- Email: [email protected]
Large accelerator vacuum system engineering
Vacuum 67 (2002) 347–357
Large accelerator vacuum system engineering Yves Dabin* European Synchrotron Radiation Fac., Mechanical Engineering TSD, 6 rue Jules Horowitz, BP 220, 38043 Grenoble c!edex, France
Abstract This paper presents an overview of vacuum and engineering for particle accelerators. There are about 15 000 particle accelerators presently in operation worldwide. Those mentioned here are large facilities for which vacuum is considered for its main aspects: * * * *
what vacuum definition is involved through gas interaction, beam losses and decay process; what pumping engineering principle used and what are the current trends; basic engineering of materials, joining and assembling processes; and application of technological know-how and how it is shared between industry and laboratories. r 2002 Published by Elsevier Science Ltd.
0. Introduction
1. Vacuum for accelerators
Given the wide field of existing facilities much experience has been gained in accelerator technology. To simplify matters we shall refer to any accelerator by the generic term of machine. Vacuum systems for machines are one of the main concerns with regards to the lifetime of the beam particles. We shall see how beam particles and vacuum are related. Once vacuum has been defined the next step is to define engineering and associated trends. Machines are made of materials and the assembling processes should be presented. Last, but not the least, machine manufacturing requires a strong collaboration between industry and the laboratory. How does the customer/ supplier relation operate in specific machine fields?
1.1. Accelerators and beam types
*Tel.: +33-476-882276; fax: +33-476-882585. E-mail address: [email protected] (Y. Dabin).
A simple method to present vacuum for accelerators is to categorise them according to the particle beam stay in the machine. Thus, we can distinguish accelerators as ‘‘single pass machines’’ when the accelerated particles pass only once in the facility and for a short time (from 1 to 103 ms) and ‘‘storage ring machines’’ when accelerated particles perform a long stay in the machine (from 10 to 102 h). In the text below, the term ‘‘machines’’ designates any type of accelerator. It is almost impossible to present an exhaustive list of machines in the world as there are at present an estimated 15 000 facilities around the world [1]. Table 1 presents the major accelerator facilities in Europe. It can be observed that specific constraints impose the presence of lifetime of UHV (typically
0042-207X/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 2 2 4 - 5
Y. Dabin / Vacuum 67 (2002) 347–357
348 Table 1 Machine name
Type
Beam
Country
Lab
Location
HERA PETRA DAFNE LEAR SPS PS Vivitron Ganil ISIS ISOLDE TORE SUPRA ESR UNILAC TESLA (proposal) CLIC (proposal) DELTA ELSA BESSY II HASYLAB ANKA ASTRID ELETTRA DIAMOND (project) SRS MAX II SLS DCI SupACO SOLEIL (project) ESRF
Col ring Col ring Col ring Col ring Col ring Col ring ion acc ion acc ion ring ion ring ion ring ion ring lin acc lin col lin col SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring
ep e ee+ Ions Any Any Ions Ions Ions Ions Ions Ions Ions ee+ ee+ e e e e e
D D I CH CH CH F F UK CH F D D D CH D D D D D DK I UK UK S CH F F F F
DESY DESY INFN CERN CERN CERN IReS Ganil RAL CERN TORE SUPRA GSI GSI DESY CERN Univ Univ BESSY HASYLAB Univ Univ ELETTRA DIAMOND RAL Max Lab. PSI Lure Lure Soleil ESRF
Hamburg Hamburg Frascati Geneva Geneva Geneva Strasbourg Caen Appleton Geneva Cadarache Darmstadt Darmstadt Hamburg Geneva Dortmund Bonn Berlin Hamburg Karlsruhe Arhus Trieste Didcot Appleton Lund Villingen Orsay Orsay Saclay Grenoble
e e? e
e+ e+ e? e
Col ring: collider ring, Lin col: linear collider, Ion acc: ion linear accelerator, SR ring: synchrotron radiation ring, Ion ring: ion storage ring.
109–1011 mbar) when machines are designed for beam storage, while single pass machines require a vacuum imposed by the electrical discharges (typically 107–109 mbar) except for super conducting linear accelerators. We will focus on the major difficulties imposed by UHV in storage machines. 1.2. Global lifetime and beam decay To have an idea of how vacuum conditions are handled, we have to note the behaviours of stored particles. For many physical reasons that will be highlighted, particles beam decay in time roughly according to the simple following model:
N ¼ No et=t ; where No is the original stored particle quantity, N the circulating particles at t time, and t the socalled lifetime (decay constant) Fig. 1 shows the operation scheme. Management of lifetime is complex as it involves not only vacuum but also all the other fields of particle beam physics such as radio frequency (RF), beam optics, material properties and pumping systems. 1.3. Physical stored beam decay phenomenon Only a simple presentation of the decay overview will be presented here, the effects of which are
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to the residual gas atoms (either nuclear or electron fields). The specific lifetime related to this process is proportional to E 2 A2 ; t½hp P/ZS2 where E is the particle energy, P the residual gas pressure, A the vessel cross-section, Z the residual gas atomic number. Fig. 1. A typical beam steady period at the ESRF in May 2001. Lifetime: 72 h, two storage ring re-fillings per day. Table 2
1.3.2. Bremsstrahlung The inelastic Coulomb scattering (pressure, aperture), also widely called Bremsstrahlung, can be written as Par þ Pgas -Par þ Pgas þ g
Process
Pressure influence (P)
Ions from residual gas Coulomb scattering (elastic) Wake field Dust particles SR in Ids Bremsstrahlung inelastic Intra beam scattering Touschek Quantum fluctuations Beam beam blow up Luminosity of collision
P P P P P
Aperture influence (A)
A A A A A A A
P is the pressure of the residual gas, A is the aperture of the vacuum vessel.
well documented. Reference to only well-known authors such as Wiedemann [2] is made. Table 2 [3] highlights the effects according to relations with either the gas pressure of the machine or the aperture of the vacuum chamber. The process of particle loss consists in the slight change of trajectory of the disturbed particle that finally moves out of the machine acceptance at a given location of the machine. A short overview of vacuum topics is presented below.
