The charge reduction effect is studied in dense dust clouds. The saturation currents of Langmuir probes are used to derive the density of ions and electrons, which are calibrated with the plasma oscillation method. The plasma potential inside the dust cloud is measured with an emissive probe, which also yields the floating potential in a heated nonemitting mode. The presence of the dust also affects the density and the plasma potential of the ambient plasma. The ion densities inside the dust cloud and in the ambient plasma are found equal, while the electron density is reduced inside the dust cloud. The measured potentials are compared with current models. Inclusion of the bi-Maxwellian distribution of the electrons leads to an improved description in the limit of low dust density. The strong increase of the floating and cloud potential for high dust density, predicted by the constant ion density model, is not confirmed.

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Effects of charge depletion in dusty plasmas

Nuclear electric propulsion (NEP) has long been recognized as a major enabling technology for scientific and human exploration of the solar system, and it may conceivably form the basis of a cost-effective space transportation system suitable for space commerce. The chief technical obstacles to realizing this vision are the development of efficient, high-power (megawatt-class) electric thrusters and the development of low specific mass (less than 1 kg/kWe) power plants. Furthermore, comprehensive system analyses of multimegawatt class NEP systems are needed in order to critically assess mission capability and cost attributes. This Technical Publication addresses some of these concerns through a systematic examination of multimegawatt space power installations in which a gas-cooled nuclear reactor is used to drive a magnetohydrodynamic (MHD) generator in a closed-loop Brayton cycle. The primary motivation for considering MHD energy conversion is the ability to transfer energy out of a gas that is simply too hot for contact with any solid material. This has several intrinsic advantages including the ability to achieve high thermal efficiency and power density and the ability to reject heat at elevated temperatures. These attributes lead to a reduction in system specific mass below that obtainable with turbine-based systems, which have definite solid temperature limits for reliable operation. Here, the results of a thermodynamic cycle analysis are placed in context with a preliminary system analysis in order to converge on a design space that optimizes performance while remaining clearly within established bounds of engineering feasibility. MHD technology issues are discussed including the conceptual design of a nonequilibrium disk generator and opportunities for exploiting neutron-induced ionization mechanisms as a means of increasing electrical conductivity and enhancing performance and reliability. The results are then used to make a cursory examination of piloted Mars missions during the 2018 opportunity.

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Prospects for Nuclear Electric Propulsion Using Closed-Cycle Magnetohydrodynamic Energy Conversion

Particle trapping in different areas of a parallel-plate, radio frequency silane discharge, and its effect on plasma optical emission of SiH and Hα, has been studied under high gas-flow and low power-density conditions, as used for “device-quality” hydrogenated amorphous silicon (a-Si:H) film deposition. The largest density of particles occurs between the electrodes, near the downstream corners of the rectangular electrodes. Particles are trapped in these positions by sheath fields, until reaching sufficient size to escape with the flow. The region of strong particle trapping has an increased intensity of optical emission, with Hα increased nearly fourfold. Slow oscillatory behavior of particle scattering and discharge emission was observed for pressures near 30 Pa. Power deposited in the discharge has also been measured; for a constant rf voltage and gas-flow speed it changes weakly with pressure, with the maximum at ∼40 Pa. Combined with film growth-rate measurements, this yields a discharge energy depo...

Particle accumulation in a flowing silane discharge

The effects of particles on helium-diluted silane RF discharges are studied using a power modulation method for various Values of relevant parameters. Compared with CW discharge cases, close correlation is clearly found between the particle growth, the self-bias voltage and the phase shift between the current and voltage of the RF discharge. Total particle number and optical emission intensity in the bulk plasma region increase monotonically after RF power-on. With these increases, the magnitude of self-bias voltage and the current-voltage phase shift decrease considerably to their minimum values and then slightly increase to their quasi-steady values. The decreases can be explained by the fact that particles in plasmas behave as very heavy negative ions. The increases may be related to the increase in the diffusion rate of electrons. It is also confirmed through this study that the modulation is very effective in the suppression of particle growth. In particular, for a duty cycle of 20%, the discharge parameters are close to those for a pure He discharge. This tendency is consistent with the result that no particles can be observed for this duty cycle.

Effects of particles on He-SiH4 modulated RF discharges

The characteristics of emissive probes in unmagnetized high pressure (≤1 Torr) argon and helium plasmas, produced by inductively coupled 13.56 MHz rf power, are studied. A procedure is given for interpreting emissive probe current–voltage (I–V) characteristics. The I–V curves indicate the amplitude of the rf fluctuation of the plasma potential. They also show ionization near the emissive probe when the potential drop between the emissive probe and the plasma potential is more than the ionization potential. Experiments show that when the temperature of the emissive probe wire and/or the neutral pressure is increased, ionization becomes significant. An increase in the local ion density due to the additional ionization was demonstrated by the I–V curves of an emissive probe and a nearby Langmuir probe. A simple procedure is presented for interpreting these results.

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Use of emissive probes in high pressure plasma

Potential structures in asymmetrical radio frequency discharges containing negative ions

Potential structures are clarified experimentally with an emissive probe in silane‐helium radio‐frequency (rf) discharges containing particles. The results reveal, compared with a pure helium rf discharge, that both the crest and trough instantaneous plasma potentials in an rf oscillation shift toward the negative side, especially in the particle localized region near the plasma‐sheath boundary. The absolute value of the self‐bias potential of the powered electrode is reduced. A considerable electric field exists even in the bulk plasma region and the strength increases with the concentration ratio of silane to helium.

Potential structure in silane radio-frequency discharge containing particles

Advantages of implementing high magnetic field up to 10 T for a closed cycle disk MHD generator (thermal input of 1000 MW) are discussed, and technical feasibility of its superconducting magnet is studied. The present study suggests a high performance of the generator with high magnetic field. A possible construction of the magnet, in which a special structure is arrenged to sustain a large electromagnetic force induced between the coils, is shown.

High Field Superconducting Magnet for a Closed Cycle Disk MHD Generator

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Feasibility of a Disk Type Non-equilibrium MHD Generator with Subsonic Flow

In spite of many experimental studies of low-power applied-field magnetoplasmadynamic (AFMPD) thrusters, thrust efficiencies of the past thrusters are very low. Hence, drastic improvement in thrust performance is required for AF-MPD thrusters to compete against other types of electric propulsion in a moderate power regime around 10 kW. For the optimization of AF-MPD thrusters, a numerical code for the flowfield simulation is now under development. A preliminary result shows that the code can deal with a complicated mixture of the induced and applied magnetic fields, which will lead to a combination of the self-field, swirl, Hall, as well as electrothermal accelerations.

Yoshihiro Okuno

Papers

Potential structures in asymmetrical radio frequency discharges containing negative ions

Potential structure in silane radio-frequency discharge containing particles

High Field Superconducting Magnet for a Closed Cycle Disk MHD Generator

Feasibility of a Disk Type Non-equilibrium MHD Generator with Subsonic Flow

Numerical Study of Low-Power MPD Arcjet