Nuclear Instruments and Methods in Physics Research A317 (1992) 445-452North-Holland

A superconducting radio-frequency cavity for manipulating the phasespace of pion beams at LA

PFJ.M . O'Donnell, J. Davis, R.A. DeHaven, E. Gray, R. Johnson, R.E. Lomax, B.J . McCloud,J .A . McGill t , C.L . Morris, J. Novak, B. Rusnak and G. TubbLos Alamos National Laboratory, Los Alamos, NM 87545, USA

J.M. Applegate, T.D. Averett, J. Beck and B.G. RitchieArizona State UniversM Tempe, AZ 85287-1504, USA

E. HaebelCERN, Geneva, Stivitzerland

D. Kiehlmann, U. Klein, M. Peiniger, P. Schäfer and H. VogelSiemens AG, Accelerator and Magnet Technology, PO Box 100100, D5060 Bergish Gladbach, Germany

H. Ward and C. Fred MooreUniversity of Texas at Austin, Austin, TX 78712-1081, USA

Received 14 February 1992

The SCRUNCHER is a superconducting radio-frequency cavity for manipulating the longitudinal phase space of the secondarypion beam from the low energy pion channel at LAMPF. Test results of the cavity performance and initial results from in-beamtests are presented .

1 . Introduction

Radio-frequency (rf) cavities can be used for manip-ulating the longitudinal phase space of charged particlebeams. This new technique is ideally suited to sec-ondary beams produced by a linac such as the protonaccelerator at the Clinton P . Anderson Meson PhysicsFacility (LAMPF), because of the very small timespread of the primary beam and consequent smalllongitudinal phase space of the resulting secondarybeam, as noted previously [1]. Many experiments usingthe LEP channel have count rates limited by the beamintensity available at the required resolution . The abil-ity to compress a medium resolution beam into anarrow energy band enables these experiments to use aconsiderably larger beam intensity . This paper de-scribes the use of such a device, a SuperConducting Rf

Current address : The Superconducting Supercollider Labo-ratory, Texas, USA.

0168-9002/92/$05 .00 G 1992 - Elsevier Science Publishers B.V . All rights reserved

Unit for Changing the Energy Resolution (theSCRUNCHER) for longitudinal focusing of the sec-ondary pion beam from the low energy pion (LEP)channel at LAMPF [1].

The beam envelope on exiting from the LEP chan-nel is a tilted ellipse when plotted as delta (momen-tum) against phase (time), as displayed in fig. 1 for a100 MeV beam with S = ± 1% (S = (p - po)lpo, wherepo is the central momentum) . The velocity terms and anegative correlation between momentum and pathlength through the LEP channel contribute to theobserved correlation . The use of an rf cavity allows thebeam to be focused in the momentum-time (longitudi-nal) coordinates much as quadrupole magnets are usedto focus the beam in the two spatial (transverse) coor-dinates. An rf cavity can be used to decelerate the highenergy pions which arrive at an earlier time and accel-erate the low energy pions which arrive later. Resultsof such a phase-space rotation are also shown in fig. 1,where a single cell rf cavity operating at 402.5 MHz

446

50

Scruncher off

-100-1.5 -1 -05 0 0.5

1

1.5-45 -1 -0.5 0 05

1

15dPVp (7)

ap/p (%)Frog. 1 . Simulations of the longitudinal phase space at the exitfrom the LEP channel with the SCRUNCHER off andSCRUNCHER on . The incident beam has T_ =100 MeV andS= ±l%. The figure shows scatter plots ~of pion relativephase against momentum, and the projection of the longitudi-

nal phase space onto the momentum axis.

J.M. O'Donnell et al. /A superconducting radiofrequency cavity at LAMPF

Scruncher on

and at 5 MV/m (x.86 MV) is seen to be capable ofreducing the energy spread in the beam by a factor ofmore than 5.

For this purpose a 402.5 MHz superconducting ra-dio-frequency (rf) cavity was procured . It has beeninstalled in the LEP channel, instrumented, tested, andrecently used for several experiments. Several newtechnological innovations were implemented in thiscavity, including vacuum windows/heat shields in thebeam pipes, variable coupling to the cavity, and roomtemperature piezoelectric tuning . These are describedfurther below.

