Effect of High Ion Temperature on the Polytropic Coefficient in the End Region of GAMMA 10/PDX

Abstract

The effect of high ion temperature on the ion polytropic coefficient (γ) was experimentally investigated in the end region of the GAMMA 10/PDX tandem mirror. The ion temperature parallel to the magnetic field (T i||) was varied using the ion cyclotron range of frequencies (ICRF) waves, which heat the plasma in the central cell, and by applying an additional gas puff in the central cell. Assuming the ion sound speed, γT i|| was evaluated by a Langmuir probe, and T i|| was evaluated in an ion energy analyzer. These parameters were combined to evaluate γ. The time evolutions of γT i|| obtained from the probe and T i|| obtained from the energy analyzer were similar. The plasma in the end region can be regarded as collisionless, but γ was lower with ICRF heating than without the heating. A high T i|| component was identified in the heated case. As mentioned in a previous numerical study [B. Lin et al., Phys. Plasmas 23, 083508 (2016)], it is experimentally suggested that γ can be reduced by the high T i|| component resulting from the ICRF heating.

Analysis of the Contribution of Magnetic Moment Conservation to Ion Energy Transport in the GAMMA 10/PDX Divergent Field Region

“…In the present experiments, plasmas with different DM are generated using the ICRF heating system, in a manner similar to that in Ref. [15]. Figure 3…”

“…In this study, ion heating is performed similarly to that described in Ref. [15], where two different ICRF antennas (RF1 and RF2) were used. RF2 resonates in the CC and RF1 resonates in the anchor cells.…”

“…We developed a retarding field analyzer (RFA) to investigate the contribution of magnetic moment conservation to ion energy transport in inhomogeneous magnetic field systems. In this study, we inserted it upstream, comparably strong magnetic field position (∼ 1.2 T) within one of the end divergent field regions of a tandem mirror GAMMA 10/PDX [11][12][13][14][15][16]. In GAMMA 10/PDX, arrays of end-loss ion energy analyzers (ELIEAs) at both end plates have already been installed [11,14,15].…”

“…In this study, we inserted it upstream, comparably strong magnetic field position (∼ 1.2 T) within one of the end divergent field regions of a tandem mirror GAMMA 10/PDX [11][12][13][14][15][16]. In GAMMA 10/PDX, arrays of end-loss ion energy analyzers (ELIEAs) at both end plates have already been installed [11,14,15]. Therefore, a diagnostic condition similar to that described in Ref.…”

“…The material and mesh size of the grids are Mo and 150 mesh/inch, respectively. Note that the plasma-facing plate and cover of the RFA are in floating potential, the absolute value of which is assumed to be one order of magnitude smaller than that of the entrance grid potential [15]. The voltage-current (VI) characteristics are obtained by sweeping the ion repeller voltage at 160 Hz.…”