Measurements of electron density and temperature in a miniature microwave discharge ion thruster using laser Thomson scattering technique

Abstract

In order to improve the thrust performance of a miniature microwave discharge ion thruster, the relationship between electron number density/temperature and operational conditions, mass flow rate, incident microwave power and magnetic field strength were measured by means of laser Thomson scattering. A photon counting method and a triple grating spectrometer were used against a small Thomson scattering signal and a strong stray laser light. Electron number density increased with incident microwave power and was saturated at critical incident microwave power; it was about 1.2×10 18 m-3 at incident microwave power > 8 W. In addition, electron number density increased with mass flow rate and became saturated; it was about 1.7×10 18 m-3 at mass flow rate > 0.04 mg/s. The electron number density gradually increased with an increase in the number of magnets, i.e., magnetic field strength. There was a sudden jump at thirteen magnets, although the thruster failed to ignite at fourteen magnets. This is because there is an optimum distance between the antenna and the electron cyclotron resonance layer. These results suggest that future improvement in thrust efficiency in miniature microwave discharge ion thrusters may come from the fine adjustment of the magnetic field configuration inside the discharge chamber.

Thomson Scattering Diagnostics in the Plasma of an Ion Thruster

“…This is because the chance of the ionization is increased with the number density of neutral atoms in the discharge chamber; the neutral atom number density is proportional to pressure inside the discharge chamber and this is proportional to {1-exp [α (1-η u ) m & ]}. The increase in N e at the vicinity of the screen grid would be due to the increase in N e in the discharge chamber; N e in the discharge chamber with increase in m & is due to the increase in pressure in the discharge chamber 17) as above mentioned. Therefore, the ion beam current increase with increase in m & 19) .…”

“…Figure 4 shows the relation between plasma parameters (electron number density and temperature) and incident microwave power at m & =9.3 μg/s and V b = 1200 V and V a = -200 V. The number density is linearly increased with incident microwave power; the electron temperature is almost constant. The increase in N e at the vicinity of the screen grid would be due to the increase in N e in the region between magnetic mirrors (between the front yoke and the central yoke), where ionization occurs 17) . On the other hand, electrons lose energy while diffusion from the ionization region to the measurement point, so the electron temperature at the measurement point remains almost constant.…”

Numerical Analysis of a Miniature Microwave-discharge Ion Thruster Using Water as the Propellant

“…It should be noted that the number of high-energy electrons in the ECR region tends to be overestimated and the resultant electron temperature also tends to be high because (electron-electron) Coulomb collisions are not taken into account, as described in Section 2.2 and in our previous paper. 7) If the effect of Coulomb collisions was considered, a somewhat lower electron temperature would be obtained owing to the relaxation of electron-electron collisions, as shown in Yamamoto et al 18) We also investigated the time-varying distribution of the electron density. The results in this section were averaged over 100 microwave cycles.…”

The Effect of the Discharge Chamber Structure on the Performance of a 5 Cm-Diameter Ecr Ion Thruster

“…A 3 cm-diameter ECR ion thruster was developed by the Kyushu University of Japan, whose microwave power, thrust, specific impulse and propellant utilization ratio were 8 W, 0.73 mN, 3060 s, and 67%, respectively [11]. Takao et al of the Nishinippon Institute of Technology tested a 5 cmdiameter ECRIT on the basis of the μ1 [19].…”