Wall Bombardment Research Articles

Particle‐in‐cell simulation of the effect of curved magnetic field on wall bombardment and erosion in a hall thruster

A curved, convex towards the channel bottom magnetic field is an important feature of an advanced Hall thruster that allows confining the plasma flow in the channel center, reducing the divergence angle of the ejected ion beam, and improving the discharge performance. In this article, the discharge behaviour of a Hall thruster in magnetic fields with different degrees of curvature is simulated with a particle‐in‐cell numerical method, and the effect of curved magnetic field on the ion bombardment and wall erosion and the associated mechanisms are studied and analysed. The results show that, as the curvature of the magnetic field increases, the propellant ionization becomes more confined at the channel center, the potential drop inside the channel decreases, and the acceleration region shifts outside the channel, which lead to the attenuation of the ion energy bombarding the wall and the deviation of the bombardment angle from the optimal sputtering angle. Conversely, the ion flux bombarding the wall near the channel exit increases. Nevertheless, the bombardment energy and angle are the dominant factors for the wall erosion, and the wall erosion rate clearly decreases with the increasing curvature of the magnetic field. These findings are closely related to the behaviour of electron conduction under a curved magnetic field; the relevant mechanisms are clarified in this article.

Analyzing the Effects of Produced Water on Asphaltene Deposition in a Vertical Production Tubing

The reduction of asphaltene deposition flux in a production tubing, caused by produced water, is studied by an engineering modeling approach. Two phenomena, which according to our analysis have the most significant potential to reduce the asphaltene deposition, are considered. First, decreasing the asphaltene concentration in a hydrocarbon fluid due to the asphaltene absorption on droplet surfaces is estimated. Second, it is considered that droplets colliding with a pipe wall partially block the surface area available for asphaltene deposition, which in return reduces the deposition flux. The fraction of the pipe wall surface dynamically coated with water droplets is calculated from analyzing the wall bombardment by droplets fluctuating in a turbulent flow. The results reveal that the asphaltene deposition flux is not very influenced by the decrease due to the asphaltene absorption on droplet surfaces or the droplet–pipe wall interaction.

Can electron multipacting explain the pressure rise in a cold bore superconducting undulator?

Preliminary studies performed with the cold bore superconducting undulator installed in the ANKA (Angstrom source Karlsruhe) storage ring suggest that the beam heat load is mainly due to the electron wall bombardment. Electron bombardment can both heat the cold vacuum chamber and induce an increase in the pressure because of gas desorption. In this contribution we compare the measurements of the pressure in a cold bore performed in the electron storage ring ANKA with the predictions obtained using the equations of gas dynamic balance in a cold vacuum chamber exposed to synchrotron radiation and electron bombardment. The balance results from two competing effects: the photon and electron stimulated desorption of the gas contained in the surface layer of the chamber wall and of the gas cryosorbed, and the cryopumping by the cold surface. We show that photodesorption alone cannot explain the experimental results and that electron multipacting is needed to reproduce the observed pressure rise. Electron bombardment can at the same time explain the observed beam heat load.

Analytic solution for low-frequency rf sheaths in pulsed discharges.

The equations governing the evolution of rf-driven sheaths are solved analytically in the regime where the rf frequency is small compared to both the ionic plasma frequency and the ion transit time in the sheaths. Poincaré's map of first return is used to gain geometric insight into the dynamics of the circuit-sheath system. The requirements of minimizing wall bombardment while maximizing the efficiency of the coupling to the substrate sheath are shown to lead to an optimum value for the blocking capacitance in asymmetric discharges. This optimum value is also favorable for rapid relaxation to the steady state in pulsed discharges. The analytic solution is applied to the problem of negative-ion extraction in afterglow plasmas.

Forming and maintaining a potential well in a quasispherical magnetic trap

The formation and maintenance of an electrostatic potential well by injecting electrons into a quasispherical cusped magnetic confinement geometry is studied experimentally, as a function of plasma fill density and of the energy and current of the injected electrons. A model is developed to analyze the experiment. It is found that the potential is comparable to the energy of the injected electrons at low density, and decreases as an increasing density of cold plasma fills the device because of ionization or wall bombardment. Implications for fusion based on electrostatic/magnetic confinement are discussed.

He wall bombardment and wall erosion in fusion devices

Abstract A small fraction of the α-particles produced in a future (D, T)-fusion plasma will escape directly and hit some areas of the surrounding solid walls with their full energy, while the majority will be confined in the magnetic field, slowed down in the plasma and reach the first wall with about the same energy distribution as the other plasma particles. During bombardment of a solid by energetic α-particles the implanted surface layers undergo several modifications leading finally to a rough surface with a spongy microstructure. Some of these modifications, i.e. blistering and flaking, are only observed if the critical gas concentration builds up first at some depth below the surface. This condition is met only if simultaneous erosion by sputtering stays below a certain limit. In fusion devices with loss fractions of ≲ 1% high energy α-particles and a plasma temperature near the first wall above some 10 eV, blistering and flaking are likely to be avoided.

