Low Frequency Plasma Wave Dispersion and ... - Semantic Scholar

Low Frequency Plasma Wave Dispersion and Propagation in Hall Thrusters*† Nicolas Gascon, Nathan B. Meezan and Mark A. Cappelli Thermosciences Division, Mechanical Engineering Department Stanford University Stanford, CA 94305-3032 650-723-1745 [email protected] IEPC-01-56 Low-frequency oscillations ( 0), with a mean value of kθ ≈ 5 m-1. This value is consistent with an m = 1 (kθ = 3 m-1) or m = 2 (kθ = 6 m-1) azimuthal mode, perhaps attributable to an ionization instability driven by axial gradients in plasma density. Further upstream, this mode drifts closer towards the axis (kθ ≈ 0 at z = 0), and then, by close examination of the 83V panels in Figure 11, seems to have drifted towards negative values of kθ, with a centroid of about kθ ≈ -5 m-1 at z = -20 mm (2 cm upstream of the exit plane). In almost all cases, this disturbance appears as a resonant azimuthal mode (locus is untilted in the dispersion map), although the precise mode number is difficult to define. An examination of the wavenumber maps shown in Figure 12 also shows that the azimuthal wavenumber passes from positive to negative values, while the axial component wavenumber is close to zero (within error) with the exception of the region in the vicinity of the exit plane (z = 0 mm), where kz ≈ 10 – 20 m-1. At 83V, the higher

We now move to discuss the results for the slightly higher discharge voltage, 110V (in the negative resistance region of the I-V characteristic). At this voltage, there is clear evidence of the existence of the breathing mode at nearly all axial positions. Also, an examination of the dispersion maps of Figures 10 and 11 shows that downstream of the position z = -10 mm, where the gradient in the inhomogeneity parameter is negative, i.e., where:

Br ∂  ne   