Vacuum Chamber Wall Research Articles

Two-dimensional diagnostic of edge plasma structure using a lithium beam probe in a compact helical system

A neutral lithium beam probe for two-dimensional diagnosis of edge plasmas has been designed and installed on the compact helical system. A lithium beam with an energy of 15keV and a current of 0.1mA is used. The spatial resolution is about 10mm, and the time response is about 10ms. The beam penetration depth is expressed in terms of the line integral density ⟨nel⟩, which is about 2×1018m−2. The beam injection angle can be varied and the observation point covers the edge and separatrix region of the helical diverter configuration. Two-dimensional electron density profiles for electron cyclotron heating and neutral beam injection (NBI) heated plasmas are obtained near and outside the last closed flux surface (LCFS). Analysis for two-dimensional density profile reconstruction indicates that significant amounts of surface plasma are confined outside the LCFS for NBI plasmas even though the ergodic layer is cut by the vacuum chamber wall (inboard limiter configuration). The usefullness of this new two-dimensional diagnostic in the edge region is demonstrated.

Beam-loss induced pressure rise of Large Hadron Collider collimator materials irradiated with158 GeV/uIn49+ions at the CERN Super Proton Synchrotron

During heavy ion operation, large pressure rises, up to a few orders of magnitude, were observed at CERN, GSI, and BNL. The dynamic pressure rises were triggered by lost beam ions that impacted onto the vacuum chamber walls and desorbed about 10 4 to 10 7 molecules per ion. The deterioration of the dynamic vacuum conditions can enhance charge-exchange beam losses and can lead to beam instabilities or even to beam abortion triggered by vacuum interlocks. Consequently, a dedicated measurement of heavy-ion induced molecular desorption in the GeV/u energy range is important for LHC ion operation. In 2003, a desorption experiment was installed at the SPS to measure the beam-loss induced pressure rise of potential LHC collimator materials. Samples of bare graphite, sputter coated (Cu, TiZrV) graphite, and 316 LN stainless steel, were irradiated under grazing angle with 158 GeV/u indium ions. After a description of the new experimental set-up, the results of the pressure rise measurements are presented, and the derived desorption yields are compared with data from other experiments.

Research of a condition of shields of the RF-antenna tokamak T-11M after four years of work

In the present paper, the results of the research into the behaviour of protective dielectric elements of the T-11M tokamak RF-antenna after four years of operation in working conditions are presented. In a modernized RF antenna, all its structural plasma facing metal elements are changed by graphite ones with a 40 µm thick boron carbide coating deposited from the gas phase, the lateral metal surfaces are protected by shields, 2 mm thick flat plates made from pyrolytic boron nitride [1] Azizov E. A. Belov A. M. Buzhinskij O. I. et al. Investigations of ion-cyclotron heating in the T-11 M tokamak Fizika Plazmy 20 1060 1064 (1994) (in Russian) [Google Scholar]. For boronization of the vacuum chamber walls, a new manner based on the decomposition of deuterated carborane (C2B10D12) in helium glow discharge was used. After completion of work, the RF-antenna and all dielectric elements were investigated with a standard set of surface diagnostics (scanning electron microscopy, Auger electron spectroscopy, X-ray and neutron diagnostics, profilometer etc.). The analysis has shown that all protective elements after four years of work have kept the coating integrity as well as chemical and phase structure. After four years of operation, the B4C coating thickness has decreased 10 times on the electron side of protective plates and 5 times on the ion side and is, accordingly, 5 and 10 µm.

Effect of Vacuum-Chamber Wall Gassing on the Evolution of an ECR Discharge in a Magnetic Trap

We study the evolution of the ECR discharge sustained in a simple mirror magnetic trap by a powerful millimeter-wave radiation. Specific features of the discharge behavior are determined by the intense gassing of the vacuum chamber walls affected by a plasma that flows out of the trap. A model of the discharge dynamics is proposed. Solutions with explosive plasma-density increase were found and analyzed, and qualitative agreement between the model and experimental results was obtained.

Properties of the electron cloud in a high-energy positron and electron storage ring

Low-energy, background electrons are ubiquitous in high-energy particle accelerators. Under certain conditions, interactions between this electron cloud and the high-energy beam can give rise to numerous effects that can seriously degrade the accelerator performance. These effects range from vacuum degradation to collective beam instabilities and emittance blowup. Although electron-cloud effects were first observed two decades ago in a few proton storage rings, they have in recent years been widely observed and intensely studied in positron and proton rings. Electron-cloud diagnostics developed at the Advanced Photon Source enabled for the first time detailed, direct characterization of the electron-cloud properties in a positron and electron storage ring. From in situ measurements of the electron flux and energy distribution at the vacuum chamber wall, electron-cloud production mechanisms and details of the beam-cloud interaction can be inferred. A significant longitudinal variation of the electron cloud is also observed, due primarily to geometrical details of the vacuum chamber. Such experimental data can be used to provide realistic limits on key input parameters in modeling efforts, leading ultimately to greater confidence in predicting electron-cloud effects in future accelerators.