1.3.1. Elastic Coulomb scattering (pressure) Elastic Coulomb scattering (pressure) is the angular deviation of the incident particles close
where g is the high-energy photon Par the particles and Pgas the pressure. The lifetime is limited by Z and by the energy acceptance, scaled as 50 for N2 gas: tB ½hE 9 10 P½mbar The result of the Bremsstrahlung processes and its counterpart radiation is a severe constraint for synchrotron radiation facilities. 1.3.3. The Touschek effect (aperture) This is the particle bunch growth due to internal collisions. Roughly the lateral oscillation due to the trajectory causes a collision that induces longitudinal momentum to the particles. This effect is a major component of lifetime. As it does not concern vacuum, it will be summarised as e3 tT ½hp acc ; r where eacc is the machine acceptance, and r the particle density. 1.3.4. Intra beam scattering (aperture) It is generally a low component of the lifetime. This is the multiple scattering of particles inside the bunch. The lifetime is given by E tIBS p : P
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1.3.5. Dust particles The presence of dust is generally attributed to either machining and/or from venting procedures. The dust may contain Silica, Alumina (Al2O3) but also steel, etc. Dust particles react differently according to their sensitivity to electrical or magnetic fields. Non-evaporable getter (NEG) powder on strips has been considered as a form of metallic dust in the past [4]. 1.3.6. Other beam particle loss processes These do not necessarily imply vacuum but their importance remains high. The wake field (pressure indirectly, aperture) is basically related to the RF vacuum vessel impedance. The description of the process will not be developed in this paper; an RF mismatch leads to a heat deposition on the vessel walls that increase the Pgas (this is an efficient and quick method to detect RF coupling with beam). The quantum fluctuation (aperture): the particles are lost because they exit from the machine acceptance (Gaussian beam). More complex processes are the beam–beam blow up and luminosity collision in the storage ring. They do not involve vacuum. 1.3.7. Global lifetime Summing up the various lifetime and keeping the essential effect leads to the global lifetime according to the following sum of inverse: 1 1 1 1 1 ¼ þ þ þ S : tGL tES tB tT tothers An order of magnitude gives tGL C5250 h; tES C100 h; tB C702100 h;
tT C10250 h:
1.4. The vacuum problem
Par-g (energy loss mainly), gþwalls-e þ Pgas (heavy molecules extracted), e þwalls-Pgas (heavy molecules extracted). It can be scaled as QPID pEIlwall k; where QPID is the molecular flow rate (mbar l/s), E the beam particle energy (GeV), I the beam particle intensity (mA), lwall the material desorption rate (molecule/photon incident), k the conversion constant (3 1020 mbar l/molecule). Fortunately, this molecular flow decreases in time according to the deposited photon dose. An example is given in Fig. 2 below showing the ESRF vacuum conditioning [5] equivalent to the coefficient lwall : This decrease in time is called the ‘‘beam conditioning’’. It is completely related to the walls. Such natural physical cleaning also has severe drawbacks; a high intensity of Bremsstrahlung leading to a poor lifetime. For storage ring sources this phase constitutes a critical period, as most of the scientific instruments are located along the source axis, this situation leads to pressure, intensity and also safety protocols. Recently, radiation level standards impose a progressive decrease of admissible doses.
2. Vacuum system design 2.1. Progress in the last 30 years Vacuum system design has progressed with 30 years of machine design experience, highlights of the past three decades are [6]: *
The key phenomena leading to gas interaction and finally a beam particle loss have been discussed above, nonetheless there are other effects, which strongly influence vacuum and impart a significant constraint on the pumping design. The photon induced desorption (PID) whose effect consists in the following:
*
Until the 1970s the basis of vacuum system designs was the ‘‘lumped pumping’’ technique. To obtain an average pressure beam pipe conductance had to be as large as possible and pumping units to be located as far away as possible from each other for a given pressure. Until the 1980s a large part of the pumping was provided in the dipole magnet (fields were rather high). This configuration was quite
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Fig. 2. ESRF vacuum conditioning.
Fig. 4. LEP vessel with NEG strip (St 101).
0.02 T for the dipoles. Fig. 4 illustrates this type of configuration. Fig. 3. Integrated sputter ion pump in HERA dipole vessel (further replaced by NEG strips).
