55-73/221 NOTE ON THE VACUUM SYSTEM FOR A�

LARGE ELECTRON STORAGE RING�

H. Winick *� Cambridge Elee tron Accelerator - Harvard University�

ABSTRACT

Rough estimates are made of the total gaa production including that

due to synchrotron radiation 1n large high energy electron ringa. The

e><pected gas loads are low enough that dis tributed pumping is not nec 4

esuty. The requirements of II lumped pumping syatem capable of pro­

du.: i ng an average E-ressure -p <_ 1 x Ie -8 torr as requ i red f or a beam

lifetime of > I hour :1re evaluated. Lower pressures may be required at

int er tiC t ion reg ions to reduce background due to i ne I as t ie e -p proc e ue s

and special syatems would be required at those locations.

Introduction

In considering a vacuum system for a lsrge radius, high energy elec­

tron storage ring one thinks first of a distributed pumping system, aince

such a system t!lOst efficiently pumps t~e gaa liberated by synchrotron

radiation liS vell as gas from chamber Burfllce outgass1.ng with no beaD'l.

A diatributed ion pumping system utiliZing the Ulagnetic field of the

bending magnets of the s torllge ring to stabilize the ion pump dillcharge

l 2has been used succen fully lie Novosibirsk lind Spear.

However the electron ring currently under considerstion for NAL has

such a low guide fie ld (see Tab Ie I) that a distributed ion pWllp utiliZing

this guide field would be inefficient. In particular, a pump of the SPEAR

design would have about 40'; of its mllxiroum speed in a field of 1000 Gauss

and less than lO~ at 500 Gauss.2

The loss of apeed could he ameliorated

to some extent by increasing the radius of the pump cells, but this would

require an increase in useable lIIIlgnet aperture.

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A distributed cryopumping system might also be considered. Such a

system was originally proposed for SPEAR. ~ Also· a large lumped cryo­

4pUlllping system was proposed fur the OrDnitron. A cryopumping system for

the large NAL electron ring might be considered especially if this ring

would make use of part of the large refrigeration capacity that may be

available from a superconduct ing-magnet proton ring that might be nearby.

However a cryopumping sys tem of such large phys ica 1 extf!nt would require

an extensive research and development program to solve the engineering

problems a!lsociated with it and might, in the end, prove economically

less lIttractive than the distributed system.

In thta note we sh<lll show that the expected gas loud will very

1 ikely be small enough to be hllndled by a conventional lumped ion pUl!lp

system and discuss the design parmneters of such a system.

Pressure Regu irements

Scattering and bremsstrahlung interactions of circulating electrons

with the residual gas will cause loss of particles and effect beam

lifetime. We do not consider here intera!: tions among the partie les of

ORe beam and beam-heam interactlons which also affect the lifetime but

are not pressure dependent.

For a given ma.:hine aperture, energy acceptance) snd operating pressure,

the pntial lifetime due to scattering increases wieh increasing energy

while the partisl lifetime due to bremsstrahlung decreaaes logarithllll.cally

with energy) Thus bremss trah lung ",lll be the dominant pressu re dependent

faceor determining the single beam lifetime. This is already true for

SPEAR at 3-4 CeV. Scaling from the numbers g~ven in the SPEAR proposa13

8results in the .equirement for an operating average pressure of:S 1 x 10.

torr for 8 lifetime of ;:: 1 hour at 25 CeV. This assumes that the gas

conslsts almost exclusively of CO, CO2 and other gases of similar or lower

values of atomic number, Z. The bremsstrahlung cr08s~gection increases

as Z2 sO it is important that heavy gsses, such as argon, be excluded.

This should be no problem in a .,.,11 constructed vacuum system and mass

speetrometer analyses of residual gases on operating storage rings have

shown no s ignificanc contribution by hesvier gases.

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RP PoveX' and Cooling Requirements

The totsl rf power that goes into synchrotron radiation is�

4�88.5 E (GeV) p .. l(A) KW

p(m)

For the purpose of thb note we have adapted a figure of 7 HW of beam

pover. The total power of the rf system would be somewhat higher because

of the excitstion power requirements.

