Vacuum Chamber Wall Research Articles

Review of heavy-ion induced desorption studies for particle accelerators

During high-intensity heavy-ion operation of several particle accelerators worldwide, large dynamic pressure rises of orders of magnitude were caused by lost beam ions that impacted under grazing angle onto the vacuum chamber walls. This ion-induced desorption, observed, for example, at CERN, GSI, and BNL, can seriously limit the ion intensity, luminosity, and beam lifetime of the accelerator. For the heavy-ion program at CERN's Large Hadron Collider collisions between beams of fully stripped lead ($^{208}\mathrm{Pb}^{82+}$) ions with a beam energy of $2.76\text{ }\text{ }\mathrm{TeV}/\mathrm{u}$ and a nominal luminosity of ${10}^{27}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ are foreseen. The GSI future project FAIR (Facility for Antiproton and Ion Research) aims at a beam intensity of ${10}^{12}$ uranium ($^{238}\mathrm{U}^{28+}$) ions per second to be extracted from the synchrotron SIS18. Over the past years an experimental effort has been made to study the observed dynamic vacuum degradations, which are important to understand and overcome for present and future particle accelerators. The paper reviews the results obtained in several laboratories using dedicated test setups, the mitigation techniques found, and their implementation in accelerators.

A road map to extreme high vacuum

Ultimate pressure of a well-designed vacuum system very much depends on pretreatments, processing and procedures [1, 2]. Until now much attention has been paid to minimizing hydrogen outgassing from the vacuum chamber wall materials, however, procedures and processing deserve further scrutiny. For reducing the gas load, high sensitivity helium leak detection techniques with sensitivities better than 1×10−12 Torr l/sec should be used. Effects that are induced by vacuum instrumentation need to be reduced in order to obtain accurate pressure measurements. This paper presents the current status of the CEBAF DC photogun. This state of the art technology is driving the need for Extreme High Vacuum (XHV). We also present sensitive helium leak detection techniques with RGA's, vacuum gauge and RGA calibration procedures, metal sponges for cryosorption pumping of hydrogen to XHV, low cost surface diffusion barriers for reducing the hydrogen gas load and clean assembly procedures. Further, alternative backing pump systems based on active NEGs [3] for turbo molecular pumps are also discussed.

Bismuth Hollow Cathode for Hall Thrusters

B ISMUTH has several qualities that make it well suited for development as a Hall-thruster propellant. When compared with more conventional propellants such as xenon, bismuth holds significant advantages from both an energetics (lower ionization energy) [1] and cost standpoint. In addition, there are significant ground-test-facility cost savings, because bismuth does not require the use of cryogenic pumps. Unlike traditional propellants, bismuth is solid at room temperature; thus, the exhausted bismuth solidifies on the room-temperature vacuum chamber walls, and consequently the entire vacuum chamber becomes an effective pumping surface. With this in mind, operating a high-power bismuth Hall thruster would require only enough pumping speed to keep up with facility outgassing and minor vacuum leaks. However, there are some difficulties that need to be addressed when using a condensable propellant. Some of the issues include sustaining elevated temperatures for bismuth evaporation, regulating bismuth mass flow, mechanical limitations inherent with using refractory metal components, and bismuth plating of thruster and spacecraft components. Recently, there have been three new programs to develop bismuth Hall thrusters, with the first successful demonstration occurring in the spring of 2005 [2–4]. In this work, the bismuth thruster was operated using a xenonLaB6 cathode. The encouraging results of the bismuth thruster motivated a study to examine the feasibility of an all-bismuth system using a bismuth cathode. In addition to all of the physical and economical gains, it would be advantageous to incorporate a bismuth cathode to eliminate the need for multiple propellant supplies on an eventual flight unit. In 2005, a functioning prototype bismuth cathode was developed and a limited number of operating characteristics were reported [5]. The primary goals of the present research were to evaluate the operating characteristics of a bismuth LaB6 cathode at different mass flow rates, compare bismuth data with xenon and krypton performance, and to reduce the amount of power required for cathode operation.

