The ultra high vacuum system of Cryring

uum/volume 44/numbers 5-7/pages 497 to 499/1993 ~ nted )n Great Britain

0042-207X/93S6 00+ O0 © 1993 Pergamon Press Ltd

The ultra high vacuum system of CRYRING ~lBagge, H D a n a r e d , K Ehrnst6n, C J H e r r l a n d e r , J Hilke, A Nilsson and K - G R e n s f e l t , Manne Smgbahn I

~stttute of Phys/cs, Frescativ 24, S- 104 05 Stockholm, Sweden

e features of the vacuum system of the accelerator~storage rmg CRYRING are descnbed. Using NEG-pumps sed on the St 707 getter developed by SAES Getters (Milan, Italy) of low temperature type activated at 300"C d operating at room temperature in combination with sputter ton-pumps, a pressure m the low 10-11 mbar eqv) region has been achmved wtthout any bake-out of the vacuum system. The design value, 5 x 10-12 mbar eqv), is expected to be reached after a complete bake-out of the system at 300°C

i- Introduction Y R I N G is an accelerator faclhty (Figure 1) dedicated to lowrgy research mainly using highly charged, heavy ions 1 The m part of C R Y R I N G is a synchrotron/storage ring, which ws a heavy 1on beam to be accelerated to a maximum energy 6 (q/A) 2 MeV u 1 The ions are produced in different 1on 10urces placed on high voltage terminals ( < 50 kV) and are pre-

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accelerated to 300 keV u-~ (for q/A > 0 25) m a four-rod type R F Q before injection into the nng The nng circumference is about 52 m and the total length of the injecnon hnes Is about the same The very low mjecnon energy and the ambition to work with highly charged, low energy ions calls for speoal precautions m order to mammize losses due to rest-gas collisions This implies that the rang has to be operated in the 1 0 - | 2 m b a r region Also, fast magnet ramping (7 T s - J ) allows the ion energy to be

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L Baggeetal CRYRING increased to its m a x i m u m value in only 150 ms, thereby decreasing the losses m those cases where the rest-gas collisions are still disturbing

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2. The ring All specially designed vacuum chambers, tubes and flanges are made from refined stainless steel o f AISI 316 LN quality This quality o f steel has a very low magnetic permeabihty, which is o f great importance in accelerators o f the C R Y R I N G type High mechanical strength is also an i m p o r t a n t property o f this kind of steel Due to the fact that C R Y R I N G is a fast ramping machine, the dipole c h a m b e r s need to have water-cooling to c o m p e n s a t e for eddy current heating Here should also be menh o n e d that water-cooling has been a d d e d to the other vacuum chambers as well By keeping the whole system slightly above the dew point, a r o u n d 12 C, we will lower the desorbtlon significantly and thus also improve the final vacuum After careful chemical cleaning o f all stainless steel c o m p o n e n t s , they are subjected to heat treatment, so-called vacuum firing, m an atmosphere o f 10 5 m b a r and a temperature o f 950'C for 2 h Afterwards, the surfaces that are to be exposed to the vacuum are well protected in order to a~old c o n t a m l n a h o n In parncular any contact with organic materials has to be avoided A s u m m a r y o f significant data is given in Table 1 All vacuum joints are o f Conflat type except the d o w n s t r e a m port o f every second dipole c h a m b e r where rectangular-shaped flanges o f 380 x 180 m m have been used to allow the addition o f experimental equipment Special Hehcoflex (St Etienne, France) gaskets have m a d e th~s c o n s t r u c u o n possible The ring is divided into six sectors using all-metal gate valves Each sector is e q m p p e d with an all-metal right angle valve for the p u m p - d o w n operation P u m p i n g d o w n from atmospheric pressure to approximately 10 6 m b a r is carried out with turbomolecular p u m p s having a p u m p i n g speed o f 80 1 s ~ each To p u m p below 10 7 m b a r 12 Star-cell (trade name by Varlan) 1on p u m p s o f 60 or 120 1 s ~ capacity and 50 N E G - p u m p units (Figure 2) are distributed around the rlng

