Is the bulk mode conversion important in high density helicon plasma?

Abstract

In a high-density helicon plasma production process, a contribution of Trivelpiece-Gould (TG) wave for surface power deposition is widely accepted. The TG wave can be excited either due to an abrupt density gradient near the plasma edge (surface conversion) or due to linear mode conversion from the helicon wave in a density gradient in the bulk region (bulk mode conversion). By numerically solving the boundary value problem of linear coupling between the helicon and the TG waves in a background with density gradient, we show that the efficiency of the bulk mode conversion strongly depends on the dissipation included in the plasma, and the bulk mode conversion is important when the dissipation is small. Also, by performing FDTD simulation, we show the time evolution of energy flux associated with the helicon and the TG waves.

Review of Helicon High-Density Plasma: Production Mechanism and Plasma/Wave Characteristics

“…conditions [1,44,45], the damping length of the TG wave is much shorter than for the plasma column k ⊥+,imag r 0 1, where k ⊥+,imag is the imaginary part of the k ⊥+ , and r 0 is the plasma radius, while that of the helicon wave is longer than the plasma column k ⊥+,imag r 0 < 1 except for in the highdensity region (n ∼ n up ), where the damping length of the helicon wave becomes comparable to that of the TG wave. Therefore, the collisional damping of the TG wave plays a dominant role on the surface power absorption while the helicon wave is important for power absorption in the central high-density region [9,12,13,46].…”

“…The helicon wave penetrates into the high-density region due to its weak damping rate, and when the helicon wave reaches n = n up , it is reflected and naturally transformed to the backward propagating TG wave from this merging point. However, when a finite collision is introduced, the efficiency of the bulk mode conversion becomes strongly dependent on the magnitude of the collision frequency [13].…”

“…Based on the dispersion relation, Kim and Hwang [94] pointed out that for bulk mode conversion, the collisional dissipation near the MCS plays an important role. Isayama et al [13] report the quantitative dependence of the bulk mode conversion efficiency on the collision frequency. Figures 4 (a) and (b) show the spatial variations of the real wavenumber k ⊥real and the normalized imaginary wavenumber k ⊥imag x 0 (where x 0 = 7.5 cm is the plasma size) of the helicon wave (red line) and the TG wave (blue line), respectively.…”

“…One part of the efficient power absorption can be explained by the strong collisional damping of the quasi-electrostatic wave or so called "Trivelpiece-Gould" (TG) wave [8]. The TG wave is excited dominantly near the radio frequency (RF) antenna and deposits its energy near the plasma surface [9][10][11], while the helicon wave penetrates into the core region and deposits its energy via collisions in the core plasma [9,12,13]. The existence of the TG wave is experimentally author's e-mail: isayamashogo@gmail.com verified [14], and the dispersion relation and the analysis of the power balance explain, at least qualitatively, some distinct features observed in the helicon discharge.…”

“…a) Real and (b) normalized imaginary parts of transverse wavenumbers of the helicon (red) and TG waves (blue) with ν/ω (ω/2π = 7 MHz) = 0 (solid), 0.01 (dashed), and 0.08 (dotted). (c) The dependence of bulk mode conversion efficiency Λ on the collision frequency ν/ω[13].…”