The 7 MW sets 8 limit on the current that can be achieved at a given

E t p. This is the current lilted in Table 1.

In addition the 7 HW !DUst be removed by water cooling. This powel'

11 d1stX'ibuted ovel' the circumference such that the power per meter is

7HW P/_tel' K

For the smallest ring under consideration (P .. 750 m)JP/_tel'

1.5 KW/meter.

The� synchl'otron radiation strikes the vacuum chamber along a stripe

2that is -lllllll high. Thus the power density is _ 1.5 KW/cm • This is a

rather high power density and carefut consideration wilt have to be given

to the cooling system.

VacuWll Des ign Reguirements

In a lumped pumping system cons18 t ing of pumps of speed S(l/sec)

seps r.a te d by tube 8 0 f c .:>nduc t snc e C( l / sec) the sve l'age press U re is

~ [1 + _s_ ] Torr (See Appendix A) ( 1) S 12 c

where Q (t~:;- t) is the gas load per pump, assumed to be uniformly dis-o _ 2Q

otributed. A reasonable vacuum system therefore has S .... 12 C snd p "" S­

8�and, 811 explained earlier, we need an average pressure P::: 1 X 10- Torr.

As will be shown later a reasonable vacuum chamber might be 6 m long

and of elliptical cross-section with s'emi-axes of 2.5 cm and 7.5 em, total

surface srea of .... 2 x 104cm2 and a conductance of 10 l~~:rs (see Appendix B).

Assuming a specific outgassing rate of 6 x 10-12 tor~-liters this chamber cm -sec

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would have a total gas load of 1.2 x 10-1 tor~;~ i ters. Such outgassing

rates are achievable in well constructed bakable high vacuum systems and

SPEAR achieves rates of tbb order with aluminum chambe-rs that are baked

before installation but tlot baked in place-even after release to atlllOs­

pheric preas ure • ',lith a120 1~:~r pump on th 19 chamber (S = 12 C) the

pump pressure would be 1 x 10-9 torr and the average preuure 2 x 10-9 torr

in the absence of beam.

Next WI! consider the goll8 load due to the effects of synchrotron radia­

tion ~ieh has been studied by many investigators ,5-8 The dominant mechanism

for the production of gas has been shoWl'l to be the following two step

proceu:9

(a)� It. synchrotron-radiation photon releases an electron from�

the wall it strikes (Photo-electric effect)�

(b)� This photo-e lee tron desorbs s gas molecule upon leaving�

and then re - en te r lng the aur face (e lee tro -de gorp t 101'1)�

A reliable pred ic tion of the gas product ion reqUires therefore a

knowledge of the magnitude and energy dependence of the photo-electric

effect10,1l on the material concerned at all energies up to several times the

critical energy, ~ c' plus a similar knowledge of the electro-desorpt ion

process. In reViewing the experimental resul ta from d liferent laboratories I

one finds variationa in surf ace material, angle of incidence, cleanliness

of surface I preVious hh tory of vacuum system etc I all of wh ieh somewhat

ohacura direct cOIIlparison,

The re"ult" ll'sy be expresaed in terms of .' desorpt ion coeff ic ient,

D(E� ) which gives the number of gas molecules produced by one synchrotron r

radiation photon. D(£7) is of course 8 function of photon e.nergy and

alao depends on photo-electron energy, lingle of incidence, surface material,

cleanllne8ll, etc:.

The number of synchrotron radiation photons emitted per aecond may

be eatimated roughly by assuming that all of the energy is radiated at

the critical energy €c

N t§./turnr 40 E(GeV)

elect-turn

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converting to circulating current in amperes gives

N 40 IE 20-.1. ltO 2.5 x 10 leA) E(GeV) photons/sec sec e

12 Actual numerical integration over the synchrotron radiation spectrum

yields a higher number

20N/sec ... 8.0 x 10 leA) t(GeV) photons/sec (2)

At the CEA measurements made with small beam currents 8

(several mAl

showed that the pressure rise near a pump in a ring straigh t sec t ian was

quite linear with stored heam energy and current from 1.1 to :5.0 GeV (correa­

ponding to critical energies of 0.1 to 2.;:1 KeV). Typical values of preuure

rise. were

boP "" 1.0 x 10-7 I(A) E(GeV) Torr

although lower values were obtained after prolonged operation without

opening the system to stm()S(ileric pressure. Since the straight section

pumping speed in this case was - 100 t/see the gss load for the entire

ring (48 II tnigh t sec t ions) is

Q ... 5 x 104 I(A) E(GeV) Torr-liters/secr

16 ... 1.75 x 10 I(A) E(GeV) molecules/sec.

