Plasma Density Buildup Research Articles

Investigation of multipactor-induced surface plasma discharge and temporal mode transition

Multipactor over a dielectric in vacuum inclines to engender interfacial gas desorption or evaporation, precipitating surface flashover and insulator failure. However, no consensus has been achieved regarding the exact mechanism during final breakdown stage, an expatiation of which therefore serves as our major motivation for this letter. By implementing the particle-in-cell simulation code, we investigate the microscopic evolution of the discharge development process and confirm the major component escalating the explosive space charge accumulation. The obtained current waveform validates the balance of charged particles between electrodes, corroborated by experimental results. A theoretical discharge model is then constructed to elucidate the physical reasoning of the previous phenomenon. Two distinct discharge modes are defined correspondingly, and the transition therein is found to be induced by rapid plasma density build-up.

Simulation and experimental studies of 20 kV/200 A PCE-Gun for discharge current analysis

In this paper, an effort has been made to study the recently developed pseudospark discharge-based 20 kV/200 A plasma cathode electron gun by using 3-D particle-in-cell simulation code VORPAL for discharge current analysis to validate the experiments. The experimental and simulation studies have been performed at different working pressures between 50 and 100 mTorr and at different applied voltages between 10 and 20 kV. Argon has been used as the working gas. The temporal behavior of the discharge current, discharge voltage, potential distribution within the hollow cathode region and the electron plasma density buildup has been analyzed from the simulations. The experiments have been performed to measure the dynamic nature of the discharge voltage and discharge current at different operating conditions. An agreement between the simulated and experimental results has been obtained well within 10 % for the discharge currents at different applied voltages, qualifying the design.

Current sheet formation near a hyperbolic magnetic neutral line in a variable-density plasma: An exact solution

Current-sheet formation near a hyperbolic magnetic neutral line in the MHD model is investigated. In this model, the magnetic field lines are swept by the plasma flow toward the X-point. Effects of density-variation of the plasma are included. An exact analytic solution is given. This solution exhibits a finite-time singularity that is peculiar to the variable-density plasma case and this finite-time singularity occurs when there is a plasma density build-up near the magnetic neutral line.

A numerical study of density limit mechanisms in tokamaks

A core particle and power balance model with radiative cooling, a ballooning-mode-limit pedestal model, a “two-point” plasma divertor model with radiative and atomic recycling cooling, and a two-dimensional (2-D) neutral transport model have been combined to model the coupled plasma core-edge physics in a tokamak. This model has been applied to examine the physical mechanisms which limit the attainable core density. The first limitation is fueling rate and core penetrability. For fueling that is able to penetrate the core, the core density increases with a fueling rate up to a limit set by one of two other mechanisms. When a sufficient impurity concentration is present, the mechanism that limits the maximum density attainable in the core seems to be an impurity-driven thermal instability that produces a thermal collapse of the core plasma. At lower impurity concentrations, the buildup of plasma density in the divertor and the associated increase in atomic recycling and ionization cooling causes a collapse of the divertor plasma temperature, leading to a thermal collapse of the core plasma. An increase in core heating input power or in plasma confinement extends the point of thermal collapse to higher density.

Plasma density buildup after pellet injection

Details of the increase of the plasma density following a pellet injection have been measured with high temporal resolution (16 μs) and the parallel expansion of ablated matter modeled with a four-fluid hydrodynamic code. The driving force of the expansion is the parallel pressure gradient, progressively balanced by the compression of the background plasma. It is shown that the ablated material experiences a strong transient poloidal motion (≊5×103 ms−1, ≊100 μs) as it expands along the field lines. This motion, which is induced by the pellet itself, results from the conservation of kinetic momentum: the convective motion in the sheet of ablated material (due to its positive potential with respect to the plasma) is compensated by a global drift of the whole magnetic surface. This model reproduces the main observations concerning the parallel propagation of the ablatant in the discharge. In particular, it shows that the stretching of the sheet of ablated material by the magnetic shear and the poloidal rotation is responsible for the homogenization of the density in a characteristic time of ≊1 ms. The plasma rotation measured immediately after a pellet injection is therefore not, in general, simply linked to the background radial electric field.

