Vacuum Chamber Wall Research Articles

Numerical analysis of heat load distribution in Heliotron J with magnetic field tracing and plasma transport modeling

We model and characterize the heat load patterns on the vacuum chamber wall in an advanced helical device, Heliotron J, based on the peripheral transport code EMC3-EIRENE, in three typical magnetic configurations with different toroidal mirror ratios (bumpiness). The heat flux distribution on the vacuum chamber wall evaluated with the EMC3-EIRENE code shows several groups of heat flux footprints, some of which are not seen in the connection length distribution obtained with the magnetic field tracing code. This clearly shows the necessity for plasma modeling in addition to magnetic field line tracing in the design of three-dimensional devices. At high electron density, a displacement and an expansion of the heat flux distribution in both the toroidal and poloidal directions are observed. As a result of the heat flux expansion, the peak value of the heat flux decreases at high electron density. Further, an approach that uses the heat distribution function is proposed to evaluate the global power distribution in the entire vacuum chamber. The evaluation results confirm a decrease in the peak heat flux and an increase in the plasma-wetted area at high electron density. The heat flux distribution expansion is larger for the low-bumpiness configuration, in which the peak heat value is by more than lower than that for the high-bumpiness configuration.

Development of a multi-channel 320GHz interferometer for high density plasma measurement in Heliotron J.

We report the development of a new interferometer with two stable, high-power, 320GHz solid-state sources in Heliotron J. A heterodyne Michelson interferometer optical scheme is employed. Two solid-state oscillators are utilized as sources with a fixed frequency at 320GHz and frequency tunable of 312-324GHz. Quasi-optical techniques are used for beam transmission. The beam is elongated in the vertical direction with two off-axis parabolic mirrors and injected into the plasma as a sheet beam for the multi-channel measurement (>5 ch.). Passing through the plasma, the beam is reflected at a retroreflector-array installed at the vacuum chamber wall. The retroreflector-array is a bunch of retroreflector structures, which can suppress the beam refraction caused by plasma without much space inside a vacuum chamber unlike a single retroreflector and can facilitate the system design. The source, detectors, and the retroreflector-array are tested to evaluate their basic performance on a tabletop experiment.

Finite element analysis of blackbody radiation environment for an ytterbium lattice clock operated at room temperature

The Stark shift due to blackbody radiation (BBR) is a key obstacle limiting the frequency uncertainty of optical lattice clocks. A well-characterized BBR environment is necessary to know exactly the temperature felt by the cold atoms. In our ytterbium clock, the lattice-trapped atoms are exposed to the thermal radiation of the surrounding vacuum chamber walls and optical windows. Calibrated platinum resistance temperature detectors are used to monitor the vacuum chamber temperature in real time. In order to obtain the effective temperature T eff in the position of the atoms, we perform finite element (FE) analysis to the thermal radiation of the vacuum chamber. Due to the temperature inhomogeneity existing in our vacuum chamber, the limited knowledge of the air convection contributes the largest part of the uncertainty in T eff. For our typical room temperature environment, T eff can be determined with an accuracy level of 160 mK, corresponding to a fractional frequency uncertainty of 5.3 × 10−18 for the BBR Stark shift. Additionally, we use a simple formula to relate T eff to the temperatures at the monitored points, which allows us to know the value of T eff without using FE analysis, and thus enables the real-time correction to the BBR Stark shift.

The energy of ions bombarding the vacuum chamber walls. Round beams

The ion induced pressure instability is a phenomenon, which could compromise the performance of positively charged machines. To mitigate this phenomenon, a vacuum system should be properly designed to provide sufficient pumping at correct locations, this includes an accurate gas dynamics modelling of beam vacuum chamber. The gas dynamics model requires a number of input parameters, among whose the energy of the ions bombarding vacuum chamber walls is the most critical one. This paper studies the energy of ions bombarding the vacuum chamber walls as a function of round beam parameters such as beam/bunch sizes, beam population and bunch spacing, as well as the effect of beam chamber aperture.

