at Novosibirsk havebeen delivered to SLAC for installa-tion, and the remaining magnets areexpected to be completed and in-stalled by spring of next year. It isanticipated that the beam line willbegin operations at the beginning of1993.

The participants separated intofour working groups and continuedmore detailed discussions of theproblems confronting the buildingblocks of linear colliders. A groupchaired by Jean-Pierre Delahaye(CERN) reviewed the preparation of

low-energy, low-emittance beams.Integration of the properties ofelectron guns and bunching systems,positron sources, damping rings, andbunch compressors to achieveoptimum performance is an intricate

problem that received muchattention in this group. Newapproaches to positron productionusing wigglers or channeling crystalswere discussed, and the use of pre-damping or collector rings with largeacceptance was reviewed.

A group chaired by GilbertGuignard (CERN) worked on prob-lems of beam dynamics and controlduring acceleration in variousstructures. The main topics weresingle and multibunch beam stabilityand alignment and trajectorycorrection techniques. Much of thediscussion centered on beam-basedalignment techniques that haveworked well at the SLC. It isimportant that the ultimate accuracyof these procedures and the instru-mentation required to make themsuccessful be fully understood. Morework remains to be done. Strategiesfor controlling multibunch insta-bilities using tuned acceleratingstructures were discussed at somelength. Here, too, more work iswaiting to be done. Optimization ofBNS damping and linac focusing tomaintain control over emittancegrowth generated by single-bunchwakes was also discussed. This is, bynow, a well established techniquethat is incorporated into nearly everymachine design.

A large group organized bySeishi Takeda (KEK) reviewedproblems in the development of high-power rf sources and acceleratingstructures. Everyone was interestedin the VLEPP 14 GHz klystron.Inefficiencies in beam transmissionhave prevented these tubes fromreaching full voltage, but the Protvinogroup is optimistic that additionaldesign changes will alleviate thisproblem. SLAC and KEK are makingsteady progress with tubedevelopment at X-band. The latesttubes in this series have generatedmicrosecond pulses with 40 MW ofdelivered power; 50 MW is the initialgoal. Everybody is experimentingwith high-power output windows as

damage is a problem already at40 MW. There are several designs forlarge area windows that lookattractive.

Careful examination of calcula-tions of wakefields created by in-tense bunches in detuned or dampedaccelerating structures led to opti-mism that the complexity of addingdamping slots can be avoided andthat detuned structures will provideadequate control of these wakefieldsto allow multiple bunches to be ac-celerated on each rf pulse. There alsowas useful discussion on manufac-turing techniques for acceleratorstructures, and in particular, the ad-vantages and disadvantages of cell-by-cell tuning strategies and the prob-lems and costs of maintaining preci-sion during fabrication.

John Irwin (SLAC) organized a

fourth group to concentrate onproblems particular to the final-focusand interaction region. This groupcovered several topics includingoptics designs, problems associatedwith the beam-beam interaction,detector backgrounds, and instru-mentation. There was an interestingdiscussion of wide-bandwidth opticsdesigns [Reinhard Brinkman (DESY)],

and in particular, evaluation ofmechanical and electrical tolerancesneeded to make them work. Alsodiscussed was a new concern thatthe finite resistance of the metal inthe beam pipe of the final quadrupolelens might destroy the emittance ofthe beam. This was found not to be aproblem, but it did place a limit onthe aperture of the final lenses usednear the interaction point. This limitwill have to be incorporated intofuture designs.

Experience with the SLC has madeit clear that the experimentaldetectors are an integral part of themachine, and control of non-physicsbackgrounds is a challenge foraccelerator and particle physicistsalike. Problems in beam collimationand detector shielding are beingattacked by a growing number of

16 FALL 1991

accelerator and particle physicists,and much work has gone forward inthe calculation of the backgroundsthat can be produced in theinteractions of small intense beams.The peripheral photon flux thataccompanies the electron beamsgrows with increasing energy, andcan become a troublesome source ofbackground if the luminosityproduced by the collider within theresolving time of the detectorbecomes too large. This ultimatelywill constrain machine designs tofavor higher repetition rates andlower luminosities per bunchcrossing. Of particular interest at theworkshop were new calculations ofhadronic photon-photon processes[Manual Drees (DESY)] that may posefurther limitations on machine anddetector designs. It remains for futurework to sharpen these issues, but itis apparent that Robert Palmer'sGuide to Linear Collider Design onpage 15 is more complex than onemight have imagined!

