Beam Screen Research Articles

Simulation of the electron-cloud build up and its consequences on heat load, beam stability, and diagnostics

Photoemission and secondary emission are known to give rise to a quasistationary electron cloud inside the beam pipe through a beam-induced multipacting process. We investigate the electron-cloud build up and related effects via computer simulation. In our model, macroparticles representing photoelectrons are emitted synchronously with the passing proton or positron bunch and are subsequently accelerated in the field of the beam. As they hit the beam pipe, new macroelectrons are generated, whose charges are determined by the energy of the incoming particles and by the secondary emission yield of the beam pipe. A quasistationary state of the electron cloud is eventually reached due to space charge. The equilibrium density is used as an input parameter for a second program that analyzes the electron-cloud driven single-bunch instability. The electron cloud simulation also allows the evaluation of the heat load on the cold Large Hadron Collider beam screen, which must stay within the available cooling capacity, and the electron charge deposited on or emitted from the electrodes of the beam-position monitors.

Overview of the LHC vacuum system

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 field 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 2 and He are the most critical species, remains within acceptable limits.

Photon-stimulated desorption and the effect of cracking of condensed molecules in a cryogenic vacuum system

The design of the large hadron collider (LHC) vacuum system requires a complete understanding of all processes which may affect the residual gas density in the cold bore of the 1.9 K cryomagnets. A wealth of data has been obtained which may be used to predict the residual gas density inside a cold vacuum system exposed to synchrotron radiation. In this study, the effect of cracking of cryosorbed molecules by synchrotron radiation photons has been included. Cracking of the molecular species CO 2 and CH 4 has been observed in recent studies and these findings have been incorporated in a more detailed dynamic gas density model for the LHC. In this paper, we describe the relevant physical processes and the parameters required for a full evaluation. It is shown that the dominant gas species in the LHC vacuum system with its beam screen are H 2 and CO. The important result of this study is that, while the surface coverage of cryosorbed CH 4 and CO 2 molecules is limited due to cracking, the coverage of H 2 and CO molecules may increase steadily during the long-term operation of the machine.

Magnetic and electric field effects on the photoelectron emission from prototype LHC beam screen material

This paper describes the experimental studies of the effect of a dipole field on photoelectron emission and on the photon reflectivities from LHC beam screen material. These studies were performed using synchrotron radiation from the VEPP-2M storage ring at BINP (Novosibirsk). The particular surface roughness and geometry of the prototype LHC beam screen material requires dedicated experimental measurements. The experiments were performed under conditions close to those expected in the LHC. An important result obtained is that a dipole magnetic field attenuates the photoelectron emission from the surface by more than two orders of magnitude with the magnetic field aligned parallel to the surface. The measurements of photon reflectivities, forward scattered and diffuse, and the azimuthal distribution of emitted photoelectrons from the same material are reported. These experimental results are important input for the final design of the LHC beam screen.

Cryogenics for the Large Hadron Collider

The Large Hadron Collider (LHC), a 26.7 km circumference superconducting accelerator equipped with high-field magnets operating in superfluid helium below 1.9 K, has now fully entered construction at CERN, the European Laboratory for Particle Physics. The heart of the LHC cryogenic system is the quasi-isothermal magnet cooling scheme, in which flowing two-phase saturated superfluid helium removes the heat load from the 36000 ton cold mass, immersed in some 400 m/sup 3/ static pressurised superfluid helium. The LHC also makes use of supercritical helium for nonisothermal cooling of the beam screens which intercept most of the dynamic heat loads at higher temperature. Although not used in normal operation, liquid nitrogen will provide the source of refrigeration for precooling the machine. Refrigeration for the LHC is produced in eight large refrigerators, each with an equivalent capacity of about 18 kW at 4.5 K, completed by 1.8 K refrigeration units making use of several stages of hydrodynamic cold compressors. The cryogenic fluids are distributed to the cryomagnet strings by a compound cryogenic distribution line circling the tunnel. Procurement contracts for the major components of the LHC cryogenic system have been adjudicated to industry, and their progress will be briefly reported. Besides construction proper, the study and development of cryogenics for the LHC has resulted in salient advances in several fields of cryogenic engineering, which we shall also review.

