Yasushi Ohkawa

Graphite Orificed Hollow Cathodes for Xenon Ion Thrusters

[Abstract] Two sizes of graphite orificed hollow ca thodes are described: one is for the main cathode of a 150-mN ion thruster and the other is for its neutralizer. One of the main cathodes is running to fail using a discharge chamb er, with which the discharge voltage, discharge current and propellant flow rate for the cathode can be simultaneously the same as those of a real thruster in nominal operation. T he cumulative operation time has reached 9,000 h and no signs of performance degradation have ever been found so far. Neutralizers have been operated as a component of a thruster and evaluated by coupling voltages, which have been kept low enough. Graphite orifice plates have shown high resistance to wear.

Thermally Stress-Free Grid Mounting with Hinges for Ion Thrusters

This paper describes a new grid mounting system using hinges for ion thrusters. Hinged grid spacers apply no stress to a grid concerning i ts thermal expansion and yield sufficient stiffness cooperatively with the grid against accel erations in any directions. The grids’ displacements are thermo-elastically analyzed by fi nite element method and the hinged mounting is compared with a conventional flexible o ne. Three-dimensional solid elements, which were found to provide more accurate axial displacements than plate and shell elements, are adopted in the analysis. The axial di splacements of the grids with the hinged mounting are generally larger than those with the f lexible one; however, the differential displacements between screen and accel grids can be smaller with the hinged one. An ion extraction system was assembled with hinges and tested.

Thermo-Elastic Analyses of Ion-Thruster Grids

This paper describes a method for estimating the temperature distributions over ion thruster’s grids and their thermal displacements. A thermal model in the method deals with heat transfer from discharge plasma to the grids and an elastic model provides their thermal displacements. Since the parameters needed for an analysis are currents, voltages and the screen-grid-edge temperature, this method is applicable not only to beam-off mode but also to beam-on mode. Temperature distributions and thermal displacements were estimated at four operating points of JAXA’s 35-cm ion thruster. Thermal input to the screen grid was much larger in beam-off modes than that in a beam-on mode and the thermal displacements of the screen grid were up to 2.2 mm. On the other hand, the displacements of the accel grid were up to 0.14 mm. Both grids would contact with each other in the beam-off modes if their separation were set to be appropriate in the beam-on mode. Nomenclature c A = area of cathode potential surface in discharge chamber excluding screen grid, m 2 dc A = area in discharge chamber, m 2 g A = area of grid in discharge chamber, m 2 1 C , 2 C = constant e = electronic charge, C a I = accel grid current, A b I = beam ion current, A d I = discharge current, A s I = screen grid current, A sat

An Experimental Study of Single Cathode Gridded Ion Thrusters

A Single Cathode gridded Ion Thruster (SCIT) has no cathodes in its discharge chamber and generates plasma using back-streamed electrons from a cathode outside the discharge chamber through ion accelerating grids. This paper describes a series of experiments that demonstrated the above principle. The experiments showed that discharge was stably sustained only when high propellant flow rate was given and propellant utilization efficiency was about several percent at most.

Current Statuses of Graphite Hollow Cathode Life Tests

The current statuses of life tests of graphite hollow cathodes at the Japan Aerospace Exploration Agency (JAXA) are described. In order to achieve long life with stable performance, the keeper disk, orifice plate, and cathode tube in these cathodes are made of high-density graphite. A life test of the discharge cathode was started in March 2006, and cumulative operation time reached 31,900 hours in June 2010. A life test of the neutralizer cathode was started in June 2008 and has accumulated 15,500 hours of operation. Microscopic observation indicated that the graphite electrodes survived 30,000-hour operation without severe erosion. The test results demonstrated that the graphite parts are effective in making hollow cathodes durable with less performance degradation.

Validation of an Ion-Thruster Grid Thermal Model with Experiments

Ion-thruster grid temperatures were measured in a beam-off condition to validate a grid thermal model, which calculated temperatures over the grids from voltages, currents and a temperature of each grid’s edge. The model was validated only by the data on the accel grid because those on the screen grid were found unreliable. Though parameters such as the emissivity of grids’ surface had been undetermined, the emissivity was obtained through the validation and the other undetermined parameters were found to have little effects on the results. The temperatures over the grids can be calculated using this model even in a beamon condition and the gap between the grids can be calculated from the temperatures by solving a thermo-elastic equation.