Difference in the photocurrent of semiconductor photodiodes depending on the polarity of current measurement through a contribution from the photoemission current

When using semiconductor photodiodes in the VUV, photoelectrons emitted from the photodiode surface form an additional current circuit when electrons reach a vacuum chamber wall surrounding the photodiode. In the case where the rear electrode of the photodiode is grounded, and hence the sensing terminal of an electrometer is connected to the front electrode, the electrometer measures the superimposed current, that is, the photocurrent internally generated in the p–n junction plus the photoemission current. Experiments done for diffusion-type silicon photodiodes and Schottky-type GaAsP photodiodes in the wavelength range from 50 nm to 200 nm show that the photoemission contribution has peaks of about 10% and 25%, respectively, of the internal photocurrent at around 100 nm for both without an extraction voltage. Therefore, for precise measurements of semiconductor photodiode photocurrents, the sensing terminal of an electrometer should be connected to the rear electrode of a photodiode to avoid any contribution from the photoemission current.

Behavior of helium gas in the LHD vacuum chamber

In general, helium gas does not remain on vacuum chamber walls because of its small activation energy. However, outgassing of helium gas from the walls has been observed in the LHD plasma vacuum chamber with a very long time constant after helium glow discharge cleaning (GDC), and absorption of helium atoms has been observed in plasma discharge experiments using helium gas. The helium partial pressure before the daily experiments was determined only by the gas species of the last GDC. No dependence has been found between the helium partial pressure and the species of the fueling gas of the last plasma experiment fueling gas. Considering that the outgassing rate of the helium gas is almost the same each morning after He GDC, the retention of helium atoms in the wall after the GDC is almost at the same level. The concentration of helium atoms in the wall before the daily experiments is estimated. The outgassing rate after the GDC is 2×10 −4 Pa m 3/s and the concentration is 4.7×10 16 atoms/cm 2. These results are of the same order as in another experiments. During helium gas plasma experiments, about a half of the amount of the inlet gas disappears with the missing particles remaining in the wall. The stainless steel wall, which is saturated with He GDC, may still have the capacity to trap high energy helium atoms. However, the energy dependence of trapping helium atoms presently is not clear.

Universality of intermittent convective transport in the scrape-off layer of magnetically confined devices

The nature of intermittency, long observed in magnetic fusion devices, was revisited lately [G. Antar et al., Phys. Rev. Lett. 87, 065001 (2001)]. It was shown that intermittency is caused by large-scale events with high radial velocity reaching about 1/10th of the sound speed. These type of structures were named “avaloids.” In the present article, the universality of convective turbulence in magnetically confined plasmas is investigated. Turbulence properties in the scrape-off layer of four different magnetic fusion devices are compared. Namely, the Tore Supra tokamak [Tore Supra Team, Nuclear Fusion, 40, 1047 (2000)] with circular cross-section limiter-bounded plasma, the Alcator C-Mod tokamak [B. LaBombard et al., Phys. Plasmas 8, 2107 (2001)] which is a divertor device, the Mega-Ampere Spherical Tokamak (MAST) [A. Sykes et al., Phys. Plasmas 8, 2101 (2001)] with vacuum chamber walls far from the plasma last closed flux surface and the PISCES linear plasma device [D. Geobel et al., Rev. Sci. Istrum. 56, 1717 (1985)]. The statistical properties of the turbulent signals in the four devices are found to be identical allowing us to conclude that intermittent convective transport by avaloids is universal in the sense that it occurs and has the same properties in many confinement devices with different configurations.

Formation and Dynamics of Very Deep Negative Potential Well in the Small Tokamak Device CSTN-IV

We achieved a potential well more than fifty times deeper than the plasma electron temperature, down to -0.3 kV with the electrode biasing, by the cross-field radial arcing current of up to 150 A which flows between the small electron-emitting electrode (LaB6) inserted to the plasma center and the stainless-steel wall of vacuum chamber. The empirical scaling of the radial resistance of plasma Rr on the radial arcing current Ib and the toroidal magnetic field BT is found to be Rr ∼ Ib-1.2BT1.0. Structural dynamics of negative potential well formation is observed: the potential dip propagates from the plasma core to the periphery. The ion saturation current also increases, starting from the inner region. The fluctuation frequency in Isat changes abruptly from 200 kHz to 30 kHz in a time of around 0.25 ms after negative biasing.

