Vacuum Chamber Wall Research Articles

Floating potential probes for process control during reactive magnetron sputtering

Long term deposition of aluminum oxide by reactive magnetron sputtering results in a drift of the discharge voltage. A similar drift of the floating potential is observed. The latter observation is further investigated and it is shown that the change of the floating potential can be linked to a change of the electric properties of the vacuum chamber walls. Optimization of the floating potential probe is performed to use the difference between the discharge voltage and the floating potential as a parameter for process control. The best results were obtained with a planar probe sufficiently far positioned opposite to the magnetron, and close enough to the chamber wall facing the magnetron. This choice can be understood in terms of the ability of the probe to sense the discharge while being protected from deposition. The usage of the aforementioned difference to control the process is demonstrated with the measurement of process curves, more specifically IV-characteristics and hysteresis experiments. The shown reproducibility of these measurements opens a pathway for more precise quantification of reactive sputter deposition simulations and enhanced process control.

Selective electron beam sensing through coherent Cherenkov diffraction radiation

We exploit the coherent emission of Cherenkov diffraction radiation (ChDR) by a relativistic electron beam to sense its position even in the presence of other particle beams. ChDR is produced in alumina inserts embedded in the vacuum chamber walls and recorded in a narrow band centered at 30 GHz. This nontrivial solution has been implemented for plasma wakefield accelerators, where the electron beam to be sensed can copropagate with another high-energy proton beam that generates the plasma wakefield. In addition, at variance with most existing position detectors, this method is insensitive to spurious electric charges due to the presence of plasma. We present the overall design of the detector as well as experimental results obtained in the AWAKE facility at CERN. Published by the American Physical Society 2024

Development of a toroidal soft x-ray imaging system and application for investigating three-dimensional plasma on J-TEXT

A toroidal soft x-ray imaging (T-SXRI) system has been developed to investigate three-dimensional (3D) plasma physics on J-TEXT. This T-SXRI system consists of three sets of SXR arrays. Two sets are newly developed and located on the vacuum chamber wall at toroidal positions of ° and °, respectively, while one set was established previously at °. Each set of SXR arrays consists of three arrays viewing the plasma poloidally, and hence can be used separately to obtain SXR images via the tomographic method. The sawtooth precursor oscillations are measured by T-SXRI, and the corresponding images of perturbative SXR signals are successfully reconstructed at these three toroidal positions, hence providing measurement of the 3D structure of precursor oscillations. The observed 3D structure is consistent with the helical structure of the m/n = 1/1 mode. The experimental observation confirms that the T-SXRI system is able to observe 3D structures in the J-TEXT plasma.

Elettra 2.0: The Vacuum System Design for a New Generation Storage Ring

Next-generation synchrotron facilities currently under development and construction are promising and delivering great advances in terms of machine design and research physics. At the same time, they are pushing the limits of technological solutions for various subsystems: to achieve the desired ultra-low electron beam emittance required in the next generation machines, the magnetic lattice has to adopt a compact design with reduced magnet apertures and other constraints like high heat load from synchrotron radiation, dynamic pressure inside vacuum chambers and chamber wall impedance are presenting unique challenges for the vacuum and engineering teams. At Elettra-Sincrotrone Trieste (Italy), the Elettra 2.0 project aims to develop a new-generation storage ring. Taking into consideration the above-mentioned constraints, we decided to adopt a new design of a vacuum chamber, while utilizing novel pumping solutions to overcome hugely reduced conductance compared to the current machine. Large sputter ion pumps (SIP) will be in the majority replaced by distributed non-evaporable getter (NEG) coatings and small NEG cartridges and SIP pumps. For the synchrotron radiation handling, due to the tight space constraints imposed by the compact lattice, the photon absorption will be managed jointly with discrete and distributed solutions: photon absorbers have been carefully studied for combining compact form and high power density loads, while key sections of the new storage ring will be water-cooled. Throughout the whole development phase, we were using Monte-Carlo simulation codes like SynRad and MolFlow+ as effective tools supporting the design of new vacuum chambers and photon absorbers. The current state of development for the Elettra 2.0 vacuum system, the challenges we faced and the solutions we adopted are presented here.

