Observation of Gentle Hump Structure on Energy Distribution Functions of End-Loss Ions in the rf-Driven Tandem Mirror Plasma

Abstract

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 Transport in Real and Velocity Space and Confinement in the GAMMA 10 Tandem Mirror.

“…From the measurement of the end-loss ions in the decay phase of the confining potential, we clearly found the boundary of the loss region created in the sustaining phase of the confining potential and also obtained information regarding the radial transport of the bounce ions. On the other hand, we observed the enhancement of ion transport to the loss region across the loss boundaries in the velocity space [6], which was caused by electromagnetic fluctuations of the Alfvén Ion Cyclotron (AIC) mode excited spontaneously in the central cell due to the anisotropic ion heating [7]. The AIC fluctuations were measured using a gold neutral beam probe, magnetic probes, and reflectometers, and closely correlated with the hump structure on the energy distribution function of the end-loss ions.…”

“…Most of the f ε (ε) curves showed the hump structure, and the background curve was estimated approximately as a summed line of two types of straight lines in the semi-logarithmic scale as shown in Fig.16. The hump structure on the f ε (ε) curves was explained successfully in consideration of the heating characteristic curves and the ion cyclotron resonance condition [6]. As the angles of ion scattering to the loss region are different in cases of Coulomb collision and particle-wave interaction, the hump structure was resolved in the velocity space by measuring the velocity distribution function f(v || , v ⊥ ) of the end-loss ions.…”

Study of Fluctuation Suppression during Potential Formation in the Tandem Mirror GAMMA 10

“…The reason for the low-frequency oscillation was not clear, but we believe that the 100-Hz oscillation may depend on the relaxation time of the fluctuations. The end-loss ion current measured by the end-loss energycomponent analyzer [12] installed in the end region was constant. This indicates that one source of the anomalous radial transport in the central region is the potential and density fluctuations and that the fluctuations decrease the plasma stored energy.…”

Transport of the ion bounced by the plug potential caused by radial electric field in the tandem mirror

“…As a subsequent subject of the improvement of the axial confinement due to the potential plugging, radial transport across the magnetic field, has recently attracted interest. The radial transport of the ion bounced across the magnetic field seems to be induced mainly at the anchor cell having a non-axisymmetric magnetic field configuration and at the plug/barrier cell with a potential hill and a potential dip, and the ion diffusion across the loss boundary seems to be enhanced by fluctuations excited in the plasma [4].…”