- Email: [email protected]
Overview of the LHC vacuum system
Vacuum 60 (2001) 25 } 34
Overview of the LHC vacuum system Oswald GroK bner* 1211 CERN, Geneva 23, Switzerland
Abstract The Large Hadron Collider (LHC) project, now in the advanced construction phase at CERN, comprises two proton storage rings with colliding beams of 7#7 TeV energy. The machine is housed in the existing LEP tunnel with a circumference of 26.7 km and requires a bending magnetic "eld of 8.3 T with 14 m long superconducting magnets operating at 1.9 K. The beam vacuum system comprises the inner &cold bore' walls of the magnets which provide a nearly perfect cryopump. In view of reducing the cryogenic power consumption, both the heat load from synchrotron radiation emitted by the proton beams and the resistive power dissipation by the beam image currents have to be absorbed on a &beam screen', which operates between 5 and 20 K and is inserted inside the cold bore. The design operating pressure must provide a beam lifetime of several days and this requirement comes from the power deposition in the superconducting magnet coils due to protons scattered on the residual gas which could lead to a magnet quench and interrupt the machine operation. Cryopumping of gas on the cold surfaces provides the necessary low gas densities but it must be ensured that the vapour pressure of cryosorbed molecules, of which H and He are the most critical species, remains within acceptable limits. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction The Large Hadron Collider (LHC) [1] consists of a pair of superconducting storage rings with a circumference of 26.7 km. Two proton beams, of 560 mA and an energy of 7 TeV, circulate in opposite directions. Of the &54 km total length, the arcs account for almost 48 km and will be at 1.9 K, the temperature of the superconducting magnets. The basic layout of LHC follows the existing LEP machine, with eight long straight sections and eight bending arcs. The LHC will include two high lu-
* Corresponding author. Tel.: #41-22-7675989; fax: #4122-767-5100. E-mail address: [email protected] (O. GroK bner). O. GroK bner for the LHC Vacuum Group.
minosity experiments (ATLAS and CMS) as well as a heavy-ion experiment (ALICE) and a B-physics experiment (LHCb). The existing SPS accelerator will be used to inject the beams at 450 GeV. To "t the 7 TeV rings into the existing LEP tunnel special design concepts are needed for the machine components and, most speci"cally, superconducting magnets with 8.3 T "eld. The magnetic bending "eld of 8.3 T implies that the superconducting magnets which use commercially available NbTi superconductors must operate at a temperature well below 4.2 K. A very attractive technical solution for the LHC has been to immerse the magnets in a bath of super#uid helium at 1.9 K. The magnets for LHC are based on a two-in-one design, where the two-beam channels are incorporated into a common iron yoke in a compact single cryostat [2]. Fig. 1 illustrates this design concept
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showing the cross-section of a dipole cryomagnet with its cryostat. The compact construction allows the installation of the LHC machine inside the existing tunnel section of LEP. In the arcs of LHC the machine lattice consists of a succession of regular 53.4 m long &halfcells': three 14.2 m long twin-aperture dipole magnets, and one 6.3 m long short straight section. In the long straight sections the machine layout is adapted to the speci"c requirements of the experiments and the beams converge into one common beam pipe, where they are tightly focused and steered into head-on collisions at the interaction points.
2. The vacuum system The vacuum system for LHC consists of two main parts which are kept entirely separated in
terms of vacuum since they must meet very di!erent requirements: the cryogenic insulation vacuum necessary to avoid heat load by gas conduction requires a pressure of only about 10\ mbar. The beam vacuum proper, which must provide a good beam lifetime and low background for the experiments, requires at least three orders of magnitude better vacuum to avoid excessive particle loss by nuclear scattering on the rest gas.
2.1. Cryogenic insulation vacuum The cryogenic insulation vacuum for the machine arcs consists of a nearly 3 km long continuous cryostat which is subdivided into 14 sectors each 212 m long [3]. Vacuum separation is achieved by thin, metallic membranes between the outer room temperature wall of the cryostat and the shell
Fig. 1. Dipole cryomagnet cross-section with cryostat.
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of the cryomagnets, called `vacuum barriersa so that individual sub-sections can be pumped, commissioned and leak checked independently. These `vacuum barriersa also separate the vacuum for the continuous cryogenic distribution line (QRL) from the machine cryostat as shown in Fig. 2. While the initial pump down can be done with conventional turbomolecular pumping groups shown in the inset of Fig. 2, the cryopumping on the large external surface of the magnet cold mass and on the thermal screens will be entirely su$cient for maintaining the static insulation vacuum once the system has been cooled down. Provision for any additional pumping during machine operation is required only if excessive helium leaks occur. This additional pumping can be achieved either by connecting external pumps or by commercial cryopanels with a large pumping capacity for helium. Whether this option will be needed to cope
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with a degraded vacuum pressure will be decided based on the experience with a test string of the machine. 2.2. Beam vacuum and beam related heat loads The beam vacuum system has been the subject of extensive studies over recent years and has been described elsewhere [4}6]. The beam pipe in the centre of the superconducting magnet coils is in direct contact with the helium bath at 1.9 K (coldbore beam pipe) so as to optimise the aperture for the beam. At this temperature, the vacuum chamber wall becomes a very e$cient cryopump with practically in"nite capacity for all condensable gas species with the sole exception of helium. External pumping should not be required and could even be rather ine$cient because of the cryosorption on the walls. Unfortunately, this concept has the
Fig. 2. Layout of cryogenic insulation vacuum.
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drawback that the cryogenic system will have to remove even very small amounts of beam-induced power which, for conventional room temperature accelerators would be insigni"cant. Indeed, since the removal of 1 W at 1.9 K requires nearly 1 kW of electric power, there is a strong incentive to avoid or to reduce to the bare minimum any source of heat in-leak to the cold beam pipe. Four distinct beam and vacuum related heat sources to the 1.9 K system can be identi"ed: (i) Synchrotron radiation: Due to the centripetal acceleration in the bending magnets the beams will emit a synchrotron radiation #ux of about 10 photons s\ m\ equivalent to a distributed linear power of 0.2 W m\. For the LHC as a whole, this amounts to 7.6 kW. (ii) Image currents from the beam: The inner wall of the beam pipe must conduct the image currents of the beams and this power depends directly on the resistivity, o of the vacuum chamber wall material. U To limit this heat load, and also to avoid resistive wall instability of the beam, the resistivity of the vacuum chamber has to be low. For the LHC this can be achieved by coating the inner surface of the stainless-steel beam pipe with a thin copper layer. It is an additional advantage that, at this low temperature, the resistivity of high-purity copper is a factor of 10 lower than for stainless steel. (iii) Photoelectrons and multipacting: Beam-induced electron multipacting can arise through an oscillatory motion of a cloud of photoelectrons and low energy secondary electrons bouncing back and forth between opposite walls of the vacuum chamber during successive passages of proton bunches [7]. Due to the strong electric "eld of the dense proton beam these electrons will be accelerated and transfer their energy to the cold wall [8]. The magnitude of the heat load depends on several parameters and a crude estimate gives 0.2 W m\ for the LHC. (iv) Beam loss by nuclear scattering: Nuclear scattering of the high-energy protons on the residual gas generates a continuous loss from the circulating beams. A small fraction of scattered protons which cannot be absorbed in a controlled way by a dedicated set of collimators escapes from the aperture of the machine and penetrates into the magnets where they are lost. This #ux of high-energy protons cre-
Fig. 3. Actively cooled beam screen in the magnet cold bore.