*
efficient for pumping requirements, close to the pressure increase under synchrotron radiation. This arrangement is presented in Fig. 3 where integrated sorption ion pumps (In-SIP) were inserted in HERA dipole field. In the 1990s In-SIPs were progressively doubled or simply replaced by NEGs. This technology was highly promoted by the needs of the LEP machine at CERN, where magnetic components were designed at very low field (typically
NEG does not require any magnetic field and therefore pumping can be inserted in any configuration. The only major concerns are the radiation heating as well as thermal deformations of NEG strips. Present and future designs. Again CERN is at the forefront of this technology with its demanding requirements for niobium coating for super conducting cavities. The NEG coating technology is far from straightforward as it allows NEG deposits exactly where gas desorption is emitted. This breakthrough has lead to impressive pumping speeds of the order of 104 l/s (N2). The main
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advantage of NEG coating is not really the pumping speed but rather the low PID outgassing yield. Fig. 5 shows the ESRF small gap chamber having undergone a NEG coating at CERN [7].
tance of the beam pipe (m l/s), S the pump speed (l/s), x the local distance, and Plim the limit pressure of the unit (mbar). For distributed pumping the equation becomes [9]
2.2. A basic accelerator design: lumped pumping PðxÞ ¼ k1 erx þ k2 erx þ
Aq ; s
A regular pumping pattern is critical for accelerators and is therefore the key design criterion. Pumping is a compromise between linear conductance of the beam pipe and pumping capabilities of the unit. The lumped pumping system is defined as [8] Lx x2 L PðxÞ ¼ Aq þ þ Plim ; S 2o
k1 ¼
Aq rL e ð1 þ erL Þ1 ; s
k2 ¼
Aq ð1 þ erL Þ1 ; s
where A is the specific linear surface (m2/m), q the specific desorption rate (mbar l/sm2), L the distance between pumps (m), o the linear conduc-
r¼
Fig. 5. NEG coated extruded aluminium vessel for the ESRF undulators.
s is the linear integrated pumping speed (l/s m), typically sC100 l/s m Other more accurate tools are nowadays currently operated: finite element method programs are edited in different laboratories (CERN, SLAC, etc.) and more sophisticated the Monte-Carlo based method for which the real 3D vessel geometry is implemented.
where
rffiffiffiffi s ; o
Fig. 6. The three modules of the ESRF BL front end.
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Fig. 7. The high power beam slits of the ESRF front end. The component acts as a differential pumping.
2.3. Differential pumping
*
This arrangement is widely used on a large scale in accelerator technology. It is also a familiar design found in many other fields. Differential pumping will be highlighted in two cases found in a synchrotron radiation facility (ESRF). Fig. 6 shows the scientific beamline (BL) front end. The pressure breakdown is as follows: Storage ring BL front end BL optical hutch BL experimental hutch
1011 pPmbar p1010 1010 pPmbar p109 109 pPmbar p107 106 pPmbar p105
slowing down and stopping sudden accidental venting from the BL.
A typical ESRF front-end beam slit is shown in Fig. 7. A second example of differential pumping installed on a UHV BL-ESRF ID 20. Fig. 8 shows a more classical arrangement of differential pumping, the purpose of which is to match pressure levels between the front end and the BL limit mirror. The X-ray beam size defines the aperture of the central channel in this case [10].
3. Materials and technology From a vacuum point of view, the front end is designed to enable:
*
*
management of high power and high power density X-ray beams; matching pressures and molecular flow; and
3.1. Choice of technology The choice of vacuum engineering technology is not a simple compromise. It is the result of a large number of factors both technical and human. In the past a laboratory would develop in-house all
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know-how is assembled. A map of influent components for technology could be as given in Fig. 9. Table 3 gives an idea of how and when technology has been used according to the trilogy base: aluminium, copper, stainless-steel. 3.2. Materials and accelerator components Fig. 8. A UHV differential pumping (three decade isolation) at the ESRF ID20 BL.
Fig. 9. Chart presenting technical choice criteria.
technologies used on its machine. Nowadays, with the exception of CERN, few institutes have the capacity to process a wide universal activity. International projects often involve collaboration and transfer of technology when complimentary
Many materials are involved in the design of accelerator components (Fig. 10); each has its specific supply, treatment and process specification. A non-exhaustive list in order of decreasing importance are: (a) Aluminium alloys (Al Mg Si 6000 and 5000 ISO series): * vacuum vessels, * flanges, * gaskets, * bellows, * RF cavities (at room temperature), * absorbers (indirect radiation), and * all mechanical structural works. (b) Stainless-steel, Ni Cr with very low carbon (300 series AISI 1.4 Euronorm series): * vacuum vessels, * flanges, * bellows, * all mechanical structural works, and * cryostats.
Table 3 Gives an idea of how and when technology has been used according to the trilogy base: aluminium, copper, stainless steel Aluminium technology for vacuum vessels Copper technology for vacuum vessels Stainless steel technology for vacuum vessels PETRA SPS TRISTAN LEP Photon Factory APS ALS Spring8 SSRC DAfNE PLS ESRF-ID vessels Italic indicates breakthrough technology.
78HERA (bronze) 76LEP cavity (+Nb coating) 81PEP (B-factory) 83All linacs room T1 85 90 92 93 94 97 98 99
85PS 91ISR 96DCI ELETTRA ESRF LHC BESSY II LHC (+copper coating) Most of SR and colliders (VIRGO)
65 73 75 90 92 97 98
97
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FPM (VITONs, etc.), pure copper (OFE, silver bearing), Nickel, pure Al (corrosion sensitive), lead, indium. Adequate specifications must be applied to gasket material processing; hardness and cleanliness are the two vital issues here. 3.3. Vacuum sealing technology
Fig. 10. The ALS Berkeley machined vessel (typical sample) [11].