Research on the deposition and properties characterization of novel conductive getter films on the inner wall of accelerator vacuum chamber

The new generation diffraction-limited storage ring has small vacuum chamber apertures, resulting in a distributed vacuum problem. Depositing a Ti-Zr-V NEG film layer on the inner face of vacuum chambers is an ideal solution for resolving the non-uniform vacuum distribution problem. However, Ti-Zr-V NEG coatings on the surface of the vacuum chamber increase its surface resistivity, which can intensify the wakefield impedance effect. This article proposes adding conductive Ag to the Ti-Zr-V NEG film. A novel Ti-Zr-V-Ag NEG film not only has good vacuum performance, but also has good conductivity. Ti-Zr-V-Ag NEG film is successfully prepared by magnetron sputtering. The activation kinetic processes and surface resistivity of Ti-Zr-V-Ag and Ti-Zr-V NEG films are detected in detail, and comparative research on the activation behavior and surface resistivity of Ti-Zr-V-Ag films deposited with various sputtering pressures is investigated. The results show that adding pure metallic silver to the Ti-Zr-V film effectively decreases the DC resistivity of the NEG films, and this reduction can be further enhanced by reducing the sputtering pressure. The activation kinetics behavior of the Ti-Zr-V-Ag film shows minor degradation compared with that of the Ti-Zr-V film.

Kinetic simulation of ion thruster plume neutralization in a vacuum chamber

The electrical environment of a ground vacuum testing chamber creates facility effects for gridded ion thrusters. For example, it is well known that the plume from the thruster generates current paths that are very different from what occurs in space, and the neutralization of this plume is also different. For reasons such as this, it is important to clarify how the experimental testing environment affects plasma flows, but understanding this effect solely through ground experiments is difficult. To that end, this study utilizes particle-in-cell and direct simulation Monte Carlo methods to simulate xenon beam ions and electrons emitted from a neutralizer. First, we compare simulations conducted within the chamber to those conducted in space, demonstrating that grounded chamber walls increase the electric potential and electron temperature. Next, we investigate the impact of the neutralizer’s position and the background pressure on the plume in the vacuum chamber. We find that as the neutralizer position moves closer to the location of maximum potential, more electrons are extracted, resulting in increased neutralization of the plume. We also observe that high background pressure generates slow charge-exchange ions, creating ion sheaths on the side walls that alter ion current paths. Finally, we discuss how the potential at the thruster and neutralizer exits affects the plume. The relative potential of the neutralizer to the vacuum chamber wall is observed to significantly influence the behavior of the electrons, thereby altering the degree of plume neutralization. These findings are shown to be consistent with experimental results in the literature and demonstrate the promise of high-performance simulation.

A novel remote detection method for inner leak location in a tokamak: Theory and preliminary experimental validation

In this study, a detection scheme was investigated for small leaks in tokamak vacuum chambers, and the scheme was validated theoretically and experimentally. An ultra-high vacuum should be maintained in the tokamak chamber. However, traditional methods of locating leaks in the inner vacuum chamber are usually less efficient. A leak detection scheme in the vacuum chamber was proposed in this study. Locating leaks by scanning the vacuum chamber wall with a multi-degree-of-freedom robot arm. Moreover, a small vacuum chamber was adopted to perform a leak test at a small flow rate. The results were compared with those achieved through COMSOL, Molflow simulations, as well as theoretical calculations. Furthermore, the scenario of a small leak in the EAST vacuum chamber was simulated. Significant pressure value fluctuations were detected when the detection device passed through the area close to the leak at a reasonable detection distance.