ates a shower of secondary particles which is ultimately absorbed in the 1.9 K magnet system. The continuous heat input to the cryogenic system is directly proportional to the gas density and hence the maximum allowed value de"nes an upper limit of the gas density in the beam pipe. The beam lifetime q, due to nuclear scattering (the cross-section for 7 TeV protons on hydrogen atoms is p"&5;10\ m) is given by 1 "cpn, q where c is the speed of light and n the gas density. The linear power load, P(W m\), is proportional to the beam current I, the beam energy E and can be expressed in terms of the beam-gas lifetime IE E(¹e<)I(A) P" "0.93 . cq q(h) The LHC is the "rst accelerator where beam loss due to nuclear scattering on the residual gas represents a non-negligible heat load. The machine design therefore includes a nuclear scattering allowance of &0.2 W m\ for the two beams. Consistent with this requirement, a beam lifetime of &100 h has been chosen, which in turn implies that the average gas density must satisfy the &lifetime limit' )1;10 H molecules m\. Of the four heat sources, synchrotron radiation, image currents and photoelectrons will be intercepted
O. Gro( bner / Vacuum 60 (2001) 25}34
by the actively cooled &beam screen' at 20 K. The fourth component due to the scattered high-energy protons cannot be intercepted by this screen and must be included in the cryogenic budget for the 1.9 K system. The section of the beam screen with its two cooling tubes is shown in Fig. 3. In order not to lose the bene"t of the cryopumping on the 1.9 K cold bore, the screen has narrow pumping slots along its length which enable gas molecules to leave the beam channel proper and to be adsorbed on the cold bore wall. The importance of cryosorbing gas molecules on a surface that is screened from the e!ects of the beam will be discussed in the following section.
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At 7 TeV, the critical energy of the synchrotron radiation spectrum is &45 eV. The actively cooled beam screen intercepts this radiation so that it does not load the 1.9 K system. The synchrotron radiation which strikes the copper-coated inner surface at a grazing incidence liberates a very large amount of gas, proportional to the photon #ux and to the molecular desorption yield. The molecular yield for H from a copper surface at 10 K is typically 5;10\ molecules photon while other species have smaller values [9]. To provide adequate pumping for the vacuum system, the screen has to be partially transparent so that molecules desorbed by the synchrotron radiation can escape from the beam channel and can be permanently cryosorbed on the cold bore surface.
[10]. By using pumping slots however, hydrogen, and even more so all other gas species, will be cryosorbed on the 1.9 K surface with a negligibly low vapour pressure and hence with a practically unlimited capacity. Since direct and scattered/re#ected photons as well as photoelectrons continuously strike the interior surface of the beam screen, cryosorbed gas molecules are re-desorbed. This &recycling' e!ect increases with surface coverage and may dominate other sources of outgassing. The global result of thermal desorption and of recycling of gas implies that only a small quantity of gas can be accommodated on the inner surface of the beam screen. Thereafter any additional molecules will gradually escape through the slots and the pressure rise stabilise when the rate of production of gas molecules equals the transmission through the slots. The hydrogen vapour pressure in a condensation cryopump has been shown to depend on the exposure to room temperature thermal radiation that may desorb hydrogen condensed on a substrate at 2.3 K [11]. Indeed, extrapolating from these earlier measurements to LHC, the synchrotron radiation power density on a bare cold bore without screen (&0.14 W cm\) could increase the saturated vapour pressure to a value comparable to the lifetime limit. The evolution of the vacuum pressure has been studied extensively at the VEPP-2 M photon beam line at BINP [12] and at CERN [13]. With a &2% area of pumping slots, hydrogen will saturate at &3;10 m\ during the initial running of LHC, well below the speci"ed lifetime limit.
4. Vapour pressure and recycling of physisorbed gas
5. Dynamic vacuum model for LHC
Among the desorbed molecules, hydrogen, which has the largest molecular desorption yield, is the most critical species for which the thermal desorption is high and the adsorption capacity at the temperature of the beam screen surface is rather small. Without the pumping slots, the surface of the screen would therefore not provide any useful pumping capacity since the equilibrium vapour density at 5 K for a monolayer of hydrogen exceeds by several orders of magnitude the acceptable limit
The dynamic vacuum behaviour for hydrogen in the cold parts of LHC with a perforated beam screen has been described by the interaction between the volume density, n (m\), and the surface density, h (m\), of adsorbed molecules on the wall of the beam screen. The volume and the surface gas interact according to the set of linear equations:
3. Primary desorption by synchrotron radiation
<
dn dH "q!an#bH and F "cn!bH. dt dt
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Here V is the volume and F is the net wall area per unit length of the vacuum system. The parameters a, b, c and q represent the following physical quantities: (i) q represents a source of gas, here in particular the photon-induced desorption rate determined by the product of the desorption yield, g (molecules photon\) and of the photon #ux, CQ , hence q"gCQ (molecules s\ m\). In more general terms q can include any source of gas generated, e.g. by ion or by electron stimulated desorption. (ii) a (m s\) describes the total pumping on the surface of the wall and, in particular, through the area of the slots in the beam screen. It can be expressed as a" v F(s#f ), where v is the average molecular velocity, s the average sticking probability of the molecules on the beam screen and f the fraction of the total surface of the beam screen with pumping slots ( f 1). (iii) b represents the gas originating from a physisorbed surface phase, h, by thermal and by photon-induced desorption. In contrast to q, this is not a source of &new' gas but constitutes a surface phase interacting through adsorption and desorption with the volume gas. Desorption from this surface phase simply &re-cycles' molecules adsorbed on the inner surface of the beam screen. This process can be expressed as iCQ , where i is the recycling cross-section in m photon\. (iv) c is the rate of adsorption of gas molecules on the surface of the beam screen, proportional to the sticking probability of the molecules, s, c" v sF. In steady state, when the volume and surface densities no longer change with time, the volume density is simply given by q gCQ n " " COS a!c v Ff and depends solely on the primary desorption by synchrotron radiation and on the pumping through the slots in the beam screen. More recently, this model has been extended to include the e!ects of ion stimulated desorption and
of cracking of molecules, e.g. of CH and of CO by photons and photoelectrons [14].
