*Corresponding author. Fax: #86-931-8277103.E-mail address: yangxt@ns.lzb.ac.cn (X.T. Yang).

Vacuum 61 (2001) 55}60

The ultra-high vacuum system of HIRFL-CSR

X.T. Yang��*, J.H. Zhang�, X.J. Zhang�, H.M. Wu�, Z.W. Niu�, S.J. Hou�,J. Meng�, W. Jacoby�

�Institute of Modern Physics, Chinese Academy of Sciences, 363 Nanchang Road, 730000, Lanzhou, People's Republic of China�GSI, Postfach 110552, D-64220 Darmstadt, Germany

Received 25 May 2000

Abstract

HIRFL-CSR, a new accelerator project planned at the Heavy Ion Research Facility in Lanzhou (HIRFL), isa multipurpose cooling storage ring (CSR). To minimize the beam loss due to charge exchange of very heavy ions with theresidual gas molecules, ultra-high vacuum of 3�10��Pa is required for the HIRFL-CSR facility, which is the lowestpressure in a large vacuum system in China up to now. In this article we describe: (1) design of the system; (2) vacuumchambers; (3) reduction of the outgassing rate; (4) vacuum equipment; (5) pressure distribution; (6) future pro-gramme � 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Ultra high vacuum; Material outgassing rate; Main pump; Bake-out system

1. Introduction

HIRFL-CSR, a new accelerator project plannedat the Heavy Ion Research Facility in Lanzhou(HIRFL), is a multipurpose cooling storage ring(CSR) which consists of a main ring (CSRm), anexperimental ring (CSRe) and two transport beamlines. The two existing cyclotrons SFC (K"69)and SSC (K"450) of the HIRFL will be used as itsinjector system. The heavy ion beams in the energyrange of 10}50MeV/u from the HIRFL-SSC willbe accumulated, cooled and accelerated to the highenergy of 100}600MeV/u in the main ring CSRm,and then fast extracted to produce radioactive ionbeams (RIB) or highly charged heavy ions.

The secondary beams (RIB or highly charged heavyions) can be accepted by the experimental ringCSRe for many internal-target experiments or highprecision spectroscopy with beam cooling. On theother hand, beams with the energy range of100}900MeV/u will also be provided by the CSRmfor many external-target experiments.

In heavy ion accelerators, the cross-sections forcharge exchange (electron stripping and electroncapture) between partially stripped, low �, veryheavy ions and the residual gas molecules are usu-ally rather large. To minimize the beam losses dueto these charge exchanges, for instance to minimizethe beam loss of the heaviest uranium ions to lessthan 10% [1], a pressure of 3.5�10��Pa is re-quired in the CSRm and CSRe (N

�equivalent)

during operation. To obtain this low pressure in thetwo rings, precautions have to be taken includingmaterial selection, vacuum "ring, UHV cleaningand in situ bake-out.

0042-207X/01/$ - see front matter � 2001 Elsevier Science Ltd. All rights reserved.PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 4 4 7 - 4

Fig. 1. The vacuum equipment layout of HIRFL-CSR.

A pressure of 1�10��Pa is su$cient in the twobeam lines since beams pass through the lines veryquickly and the beam losses are much less than theyare in the two rings.

2. Design of the system

The total length of the HIRFL-CSR vacuumsystem is 450m and the total inner surface is about263m� (not including the equipment inside the vac-uum system). The four subsystems (CSRm, CSRe,SSC-CSRm beam line and CSRm-CSRe beam line)all have di!erent dipole and quadrupole chambers.The electron coolers, RF cavities, internal targets,injection and extraction elements such as kickers,bumpers and septa are installed in the straightsections of the two rings. Various beam diagnosticelements are mounted in the appropriate chambers.

The vacuum equipment layout is shown in Fig. 1.More than 500 standard vacuum components areneeded for the whole system and more than 400di!erent chambers have to be manufactured. Allmetal gate valves divide CSRm into "ve sections,CSRe into four sections, SSC-CSRm beam line intotwo sections and CSRm-CSRe beam line into foursections. For each section there are two or threepump-down stations where movable turbo pumpscan be mounted.

Fast closing valves are installed in the injectionand extraction lines to prevent the two rings frompossible vacuum breakdown. A pressure measure-ment device is installed in each vacuum section.Bellows allow for adjustment of the di!erent cham-bers and avoid damage by the increase of chamberlength during the bake-out process.

The rings will be equipped with permanentheater jackets and thermocouples. In the two trans-fer lines there is bake-out only on the last sectorbefore and after the two rings. Near the rings, thereare large pumps installed to reduce the pressurefrom 10�� to 10��Pa.

3. Vacuum chambers

3.1. Material selection

For chambers where the diameter is less than250mm, Con#at�� #anges are used. (Con#at is

a registered trademark of Varian Associates, Inc.)For larger diameter vessels, rotatable DESY��-type #anges are used. (DESY is a registered trade-mark of DESY, Germany. Rotatable DESY type#anges are new type modi"ed by GSI, Germany.)Both are manufactured from stainless steel, type316LN (forged, ESU processed stock). The cham-bers are made of 304L or 316L.