N VIEW OF THE VERY difficultand uncertain conditions in the

host country, the Workshop atProtvino was organized and con-ducted in a remarkably gracious at-mosphere well-suited to intellectualexchange and growth. The confer-ence fulfilled an important functionfor the international community, andserved as a useful professional expe-rience for all who attended. As thepace of work on linear colliders ac-celerates, so too will the need forphysicists and engineers to gather todiscuss and redirect their efforts. Thenext in this series of workshops(LC92) will be held next Summer inGarmisch Partenkirchen, Germany.

0

BEAM LINE 17

TOWA 10 THI3 NEXT 2 INEAR CO LIER

The Accelerator Test Facility at KEKKOJI TAKATA

HE ACCELERATOR TEST FACILITY (ATF) wascreated to promote TeV linear collider re-search at KEK, the National Laboratory forHigh Energy Physics in Tsukuba, Japan. TheJapanese high energy physics community hasbeen investigating the feasibility of next gen-eration TeV electron-positron linear colliders

-imilr .1 n h+-r\ 'NT,---- T i .n-r -11-A-,i,, /NTT r\Oll1lC1111 LU L I -, INVLAL -llldal l UlllULUCi I1 L k.

under study in the United States. We call our collider the JapanLinear Collider or JLC. Studies for the JLC were initiated as earlyas 1982, when construction of the 30 GeV TRISTAN storage ring,the first high energy electron-positron collider in Japan, was at itsheight.

We have been examining the realistic possibilities that wouldenable us to reach a luminosity greater than 103 3cm-2 s-l atcenter-of-mass energy around 1 TeV. In a linear collider there aretwo identical main linacs that accelerate beams from a few GeVto several hundred GeV facing each other-one for electrons andthe other for positrons. We decided that the main linac should usethe well-established technology of conventional linacs. The ac-celeration gradient should be 100 MV/m or more so that the linacsare not too long, and the wall plug power should be less than200 MW, the maximum available at KEK. Those considerationshave led us to the following JLC design parameters: (1) the mainlinacs are at X-band frequency (11.4 GHz, four times the SLAC S-band frequency of 2856 MHz); (2) these linacs accelerate ten beambunches and have a repetition rate of 150 Hz; and (3) the beam sizeat the collision point would be about 5 nm high by 600 nm wide.

18 FALL 1991

i

ONVENTIONAL S-BAND LINACSstill play important roles in high

energy linear colliders as injectorsand positron production linacsbecause of their capability of accel-erating high currents. Therefore,although research for the X-bandlinac is key, S-band technology isstill being pursued intensively. A 2.5GeV S-band linac has been inoperation at KEK since 1982 as theinjector for the Photon Factory andlater on for the TRISTAN Accumu-lation Ring. Such a busy machine isnot useful for experimental accel-erator studies, and when we foundedthe ATF in 1987 we started to build atest S-band linac. We wanted thislinac to accelerate the very short,high current bunches required bythe JLC design and to have an accel-erating gradient higher than theconventional 10 MV/m so we couldgain experience with high gradientoperation. We plan to test a 1.5 GeVdamping ring (also an important partof a linear collider complex), and thelinac was to be extended to the damp-ing ring injection energy if enoughspace was available.

Even at S-band the rf source was acentral issue. The most powerful S-band klystron then available in Ja-pan was a 30 MW tube that was notreliable partly due to the old designof the electron gun. Therefore, wewere grateful to SLAC when several5045 tubes, the state-of-the-art klys-tron used for the SLC, were shippedto KEK in 1987 and 1988 according tothe US/Japan Cooperative Agreement

in high energy physics. We designedthe test linac to use these powerfultubes. It has a bunching section withsubharmonic cavities to capture thebeam and a short SLAC-type struc-ture to accelerate it to 40 MeV. Theregular part of the linac has 3 m longSLAC-type structures with a gradi-ent of 40 MeV/m. The peak rf powerrequired is high, and the SLED sys-tem is used to compress the klystronoutput power. Using the 5045 tubesand the SLED system, only six klys-trons will be needed to get to 1.5 GeV.This should be compared with morethan 40 klystrons for the KEK 2.5 GeVlinac.