(c) Copper alloys Cu pure, Cu OFE (ISO), Cu Si (3 to 8%), Cu reinforced AL2O3, Cu Cr: * RF cavities (room temperature and super conducting), * RF wave guides, * gaskets, * cooling devices, and * beam absorbers (direct radiation). (d) Ceramics Alumina Al2O3, Glass ceramic (machinable), glasses (o4001C): * vessels (highly pulsed dipoles in injection sectors): * feedthrough, plugs, * electrical insulation parts, and * RF window (Ti coating). Highly specialised companies braze all these materials. (e) Engineering polymers (*): * polyimide—VESPELs, KAPTONs, * peek—KETRONs. Used as isolators, windows, friction gears (*) These have a high water content and a bakeout specification must be processed. (f) Exotic materials: * beryllium (PF6Os) for SR window vessels interaction region, absorbers (direct beam); * pyrolitic graphite for windows and absorbers. Windows can also be provided in ‘‘vitreous carbon’’, or PVD Diamond (recently). (g) Gasket materials: Gaskets are essential in vacuum engineering and the choice of material covers a wide range such as:
A lot of experience has been gained in this field and nowadays users and designers have a wide choice of standard technologies. Fig. 11 presents details of the mostly widely available technologies. On a regular basis, accelerator requirements impose custom treatment, be it for reasons of size, budget or shape constraints. Specific designs have been created for these purposes: *
*
*
More sophisticated all aluminium CF system for the TRISTAN (1983) machine in Japan, the system designed by Ishimaru. Simpler alternatives are now available in industry with products such as Atlass flanges, offering a bimetal CF flange Al/stainless-steel. This metal transition is now currently used in accelerator technology. Large dimension connection (diameter 4 m) was required for TORE SUPRA (Cadarache, France); Fig. 12 shows the designed system (also used at ESRF for mirror vessels). Electrically isolated flange. This technology is specific to the pulsed machine like synchrotron booster used in the injector chain of storage rings. This was developed at CERN for SPS in 1978 (see Fig. 13).
3.4. Material and structural processes Apart from vacuum sealing, structural engineering processes are also a key part of accelerator design. The two major fields will be discussed here: welding and brazing. 3.4.1. Assembling with welding Several processes are compatible for UHV vessels: TIG (tungsten electrode with gas shielding), plasma welding, electron beam welding and more recently laser welding. Practical qualities for
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Fig. 11. The vacuum sealing alternative.
Fig. 12. The Cadarache flange used at ESRF for sarcophagus mirror vessels.
UHV are completely dependent on the know-how of the manufacturer. To implement accelerator components, laboratories generally proceed with a two-step strategy: * *
validation of a prototype, and validation of the manufacturer with a shared protocol.
3.4.2. Assembling with brazing Vacuum brazing techniques are a critical field and specialised companies are few and far between in Europe (o10). This assembling process requires
Fig. 13. Isolated flanges used at 10 Hz for booster injectors.
a special skill from the designers and manufacturers. The braze alloys involved are generally bi-, tri- or quadri-copper based and a sound mastery of fine metallurgy techniques and a good coherency of all junction gaps. In conclusion, the golden rules for brazing for a safe accelerator design are: *
no brazed joints between water to vacuum,
Y. Dabin / Vacuum 67 (2002) 347–357 *
*
*
materials must be qualified with adequate specifications, cooling: deionised water should be used—no boiling phase allowed ( except for very special cooling processes), and safe heating: Alo1501C, copper o1501C (OFHC) large size heating reinforced coppero4001C large size heating water o1401C at 8 bars.
4. Accelerator manufacture 4.1. A quality system The accelerator components require a quality system, which guarantees that the vacuum performances are acceptable. This is more or less obvious for industry which is precisely organised in this way (ISO 9000 series), this is not always the normal case for laboratories as their quality systems are more geared to the size of the machine or to the complexity of their collaborations. The result is always the sum of a fruitful collaboration between industry and laboratories. The system is operated in a parallel manner through the supplier workflow and in the laboratory follow up. 4.2. Major determining areas *
*
*
*
A sound technical base is essential and is related to machining, welding, brazing,y. Precise management. In many cases there are contracts, manufacturer’s proprietary process (restricted know-how), manufacturing flow chart, material specification, hold points. A good mutual understanding between parties based on a good exchange of know-how. Rigorous measurement protocol. Accelerator for manufacturing requires mixing of mechanical fields, UHV, RF electrical discharge sensitivity, cleanliness, etc. These measurements are recorded to follow their evolution during series manufacture. Each item produced, RF cavity, vacuum vessel, wave guides, accelerating structure, although identical are submitted to a
357
history which implies an individual follow up during their lifetime. This is where the lab’s quality system applies.
5. Conclusion Laboratories which operate accelerators undergo a permanent evolution on their vacuum systems. Concerns for a long lifetime are high due to the many scientific and safety issues. On the one hand, the design know-how proposed to industry leads to high level requirements and on the other hand industry also has know-how of new techniques. This engineering know-how should be carefully preserved as it is the fruit
Acknowledgements The author would like to thank ESRF colleagues R. Kersevan and P. Thiry for fruitful discussions and J.-L. Bersier for bibliographic research.
References [1] Amaldi K. P3 the importance of particle accelerators. EPAC 2000, Vienna. [2] Wiedemann H. Particle accelerator physics primer. Springer. [3] Wiederman H. International workshop on vacuum systems for b factories and high synchrotron light sources. Cornell; 1992. [4] Debut G, et al. Vacuum conditioning of the ESRF storage ring. EPAC 94, London. [5] Kersevan R. Performance of a narrow gap, NEG-coated, extruded aluminium chamber at the ESRF, EPAC 2000. [6] Benvenuti C. Non evaporable getters: from pumping strips to thin film coating. EPAC 98, Stockholm. [7] Kersevan R. Status of the ESRF vacuum system. EPAC 2000 Stockholm. [8] Westerberg L. Proceedings CERN Accelerator School, Vacuum technology, CERN 99.05, 19 August 1999. [9] Poncet A. JUAS course, 1996. [10] Renier M, Draperi A. A differential pumping system for one of the ESRF UHV beamlines. Vacuum 1997;48:405. [11] Henderson T. LBL Berkeley, private communication.
