Richard A. Gerwin

Lower hybrid drift instability at low drift velocities

The behavior of the lower hybrid drift instability is investigated in the low drift velocity regime by including the effects of the magnetic field on the ions. It is found that the mode is transformed from the nonresonant lower hybrid drift instability to the resonant ion cyclotron drift instability as the drift velocity Vd decreases and is finally stabilized when Vd/Vti∼ (me/mi)1/2.

Electromagnetic current instabilities

Linear electromagnetic waves in an infinite, homogenous current‐carrying Vlasov plasma with ion β≈1 are considered. Three instabilities emerge: a long wavelength, ’’kink‐like’’ mode, and two instabilities with wavenumbers greater than the reciprocal ion Larmor radius, the ’’electromagnetic ion acoustic instability’’ and the ’’whistler current instability.’’ The linear properties of these instabilities are investigated in detail; they have substantially lower thresholds than the electrostatic ion‐acoustic instability, and are generally favored by increasing ion β. In the regime of linear growth, the whistler current instability can give rise to an anomalous resistivity, and preferentially heats the ions.

Magnetohydrodynamic flow physics of magnetically nozzled plasma accelerators with applications to advanced manufacturing

The coaxial plasma accelerator is a simple, compact, and mechanically robust device that utilizes the Lorentz J×B force to accelerate plasma to high velocity. Originally developed in the 1950s for the purpose of providing energetic plasmas for fusion energy experiments, coaxial plasma accelerators are presently being investigated as an environmentally sound and economical means of materials processing and advanced manufacturing. While commercial applications of this technology are already on line, future commercial applications will require improving accelerator reproducibility and efficiency, better controlling the accelerated plasma flow velocity or energy, and better controlling the distribution of directed energy or power on target. In this paper, the magnetohydrodynamic flow physics of magnetically nozzled plasma accelerators is presented with a view to achieving the accelerator control necessary for future industrial applications. Included is a fundamental description of plasma production, accelerat...

Resistive plasma detachment in nozzle based coaxial thrusters

Nozzle based coaxial plasma thruster constitute potentially attractive electric propulsion engines that are both compact (with high thrust density) and robust. The coaxial plasma thruster can be viewed as an evolutionary state of magnetoplasmadynamic (MPD) thruster development that may satisfy the demanding performance requirements of Space Exploration Initiative (SEI) relevant cargo or piloted missions. Previous work (Schoenberg et al., AIAA Technical Report 91‐3570) has shown that ideal magnetohydrodynamics (MHD) plays a major role in multi‐megawatt coaxial plasma thruster dynamics, particularly in the behavior of high‐grade plasma acceleration by a converging‐diverging magnetic nozzle. In this paper, we examine the detachment of high‐grad MHD plasma from a magnetic nozzle by resistive diffusion. A quantitative description of the resistive detachment process is derived. Included is a discussion of non‐ideal MHD effects, including classical and anomalous resistivity, that can enhance the detachment. This analysis supports the hypothesis that magnetic nozzle design for high‐performance thruster operation requires an optimization between the conflicting requirements of efficient plasma acceleration and plasma detachment. A qualitative prescription for such an optimized magnetic nozzle design is discussed.Nozzle based coaxial plasma thruster constitute potentially attractive electric propulsion engines that are both compact (with high thrust density) and robust. The coaxial plasma thruster can be viewed as an evolutionary state of magnetoplasmadynamic (MPD) thruster development that may satisfy the demanding performance requirements of Space Exploration Initiative (SEI) relevant cargo or piloted missions. Previous work (Schoenberg et al., AIAA Technical Report 91‐3570) has shown that ideal magnetohydrodynamics (MHD) plays a major role in multi‐megawatt coaxial plasma thruster dynamics, particularly in the behavior of high‐grade plasma acceleration by a converging‐diverging magnetic nozzle. In this paper, we examine the detachment of high‐grad MHD plasma from a magnetic nozzle by resistive diffusion. A quantitative description of the resistive detachment process is derived. Included is a discussion of non‐ideal MHD effects, including classical and anomalous resistivity, that can enhance the detachment. This...

Preliminary investigation of power flow and performance phenomena in a multimegawatt coaxial plasma thruster

Preliminary experimental and theoretical research that was directed toward the study of quasi-steady-state power flow in a large, unoptimized, multimegawatt coaxial plasma thruster is summarized. Large coaxial thruster operation is discussed, and the experimental results are evaluated and interpreted with a view to the development of efficient, steady-state, megawatt-class magnetoplasmadynamic (MPD) thrusters. >

Mode coupling effects on resistive wall instabilities

It is shown that destabilization of resistive wall magnetohydrodynamic (MHD) modes in the presence of rotation is a mode coupling phenomenon. Based on this observation, certain unanticipated effects are readily explained. These include the fact that resistive wall modes with rotation can be unstable, even for parameters for which the MHD modes are stable with the wall at infinity, and the fact that this destabilization depends critically on the plasma parameters.

Transport implications of current drive by magnetic helicity injection

It is shown that in fusion plasma configurations sustained by electrode helicity injection, the core electron temperature (in electron volts) can, at most, be 25% to 40% of the electrode voltage (in volts). This result is obtained by assessing magnetic helicity injection as a driver of macroscopic steady-state plasma currents in magnetic confinement devices. Coaxial helicity injection using electrodes (CHI) and oscillating-field current drive (OFCD) are compared to inductive current drive. Magnetic helicity, K, is uniquely defined as the time-dependent volume integral of A⋅B when the surface components of A are purely solenoidal. Using an Ohm’s law including Hall terms, magnetic helicity transport modeling shows that no closed magnetic surfaces with time and volume averaged parallel currents can exist continuously within a plasma sustained only by CHI or OFCD. The 25% to 40% limitations are obtained by considering long and short electron mean-free-path models of parallel energy transport.

Hydromagnetic Surface Waves in a Conducting Liquid Surrounded by a Flowing Gas

The stability of the interface between a highly conducting liquid and a flowing, nonconducting gas is studied, supposing a magnetic field to be parallel to the interface. Arbitrary angles are allowed between magnetic field, flow direction and wave vector.

Coaxial plasma thrusters for high specific impulse propulsion

A fundamental basis for coaxial plasma thruster performance is presented and the steady-state, ideal MHD properties of a coaxial thruster using an annular magnetic nozzle are discussed. Formulas for power usage, thrust, mass flow rate, and specific impulse are acquired and employed to assess thruster performance. The performance estimates are compared with the observed properties of an unoptimized coaxial plasma gun. These comparisons support the hypothesis that ideal MHD has an important role in coaxial plasma thruster dynamics.

Magnetic nozzle design for coaxial plasma accelerators

Magnetic nozzles have great potential for improving the efficiency and performance of coaxial plasma accelerators in applications such as space propulsion and advanced manufacturing. Proper design of magnetic field geometry can improve coaxial accelerator performance in three ways. First, the applied field which intercepts the anode surface without directly connecting the two electrodes can minimize anode fall inefficiencies by improving electron conduction across the anode sheath and by opposing the Hall-induced starvation effect. Second, a properly designed magnetic geometry can provide a nozzling mechanism to permit the plasma to accelerate smoothly and efficiently from sub to super-magnetosonic flow. Third, a magnetic nozzle provides control over the flow of plasma from the accelerator. For applications such as surface modification and etching, magnetic nozzles can maximize the treatable surface area and tailor the downstream plasma energy distribution. For thrust generation, proper design of a magnetic nozzle can enable efficient detachment of the plasma from the magnetic field. >