Helicon plasma is one of the radio frequency plasma source that can generate high-density and lowtemperature plasmas by utilizing the helicon wave, i.e., the electromagnetic whistler wave in a bounded geometry. Helicon plasma is very useful for various applications due to its extremely efficient production of high-density plasma. Conversely, many unsolved physical issues remain regarding how an efficient production of the helicon plasma is realized in laboratories and what determines the density maximum. The past decades of the helicon studies have revealed that the "Trivelpiece-Gould" wave is responsible for the efficient power absorption. In recent years, the drift-wave type and the parametric-decay instabilities have been extensively studied, by using the linear magnetized helicon plasma sources. The present helicon study considers these non-linear effects. Consequently, it includes many interests for both industry and research fields. The mechanism of the helicon production is discussed, based on the several critical physical issues. In addition, some recent topics and the efforts to build a more refined physical model of the helicon plasma are highlighted.

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.

Abrupt jumps in the density of helicon discharge have been observed following continuous variation of parameters such as the external magnetic field and the radio frequency (RF) input power. In this study, we show the spatio-temporal behavior of such density jumps and the mode transition in a helicon plasma. It is found that the density jump process is characterized by two temporal phases with the contribution of higher axial modes, changing the antenna current (and thus the RF power). We also investigated the effect of the neutral depletion on the local plasma equilibrium. The temporal fluctuation of the plasma density caused by the neutral depletion was observed.

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The formation mechanism of the density profile of helicon discharge, which has been a dispute for a long time, is investigated by using a careful self-consistent model. A detailed investigation of the local balance between the source and the loss fluxes reveals how the centrally peaked density profile is generated, despite the strong surface power absorption by the mode-converted Trivelpiece-Gould (TG) wave from the helicon wave, without any assumption of anomalous diffusion. Our results suggest that the flux transport toward the wall balances out the surface source flux by the TG wave, while the plasma core grows by the power of helicon wave deposition, resulting in the centrally peaked density profile. It is also found that the density profile can be controlled successfully to produce centrally peaked, flat, or hollow profiles by adjusting the contribution of the higher axial mode number of the TG wave.