Fine structure of the electromagnetic fields formed by backward surface waves in an azimuthally symmetric surface wave-excited plasma source

Abstract

The electromagnetic fields and plasma parameters have been studied in an azimuthally symmetric surface wave-excited plasma (SWP) source, by using a two-dimensional numerical analysis based on the finite-difference time-domain (FDTD) approximation to Maxwell's equations self-consistently coupled with a fluid model for plasma evolution. The FDTD/fluid hybrid simulation was performed for different gas pressures in Ar and different microwave powers at 2.45 GHz, showing that the surface waves (SWs) occur along the plasma-dielectric interfaces to sustain overdense plasmas. The numerical results indicated that the electromagnetic SWs consist of two different waves, Wave-1 and Wave-2, having relatively shorter and longer wavelengths. The Wave-1 was seen to fade away with increasing pressure and increasing power, while the Wave-2 remained relatively unchanged over the range of pressure and power investigated. The numerical results revealed that the Wave-1 propagates as backward SWs whose phase velocity and group velocity point in the opposite directions. In contrast, the Wave-2 appeared to form standing waves, being ascribed to a superposition of forward SWs whose phase and group velocities point in the same direction. The fadeaway of the Wave-1 or backward SWs at increased pressures and increased powers was seen with the damping rate increasing in the axial direction, being related to the increased plasma electron densities. A comparison with the conventional FDTD simulation indicated that such fine structure of the electromagnetic fields of SWs is not observed in the FDTD simulation with spatially uniform and time-independent plasma distributions; thus, the FDTD/fluid hybrid model should be employed in simulating the electromagnetic fields and plasma parameters in SWPs with high accuracy.

A miniature electrothermal thruster using microwave-excited microplasmas: Thrust measurement and its comparison with numerical analysis

“…The microplasma is probably sustained by surface waves, demonstrated by a numerical model. 15,16,23 To estimate the gas temperature of the microplasma source, i.e., the reservoir temperature of the microthruster, a small amount of nitrogen is added to the argon plasma to observe the rotational spectra. Since electrons are inefficient in providing the rotational energy to molecules owing to their small mass, the population of rotational states is controlled by collisions with heavy particles.…”

“…[3][4][5][6][7][8][9] Assuming that plasmas are excited by surface waves, the major feature of such plasmas is that microwaves penetrate into the plasma chamber along the plasma-dielectric interfaces even in the overdense mode and the electron heating occurs in a thin ͑a few millimeters deep or less͒ skin-depth layer, that is, most of the power is absorbed near the interfaces. [10][11][12][13][14][15][16] Such mechanism of the power deposition is a great advantage to generate plasmas in a limited space without magnetic-field confinement. These features would contribute to a simple structure and long-time operation, compared with other conventional thrusters, because the present thruster requires no electrodes, neutralizers, or magnets.…”

“…The plasma source is composed of a cylindrical dielectric tube, covered with a grounded metal. Microwaves are injected into the plasma chamber through a coaxial cable and a metal rod antenna covered with a separate quartz tube, [13][14][15][16] where propellant gases, Ar in this study, are ionized and heated up in the pressure range from 10 to 100 kPa. Such high thermal energy is converted into directional kinetic energy through the micronozzle to produce the thrust required.…”

Microwave power coupling in a surface wave excited plasma

“…[1][2][3][4] Also peoples have used slot antennas for launching of microwave [4][5][6][7][8] for higher densities. In slot antenna configuration, plasma lengths are broadened with decrease in gas pressures.…”

“…9 In dielectric-bounded SWP sources, more than 200 W microwave power is injected. [1][2][3][4][5][6][7][8][9][10][11][12] In MVP sources, microwave plasmas are produced along a dielectric-free metal antenna and the surface waves propagate along the plasma-sheath interfaces. In 2005, Kousaka et al 13 at Nagoya University reported a new SWP source which can generate over-dense plasma along the surface of a negative biased metal rod, called metal-antenna surface wave excited plasma (MASWP).…”