The production of high density plasmas is one of the important issues in the tandem mirror experiments. In GAMMA10, plasmas are mainly produced with the ion-cyclotron range of frequency (ICRF) waves. In addition to the fast Alfvèn wave with a frequency near the fundamental ion-cyclotron frequency, the high harmonic fast waves (HHFW) have been used for the plasma production, and the density increase has been observed clearly [1]. The propagation and absorption of HHFW in the central cell of the GAMMA10 tandem mirror have been analyzed to investigate the mechanism of the plasma production.
In the previous experiments, the density saturation has been observed when ICRF sources with frequencies near the fundamental cyclotron frequency were used. Only the eigenmode with a fundamental radial structure is excited and the excitation will be strongly affected by the density so as to satisfy the boundary conditions in the axial direction. As the optimum density for the eigenmode formation exists, the density will be clamped at the density with which the eigenmode is strongly excited. On the other hand, several radial eigenmodes are excited simultaneously when HHFW is applied. Eigenmodes with different radial structures can be excited and the density clamping will be released.
To investigate the HHFW propagation and absorption in GAMMA10, the computer code based on the Finite Element Method was introduced [2]. The wave fields are calculated on the bounded, inhomogeneous, and axisymmetric plasmas in the mirror magnetic field. Eigenmodes of HHFW with different radial structures can be excited in the present experimental conditions. The amplitude of these eigenmodes varies according to the boundary conditions. As the density increases, a new eigenmode with higher radial mode number appears and its amplitude becomes larger. In the experiment, the radial mode transition from weakly excited mode to strongly excited one will be observed. It is suggested that the radial mode transition is essential for the density increase.

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