Analysis of the Accuracy of Measuring the Flux Density of All Hydrogen Isotopes from First Wall to Plasma Using the H-Alpha Diagnostics in ITER

The results of the analysis of the accuracy of measuring the flux density of all hydrogen isotopes from the first wall of the main vacuum chamber to the ITER tokamak plasma using the high-resolution spectroscopy of Balmer alpha lines of hydrogen isotopes (H-alpha diagnostics) are presented. Under the conditions of the expected high background radiation generated by the divertor stray light and the absence of the possibility of using the optical dumps in ITER, a differential measurement scheme is necessary, which uses the spatial variation of the coefficient of the light reflection from the natural landscape of the first wall. The measurement accuracy analysis is carried out with the help of the method of synthetic diagnostics, which uses the results of the predictive numerical modeling of different modes of hydrogen recycling for simulation of the expected diagnostic signals and determination of the desired parameters by solving an inverse problem.

Mechanism and properties of plasma nitriding AISI 420 stainless steel at low temperature and anodic (ground) potential

AISI 420 stainless steel was nitrided at low temperatures and anodic potential in a plasma atmosphere of ammonia (NH3) assisted by hollow cathode discharge. The specimens were placed on a worktable connected to the wall of vacuum chamber that was earthed (0 V) and used as anode of glow discharge. Plasma nitriding was carried out at a temperature ranging from 390 to 510 °C for 4 h in NH3 atmosphere. The thickness of white layer was in a range of 6 to 40 μm. The surface hardness reached to 1250 ± 72 HV0.05 for the specimen nitrided at 510 °C for 4 h. Both corrosion and wear resistance of AISI 420 stainless steel were simultaneously improved after plasma nitriding at 450 °C for 4 h.

The impact of synchrotron radiation at the Compact Linear Collider

Synchrotron radiation (SR) emission and interactions with the vacuum chamber walls have the potential to negatively impact the performance of future electron–positron colliders. The Beam Delivery System (BDS) of the Compact Linear Collider (CLIC) contains weak bending and multipole magnets that lead to less intense emissions than at circular colliders with similar centre-of-mass energies. However, the linear geometry more easily allows for multiple reflections of SR photons, that can travel further downstream in the accelerator and impact the detector region. In this study, the results of PLACET and Synrad+ simulations of photon emissions and reflections in the CLIC BDS at two energy stages of 380GeV and 3TeV are presented. Estimates are given for heating and outgassing caused by SR photons interacting with the vacuum chamber in the BDS. The occupancy levels in the tracking detectors coming from full-detector simulations in Geant4 are presented. Optimised beam pipe apertures are proposed for the forward detector region, as well as mitigation methods to ensure the safety and best possible performance of the detector.

Bound-free pair production in relativistic nuclear collisions from the NICA to the HE LHC colliders

We consider the electron-positron pair production in relativistic heavy ion collisions, in which the produced electron is captured by one of the nuclei resulting, thus, in the formation of a hydrogen--like ion. These ions emerge from the collision point and hit the vacuum chamber wall inside superconducting magnets. Therefore, this process may be important for the problems of beam life time and for the quenching the irradiated magnet. A theoretical investigation for such a bound-free pair production (BFPP) at the colliders from NICA to HE LHC is presented. We obtain an approximate universal formula for the total cross section of the process. We compare it with the results of available numerical calculations and estimate that an accuracy of our calculations is better than $30$ % at the energies of the NICA collider and becomes of the order of a few percent for the RHIC and HE LHC colliders. Based on the obtained results, the detailed calculations are performed for future experiments at the NICA collider. We find that the expected BFPP cross sections for the Au$^{79+}$-Au$^{79+}$ and Bi$^{83+}$-Bi$^{83+}$ collisions are in the range from $10$ to $70$ barn, while for the p-Au$^{79+}$ and p-Bi$^{83+}$ collisions they are in the range of a few mbarn.

Влияние электродной поляризации и динамического эргодического дивертора на характеристики прерывистых всплесков плотности плазмы в токамаке

