Fast‐Ion Response to Energetic‐Particle‐Driven MHD Activity in Heliotron J

Abstract

In Heliotron J, low magnetic shear configuration, instabilities with frequency chirping in the frequency range of Alfvén eigenmodes have been observed in tangentially injected neutral beam plasmas. These modes are induced by energetic-particle driven magnetohydrodynamic (MHD) instabilities such as global Alfvén eigenmode or energetic particle mode. A hybrid directional Langmuir probe system has been installed into Heliotron J to investigate the response of fast-ion fluxes to the MHD modes. A high coherent response of the ion flux to the bursting modes has been observed not only by the co-directed probe but also by the counter-directed one. A linear correlation between the response of the co-directed ion flux and the mode amplitude has been found. The radial profile of the response of the co-directed ions has decreased with the minor radius and has not been obtained significantly outside last closed flux surface. These results indicate that the fast-ion response is due to a resonant convective oscillation. The ion flux response of the counter-directed probe has appeared in the growth phase of the mode burst. Its phase relation is different from that of co-directed one and magnetic probe located at the Heliotron J vacuum vessel. Two candidates of the detected ion flux of the counter-directed probe have been discussed.

Edge plasma responses to energetic-particle-driven MHD instability in Heliotron J

“…Depending on the radial position of the probe, the response has a time delay as the probe moves outward. A similar phenomenon was observed in a past experiment using the directional probe in Heliotron J and also in CHS, which was considered to be radial transport of fast ions [25,26]. The responses observed here also correspond to the radial transport of fast ions from a comparison with past experimental data, although the signal was not obtained by a directional probe but by a conventional probe.…”

Design of Lost Fast-Ion Probe Based on Thin Faraday Films in Heliotron J

“…The separation of two apertures is 5.5 mm. The size of the aperture is determined using the grid calculation program [3] so as to obtain an aimed particle whose energy is up to 30 keV. Figure 5 shows the typical energy and pitch angle map of FLIP.…”

“…In previous experiments performed in Heliotron J, characteristics of AEs excited by neutral beam (NB) injected fast ions have been studied [2]. In these experiments, increases of fast-ion loss due to these AEs were measured with a Langmuir probe placed on the outside of the last closed flux surface [3]. To clarify causality between AEs and consequent fast-ion losses, velocity-space information of lost ions i.e.…”

Improvement of core heat transport in NBI plasmas of heliotron J using high-intensity gas puffing

“…In recent experiments with Heliotron J, a new phenomenon was observed. Using a newly developed fuelling method called high-intensity GP (HIGP) [16][17][18][19][20][21], a fuelling technique which puffs a much larger amount of gas compared with the traditional GP in a very short period (normally 10-15 ms), a peak electron temperature profile was formed in a plasma heated by neutral beam injection (NBI). As the electron temperature profile was formed, the density profile remained similar to the conventional GP discharge.…”