All metal seals are adopted throughout the vac-uum system. Oxygen-free copper gaskets and cop-per wires are used for the two types of #anges.Screws and nuts are made of stainless steel. Thescrews are silver plated to avoid sticking after thebake-out process. All materials inside the vacuumchambers have to be chosen for low outgassing rateand for being able to withstand temperatures of3003C.

56 X.T. Yang et al. / Vacuum 61 (2001) 55}60

Fig. 2. The cross-section of CSRm dipole chamber.

Fig. 3. The cross-section of CSRm quadrupole chamber.

3.2. The structure of dipole and quadrupolechambers

There are 16 dipole chambers and 16 quadrupolechambers in each ring. The CSRm dipole chambershave a curve length of 3.5m with a radius of 7.6mand an angle of 22.53. The CSRe dipole chambershave a curve length of 2.9m with a radius of 6mand the same angle. Rectangular cross-sectionsof 152�60mm and 234�70mm are requiredfor CSRm and CSRe, respectively. An octagonalcross-section is chosen for the quadrupole cham-bers according to the magnet gaps. Figs. 2 and3 show the cross-sections of the dipole and quadru-pole chambers in the CSRm.

3.3. The structure of the other chambers

The other chambers (beam diagnostic chambersand pumping chambers) are made of stainless steel304L with wall thickness of 2}3mm. Bellows arewelded to some chambers for adjustment and forbake-out. Specially designed supports are neededbecause of the changes in length during the bake-out process.

4. Reduction of the outgassing rate

In an ultra-high vacuum system, the gas load ismainly thermal outgassing of the chamber mater-ials. Hence, the pretreatment of the materials iscritical for obtaining minimum outgassing rate inorder to obtain a low pressure. The steps to reducethe outgassing rate are described below.

4.1. Degassing the materials

In a vacuum of 3�10��Pa the residual gasmainly consists of H

�(about 90%) [2]. To achieve

a low outgassing rate the H�

can be reduced byvacuum "ring. Copper gaskets can be "red in a vac-uum furnace, at a pressure of 10��Pa and a tem-perature of 4503C for 6 h. The stainless-steelmaterials are "red for 1 h/mm of wall thickness ata pressure of 10��Pa and a temperature of 9503C.At the end of the "ring process the temperatureshould be reduced quickly (in about 15min) from900 to 6003C to prevent segregation of carbon atthe surfaces.

A vacuum furnace with an operating pressure ofp)10��Pa, with a diameter of 800mm anda length of 3000mm has been fabricated in Lan-zhou for this process.

4.2. Ultra-high vacuum cleaning procedure

The following procedures are used:(a) rough cleaning;(b) immersion in perchloroethylene (C

�Cl

�) va-

pour (1213C) for 15min;(c) ultrasonic cleaning in alkaline detergent

(PH"10}11, 653C) for 15min;

X.T. Yang et al. / Vacuum 61 (2001) 55}60 57

(d) rinsing by immersion in demineralized water(603C) until no foam in the water;

(e) drying in an oven at 1503C;(f ) back "lling the chamber with nitrogen and clos-

ing it with metal sealed #anges.

4.3. In situ bake-out

The chambers and the vacuum components in-side are designed to be bakeable in situ to 3003C.Dipole and quadrupole chambers are baked bycoaxial heaters with a diameter of 2}3mm. The rateof temperature change is 303C/h. The other cham-bers are heated by heating tapes, insulated by glass"bre. Thermocouples will be used for each heatercircuit to control the temperature during bake-outso that temperature di!erences do not exceed 103C.To avoid thermal losses and to protect the magnetcoils, the dipole and quadrupole chambers have tobe insulated. The tight space of the magnet gapslimits the thickness of the insulation to less than5mm. Microtherm��, a super-low thermal con-ductivity material, has to be used for dipole andquadrupole magnet chambers. This keep the out-side temperature of the insulation lower than 803Cwith a thickness of only 3}5mm. (Microtherm isa registered trademark of Microtherm Interna-tional Limited, England.)

The bake-out process of each vacuum section iscontrolled by a computer and a set of controlmodules handle the data to and from the computer.After the bake-out the control station will bemoved to the next vacuum section.

Operational experiences of many particle accel-erators in the world suggests that, after thepretreatment mentioned above, the outgassingrate of the materials should be lower than5�10�Pa l/s cm� [3}6].

5. Vacuum equipment

5.1. Main pumps

The two rings are pumped by titanium sublima-tion pumps and sputter ion pumps. Sputter ionpumps with pumping speeds of 200}400 l/s removenon-getterable gases such as methane and argon.

The ultimate pressure of the pumps is lower than1�10��Pa. The pumping speed in this pressurerange should be more than 30% of the nominalpumping speed for nitrogen.