2 . System design

2.1 . Cryogenics subsystem

The superconducting cavity, constructed from 3 mmthick copper with a 3 mm explosively bonded niobiumlayer, is housed in a cryostat cooled by liquid helium(LHe) flowing in copper pines attached to the outercopper surface . A schematic diagram of the refrigera-tion system is shown in fig. 2 . A 60 1 LHe storagedewar is installed at the top of the cavity and is used asa buffer for an automatic filling system supplied from a

Fig . 2 . Cooling system for the SCRUNCHER, showing the cavity (at liquid helium temperatures), the heat shield (at 50 K), theintermediate storage vessel and Pine 5001 dewar.

2.2. RF system

J.M. O'Donnell et al. /A superconducting radiofrequency cavity at LAMPF

500 1 dewar. The boiled-off cold gas from the cavitycools the heat shields surrounding the cavity .

Pions from the LEP channel enter and exit thecavity through beam tubes sealed with aluminized my-lar windows mounted at the location of the heat shield .The original 76 p.m windows have been replaced by 25p,m windows to reduce the multiple scattering andstraggling of the pion beam . These windows also helpreduce the radiative heat load on the cavity by a factorof 200 (calculated) from that without the windows(= 10 W). The windows also isolate the cavity vacuumfrom the upstream and downstream beam-line vacuum,preventing contamination of the superconducting sur-faces with dust or oil.

The static heat load on the cavity was measured bythe LHe consumption rate (not including transfer lossesin the LHe system) with no rf power applied to thecavity to be 11 1/h, corresponding to a heat load ofabout 8 W, consistent with design calculations, predict-ing about 7 W. This heat load is primarily due toconduction through the beam tubes and the coupleralone. More recently improved insulation has broughtthe static heat load down to 5 W. Typical rf powerlevels of 6 W for operation with a 50 MeV beamenergy increases consumption to 15 1/h LHe, increas-ing to almost 50 1/h at the max:mum fields obtainablefor higher beam energies .A period of about eight hours is required to cool

the cavity to operating temperatures (= 4 K) fromroom temperature . Rapid cooling as the temperaturecrosses the critical temperature for niobium (11 K) wasfound to give the best final quality factors for thecavity, attained by forcing LHe through the coppercooling 'tines, rather than relying on gravitational feedfrom the cryostat . Several days are required to warmthe device to room temperature with a procedure in-cluding operation of heaters mounted at the bottom ofthe cavity, and flowing hot gas through the coolingsystem .

RF power is fed to the cavity through a coax-wave-guide-coax transition with the final coaxial line cou-pling electrically to the cavity. The input coupler qual-ity factor, Qcoup, is adjustable between 1 x 10 7 and3.6 x 10 9 by mechanically moving the center conductorof the final coaxial line with respect to the cavity. Aplot of Qcoup as a function of the position of the centerconductor is shown in fig. 3. The ability to easily adjustthe coupling was used in conditioning the cavity, whereit is necessary to drive the cavity at high power levelswith low Qcoup values, testing different phase andamplitude control schemes, and accurately measuringthe cavity quality factor at high Qcaup values .

The central frequency of the cavity is tuned by

Ûv

10'° r

109

108

i° Lo

3. RF control

_

Vcav

f-f0) J

PI

2( rlq)Qcav

1+ (

fU

Qcav

2

447

20 40 60 80 100 120Coupler position (mm)

Fig. 3. Q of the coupler as a function of the position of thecenter rod in the coupler.

mechanically changing its length which decreases at arate of 200 Hz/Wm. Three mechanical screws, each inline with one piezoelectric crystal, were mounted onone end of the cryostat and compressed one of thebeam tubes to tune the cavity . The breakdown prob-lems associated with operating piezoelectric crystals ina cryogenic environment were solved by mounting thepiezoelectric crystal external to the cryostat. The rangeof the mechanical tuner was ±0.8 MHz about thecentral frequency of 402.5 MHz, while the piezoelectrictuner had a range of ± 12 kHz.

The cavity is driven by an rf amplifier equipped withan isolator such that the cavity resembles a matchedresistive load to the amplifier. The minimum amountof power ?, needed to drive the cavity at a givenvoltage Vca� and at a frequency f is given by

where r/q is the shunt impedance of the cmity, fo isthe resonant frequency of the cavity, and Q~av is thequality factor . A value of r/q = 76.5 SZ was obtainedfor the geometry of the SCRUNCHER cavity using thecode URMEL-T [2]. From eq . (1) only 11.34 W ofpower is needed to reach the design gradient of Eaa =5MV/m =Vca�/37.26 cm, at the design Qcav of 2x 109if the cavity is critically coupled so that all of thispower is delivered to the cavity.