Ion desorption stability in superconducting high energy physics proton colliders

In this article we extend our previous analysis of a cold beam tube vacuum in a superconducting proton collider to include ion desorption in addition to thermal desorption and synchrotron radiation induced photodesorption. The ion desorption terms introduce the possibility of vacuum instability. This is similar to the classical room temperature case but is now modified by the inclusion of ion desorption coefficients for cryosorbed (physisorbed) molecules which can greatly exceed the coefficients for tightly bound molecules. The sojourn time concept for physisorbed H2 is generalized to include photodesorption and ion desorption as well as the usually considered thermal desorption. The ion desorption rate is density dependent and divergent so at the onset of instability the sojourn time goes to zero. Experimental data are used to evaluate the H2 sojourn time for the conditions of the Large Hadron Collider (LHC) and the situation is found to be stable. The sojourn time is dominated by photodesorption for surface density s(H2) less than a monolayer and by thermal desorption for s(H2) greater than a monolayer. For a few percent of a monolayer, characteristic of a beam screen, the photodesorption rate exceeds the ion desorption rate by more than two orders of magnitude. The photodesorption rate corresponds to a sojourn time of approximately 100 s. The article then turns to the evaluation of stability margins and the inclusion of gases heavier than H2 (CO, CO2, and CH4), where ion desorption introduces coupling between molecular species. Stability conditions are worked out for a simple cold beam tube, a cold beam tube pumped from the ends, and a cold beam tube with a coaxial perforated beam screen. In each case a simple inequality for stability of a single component is replaced by a determinant that must be greater than zero for a gas mixture. A connection with the general theory of feedback stability is made and it is shown that the gains of the diagonal uncoupled feedback loops are first order in the ion desorption coefficients whereas the gains of the off-diagonal coupled feedback loops are second order and higher. For this reason it turns out that in practical cases stability is dominated by the uncoupled diagonal elements and the inverse of the largest first-order closed loop gain is a useful estimate of the margin of stability. In contrast to the case of a simple cold beam tube, the stability condition for a beam screen does not contain the desorption coefficient for physisorbed molecules, even when the screen temperature is low enough that there is a finite surface density of them on the screen surface. Consequently there does not appear to be any particular advantage to operating the beam screen at a high enough temperature to avoid physisorption. Numerical estimates of ion desorption stability are given for a number of cases relevant to the LHC and all of the ones likely to be encountered were found to be stable. The most important case, a 1% transparency beam screen at ∼4.2 K, was found to have a stability safety margin of approximately 17 determined by ion desorption of CO. Ion desorption of H2 is about a factor of 75 less stringent than CO. For these estimates the beam tube surface was assumed to be chemically cleaned but otherwise untreated, for example, by a vacuum bakeout or by glow discharge cleaning.

Synchrotron radiation induced gas desorption from a Prototype Large Hadron Collider beam screen at cryogenic temperatures

The performance of the vacuum system of the Large Hadron Collider will depend critically on the synchrotron radiation induced gas desorption and on the readsorption of molecules on the cold surfaces. The present design of the system is based on a so-called beam screen inserted in the 1.9 K cold bore of the magnets. Gas molecules desorbed will therefore readsorb on the beam screen which is held at a temperature between 5 and 20 K. Pumping slots in the beam screen enable some of the desorbed gas to be pumped onto the 1.9 K surface of the cold bore.

Technological problems related to the cold vacuum system of the LHC

The Large Hadron Collider (LHC) project, now in its design phase at CERN, comprises two proton storage rings with colliding beams of 7 TeV energy. The machine will be housed in the existing LEP tunnel with a circumference of 26.7 km and requires a bending magnetic field of 8.4 T with 14 m long superconducting magnets. The beam vacuum chambers comprise the inner “cold bore” walls of the magnets. These magnets operate at 1.9 K, and thus serve as very good cryo-pumps. 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 vacuum chamber. The design of this beam screen represents a technological challenge in view of the numerous and often conflicting requirements and because of the very tight mechanical tolerances imposed. The synchrotron radiation produces strong outgassing from the walls. The design pressure necessary for operation must provide a beam lifetime of several days. An additional stringent 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, therefore, interrupt the machine operation. Cryo-pumping of gas on the cold surfaces provides the necessary low gas densities but it must be ensured that the vapour pressures of cryo-sorbed molecules, of which H 2 and He will be the most critical species, remain within acceptable limits. In the warm straight sections of the LHC the pumping speed requirement is determined by ion induced desorption and the resulting vacuum stability criterion.