Stannane in extreme ultraviolet lithography and vacuum technology: Synthesis and characterization

In extreme ultraviolet (EUV) lithography, tin droplets evaporate and subsequently coat various surfaces including the collector mirrors. To clean off the tin, a hydrogen plasma is often used, but as a result, an unstable by-product, stannane (SnH4) is formed. The physicochemical characteristics of this gas, its formation in a plasma process, and its interaction with various materials have not been explored and understood completely. Here, the electron ionization mass spectrum of SnH4 is presented. All ten natural abundance isotopes were observed experimentally for each fragment, i.e., Sn+, SnH+, SnH2+, and SnH3+. Density functional electronic structure theory was used to calculate the optimized ground state geometries of these gas phase species and their relative stabilities and helped explain the absence of SnH4+ in the observed signals. The density of the liquid, its cracking pattern, and the surface morphology of its deposits were examined. The surface of the deposited tin film resulting from the decomposition and subsequent oxidation was characterized by x-ray photoelectron spectroscopy. The main species found at the surface were metallic tin and tin (II) oxide (SnO). The detailed characterization of stannane should help correctly identify it in EUV lithographic processes and develop approaches in the future to mitigate its decomposition and redeposition on the collector mirrors or vacuum chamber walls.

Characterization of a retroreflector array for 320-GHz interferometer system in Heliotron J.

A retroreflector array, composed of a cluster of small retroreflectors, is experimentally studied for application to a Michelson-type interferometer system in the fusion plasma experiment. Such a new-type reflector has the potential to be a vital and effective tool at a spatially limited location, such as on the vacuum chamber wall of plasma experimental devices. To investigate the effect of retroreflector array on the reflected beam properties, a tabletop experiment is performed with the retroreflector array composed of 4mm corner-cube retroreflectors and with a 320-GHz (λ ∼ 0.937mm) submillimeter wave source. An imaging camera is utilized to measure the submillimeter wave beam profile and is scanned perpendicularly to the beam propagation direction if necessary. The experimental result exhibits a diffraction effect on the reflected beam, resulting in the emergence of discrete peaks on the reflected beam profile, as predicted in the past numerical study; however, the most reflected beam power converges on the one reflected into the incident direction, resulting from a property as a retroreflector. Furthermore, the dependence of the reflected beam on the incident beam angle is characterized while fixing the detector position, and the retroreflection beam intensity is found to vary due to the diffraction effect. Such an undesired variation of beam intensity induced by the diffraction can be suppressed with a focusing lens placed in front of the detector in the practical application to an interferometer.

Development of ceramic vacuum chamber for collider ring kicker of Circular Electron Positron Collider (CEPC)

The fast-pulsed kicker of the collider ring of Circular Electron Positron Collider (CEPC) uses a ceramic vacuum chamber to reduce eddy currents and maintain the magnetic field waveform at the beam's track. But the image charges induced by beams will accumulate on the insulating ceramic to increase the beam impedance. Eddy current shielding and beam impedance are traded off by coating a discontinuous “pattern” of TiN (titanium nitride) film on the inner wall of the ceramic vacuum chamber. Several potentially effective coating patterns are proposed. The eddy current shielding effect and the beam impedance are analyzed by simulation software. The simulation results show that coating in a ladder-shaped pattern maintains the magnetic field waveform well and helps to reduce the impedance. Properties such as electrical conductivity and SEY (secondary electron yield) of the films are improved in coating experiments and the technical feasibility of coating in a racetrack-shaped ceramic vacuum chamber is verified. Finally, the impedance of a 1200 mm-long ceramic vacuum chamber coated with ladder-shaped pattern film and the effect on the kicker magnetic field are measured.

A Ray Tracing Model for Electron Optical Imaging in Electron Beam Powder Bed Fusion

The recent success of the process monitoring method Electron Optical Imaging, applied in the additive manufacturing process Electron Beam Powder Bed Fusion, necessitates a clear understanding of the underlying image formation process. Newly developed multi-detector systems enable the reconstruction of the build surface topography in-situ but add complexity to the method. This work presents a physically based raytracing model, which rationalises the effect of detector positioning on image contrast development and masking. The model correctly describes the effect of multiple scattering events on vacuum chamber walls or heat shields and represents, therefore, a predictive tool for designing future detector systems. Most importantly, this work provides a validated method to compute build surface height gradients directly from experimentally recorded electron-optical images of a multi-detector system without any calibration steps. The computed surface height gradients can be used subsequently as input of normal integration algorithms aiming at the in-situ reconstruction of the build surface topography.