448

If the cavity is driven as little as 20 Hz away fromthe resonant frequency, the rf driving power is in-creased by a factor of 10000. The amount of rf powerrequired to drive the cavity depends critically upon thebandwidth needed to keep the cavity phase locked tothe desired accuracy 41+51 . 11he needed bandwidth isinfluenced by acoustic vibrations of the cavity at fre-quencies above the first mechanical resonance (near 80Hz) where a feedback loop closed with the mechanicaltuner can no longer be used to control the cavityphase .

Provision for 3 kW of rf power was made in design-ing the rf control loop, because little guidance concern-ing the expected level of acoustic vibrations was found .Acoustic vibrations were minimized by mounting thecavity on vibration isolation pads, connected to a framewhich was bolted to a concrete pad mechanically iso-lated from the floor of the experimental area. With thissimple precaution, it was found that the phase of thecavity could be controlled within the required toler-ance with a feedback loop using only the piezoelectric(mechanical) tuner with much lower rf po-er levels(less than 30 W for 5 MV/m). Two advantages accruedfrom this discovery. The maximum field strength, andresulting X-ray production_, could be limited by limitingthe rf power without interfering with the phase controlloop . Also rf cabling of the cavity was simplified be-cause of the lower rf power levels. These lead tosimplifications in the interlock and personnel protec-tion systems needed for routine cavity operations.

While the controller for the rf power originally

J.M. O'Donnell et al. / A superconducting radiofrequency cavity at LAMPF

Mechanical Une Stretcher 20dbFreq Adj

Fig . 4 . RF phase and amplitude (dashed

incorporated three control modes (a mode where thecavity is locked to voltage-controlled oscillator, a modewhere the cavity is driven from the LAMPF referencerf and a self-excited mode), final operation with theself-excited mode was adopted because of the ease ofoperation of the cavity in this mode when bringing upthe fields and also locking the control loops . The rfcontrol system used is illustrated in fig . 4. It consists ofa self-excited amplitude control loop for providing rfpower to the cavity, and a phase-lock loop, closedaround the piezoelectric tuner and locked to the 201.25MHz accelerator frequency through a frequency dou-bler and a motor-driven mechanical delay line . Thefrequency of the cavity was compared to a beam pickupand phase adjustments made by an electronic phaseshifter and a sample-and-hold circuit . The frequency ofthe self-excited loop was adjusted via a manual trom-bone, setting it for a minimum reflected power orforward power if the amplitude loop was locked . Thismethod was very stable, and did not need adjustmentafter initial setup . The rf amplifier used in the self-ex-cited loop was a 25 W solid-state amplifier . This is thefirst instance that operation of a cavity with a highloaded quality factor has been performed in a produc-tion situation . The phase and amplitude were adjustedfor each pion energy, in order to minimize the beamenergy spread . The procedures used to find the opti-mum values for these parameters are described later inthis paper. The range of the piezoelectric crystals re-quired that the mechanical tuner be readjusted typi-cally every several hours .

3db

_

J

SBiasline) ccntru! Circuit tOr th,.; SCRUh1CHER.

4. Measurement of Q,,a,,

The design goal for the cavity was to achieve aquality factor Qcav of 2 X 109 at a field gradient of 5MV/m.On delivery the cavity was cooled and low level rf

power was applied using the self-excited loop shown infig . 4 . The variable coupler allowed the Q of the cavityto be accurately measured by varying the coupler tofind the minimum reflected power while the cavity wasexcited at low power in a self-excited loop, correspond-ing to critical coupling and Qcav = 2Qioad*

Qload was determined by measuring the decay timeof the cavity . Power levels in the cavity were measuredusing the diodes shown in fig . 4, which were calibratedagainst do power measurements of rf levels in thecavity using a calorimetric power meter. Typical powerdecay times (-r) of 1.2 s were obtained, correspondingto Qcav = 3.0 X 109, using

Qcav = 2Qload = 2'rrTf0 . -

(2)Since the minimum in the reflected power as a

function of coupler position could be determined withan accu:acy of about 0.1 mm, the uncertainty in theQcav measurement is about 1%, with a correspondinguncertainty in the calibration of the cavity field ofabout 0.5% (see eq. (1)). Although precise measure-ments have not been made, coarse measurements indi-cate that the field amplitude is being maintained tobetter than 1 or 2%, and phase lock to better than± 2°, using the circuit shown in fig . 4 . (We note thatthe phase error deteriorates when thermoacoustic os-cillations occur).

