Neutral Particle Dynamics Research Articles

Hall effect thruster impedance characterization in ground-based vacuum test facilities

Hall effect thrusters (HETs) are typically regarded as DC electric propulsion devices as they are operated with isolated DC power supplies. However, it is well known that the HET’s discharge current possesses oscillations of varying magnitudes and frequencies and is thus a function of time with AC characteristics. The observed oscillations are caused by plasma processes associated with ion, electron, and neutral particle dynamics that occur inside the HET’s discharge channel and in the plume as the HET electrically interacts with its local operating environment. The extent to which plasma oscillations impact HET discharge dynamics is difficult to quantify due to the complexity of analyzing AC signals, given that the HET is a nonlinear, time-variant electrical load. In this work, we overcome the challenge of nonlinearity and time-variance of HETs by conducting a small-signal impedance analysis to characterize the effective resistance and reactance of the HET discharge with a novel and versatile impedance measurement diagnostic. The impedance magnitude and phase of a 7-kW class HET were measured from 100 Hz to 300 kHz with an excitation signal of ± 2 Vpk for two discharge operating conditions on krypton: 4.5 kW, 15 A and 6 kW, 20 A. The results were used to quantify resistive, capacitive, and inducive characteristics present within the HET discharge signature. For the 4.5 kW, 15 A thruster operating condition, the breathing mode capacitance was estimated to be 12.6 µF with an inductance of 15.3 µH. Furthermore, the impedance characteristics of the breathing mode are within ± 2.4 kHz of the power spectral density plots independently generated by time-resolved oscilloscope traces indicating good agreement in the frequency domain. Thus, the impedance measurement tool is a new diagnostic for characterizing the impedance and associated AC characteristics of HETs.

Investigation of neutral particle dynamics in Aditya tokamak plasma with DEGAS2 code

Neutral particle behavior in Aditya tokamak, which has a circular poloidal ring limiter at one particular toroidal location, has been investigated using DEGAS2 code. The code is based on the calculation using Monte Carlo algorithms and is mainly used in tokamaks with divertor configuration. This code has been successfully implemented in Aditya tokamak with limiter configuration. The penetration of neutral hydrogen atom is studied with various atomic and molecular contributions and it is found that the maximum contribution comes from the dissociation processes. For the same, Hα spectrum is also simulated and matched with the experimental one. The dominant contribution around 64% comes from molecular dissociation processes and neutral particle is generated by those processes have energy of ~2.0 eV. Furthermore, the variation of neutral hydrogen density and Hα emissivity profile are analysed for various edge temperature profiles and found that there is not much changes in Hα emission at the plasma edge with the variation of edge temperature (7–40 eV).

Neutral gas heating in helium microplasmas

The present study details a self-consistent model of charged and neutral particle dynamics which is applied to atmospheric small-space (200 μm) discharges in helium. Hydrodynamic transport equations of the self-consistent and time-dependant model are described with an emphasis on the different terms involved in the close coupling among charged species, neutral species, and the electric field. Those equations are solved by two-dimensional numerical schemes for both species transport and electric field. The microplasmas are studied from an initial cloud until the stages of charged particle overamplification in small spaces, where transients are particularly important. Gas heating, neutral depletion initiation, and electric field reversal are observed, highlighting the close interaction between neutral gas and charged species in governing the evolution of the microplasma.

Self‐Consistent Simulations of the Plasma‐Wall Transition Layer

AbstractIn the present work we develop a 1d3v (1D in space and 3D in velocity) kinetic Particle‐in‐cell model of the magnetised plasma‐wall transition layer, where the charged and neutral particle dynamics and interaction between them is included in a fully self‐consistent way. The plasma‐surface interactions are described via fixed emission coefficients for the secondary particles. We obtained different profiles of plasma and neutral particles and cross‐checked classical boundary conditions, which are usually formulated inside the PWT. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of the Zeeman patterns to the measurement of local neutral behaviour in the edge plasma of TRIAM-1M tokamak

Neutral particle dynamics is measured in the edge plasma of the TRIAM-1M tokamak by means of the Zeeman patterns in the spectral shape. This measurement can provide the emission position with high spatial resolution, as well as the local drift velocity and temperature. In the TRIAM-1M, the inward neutral flow velocity driven by the radial neutral pressure gradient is observed. The temporal evolution of the flow velocity suggests that the distance between the last closed flux surface and the first wall changes the recycling flux. Thereby, the neutral pressure gradient and the neutral flow velocity are varied. This technique can be an effective way for the measurement of the local neutral behaviour in the edge region.