Neutral Molecule Desorption from SS(304) and A1(6061) Due to Particle Bombardments

The pressure in both electron and proton storage rings is dependent on the particle wall bombardment molecular desorption rates due in one case (proton storage rings) to ~ keV ions while in the other to ~ 100 eV electrons. Also in order to minimize secondary electron amplification effects in both types of machines the secondary electron emission coefficient should be as low as possible. The above quantities have been measured on A1(6061) and SS(304) materials having had various treatments including a sputter deposition of Ti and TiN in the case of A1. The results indicate for both electron and proton storage rings that SS(304) materials properly prepared have particular advantages over A1 considering both molecular desorption yields and secondary electron emission effects.

An upper bound to the pressure (outgassing rate) in a dynamic vacuum system (1) Uniform wall bombardment

Abstract

Superthermal alpha transport and wall bombardment in tokamaks

Fusion-product transport, from an axisymmetric tokamak plasma to the first wall, is modelled. Prompt-loss profiles (i.e. prior to slowing) are calculated as a function of poloidal angle. There is a significant flux incident at near-grazing angles, thus enhancing sputtering. Non-prompt losses are also obtained; the resulting wall bombardment is ≲ 1 10 of the prompt contribution but is localized at the outboard edge of the tokamak. To verify fusion-product transport, a detection experiment has been proposed and modelled for a device like PLT or TFTR. Blistering is the dominant wall erosion mechanism. Serious impurity contamination of the plasma can result: Δ〈Z〉 eff/s ~ 6.5 in a TFTR-size device, and 0.014 in UWMAK-I; this poses a potential limit on reaction burn time. These effects can be minimized by: 1) periodic B-field reversal, 2) special wall materials and/or rapid wall replacement designs in peak flux areas, 3) increased plasma-wall separation, and 4) increased plasma current.

Aspects of plasma-wall interaction in the ASDEX divertor experiment

ASDEX (Axially SymmetriC Divertor Experiment) is a large tokamak now under construction at IPP Garching. The main parameters are: major radius 1.65 m, plasma radius 0.4 m, toroidal field on axis 2.8 T and plasma current 500 kA. The experiment is scheduled to go into operation in about two years. The first aim of the experiment is to test the divertor action, i.e. plasma stability without material limiter and reduction of impurity influx. Since the divertor will essentially be of the unload-type, the latter problem should be solved by reducing wall bombardment. On the premises of sufficient stability the plasma-wall interaction will occur on the divertor slits, in the main divertor chamber and (with refuelling) on the wall owing to charge exchange neutrals. Plans for surface studies are being discussed.

Emissionless power cathode

The combination of a weak rf discharge and a plasma current multiplier constitutes an effective dc power cathode which operates without electron emission, elevated materials temperature, or wall bombardment, etc. Such power cathodes may find application wherever filament cathode operation poses difficulties, as with reactive gases, or where filament replacement is undesirable.

Reemission of Ionically Pumped H2 and He from a Stainless Steel Surface

H 2 and He discharges in a stellarator produce copious wall bombardment by the plasma ions. The reemission rate of the gas was measured for times of 0.06 to 105 see after bombardment. The rate was approximately proportional to t−0.6 for He and t−0.75 for H2. The reemission is also calculated assuming diffusion of the gas in the solid and taking into account the finite nonuniform initial distribution. It follows from the analysis that the rate is proportional to t−n where n approaches 0.5 or 1.5 as the ratio of reemission to bombarding time approaches zero or infinity.

The Divertor, a Device for Reducing the Impurity Level in a Stellarator

A divertor, designed to reduce the flow of impurities from the wall into a gas discharge, has been used with a small stellarator. In the divertor an outer shell of magnetic flux is bent away from the discharge channel into a large auxiliary chamber. Ions diffusing toward the wall tend to follow the lines of force of this outer shell into the divertor chamber. Impurities produced by wall bombardment in this chamber do not readily return to the discharge. The magnetic design of this device is described, and a phenomenological theory of its performance is outlined. The spectroscopic data with and without the divertor activated indicate that the divertor reduces the influx of impurities by a factor of two to three, while the impurity concentration at the core, or central region, may be diminished by an order of magnitude when the divertor is activated. Kinetic temperatures of positive ions determined from spectroscopic measurements of Doppler broadening increase from 40 ev without the divertor to 60 ev for He+ and to 130 ev for O++++ with use of the divertor.