The process of space current neutralization of intense relativistic electron beam under an externally applied magnetic guide field

The process of space current neutralization of intense relativistic electron beam under an externally applied magnetic guide field is discussed in this paper. Ionization by electron avalanching and by beam electrons impact and recombination is included in the calculation of plasma density buildup, with plasma heating by return current and two- stream instability taken into account. A code to evaluate the process of space current neutralization was set up. The calculations demonstrate that the optimum gas pressure increases as peak beam current increases, and it decreases as the risetime of beam pulse increases.

Transport and interaction of a relativistic electron beam in low pressure neutral gases

A numerical study of the transport of a 0.27-MeV, 6.6-kA, 40-ns relativistic electron beam in argon and hydrogen in the pressure range of 0.01–1.0 Torr taking into account charge and current neutralization effects is presented. Ionization by avalanching and by beam and plasma electrons is included in the calculation of plasma density buildup. Plasma heating resulting from return current heating and two-stream instability is taken into account. The computed results of charge transport, net current, and breakdown time are compared with experimental results obtained in this laboratory. The results are in reasonable agreement with the experiment and show a maximum charge transport of 75% at the optimum pressure of 0.1 and 0.6 Torr in argon and hydrogen, respectively. The calculations indicate beam-generated plasma parameters of 1019–1020 m−3 density and 1–5 eV electron temperature.

A numerical model of the electrodynamics of plasma within the contaminant gas cloud of the Space Shuttle Orbiter at low Earth orbit

This paper presents results from a two‐dimensional, finite‐difference model used to solve for the time evolution of low beta plasma within the neutral contaminant cloud in the vicinity of space platforms in low earth orbit. The model of the ambient and contaminant plasma dynamics takes into account the effects of the geomagnetic field, electric fields, background ionosphere, ion‐neutral collisions, chemistry, and both Pederson and Hall currents. Net ionization and charge exchange source terms are included in the fluid equations to study electrodynamic effects of chemistry within a moving neutral cloud in the low earth orbit ionosphere. The model is then used with complete water cloud chemistry to simulate the known outgassing situation of the space shuttle Orbiter. A comparison is made of the model results with plasma observations made during daytime on OSS‐1/STS‐3 mission. The reported density enhancements of the OSS‐1 mission are unattainable with normal photoionization and charge exchange rates of simple water cloud chemistry used in the two‐dimensional model. The enhanced densities are only attained by a generic chemistry model if a net ionization rate 1000 times higher than the photoionization rate of water is used. It is also shown that significant plasma buildup at the front of the contaminant neutral cloud can occur due to momentum transfer from the neutral outgas cloud to the plasma through elastic collisions and charge exchange. The currents caused by elastic and reactive collisions result in the generation of a small polarization electric field within the outgas cloud. The polarization electric field, in combination with the geomagnetic field, causes the plasma to drift with the contaminant neutral cloud but at a slower speed in the stationary reference frame. The slow drifting is consistent with OSS‐1 plasma probe interpretations where a high relative speed between the Orbiter and ions is inferred. Also, if there is a fast net ionization process within the outgas cloud, the drifting plasma allows for nonlinear buildup of plasma density with respect to the net ionization rate.