Vacuum system for LHC

The Large Hadron Collider (LHC) which is planned at CERN will be housed in the tunnel of the Large Electron Positron collider (LEP) and will store two counter-rotating proton beams with energies of up to 7 TeV in a 27 km accelerator/storage ring with superconducting magnets. The vacuum system for the LHC will be at cryogenic temperatures (between 1.9 and 20 K) and will be exposed to synchrotron radiation emitted by the protons. A stringent limitation on the vacuum is given by the energy deposition in the superconducting coils of the magnets due to nuclear scattering of the protons on residual gas molecules because this may provoke a quench. This effect imposes an upper limit to a local region of increased gas density (e.g. a leak), while considerations of beam lifetime (100 h) will determine more stringent requirements on the average gas density. The proton beam creates ions from the residual gas which may strike the vacuum chamber with sufficient energy to lead to a pressure ‘run-away’ when the net ion induced desorption yield exceeds a stable limit. Synchrotron radiation induced gas desorption, well known from electron rings, also affects the dynamic vacuum in the cold LHC by the gradual accumulation of easily re-desorbable, condensed gas and by the steeply rising H 2 vapour pressure as the coverage exceeds a monolayer. These dynamic pressure effects will be limited to an acceptable level by installing a perforated ‘beam screen’ which shields the cryopumped gas molecules at 1.9 K from synchrotron radiation and which also absorbs the synchrotron radiation power at a higher and, therefore, thermodynamically more efficient temperature.

Monte Carlo study of light transmission through a cylindrical tube.: D Blechschmidt,J Vac Sci Technol, 11 (3), 1974, 570–574.

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 field 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 H2 and He are the most critical species, remains within acceptable limits.

Study of a cryopump for possible use on the ISR.: C Benvenuti, Vide, 28 (168), 1973, 235–241.

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 field 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 H2 and He are the most critical species, remains within acceptable limits.

Emission properties of alloys of refractory metals with zirconium carbide.: T L Matskevich and T V Krachino, Izv AN SSSR Ser Fiz, 37 (12), 1973, 2538–2542 ( in Russian).

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 field 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 H2 and He are the most critical species, remains within acceptable limits.

Myelography in Intervertebral Disk Protrusion

The advantages of contrast myelography in the diagnosis of protruded intervertebral disk are considered by many neurosurgeons to be too slight to offset the possible (though unproved) risks of introducing lipiodol into the subarachnoid space, and they have preferred to base their indications for operation on careful evaluation of the clinical findings. Whether or not this attitude is justifiable, it is at least a signal for the radiologist to review his results more critically. Upon doing so, he cannot but confess that x-ray diagnosis of disk protrusion as generally practised has certain shortcomings which detract from its efficiency. Several investigators have sought to determine the cause of these shortcomings. Among the factors mentioned, the most important would seem to be the narrowness of the dural sac and lateral protrusion of the disk. That these do not account for all failures of the protruded disk to produce a characteristic defect in the oil column is, however, clear to anyone who has seen many of these cases in the operating room. With these considerations in mind, we sought to determine whether the dural sac is always in direct contact with the posterior surface of the vertebral bodies and the intervertebral disks, as has been commonly assumed. We found that, especially in the region of the fifth lumbar vertebra, where most protrusions occur, this is frequently not the case. For our investigation, the patient was placed in the prone position and examined with the horizontal x-ray beam and vertical film or screen (Fig. 1). In this position the best possible filling of the sac in the region under consideration is obtained, and at the same time opportunity is afforded to glance between the sac and the posterior surface of the vertebrae. It was found that the sac is, indeed, usually in direct contact with the body of the vertebra, but that in a certain percentage of cases (about 15 per cent) they are separated by a space averaging 2 to 4 mm. in width (Fig. 2), and in extreme instances (5 to 8 per cent) reaching as much as 10 mm. This space, the so-called epidural space, attains its maximum width between the fifth lumbar and the first sacral vertebrae. At this site the majority of spines show a sudden lordotic curve, and this angulation may give rise to an abnormal relation between sac and vertebra, as will be explained. So long as the length of the dural sac is ample, it can follow the ventral side of the canal, although this is the longer path, and it will do so because the nerve roots pull it ventrally. In cases of marked widening of the space, however, the dural sac appears to be stretched like a hammock between its attachment to the dorsal side of the sacral canal and its cranial attachment, following the dorsal rather than the ventral margin of the canal (Fig. 3). The significance of this observation is obvious.