Shigeru Yokota

Development of a two-dimensional dual pendulum thrust stand for Hall thrusters

A two-dimensional dual pendulum thrust stand was developed to measure thrust vectors [axial and horizontal (transverse) direction thrusts] of a Hall thruster. A thruster with a steering mechanism is mounted on the inner pendulum, and thrust is measured from the displacement between inner and outer pendulums, by which a thermal drift effect is canceled out. Two crossover knife-edges support each pendulum arm: one is set on the other at a right angle. They enable the pendulums to swing in two directions. Thrust calibration using a pulley and weight system showed that the measurement errors were less than 0.25 mN (1.4%) in the main thrust direction and 0.09 mN (1.4%) in its transverse direction. The thrust angle of the thrust vector was measured with the stand using the thruster. Consequently, a vector deviation from the main thrust direction of +/-2.3 degrees was measured with the error of +/-0.2 degrees under the typical operating conditions for the thruster.

Laser energy deposition effectiveness on shock-wave boundary-layer interactions over cylinder-flare combinations

The effects of repetitive laser-pulse energy depositions (5.5 mJ/pulse) onto a shock wave-boundary layer interaction region over cylinder-flare model in a Mach 1.92 flow are experimentally investigated. Depending on the nose shape and the flare angle, the flow patterns are subdivided to two; separated flow in which a slip line and a strong separation shock wave originated in the nose-cylinder junction appears, and a non-separated flow in which a slip line is not observed and the re-attachment shock wave is much weaker. At flare angles around 30°, the separation can be suppressed by laser energy deposition even of down to 5 kHz. The Schlieren-visualized flow patterns are well correlated to the drag characteristics, in which a larger drag is obtained without separation. A possible scenario of the separation control is that the disturbance introduced by the baroclinic vortex ring induced the boundary layer transition so that it became robust against the adverse pressure gradient. Under marginal conditions, dual mode flow patterns, that is, a partial and full suppression modes are obtained under the same operation conditions.

Hall Thruster Channel Wall Erosion Rate Measurement Method Using Multilayer Coating Chip

Lifetime problem is extremely important for a Hall thruster because most missions require more than 10,000 hr operation. Hall thruster’s primary lifetime-limit is caused by channel wall erosion. Since conventional erosion measurement methods require long time operation, it is important to develop a fast erosion measurement method to investigate erosion problem and to reduce erosion rate. An erosion rate measurement method using multilayer coating chips is developed for this purpose. This method accelerates the channel wall reduction rate measurement by using small chips with alternate very thin coating layers. The objective of this study is to demonstrate the usefulness of this method. The relationship between magnetic flux density and channel wall erosion rate was examined, and the result was compared with that of conventional emission spectroscopy. In addition, axial distribution measurement of channel wall reduction rate was also conducted.

Electrostatic acceleration of helicon plasma using a cusped magnetic field

The electrostatic acceleration of helicon plasma is investigated using an electrostatic potential exerted between the ring anode at the helicon source exit and an off-axis hollow cathode in the downstream region. In the downstream region, the magnetic field for the helicon source, which is generated by a solenoid coil, is modified using permanent magnets and a yoke, forming an almost magnetic field-free region surrounded by an annular cusp field. Using a retarding potential analyzer, two primary ion energy peaks, where the lower peak corresponds to the space potential and the higher one to the ion beam, are detected in the field-free region. Using argon as the working gas with a helicon power of 1.5 kW and a mass flow rate of 0.21 mg/s, the ion beam energy is on the order of the applied acceleration voltage. In particular, with an acceleration voltage lower than 150 V, the ion beam energy even exceeds the applied acceleration voltage by an amount on the order of the electron thermal energy at the exit of the helicon plasma source. The ion beam energy profile strongly depends on the helicon power and the applied acceleration voltage. Since by this method the whole working gas from the helicon plasma source can, in principle, be accelerated, this device can be applied as a noble electrostatic thruster for space propulsion.