6. Ion stimulated desorption and pressure instability Ionization of the residual gas molecules will produce ions, which are ejected by the positive beam potential and accelerated towards the beam screen. The "nal impact energy of the ions may be up to 300 eV in the arcs of the LHC and can be several keV in the experimental regions with tightly focussed beams. As with photons, the energetic ions are very e!ective in desorbing tightly bound gas molecules, a process which is characterised by the molecular desorption yield g (molecules ion\). Desorbed molecules may in turn be ionised and thus increase the desorption rate. This avalanche e!ect can lead to the so-called &ion-induced pressure instability' known from previous experience with the intersecting storage rings (ISR machine) at CERN [15]. As a result of this feedback mechanism the pressure increases with beam current from the initial value, P , as P M P(I)" , 1!pg I/eS G CDD where e is the electron charge and p the ionisation cross-section of the residual gas molecules for high-energy protons; for common gas molecules typically of the order of 10\ m [16]. S is the CDD e!ective, linear pumping speed of the system. The pressure runaway depends critically on the local cleanliness of the surface, through the ioninduced molecular desorption yield, g , and on the G local pumping speed. The stability limit is expressed by the condition that the product of beam current I and of the molecular desorption yield must be less than a critical value given by the e!ective pumping speed: e (g I) " S . G APGR p CDD In the cold sections of the LHC the pumping speed can be very large for molecules condensing on the wall of the beam screen. Nevertheless, it has a wellde"ned lower limit determined by the pumping
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slots in the beam screen. The lowest condition for vacuum stability is thus de"ned by the area f of pumping holes per unit length: 1e v f. (g I) " G APGR 4 p From this equation, the stability limit for the cold arc is typically 10 A for hydrogen and somewhat less for the other gas species. The gradual accumulation of physisorbed gas in the case of a bare cold bore leads to an increasing desorption yield which may reach values as large as 10 H molecu les ion\ at a monolayer [17]. With the use of the beam screen that protects the cryosorbing surfaces from the direct view of synchrotron radiation and of ions, the ion-induced pressure instability can be avoided.
7. Beam-induced electron multipacting Beam-induced multipacting [18], which can arise through oscillatory motion of a cloud of photoelectrons and low-energy secondary electrons bouncing back and forth between opposite walls of the vacuum chamber during successive proton bunches, represent a potential problem for the machine [19]. The key parameters for this process are: the bunch intensity and bunch spacing, the synchrotron radiation intensity and the photoelectric and secondary electron yields. The build-up and the steady-state intensity of the electron cloud inside the beam pipe is a function of the energy distribution of secondary electrons and of external electric and magnetic "elds. Inside the strong magnetic "eld of the bending magnets, photoelectrons that are produced in the horizontal plane are strongly suppressed since their movement is con"ned to the vertical direction along the magnetic "eld lines. For this reason only those synchrotron radiation photons which are re#ected and scattered as to hit the top and bottom surfaces of the beam screen will contribute to the electron multipacting. In the optimum design of the beam screen the photon re#ectivity should therefore be low. The average energy of photoelectrons may be of the order of 200 eV. Under situations where the
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secondary electron yield exceeds unity, the electron cloud may grow inde"nitely and, as a consequence, produce electron stimulated gas desorption. A "rst criterion for the onset of multipacting is obtained from the necessary condition that the transit time of electrons from wall to wall must be equal to or less than the time between the passage of successive bunches so that the electron cloud can move in synchronism with the beam: r N " N . @ r ¸ C @@ Here r (m) is the beam pipe radius, ¸ is the N @@ distance between proton bunches and r the C classical electron radius. In the LHC the determining parameters (the beam screen radius, the bunch intensity and the bunch spacing) are such that the wall-to-wall multipacting condition is satis"ed above about to of the nominal current [20]. The consequence for the machine operation can be an additional heat load due to the power deposited by the photoelectrons in the wall of the cold vacuum system, electron stimulated gas desorption and coherent beam oscillations of the proton beam with the electron cloud leading to emittance growth and ultimately to beam loss. These e!ects have been studied extensively over the past years and it was found that an average heat load of 0.5 W m\ must be included in the cryogenic budget. To reduce the electron cloud e!ect, one must "nd ways to reduce the production rate of photoelectrons and to achieve a low secondary electron yield at the internal surface of the beam screen [21]. The pressure rise due to multipacting electrons has recently been observed in the SPS accelerator at CERN, which will be used as the injector for the LHC. When operating the SPS with LHC-type beams, the pressure starts to increase rapidly above some threshold value that corresponds to about half of the nominal intensity, see Fig. 4. The rate of gas desorption by the electron cloud, Q (Pa m s\ m\), can be related with the heat AJMSB load to the vacuum system, P (W m\): JGL g P Q "k C JGL , AJMSB 1E 2 AJMSB
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O. Gro( bner / Vacuum 60 (2001) 25}34
tively, as shown in Fig. 5 [20]. Therefore, to be safe against electron multipacting, it will be essential to provide a vacuum chamber surface with a low secondary electron yield and primarily for this reason bare aluminium has been excluded as a material for beam pipes. Even for the more favourable materials like copper or stainless steel, it will be necessary to reduce their secondary electron yield below a critical value ((1.3) by in situ conditioning, i.e. &beam scrubbing' with synchrotron radiation [22]. Fig. 4. Pressure rise in the SPS vacuum system due to gas desorption by multipacting.
8. Mechanical design 8.1. Beam screen
Fig. 5. Secondary electron yield curves for technical aluminium and copper surfaces.
The beam screen is cooled by two longitudinal tubes placed in the vertical plane where it is acceptable for the beam aperture. For mechanical and magnetic requirements, the screen material must be a low permeability stainless steel, approximately 1 mm thick [23,24]. The inner wall is coated with 0.05 mm high conductivity copper to keep the electric impedance low [25]. Among the di!erent methods for producing the copper layer roll bonding of copper onto stainless-steel strips is the most cost e!ective for series production. The coated strip is shaped into the "nal pro"le in a continuous tube-forming machine with a single longitudinal laser weld. 8.2. Aperture
where g is the electron stimulated molecular deC sorption yield (&10\}10\ molecules electron\) and 1E 2 the average energy of elecAJMSB trons striking the vacuum chamber wall. The factor k converts from molecular density to pressure units. It is important to note that for a typical set of parameters, this additional gas load will be of the order of the synchrotron radiation-induced gas load. Secondary electron yield data for &technical' Cu and Al surfaces measured on samples in the laboratory indicate that for both materials a yield of unity is exceeded at an electron energy of less than 50 eV and that the yield versus energy curves peak at 2.2 and above 3 between 300 and 450 eV, respec-
Since the diameter of the cold bore is limited, a design that maximises the aperture available for the beam must be found. The section of the beam screen is the result of an extensive work which takes into account the beam requirements at all stages of operation from injection, beam cleaning, colliding beams, to beam abort. An individual beam screen for a dipole magnet is approximately 16 m long and its nominal clearance to the cold bore is less than 0.5 mm. It will be a technical challenge to meet these very tight dimensional tolerances and the required straightness. The beam screens in one half-cell will be cooled in series by helium with 5 K inlet and 20 K outlet temperature. Laser welding is
O. Gro( bner / Vacuum 60 (2001) 25}34
the preferred method for the attachment of the cooling tubes to minimise distortion and to keep the heat a!ected zone of the welds as small as possible. After the screen has been inserted into the magnet there should a small annular gap to obtain good centering and low thermal contact.
