Plume Characteristics of a Quasi-Steady Magnetoplasmadynamic Arcjet

The plasma plume of a 1-MW-class quasi-steady magnetoplasmadynamic (MPD) arcjet is studied to determine the plume structure and plasma plume fluctuations in a downstream plume region (250-1250 mm away from the MPD arcjet). By using a double probe and a high-electron-temperature (similar to 5 eV) and high-number-density (similar to 8 x 10(19) m(-3)) region, so-called "cathode jets" are found along the central axis of plasma plume close to the MPD arcjet. Moreover, the plasma plume radial profile is constant in a downstream plume region (>= 750 mm from the MPD arcjet). The power spectrum density (PSD) of the ion saturation current is proportional to 1/f(1.6) (f: frequency) for f > 80 kHz, and in this frequency range, PSD decreased with distance from the MPD arcjet. In contrast, in the frequency region < 80 kHz, the PSD-f curve is at a constant value. Although a peak in the discharge voltage is attributed to the generalized lower hybrid drift instability (GLHDI), this instability is not found in the plasma plume near the MPD arcjet. The influence of the GLHDI is limited only to the discharge chamber, and fluctuations caused by the instability are random in the downstream region.