Measurements of Qcav versus Eacc are presented infig. 5, showing cavity performance as delivered, afterone day of conditioning with slowly pulsed rf power atlevels of near 400 W and a duty factor of 10%, andafter six hours of further conditioning with about 1 X10-5 mbar of helium in the cavity. After several monthsof use, the cavity became contaminated with oil andaluminized Mylar, following a rupture of one of the 76Rm windows . The final results obtained after cleaningthe niobium in the cavity with a series of Freon,ultraclean water, acetone and methyl alcohol rinses arealso shown .

The accelerating gradients obtained in the cavityare related to the peak electric fields by EP = 3.2E.,This factor was obtained from URMEL-T [2] calcula-tions . Thus EP = 20 MV/m was the highest fieldreached in the tests .

5. Measurements with LEP beam

J.M. O'Donnell et al. /A superconducting radiofrequency cavity at LAMPF

The SCRUNCHER was installed at the LEP chan-nel [3], and vacuum coupled to the CLAMSHELL

Ç' 109

1080

0 as delivered0 after RF processingo after He processing® after LANL cleaning

0

'911 19 ~r

a0 0 0

6

0

06

0I A A 1- I . A 1 1 I

2 4 6 8Ea~ (MV/m)

Fig . 5 . The quality factor of the cavity (Qcav) is plotted as afunction of the field gradient in the cavity . The points showdata for the cavity as delivered (), after one day of highpower conditioning (o), after a further six hours off conditioning with helium gas in the cavity C0, and after cleaning the

cavity as described in the text (

).

449

spectrometer [4] . The channel consists of an initial pairof quadrupole magnets, four dipole magnets to mo-mentum analyze the pions from the production target,and a final pair of quadrupole magnets. Typically thefirst two quadrupoles are set to give point-to-paralleloptics from the production target to the center of thechannel (vertical). A pair of collimator momentum jawsat the center of the channel then selects a givenmomentum bite for the beam, which is focused to aspot on the experiment target by the second pair ofquadrupoles . The pion flux from the channel is propor-tional to the momentum bite, and is also a function ofthe pion momentum .

The SCRUNCHER adds approximately 1.4 m offlight path to the pion channel . As no further focusingwas added to the channel, the extra drift causes thebeam-spot size at the target location to increase, inboth the horizontal and vertical planes, even afteroptimizing the final quadrupoles in the channel. Inorder to allow the CLAMSHELL spectrometer to ac-comodate the larger beam spot, without compromisingthe resolution, a position-sensitive, cathode-strip read-out wire chamber was added in the front of the spec-trometer . The size of the target image at the spectrom-eter focal plane could then be corrected in software tocompensate for the larger beam spot . This is particu-larly important in the verical plane, which is the disper-sion plane of the spectrometer .

5.1 . Optimum rfgradient and Qhli~se

1F-m-2âf

onmeld et 11, 0A stil~etco~,ducting radiofrequency cavity at LAMPF

-ßoA /4

F'0A I

#A

a c~naRmet (50Fig. ~6. Calculated beam momentum after tpe SCRU'hCHlrit(d ~Çn«) as a function of the bcan rnomentvrt~ into theSCRUNCHER (acnann~a) for a 50 I+~1eV' beam. The VoRwes arefor afield of 1 .64 MV/m (dashed) ~~hich g ives R®cc~tregati®nat S = 0~e) and for 1 .98 MV/m (s® i~). Voir the Ic~~,+'er fieldsetting, the dotted lines show a b~at1, resolution Of i 0.5417ccompressed to approximately ±0.®9cle-, the dmr-dashed ligeshows the momentum of the [; tnal beam With the

SCRUNCHES vFf,

The field gradient, E;,,C , required is the cavity maybe esümated from

L

' -'Poß'cEa~ _

_, +R;6

2TrzF-rr=

4,fR°~/4 dz cas

co~

~(

) .