Modeling of Reactive Sputtering-History and Development.

This work critically reviews the evolution of reactive sputtering modeling that has taken place over the last 50 years. The review summarizes the main features of the deposition of simple metal compound films (nitrides, oxides, oxynitrides, carbides, etc.) that were experimentally found by different researchers. The above features include significant non-linearity and hysteresis. At the beginning of the 1970s, specific chemisorption models were proposed. These models were based on the assumption that a compound film was formed on the target due to chemisorption. Their development led to the appearance of the general isothermal chemisorption model, which was supplemented by the processes on the surfaces of the vacuum chamber wall and the substrate. The model has undergone numerous transformations for application to various problems of reactive sputtering. At the next step in the development of modeling, the reactive sputtering deposition (RSD) model was proposed, which was based on the implantation of reactive gas molecules into the target, bulk chemical reaction, chemisorption, and the "knock-on effect". Another direction of the modeling development is represented by the nonisothermal physicochemical model, in which the Langmuir isotherm and the law of mass action are used. Various modifications of this model allowed describing reactive sputtering processes in more complex cases when the sputtering unit included a hot target or a sandwich one.

Compact, ultra-high vacuum compatible, high power density conductive heaters

We present a design and evaluation of a simple and easy-to-fabricate conductive heater intended for ultra-high vacuum experiments. We demonstrate a compact and power-dense heater that has minimal outgassing up to nearly 200 °C. We further detail a method for using the heater in air to heat an object in vacuum by partially replacing a glass vacuum chamber wall with silicon, avoiding some possible outgassing issues. This method has successfully loaded a 2D magneto-optical trap feeding a 3D magneto-optical trap in rubidium 87.

Specific features of focused electron beam transport through a narrow metal tube at negative potential in the forevacuum pressure range

We describe our investigations on the transport of a focused electron beam through a long and narrow metal tube, held at negative potential with respect to the vacuum chamber walls, in the elevated forevacuum pressure range. We have explored the effect of electron beam parameters and tube potential on beam transport. It is shown that the strength of this effect is connected to the initiation of a non-self-sustained glow discharge within the tube, as well as to the beam magnetic focusing parameters. We also show that increase in the fraction of electrons transported through the tube is conducive to uniformity of the distributions of the non-self-sustained discharge current and the discharge plasma density along the tube length.

Моделирование реактивного распыления. Обзор

This paper takes a critical look at the evolution of reactive sputtering modeling over the past 50 years. The review summarizes the main regularities of the processes of film deposition of simple metal compounds (nitrides, oxides, oxynitrides, carbides, etc.) discovered experimentally by various researchers. The above features include significant non-linearity and hysteresis. In the 1970s, specific models of isothermal chemisorption were proposed. These models were based on the assumption that a compound film is formed on the target due to chemisorption. Their development led to the emergence of a general model of isothermal chemisorption, which was supplemented by processes on the surfaces of the vacuum chamber wall and the substrate. The model has undergone numerous transformations for application to a variety of reactive spray applications. At the next stage in the development of modeling a reactive sputtering (RSD) model was proposed based on the implantation of reactive gas molecules into the target, bulk chemical reaction, chemisorption, and the «knock-on» effect (implantation of adsorbed recoil atoms or molecules that received an impulse from an accelerated argon ion). Another direction in the development of modeling is represented by a non-isothermal physical and chemical model, in which the Langmuir isotherm and the law of mass action were used. Various modifications of this model made it possible to describe reactive sputtering processes in more complex cases, when a hot or sandwich target is included in the sputtered unit.

