In Japan, the thruster with anode layer (TAL) has been investigated for a long time. RAIJIN94 thruster has been developed based on the acquired knowledge of TALs. This thruster has been designed by Kyushu University and University of Tokyo, using many Japan-developed elemental technologires. RAIJIN94 is operated at 5.0 kW, and achieved the maximum thrust efficiency of 0.64 and maximum specific impulse of 2,200 seconds. This thrust performance is competitive with the stationary plasma thrusters (SPTs) of the same power level developed in foreign companies. Further design optimization is persuit including the remaining issue of thruster lifetime. It is expected that a competitive TAL will be developed as the main propulsion for next-generation space missions and staellites systems.

In the University of Tokyo, we designed and fabricated the RAIJIN66, which has a 70% outer diameter of the RAJIN94. RAIJIN66 has the scaled discharge channel and magnetic field geometries similar to those of RAIJIN94. A scaling method to maintain the thruster characteristics of TAL is persuit. The technology called magnetic shielding is being applied to TAL for better thruster lifetime. We also perform internal plasma measurements as well as thruster testing. We are conducting our researches by associating thruster performance and plasma physics.

Basically, Hall thrusters have an axisymmetric geometry, but there are plasma oscillation phenomena propagating in the azimuthal direction. These plasma oscillations have been concerned as they would be related to the cross-field axial electron transports. The basic principle of Hall thruster is to induce the azimuthal Hall current by the ExB drift of axial electric field and radial magnetic field. Based on the same physics, if an azimuthal electric field is generated by the azimuthal plasma oscillations, an electron flow is induced in the axial direction (toward the anode). The induced electron current in the axial direction results in the increased power consumption. How do the azimuthal plasna oscillations evolve, and how do the azimuthal electric fields affect axial electron transport? Elucidation of these mechanism is necessary for better understanding and performance improvement of Hall thrusters

We investigate azimuthal plasma physics and cross-field electron transport from both the numerical simulation and experiment. Numerical and linear stability analyses are used on how azimuthal plasma oscillations are induced. Regarding the effect of azimuthal plasma distribution on axial electron transport, we are doing a unique experiment. Azimuthal inhomogeneity in plasma property is artificially formed by applying azimuthally nonuniform propellant supply or magnetic flux density. The effects of these artificial nonuniformities on azimuthal plasma property and thruster performance are measured.

Hall thruster uses a magnetic field for trapping electron motions, and this magnetic field has a big influence on the thruster performance. The trapped electrons cannot travel across magnetic field, whereas they can freely move along magnetic lines. Owing to this characteristic, the plasma potential is formed to make the equipotential lines close to magnetic lines, and the electric field directs in the perpendicular of magnetic lines. To prevent ion losses to the channel walls, a lens-shape magnetic field has been employed for Hall thrusters.

A technology called "magnetic shielding" has been proposed to use this characteristic effectively. By aligning the magnetic lines with the discharge channel walls, the number of ions colliding with channels walls are drastically decreased. The channel wall erosion due to ion bombardment is one of the main factors limiting the Hall thruster lifetime. The lifetime of Hall thruster would be significantly improved by using the magnetic shielding

We conduct a research to apply the magnetic shielding to TALs. TAL has a characteristic that the ion flux flowing to metallic channel walls is measured as current. We are investigating magnetic field and channel configurations that minimize the wall ion fluxes.

Modeling technilogies of Hall thruster is being matured, and the Computer Aided Engineering (CAE) is applied in the thruster design processes. Especially, the numerical modeling of plasma flow involves various plasma physics, and the development of plasma simulation code is competitvely conducted around the world to achieve accurate performance prediction.

In addition to a standard axisymmetric two-dimensional model for Hall thruster perfromance prediciton, We develop axial-azimuthal two-dimensional model for analyzing azimuthal plasma oscillations and their effects on cross-field electron transport. The plasma flow is modeled by the hybrid model where the ions and neutral atoms are handled by particle method and the electrons are approximated as fluid. By coupling these two-dimensional models, we are trying to achieve a self-consistent simulation of the Hall thruster discharge.

A large vaccum chamber of 2 m diameter and 3 m length is available.

Rotary pump : ULVAC　PKS-070, 7,000 l/min × 2

Mechanial booster pump : ULVAC　PMB-060B, 103,300 l/min

Oil diffusion pump : ULVAC　PFL-36, 37,000 l/s

Cryopump : ULVAC　CRYO-U20H, 10,000 l/s (Ns)

Also, this vacuum facility is equiped with shroud panels cooled by liquid nitrogen for space thermal environment.

A thrust stand measures the thrust generated by plasma thrusters. We developed a dual-pendulum thrust stand which has merits as follows:

• Thermal Drift Free: A normal thrust stand has an issue called the thermal drift. The heat from thruster causes a thermal deformation on the stand, generating an error in the thrust measurement. The dual-pendulum thrust stand cancels the effects of thermal deformation between the double pendulums, and the errors from thermal deformation are minimized.

• Real-time monitoring: Zero-displacement control is applied to maintain the relative positions of two pendulums by using a electromagnetic actuator. The response time for the control is within 10 seconds, and the thrust can be measured quickly for various conditions.

• Portability: The stand system is independent of the vacuum facility, and it can be transported to other vacuum chambers. In fact, we have used this stand for the Hall thruster permance testing in ISAS/JAXA.

We conduct plasma diagnostics of the Hall thruster discharge plasma inside the discharge channel and in the plume. The obtained data are utilized for thruster design optimization and validation of plasma models.

• Faraday Probe : Faraday probe measures the ion beam exhausted from the thruster. The probe is mounted on a movable stage, and the spatial distribution of the ion beam current is obtained by sweeping the stage.

• Single Probe : A single probe measures the plasma properties inside or outside of the thruster. A V-I characteristics is obtained by sweeping the voltage of probe surface, and the electron density and electron temperature can be measured.

• Emissive Probe : An emissive probe is used for measuring the plasma potential around the thruster. This probe is heated by the applied current. The emissive probe emits thermal electrons, and the floating potential of the probe reaches the plasma potential. Owing to this characteristics, the plasma potential can be measured instantaneously, and the space potential distribution can be obtained in high space resolution.