8.3. Magnetic properties The magnetic permeability of the beam screen and its components must be very low since any asymmetry in the distribution of magnetic material could give rise to undesirable harmonics of the magnetic guiding "eld of the beam. The speci"cation imposes a magnetic permeability not exceeding 1.005 at low temperature. This requirement eliminates more conventional types of stainless steels used in vacuum applications, e.g. 304 and 316 type steel, and imposes the use of highly stable austenitic stainless steels alloyed with Mn and N [26].
8.4. Copper coating and magnet quench The inner surface of the screen must be coated with high conductivity copper to obtain good electric conductivity. The thickness chosen for this copper layer results from an optimisation of several con#icting requirements: A su$ciently thick, high conductivity layer will reduce the power dissipated by image currents, provide a low electric impedance and thus limit the growth rate for the transverse resistive wall beam instability. A larger power dissipation would require more or larger helium cooling channels and thus complicate the design or reduce the vertical aperture. On the other hand, a thicker copper layer will entail larger eddy currents and electro-mechanical forces during a magnet quench and hence require a mechanically stronger, i.e. thicker beam screen, which again reduces the aperture. The compromise is 0.05 mm copper and a wall thickness of about 1 mm for the stainless-steel tube. The stresses induced in the beam screen during a magnet quench, results in several tens of millimeters de#ection in the horizontal plane.
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8.5. Beam screen interconnects The mechanical design of the cold arc vacuum system requires a large number of interconnections for the beam vacuum between the beam screens of adjacent magnets. Following the proven design of the LEP vacuum system an improved bellows concept has been developed which takes into account the large thermal contraction (approx. 45 mm) between room temperature and operating temperature [27]. An important design criterion to be met is the requirement of installation and alignment of the long cryomagnets without the risk of damaging the delicate components even in case of lateral o!sets of up to 4 mm. The beam screen cooling tubes are joined together in the insulation vacuum enclosure by a #exible tubing to reduce the risk of a helium leak into the beam vacuum.
9. Summary The design of the LHC vacuum system must satisfy a number of requirements, which depend not only on vacuum issues but also on considerations of both electric impedance and heat losses into the cryogenic system and/or the superconducting magnets. The pumping relies entirely on crypo-pumping of the 1.9 K cold bore surface, which o!ers a practically unlimited capacity for all gases except helium. The detailed design of the vacuum system is strongly in#uenced by the large amount of synchrotron radiation, which represents a signi"cant source of photon stimulated outgassing, creates photoelectrons and is thus a driving mechanism for beam-induced multipacting. The beam-induced heat load to the 1.9 K system has great importance for the design and imposes by itself the presence of an actively cooled beam screen. The intense proton bunches with their high peak current lead to energetic ions which strike the walls and may cause ion-induced desorption and ion-induced pressure rise. Due to the cryogenic temperature of the walls, most molecular species are strongly cryosorbed and stick to the surface. This e!ect leads to a buildup of layers of cryosorbed gas, which in turn may desorb either thermally (vapour pressure e!ect) or by photon stimulated desorption (re-cycling e!ect).
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More recent studies have shown that in addition cracking of condensed gas species may occur.
Acknowledgements The description of the LHC vacuum system presented here re#ects the work by many colleagues in the LHC Vacuum Group. The important contributions, in particular from the groups LHC-CRI, EST-SM and SL-AP are also gratefully acknowledged.
References [1] The LHC Study Group. The Large Hadron Collider, Conceptual Design, CERN/AC/95-05, 1995. [2] Evans L, Lebrun Ph. Progress in construction of the LHC. LHC Project Report 239, 1998. [3] Brunet JC et al. LHC-Project Report 280 and PAC-99, New York, 1999. [4] Mathewson AG et al. EPAC-94, London, 1994. [5] Mathewson AG et al. PAC-95, Dallas, 1995. [6] GroK bner O. Vacuum 1995;46:767. [7] GroK bner O. 10th International Conference on High Energy Accelerators, Protvino, July 1977. [8] GroK bner O. PAC-97, Vancouver, 1997.
[9] Anashin VV, Malyshev OB, Calder R, GroK bner O. Vacuum 1999;53:269}72. [10] WalleH n E. J Vac Sci Technol A 1997;15(2):265}74. [11] Benvenuti C, Calder R, Passardi G. J Vac Sci Technol 1976;13(6):1172}82. [12] Calder R, et al. J Vac Sci Technol 1996;A14(4):2618}23. [13] Baglin V. Private communication, December 1999. [14] Anashin VV, Malyshev OB, Collins I, GroK bner O. Photonstimulated desorption and the e!ect of cracking of condensed molecules in a cryogenic vacuum system. Vacuum 2001;60:15}24. [15] GroK bner O, Calder R. IEEE Trans Nucl Sci 1973;NS20:760. [16] Rieke F, Prepejchal W. Phys Rev A 1972;6:1507. [17] Barnard JC, Bojko I, Hilleret N. Vacuum 1996;47(4):347. [18] GroK bner O. 10th International Conference on High Energy Accelerators, Protvino, July 1977. [19] Zimmermann F. LHC Project Report 95, 27 February 1997. [20] GroK bner O. PAC-97, Vancouver, 1997. [21] Baglin V, Collins IR, GroK bner O. EPAC-98, Stockholm, 1998. [22] BruK nning O, et al. PAC-99, New York, 1999. [23] Poncet A, et al. EPAC-94, London, 1994. [24] Cruikshank P, et al. PAC-97, Vancouver, 1997. [25] Ruggiero F, Caspers R, Morvillo M. PAC-97, Vancouver, 1997. [26] Sgobba S, Hochoertler G. International Congress Stainless Steel, 1999, Science and Market, Chia Laguna (I), 6}9 June 1999, Vol. 2, pp. 391}401. [27] Veness R, et al. PAC-99, New York, 1999.
