Yasushi Okawa

Three-Dimensional Divergence Characteristics of Ion Beamlets in an Ion Thruster

Three-dimensional divergence characteristics of ion beamlets in an ion thruster were investigated using numerical simulation. A three-dimensional particle-in-cell code was developed to explain the formation mechanism of non-circular grid hole erosion observed in an endurance test of a thruster. Calculation results demonstrated that the ion beamlets cross over the hole axis and possess a sub-hexagonal cross-section in the downstream of the crossover point under low perveance condition. This beamlet shape is formed because of the hexagonally symmetric hole arrangement of ion optics. The calculation results also showed that the calculated beamlet cross-sections were similar to the sub-hexagonal erosion patterns observed in the experiment. This agreement indicates that the non-circular erosion patterns were caused by the direct impingement of the diverged ion beamlets possessing the subhexagonal cross-section.

Experiments on ion beam deflection using ion optics with slit apertures

An experimental investigation on ion beam deflection by grid translation was performed. The ion beam deflection in ion optics is a desired technology for ion thrusters because thrust vector control utilizing this technique can eliminate the need for conventional gimbaling devices and thus reduce propulsion system mass. A grid translation mechanism consisting of a piezoelectric motor, a ceramic lever, and carbon-based grids with slit apertures was fabricated and high repeatability in beam deflection characteristics was obtained using this mechanism. Results showed that the beam deflection angle was proportional to the grid translation distance and independent of slit width and grid voltage. A numerical simulation successfully reproduced the beam deflection characteristics in a qualitative and quantitative sense. A maximum beam deflection angle of approximately plus or minus 6 degrees, which was comparable to that of the ordinary gimbaling devices used in space, was obtained without a severe drain current. Therefore, the beam deflection by grid translation is promising as a thrust vectoring method in ion thrusters.

Guided Modes in a Si-Quantum Wire and Their Control

The eigenmodes of an electron wave propagating through a Si-quantum wire are analyzed self-consistently by a finite-element method. The anisotropic effective mass tensor corresponding to the three inequivalent alignments of conduction band valleys in Si and the continuity of the probability current densities at the Si-SiO 2 interface are taken into account. As a result, the eigenenergies of the electron wave and the electron density distribution are found to be greatly affected by the electron-electron interaction. It is also demonstrated that the number of guided modes in the Si-quantum wire is controlled by the gate bias voltage.

Experiments to Find the Origins of Highly Diverged Ions

Where highly diverged ions, which might erode accelerator holes in noncircular shapes, in a beamlet from a multiple-hole-ion-extraction system originated from was investigated. Changes in beamlet profiles were observed putting obstacles at the centers of every three adjacent holes on the upstream side of a screen grid, and the obstacles would occupy the space where ions would pass through or be produced. In addition, a barrel electrode was embedded in an accelerator hole to facilitate the detection of ion impingement on the accelerator grid. The barrel current was found to be not proportional to the beamlet current even when no beamed-ion impingement was expected, though the gridlet total current, which included the barrel current, was proportional. Putting certain obstacles reduced the ion impingement on the barrel in a very-low-perveance region but increased it in a middlelow-perveance region. The origin of the highly diverged ions may exist over the upstream surface of the screen grid but how much such ions contribute to the accelerator-hole erosion is still unknown.

Numerical Study of Beam Extraction Phenomena in an Ion Thruster

Ion beam extraction phenomenon is one of the most important processes in an ion propulsion system because ion beam current and beam accelerating voltage dominate thrust. In order to evaluate the beam extraction capability, which is determined by the interrelation between discharge plasma properties and beam accelerating conditions, there is a need to understand the problems of ion sheath formation. This study has been performed to develop a numerical simulation code which is capable of investigating the ion beam extraction phenomena in the ion thruster. In the calculation, both ions and electrons were treated as particles in a particle-in-cell routine to determine the ion sheath self-consistently. The calculation results of electric potential contours and extracted currents were compared with the experimental ones and they were found to be in good agreement. In addition, it was confirmed that the ion beam extraction capability can be simulated in a wide range of beam accelerating voltage.

Guided modes in an arbitrarily oriented Si-quantum wire and their control

Abstract The eigenmodes of electron wave propagating through a Si-quantum wire are aanalyzed self-consistently by using a finite element method. The anisotropic effective mass tensor of the conduction band valleys in Si and the continuity of the probability current densities at the SiSiO 2 interface are duly taken into account. As a result, the eigenenergies of electron wave, the electron density distribution and their control by the gate voltage are presented.