A new test stand for heavy ion induced gas desorption measurements at TSL

In several experiments at CERN, GSI and BNL it has been found that the lifetime of highly energetic heavy ions in synchrotrons decreases with increasing number of injected ions. This phenomenon occurs due to the collisions of beam ions and residual gas molecules leading to the change of charge of the ions and their loss on the vacuum chamber walls, which in turn cause ion-induced gas desorption and further pressure increase. To gain a deeper understanding of the ion-induced desorption process in the energy range 5–45MeV/u, a dedicated test stand was built at the end of the K beamline at The Svedberg Laboratory (TSL) in Uppsala, Sweden. The energy range was chosen due to the fact that the injection energy of the heavy ion synchrotron SIS18 at GSI will be 10MeV/u, and that there are insufficient data in this energy range. A Test Particle Monte-Carlo model of the experimental set-up was build-up, run and analysed for different sample configurations. An important result is that for the same sample material the desorption yield from a flat sample causes a 1.58 times larger pressure increase than that of a tubular sample. A detailed explanation of the set-up is presented.

Transition from Bohm to classical diffusion due to edge rotation of a cylindrical plasma

The outer region of the plasma column of a large, linear plasma device is rotated in a controlled fashion by biasing a section of the vacuum chamber wall positive with respect to the cathode (Er<0). The magnitude and temporal dependence of the observed cross-field current, produced when the bias voltage is applied, is consistent with ion current arising from ion-neutral collisions. Flow speeds in the outer regions of the plasma column exceed the local sound speed. In the nonrotating plasma column, cross-field, radial particle transport proceeds at the Bohm diffusion rate. Rotation, above a threshold bias voltage, reduces cross-field transport from Bohm to classical rates, leading to steeper radial density profiles. Reduction of particle transport is global and not isolated to the region of flow shear. The transition from the nonrotating to the rotating plasma edge in the linear plasma column is similar to the low confinement to high confinement mode transition observed in tokamaks when Er<0.

Characterization of the CEBAF 100 kV DC GaAs photoelectron gun vacuum system

A vacuum system with pressure in the low ultra-high vacuum (UHV) range is essential for long photocathode lifetimes in DC high voltage GaAs photoguns. A discrepancy between predicted and measured base pressure in the CEBAF photoguns motivated this study of outgassing rates of three 304 stainless steel chambers with different pretreatments and pump speed measurements of non-evaporable getter (NEG) pumps. Outgassing rates were measured using two independent techniques. Lower outgassing rates were achieved by electropolishing and vacuum firing the chamber. The second part of the paper describes NEG pump speed measurements as a function of pressure through the lower part of the UHV range. Measured NEG pump speed is high at pressures above 5×10 −11 Torr, but may decrease at lower pressures depending on the interpretation of the data. The final section investigates the pump speed of a locally produced NEG coating applied to the vacuum chamber walls. These studies represent the first detailed vacuum measurements of CEBAF photogun vacuum chambers.

Heavy-Ion-Induced Electronic Desorption of Gas from Metals

During heavy-ion operation in several particle accelerators worldwide, dynamic pressure rises of orders of magnitude were triggered by lost beam ions that bombarded the vacuum chamber walls. This ion-induced molecular desorption, observed at CERN, GSI, and BNL, can seriously limit the ion beam lifetime and intensity of the accelerator. From dedicated test stand experiments we have discovered that heavy-ion-induced gas desorption scales with the electronic energy loss (dE_{e}/dx) of the ions slowing down in matter; but it varies only little with the ion impact angle, unlike electronic sputtering.

Effect of Magnetic Configuration on the Neutral Particle Recycling in Compact Helical System

In devices such as Stellarator/Heliotron, nested magnetic surfaces are surrounded by the so-called separatrix layer, which consists of open field lines with various connection length to the vacuum chamber wall and/or divertor plates. In this paper, we study the strike point distribution of field lines on Compact Helical System chamber wall, especially under divertor configuration. Then, by using these distribution as the measure of neutral recycling source, Monte Carlo simulation of 3-D neutral transport is done. The results are compared with those obtained for different magnetic configuration.