Overview of the LHC vacuum system Oswald GroK bner* 1211 CERN, Geneva 23, Switzerland
Abstract The Large Hadron Collider (LHC) project, now in the advanced construction phase at CERN, comprises two proton storage rings with colliding beams of 7#7 TeV energy. The machine is housed in the existing LEP tunnel with a circumference of 26.7 km and requires a bending magnetic "eld of 8.3 T with 14 m long superconducting magnets operating at 1.9 K. The beam vacuum system comprises the inner &cold bore' walls of the magnets which provide a nearly perfect cryopump. In view of reducing the cryogenic power consumption, both the heat load from synchrotron radiation emitted by the proton beams and the resistive power dissipation by the beam image currents have to be absorbed on a &beam screen', which operates between 5 and 20 K and is inserted inside the cold bore. The design operating pressure must provide a beam lifetime of several days and this requirement comes from the power deposition in the superconducting magnet coils due to protons scattered on the residual gas which could lead to a magnet quench and interrupt the machine operation. Cryopumping of gas on the cold surfaces provides the necessary low gas densities but it must be ensured that the vapour pressure of cryosorbed molecules, of which H and He are the most critical species, remains within acceptable limits. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction The Large Hadron Collider (LHC) [1] consists of a pair of superconducting storage rings with a circumference of 26.7 km. Two proton beams, of 560 mA and an energy of 7 TeV, circulate in opposite directions. Of the &54 km total length, the arcs account for almost 48 km and will be at 1.9 K, the temperature of the superconducting magnets. The basic layout of LHC follows the existing LEP machine, with eight long straight sections and eight bending arcs. The LHC will include two high lu-
* Corresponding author. Tel.: #41-22-7675989; fax: #4122-767-5100. E-mail address: [email protected] (O. GroK bner). O. GroK bner for the LHC Vacuum Group.
minosity experiments (ATLAS and CMS) as well as a heavy-ion experiment (ALICE) and a B-physics experiment (LHCb). The existing SPS accelerator will be used to inject the beams at 450 GeV. To "t the 7 TeV rings into the existing LEP tunnel special design concepts are needed for the machine components and, most speci"cally, superconducting magnets with 8.3 T "eld. The magnetic bending "eld of 8.3 T implies that the superconducting magnets which use commercially available NbTi superconductors must operate at a temperature well below 4.2 K. A very attractive technical solution for the LHC has been to immerse the magnets in a bath of super#uid helium at 1.9 K. The magnets for LHC are based on a two-in-one design, where the two-beam channels are incorporated into a common iron yoke in a compact single cryostat [2]. Fig. 1 illustrates this design concept
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showing the cross-section of a dipole cryomagnet with its cryostat. The compact construction allows the installation of the LHC machine inside the existing tunnel section of LEP. In the arcs of LHC the machine lattice consists of a succession of regular 53.4 m long &halfcells': three 14.2 m long twin-aperture dipole magnets, and one 6.3 m long short straight section. In the long straight sections the machine layout is adapted to the speci"c requirements of the experiments and the beams converge into one common beam pipe, where they are tightly focused and steered into head-on collisions at the interaction points.
2. The vacuum system The vacuum system for LHC consists of two main parts which are kept entirely separated in
terms of vacuum since they must meet very di!erent requirements: the cryogenic insulation vacuum necessary to avoid heat load by gas conduction requires a pressure of only about 10\ mbar. The beam vacuum proper, which must provide a good beam lifetime and low background for the experiments, requires at least three orders of magnitude better vacuum to avoid excessive particle loss by nuclear scattering on the rest gas.
2.1. Cryogenic insulation vacuum The cryogenic insulation vacuum for the machine arcs consists of a nearly 3 km long continuous cryostat which is subdivided into 14 sectors each 212 m long [3]. Vacuum separation is achieved by thin, metallic membranes between the outer room temperature wall of the cryostat and the shell
Fig. 1. Dipole cryomagnet cross-section with cryostat.
O. Gro( bner / Vacuum 60 (2001) 25}34
of the cryomagnets, called `vacuum barriersa so that individual sub-sections can be pumped, commissioned and leak checked independently. These `vacuum barriersa also separate the vacuum for the continuous cryogenic distribution line (QRL) from the machine cryostat as shown in Fig. 2. While the initial pump down can be done with conventional turbomolecular pumping groups shown in the inset of Fig. 2, the cryopumping on the large external surface of the magnet cold mass and on the thermal screens will be entirely su$cient for maintaining the static insulation vacuum once the system has been cooled down. Provision for any additional pumping during machine operation is required only if excessive helium leaks occur. This additional pumping can be achieved either by connecting external pumps or by commercial cryopanels with a large pumping capacity for helium. Whether this option will be needed to cope
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with a degraded vacuum pressure will be decided based on the experience with a test string of the machine. 2.2. Beam vacuum and beam related heat loads The beam vacuum system has been the subject of extensive studies over recent years and has been described elsewhere [4}6]. The beam pipe in the centre of the superconducting magnet coils is in direct contact with the helium bath at 1.9 K (coldbore beam pipe) so as to optimise the aperture for the beam. At this temperature, the vacuum chamber wall becomes a very e$cient cryopump with practically in"nite capacity for all condensable gas species with the sole exception of helium. External pumping should not be required and could even be rather ine$cient because of the cryosorption on the walls. Unfortunately, this concept has the
Fig. 2. Layout of cryogenic insulation vacuum.
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drawback that the cryogenic system will have to remove even very small amounts of beam-induced power which, for conventional room temperature accelerators would be insigni"cant. Indeed, since the removal of 1 W at 1.9 K requires nearly 1 kW of electric power, there is a strong incentive to avoid or to reduce to the bare minimum any source of heat in-leak to the cold beam pipe. Four distinct beam and vacuum related heat sources to the 1.9 K system can be identi"ed: (i) Synchrotron radiation: Due to the centripetal acceleration in the bending magnets the beams will emit a synchrotron radiation #ux of about 10 photons s\ m\ equivalent to a distributed linear power of 0.2 W m\. For the LHC as a whole, this amounts to 7.6 kW. (ii) Image currents from the beam: The inner wall of the beam pipe must conduct the image currents of the beams and this power depends directly on the resistivity, o of the vacuum chamber wall material. U To limit this heat load, and also to avoid resistive wall instability of the beam, the resistivity of the vacuum chamber has to be low. For the LHC this can be achieved by coating the inner surface of the stainless-steel beam pipe with a thin copper layer. It is an additional advantage that, at this low temperature, the resistivity of high-purity copper is a factor of 10 lower than for stainless steel. (iii) Photoelectrons and multipacting: Beam-induced electron multipacting can arise through an oscillatory motion of a cloud of photoelectrons and low energy secondary electrons bouncing back and forth between opposite walls of the vacuum chamber during successive passages of proton bunches [7]. Due to the strong electric "eld of the dense proton beam these electrons will be accelerated and transfer their energy to the cold wall [8]. The magnitude of the heat load depends on several parameters and a crude estimate gives 0.2 W m\ for the LHC. (iv) Beam loss by nuclear scattering: Nuclear scattering of the high-energy protons on the residual gas generates a continuous loss from the circulating beams. A small fraction of scattered protons which cannot be absorbed in a controlled way by a dedicated set of collimators escapes from the aperture of the machine and penetrates into the magnets where they are lost. This #ux of high-energy protons cre-
Fig. 3. Actively cooled beam screen in the magnet cold bore.