Measurement of erosion rate by absorption spectroscopy in a Hall thruster

The erosion rate of a Hall thruster was estimated with the objective of building a real-time erosion rate monitoring system using a 1kW class anode layer type Hall thruster. This system aids the understanding of the tradeoff between lifetime and performance. To estimate the flux of the sputtered wall material, the number density of the sputtered iron was measured by laser absorption spectroscopy using an absorption line from ground atomic iron at 371.9935nm. An ultravioletAlxInyGa(1−x−y)N diode laser was used as the probe. The estimated number density of iron was 1.1×1016m−3, which is reasonable when compared with that measured by duration erosion tests. The relation between estimated erosion rate and magnetic flux density also agreed with that measured by duration erosion tests.

Plasma Density Fluctuation inside a Hollow Anode in an Anode-layer Hall Thruster

Electrical sheath structure and plasma dynamics inside a hollow anode are numerically computed using a fully kinetic 2D3V Particle-in-Cell (PIC) / Direct Simulation Monte Carlo (DSMC) code. By treating electrons and ions as particles, temporal and spatial variations of the non-neutral plasma near the anode surface were analyzed. The code was verified in terms of the neutral decay time and discharge current waveform. The simulation results show that in the stable region, which corresponds to the low magnetic flux density case, electrons penetrated deeply into the anode hollow. Then, an ion sheath is created on the anode surface due to the high ionization rate in the hollow. The propellant utilization by the anode exit is estimated at 45%. In the “ionization oscillation” region, which corresponds to the high magnetic flux density case, an ion sheath and electron sheath appeared alternately. An electron sheath was formed in the vicinity of the anode surface because of the lack of electron density for sustaining discharge.

Diagnosing on plasma plume from xenon Hall thruster with collisional-radiative model

The collisional-radiative model for xenon is used to calculate the electron density and temperature, and the atom population distribution in the plasma plume from a xenon Hall thruster. In the calculation, 173 levels of atom population are considered; only the processes of electron induced excitation and deexcitation, and spontaneous decay are simulated. The plasma plume is assumed to be optically thin. Consequently, the reasonable parameters of plasma plume along the outside center line of the thruster channel are obtained by making the calculated emission spectrum corresponding to measured ones and based on the atomic data available on site and by codes.

Diagnostics of hall thruster plume by laser absorption spectroscopy

Plume characteristics of a magnetic layer type Hall thruster were evaluated by laser absorption spectroscopy and single probe measurement. Translational temperature and total number density distributions of xenon atom were deduced using an absorption line of XeI 823.16 nm and electron temperature. As a result, the temperature was around 430±50 K in the almost all measured region, which agreed with the thermocouple measurement though it might be overestimated 850 K at the channel exit due to the Zeeman effect. The maximum total number density was 3.9x10 m at the channel exit. Then, the number density decreased by one order at 200 mm away from the exit.

Number Density Distributions of Xenon Atom in Hall Thruster Plumes

Laser absorption spectroscopy was applied to a magnetic layer type hall thruster plume in the different ambient pressure conditions. An influence of the ambient pressure on the number density measurement was evaluated. As a result, the measured number density was found to contain the background xenon. Then, the number density distribution of the propellant xenon was estimated separately from the background one.

Development of a Dual Pendulum Thrust Stand for Hall Thrusters

A 2D dual pendulum thrust stand was developed to measure thrust vector of a Hall thruster without thermal effect. A thruster is mounted on a pendulum and gap sensors are mounted on the other. By measuring thrust from the displacement between two pendulums, a thermal drift effect is canceled out. Two crossover knife-edges enable the pendulums to swing in both main thrust direction and its transversal direction. Thrust calibration using a pulley and weight system showed that the measurement errors were less than 0.25 mN (1.4%) in the main thrust direction and 0.09 mN (1.4%) in its transversal direction. The vector angle of a steering Hall thruster was measured with the stand. Thrust signal shows that the thermal effect is negligible on this stand. A vector angle of 2.3(±0.2) deg was measured.