NCSX Plasma Heating Methods

The National Compact Stellarator Experiment (NCSX) has been designed to accommodate a variety of heating systems, including ohmic heating, neutral beam injection, and radio-frequency (rf). Neutral beams will provide one of the primary heating methods for NCSX. In addition to plasma heating, neutral beams are also expected to provide a means for external control over the level of toroidal plasma rotation velocity and its profile. The experimental plan requires 3 MW of 50-keV balanced neutral beam tangential injection with pulse lengths of 500 ms for initial experiments, to be upgradeable to pulse lengths of 1.5 s. Subsequent upgrades will add 3 MW of neutral beam injection (NBI). This paper discusses the NCSX NBI requirements and design issues and shows how these are provided by the candidate PBX-M NBI system. In addition, estimations are given for beam heating efficiencies, scaling of heating efficiency with machine size and magnetic field level, parameter studies of the optimum beam injection tangency radius and toroidal injection location, and loss patterns of beam ions on the vacuum chamber wall to assist placement of wall armor and for minimizing the generation of impurities by the energetic beam ions. Finally, subsequent upgrades could add an additional 6 MW of rf heating by mode conversion ion Bernstein wave (MCIBW) heating, and if desired as possible future upgrades, the design also will accommodate high-harmonic fast-wave and electron cyclotron heating. The initial MCIBW heating technique and the design of the rf system lend themselves to current drive, so if current drive became desirable for any reason, only minor modifications to the heating system described here would be needed. The rf system will also be capable of localized ion heating (bulk or tail), and possibly IBW-generated sheared flows.

Optical-readout micro-cantilever array IR imaging system and performance analysis

The uncooled IR imaging technology has wide applications. Using the optical readout uncooled IR imaging system built in our laboratory, thermal images of human body are obtained successfully by placing a bi-material micro-cantilever array (FPA) without silicon substrate in air. By comparing the IR imaging results obtained by placing FPA under different vacuum pressures, the pressure dependence of the thermal conductance and system noise is modeled, and the influence to the system performance is analyzed. The model of thermal conductance of gas is analyzed and verified experimentally on condition that the mean-free path of the gas molecules is larger than the gas gap between the bi-material cantilever and the wall of vacuum chamber. Experimental analysis indicates that for FPA working in air, the random vibration noise of the cantilever impacted by the gas molecules' thermal motion is the main source of the system noise. When the pressure decreases to lower than 10-2 Pa, the influence of gas conductance can be neglected, the total thermal conductance decreases to the limit of radiative conductance, and the random vibration noise of the cantilever is reduced to a minimum. The experimental results and theoretical analysis agree well.

위성 열평형 시험에서 챔버 벽 영향에 관한 연구

위성 열평형 시험에 사용하는 열진공 챔버의 벽은 흑체 거동을 하는 우주공간과는 달리 위성의 복사 에너지를 완벽하게 흡수하지 못하고 일부를 반사한다. 챔버의 크기가 작을수록 이러한 챔버 벽 효과는 커지는 것으로 알려져 있다. 이것은 시험비용을 줄이기 위해 대형 챔버를 사용하기 힘든 소형위성의 개발에 걸림돌이 된다. 본 연구에서는 챔버 벽 효과를 예측하고 이를 보정하기 위한 정량적인 분석을 진행하였다. 그 결과로 챔버 벽 효과에 의한 온도 오차를 계산하여 시험 데이터를 보정할 수 있게 하였다. 또, 최적 면적비를 정의하여 소형위성의 열평형 시험용 열진공 챔버의 크기를 정하는데 기준을 마련하였다. 덧붙여 챔버 벽을 투명한 재질로 코팅하여 챔버 벽 효과를 줄일 수 있는 방안에 대해 이론적인 분석을 수행하였다. The wall of thermal vacuum chamber which is used for the satellite thermal balance test doesn't absorb satellite's IR emission perfectly and reflects some part of that. It is estimated that small thermal vacuum chamber has relatively larger wall effect than the big one. The small thermal vacuum chamber is required for the small satellite test to reduce the test cost. A quantitative analysis was carried out to investigate the chamber wall effect. As a result, temperature errors caused by chamber wall effect was calculated, and the temperature data acquired in the thermal balance test have been compensated. By defining the optimized area ratio between chamber surface and satellite surface area, the baseline to be able to determine the minimum size of thermal vacuum chamber was established to minimize the wall effect. Also, theoretical analysis about transparent material coating which can reduce the chamber wall effect is conducted.