We have set-up the linac on the25 m by 25 m floor of an unusedTRISTAN experimental hall, whereradiation safety regulations can be

The high gradient test setup at the ATF.Using this 66.5 cm long S-band struc-ture an average gradient of 92 MV/mwas attained with an input power ofmore than 160 MW. Beside the structureHelmholtz coils are shown that are partof the test S-band linac beingconstructed at the A TF.

BEAM LINE 19

easily satisfied. We prepared an areasurrounded by shielding blocks forthe injection section and one or tworegular structures in addition. Wehave used precision beds to easilyplace linac components to a preci-sion of a few tens of microns. Theinjection section has been completed,and a couple of regular structures arebeing manufactured. Outside theshield we set up four rf systems, eachconsisting of a modulator, klystronand evacuated waveguides, capableof delivering a peak power of 100 MWwith 1 tus duration.

The test linac construction is notthe only activity at the ATF. The rfsystems have been very useful forother R&D including having bigmodulators for developing X-bandklystrons and developing low powerrf components and control circuitsto meet the stringent specificationsof the JLC.

HE MOST IMPORTANT research

at the ATF has been high gradi-ent tests with S-band structures.Without high power X-band klys-trons, such tests are usually done atS-band. Information obtained is valu-able for estimating the performanceof X-band structures at 100 MV/m.When the 30 MW klystron was ouronly high power source, we tested ashort structure using a special wave-guide system to boost the rf power.The rise time of the power pulse waslong and the flat top short, but wereached 100 MV/m anyway. The 5045

tubes have enabled us to test a longerstructure by driving it directly withsufficiently long, rectangular rfpulses. A peak power of 200 MWwith a 1 gis duration is possible if wecombine outputs from two klystrons.We fabricated a SLAC-type structure66.5 cm long with a high shunt im-pedance to try to reach 100 MV/mwith this rf pulse.

We began rf processing the struc-ture mid-1989, and after 900 inte-grated hours of processing at 25 Hzwe reached 92 MV/m. Beam accel-eration was tested with these fields.We were particularly concerned byfield-emitted dark currents, sincethey are a crucial factor in operatingat such high gradients. Currents in-creased drastically as the gradientapproached 100 MV/m. Their behav-ior was in good agreement with thatobserved in experiments at SLAC.The energy spectra had a low-energycomponent due to local dischargesand a broader one that was the resultof acceleration over the structure.This structure was fabricated by aJapanese firm in the same way con-ventional linacs for medical use were.We are now testing a second one thathas the same geometrical shape butwas fabricated more carefully withless surface contamination. The pro-cessing time seems much shorter asexpected.

A S MENTIONED ABOVE one goalof the ATF is to construct a

1.5 GeV damping ring. The beam

emittance required at the JLC is 1 to2 orders of magnitude smaller thanthat of presently operating storagerings. A promising way to achievethis is to use wiggler magnets. Thewiggler fields are complicated, andwe want to study the beam dynamicsand verify reaching low emittanceexperimentally. This is the reason,which we consider a strong one, thatwe wish to construct a test dampingring. The ring will have a racetrackshape about 60 m long by 30 m wide.

One of problems associated withthis plan has been finding a spacelarge enough for the linac and ring.To solve it, we are clearing a buildingthat had been used to assembleTRISTAN rf cavities. Substantial civil

construction is needed to reinforcethe floor to hold shielding and tomeet radiation safety regulations. Wehope that the construction of thelinac and ring complex will be com-pleted by the end of 1994.

Although the main activity isconstruction of the S-band linac, weare pursuing other R&D for the JLC.

The X-band klystron and structureare particularly important issues andneed a great deal of fundamentalstudy. If we are fortunate in carryingout this work, we will set up a shortsection of the X-band linac besidethe damping ring and acceleratesmall emittance beams extractedfrom it. At that point it will be pos-sible to judge whether technologieshave matured enough to constructthe JLC.