The effect of a single blade limiter on energetic neutral beam particles in doublet III

Energetic beam ion collisions with the main limiter can be a significant power loss process under certain operating conditions in Doublet III. Furthermore, these collisions may cause measurable damage to the limiter itself.Under low current and low toroidal field conditions (e.g., Ip = 290 kA and BT = 6.3 kG), 20–38% of the inferred absorbed beam power may be deposited directly on the ion drift side of the limiter by the beam ions. However, for higher plasma current and toroidal fields (e.g., Ip = 290 kA and BT = 15 kG), the fraction of inferred absorbed beam power deposited on the limiter is reduced to < 10%. Monte Carlo code simulations show that this loss of beam power is primarily a result of the large poloidal and toroidal gyro-orbits of the energetic beam ions. Other factors which may enhance beam ion losses to the limiter are (1) large separation distances between the primary limiter and the (outboard) vacuum vessel wall, and (2) plasma density buildup near the plasma edge during high gas puff operation.In addition, our data suggests enhanced plasma density and recycling near the limiter. This localized density can cause appreciable premature ionizations of the incoming beam neutrals and thus reduce the effective plasma heating of the beamline which is immediately upcurrent of the limiter. The prematurely-ionized beam particles from this adjacent beamline are responsible for much of the damage to the ion drift side of the limiter. We have found that under certain operating conditions (1) the direct beam heating of the limiter is 50% greater and (2) the stored plasma energy is 10% less when the beamline immediately upcurrent of the limiter heats the plasma. Thus, the relative positions of the limiters to the beamlines are important in designing future tokamaks.

Recycling processes in tokamaks

Recycling of the working gas at the walls and limiter of a tokamak discharge plays a pivotal role in the dynamics of the plasma. Because of large recycling rates, plasma can generally be maintained for many particle confinement times without the introduction of new gas from sources external to the vacuum system. The details of recycling are also intimately tied in to the question of plasma density build-up with pulsed-gas. The mechanisms responsible for desorption of working gas from vessel surfaces also cause desorption of impurity gases and thus they are important in both the achievement of high density plasma and the cleanliness of the discharge. Recycling has been studied on numerous tokamaks through various experimental means. In this discussion several of these are reviewed. One technique involves changing the working gas from hydrogen to deuterium and back again, with consequent changes in plasma and wall constituents which are observed by spectroscopic, residual gas analysis, and sample surface probe diagnostics. Absolute spectroscopy of neutral hydrogen emission can yield estimates for the flux of recycling working gas at the edge of the plasma and particle confinement times. It is found that toroidal variations must be carefully taken into account in this case, since under some conditions the recycling at the limiter is important, while under others, recycling at the wall dominates. The condition of the wall itself as regards gases lightly bound on the surface, as well as those more tightly bound in the bulk near the surface, critically influences the recycling processes. This condition is determined by many factors, including various discharge cleaning schemes, tokamak discharges themselves, wall temperature, and titanium gettering.

Diagnostic Techniques of the High Compression and Neutral Injection Experiments

In the Multi-Stage Compression Experiment (Toy Top III) we wish to determine the energy and spatial distribution of the plasma components - ions and electrons - from the time of their initial injection into the machine through the heating and containment cycle. An ion analyzer, consisting of a magnetic momentum analyzer followed by an electrostatic analyzer, has been valuable in finding the energy distribution of the plasma ions. The density distri-of the plasma in the experiment's magnetic field has been largely determined with electrostatic extraction probes. Microwave radiometers and double probes have been utilized to find electron temperatures. Li6 (Eu) and plastic scintillation counters assess the total number and rate of production of neutrons from D-D reactions, which may be confirmed with nuclear emulsions. Electron temperatures in the Single-Stage High Compression Experiment (Table Top II) have been determined from an analysis of the escaping electron flux. The initial experiments on neutral atom injection into a mirror field geometry (ALICE) require a knowledge of the initial background gas density, neutral beam current, buildup of plasma density, loss rate from the confinement region, and the spatial distribution of the plasma. A magnetically shielded, nude Bayard-Alpert gauge determines the total initial gas pressure, the partial pressure of each atomic specie being found with a mass spectrograph. A copper target calorimeter indicates beam current; while neutral particle detectors on the chamber walls and screened Faraday collectors outside of the mirrors show the buildup and loss of plasma.