where -11 (= 37.26 cm) is the 109th ofthe cavity, F.t-I- isthe transit-time factor (below), p,, i s the cerival mo-mentum of the beam, ß and y a,re the normal kine-matic factors for the central blearx energY, f, (- 16.7m) is the length of the LEP ch4ninel and -Pst, (- 6.45m) is a TRANSPORT matrix ekttient [5]. The transit-time factor corrects for the varüatibn of the field in thecavity with position and tirno, as the beach passesthrough the cavity:

Here, a is the wavelength of the rf and ß4 I co/c,where vo~ is the design velocity o f the cavity. F_~q, (3)gives the field strength required o remove any cOgtel a-tion between the pion momentuM before uind c*aftet theSCRUNCHER, at S = 0%. The dished curvy iR fig . 6shows the effect of the SCRiINC HER for a SD Nie'Vbeam with the field given by eQ, (3), The 'dot cd ones

show an input beam with a resolution of ±0.54%(filling 120° of the rf) compressed to approximately±0.09% . It is often possible to use a higher gradient .With a cavity field given by

aT"21FT ,r sin 60'

where AT, is the maximum energy spread in the beam,such that the beam fills ±60° of the rf, theSCRUNCHER over-compresses the momenta within±60°. (These estimates were presented in ref. [1] .) It isseen (solid curve, fig . 6) that now the momentum jawsmay be opened up to = ±0.7%, without degrading theoverall resolution . For 50 MeV, this would be 1.98MV/m, about 20% higher than from eq . (3) .

However, the above estimates assume an ideal cav-ity with no fringe fields . The fringe fields of theSCRUNCHER extend out of the cavity into the beamline so that a slightly lower field is more suitable . Onthe other hand, since the finite resolution of the spec-trometer has not been included in the estimates aslight over-compression is not detrimental .

Though eq . (5) gives a reasonable initial estimatefor Ea,:c, we used an empirical method to determinethe optimum gradient which also determines the cor-rect phase for the cavity . The optimum phase andgradient for the SCRUNCHER were obtained by mea-suring the momentum of the transmitted beam withthe spectrometer and with the cavity off. The pionbeam used had a small momentum spread (= 0.2%),obtained by closing down the momentum jaws in themiddle of the channel . In the first step in this proce-dure the SCRUNCHER phase was adjusted so thatthe momentum of the beam was not changed when therf power was turned on . Next, the field strength wasadjusted to minimize the correlation between thetransmitted pion momentum after the SCRUNCHERand the injected pion momentum as selected by themomentum jaws over the range of the input momen-tum desired . Curves similar to fig . 6 could then bemeasured . An optimum gradient of 1 .9 MV/m wasfound appropriate for a 50 MeV beam, in very goodagreement with eq. (5). Finally, the maximum momen-tum which could be accepted by the SCRUNCHERwas empirically determined by the limits of the mo-mentum jaws beyond which the correlation could notbe removed .

Not all experiments at LEP benefit from theSCRUNCHER, depending on the required beam en-ergy and resolution, and so the device is installed andremoved typically once a year. We have consideredhow the optimum gradient and phase change, as theSCRUNCHER is placed at different locations alongthe beam axis . For a 10 cm change in the location ofthe cavity, the correlation between flight time and pion

J.M. O'Donnell et al. /A superconducting radiofrequency cavity at LAMPF

momentum will produce a 0.6% change in time spreadof the beam . For particle velocity /3 = 0.67, this amountsto a change of 0.4% in the pion energy which can beaccepted by the rf. The correlation between path lengthand pion momentum through the channel does notchange with small changes in the location of the cavity .As the cavity position is not expected to change bymore than this magnitude between different experi-ments, only one series of scans (outlined previously)need be performed to determine the gradient . Theoptimum rf phase is very sensitive to the location of theSCRUNCHER along the beam axis . Indeed, a shift of10 cm corresponds to a phase change of 48°.A quick procedure has been developed to deter-

mine the correct rf phase if the gradient is alreadyknown. Using a beam with a narrow momentum spreadwith the rf phase lock circuit disabled, the input phaseis varied until a point is reached where a strong corre-lation is found between the final pion momentum andthe SCRUNCHER rf phase. At this point, the correla-tion between the final pion momentum and the time offlight through the channel should be minimized . (Thepoint which is 90° out of phase is easily found bylooking for no correlation between the final momen-tum and the SCRUNCHER rf phase.) Once the phaseis set, the maximum momentum jaw settings are thenquickly found by looking for the point where correla-tions are just visible between the final pion momentumand time of flight through the channel .