ates a shower of secondary particles which is ultimately absorbed in the 1.9 K magnet system. The continuous heat input to the cryogenic system is directly proportional to the gas density and hence the maximum allowed value de"nes an upper limit of the gas density in the beam pipe. The beam lifetime q, due to nuclear scattering (the cross-section for 7 TeV protons on hydrogen atoms is p"&5;10\ m) is given by 1 "cpn, q where c is the speed of light and n the gas density. The linear power load, P(W m\), is proportional to the beam current I, the beam energy E and can be expressed in terms of the beam-gas lifetime IE E(¹e<)I(A) P" "0.93 . cq q(h) The LHC is the "rst accelerator where beam loss due to nuclear scattering on the residual gas represents a non-negligible heat load. The machine design therefore includes a nuclear scattering allowance of &0.2 W m\ for the two beams. Consistent with this requirement, a beam lifetime of &100 h has been chosen, which in turn implies that the average gas density must satisfy the &lifetime limit' )1;10 H molecules m\. Of the four heat sources, synchrotron radiation, image currents and photoelectrons will be intercepted
O. Gro( bner / Vacuum 60 (2001) 25}34
by the actively cooled &beam screen' at 20 K. The fourth component due to the scattered high-energy protons cannot be intercepted by this screen and must be included in the cryogenic budget for the 1.9 K system. The section of the beam screen with its two cooling tubes is shown in Fig. 3. In order not to lose the bene"t of the cryopumping on the 1.9 K cold bore, the screen has narrow pumping slots along its length which enable gas molecules to leave the beam channel proper and to be adsorbed on the cold bore wall. The importance of cryosorbing gas molecules on a surface that is screened from the e!ects of the beam will be discussed in the following section.
29
At 7 TeV, the critical energy of the synchrotron radiation spectrum is &45 eV. The actively cooled beam screen intercepts this radiation so that it does not load the 1.9 K system. The synchrotron radiation which strikes the copper-coated inner surface at a grazing incidence liberates a very large amount of gas, proportional to the photon #ux and to the molecular desorption yield. The molecular yield for H from a copper surface at 10 K is typically 5;10\ molecules photon while other species have smaller values [9]. To provide adequate pumping for the vacuum system, the screen has to be partially transparent so that molecules desorbed by the synchrotron radiation can escape from the beam channel and can be permanently cryosorbed on the cold bore surface.
[10]. By using pumping slots however, hydrogen, and even more so all other gas species, will be cryosorbed on the 1.9 K surface with a negligibly low vapour pressure and hence with a practically unlimited capacity. Since direct and scattered/re#ected photons as well as photoelectrons continuously strike the interior surface of the beam screen, cryosorbed gas molecules are re-desorbed. This &recycling' e!ect increases with surface coverage and may dominate other sources of outgassing. The global result of thermal desorption and of recycling of gas implies that only a small quantity of gas can be accommodated on the inner surface of the beam screen. Thereafter any additional molecules will gradually escape through the slots and the pressure rise stabilise when the rate of production of gas molecules equals the transmission through the slots. The hydrogen vapour pressure in a condensation cryopump has been shown to depend on the exposure to room temperature thermal radiation that may desorb hydrogen condensed on a substrate at 2.3 K [11]. Indeed, extrapolating from these earlier measurements to LHC, the synchrotron radiation power density on a bare cold bore without screen (&0.14 W cm\) could increase the saturated vapour pressure to a value comparable to the lifetime limit. The evolution of the vacuum pressure has been studied extensively at the VEPP-2 M photon beam line at BINP [12] and at CERN [13]. With a &2% area of pumping slots, hydrogen will saturate at &3;10 m\ during the initial running of LHC, well below the speci"ed lifetime limit.
4. Vapour pressure and recycling of physisorbed gas
5. Dynamic vacuum model for LHC
Among the desorbed molecules, hydrogen, which has the largest molecular desorption yield, is the most critical species for which the thermal desorption is high and the adsorption capacity at the temperature of the beam screen surface is rather small. Without the pumping slots, the surface of the screen would therefore not provide any useful pumping capacity since the equilibrium vapour density at 5 K for a monolayer of hydrogen exceeds by several orders of magnitude the acceptable limit
The dynamic vacuum behaviour for hydrogen in the cold parts of LHC with a perforated beam screen has been described by the interaction between the volume density, n (m\), and the surface density, h (m\), of adsorbed molecules on the wall of the beam screen. The volume and the surface gas interact according to the set of linear equations:
3. Primary desorption by synchrotron radiation
<
dn dH "q!an#bH and F "cn!bH. dt dt
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Here V is the volume and F is the net wall area per unit length of the vacuum system. The parameters a, b, c and q represent the following physical quantities: (i) q represents a source of gas, here in particular the photon-induced desorption rate determined by the product of the desorption yield, g (molecules photon\) and of the photon #ux, CQ , hence q"gCQ (molecules s\ m\). In more general terms q can include any source of gas generated, e.g. by ion or by electron stimulated desorption. (ii) a (m s\) describes the total pumping on the surface of the wall and, in particular, through the area of the slots in the beam screen. It can be expressed as a" v F(s#f ), where v is the average molecular velocity, s the average sticking probability of the molecules on the beam screen and f the fraction of the total surface of the beam screen with pumping slots ( f 1). (iii) b represents the gas originating from a physisorbed surface phase, h, by thermal and by photon-induced desorption. In contrast to q, this is not a source of &new' gas but constitutes a surface phase interacting through adsorption and desorption with the volume gas. Desorption from this surface phase simply &re-cycles' molecules adsorbed on the inner surface of the beam screen. This process can be expressed as iCQ , where i is the recycling cross-section in m photon\. (iv) c is the rate of adsorption of gas molecules on the surface of the beam screen, proportional to the sticking probability of the molecules, s, c" v sF. In steady state, when the volume and surface densities no longer change with time, the volume density is simply given by q gCQ n " " COS a!c v Ff and depends solely on the primary desorption by synchrotron radiation and on the pumping through the slots in the beam screen. More recently, this model has been extended to include the e!ects of ion stimulated desorption and
of cracking of molecules, e.g. of CH and of CO by photons and photoelectrons [14].