Study of optical dumpers used in high vacuum system of interferometric gravitational wave detectors

Outgassing velocities and optical absorption coefficients of black colored optical absorbers (UB-NiP, Phosblack II, Raydent, ECB+DLC and Alumite) were studied to reduce scattered and strayed light in interferometric gravitational wave detectors. The measured results showed that the UB-NiP had the largest optical absorption coefficient of 99.89% for 1064nm wavelength of light. For the outgassing velocity, the ECB+DLC had the lowest value, which was 1 × 10−8 Pam3/s/m2 at 20 hours. This value was about two orders of magnitude smaller than that of the UB-NiP, however, its optical absorption coefficient was only 65 %. Therefore, we concluded that the UB-NiP is suitable as an optical dumper with small area and high efficiency, and the ECB+DLC is suitable as large area coating such as a use for vacuum chamber walls.

Electron Cloud in Various Kinds of Magnetic Field of BEPCII

Electron cloud instability (ECI) may take place in a positron storagering when the machine is operated with a multi-bunch positron beam.According to the actual shape of the vacuum chamber in the BEPCII, aprogramme which is different from the other simulation codes has beendeveloped. Because of the distance between dipole magnet and sextupole,the quadrupole magnet of BEPCII is very short, much of thephotoelectrons can be produced and can move in magnetic fields. Themotion of electrons in various kinds of magnetic fields is studied indetail, especially for the solenoid field which will be wound in the vacuumpipe of BEPCII. Simulation shows that the solenoid field is very effectiveto confine the electrons to the vicinity of the vacuum chamber wall andto make an electron free region at the vacuum pipe centre.

A Dispersion Interferometer Based on a CO2 Laser

A double-pass dispersion interferometer based on a 9.6-µm CO2 laser with a sensitivity of 〈 nel〉min ∼ 1 × 1013 cm−2 and a temporal resolution of ∼50 µ s, designed to measure linear plasma density, is described. A ZnGeP2 nonlinear crystal is used as the frequency doubler. The main advantages of the interferometer are its compactness and a low sensitivity to vibrations of optical elements. The interferometer requires no special vibration isolation. Its main components are arranged compactly on an optical bench outside the apparatus, except for a window for radiation injection and a retroreflector; these are mounted on the wall of the experimental facility's vacuum chamber. The advantages of the dispersion interferometer have been demonstrated in an experiment with a gas-dynamic trap.

Evidence of the influence of reflections on the Zeff profile measurements in the Tore Supra tokamak and their mitigation

Reliable experimental determination of the mean effective charge (Zeff) of the plasma is of enormous importance for the impurity control in high temperature plasmas, allowing the performance of various discharge scenarios to be assessed and the benchmarking of model calculations, ideally accompanied by measurements of the Zeff profile. The Zeff is usually calculated using data from line-integrated visible bremsstrahlung measurements, electron density and temperature profiles, and the plasma geometry. On Tore Supra (TS) it had been noticed in the past that hot spots on the inner wall, MARFEs or supra thermal electrons can falsify the signal of the visible bremsstrahlung. More significant, however, was a systematic overestimation of the edge channels, giving unreasonably high edge Zeff values of 30 or even higher. The analysis of dedicated discharges in the present configuration of TS leads us to conclude that this is due to the influence of vessel wall reflections. A newly developed inversion method which takes reflections into account shows that the presence of reflections leads to an overestimation of the outer channels in the reconstruction of the Zeff profile. This is due to the larger relative contribution of the reflections on the edge channels due to the shorter path length through the plasma and the lower bremsstrahlung emission at the edge. We present a model which takes into account this effect and allows a correction procedure to be applied when calculating the Zeff profile. The TS data can be modeled assuming Lambertian (diffusive) reflections with a reflectivity coefficient of 20%–25%, which is in agreement with measurements of the reflectivity of the inner wall of the TS vacuum chamber.