5.2. Influence of the SCRUNCHER on scattering experi-ments

The effects of improved beam momentum resolu-tion obtained with the SCRUNCHER in a scatteringexperiment can be viewed as either improved resolu-tion in measuring missing mass (with flux kept con-stant) in a spectrometer, or as a gain in flux (byopening the channel momentum jaws) while keepingthe experiment resolution constant.

Fig. 7 shows rr' elastic scattering data from a thin93Nb target at 50 MeV with the SCRUNCHER on andoff; all other experimental parameters are unchanged .The momentum jaws in the channel were kept fixed at0p/po = ±2%. The dramatic change in resolution (bya factor of 4) is evident . The overall system (channeland spectrometer) resolution contains equal contribu-tions from the beam and the spectrometer . To acheivecomparable resolution using only the momentum jawswould require cutting the incident pion flux down by afactor of 6.

This improvement in the quality achievable in lowenergy experiments shows clearly in fig . 8, where acomparison of ir' inelastic scattering from 12C at 50MeV and a scattering angle of 60° using theCLAMSHELL spectrometer is shown with and without

30

'J 10v°

-5 -4 -3 -2 -1 0

1

2

3

-Q (MeV)Fig . 7. Comparison of 30°, -rr' elastic scattering from 93Nb at50 MeV with the SCRUNCHER on (solid) and off (dashed).

the SCRUNCHER. With the SCRUNCHER off, thestrong well-separated states are visible (e.g. groundstate, 2+ at 4.44 MeV, 0+ at 7.66 MeV and the 1 +

360

240

120

48

24

0-5

Resolution Test93Nb (7-r+ 7T+)Tn = 50 MeV

Scruncher on

Scruncher off

;- 0.4 MeV

-a.

: -1 .6 MeV

f

Scruncher on

f

Scruncher off

-Q (MeV)

. . . . . . . . . I . . . . . . . .

5

15

25

451

Fig . 8. Raw data for 30 MeV inelastic a+ scattering from 12Cat 60° with the SCRUNCHER on and off.

452 J.M. O'Donnell et al. /A superconducting radiofrequency cavity at LAMPF

(T= 0) at 12.71 MeV). However, it is not until theSCRUNCHER is turned on that some of the weakerstates become observable . Thus, the 15.11 MeV(1 }, T= 1), 18.25 MeV (2 - , T= 0) and the 16.11MeV (2', T= 1) states are clearly observed in the"SCRUNCHER on" histogram . Many of these weakerstates are of interest, e.g. the 15.11 MeV mixes, throughthe Coulomb interaction, with the strong 1 + state at12.71 MeV [6-8]. The difference between the twohistograms arises solely from the improved resolutionavailable with the SCRUNC E , making it possible toseparate close states and improving the signal-to-noiseratio. We note that data similar in quality to the"SCRUNCHER on" histogram were obtainable priorto installing the SCRUNCHER [7]. However, this re-quired setting the channel momentum jaws to Ap/p®= +0.4%, which leads to long run times. This con-trasts with the present work, which used the flux fromthe channel set to Ap/po = ± 2.0%, and compressedthe beam resolution using the SCRUNCHER.

any experiments at LEP are hindered by lowcount rates . The SCRUNCHER now enables manysuch experiments to run with a larger flux obtained byopening the momentum jaws, and momentum com-pressing the beam back down to an acceptable resolu-tion. Some recent double-charge-exchange experimentsusing thick targets and trying to separate peaks 3.5MeV apart [9] have gained a factor of 2 in count rate .A gain of over four has been shown to be achievablefor experiments requiring better resolution (down toaround 0.3 MeV).

6. Conclusions

The SCRUNCHER, a superconducting rf cavitywhich can be used to rotate the longitudinal phase

space of low energy secondary beams from a pulsedsource, has been successfully developed . The deviceincorporates several new and innovative technologiesenabling very stable operation from a small powersupp:y . Most aspects of the SCRUNCHER have eithermet or exceeded design expectations . The cavity isexpected to have a strong influence on the low energypion program at LAMPF, enabling many experimentswhich were limited by the available flux to run in aconsiderably shorter time .

Acknowledgements

This work is supported by the Robert A. WelchFoundation and the US Department of Energy.

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