Using Eq. (2) this corresponds to sn average desorption coefHci.ent

of

1.75 x lOa; D(E ) ~ 2.2 x 10-5 molecules/photon

., "" 8.0)( 1020

In the etA ring the synchrotron radiation s trikes an uncooled wall of

II ceramic vacuum chamber at grazing incidence (qJ "" 80 lIlilliradians) and the

ba&e pres lIure is - 5 >' 10.9 torr.

Desorption cae Hie ients of :: 1.0 )( 10-5 molecule9/phocon were measured

in the CEA bypass for electron energies up to 2.5 CeV (E = 3.4 KeV). The e

synchrotron radist ion 8~rikes the uncoaled s ta1n1ess s tel'll wall of the bypass

vacuum chamber partly at graZing incidence and partly near normal inc idence. The

10base pressure in the bypass is _ 4 x 10. torr.

Other data comes from studies of gaa product ion at SPEAR. '7 Measurements on

this storage ring resul t in va lues of D(E y ) of ~ J,.O x 10-5 molecules/photon

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for stored beallls of energy between 1.2 and 2 -55 CeV (corresponding to

critical energies of .30 and 2.9 KeV}. In SPEAR the synchrotron radiation

2 strikes a water cooled aluminum surface at near perpend icular inc idence.

The SPEAR results show chat the desorpt.ion coefficient appears to be

dropping rapidly above about 3 KeV.

This is in accord with cesults on che photo-electric yield for

x-rays m ahtminum10,1l which show a yield that d",cresses as llE for r

photons between 3 and 30 KeV. For other materials che yield is approy­

imately constant over this energy range. The yield is lowest for nonnal

in¢idence, varying 8S 1/sio qJ •

Thus it appears that higher energy photons (> 3 KeV} are less efficient

in desorbing gas molecules on aluminum, If this 19 true the total gas production in

high energy storsge rings may be considerably less than one would expect

from scaling gas production v~th the total number of photons or with the

synchrotron radiation power.

It muat be stressed however that 8 desorption coefficient for aluminum thac

decreases with photon energy above 3 KeV is based on some lit tIe data on the storage

ring SPEAR and on data un photo-electric yield up to 30 KeV. We 101111

extrapolate this darn to make rough estimates of the g88 production fror.t

synchrotron radiation spectra with much higher critical energies, but these

estimates !tlould be regarded as preliminary unt it better data become s ava1.lsb Ie.

Theoretical and eKperimental work on this problem is presently underway at

1the Stanford Linear Accelerator Center. il. ThiS, coupled with data that will

be available wtIen SpeAR opaules at higher energy (e.g., at )\ GeV, (c = 11 KeV)

should lead to a much improved understanding oC the problem Ln the next

year.

Prediction of Synchrotron Radiation Gas Production in Large Electron Rings

To arrive at a definite prediction of gas production in large electron

storage rings we will !!lske use of a computer program developed at the

Stanford Linear Accelerator Center. This program divides the synchrotron

radiation spectrum up into three regions:

(a) E < 20 eV • . These photons Bre ignored on the assumptiony

that they have a negligib ly small desorption coef fic ient.

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(b)� 20 eV < E < 3 KeV. This region is divided into about� r�

12� intervsls. The program calculates the number of photons

12in each interval and uaes an energy dependent desorption

coe f f 1c ient (as de termin~d from pressure rise mess urement s

on SPEAR operating at different energies) to calculate the

gas production. The sverage value of the desorption co­

efficient in thb region is about 2.5 )( 10-5 mol-eculea/photon.

