A gentle hump structure on energy distribution function of end-loss ions was observed in the rf-driven tandem mirror plasma by use of an end-loss energy component analyzer. Alfvén ion-cyclotron (AIC) fluctuations are excited spontaneously due to anisotropic ion heating in the central cell of the tandem mirror. We observed that the excitation of the AIC waves caused the hump structure on the energy distribution function. The enhancement factor of the end-loss ions is estimated, and the gentle hump structure is discussed from the viewpoint of the heating characteristics of the AIC waves. It is pointed out that the hump structure decreases the ion confinement. PACS numbers: 52.55.Jd, 52.25.Fi, 52.70.Nc In open systems, loss regions exist necessarily in the velocity space of the plasma. The plasma scattered from the trapped region to the loss region flows to the end walls along the magnetic field line. In order to improve the plasma confinement in the tandem mirror, the hyperboloidal loss boundaries in the velocity space are shifted in the direction of higher energy along a velocity component axis parallel to the magnetic field by creating electrostatic potentials on both sides of the plasma [1,2]. Effective plasma heating is also essential to achieve controlled thermonuclear fusion. Usually ions are heated anisotropically with respect to the direction of the magnetic field line, and various fluctuations are excited due to the anisotropic temperature. In the tandem mirror GAMMA 10 [3,4] ion-cyclotron-range-of-frequency (ICRF) waves are injected into the target plasma in the central cell. Ions are heated perpendicularly by the excited Alfvén slow waves [5]. At the same time, the Alfvén ion-cyclotron (AIC) fluctuations are also excited spontaneously in the central cell by reason of the anisotropic ion temperature [6]. The electromagnetic fluctuations cause diffusion of ions in the velocity space [7], therefore enhancement of the ion transport across the loss boundaries in the velocity space by carrying out the ion heating should be investigated on the same level as the effect of the electrostatic potential on the axial confinement in the tandem mirror. Here, we notice the hump structure on the energy distribution function of the end-loss ions in the rf-driven tandem mirror plasma and focus our discussion to the ion transport from the trapped region to the loss region caused by the AIC fluctuations. The energy and velocity distribution functions of the end-loss ions are measured by the end-loss energy component analyzer (ELECA) [8], and the AIC fluctuations are measured by reflectometers and magnetic probes. In this Letter, the energy flux is described briefly, next the observation of the hump structure is mentioned, and finally the mechanism of the hump structure and the influence of the hump on the ion confinement are discussed.The tandem mirror GAMMA 10 consists of many mirror cells including a central cell, two anchor, and two plug/barrier cells. Figure 1 shows an axial profile of the magnetic field strength toge...

Ion distributions in a loss cone are measured in the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985)]. When Alfvén ion cyclotron (AIC) modes are excited in the central cell, enhanced end-loss of ions is observed for a characteristic energy region which agrees well with the resonant ion cyclotron condition with the AIC wave. From measurements of pitch angle distributions of end-loss ions, it is found that the loss cone is filled for this characteristic energy region, indicating an ion diffusion in a velocity space induced by the AIC wave. Magnitude of the observed additional end-loss increases with increase in the amplitude of the AIC modes. In the present experimental parameters in GAMMA 10, the AIC wave effect on the axial ion confinement is not serious.

A gold neutral beam probe system was improved by adopting a multichannel ion detector and adding sweeping functions of beam energy, deflector voltage, and electrode voltage of the analyzer to the system to measure the fluctuations and time evolution of the two-dimensional space potentials with fast resolving time during one shot. Positions of the beam spot on the multichannel detector corresponding to the ionizing points of the beam in the plasma were simulated precisely as a function of the beam energy and the injection angle. A potential derivation formula was determined taking into consideration both the numerical and experimental results, and the reproducibility of the potential profile was checked. Two-dimensional potential profiles were measured in the optimization experiment of the microwave injection angles for formation of the plug potential.

A microchannel plate (MCP) is used as an ion detector in the end-loss energy component analyzer, which has been developed in order to measure velocity distribution functions of end-loss ions in the tandem mirror GAMMA10. The gain of the MCP depends on variable parameters of the incident ion energy, the incident ion current density, and the MCP bias voltage. The gain characteristics was obtained using a hydrogen beam test stand, and the gain curves were described as a function of the parameters for the purpose of the quantitative evaluation of end-loss ions.

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