Effects of neutral particle dynamics in the active medium of discharge pumped XeCl lasers

A powerful fluid model to study the role of gas dynamics in a glow discharge considered as an excitation medium for XeCl lasers is presented. This model was employed using a numerical code including four strongly coupled parts: electric circuit equations, electron Boltzmann equation, kinetic equations and conservation equations. The theoretical formalism presented here is based on the two-dimensional Euler equations describing the perfect compressible fluid. The numerical calculations have been carried out considering the single shot regime and neglecting the gas flow. The monotonic upstream centred scheme for conservation laws corrections has been used in order to estimate the exchange flow surrounding the control volume walls. We show that the crossing discharge induces a thermal energy density enhancement located principally in the cavity centre.

Self-consistent modelling of charged and neutral particle dynamics in short-gap helium and hydrogen discharges

A self-consistent model of charged and neutral particle dynamics is developed for the case of high pressure short-gap discharges in helium and hydrogen and . Boundary wall effects on the electron swarm parameters are first investigated by a Monte Carlo method in order to verify the validity of the classical local field approximation in short-gap discharges. The hydrodynamic transport equations of the self-consistent model are then described with an emphasis on the different terms involved in the close coupling between charged and neutral particles and the electric field. These equations are solved by powerful two-dimensional numerical schemes for both transport and electrical field equations. The discharges are studied from an initial electronic cloud to the first stages of breakdown. Cathode emission is discussed in terms of its prime importance in the spatio-temporal evolution of the short-gap discharges and it is shown that the principal difference between helium and hydrogen discharges is due to the mode of cathode emission. The particular observations in the luminosity in hydrogen are discussed in terms of ionization of the gas and secondary emission processes at the surface. A detailed analysis reveals a complex distribution of charged particles due to the superposition of ionization and transport effects. Furthermore, Joule heating of the neutral medium is evaluated in the entire time scale of the discharge and its influence on the discharge evolution is discussed.

Neutral particle dynamics in the Alcator C-Mod tokamak

An experimental study of neutral particle dynamics has been carried out over a wide range of operating conditions in the Alcator C Modified (Alcator C-Mod) tokamak using special purpose pressure gauges. Both divertor and midplane pressures are found to depend primarily on the edge plasma regimes defined by the scrape-off layer heat transport. While the maximum divertor neutral pressure (30-60 mtorr) is achieved at high core plasma densities (ne=(2-4)*1020 m-3), corresponding to the detached divertor state, the maximum ratio of divertor to midplane pressure (~70) is achieved in the high recycling transport regime. Variations in the divertor geometry have a weaker effect on the neutral pressures. For otherwise similar plasmas, the divertor pressure and compression are maximized when the strike point is located at the bottom of the vertical target plate. Modelling shows that the high pressure sustained during detached divertor operation (despite a considerable drop in the recycling source) can be explained by scattering of neutrals off the cold plasma plugging the divertor throat

Modelling of the ASDEX scrape-off and divertor

For the divertor configurations of ASDEX a two-fluid model describing the plasma flow and energy transport in the scrape-off layer together with models for the neutral particle dynamics in the divertor chamber have been used to study the relation between the neutral molecule density and plasma density in the divertor and the scrape-off density at the midplane. Results of DEGAS Monte Carlo calculations prove the divertor entrance slits to be nearly impenetrable for neutrals in the presence of a plasma. A global recycling picture is suggested relying on edge density control by the divertor and a hydrogen ion inward drift in the main discharge region to maintain the bulk plasma density even in the absence of neutrals in the main chamber.