6. Ion stimulated desorption and pressure instability Ionization of the residual gas molecules will produce ions, which are ejected by the positive beam potential and accelerated towards the beam screen. The "nal impact energy of the ions may be up to 300 eV in the arcs of the LHC and can be several keV in the experimental regions with tightly focussed beams. As with photons, the energetic ions are very e!ective in desorbing tightly bound gas molecules, a process which is characterised by the molecular desorption yield g (molecules ion\). Desorbed molecules may in turn be ionised and thus increase the desorption rate. This avalanche e!ect can lead to the so-called &ion-induced pressure instability' known from previous experience with the intersecting storage rings (ISR machine) at CERN [15]. As a result of this feedback mechanism the pressure increases with beam current from the initial value, P , as P M P(I)" , 1!pg I/eS G CDD where e is the electron charge and p the ionisation cross-section of the residual gas molecules for high-energy protons; for common gas molecules typically of the order of 10\ m [16]. S is the CDD e!ective, linear pumping speed of the system. The pressure runaway depends critically on the local cleanliness of the surface, through the ioninduced molecular desorption yield, g , and on the G local pumping speed. The stability limit is expressed by the condition that the product of beam current I and of the molecular desorption yield must be less than a critical value given by the e!ective pumping speed: e (g I) " S . G APGR p CDD In the cold sections of the LHC the pumping speed can be very large for molecules condensing on the wall of the beam screen. Nevertheless, it has a wellde"ned lower limit determined by the pumping
O. Gro( bner / Vacuum 60 (2001) 25}34
slots in the beam screen. The lowest condition for vacuum stability is thus de"ned by the area f of pumping holes per unit length: 1e v f. (g I) " G APGR 4 p From this equation, the stability limit for the cold arc is typically 10 A for hydrogen and somewhat less for the other gas species. The gradual accumulation of physisorbed gas in the case of a bare cold bore leads to an increasing desorption yield which may reach values as large as 10 H molecu les ion\ at a monolayer [17]. With the use of the beam screen that protects the cryosorbing surfaces from the direct view of synchrotron radiation and of ions, the ion-induced pressure instability can be avoided.
7. Beam-induced electron multipacting Beam-induced multipacting [18], which can arise through oscillatory motion of a cloud of photoelectrons and low-energy secondary electrons bouncing back and forth between opposite walls of the vacuum chamber during successive proton bunches, represent a potential problem for the machine [19]. The key parameters for this process are: the bunch intensity and bunch spacing, the synchrotron radiation intensity and the photoelectric and secondary electron yields. The build-up and the steady-state intensity of the electron cloud inside the beam pipe is a function of the energy distribution of secondary electrons and of external electric and magnetic "elds. Inside the strong magnetic "eld of the bending magnets, photoelectrons that are produced in the horizontal plane are strongly suppressed since their movement is con"ned to the vertical direction along the magnetic "eld lines. For this reason only those synchrotron radiation photons which are re#ected and scattered as to hit the top and bottom surfaces of the beam screen will contribute to the electron multipacting. In the optimum design of the beam screen the photon re#ectivity should therefore be low. The average energy of photoelectrons may be of the order of 200 eV. Under situations where the
31
secondary electron yield exceeds unity, the electron cloud may grow inde"nitely and, as a consequence, produce electron stimulated gas desorption. A "rst criterion for the onset of multipacting is obtained from the necessary condition that the transit time of electrons from wall to wall must be equal to or less than the time between the passage of successive bunches so that the electron cloud can move in synchronism with the beam: r N " N . @ r ¸ C @@ Here r (m) is the beam pipe radius, ¸ is the N @@ distance between proton bunches and r the C classical electron radius. In the LHC the determining parameters (the beam screen radius, the bunch intensity and the bunch spacing) are such that the wall-to-wall multipacting condition is satis"ed above about to of the nominal current [20]. The consequence for the machine operation can be an additional heat load due to the power deposited by the photoelectrons in the wall of the cold vacuum system, electron stimulated gas desorption and coherent beam oscillations of the proton beam with the electron cloud leading to emittance growth and ultimately to beam loss. These e!ects have been studied extensively over the past years and it was found that an average heat load of 0.5 W m\ must be included in the cryogenic budget. To reduce the electron cloud e!ect, one must "nd ways to reduce the production rate of photoelectrons and to achieve a low secondary electron yield at the internal surface of the beam screen [21]. The pressure rise due to multipacting electrons has recently been observed in the SPS accelerator at CERN, which will be used as the injector for the LHC. When operating the SPS with LHC-type beams, the pressure starts to increase rapidly above some threshold value that corresponds to about half of the nominal intensity, see Fig. 4. The rate of gas desorption by the electron cloud, Q (Pa m s\ m\), can be related with the heat AJMSB load to the vacuum system, P (W m\): JGL g P Q "k C JGL , AJMSB 1E 2 AJMSB
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O. Gro( bner / Vacuum 60 (2001) 25}34
tively, as shown in Fig. 5 [20]. Therefore, to be safe against electron multipacting, it will be essential to provide a vacuum chamber surface with a low secondary electron yield and primarily for this reason bare aluminium has been excluded as a material for beam pipes. Even for the more favourable materials like copper or stainless steel, it will be necessary to reduce their secondary electron yield below a critical value ((1.3) by in situ conditioning, i.e. &beam scrubbing' with synchrotron radiation [22]. Fig. 4. Pressure rise in the SPS vacuum system due to gas desorption by multipacting.
8. Mechanical design 8.1. Beam screen
Fig. 5. Secondary electron yield curves for technical aluminium and copper surfaces.
The beam screen is cooled by two longitudinal tubes placed in the vertical plane where it is acceptable for the beam aperture. For mechanical and magnetic requirements, the screen material must be a low permeability stainless steel, approximately 1 mm thick [23,24]. The inner wall is coated with 0.05 mm high conductivity copper to keep the electric impedance low [25]. Among the di!erent methods for producing the copper layer roll bonding of copper onto stainless-steel strips is the most cost e!ective for series production. The coated strip is shaped into the "nal pro"le in a continuous tube-forming machine with a single longitudinal laser weld. 8.2. Aperture
where g is the electron stimulated molecular deC sorption yield (&10\}10\ molecules electron\) and 1E 2 the average energy of elecAJMSB trons striking the vacuum chamber wall. The factor k converts from molecular density to pressure units. It is important to note that for a typical set of parameters, this additional gas load will be of the order of the synchrotron radiation-induced gas load. Secondary electron yield data for &technical' Cu and Al surfaces measured on samples in the laboratory indicate that for both materials a yield of unity is exceeded at an electron energy of less than 50 eV and that the yield versus energy curves peak at 2.2 and above 3 between 300 and 450 eV, respec-
Since the diameter of the cold bore is limited, a design that maximises the aperture available for the beam must be found. The section of the beam screen is the result of an extensive work which takes into account the beam requirements at all stages of operation from injection, beam cleaning, colliding beams, to beam abort. An individual beam screen for a dipole magnet is approximately 16 m long and its nominal clearance to the cold bore is less than 0.5 mm. It will be a technical challenge to meet these very tight dimensional tolerances and the required straightness. The beam screens in one half-cell will be cooled in series by helium with 5 K inlet and 20 K outlet temperature. Laser welding is
O. Gro( bner / Vacuum 60 (2001) 25}34
the preferred method for the attachment of the cooling tubes to minimise distortion and to keep the heat a!ected zone of the welds as small as possible. After the screen has been inserted into the magnet there should a small annular gap to obtain good centering and low thermal contact.