Design and Development of a Short-Period Superconducting Undulator at the APS

A planar superconducting undulator (SCU) with a period of 15 mm is under development at the Advanced Photon Source (APS). The SCU is designed to achieve a peak field of 0.8 T on the beam axis for an 8 mm pole gap and a current density in the coil of 1.0 kA/mm/sup 2/. Short sections of low-carbon-steel cores with 12 and 22 periods were fabricated, and coil windings were completed with NbTi superconducting wire. After training by means of quenches, the SCUs were able to charge up to near the critical current density j/sub c/ of 1.43 kA/mm/sup 2/. Using a thin-film heater attached to the inner surface of a vacuum chamber wall, steady-state heat fluxes were applied to the coil/pole face of the 12-period SCU in 4.2 K liquid He (LHe). The heat flux densities needed to quench the SCU were measured. At 0.998j/sub c/ and 0.8j/sub c/ the thermal stability margins were about 1.3 mW/mm/sup 2/ and 2 mW/mm/sup 2/, respectively. The thermal stability margin of the SCU was affected by the latent heat of vaporization of LHe.

One-step synthesis of porous palladium nanostructures by H2+He arc plasma method

Porous palladium nanostructures were prepared on a copper sheet previously fixed over the plasma high-temperature area by H2+He arc plasma method. However, dispersed palladium nanoparticles with diameters in 50–90 nm were obtained on the inner wall of vacuum chamber. SEM, TEM, EDXA, XRD and BET were employed to characterize the palladium porous materials. SEM images show that porous palladium nanostructures are composed of spherical particles with diameters of 35 ± 3 nm. This special structure with large surface to volume ratio can be recycled easily for application in the field of catalysis.

On the Theory of Diamagnetic Measurements in Open Traps

A formula for the plasma pressure as a functional of the diamagnetic loop voltage is derived for a general case when the time for the magnetic flux to percolate through conducting wall of vacuum chamber is not small as compared to the plasma confinement time. An arbitrary shape of the conducting chamber and the plasma column are allowed in contrast to earlier theories based on axial symmetry of the system.

ＳＰｒｉｎｇ‐８電子蓄積リング真空チェンバーの８ＧｅＶ蓄積電子廃棄時における損傷の調査

The 8 GeV electron beam stored on the SPring-8 storage ring finally hits the vacuum chamber wall, especially stainless steel chamber wall of 0.7 mm thickness in the injection section, when the operation is suddenly stopped by interlock system for safety. At that time the localized heat load is generated in the stainless steel material as a result of the dense electro-magnetic shower. Because of the several times such heat loads, the stainless steel chamber has been broken and vacuum also has. On the cross section of the broken stainless steel wall, one can see the evidence of the degeneration or melting down of the stainless steel which is well described by the cascade calculation for the high energy electro-magnetic events using GEANT31). To avoid such a vacuum break, a thicker stainless steel chamber has been installed and an Al damper, in which density of electro-magnetic shower is low because of its small atomic number, has been developed.

Modeling of three-dimensional neutral transport in tandem mirror plasmas using a Monte-Carlo code

Modeling of neutral transport in the tandem mirror plasmas has been performed based on simulation studies using a Monte-Carlo code. A three-dimensional neutral transport simulation in both the central-cell (an axisymmetric simple mirror) and anchor-cell (a non-axisymmetric minimum-B configuration) was successfully carried out using the DEGAS version 63 code. In order to realize the geometrical structure in the simulation space, a ‘second wall’ is introduced in the code, which enabled us to perform a more detailed simulation. The simulation results agreed well with the Hα measurements of the experiment. The effect of cold gas influx from the beamline of the neutral beam injection (NBI) system on the particle fueling is also investigated and it is found that the azimuthally uniform particle source due to the recycling on the central vacuum chamber wall prevails over the localized fueling from cold gas influx accompanying the NBI.