Large accelerator vacuum system engineering Yves Dabin* European Synchrotron Radiation Fac., Mechanical Engineering TSD, 6 rue Jules Horowitz, BP 220, 38043 Grenoble c!edex, France
Abstract This paper presents an overview of vacuum and engineering for particle accelerators. There are about 15 000 particle accelerators presently in operation worldwide. Those mentioned here are large facilities for which vacuum is considered for its main aspects: * * * *
what vacuum definition is involved through gas interaction, beam losses and decay process; what pumping engineering principle used and what are the current trends; basic engineering of materials, joining and assembling processes; and application of technological know-how and how it is shared between industry and laboratories. r 2002 Published by Elsevier Science Ltd.
0. Introduction
1. Vacuum for accelerators
Given the wide field of existing facilities much experience has been gained in accelerator technology. To simplify matters we shall refer to any accelerator by the generic term of machine. Vacuum systems for machines are one of the main concerns with regards to the lifetime of the beam particles. We shall see how beam particles and vacuum are related. Once vacuum has been defined the next step is to define engineering and associated trends. Machines are made of materials and the assembling processes should be presented. Last, but not the least, machine manufacturing requires a strong collaboration between industry and the laboratory. How does the customer/ supplier relation operate in specific machine fields?
1.1. Accelerators and beam types
*Tel.: +33-476-882276; fax: +33-476-882585. E-mail address: [email protected] (Y. Dabin).
A simple method to present vacuum for accelerators is to categorise them according to the particle beam stay in the machine. Thus, we can distinguish accelerators as ‘‘single pass machines’’ when the accelerated particles pass only once in the facility and for a short time (from 1 to 103 ms) and ‘‘storage ring machines’’ when accelerated particles perform a long stay in the machine (from 10 to 102 h). In the text below, the term ‘‘machines’’ designates any type of accelerator. It is almost impossible to present an exhaustive list of machines in the world as there are at present an estimated 15 000 facilities around the world [1]. Table 1 presents the major accelerator facilities in Europe. It can be observed that specific constraints impose the presence of lifetime of UHV (typically
0042-207X/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 2 2 4 - 5
Y. Dabin / Vacuum 67 (2002) 347–357
348 Table 1 Machine name
Type
Beam
Country
Lab
Location
HERA PETRA DAFNE LEAR SPS PS Vivitron Ganil ISIS ISOLDE TORE SUPRA ESR UNILAC TESLA (proposal) CLIC (proposal) DELTA ELSA BESSY II HASYLAB ANKA ASTRID ELETTRA DIAMOND (project) SRS MAX II SLS DCI SupACO SOLEIL (project) ESRF
Col ring Col ring Col ring Col ring Col ring Col ring ion acc ion acc ion ring ion ring ion ring ion ring lin acc lin col lin col SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring SR ring
ep e ee+ Ions Any Any Ions Ions Ions Ions Ions Ions Ions ee+ ee+ e e e e e
D D I CH CH CH F F UK CH F D D D CH D D D D D DK I UK UK S CH F F F F
DESY DESY INFN CERN CERN CERN IReS Ganil RAL CERN TORE SUPRA GSI GSI DESY CERN Univ Univ BESSY HASYLAB Univ Univ ELETTRA DIAMOND RAL Max Lab. PSI Lure Lure Soleil ESRF
Hamburg Hamburg Frascati Geneva Geneva Geneva Strasbourg Caen Appleton Geneva Cadarache Darmstadt Darmstadt Hamburg Geneva Dortmund Bonn Berlin Hamburg Karlsruhe Arhus Trieste Didcot Appleton Lund Villingen Orsay Orsay Saclay Grenoble
e e? e
e+ e+ e? e
Col ring: collider ring, Lin col: linear collider, Ion acc: ion linear accelerator, SR ring: synchrotron radiation ring, Ion ring: ion storage ring.
109–1011 mbar) when machines are designed for beam storage, while single pass machines require a vacuum imposed by the electrical discharges (typically 107–109 mbar) except for super conducting linear accelerators. We will focus on the major difficulties imposed by UHV in storage machines. 1.2. Global lifetime and beam decay To have an idea of how vacuum conditions are handled, we have to note the behaviours of stored particles. For many physical reasons that will be highlighted, particles beam decay in time roughly according to the simple following model:
N ¼ No et=t ; where No is the original stored particle quantity, N the circulating particles at t time, and t the socalled lifetime (decay constant) Fig. 1 shows the operation scheme. Management of lifetime is complex as it involves not only vacuum but also all the other fields of particle beam physics such as radio frequency (RF), beam optics, material properties and pumping systems. 1.3. Physical stored beam decay phenomenon Only a simple presentation of the decay overview will be presented here, the effects of which are
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to the residual gas atoms (either nuclear or electron fields). The specific lifetime related to this process is proportional to E 2 A2 ; t½hp P/ZS2 where E is the particle energy, P the residual gas pressure, A the vessel cross-section, Z the residual gas atomic number. Fig. 1. A typical beam steady period at the ESRF in May 2001. Lifetime: 72 h, two storage ring re-fillings per day. Table 2
1.3.2. Bremsstrahlung The inelastic Coulomb scattering (pressure, aperture), also widely called Bremsstrahlung, can be written as Par þ Pgas -Par þ Pgas þ g
Process
Pressure influence (P)
Ions from residual gas Coulomb scattering (elastic) Wake field Dust particles SR in Ids Bremsstrahlung inelastic Intra beam scattering Touschek Quantum fluctuations Beam beam blow up Luminosity of collision
P P P P P
Aperture influence (A)
A A A A A A A
P is the pressure of the residual gas, A is the aperture of the vacuum vessel.
well documented. Reference to only well-known authors such as Wiedemann [2] is made. Table 2 [3] highlights the effects according to relations with either the gas pressure of the machine or the aperture of the vacuum chamber. The process of particle loss consists in the slight change of trajectory of the disturbed particle that finally moves out of the machine acceptance at a given location of the machine. A short overview of vacuum topics is presented below.
