arXiv subjects

Shaun Andrews

Publications and source records attributed to Shaun Andrews.

4 recordsLinked to original sources

arXiv2026

A Reduced-Order Particle-in-Cell Method with Azimuthal Fourier-Decomposed Fields for Nominally Axisymmetric Plasmas

A reduced-order Particle-in-Cell method is introduced for kinetic simulation of otherwise axisymmetric cylindrical plasmas that exhibit azimuthal instabilities. The method spatially decomposes all field quantities into a small number of azimuthal Fourier modes $m=0,...,N_m,\quad N_m \ll N_\theta$, reducing the costly three-dimensional field solve $\mathcal{O}(N_zN_rN_\theta)$ to a family of decoupled independent two-dimensional problems $\mathcal{O}((N_m+1)N_zN_r)$ on the meridional plane - one per mode - while particles continue to move in full three-dimensional space. Fields are reconstructed at particle positions by coherent superposition of these modal contributions, preserving complete azimuthal variation at a fraction of the cost of a conventional three-dimensional simulation. For initial validation, the method is tested against two benchmark problems with restricted axial dimension: The diocotron instability of a hollow electron annulus across three geometrically distinct configurations, recovering linear growth rates and eigenpotential structures within 7% of closed-form analytic predictions, and reproducing the non-linear vortex dynamics characteristic of the instability saturation; and a further benchmark against the community-standard Landmark Penning discharge problem recovers the rotating-spoke frequency, radial plasma profiles, and modal energy hierarchy in quantitative agreement with long-time reference simulations, at approximately 640 CPU-hours - a factor of 46 speed-up compared to the median benchmark cost. The approach represents a step toward addressing the important gap between computationally prohibitive full three-dimensional kinetic simulation and the physically limited reduced-dimensionality models on which predictive modelling of anomalous transport in magnetised plasma devices currently relies.

physics.plasm-ph↗

arXiv2024

Axial-radial plasma transport and performance of a plasma thruster magnetic nozzle under Bohm's anomalous diffusion scaling

Magnetic nozzles (MN) are known to be subject to anomalous non-collisional diffusion mechanisms driven by instabilities and wave-particle interactions. This study therefore employs a fully kinetic axial-radial particle-in-cell (PIC) model to examine the impact of this anomalous diffusion on plasma transport and the propulsive performance of MNs typical of low-power cathode-less radio-frequency (RF) plasma thrusters. A Bohm-type anomalous collisionality scaling ($\nu_{an}=\alpha_{an}\omega_{ce}$) is implemented to simulations of the 150 W-class REGULUS-150-Xe thruster, evaluating both low-power (30 W) and high-power (150 W) operating conditions. The impact on azimuthal electron current formation is assessed, as well as its subsequent effect on thrust generation, momentum and power balance, and overall propulsive efficiency. A critical value of the Bohm coefficient was found to exist, where the MN expansion transitions from a well-collimated to an under-collimated state and electron transport shifts from being dominated by magnetic advection to being dominated by cross-field diffusion. This critical transition was found to occur within a narrow interval between $\alpha_{an}$=1/128 and 1/64. Beyond this threshold, it is found that the enhanced cross-field transport of electrons inhibits the formation of the typical MN potential barrier, reducing the radial confinement. The downstream potential drop is reduced by up to 15\%. Diamagnetic electron current is diminished in the absence of steep pressure gradients and the $E\times B$ current becomes purely paramagnetic. The MN efficiency is cut from circa 0.5 to 0.2 due to loss of electron thermal energy conversion and increased plume divergence. At the Bohm limit of $\alpha_{an}=1/16$, agreement to experimental thrust profiles of $<20\%$ is achieved in contrast to 48\% overestimation at high-power in the classical case.

physics.plasm-ph↗

arXiv2023

Fully kinetic study of facility pressure effects on RF-source magnetic nozzles

A fully kinetic 2D axisymmetric Particle-in-Cell (PIC) model is used to examine the effects of background facility pressure on the plasma transport and propulsive efficiency of magnetic nozzles. Simulations are performed for a low-power (150 W class) cathode-less radio-frequency (RF) plasma thruster, operating with xenon, between background pressures up to 10$^{-2}$ Pa and average electron discharge temperatures of 4 - 16 eV. When the electron temperature within the near-plume region reaches 8 eV, a decisive reduction in performance occurs: at 10$^{-2}$ Pa, in-plume power losses surpass 25% of the discharge energy flux. Given that the ionization energy for Xe is 12 eV, the 8 eV threshold indicates that a consistent percentage of electrons has energy enough to trigger ionization. On the other hand, when the temperature is below such threshold, the primary collisions are charge-exchange and inelastic ion scattering, and the power loss remains less than 10%. It is established that losses in the considered HPT are significant if the facility pressure is greater than 10$^{-3}$ Pa, at absorbed powers larger than 130 W. At the nominal 150 W, this results in a 15% thrust reduction. When facility pressure is taken into consideration over ideal vacuum simulations, numerical error is reduced to <30% when compared to experimental thrust measurements at 10$^{-3}$ Pa.

physics.plasm-ph↗

arXiv2022

Fully kinetic model of plasma expansion in a magnetic nozzle

A self-consistent model is presented for performing steady-state fully kinetic Particle-in-Cell simulations of magnetised plasma plumes. An energy-based electron reflection prevents the numerical pump instability associated with a typical open-outflow boundary, and is shown to be sufficiently general that both the plume kinetics and plasma potential demonstrate domain independence (within 4%). This is upheld by non-stationary Robin-type boundary conditions on the Poisson's equation, coupled to a capacitive circuit that allows physical evolution of the downstream potential drop in the transient. The method has been validated against experiments, providing results that fall within the uncertainty of measurements. Simulations are then carried out to study collisional xenon discharges into axisymmetric diverging magnetic nozzles. Particular discussion is given to the identification of a potential well arising from charge separation at the edge of the plume, the role of ion-neutral charge exchange, and a three-region piecewise polytropic cooling regime for electrons. The polytropic index is shown to depend on the degree of magnetisation. Specifically, in the region near the thruster outlet, the plume is weakly-magnetised due to the cross-field diffusion of electron-heavy particle collisions. Downstream, a strongly-magnetised region of near-isothermal expansion occurs. Finally, in the detached region, the polytropic index tends to that of a more adiabatic unmagnetised case. With an increasing magnetic nozzle field strength, an inferior limit is found to the average polytropic index of $\barγ_e\sim1.16$.

physics.plasm-ph↗