O

20 FALL 1991

FROM THE EDITORS' DESK

THIS IS THE SIXTH ISSUE of the new version of the Beam Line,which may be a time for us to pause briefly to take stock. Our masthead says"A Periodical of Particle Physics," and particle physics has in fact been thesubject of most of the articles that have appeared since our first issue in thesummer of 1990. But the niche we are trying to carve for the Beam Linemight better be described as "A periodical of particle physics and other a

things that the particle physics community is likely to be interested in."(Not a very elegant masthead.) Thus in the present issue we have articles onwhite dwarfs and science education, and in earlier issues there have been |nie.ces on onsmnlnov and sociPnrPc nnlirv........- V.L.. . CL. L ....... -,'1 · tL-cwin IVI. IV/cvllan, 1907/-1991

This is all to the good. We hope to continue this mix of high-energyphysics (HEP) and related fields, of the factual and the speculative, of thespecific and the general. We have articles in train on possible new accelera-tors, on detectors for the SSC and for gravity waves, and on computationdevelopments in particle physics. We continue to be very interested insoliciting articles and ideas for articles. We have a special interest inspeculative or opinion pieces about where the field of HEP is going, or shouldbe going. Give us a call, electronic mail, or FAX and let's talk about it.

WE TAKE NOTE OF THE RECENT DEATH of Edwin M. McMillan,one of the great names in our field of physics. A particularly warm andinteresting tribute to McMillan, written by J. David Jackson, appeared inNature 353, 602 (1991).

We also take note of the devastating fire in mid-October thatdestroyed several thousand homes in the hills above Oakland and Berkeley,California. We know of ten of our particle-physics colleagues and 45 othersfrom Lawrence Berkeley Laboratory who lost their homes in this inferno.And this rather hard upon the heels of the large earthquake almost two yearsearlier. Our sincere sympathies to those affected.

BEAM LINE 21

LETTERS TO THE EDITORS

Why SSC?

In most cases I write letters to the editorout of anguish, to protest some thing orthe other. This time it is praise. I find thearticle on "Why SSC" (for short) byRiordan to be excellent. Probably thebest I read on the subject so far. I urge youto try to give it wider circulation bysending it to other journals. Would theCERN Courier take it?

DRASKO JOVANOVICFermilab

We give our permission to anyone whowishes to reprint Riordan's article"Why Are We Building the SSC?" pro-viding they acknowledge the BeamLine. We do not know what the CERNCourier's reaction would be to reprint-ing the article; however, we would behappy to publish an article on the LHC.

The Beam Line has a distributionof about 4500. If you would like toreceive it, send electronic mail toBEAMLINE@SLACVM or regular mail to theBeam Line Editors, SLAC, P.O. Box

4349, Stanford, CA 94309 USA.

91 PAC

It's not that letter writing is a lost art,this [electronic mail] is just a lot easier.

In the Summer 1991 issue, in a re-view of the '91 Particle Accelerator Con-ference, Robert Siemann said "The sec-ond absence is disturbing: Advanced ac-celerator concepts were not well repre-sented." I hope this was in reference tothe number of papers and not to theircontent. The AWA (Argonne WakefieldAccelerator) group had a few papers atthe conference, and we had a few col-laborators at SLAC who also used some"advanced techniques" to measure new

concepts in accelerator design. We thinkthe ideas were represented well.

It is true that new concepts are lost inthe ocean of engineering work being doneon accelerators. One factor which both-ers me is the lack of accelerator engineer-ing programs at universities. The tech-nology is stable enough and the applica-tions are growing to the point wherebusinesses are buying accelerators forproduction work. The search for newmethods of building accelerators shouldbe a much smaller group than the im-proved engineering design of applicationsalready in place.

I don't think there is an absence ofadvanced accelerator concepts. There isso much room for improvement of all theold concepts that it just swamps out thenew ideas by volume. Once a new idea isshown to work it becomes an old one andmore people work on improving it. Withthe number of accelerators on line in theworld for specific purposes it's hard tosee where there is time for pure accelera-tor research.

The AWA mission is higher gradients,an "advanced accelerator activity." Weare very lucky to be able to focus onresearch without having to worry aboutusers. I agree with Siemann that moregroups similar to the AWA should exist,but I doubt that they will ever seem to be"well represented" in numbers comparedto all the other accelerator work nowtaking place.

MIKE ROSING

Argonne National Laboratory

Robert Siemann replies: About tenyears ago was a time of ferment inadvanced accelerators. Plasma-basedaccelerators, grating accelerators,inverse free electron lasers, two-beamaccelerators, collective implosionaccelerators, wakefield accelerators,.... were actively discussed. The

presentations of those ideas and thediscussions about them were, to me,highlights of the PAC's of the 1980s.They were never a large fraction of thepapers, but they were among the mostinteresting.