Intermittent bursts of plasma density measured by Langmuir probes at the edge of TEXTOR tokamak are studied. These bursts appear as a result of turbulent plasma transport processes involving the formation and propagation of various coherent turbulent structures. Such processes impede controlled thermonuclear fusion: they degrade plasma confinement and entail increased heat load on the vacuum chamber walls and other components located near plasma; they also entail strong erosion of these components along with unwanted capture of tritium. Therefore, investigation of turbulent plasma transport processes and dynamics of coherent turbulent structures is one of the most important tasks to be solved for implementing controlled thermonuclear fusion. This is especially important in the context of elaborating and improving the methods for externally controlling the turbulent plasma transport processes. The electrode biasing method and a dynamic ergodic divertor are frequently used for externally influencing thermonuclear plasma and controlling turbulent transport processes. It should be noted that by studying the temporal characteristics of plasma density bursts together with their radial dependence it becomes possible to get better understanding of and deeper insight into the physical nature of turbulent plasma transport processes and dynamics of coherent turbulent structures. In this article, the temporal characteristics of plasma density bursts and their radial dependence are studied in two different modes: with electrode biasing and with a dynamic ergodic divertor. Conformable changes in the characteristics of intermittent bursts are observed in both cases. Namely, the average burst rate increases, and the average burst duration decreases in comparison with the ohmic regime. This is due to the fact that electrode biasing and certain regimes of the dynamic ergodic divertor cause changes in the radial electric field. This has a conformable effect on the dynamics of coherent turbulent structures and plasma transport processes through a shear poloidal flow, which emerges as a consequence of electric drift due to nonuniform radial electric field and toroidal magnetic field, which are perpendicular to each other. After detailed investigations and refinement, it should become possible to use certain regimes of the dynamic ergodic divertor as a means of contactless biasing for externally controlling the turbulent plasma transport in thermonuclear installations.

First Experimental Data on a New SR Beamline of the VEPP-2000 Complex for the HL-LHC Vacuum System

In the framework of the large hadron collider (LHC) upgrade project, new materials are proposed for the vacuum. Amorphous carbon deposited onto the vacuum-chamber wall is examined as a coating with a low secondary electron emission rate for superconducting systems designed to upgrade the LHC, i.e., to increase the luminosity of the machine (HL-LHC). Since protons will generate synchrotron radiation with a critical energy of ~10 eV and a flux of 1016 photon m–1 s–1, it is important to study the effect of photons on a surface covered with amorphous carbon at room and cryogenic temperatures. The construction and parameters of the setup on the new synchrotron beamline of the VEPP-2000 booster are described. The first results of measuring the photodesorption coefficient are also presented.

Modern Diagnostics for Investigation of Lithium Element Behavior in Tokamaks

The introduction of lithium as a material for production of in-vessel plasma-facing elements of the tokamak necessitates the development of the corresponding diagnostic instruments. A series of diagnostic devices have been developed and fabricated for tokamaks T-10 and T-11 M which make it possible to investigate the processes of lithium transport in the tokamak plasma scrape-off layer (SOL), real-time dynamics of lithium deposition at various temperatures of a collecting surface by means of the microbalance technique, adsorption/desorption process of hydrogen isotopes on the lithium surface, and influence of an electric field on lithium trapping. Scanning of plasma parameters is provided by Langmuir probes. Such devices can be used to extract the lithium deposited on the inner walls of the tokamak vacuum chamber without opening it. For these purposes, a lock chamber and bellows-free vacuum input allowing movement and rotation is provided. It is planned to perform the study of the tokamak plasma interaction with in-vessel lithium-based elements by means of infrared (IR) thermometry. It is planned to try out this technique on the Т-11М tokamak using a special device based on a IR camera.

Characterization of amorphous hydrocarbon CDx films (x ∼ 0.5) for energy storage applications

Smooth hydrocarbon films formed on the vacuum chamber walls in tokamaks under the erosion of graphite elements during deuterium plasma discharges are the main accumulators of hydrogen isotopes in carbon matrices. Therefore, they can be considered as a promising material for hydrogen storage applications. In the present work, such hydrocarbon CDx films (x ∼ 0.5) produced in T-10 tokamak (NRC Kurchatov Institute, Russia) were studied using scanning electron microscopy (SEM), NEXAFS and EXAFS spectroscopies together with thermal desorption (TD) for the films’ characterization and estimation of their hydrogen storage capacity. The C 1s X-ray absorption spectra of the CDx films were recorded for the first time using the BESSY II storage ring facility (Germany). The obtained NEXAFS spectra were found typical for CK-spectra of sp3 + sp2 hydrocarbon systems with a high H/C ratio. The role of Fe impurities from the tokamak chamber walls was elucidated as a catalytic effect “facilitating” the thermal desorption of hydrogen (deuterium) from CDx films. The Fe K-edge spectra allowed to confirm a fractal (self-affined) structure of CDx films, with a minimal sp2 fractal aggregate ∼2–3 nm, and formation of 3D carbon sp3 + sp2 network, accumulated a large number of H-isotopes and CxHy hydrocarbons. It was found that the H storage and thermal desorption properties of the CDx films can be improved by their inherent Fe impurities, capable to reduce the barrier of TD and to increase the H/C storage capacity by several times even at room temperature.