Thermal desorption study of selected austenitic stainless steels

Residual hydrogen in stainless steel results in a steady outgassing from vacuum chamber walls, hindering the achievement of ultrahigh vacuum conditions. The total content, the binding states, and the diffusivity of residual hydrogen in austenitic stainless steels, which together define the room temperature hydrogen outgassing rate, have been investigated by thermal desorption spectroscopy (TDS). Seven different steel types have been studied by means of two different TDS systems. The study has been extended to include the effects of various postproduction treatments, aimed at reducing the hydrogen content and/or outgassing, namely vacuum firing and baking both in air and under vacuum. A large variety of hydrogen desorption peaks have been observed, which have been attributed to diffusible hydrogen, hydrogen trapping in the surface oxides or in lattice defects induced by precipitates, and steel recrystallization. The hydrogen depletion effectiveness of vacuum baking at different temperatures has been quantified, and the consequences of air baking have been clarified, leading to practical guidelines for technological applications.

Numerical study of the photoelectron cloud in KEKB Low Energy Ring with a three-dimensional particle in cell method

A three-dimensional particle in cell simulation code has been developed to study the photoelectron cloud instabilities in KEKB LER. In this report, the program is described in detail. In particular, typical simulation results are presented for the photoelectron motion in various kinds of magnetic fields. The simulation shows that a solenoid is very effective in confining the photoelectrons to the vicinity of the vacuum chamber wall and in creating a region free of photoelectrons at the vacuum pipe center. The more uniform the solenoid field is, the more effectively does it suppress the electron-cloud buildup. Multipacting can occur both in a drift region and in a dipole magnet, and the heat load deposited on the chamber wall due to the lost electrons is important in these two cases. Electron trapping by the beam field as well as by various magnetic fields is an important phenomenon, especially inside quadrupole and sextupole magnets. Our numerical results qualitatively agree with the experimental studies.

Discharge cleaning and wall conditioning in a Novillo Tokamak

Our Novillo Tokamak is a small toroidal device magnetically confined defined by the main design parameters: R o=0.23 m, a v=0.08 m, a p=0.06 m, B T=0.05–0.47 T, I p=1–12 kA, n e=1–2×10 13 cm −3, T e=150 eV, T i=50 eV. For the initial discharge chamber cleaning we have often used vacuum baking up to 100 °C and then conditioning using Taylor discharge cleaning (TDC) in H 2 and He. In this work we report that vacuum baking is effective for obtaining a final total pressure of the order of 1.6×10 −7 Torr. We have found that a single parameter, the performance parameter (PP), can be used to optimize the TDC method. This parameter represents the quantity of electron and ion energy incident on the chamber wall during the Taylor discharge, it is equal to ( I p τ), where I p is the peak-to-peak plasma current and τ is the plasma current duration. In graphs of PP versus the gas pressure for different oscillator powers, the maximum value of PP indicates the best cleaning conditions when using TDC. The results of the vacuum chamber wall conditioning using this criterion are reported.

Shunting arc plasma generation and ion extraction

This article reviews the generation and time evolution of shunting arc plasma, which is an alternating pulsed discharge through a solid-state material and contains mainly ions of a metal or a solid-state material. The material is provided into the plasma as a rod that connects electrodes for the discharge. Optimization of the discharge condition has made it possible for the arc discharge to be self ignited with relatively low input power to the rod, which leads to a long lifetime of the rod. Because the principal of its generation is ohmically heating the rod, the droplets are not emitted. The material is deposited from the surface by the heating at the initial stage and an arc discharge is generated via a glow-like discharge and surface discharge along a rod. The shunting plasma flows towards the vacuum chamber wall due to the voltage bias of the plasma to the grounded chamber and induces a new plasma. This brings a hybrid usage of the shunting arc plasma and its induced plasma for synthesizing a new compound of the materials. As another hybrid plasma generation, the shunting arc discharge can be produced in a stationary RF (13.56 MHz)-plasma. The possibility of the shunting pulse plasma as a pulsed ion source for plasma based ion implantation is also described.

Shielding of infrared edge and synchrotron radiation

Long-wavelength edge and synchrotron radiation from an electron storage ring may be suppressed by nearby vacuum chamber walls. The suppression, known as shielding, occurs at wavelengths where the walls upstream of an observer reflect a large portion of the radiation within the near-field region. For a beamline whose vertical entrance aperture equals the vacuum chamber height, shielding is not expected at wavelengths where the radiation opening angle is smaller than that subtended by the entrance aperture.

Production of a large-area argon microwave plasma by a ring slot antenna

A large-area, uniform, low-temperature and high-density microwave argon plasma is produced using a simple flat ring slot antenna without a magnetic field. The plasma production is based on the absorption of the energy of microwave propagating in the dielectric plate (quartz glass) along the plasma surface. The power is coupled to the dielectric plate by azimuthal symmetrical mode. An electron density of >10 11/cm 3 with an electron temperature <2 eV, and a floating potential with respect to the vacuum chamber wall of <4 V are obtained at 5 mTorr with 2.45 GHz, peak input power of 500 W (duty ratio t/ T=0.5; pulse width t=50 μs, and period T=100 μs). A high ion current density (over 10 mA/cm 2) of uniformity (standard deviation/average) within ±3% over 24 cm diameter is achieved.