Titanium sublimation pumps have a high capa-city for hydrogen at very low pressure, where theresidual gas is mainly H

�(90%). Three "laments

made of titanium-molybdenum wire (85% Ti, 15%Mo, Ar-free) are mounted on holders in the pumpbodies, which are made of 316L stainless steel. Thepumps have an area of 5000 cm� of sublimatedtitanium and a pumping speed of approximately2000 l/s for active gases.

5.2. Pumpdown station

Pumpdown stations consist of a magneticallysuspended turbo-pump of 500 l/s and an oil-freeroughing pump of 20m/h. They are used to pumpthe system down to 10��Pa, to extract the gasesduring the bake-out process and to detect leaks inthe system. There are several pumpdown stationswhich can be moved as required.

5.3. Vacuum measurement

The pressure is monitored by combinationtubes consisting of a Pirani gauge with either aBayard}Alpert gauge or an extractor gauge. ThePirani gauge covers the pressure from atmosphereto 10��Pa, and the extractor gauge measures downto 10��Pa. The ion pump currents can also indi-cate pressure down to 10��Pa.

Mass spectrometers are installed in every sectionto analyze the residual gases in the system.

5.4. Vacuum valves

The valves in the two rings are bakeable to3003C. All metal gate valves of 200mm diameter inbore are used as insulation valves for the sectors ofboth CSRm and CSRe. Because of the high price,the number of valves is reduced to a minimum. Allmetal angle valves are used to connect the pump-down stations. All small metal angle valves are usedto vent the system with dry nitrogen to protect thewalls from contamination with water and dustwhen the system is opened.

58 X.T. Yang et al. / Vacuum 61 (2001) 55}60

Fig. 4. The pressure distribution in one quarter of CSRm vac-uum system.

The sector valves are controlled by gauges andion-pump currents to protect the system as far aspossible from pressure breakdowns. Fast closingvalves with a closing time of 20ms are mountedbetween the beam lines and the rings to protect theultra-high vacuum from possible failures in thebeam lines.

6. Pressure distribution

The length of four subsystems is as follows:CSRm+160m; CSRe+129m; SSC-CSRm beamline+115m; CSRm-CSRe beam line +50m. Thegas-load in CSRm is 1.53�10��Pa l/s and the ef-fective pumping speed for each main pump stationis about 1000 l/s. According to the equation:S"Q/P, 51 main pump stations are neededfor CSRm. The distance between two pumps isabout 3m. Similarly, 57 main pump stations areneeded for CSRe, which has a gas-load of1.71�10��Pa l/s. The distance between twopumps is about 2.2m. The pressure in the beamlines is not so low, so the distance between twopumps is about 6}8m.

The pressure distribution along the system withuniform gas-load (mainly thermal-stimulated out-gassing of the vacuummaterials) is expressed by theequations [7]:

P�"ql�

¸

S#

x

;!

x�

2;¸�,P�"

ql¸

S"

Q

S,

P�"P

��1#

S

2;�,where P

�,P

�,P

�are respectively the pressure of

any position, base pressure of the pump and thepressure of the farthest position from pump (Pa);q is the material outgassing rate in unit area(Pa l/s cm�); l the cross-section circumference (cm);¸ the distance between pumps (cm); Q the outgass-ing of the chamber materials (Pa l/s); ; the linearconductance of the chamber (l/s) and S the pump-ing speed. According to this calculation, the highestpressure is P

�, at the middle of two pumps and the

pressure distribution curve between two pumpsshould be a parabola.

The vacuum elements in four quadrants ofCSRm and CSRe are almost symmetrically ar-ranged. Fig. 4 shows the pressure distribution inone quarter of CSRm.

7. Future programme

The project has to be "nished in 2004. Noweverything is under way and on schedule. A CSRmdipole chamber cell as the "rst prototype is beingdiscussed and will be manufactured and tested inthe near future; necessary drawings have been "n-ished; various experiments have been done in ourlaboratory; the investigation and the studies for thebake-out system have made progress.

Acknowledgements

The authors would like to thank B.H. Shen, D.K.Jiang, S.X. Zhang, S.L. Ma, Y. Wang and P. Pengfor their advice and many useful discussions. Theauthors also thank M. Rau and the other expertswho work in GSI for their useful help.

References

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X.T. Yang et al. / Vacuum 61 (2001) 55}60 59

[3] Miertusova J. Experience with the extra vacuum systemduring commissioning. Sincrotron Trieste, S.P.A, Pad-nciano 99, 34012 Trieste, Italy.

[4] Benvenuti C. Production and measurement of extreme vac-uum. CSRN. Proceedings of the seventh InternationalVaccum Congress and third International Conference onSolid Surface, Vienna, 1997.

[5] Blechschmidt D. Basic design of vacuum system for400GEVLSRwith NormalMagnet. CERN-ISR-VA/75-29.

[6] Brouet M. The ultra-high vacuum system for the LowEnergy Antiproton Ring (LEAR) Design. cost and perfor-mances. CERN/PS-ML/83-42.

[7] Da DA. Vacuum design manual. 1995. p. 624. [inChinese].

60 X.T. Yang et al. / Vacuum 61 (2001) 55}60