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Figure 2 A NEG-pump unit

less steel tube with an inner diameter o f 10 cm and a length o f 30 cm The stainless steel tube has a D N 100 Conflat flange welded o n t o one end which is then connected to the vacuum system The tube is closed at the other end The total active getter area for one p u m p unit is 3340 cm 2 Using a F l s c h e ~ M o m m s e n test d o m e ~ the p u m p i n g speed o f such a unit has been measured to 400 1 s ~ for hydrogen, after passive actlvataon (heating from outside) at 30WC for 30 mln During the acUvatlon sequence a t u r b o - p u m p is used for evacuating the released gases from the getter The actlvauon is carried out with a temperature rise o f IOOCh

4. Electron cooler 3. NEG-pumping units The main p u m p i n g system in C R Y R I N G consists o f N o n Evaporable Getters ( N E G ) o f low temperature type, SAES St 707, collected in special p u m p units These N E G - p u m p units have been subject to different test procedures and have proven to make the low 10 ~2 m b a r (N2 eqv) region attainable A N E G - p u m p unit is built up from standard 3 cm wide N E G strips cut into pieces 25 cm in length Each p u m p unit consists o f 24 such lengths arranged in a fan-shaped geometry in a stain-

Table 1 Characteristics of the CRYRING uhv-system Ring circumference Beam tube diameter Total volume Total area Design vacuum Pumping speed hydrogen Pumping speed methane Pumping speed rare gases Expected desorptlon/dlffusmn rate 2 498

51 63 m I 10 m 750 1 30 m 2 (> 99% 316LN stainless steel) 5 × 10 2 mbar (N2 eqv) 20,000 1 s I000 1 s 300 1 s t 2 × 10 ~ mbar cm -~s

The C R Y R I N G project includes an electron cooler 4, a device for improving emlttance and energy spread o f the circulating beam It works by merging a cold electron beam 0 e with small emlttance and energy spread) with the 1on beam ovei a 1 I m long section o f the ring The main c o m p o n e n t s o f the cooler are an electron gun, magnets to guide the electron beam, and a collector In the gun, the electrons are emitted from a 40 m m diameter dispenser cathode heated to a r o u n d 900 ~C This cathode is obviously one o f the main sources o f outgassing ~ in the C R Y R I N G vacuum system A n o t h e r big gas load comes from the collector, a water-cooled vessel o f O F H C - c o p p e r , where the electrons arc d u m p e d at a m a x i m u m power dissipation o f 10 5 kW Close to the electron gun and surrounding the beam, a NEG p u m p is located, contalnmg 25 2 m o f N E G strips with an active area o f 14,000 cm 2 The strips can be activated by resistive heating, thus avoiding that the entire vacuum system has to be baked before a good vacuum can be achieved Behind the gun there is a 1500 I s ~ cryopump, to improve the p u m p i n g speed in general, and also to p u m p rare gases and methane which are not p u m p e d by the N E G On the collector side there is 19 2 in o f N E G strips in an a r r a n g e m e n t similar to the one on the gun side Behind the collector there is a 120 1 s ~ion p u m p The b a c k g r o u n d pressure at the gun with cold cathode is at

Bagge et al CRYRING

)l~sent 3 x 10 ~' mbar without any bake-out of the system At collector side, the pressure is three times higher due to the maller pumpmg speed With a hot cathode that has been out~lssed for approximately one month, the pressure at the gun ~reases by a factor of three, approaching that of the collector ide When the electron beam is switched on, the pressure acreases, mainly at the collector, dependent on the current and rgy of the beam Initially, before the collector surface has n cleaned from adsorbed gases, the pressure can increase by ral powers of ten After some time it drops again to a lower e The cooler, which came into operation in May this year, s n o t been operated for more than a couple of hours at a time us, equilibrium pressure values have not been measured over longer periods with the electron beam on