8.3. Magnetic properties The magnetic permeability of the beam screen and its components must be very low since any asymmetry in the distribution of magnetic material could give rise to undesirable harmonics of the magnetic guiding "eld of the beam. The speci"cation imposes a magnetic permeability not exceeding 1.005 at low temperature. This requirement eliminates more conventional types of stainless steels used in vacuum applications, e.g. 304 and 316 type steel, and imposes the use of highly stable austenitic stainless steels alloyed with Mn and N [26].
8.4. Copper coating and magnet quench The inner surface of the screen must be coated with high conductivity copper to obtain good electric conductivity. The thickness chosen for this copper layer results from an optimisation of several con#icting requirements: A su$ciently thick, high conductivity layer will reduce the power dissipated by image currents, provide a low electric impedance and thus limit the growth rate for the transverse resistive wall beam instability. A larger power dissipation would require more or larger helium cooling channels and thus complicate the design or reduce the vertical aperture. On the other hand, a thicker copper layer will entail larger eddy currents and electro-mechanical forces during a magnet quench and hence require a mechanically stronger, i.e. thicker beam screen, which again reduces the aperture. The compromise is 0.05 mm copper and a wall thickness of about 1 mm for the stainless-steel tube. The stresses induced in the beam screen during a magnet quench, results in several tens of millimeters de#ection in the horizontal plane.
33
8.5. Beam screen interconnects The mechanical design of the cold arc vacuum system requires a large number of interconnections for the beam vacuum between the beam screens of adjacent magnets. Following the proven design of the LEP vacuum system an improved bellows concept has been developed which takes into account the large thermal contraction (approx. 45 mm) between room temperature and operating temperature [27]. An important design criterion to be met is the requirement of installation and alignment of the long cryomagnets without the risk of damaging the delicate components even in case of lateral o!sets of up to 4 mm. The beam screen cooling tubes are joined together in the insulation vacuum enclosure by a #exible tubing to reduce the risk of a helium leak into the beam vacuum.
9. Summary The design of the LHC vacuum system must satisfy a number of requirements, which depend not only on vacuum issues but also on considerations of both electric impedance and heat losses into the cryogenic system and/or the superconducting magnets. The pumping relies entirely on crypo-pumping of the 1.9 K cold bore surface, which o!ers a practically unlimited capacity for all gases except helium. The detailed design of the vacuum system is strongly in#uenced by the large amount of synchrotron radiation, which represents a signi"cant source of photon stimulated outgassing, creates photoelectrons and is thus a driving mechanism for beam-induced multipacting. The beam-induced heat load to the 1.9 K system has great importance for the design and imposes by itself the presence of an actively cooled beam screen. The intense proton bunches with their high peak current lead to energetic ions which strike the walls and may cause ion-induced desorption and ion-induced pressure rise. Due to the cryogenic temperature of the walls, most molecular species are strongly cryosorbed and stick to the surface. This e!ect leads to a buildup of layers of cryosorbed gas, which in turn may desorb either thermally (vapour pressure e!ect) or by photon stimulated desorption (re-cycling e!ect).
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More recent studies have shown that in addition cracking of condensed gas species may occur.
Acknowledgements The description of the LHC vacuum system presented here re#ects the work by many colleagues in the LHC Vacuum Group. The important contributions, in particular from the groups LHC-CRI, EST-SM and SL-AP are also gratefully acknowledged.
References [1] The LHC Study Group. The Large Hadron Collider, Conceptual Design, CERN/AC/95-05, 1995. [2] Evans L, Lebrun Ph. Progress in construction of the LHC. LHC Project Report 239, 1998. [3] Brunet JC et al. LHC-Project Report 280 and PAC-99, New York, 1999. [4] Mathewson AG et al. EPAC-94, London, 1994. [5] Mathewson AG et al. PAC-95, Dallas, 1995. [6] GroK bner O. Vacuum 1995;46:767. [7] GroK bner O. 10th International Conference on High Energy Accelerators, Protvino, July 1977. [8] GroK bner O. PAC-97, Vancouver, 1997.
[9] Anashin VV, Malyshev OB, Calder R, GroK bner O. Vacuum 1999;53:269}72. [10] WalleH n E. J Vac Sci Technol A 1997;15(2):265}74. [11] Benvenuti C, Calder R, Passardi G. J Vac Sci Technol 1976;13(6):1172}82. [12] Calder R, et al. J Vac Sci Technol 1996;A14(4):2618}23. [13] Baglin V. Private communication, December 1999. [14] Anashin VV, Malyshev OB, Collins I, GroK bner O. Photonstimulated desorption and the e!ect of cracking of condensed molecules in a cryogenic vacuum system. Vacuum 2001;60:15}24. [15] GroK bner O, Calder R. IEEE Trans Nucl Sci 1973;NS20:760. [16] Rieke F, Prepejchal W. Phys Rev A 1972;6:1507. [17] Barnard JC, Bojko I, Hilleret N. Vacuum 1996;47(4):347. [18] GroK bner O. 10th International Conference on High Energy Accelerators, Protvino, July 1977. [19] Zimmermann F. LHC Project Report 95, 27 February 1997. [20] GroK bner O. PAC-97, Vancouver, 1997. [21] Baglin V, Collins IR, GroK bner O. EPAC-98, Stockholm, 1998. [22] BruK nning O, et al. PAC-99, New York, 1999. [23] Poncet A, et al. EPAC-94, London, 1994. [24] Cruikshank P, et al. PAC-97, Vancouver, 1997. [25] Ruggiero F, Caspers R, Morvillo M. PAC-97, Vancouver, 1997. [26] Sgobba S, Hochoertler G. International Congress Stainless Steel, 1999, Science and Market, Chia Laguna (I), 6}9 June 1999, Vol. 2, pp. 391}401. [27] Veness R, et al. PAC-99, New York, 1999.