Dynamic pressure evolution during the LHC operation

For the accelerator community and the vacuum scientists, the understanding of the beam interactions with a vacuum chamber is fundamental to provide solutions to mitigate pressure rises induced by electron, photon, and ion molecular desorption. Moreover, beam instabilities induced by ion and electron clouds must be investigated in order to find solutions to reduce them. This study presents in situ measurements of pressure evolutions and electrical currents performed during the LHC RUN II (2018). The proton beam circulating in the LHC vacuum chamber ionizes the residual gas producing electrons as well as positive ions. These charged particles are accelerated away from the beam and reach the vacuum chamber wall, inducing, among other phenomena, stimulated desorption and secondary electron emission. Moreover, protons emit synchrotron radiations that also induce photodesorption and photoelectron production. Experimental measurements of the electrical signals recorded on copper electrodes were compared to calculations considering both the secondary electron yield of copper and the electron energy distribution. All measurements performed with the Vacuum Pilot Sector in the LHC ring show the importance of taking into account a large variety of phenomena in order to understand the pressure evolution in the LHC. Results show that the multipacting threshold, corresponding to an increase in the electron cloud density, strongly depends on the number of protons per bunch. Finally, the ion current was measured with a biased electrode lower than $\ensuremath{-}500\text{ }\text{ }\mathrm{V}$. It was much higher than expected, pointing its origin not only from simple beam-gas ionization but also from the ionization of the residual gas by the electron cloud.

Surface characterization and secondary electron yield measurement of the Ti-Zr-V films deposited on laser-etched copper

The electron multipacting effect and distributed vacuum profile, induced by the interaction of the particles in vacuum chamber with the surrounding walls, are the significant issues in modern accelerators. Laser-etching the inner surface of the vacuum chambers can effectively eliminate the electron multipacting effect, and depositing the non-evaporable getter (NEG) film on the vacuum chamber walls can provide distributed pumping properties. In the present study, Ti-Zr-V NEG films were prepared by magnetron sputtering technique on laser-etched oxygen-free copper (OFC) with different deposition times. The secondary electron yields (SEY), activation kinetics, and resistance of the samples were evaluated. The results show that although the SEYs of the coated laser-etched samples are higher than those of the uncoated laser-etched sample, the maximum SEY values are still below the threshold of the operation requirements. The Ti-Zr-V film deposited on laser-etched surface exhibits obvious nucleation phenomenon with columnar microstructure and can be activated at 180^∘C. Moreover, the coated laser-etched samples present lower resistance compared with the uncoated laser-etched sample. The combination of the laser-etching technique and the NEG thin film is aim to provide a practical treatment for the vacuum chambers in modern accelerators.

Indicators of upcoming electric breakdown in a pyroelectric accelerator

This paper describes indicators that can be used to monitor the operating mode of a pyroelectric accelerator. It is shown that the ratio of the characteristic X-ray emission lines from the target and the vacuum chamber walls is very sensitive to the state of the accelerator. Also, the peak to total count rate ratio in the electron spectrum exhibits similar properties. These parameters change sharply ahead of the electric breakdown and are very sensitive to the residual gas pressure level. Monitoring these indicators during the accelerator operation provides a fine tool aiding the implementation of pyroelectric technology for stable and reliable charged particle generation and acceleration.

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

In high intensity positively charged beam machined the beam intensity could be limited by the ion induced pressure instability. This is a phenomenon occurs when residual gas molecules can be ionised and accelerated towards beam chamber walls causing ion stimulated gas desorption and residual gas density increase. Under certain conditions residual gas density can increase to such values when the beam loss rate becomes unacceptable. 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. This model requires a number of input parameters, among whose the energy of the ions bombarding vacuum chamber walls is the most critical one. In the previous paper the energy of ions bombarding the vacuum chamber walls was studied for round beams. This paper is reporting on how the elliptic shape of beams affects these results.

Line-shape modeling in microwave spectroscopy of the positronium n=2 fine-structure intervals

We report numerical simulations of positronium experiments designed to measure the $n=2$ fine-structure intervals. The simulations include all possible interference effects between all 20 states in the $n=1$ and laser-excited $n=2$ manifolds as well as representations of the electric and magnetic fields present in the waveguides used in the experiments. We find that rf wave reflection from the vacuum chamber walls is a possible explanation of previously observed line-shape distortions and shifts. We also characterized several systematic effects, including those arising from quantum interference, that are likely to be significant for future measurements.