A pinboard by
Gary Li

Graduate Student, UCLA


Studying the plasma formation process in a next-generation high-powered plasma rocket.

Electric propulsion (EP) is a relatively new type of rocket propulsion technology that provides a massive savings in fuel compared to conventional rockets, opening doors for ambitious space missions to deep space destinations such as Mars, the asteroid belt, and the outer planets. The field reversed configuration (FRC) thruster is one of the most promising candidates for such missions because its scales efficiently to high power. Higher power provides shorter trip times for space missions, potentially reducing years to months.

In order to improve the design of state-of-the-art FRC thrusters, we need to study how to efficiently form and eject the hot ionized gas known as plasma to produce thrust. Therefore, it is critical to measure the plasma's properties, such as density and temperature. However, as a technology derived from nuclear fusion, measuring the plasma properties from such a hot plasma can be extremely difficult. Conventional methods that involve directly sampling the hot FRC plasma typically don't work. Therefore, we utilize an indirect method known as time-resolved optical emission spectroscopy (OES) to study the plasma.

By using time-resolved OES, we can obtain the plasma properties as a function of time by simply measuring the light naturally emitted from the FRC plasma. This tells us how efficiently the plasma is being formed, and how fast the plasma is being ejected from the thruster. Time-resolved OES provides insight on the the performance of the rocket and subsequently informs future designs. My present work involves the implementation of this technique on a laboratory model FRC thruster, and the resulting analysis of the data.


Electron Pre-Acceleration at Nonrelativistic High-Mach-Number Perpendicular Shocks

Abstract: We perform particle-in-cell simulations of perpendicular nonrelativistic collisionless shocks to study electron heating and pre-acceleration for parameters that permit extrapolation to the conditions at young supernova remnants. Our high-resolution large-scale numerical experiments sample a representative portion of the shock surface and demonstrate that the efficiency of electron injection is strongly modulated with the phase of the shock reformation. For plasmas with low and moderate temperature (plasma beta $\beta_{\rm p}=5\cdot 10^{-4}$ and $\beta_{\rm p}=0.5$), we explore the nonlinear shock structure and electron pre-acceleration for various orientations of the large-scale magnetic field with respect to the simulation plane while keeping it at $90^\circ$ to the shock normal. Ion reflection off the shock leads to the formation of magnetic filaments in the shock ramp, resulting from Weibel-type instabilities, and electrostatic Buneman modes in the shock foot. In all cases under study, the latter provides first-stage electron energization through the shock-surfing acceleration (SSA) mechanism. The subsequent energization strongly depends on the field orientation and proceeds through adiabatic or second-order Fermi acceleration processes for configurations with the out-of-plane and in-plane field components, respectively. For strictly out-of-plane field the fraction of supra-thermal electrons is much higher than for other configurations, because only in this case the Buneman modes are fully captured by the 2D simulation grid. Shocks in plasma with moderate $\beta_{\rm p}$ provide more efficient pre-acceleration. The relevance of our results to the physics of fully three-dimensional systems is discussed.

Pub.: 18 Aug '17, Pinned: 23 Aug '17

NIMROD calculations of energetic particle driven toroidal Alfv\'en eigenmodes

Abstract: Toroidal Alfv\'en eigenmodes (TAEs) are gap modes induced by the toroidicity of tokamak plasmas in absence of continuum damping. They can be excited by energetic particles (EPs) when the EP drive exceeds other dampings. A TAE benchmark case, which was proposed by the International Tokamak Physics Activity (ITPA) group, is studied in this work. Numerical calculations of linear growth of TAEs driven by EPs in a circular-shaped, large aspect ratio tokamak have been performed using the Hybrid Kinetic-MHD (HK-MHD) model implemented in the NIMROD code. This HK-MHD model couples a delta f particle-in-cell (PIC) representation of EPs with the 3D MHD representation of the bulk plasma through moment closure for the momentum conservation equation. Both the excitation of TAEs and their transition to energetic particle modes (EPMs) have been observed. The influence of EP density, temperature, density gradient and position of the maximum relative density gradient, on the frequency and the growth rate of TAEs are obtained, which are consistent with those from eigen-analysis calculations and gyrokinetic simulations for an initial Maxwellian distribution of EPs. The relative pressure gradient of EP at the radial location of TAE gap, which represents the drive strength of EPs, can strongly affect the growth rate of TAEs. When the density and temperature of EP distribution are above certain threshold, the transition from TAE to EPM occurs and the mode structure changes.

Pub.: 18 Aug '17, Pinned: 23 Aug '17