Following the so- called gas burn out stage, the electron temperature rises to relatively high values on the order of 10 -20eV. (6,7) and ASPL experimental results (3) indicate that there is a promising regime of the VASIMR helicon operation, when almost 100% of the gaseous propellant is ionized. Low-temperature plasma physics is often considered a discipline so different from high-temperature plasma physics that there is little overlap, but these studies show that the techniques developed in fusion and space plasma physics can be applied to technological plasmas as well. Applications of helicon discharges to such diverse fields as plasma accelerators, microwave generators, and tokamak physics are illustrated. The importance of kinetic effects and of a short-wavelength mode arising at low magnetic fields is evaluated. Experimental data on the waves and the equilibrium properties of the discharge are shown, and the status of our current understanding of the physical processes therein is summarized. How helicon waves are related to whistler waves and waves in a plasma-filled waveguide is explained, and the mystery of the high ionization efficiency is outlined. The energy cost for ionization and backwall energy losses are shown to significantly reduce the efficiency.īecause of their high density, low-pressure discharges ionized by helicon waves are being studied for their possible use in cluster tools for the fabrication of next-generation computer chips. Considering such a configuration for a plasma thruster, we calculate the expected thrust, propellant utilization, specific impulse, and efficiency. For an open system of a nonzero mass flow, analytical expressions are derived for the profiles of the plasma and the neutral flows. This is in contrast to the case in which the neutral gas is thermalized. It is shown that the pressure of the collisionless neutral gas is maximal where the density is minimal. For a closed system with zero mass flow, the density profiles of both plasma and neutrals are found, and values for the rate of depletion at asymptotic limits are derived. The neutrals and the plasma are coupled only through volume ionization and wall recombination. In this paper, we address the effects of neutral depletion on the steady state of a collisionless plasma generated in a collisionless neutral gas. Recent theoretical analyses predicted unexpected effects of neutral depletion in both collisional and collisionless regimes. Neutral depletion can significantly affect the steady state of low-temperature plasmas. The experimental program thus demonstrated the potential of the mini-helicon discharge as an excellent space thruster design, given its high throughput, absence of internal electrodes and cathodes, potentially long life time, very low divergence of the plasma plume, flexible choice of propellants, and variable specific impulse. Operation with permanent magnets was attempted as well, including a magnetic nozzle and the possibility of a double layer formation. 4MW/(square meter), power density throughput has been achieved in a steady-state regime. Plasma plume flow velocities in the 10 to 40km/sec interval were measured, and plume acceleration could be easily varied by altering applied RF-power or propellant flow rate. The parameters of the plasmas were measured using high-resolution spectroscopic and in-situ diagnostics, including super-fine emission line structure and isotopic shift of a seeded impurity.

Stable discharges with well-collimated plume exhaust were achieved for argon (Ar), nitrogen (N2), mixtures of Ar and N2, and air. These objectives were successfully achieved. The program was designed to address several objectives: (i) to achieve a high coupling of RF-power to plasma and efficient acceleration to >20km/sec (ii) to evaluate the scaling relations between power density, flow rates and B-field strength (iii) to demonstrate operation with common molecular and atomic gases, and mixtures (iv) to provide detailed characterization of the performance using spectroscopic and invasive techniques (v) to experimentally demonstrate variable specific impulse capability. The helicon discharge was designed and built in several different configurations using low- and high-current electromagnets. An experimental program to develop and test a mini-helicon thruster is described.

Mit mini helicon plasma thruster