Quasi-direct current plasma immersion ion implantation

Quasi-dc (direct current) plasma immersion ion implantation (PIII) is demonstrated in the long-pulse mode. To prevent plasma extinction as a result of the sheath reaching the vacuum chamber wall in long-pulse experiments, a grounded grid is used to partition the chamber into two halves. The pulse width can be readily increased to 500 μs that is more than 10 times longer than that in typical low-pressure PIII experiments for monoenergetic implantation (ion mean free path≫sheath thickness). The electron saturation current measured by the Langmuir probe indicates that the grounded grid indeed stops the propagation of the plasma sheath. After the plasma sheath reaches the grounded grid, the pulse current drops to a smaller value indicative of the quasi-dc PIII mode. The plasma recovery time is found to be 800 μs thereby limiting the maximum pulsing frequency to below 1 kHz, and the preferred pulse duration window is between 100 and 500 μs. The secondary ion mass spectrometry profiles show that low energy ions are reduced using long pulses. This operation mode thus offers the unique advantage of a smaller low-energy ion component, that is, more monoenergetic ion distribution, and less surface damage compared to conventional short-pulse PIII. When compared to dc-PIII, this mode retains the discharge characteristics and works well for insulators.

Experimental investigation of the ripple induced losses of perpendicularly injected beam ions in the low aspect ratio helical system CHS

Confinement of perpendicularly injected beam ions has been investigated experimentallyboth in the standard configuration and in the drift optimized configuration of theCHS heliotron/torsatron by means of the neutron technique. The experimental resultsindicate that the confinement of trapped beam ions is poor in both configurations. Theobserved loss rate of trapped beam ions was very high and this high rate is not explainedby losses due to charge exchange or collisional scattering from the timescale perspective.Full gyro-motion following orbit calculations showed that in the peripheralregion of CHS plasmas, trapped beam ions are promptly lost because they immediatelyintersect the vacuum chamber wall due to their large Larmor radii. It also suggested thatin the core domain the collisionless transition behaviour is the key to understanding the rapid lossof beam ions. It was shown that the magnetic moment of perpendicularly injected beamions is not well conserved in CHS. The rapid losses of perpendicularly injected beamions are probably due to non-adiabatic effects resulting in the transition behaviour of energeticions.

The pressure and gas composition evolution during the operation of the LEP accelerator at 100 GeV

The operation of the electron–positron storage ring LEP at an energy of 100 GeV leads to the deposition of a high-power flux on the vacuum chamber walls. The high critical energy of the photons emitted as synchrotron light (740 keV), places the photon-stimulated desorption in this machine in a completely new energy range compared to the existing electron storage rings. At this high photon energy, the energy dissipation of the photons in aluminium is done predominantly via the Compton process. This absorption mechanism leads to a larger enhancement of the gas desorption with the photon energy compared to the photoelectric effect. Measurements made at 100 GeV to study the variation of the pressure increase, of the gas composition and of the cleaning rates as a function of the beam dose and energy are presented and discussed in the context of the absorption of high-energy photons in aluminium.

Main divertor flows in Heliotron E: their distribution and dependence on NBI and ECH

In the l = 2/m = 19 Heliotron E heliotron/torsatron device, with a currentless plasma heated by NBI and ECH, the poloidal distributions were measured of diverted plasma flows near the vacuum chamber wall in eight sections of one helical field pitch. A vertical asymmetry of these distributions was observed, contrary to what had been expected from the helical symmetry of the `ideal' (unperturbed) vacuum divertor magnetic configuration. Under NBI conditions, the density normalized divertor plasma flow Jd/n̄e was a rising function of the density normalized absorbed power Pabs/n̄e. This is considered to be a consequence of the power degradation of confinement. With second harmonic ECH, where the main part of the launched power was absorbed in the central region, a similar power dependence as for the NBI was observed. With fundamental ECH, where a considerable fraction of the microwave power was absorbed at the periphery, Jd/n̄e for some areas of the divertor trace distinctly exceeded those obtained under NBI-only conditions with the same Pabs/n̄e. On the other hand, two different time responses (fast and slow) of Jd were observed for the perpendicular NBI and fundamental ECH cases in some particular positions. The slow response is considered to be caused by a diffusion-like outflow of the bulk plasma to the divertor. The fast outflow of particles to the divertor might be caused by a loss of locally trapped high energy electrons and ions.

Gas-phase basicity of (CH3)3N+-C6H4-COO- zwitterions: a new class of organic super bases.

Gas-phase basicity of (CH3)3N+-C6H4-COO- zwitterions: a new class of organic super bases.