西山 和孝

<p>Hayabusa2 is the second asteroid sample return mission by JAXA. The ion engine system (IES) for Hayabusa2 is based on that developed for Hayabusa with modifications necessary to improve durability, to increase thrust by 20%, and to reflect on lessons learned from Hayabusa mission. Hayabusa2 will rendezvous with a near-earth asteroid 1999 JU3 and will take samples from its surfaces. More scientific instruments than Hayabusa including an impactor to make a crater and landers will be on board thanks to the thrust enhancement of the IES. An improved neutralizer with stronger magnetic field for longer life has been under endurance test in diode mode since August 2012 and has accumulated the operational hours of 25600 h ( > mission requirement: 14000 h) by July 2015. The IES flight model was developed within 2.5 years. The spacecraft was launched from Tanegashima Space Center in Kagoshima Prefecture on-board an H-IIA launch vehicle on December 3, 2014. </p>

The Small Demonstration Satellite-4 (SDS-4) of JAXA launched on May 18, 2012 (JST) is equipped with a Japan's first quartz crystal microbalance (QCM) for spacecraft surface contamination monitoring. The QCM was installed on one of the satellite outer surface and occasionally observed gradual frequency decrease (=contamination) under the ground clean room environment for about a year. The QCM frequencies just before and after the launch by the H-IIA Launch Vehicle No. 21 (H-IIA F21) were almost the same, which indicated good cleanness inside the H-IIA's payload fairing. The frequency rapidly increased to the initial level during the first week after the launch probably due to removal or erosion of contaminants on the crystal surface by attack of atoms and ions in the orbit at an altitude of about 700 km. Contamination was never dominant during seventeen months of the space operation. Long term trend of the QCM frequency seems to be affected by the upper atmosphere density changing with the F10.7 solar radio flux.

A 20-cm diameter electron cyclotron resonance xenon ion thruster for space propulsion is under development that generates 500 mA of ion beam current at a microwave discharge power of 100 W. It does not have any moving mechanical parts for microwave impedance matching. Extracted ion currents and reflected microwave powers were experimentally investigated around a nominal frequency of 4.25 GHz for different flow rates. Optimized frequency tuning within 0.6% of the nominal frequency minimized the microwave reflection and maximized the ion current at each flow rate between 0.39 and 1.27 mg/s. However, constant frequency operation at 4.266 GHz is recognized as the best strategy because it provided with fare performance in wide range of flow rate and almost minimum reflection during tentative stop of beam extraction after high voltage breakdowns.

The &mu;10 cathode-less electron cyclotron resonance ion engines, have propelled the Hayabusa asteroid explorer for seven years since its launch in May 2003. The spacecraft was focused on demonstrating the technology needed for a sample return from an asteroid, using electric propulsion, optical navigation, material sampling in a zero gravity field, and direct re-entry from a heliocentric orbit. The final stage of the return cruise and the subsequent trajectory correction maneuvers have been accomplished by using a special combined operation of neutralizer A and ion source B after the exhaustion of the other neutralizers' lives by the autumn of 2009. The total duration of the powered spaceflight was 25,590 h, which provided a delta-V of approximately 2.2 km/s and a total impulse of 1 MN&middot;s. The degradation trends of the thruster performances have been investigated. It seems that the main cause of the degradation was the decrease in effective microwave power input to the discharge plasma induced by the increase in the transmission loss of the microwave feed system, and not due to the increase in the gas leakage through the accelerator grid apertures enlarged by erosion. Unintentional engine stop events have been summarized and analyzed. Most of them occurred due to the limit check errors of the backward microwave powers. Such errors can be decreased by carefully monitoring the trend change in microwave backward power as a function of xenon flow rate in future missions.

A completely new solar electric propulsion concept, the Air Breathing Ion Engine (ABIE), has been proposed for spacecraft drag makeup at very low altitudes, ranging from 150 to 200 km. ABIE scoops up neutral atoms and molecules traveling at an orbital velocity of approximately 8 km/s, ionizes them by means of an electron cyclotron resonance plasma source that is efficient in a wide range of low gas pressures, and accelerates the ionized air particles electrostatically to exhaust velocities larger than 100 km/s. The key technology of this thruster is the design of a propellant inlet which allows the incoming flow to enter the discharge chamber, yet it prevents the thermalized gas from escaping upstream. In this system, an air-breathing-type neutralizer may also be employed, in which case the need to carry on-board xenon propellant is eliminated and results in gains in payload mass if the mission duration is longer than 2 years. This technology should give researchers access to a part of the atmosphere that is currently very difficult to measure and is thus called the "ignorosphere." Promising applications other than aeronomy include science missions involving accurate gravity and magnetic field mapping, and high-resolution Earth surveillance.

Radiated electric field emissions from the prototype model of the Ion Engine System (IES) of the MUSES-C mission were measured in accordance to MIL-STD-461 E. The average noise level exceeded the narrowband specification at frequencies less than 5 MHz. The microwave discharge neutralizer generates a broadband noise and narrowband oscillations which have a fundamental frequency of about 160 kHz and are accompanied by its harmonics up to the 5th. The leakage of the 4.25 GHz microwave for plasma production and its second harmonic were 65 dB and 35 dB above specification, respectively. The X-band receiver onboard the MUSES-C measured the noise from the IES at the up-link frequency of 7.2 GHz through a horn antenna. This susceptibility test proved that the microwave discharge ion thruster will never interfere the deep space microwave communication.

Noise and oscillatory behavior of a plasma column produced in front of the microwave discharge neutralizer developed for MUSES-C mission were experimentally investigated. Radiated electric field emissions were measured following to MIL-STD-461 E. The average noise level exceeded the narrowband specification by 30 dB&mu;V/m at frequencies less than 5 MHz. Noise in electron emission current was also measured by using a current probe and a spectrum analyzer, and was compared with the noise of a hollow cathode. The microwave discharge neutralizer generates a broadband noise and oscillations which have a fundamental frequency of about 160 kHz and are accompanied by its harmonics up to the 5th. Considering the dependence on the diameter of the plasma column, they are probably the radial oscillation modes of ion acoustic waves. Although the hollow cathode shows nearly the same noise level at frequencies less than 1 MHz, intense oscillation exists in the 1-10 MHz range, which is generated by the keeper plasma.