M. Takechi

Energetic ion driven MHD instabilities observed in the heliotron/torsatron devices Compact Helical System and Large Helical Device

Recent results of energetic ion driven MHD instabilities observed in the heliotron/torsatron devices Compact Helical System (CHS) and Large Helical Device (LHD) are presented. Alfven eigenmodes (AEs) and fishbone-like burst modes (FBs) destabilized by energetic ions were observed in NBI heated plasmas of CHS. The AEs are toroidicity induced Alfven eigenmodes (TAEs) and global Alfven eigenmodes (GAEs), where the identified toroidal mode numbers are n = 1 and 2 for TAEs and n = 0 for GAEs. The frequencies of the FBs are less than, at most, half of the minimum TAE gap frequency and do not exhibit the obvious density dependence related to Alfven velocity. The modes have characteristic features of the energetic particle modes or the resonant TAEs excited by circulating energetic beam ions produced by NBI. Bursting amplitude modulation is observed in TAEs as well as in FBs. Rapid frequency chirping is observed in each burst, by a factor of 2-6 in FBs and about 25% in TAEs. In several shots, the power spectrum of the TAEs is split into multiple peaks having the same toroidal mode number through non-linear evolution of TAEs. A pulsed increase in energetic ion loss towards the wall is induced by m = 3/n = 2 FBs, but so far not by m = 2/n = 1 FBs, TAEs and GAEs, where m is the poloidal mode number. This research has been extended to LHD plasmas heated by neutral hydrogen beams with about 130xa0keV energy. Similar to CHS, TAEs and FBs were observed in relatively low density plasmas at low toroidal magnetic field (Bt = 1.5xa0T).

Confinement physics study in a small low aspect ratio helical device: CHS

Variation of the plasma position relative to the centre of the helical coil winding is a very effective means of controlling the MHD stability and the trapped particle confinement in heliotron/torsatron systems, but improving one of these two characteristics with this parameter simultaneously has a detrimental effect on the other. The inward shifted configuration is favourable for drift orbit optimization but is predicted to be unstable according to the Mercier criterion. Various physics problems, such as electric field structure, plasma rotation and MHD phenomena, have been studied in the Compact Helical System (CHS) with a compromise intermediate position. With this standard configuration, CHS has yielded experimental results that contribute to the understanding of general toroidal confinement physics and low aspect ratio helical systems. In the recent experiments, it was found that a wide range of inward shifted configurations give stable plasma discharges without any restriction to the special pressure profile. Such an enhanced range of operation made it possible to study experimentally the drift orbit optimized configuration in heliotron/torsatron systems. The effect of configuration improvement was studied with plasmas in a low collisionality regime.

Observation of MHD induced fast ion losses on the CHS heliotron/torsatron

MHD induced beam ion losses were observed on the CHS heliotron/torsatron with an energy and pitch angle resolved scintillator detector. Beam ions escaping into the probe were characterized using orbit calculation codes. It was found that two types of ions, namely `passing boundary ions and `trapped ions, were lost periodically during an m/n = 3/2 fishbone-like instability. There are thresholds in the mode amplitude for the ejection of fast ions. Above this threshold, losses are enhanced together with an increase of the mode amplitude. The excitation of the fishbone-like instability and the enhancement of MHD induced losses strongly depend on the degree of accumulation of beam ions in the plasma. Orbit calculations show that passing boundary ions, which have large pitch angle and v close to zero (or close to the propagating velocity of the magnetic fluctuation) near the q = 3/2 surface, are ejected by the instability.

Overview of CHS experiments4

Recent experimental results are summarized. One of them is the bifurcation phenomenon of the electric potential which manifests itself in various aspects of CHS plasmas. Here, electrons heated with ECH play an essential role. The CHS will be operated, after being moved to the Toki site, so as to perform complementary experiments to LHD.

Transition from L mode to high ion temperature mode in CHS heliotron/torsatron plasmas

A high ion temperature (Ti) mode is observed for neutral beam heated plasmas in the Compact Helical System (CHS) heliotron/torsatron. The high Ti mode plasma is characterized by a high central ion temperature, Ti(0), and is associated with a peaked electron density profile produced by neutral beam fuelling with low wall recycling. Transition from L mode to high Ti mode has been studied in CHS. Ti(0) in the high Ti mode discharges reaches 1 keV, which is 2.5 times higher than that in the L mode discharges. The ion thermal diffusivity is significantly reduced by a factor of more than 2-3 in the high Ti mode plasma. The ion loss cone is observed in neutral particle flux in the energy range 1-6 keV with a narrow range of pitch angle (90° ±20°) in the high Ti mode. However, the degradation of ion energy confinement due to this loss cone is negligible.

The effect of divertor tile material on radiation profiles in LHD

In the large helical device (LHD) radiation profiles measured using arrays of resistive metal, foil bolometers are used to investigate the change in radiation resulting from a replacement of stainless steel divertor plates with graphite tiles between the second and third experimental campaigns. In particular, for the magnetic configuration of R axis = 3.6 m and at line averaged densities below 3 x 10 19 m 3 , a reduction of the radiated power fraction from 35% in the second campaign to down to 12% in third campaign was observed. Comparing similar shots (in terms of discharge parameters) from the second and third cycles the core (0 < r/a < 0.79) radiation fraction was reduced from 44% to 30% of the total radiation. Spectroscopic measurements show a corresponding decrease in radiation from iron. Comparing long pulse discharges the radiated power fraction reduced from greater than 67% during the density limiting oscillation observed in the second campaign to 20% during the third campaign with a reduction in the respective core radiation fractions from 63% to 37%.

Global MHD modes excited by energetic ions in heliotron/torsatron plasmas

Fishbone-like burst modes (FBs) and toroidal Alfven eigenmodes (TAEs) excited by energetic ions were observed for the first time in CHS heliotron/torsatron plasmas heated by co-injected neutral beams, where the rotational transform is increased by the induced beam driven current. FBs of m = 3/n = 2 (where m, n are the poloidal and toroidal mode numbers) induce a pulsed increase in the energetic ion loss flux. FBs of m = 2/n = 1 induce sawtooth oscillations in the latter half of a discharge where the plasma β becomes high. Only when the beam velocity exceeds about half of the central Alfven velocity and the net plasma current is induced to the required level are TAEs with n = 1 and n = 2 excited; these are localized in the plasma core region, where the magnetic shear is appreciably reduced by the net plasma current. So far, TAE induced energetic ion loss has not been observed.

Alfvén eigenmodes excitation by external loop antennas in the Compact Helical System heliotron/torsatron

Four loop antennas are installed in the Compact Helical System (CHS) heliotron/torsatron in order to externally excite Alfven eigenmodes (AEs). These antennas are used in two different operation methods: one is as a set of four external loop antennas, and the other two pairs of electrodes inserted near the plasma boundary. For these two operations, magnetic perturbations in the frequency range of 10–250 kHz are generated in the plasma. The plasma response for the applied magnetic perturbations is measured by magnetic probe arrays. Information of AEs can be derived from the analysis of the response. In the excitation experiment of AEs using the “electrode method,” magnetic fluctuations are resonantly enhanced in a certain electron density during the afterglow of a neutral beam heated plasma, where no energetic ions exist. The fluctuations have the toroidal mode number n=1 and the frequency lies just above the lower bound of the toroidicity-induced Alfven eigenmode (TAE) gap in the plasma core region. The e...