Kimiya Komurasaki

Two-dimensional numerical model of plasma flow in a Hall thruster

Two-dimensional numerical model of plasma flow in a Hall thruster has been made to estimate analytically the ion-loss flux to the walls of an acceleration channel, and to obtain information about desirable configurations for good thruster performance. The model presented herein is comprised of an electron diffusion equation and an ion kinetic equation, which enables one to compute electrostatic potential contours and ion-beam trajectories. In the first step ion-production distribution was assumed. From the results it was found that electric-field distortion, which is a main cause of ion-loss to the channel walls, is induced not only due to the curvature of magnetic field lines, but also due to the radial nonuniformity of ion-production distribution. In the second step, the ion-production distribution was self-consistently determined by combining an energy conservation equation with the previous two basic equations. The results indicate that the shape of ion-production distribution largely changes with the magnetic field geometry, and hence, the field geometry significantly influences the ion-loss flux to the channel walls. The computed ion-loss fraction (a fraction of ions produced that are lost to the walls) ranges from 0.30 to 0.55, and shows good agreement with the measured values. Therefore, this model should be an effective tool in both the design and improvement of Hall thrusters.

Plasma generation using high-power millimeter-wave beam and its application for thrust generation

Propagation of an ionization front in the beam channel was observed after plasma was generated using a 170GHz millimeter-wave beam in the atmosphere. The propagation velocity of the ionization front was found to be supersonic when the millimeter-wave power density was greater than 75kWcm−2. The momentum coupling coefficient Cm, a ratio of the propulsive impulse to the input energy, was measured using conical and cylindrical thruster models. A Cm value greater than 350NMW−1 was recorded when the ionization front propagated with supersonic velocity.

Development of thrust stand for low impulse measurement from microthrusters

A thrust stand has been developed to accurately measure thrust produced by two types of microthrusters—a liquid propellant pulsed plasma thruster (LP-PPT) and a diode laser ablation microthruster. The impulse of LP-PPT ranged from 20 to 80 μNs. The diode laser microthruster, which is a new type of microthruster, produces much lower impulse range of 1–10 μNs for about 1 s. The mechanical noise induced from the background vibrations becomes a crucial problem for precise estimate of thrust particularly in low impulse measurements. A data analysis method to reduce the effect of mechanical noise is proposed by introducing an additional term in a fitting function. It was verified that the analysis method used in our experimental conditions reduced variance caused by noise down to one-third that of a normal fitting method. The accuracy of the thrust stand is 2.1 μNs in the case of the LP-PPT and 0.7 μNs in the case of the diode laser microthruster.

Discharge Current Oscillation in Hall Thrusters

The discharge current oscillation at a frequency range of 10-100 kHz in Hall thrusters was investigated with the objective of extending their stable operational range. The amplitude of oscillation was measured using two types of Hall thrusters-the anode layer type and the magnetic layer type. The oscillation amplitude was found to be sensitive to the applied magnetic flux density, and this result indicated that the oscillation was affected by electron mobility. An oscillation model was proposed based on the experimental results, and the predicted frequency and stable operational range were found to agree qualitatively agreed with the experimental results. This model shows that the momentum transfer corresponding to plasma fluctuation, that is, the viscosity effect, is crucial to achieving stability. Thus, the oscillation amplitude for various acceleration channel configurations-divergent, parallel, and convergent-was measured because the momentum transfer could be affected by the channel configuration. The stable operational range was successfully extended by the adoption of the convergent configuration in each type of Hall thruster, as shown by this model.

Energy transfer from a laser pulse to a blast wave in reduced-pressure air atmospheres

Focusing a transversely excited atmospheric CO2 laser beam in air atmospheres induced a blast wave. The kinetic energy of a laser-induced blast wave was determined from shadowgraph images of shock wave expansion. Results showed that the fraction of input laser energy that is converted into the blast wave energy decreased from 0.45 to 0.2 concomitant with the decrease in ambient pressure from 100 to 10 kPa. Also, it was insensitive to input laser energy from 4 to 13 J.

Enthalpy Measurement in Inductively Heated Plasma Generator Flow by Laser Absorption Spectroscopy

Laser absorption spectroscopy was applied for diagnostics of inductively heated plasma generator flows. Temporal variation of translational temperature was deduced from measured absorption line broadening because the flow properties fluctuated at 300 Hz in synchronization with the induction current. The specific total enthalpy and mole fraction of oxygen were estimated from the deduced temperature assuming thermochemical equilibrium. Consequently, the averaged degree of dissociation of oxygen is 0.92. The specific total enthalpy was estimated at 33.7 ± 2.9 MJ/kg; 39% of it was in the form of chemical potential. The results show good agreement with intrusive measurements.

Influence of the focusing f number on the heating regime transition in laser absorption waves

Laser plasma was produced in air atmosphere using a transversely excited atmospheric CO2 pulse laser; the influence of the focusing f number (≡ focal length/laser beam diameter) on the threshold of a heating regime transition in laser absorption waves was then investigated. As a result, when the laser energy was 10 J/pulse, the transition threshold was 2.6±0.4 MW/cm2 with f=1.1 optics and 3.7±0.4 MW/cm2 with f=2.2. The difference was explained in terms of the front area and absorption layer thickness in the laser supported detonation regime.

Plasma acceleration processes in an ablative pulsed plasma thruster

Plasma acceleration processes in an ablative pulsed plasma thruster (APPT) were investigated. APPTs are space propulsion options suitable for microspacecraft, and have recently attracted much attention because of their low electric power requirements and simple, compact propellant system. The plasma acceleration mechanism, however, has not been well understood. In the present work, emission spectroscopy, high speed photography, and magnetic field measurements are conducted inside the electrode channel of an APPT with rectangular geometry. The successive images of neutral particles and ions give us a comprehensive understanding of their behavior under electromagnetic acceleration. The magnetic field profile clarifies the location where the electromagnetic force takes effect. As a result, it is shown that high density, ablated neutral gas stays near the propellant surface, and only a fraction of the neutrals is converted into plasma and electromagnetically accelerated, leaving the residual neutrals behind.

Diode-laser tomography for arcjet plume reconstruction

Diode-laser absorption tomography is described with which the spatial temperature and the atomic number density distribution of a 3-kW class arcjet can be derived simultaneously by reconstruction of the absorption coefficient field of the arcjets argon exhaust plume. One can perform various parameter measurements by changing the arcjets mass-flow rates and discharge currents. The maximum temperature and atomic number density increase with the mass-flow rate and the discharge current. The trend for increase is not always found for a specific input power, although at a fixed mass-flow rate the power increases at that rate.

Thrust Performance of Microwave Rocket Under Repetitive-Pulse Operation

An experiment was conducted on a microwave rocket (repetitive-pulse millimeter-wave-beam-powered thruster) with repetitive pulses. A thruster model with a forced-breathing system was used. The forced-breathing system supplies fresh air from the thrust wall into the thruster. The pressure histories in the thruster weremeasured and the propagation velocity of the shock wave and the thrust impulse were deduced. Results show that although the propagation velocity was identical to the result for the single-pulse operation at the first pulse, the propagation velocity increased after the second pulse. Similarly, the impulse decreased after the second pulse. The dependence of the propagation velocity of the shock wave and the thrust performance on the partial-filling rate of the fresh air was comparedwith that of the thrust-generationmodel with a forced-breathing system. The experimental results showed good agreement with those obtained using the model.