Times have changed, and advancedaccelerator physics has matured. Someideas were explored and found to bewanting. Others were found suffi-ciently interesting that they havebecome part of the mainstream ofaccelerator physics. I missed theferment at the 1991 PAC and mycomment was addressed to that. Ididn't hear new ideas for reaching1 GeV/m, for using recent advances inlaser technology for accelerators, etc.Maybe the realities are too hard, ormaybe we are in a consolidation phasefollowing a creative flurry.

Whatever the cause for my percep-tion about the 1991 PAC, I valueadvanced accelerator work and wasusing the Beam Line article to expressmy concern for it. It needs resourcesand people, and here I am in completeagreement with Dr. Rosing.

EGS

In our Beam Line article, "EGS-A Tech-nology Spinoff to Medicine," we ne-glected to mention the important contri-butions made by Hideo Hirayama of KEKand Ray Cowan of MIT. In particular, thecover photograph was produced by meansof the SHOWGRAF graphics package de-veloped for EGS by Dr. Cowan.

RALPH NELSON, SLAC

ALEX BIELAJEW, NRC, Ottawa, Canada

We are indebted to Ray Cowan for thecover of the Spring 1991 issue of theBeam Line.

22 FALL 1991

CONTRIB UTORS

VIRGINIA TRIMBLE oscillates at a frequency of 63.4 nHz between the PhysicsDepartment of the University of California, Irvine (where she has tenure)and the Astronomy Department of the University of Maryland, CollegePark (where her husband, physicist Joseph Weber, is Professor Emeritus).Her degrees come from UCLA (B.A.), California Institute of Technology(M.S., Ph.D.), and Cambridge University (M.A.). She was the 1986 recipientof the U. S. National Academy of Sciences Award for scientific reviewingand currently serves as editor of Comments on Astrophysics and associateeditor of the Astrophysical Journal. Virginia is interested in stars andgalaxies and other structures that require relatively little maintenance.

MARJORIE G. BARDEEN is Program Director of the Fermi National Accelera-tor Laboratory (Fermilab) Education Office and is the Vice President ofFriends of Fermilab, a not-for-profit corporation whose sole purpose is todevelop and conduct precollege education programs at Fermilab in Batavia,Illinois. She is also an elected member of the Board of Trustees, College ofDuPage, in Glen Ellyn, Illinois, and is currently the Chairman. Marge holdsa B.A. in Mathematics from the University of Minnesota and an EducationalCertificate (Mathematics) from Elmhurst College, Elmhurst, Illinois. Underher leadership numerous needs assesment workshops with teachers, schooladministrators, and community leaders have been conducted to developscience education programs, and 40 such programs currently make a wealthof resources available to elementary and secondary teachers and students atFermilab.

DAVID BURKE. a regular contributor to the Beam Line. is head of Exnerimen-tal Group I, established as part of SLAC's effort on the accelerator physics andtechnology of the Next Linear Collider (NLC), in addition to playing a majorrole in the construction and instrumentation of the Final Focus Test Beam(Vol. 20, No. 2, 1990). David came to SLAC in 1978 and distinguished himselfin the commissioning of the Stanford Linear Collider and as a key memberof the Mark II Collaboration. He served on the Organizing Committee of theSaariselka conference and organized the NLC study efforts at previousSnowmass (Colorado) meetings. i

BEAM LINE 23

EWAN PATERSON is a Professor at SLAC and is Assistant Director for

Accelerator Systems. He has been at SLAC for almost 20 years and has beeninvolved in the commissioning, development, and construction of all the

accelerators on the site. He is interested in many areas of advanced accelera-tor development from next-generation linear colliders to next-generationsynchrotron light sources.

KojI TAKATA is head of KEK's TRISTAN accelerator department. He came to

KEK in 1972 and became engaged in developing fast kicker magnets for the

12 GeV proton synchrotron. In 1977 he became responsible for design,construction, and operation of the rf system of the Photon Factory 2.5 GeV

electron storage ring, and then in 1980 also of the TRISTAN 30 GeV electron-

positron storage ring. Since 1982 he has been supervising R&D work at KEK

for TeV linear collider technologies.

24 FALL 1991

I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~