Dynamic pressure in the LHC: influence of ions induced by ionization of residual gas by both the proton beam and the electron cloud

For the future HL-LHC or FCC study, the understanding of the beam interactions with the vacuum chamber is fundamental to provide solutions to mitigate the pressure rises induced by electronic, photonic and ionic molecular desorption. The proton beam circulating in the LHC vacuum chamber ionizes the residual gas producing electrons as well as positive ions. In-situ measurements were carried out, on the LHC Vacuum Pilot Sector during the LHC RUN II, to monitor the dynamic pressure, and to collect the electrical signals due to the electron cloud and to the ions interacting with the vacuum chamber walls. Experimental measurements of electrical signals recorded by copper electrodes were compared to calculations taking into account both the Secondary Electron Yield of copper and electron energy distribution. Finally, it seems that copper electrodes were not fully conditioned and an ion current could be estimated.

Minimal multi-target plasma sputtering tool

A multi-target plasma sputtering tool is developed for a recently progressed new concept of a semiconductor fab system called ‘Minimal Fab’. A frequency-tuning compact helicon plasma source is attached on the side wall of the compact vacuum chamber and the plasma is transported along curved magnetic field lines to a target surface, which is mounted on a rotational target holder and negatively biased by a dc power supply. The target material is sputtered by the ions accelerated by the sheath potential drop and deposited on the substrate facing the target. The target material can be changed by rotating the holder; thereby the multi-layered metallic film can be formed in the single chamber. Substrate heating induced by secondary electrons emitted from the target surface and accelerated by the sheath is successfully inhibited by a magnetic filter near the substrate. The performance of the sputtering reactor designed as a laboratory model is shown here. The sputtering reactor and all of the components required for the sputtering tool are successfully contained within a compact frame of Minimal Fab (29.5 cm in width, 45 cm in depth, and 144 cm in height) via the development of the compact reactor, especially the compact rf plasma source.

Compositional, Optical and Electrical Characteristics of SiOx Thin Films Deposited by Reactive Pulsed DC Magnetron Sputtering

The influence of O2 flow rate on the compositional, optical and electrical characteristics of silicon oxide (SiOx) thin films (x < 2) were studied in this work. The SiOx thin films were obtained by pulsed direct current (DC) magnetron sputtering (PMS) onto n-type Si wafers (and also on glass substrates) at a vacuum of 3 × 10−3 Pa. Rutherford backscattering spectrometry (RBS) was used to check the compositional elements of deposited films and its oxidized states were analysed via Fourier-transform infrared (FTIR) spectroscopy. The optical properties of as-deposited SiOx thin films were investigated from transmittance measurements at room temperature in the wavelength range of 250–800 nm. The obtained data reveal that the Urbach energy (a measure of the band tail extension, Eu) decreased from about 523 to 172 meV as the rate of oxygen gas flow increased. On the contrary, the optical energy band-gap (Eg) increased from 3.9 to 4.2 eV. Conduction and valance band positions (relative to the normal hydrogen electrode) were also evaluated. The observed behavior is probably associated with the degree of disorder and defects presented in the as-deposited SiOx thin films, probably due to the presence of newly inserted oxidized OnSiHy species resulting from some contamination with water vapor desorbed from the walls of the deposition vacuum chamber. After deposition of a gold top electrode, the electrical characteristics of the fabricated Au/SiOx/n-Si system (i.e., a metal/insulator/semiconductor structure—MIS) were studied via characteristic I-V curves and their dependence upon the O2 flow rate are reported. It was observed that the Au/SiOx/n-Si structure behaves like a Schottky-diode exhibiting a very good diode rectifying performance with a rectification ratio of at least 300 and up to 104, which refers to the samples produced with the lower and higher O2 flow rates, respectively. It was also found that the O2 flow rate influences the rectifying performance of the SiOx/n-structures since both the diode ideality factor, n, and the diode series-resistance, RS decreases with the increase of O2 content, possibly reflecting a closer approximation to a full stoichiometric condition.