1.3.1. Elastic Coulomb scattering (pressure) Elastic Coulomb scattering (pressure) is the angular deviation of the incident particles close
where g is the high-energy photon Par the particles and Pgas the pressure. The lifetime is limited by Z and by the energy acceptance, scaled as 50 for N2 gas: tB ½hE 9 10 P½mbar The result of the Bremsstrahlung processes and its counterpart radiation is a severe constraint for synchrotron radiation facilities. 1.3.3. The Touschek effect (aperture) This is the particle bunch growth due to internal collisions. Roughly the lateral oscillation due to the trajectory causes a collision that induces longitudinal momentum to the particles. This effect is a major component of lifetime. As it does not concern vacuum, it will be summarised as e3 tT ½hp acc ; r where eacc is the machine acceptance, and r the particle density. 1.3.4. Intra beam scattering (aperture) It is generally a low component of the lifetime. This is the multiple scattering of particles inside the bunch. The lifetime is given by E tIBS p : P
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1.3.5. Dust particles The presence of dust is generally attributed to either machining and/or from venting procedures. The dust may contain Silica, Alumina (Al2O3) but also steel, etc. Dust particles react differently according to their sensitivity to electrical or magnetic fields. Non-evaporable getter (NEG) powder on strips has been considered as a form of metallic dust in the past [4]. 1.3.6. Other beam particle loss processes These do not necessarily imply vacuum but their importance remains high. The wake field (pressure indirectly, aperture) is basically related to the RF vacuum vessel impedance. The description of the process will not be developed in this paper; an RF mismatch leads to a heat deposition on the vessel walls that increase the Pgas (this is an efficient and quick method to detect RF coupling with beam). The quantum fluctuation (aperture): the particles are lost because they exit from the machine acceptance (Gaussian beam). More complex processes are the beam–beam blow up and luminosity collision in the storage ring. They do not involve vacuum. 1.3.7. Global lifetime Summing up the various lifetime and keeping the essential effect leads to the global lifetime according to the following sum of inverse: 1 1 1 1 1 ¼ þ þ þ S : tGL tES tB tT tothers An order of magnitude gives tGL C5250 h; tES C100 h; tB C702100 h;
tT C10250 h:
1.4. The vacuum problem
Par-g (energy loss mainly), gþwalls-e þ Pgas (heavy molecules extracted), e þwalls-Pgas (heavy molecules extracted). It can be scaled as QPID pEIlwall k; where QPID is the molecular flow rate (mbar l/s), E the beam particle energy (GeV), I the beam particle intensity (mA), lwall the material desorption rate (molecule/photon incident), k the conversion constant (3 1020 mbar l/molecule). Fortunately, this molecular flow decreases in time according to the deposited photon dose. An example is given in Fig. 2 below showing the ESRF vacuum conditioning [5] equivalent to the coefficient lwall : This decrease in time is called the ‘‘beam conditioning’’. It is completely related to the walls. Such natural physical cleaning also has severe drawbacks; a high intensity of Bremsstrahlung leading to a poor lifetime. For storage ring sources this phase constitutes a critical period, as most of the scientific instruments are located along the source axis, this situation leads to pressure, intensity and also safety protocols. Recently, radiation level standards impose a progressive decrease of admissible doses.
2. Vacuum system design 2.1. Progress in the last 30 years Vacuum system design has progressed with 30 years of machine design experience, highlights of the past three decades are [6]: *
The key phenomena leading to gas interaction and finally a beam particle loss have been discussed above, nonetheless there are other effects, which strongly influence vacuum and impart a significant constraint on the pumping design. The photon induced desorption (PID) whose effect consists in the following:
*
Until the 1970s the basis of vacuum system designs was the ‘‘lumped pumping’’ technique. To obtain an average pressure beam pipe conductance had to be as large as possible and pumping units to be located as far away as possible from each other for a given pressure. Until the 1980s a large part of the pumping was provided in the dipole magnet (fields were rather high). This configuration was quite
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Fig. 2. ESRF vacuum conditioning.
Fig. 4. LEP vessel with NEG strip (St 101).
0.02 T for the dipoles. Fig. 4 illustrates this type of configuration. Fig. 3. Integrated sputter ion pump in HERA dipole vessel (further replaced by NEG strips).
*
efficient for pumping requirements, close to the pressure increase under synchrotron radiation. This arrangement is presented in Fig. 3 where integrated sorption ion pumps (In-SIP) were inserted in HERA dipole field. In the 1990s In-SIPs were progressively doubled or simply replaced by NEGs. This technology was highly promoted by the needs of the LEP machine at CERN, where magnetic components were designed at very low field (typically
NEG does not require any magnetic field and therefore pumping can be inserted in any configuration. The only major concerns are the radiation heating as well as thermal deformations of NEG strips. Present and future designs. Again CERN is at the forefront of this technology with its demanding requirements for niobium coating for super conducting cavities. The NEG coating technology is far from straightforward as it allows NEG deposits exactly where gas desorption is emitted. This breakthrough has lead to impressive pumping speeds of the order of 104 l/s (N2). The main
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advantage of NEG coating is not really the pumping speed but rather the low PID outgassing yield. Fig. 5 shows the ESRF small gap chamber having undergone a NEG coating at CERN [7].