(c)� E)' > :3 KeV. For this region a deBOr?tion coeffic ient that

decrellSes as l/Er is used. It is normalized to D(E ) '" r

4 x 10-5 molecules at E '" 3 KeV and the calculation is photon r�

carried out to E = 10 E� r c

The resul ts of th is calc ulation are shown in Table I. This table

gives the maximum current (corresponding to synchrotron radiation loases

of 7 HW) Bt diffnent energies and bending rlldii, and the predicted g88

loads. These gas loads must be regarded as rough ea timates (especially

.t: the higher critical energies) and some factor should l)e included

for conservatism.

Vacuum Pumps and Chambers

In cons idering the requirements of the magnet lattice .nd lengths

of bending IllAgnets, 13 a pump spacing of 5-6 meter. uema reason.ble. We

w111 adopt 6 meters as our pump spacing. For the largest synchrotron

radiation gas loads II pump of ;: 120 liters/sec speed vill be required and

according to Eq. (1) a chamber conductance of ;;: 10 liters/sec 18 desirable.

For. 6 meter long chamber an elliptical cross~seetion with semi-axes of

2.5 cm and 7.5 cm has a 10 liter/sec conductance (see Appendix B).

As shown earlier the gas losd per pump for this chsmber in the absence

of beam is expected to be - 1.2 )( 10.7 torr~lit:ers . This should be sec

added to the synchrotron radiation,gas load per pump given in Table 1.

The highest gas load per pump thus obtained (for the case of Ii 15 GeV,

1.2 A ring with a 750 m bending radius) is 5.7 x 10.7 tor;:~1ters

With� a 120 literIsee pump the pump pressure is 5 x 10-9 torr and the

8 average pressure is 1 x 10- torr.

Thus our a.verage pres sure requirement is met with no safety margin

in this case. ()' course a larger pump and/or larger chamber conductance

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could be used to obtain some safety margin in this wors t case.

In the case of a 20 GeV, -375 A ring with 750 III bending radius (which

was selected for particular at:eention by the NAL summer study group) the

synchrotron radiation gas load per pump is 1.9 x 10-7 tor~:~i ters (from

Table I). Adding 1.2 )( 10.7 tor::~iters for chamber surface outgassing

g1vell a total gas load per pump of 3.1 )( 10 -7 tor~;~ iters With a 120

literIllec pump the average pressure would be ~ 5 X 10-9 torr: Le., a

factor of two lower than the design requirement.

Of courle IIIlIny variations on chslllber conductance, pUVlp spacing snd

PUIIlp Ipeed can be IIIlIde utilizing the data of Table I, or better data that

may become available later, in an optimization procell8. This should be

done ill detail once tina parameters are set.

The author benefitted from discuuionl with N. Dean, B. Garwin,

J. Jurow and .I. 1eel af the Stanford Linear Accelerator Center. The

COlllputar proltlllll uaed to calculate laa production was developed by

I. Garvin and J. Jutov.

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REFERENCES

* Now at Stanford University.

1 •� Budke r, Protopopov, Skrins!<y: Proc. of VI I lnt. Cemf. on High Ene rgy�

Ace" Vol. II, p. 37 (1969).�

2.� U. CuDDtlings, N. Dean, F. Johnson, J. Jurow, J. Voss; Jour. Vsc. Sc 1.�

and Tech., Vol. 8 No.1, p. 348 (1971).�

'3.� SLAC Storage Ring Group; Prop. for a High Energy e+e - ColI idinS Beam�

Stonge Ring of SLAe. (June 1965).�

4.� Omnitron Design Study Group: The Omnitron Propos a1, p. VII-4 (July 1966).

5.� M. Bernardini and i.. Halter; Journal 'lac. Sci and Tech., Vol. 2 No. "

6.� G. E. Fisher and R. A. Mack; Jour. 'lac. Sci and Tech., Vol. 2 No. j,

p. 123 (1968).

7.� N. Dean J E. Garwin, J. Jurcw, J. Rees; SPEAR Technical note on, Syn­�

chrotron Radiation Induced Gas Desorption at SPEAR, in preparation.�

8.� H. Winick, 1967, unpublhl1ed.

9.� E. Garwin, "3 BeV Colliding Beam Vacuum System," SLAC MelllO, Aug. 1963.

