No abstract

Due to large particle exhaust velocity and specific impulse, electric propulsion systems have an edge over chemical propulsion for missions targeting regions outside the Earth's atmosphere. Stationary plasma thrusters and helicon plasma thrusters (HPTs) are commonly used electric propulsion devices for a space mission. In HPTs or expanding magnetic field plasma thrusters, plasma expands from the source region to the expansion region in an externally applied expanding magnetic field. Due to plasma expansion in such a magnetic field configuration, a current free double layer is found to form, which accelerates bulk ions, and a directional ion beam is generated, which causes thrust in the opposite direction. A Particle In Cell (PIC) solver with Monte Carlo Collision (MCC) scheme which resolves the axial direction and all three velocity degrees of freedom (1D-3V PIC-MCC) that captures the 2D spatial plasma expansion effect via a 1D flux conserving model is developed to simulate an argon plasma in an expanding magnetic field. Using the 1D-3V PIC-MCC solver, double layer formation due to plasma expansion, thrust generation, and optimization of thrust studies over a large parameter set, such as fill pressure of Ar, is presented. We compare our results with a particle loss model, which is commonly used as the simplest model for HPTs.

Development of a demountable and see-through hollow cathode (HC) discharge lamp suitable for optogalvanic (OG) spectroscopy is described. The design of the HC lamp is simple, compact, and inexpensive. Lithium, investigated rarely by the OG method, is selected for cathode material as its isotopes are important for nuclear industry. The HC lamp is characterized electrically and optically for discharge oscillations free OG effect. Strong OG signals of lithium as well as neon (as buffer gas) are produced precisely upon copper vapor laser pumped tunable dye laser irradiation. The HC lamp is capable of generating a clean OG resonance spectrum in the available dye laser wavelength scanning range (627.5-676 nm) obtained with 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran dye. About 28 resonant OG lines are explicitly observed. Majority of them have been identified using j-l coupling scheme and assigned to the well-known neon transitions. One line that corresponds to wavelength near about 670.80 nm is assigned to lithium and resolved for its fine (S → P) transitions. These OG transitions allow 0.33 cm accuracy and can be used to supplement the OG transition data available from other sources to calibrate the wavelength of a scanning dye laser with precision at atomic levels.

We present a quantitative comparative study of the formation of coherent phase space structures in one dimension using two widely followed approaches in kinetic simulations of collision-less plasmas, namely, Monte Carlo based Particle-In-Cell (PIC) simulations and phase space grid based Eulerian Vlasov-Poisson simulations. Using a newly developed PIC solver, we demonstrate that, while for linear regimes, there is a ready quantitative agreement between Vlasov-Poisson and PIC solvers, whereas for weakly nonlinear regimes and late time simulations, for comparable field resolutions, Vlasov-Poisson simulation results are found to be relatively noise-free as compared to PIC results with a large number of PIC particles. As an extreme case, we address using high resolution PIC simulations, the formation of giant phase space vortices obtained recently using the Vlasov-Poisson method [P. Trivedi and R. Ganesh, Phys. Plasmas 23, 062112 (2016)] for an infinitesimal amplitude external drive. For identical parameters and numerical resolution, we present a qualitative and quantitative comparison between PIC results of driven giant phase space structures and those of the Vlasov method, for a Maxwellian plasma.