tance of the beam pipe (m l/s), S the pump speed (l/s), x the local distance, and Plim the limit pressure of the unit (mbar). For distributed pumping the equation becomes [9]
2.2. A basic accelerator design: lumped pumping PðxÞ ¼ k1 erx þ k2 erx þ
Aq ; s
A regular pumping pattern is critical for accelerators and is therefore the key design criterion. Pumping is a compromise between linear conductance of the beam pipe and pumping capabilities of the unit. The lumped pumping system is defined as [8] Lx x2 L PðxÞ ¼ Aq þ þ Plim ; S 2o
k1 ¼
Aq rL e ð1 þ erL Þ1 ; s
k2 ¼
Aq ð1 þ erL Þ1 ; s
where A is the specific linear surface (m2/m), q the specific desorption rate (mbar l/sm2), L the distance between pumps (m), o the linear conduc-
r¼
Fig. 5. NEG coated extruded aluminium vessel for the ESRF undulators.
s is the linear integrated pumping speed (l/s m), typically sC100 l/s m Other more accurate tools are nowadays currently operated: finite element method programs are edited in different laboratories (CERN, SLAC, etc.) and more sophisticated the Monte-Carlo based method for which the real 3D vessel geometry is implemented.
where
rffiffiffiffi s ; o
Fig. 6. The three modules of the ESRF BL front end.
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Fig. 7. The high power beam slits of the ESRF front end. The component acts as a differential pumping.
2.3. Differential pumping
*
This arrangement is widely used on a large scale in accelerator technology. It is also a familiar design found in many other fields. Differential pumping will be highlighted in two cases found in a synchrotron radiation facility (ESRF). Fig. 6 shows the scientific beamline (BL) front end. The pressure breakdown is as follows: Storage ring BL front end BL optical hutch BL experimental hutch
1011 pPmbar p1010 1010 pPmbar p109 109 pPmbar p107 106 pPmbar p105
slowing down and stopping sudden accidental venting from the BL.
A typical ESRF front-end beam slit is shown in Fig. 7. A second example of differential pumping installed on a UHV BL-ESRF ID 20. Fig. 8 shows a more classical arrangement of differential pumping, the purpose of which is to match pressure levels between the front end and the BL limit mirror. The X-ray beam size defines the aperture of the central channel in this case [10].
3. Materials and technology From a vacuum point of view, the front end is designed to enable:
*
*
management of high power and high power density X-ray beams; matching pressures and molecular flow; and
3.1. Choice of technology The choice of vacuum engineering technology is not a simple compromise. It is the result of a large number of factors both technical and human. In the past a laboratory would develop in-house all
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know-how is assembled. A map of influent components for technology could be as given in Fig. 9. Table 3 gives an idea of how and when technology has been used according to the trilogy base: aluminium, copper, stainless-steel. 3.2. Materials and accelerator components Fig. 8. A UHV differential pumping (three decade isolation) at the ESRF ID20 BL.
Fig. 9. Chart presenting technical choice criteria.
technologies used on its machine. Nowadays, with the exception of CERN, few institutes have the capacity to process a wide universal activity. International projects often involve collaboration and transfer of technology when complimentary
Many materials are involved in the design of accelerator components (Fig. 10); each has its specific supply, treatment and process specification. A non-exhaustive list in order of decreasing importance are: (a) Aluminium alloys (Al Mg Si 6000 and 5000 ISO series): * vacuum vessels, * flanges, * gaskets, * bellows, * RF cavities (at room temperature), * absorbers (indirect radiation), and * all mechanical structural works. (b) Stainless-steel, Ni Cr with very low carbon (300 series AISI 1.4 Euronorm series): * vacuum vessels, * flanges, * bellows, * all mechanical structural works, and * cryostats.
Table 3 Gives an idea of how and when technology has been used according to the trilogy base: aluminium, copper, stainless steel Aluminium technology for vacuum vessels Copper technology for vacuum vessels Stainless steel technology for vacuum vessels PETRA SPS TRISTAN LEP Photon Factory APS ALS Spring8 SSRC DAfNE PLS ESRF-ID vessels Italic indicates breakthrough technology.
78HERA (bronze) 76LEP cavity (+Nb coating) 81PEP (B-factory) 83All linacs room T1 85 90 92 93 94 97 98 99
85PS 91ISR 96DCI ELETTRA ESRF LHC BESSY II LHC (+copper coating) Most of SR and colliders (VIRGO)
65 73 75 90 92 97 98
97
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FPM (VITONs, etc.), pure copper (OFE, silver bearing), Nickel, pure Al (corrosion sensitive), lead, indium. Adequate specifications must be applied to gasket material processing; hardness and cleanliness are the two vital issues here. 3.3. Vacuum sealing technology
Fig. 10. The ALS Berkeley machined vessel (typical sample) [11].