10.� I. M. Izrailev; Soviet Physics - Tech. Physics, ~, No. 11, p. 1020

11.� A. S. Ganeev ancl 1. M. Izrallev; Soviet Physics ~ Tech. Phys ies, Vol. '"1,

No.3, P' 270 (Sept 1961).

12.� R. A. Hack; Spectral and Ang. Disc. of Synch. Rad. CUL, 1027, Feb. 1966.

13.� T. L. Coll ina; private cOIIIJIunication.

14.� E. Gu.,in; private commun!cat ion.

-- --

TABLE I (b)rea} 2.2£3

E p B f: =-- QT QT/N

(CeV) (til) (Causs) (Amp) C

(KeV)p

(10.5 Tor::;iterll) H(e) (10 -8 .!2E!.:.!.iters

--�sec

~O 2.0 x 103 :;00 .20 30 4.0 2000 2

1.5 670 .15 40 3.7 1500 2.5

1.0 1000 .10 60 3.~ 1000 ,.4

·75 1340 .075 79 3.4 750 4·5

25 2.0 X 103 420 .40 17 6.8 2000 ,.4 1.5 560 .30 23 6.6 1500 4.4

1.0 840 .20 34 6.4 1000 6..4 .­

-75 1120 .15 46 6.1 750 8.1

20 2.0 X 103 330 l.0 9 14 2000 ., I

-D 1.5 440 ·75 12 14 1500 9.30 I�

1.0 660 .50 18 14 1000 14

.75 890 .375 23 14 750 19

15 2.0 X 103 250 3.1 3·7 31 2000 16

1.5 333 2.4 5.0 3~ 1500 23

1.0� 500 1.6 7.4 3~ 1000 34

.75 666 1.2 9.9 34 750 45

(a)Th19 current corresponds to "7 MW of power into synchrotron radiation for one beam (for e-p colliding beams) Dr the total in two beams (for e01- e ~ colliding beams)

(b )The total gss load due to synchrotron radiation, Qy , is the reault of a computer calc.ulation. See Te1l-t. (c)N is the total number of pumps assuming 6 meter spacing between pumps.

APPENDIX A

Av@rag@ Pressure in a Lumped Pumping System

Consider a sys~em of pumps equally spaced around a large ring"

t

fi\ l:>x

0 x

,

I

I

~ I

~ J I

I�

I� II 1(=1I

x=I/2xaO

Le~: S Ilpeed of pump

c length of vacuum chamber between pumps�

B .. perimeter of vacuum chamber�

q = outgllss {ng rate per uni~ Ilrell�

res istance of vacuum chamber per unit length�

C = -!t = of�conduc tance vacuum chamber�

Q = q HI ., Outgas9 ing rate of one vacuum chamber� o�

Q(x) outgassing rate of chamber from x to 1/2 •�

p(x=o) = 0o/S�

P = average pressure along vacuum chamber�

Then Q(x) = x)Bq and the pressure drop across c.x is 6P(x) o(x)rL'>x(f ~

Integrating this' gives

x2 Jf x

~p(x) = r f x

= rBq f (~- x) rBq [~ xQ(x)dx dx - 2

x:O -0 0

P(x) = p(x"O) + rHq [f x ~]

p

l/2 '/2 [

P(x)dx £. + rBql ~. p(x=O) dx1/2 ~ (i · ~)] rBql Q 0 0p o 0p(x"O) + p(x=O) +- -+ -2.�

12� 12C S 12C

~ [1 + ~] 12c

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APPENDIX B

Calculation of Vacuum Cha:nber Condllctance

A cube of lengch L, and elliptical croSB-section (semi axes a,b)

bas * a condu:a[n;:o:i]V~J2 by a2b2

C • lL.. [., : sec

b' ]'72

where all dimensions are in cm.

Some representative calculations follow:

C 1Herss(cm) b(cm) ~

LO t..5 LO

1.25 7.5 ;').0

1.875 6.875 5.4

2.5 7.5 10.2

3.75 7.5 21.7

*S. Dushman - Scientific Foundation of Vacuum ,echoal 08r J John Wiley, 1962, p. 89.

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