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.
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