(c) Copper alloys Cu pure, Cu OFE (ISO), Cu Si (3 to 8%), Cu reinforced AL2O3, Cu Cr: * RF cavities (room temperature and super conducting), * RF wave guides, * gaskets, * cooling devices, and * beam absorbers (direct radiation). (d) Ceramics Alumina Al2O3, Glass ceramic (machinable), glasses (o4001C): * vessels (highly pulsed dipoles in injection sectors): * feedthrough, plugs, * electrical insulation parts, and * RF window (Ti coating). Highly specialised companies braze all these materials. (e) Engineering polymers (*): * polyimide—VESPELs, KAPTONs, * peek—KETRONs. Used as isolators, windows, friction gears (*) These have a high water content and a bakeout specification must be processed. (f) Exotic materials: * beryllium (PF6Os) for SR window vessels interaction region, absorbers (direct beam); * pyrolitic graphite for windows and absorbers. Windows can also be provided in ‘‘vitreous carbon’’, or PVD Diamond (recently). (g) Gasket materials: Gaskets are essential in vacuum engineering and the choice of material covers a wide range such as:
A lot of experience has been gained in this field and nowadays users and designers have a wide choice of standard technologies. Fig. 11 presents details of the mostly widely available technologies. On a regular basis, accelerator requirements impose custom treatment, be it for reasons of size, budget or shape constraints. Specific designs have been created for these purposes: *
*
*
More sophisticated all aluminium CF system for the TRISTAN (1983) machine in Japan, the system designed by Ishimaru. Simpler alternatives are now available in industry with products such as Atlass flanges, offering a bimetal CF flange Al/stainless-steel. This metal transition is now currently used in accelerator technology. Large dimension connection (diameter 4 m) was required for TORE SUPRA (Cadarache, France); Fig. 12 shows the designed system (also used at ESRF for mirror vessels). Electrically isolated flange. This technology is specific to the pulsed machine like synchrotron booster used in the injector chain of storage rings. This was developed at CERN for SPS in 1978 (see Fig. 13).
3.4. Material and structural processes Apart from vacuum sealing, structural engineering processes are also a key part of accelerator design. The two major fields will be discussed here: welding and brazing. 3.4.1. Assembling with welding Several processes are compatible for UHV vessels: TIG (tungsten electrode with gas shielding), plasma welding, electron beam welding and more recently laser welding. Practical qualities for
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Fig. 11. The vacuum sealing alternative.
Fig. 12. The Cadarache flange used at ESRF for sarcophagus mirror vessels.
UHV are completely dependent on the know-how of the manufacturer. To implement accelerator components, laboratories generally proceed with a two-step strategy: * *
validation of a prototype, and validation of the manufacturer with a shared protocol.
3.4.2. Assembling with brazing Vacuum brazing techniques are a critical field and specialised companies are few and far between in Europe (o10). This assembling process requires
Fig. 13. Isolated flanges used at 10 Hz for booster injectors.
a special skill from the designers and manufacturers. The braze alloys involved are generally bi-, tri- or quadri-copper based and a sound mastery of fine metallurgy techniques and a good coherency of all junction gaps. In conclusion, the golden rules for brazing for a safe accelerator design are: *
no brazed joints between water to vacuum,
Y. Dabin / Vacuum 67 (2002) 347–357 *
*
*
materials must be qualified with adequate specifications, cooling: deionised water should be used—no boiling phase allowed ( except for very special cooling processes), and safe heating: Alo1501C, copper o1501C (OFHC) large size heating reinforced coppero4001C large size heating water o1401C at 8 bars.
4. Accelerator manufacture 4.1. A quality system The accelerator components require a quality system, which guarantees that the vacuum performances are acceptable. This is more or less obvious for industry which is precisely organised in this way (ISO 9000 series), this is not always the normal case for laboratories as their quality systems are more geared to the size of the machine or to the complexity of their collaborations. The result is always the sum of a fruitful collaboration between industry and laboratories. The system is operated in a parallel manner through the supplier workflow and in the laboratory follow up. 4.2. Major determining areas *
*
*
*
A sound technical base is essential and is related to machining, welding, brazing,y. Precise management. In many cases there are contracts, manufacturer’s proprietary process (restricted know-how), manufacturing flow chart, material specification, hold points. A good mutual understanding between parties based on a good exchange of know-how. Rigorous measurement protocol. Accelerator for manufacturing requires mixing of mechanical fields, UHV, RF electrical discharge sensitivity, cleanliness, etc. These measurements are recorded to follow their evolution during series manufacture. Each item produced, RF cavity, vacuum vessel, wave guides, accelerating structure, although identical are submitted to a
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history which implies an individual follow up during their lifetime. This is where the lab’s quality system applies.
5. Conclusion Laboratories which operate accelerators undergo a permanent evolution on their vacuum systems. Concerns for a long lifetime are high due to the many scientific and safety issues. On the one hand, the design know-how proposed to industry leads to high level requirements and on the other hand industry also has know-how of new techniques. This engineering know-how should be carefully preserved as it is the fruit
Acknowledgements The author would like to thank ESRF colleagues R. Kersevan and P. Thiry for fruitful discussions and J.-L. Bersier for bibliographic research.
References [1] Amaldi K. P3 the importance of particle accelerators. EPAC 2000, Vienna. [2] Wiedemann H. Particle accelerator physics primer. Springer. [3] Wiederman H. International workshop on vacuum systems for b factories and high synchrotron light sources. Cornell; 1992. [4] Debut G, et al. Vacuum conditioning of the ESRF storage ring. EPAC 94, London. [5] Kersevan R. Performance of a narrow gap, NEG-coated, extruded aluminium chamber at the ESRF, EPAC 2000. [6] Benvenuti C. Non evaporable getters: from pumping strips to thin film coating. EPAC 98, Stockholm. [7] Kersevan R. Status of the ESRF vacuum system. EPAC 2000 Stockholm. [8] Westerberg L. Proceedings CERN Accelerator School, Vacuum technology, CERN 99.05, 19 August 1999. [9] Poncet A. JUAS course, 1996. [10] Renier M, Draperi A. A differential pumping system for one of the ESRF UHV beamlines. Vacuum 1997;48:405. [11] Henderson T. LBL Berkeley, private communication.