~. Vacuum measurements At present, the total pressure is measured with cold-cathode lg~Uxges of inverted magnetron type with a lower pressure limit of 10 ~ mbar Such gauges have been shown 6 to give readings 1!~good agreement with those of Bayard-Alpert gauges, down to the 10- t~ mbar range Partial pressure measurements are carried f ~ t owith a quadrupole mass analyser with special uhv perance Bayard-Alpert lOnlsatlon gauges 7 with a sensitivity of rr ~, equipped with a modulator allowing measurements down to 10 ]3 mbar, will be installed

6L Bake-out system and procedure "ghe magnet chambers are wrapped with four layers of ceramic aper on top of thermo-coax heating elements Other chambers beam tubes as well as valves are equipped with specially ~igned heating jackets For the flanges, metallic heating collars I~ used The designed baking temperature is 300°C During the ike-out procedure the temperature IS measured using about 250 ~m-magnet~c, type E, thermoelements, which are connected to a Exomatlc microprocessor control system (Teckomatorp, Weden) operating the bake-out power via 250 solid-state relays lie whole system (Figure 3) is supervised by a 386 PC providing contact with the operator Special care is exercised when erlng the temperature in order to prevent sensitive parts forg 'cold spots' where condensation can occur During the

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I I I I I I Dec Jan 91 Oct Nov Feb 92 Aug The heights of the bars represent the pressure dlstrlbubon in the ring o 3 weeks after first pumpdown b 3 weeks after installation of new equloment and elimination of minor leaks c After 4 0 weeks purnpmg d 3 weeks pump)ng with NEC-pumps *nstalled e 15 weeks pumping with NEG-pumps installed f 40 weeks pump)ng with NEG-pumps )nstalled

Figure 4. Pressure evolutmn and distribution m CRYRING from the start in December 1990

bake-out procedure the N E G pump units have a temperature of 200°C to prevent saturation of the NEGs, while the rest of the system as kept at 300°C After 24 h baking, the system temperature is lowered to 150°C At this stage the N E G pump units are activated for 30 min at a temperature of 300°C During the bakmg and activation processes the system is pumped by turbomolecular pumps When the system temperature ]s 125°C the ion-pumps are started and the turbo-molecular pumps are valved off

7. Summary An unbaked vacuum-fired stainless steel uhv-system can reach the 10 )' mbar range with reasonable pump-down (Figure 4) time, when pumped by NEG-pumps in combination with sputter ion-pumps The residual-gas composition after three weeks of pumping is dominated by H2 with a content of 70%, the remainder consisting mainly of CH4, H20 and CO

References

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Vocuum (tabor)

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K Abrahamsson, G Andler, L Bagge, E Beebe, P Carl6, H Danared, S Egnell, M Engstrom, A Filevlch, C J Herrlander, J Hllke, J Jeansson, A Kallberg, S Leonteln, L Llljeby, A Nllsson, A Pa~l, K-G Rensfelt, U Rosengfird, A Slmonsson, J Starker, M af Ugglas, Status and Commlsslomng of CRYRING, Proc 3rd European Partwle Accelerator Conf, Berhn, Germany, 24-28 March, 1992 2R Calder and G Lewln, Br J Appl Phys, 18, 1459 (1967) 3E Fmcher and H Mommsen, Vacuum, 17, 309 (1967) 4H Danared, The CRYRING electron cooler, in ECOOL 90, Legnaro, 15-17 May, 1990 5C Habfast, M GIrardml, L Hutteu, A Poncet, H Path, B Sehgmann and A Wolf, Vakuum-Techmk, 34, 195 (1985) 6B Angert, G Gontero and H Wahl, Vacuum, 34(8/7), 749-755 7C Benvenuti and M Hauer, Nucl Instrum Meth, 140, 453 (1977)

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