Role of Helium–Hydrogen ratio on energetic interchange mode behaviour and its effect on ion temperature and micro-turbulence in LHD

Abstract

In the Large helical device, a change of energetic particle mode is observed as He concentration is varied in ion-ITB type experiments, having constant electron density and input heating power but with a clear increase of central ion temperature in He rich discharges. This activity consists of bursty, but damped energetic interchange modes (EICs, X Du et al., Phys. Rev. Lett. 114 p.155003 (2015)), whose occurrence rate is dramatically lower in the He-rich discharges. Mechanisms are discussed for the changes in drive and damping of the modes with He concentration. These EIC bursts consist of marked changes in the radial electric field, which is derived from the phase velocity of turbulence measured with the 2D phase contrast imaging (PCI) system. Similar bursts are detected in edge fast ion diagnostics. Ion thermal transport by gyro-Bohm scaling is recognised as a contribution to the change in ion temperature, though fast ion losses by these EIC modes may also contribute to the ion temperature dependence on He concentration, most particularly controlling the height of an "edge-pedestal" in the Ti profile. The steady-state level of fast ions is shown to be larger in Helium rich discharges on the basis of a compact neutral particle analyser (CNPA), and the fast-ion component of the diamagnetic stored energy. These events also have an influence on turbulence and transport. The large velocity shear induced produced during these events transiently improves confinement and suppresses turbulence, and has a larger net effect when bursts are more frequent in Hydrogen discharges. This exactly offsets the increased gyro-Bohm related turbulence drive in Hydrogen which results in the same time-averaged turbulence level in Hydrogen as in Helium.

Theoretical analysis of energetic-ion-driven resistive interchange mode stabilization strategies using a Landau closure model

“…The 1/1 EIC stability can be further improved if the Deuterium is mixed with Helium. This optimization trend was already confirmed in the LHD experiments [27,60].…”

Observation of Interaction between Fast Ions and Turbulence in Large Helical Device

“…This finding supports the idea that the first transient reduction is not due to changes in confinement, but due to the injection of fast ions. The transient reduction of turbulence was also observed due to the loss of fast ions, which triggered energetic particle-driven interchange (EIC) mode [14][15][16]. In EIC mode, the transient reduction of turbulence lasted 1 msec, which was much shorter than our results.…”

“…In EIC mode, the transient reduction of turbulence lasted 1 msec, which was much shorter than our results. Additionally, the reduction was caused by the strong E r and/or shear formation [15,16]. In contrast, during these experiments, the transient reduction of turbulence was caused by the injection of fast ions rather than their loss.…”

Realization of high T i plasmas and confinement characteristics of ITB plasmas in the LHD deuterium experiments

“…The timing of the EIC occurrence is indicated by arrows and dashed lines in figures 5(c)-( f ). The EICs are driven by the increased pressure gradient of helically trapped energetic ions, which are mainly generated by the perpendicular NBI [59][60][61][62][63][64]. The neutron emission rate dropped associated with the EIC event.…”