Wave propagation and power deposition in blue-core helicon plasma

Abstract

The wave propagation and power deposition inside and outside the blue-core helicon plasma are computed, together with their transitional behaviours prior to and after the blue-core formation. Computations refer to the experiments on the CSDX (controlled shear decorrelation experiment) (Thakur et al., Plasma Sources Science and Technology 23: 044,006, 2014 and Thakur et al., IEEE Transactions on Plasma Science 43: 2754–2759, 2015). It is found that the radial profile of wave electric field peaks off-axis during the blue-core formation, and the location of this peak is very close to that of particle transport barrier observed in experiment; the radial profile of wave magnetic field shows multiple radial modes inside the blue-core column, which is consistent with the experimental observation of coherent high m modes through Bessel function. The axial profiles of wave field indicate that the decay length shortens for increased external field strength, especially when the blue-core mode has been achieved, and this length is relatively longer inside the core than that outside. The wave energy density is overall lower in two orders after blue-core formation than that prior to, and the energy distribution shows a periodic boundary layer near the edge of blue-core column. The dispersion relation inside the blue-core column suggests the presence of two radial modes, while outside the blue-core column it shows no variation, i.e. constant wave number with changed frequency. The power deposition appears to be off-axis in the radial direction, forming a hollow profile, and when the blue-core mode has been formed it shows periodic structure in the axial direction. Analyses based on the step-like function theory and introduced blue-core constant provide consistent results and more physics understanding. These details of wave propagation and power deposition during the blue-core formation are presented for the first time, and helpful for understanding the mechanism of blue-core phenomenon. The equivalence of blue-core plasma column to optical fiber for electromagnetic communication is also explored, and preliminary calculation shows that total reflection can indeed occur if the incident angle is larger than a threshold value. This may inspire a novel application of helicon plasma, and is one of the most interesting findings of present work.

Wave field structure and power coupling features of blue-core helicon plasma driven by various antenna geometries and frequencies

“…This agrees with Fig. 6 and previous study [21] that certain level of plasma density is necessary for power coupling, i.e., for blue-core mode most plasma is shrunk onto axis which lowers the plasma density near edge.…”

“…The correlation between the peak wave field and blue-core edge is also consistent with previous study. [21] Notably, the streamlines are most dense for the half helix antenna in the blue-core region, indicating that the magnitude of wave magnetic field is strongest there. Figure 10 illustrates the axial distributions of wave magnetic field.…”

“…The hollow power distribution is consistent with previous study. [21] Figure 12 shows the axial variations of normalized relative power absorption. It can be seen that the Boswell and Nagoya III antennas exhibit the same variation of power absorption in the axial direction.…”

“…This claim agrees with previous study. [21] The next step is to investigate the correlation between total loading resistance and frequency in both non-blue-core and blue-core modes. This resistance directly reflects the coupling level between the driving antenna and the plasma column.…”

“…Lei Chang et al computed the wave propagation and power deposition inside and outside the blue core and their transitional behaviors before and after the blue core formation using the EMS code. [21] However, little attention was given to the blue-core wave structure and power absorption for different types of antennas. In this paper, the main innovation is to explore and compare the influences of various antenna geometries and driving frequencies on the blue-core helicon plasma evolution, i.e., in terms of wave field structure and power coupling features.…”