Hydrogen incorporation and release from nonevaporable getter coatings based on oxygen-free Pd/Ti thin films

Oxygen-free Pd/Ti thin films have been developed as novel nonevaporable getter coatings for ultrahigh vacuum (UHV) chamber walls, which can maintain high pumping speed even after many cycles of air-vent and only require activation at a relatively low temperature of 133°C. Here, the authors clarify the hydrogen (H) incorporation and release properties of such oxygen-free Pd/Ti thin films using scanning transmission electron microscopy (STEM), energy dispersive x-ray spectroscopy (EDS), and H depth profiling by 1H(15N,αγ)12C nuclear reaction analysis (NRA). EDS mapping and STEM show that the layers of Pd (6 nm) and Ti (14 nm) prepared by sequential UHV deposition on a stainless steel 304L substrate are clearly separated and that the Ti is completely covered by Pd, which prevents oxidation of Ti. NRA reveals that H accumulates preferentially in the Ti layer, where the H-content increases almost linearly with the H2 dosage. Quantitative analysis indicates that TiH1.92 is produced in the Ti layer after an exposure of 4×104 Langmuir, where the H uptake is still not saturated. Upon heating in UHV, H starts to desorb above 170°C and the release rate increases above 200°C. Repeated cycles of H incorporation at room temperature (20°C) and release at about 240°C are possible. Although the H incorporation rate changes during the initial absorption/desorption, it stabilizes after a few (∼3) incorporation/release cycles.

INFLUENCE OF MOVABLE B4C-LIMITER ON CHARACTERISTICS OF RF DISCHARGE PLASMA IN THE URAGAN-2M TORSATRON

A variant of the B4C movable biased pumped limiter for the Uragan-2M torsatron is described. Some results of experiments with the limiter moved from the vacuum chamber wall to the central region of the plasma column in the work mode of Uragan-2M operation are reported. The effect of limiter-plasma interaction on parameters of the RF plasma discharges is discussed.

SUPERLUMINAL SYNCHROTRON RADIATION

To avoid complex computations based on wide Fourier expansions, the electromagnetic field of synchrotron radiation (SR) was analyzed using Lienard–Wiechert potentials in this work. The retardation equation was solved for ultrarelativistic movement of rotating charge at distances up to the trajectory radius. The radiation field was determined to be constricted into a narrow extended region with transverse sizes approximately the radius of trajectory divided by the particle Lorentz factor (characteristic SR length) cubed in the plane of trajectory and the distance between the observation and radiation emission point divided by the Lorentz factor in the vertical direction. The Lienard–Wiechert field of rotating charge was visualized using a parametric form to derive electric force lines rather than solving a retardation equation. The electromagnetic field of a charging point rotating at superluminal speeds was also investigated. This field, dubbed a superluminal synchrotron radiation (SSR) field by analogy with the case of a circulating relativistic charge, was also presented using a system of electric force lines. It is shown that SSR can arise in accelerators from “spot” of SR runs faster than light by outer wall of circular accelerator vacuum chamber. Furthermore, the mentioned characteristic lengths of SR in orbit plane and in vertical direction are less than the interparticle distances in real bunches in ultrarelativistic accelerators. It is indicating that this phenomenon should be taken into account when calculating bunch fields and involved at least into the beam dynamic consideration.

SUPERLUMINAL SYNCHROTRON RADIATION

To avoid complex computations based on wide Fourier expansions, the electromagnetic field of synchrotron radiation (SR) was analyzed using Lienard–Wiechert potentials in this work. The retardation equation was solved for ultrarelativistic movement of rotating charge at distances up to the trajectory radius. The radiation field was determined to be constricted into a narrow extended region with transverse sizes approximately the radius of trajectory divided by the particle Lorentz factor (characteristic SR length) cubed in the plane of trajectory and the distance between the observation and radiation emission point divided by the Lorentz factor in the vertical direction. The Lienard–Wiechert field of rotating charge was visualized using a parametric form to derive electric force lines rather than solving a retardation equation. The electromagnetic field of a charging point rotating at superluminal speeds was also investigated. This field, dubbed a superluminal synchrotron radiation (SSR) field by analogy with the case of a circulating relativistic charge, was also presented using a system of electric force lines. It is shown that SSR can arise in accelerators from “spot” of SR runs faster than light by outer wall of circular accelerator vacuum chamber. Furthermore, the mentioned characteristic lengths of SR in orbit plane and in vertical direction are less than the interparticle distances in real bunches in ultrarelativistic accelerators. It is indicating that this phenomenon should be taken into account when calculating bunch fields and involved at least into the beam dynamic consideration.

Modeling Studies for Synchrotron-Radiation-Induced Electron Production in the Vacuum Chamber Walls at CesrTA

Modeling Studies for Synchrotron-Radiation-Induced Electron Production in the Vacuum Chamber Walls at CesrTA