Direct observation of the non-locality of non-diffusive counter-gradient electron thermal transport during the formation of hollow electron-temperature profiles in the Large Helical Device

Abstract

A heating source with off-axis electron cyclotron heating (ECH) alone produced a plasma with a quasi-steady-state hollow electron-temperature profile in the Large Helical Device. The clear formation of this quasi-steady-state hollow electron-temperature profile can be explained by adding the outward heat convection term to the diffusion term, as a simple model to describe the electron heat flux, using the energy conservation equation. In addition, we directly observed the non-locality of the non-diffusive (convective) contribution in transient electron thermal transport in the condition that power-modulated on-axis ECH was applied to the plasma sustained by off-axis ECH. The experimentally evaluated flux-gradient relation shows two different positive values of the electron heat flux at zero temperature gradient by going back and forth between positive and negative temperature gradient regions in the transport hysteresis phenomenon.

Investigation of core transport changes in DIII-D discharges with off-axis T e profile peaks

“…Furthermore, the strongly negative T e gradient on both sides of the T e peaks can be explained with the presence of convective terms in electron thermal transport. This agrees with previous RTP and LHD results [4,6,12]. However, it is uncertain whether the same mechanisms give rise to convective counter-gradient transport.…”

“…It should be noted that the location of the off-axis T e peak (ρ ∼ 0.3) is different from the ECH deposition location ρ dep ∼ 0.4. This behavior is different from that of the RTP [4] and LHD [6] experiments, where the off-axis T e peaks coincide with ρ dep .…”

“…Further simulation effort confirmed the presence of negative convective heat flux in the core, which sustains the observed hollow profiles [11,12]. An outward heat convection that sustains the hollow T e profile is also observed in the LHD experiment [6], but is not linked to the rotational transform profiles. While some questions regarding hollow T e profiles have been answered for the L-mode case, similar phenomena and modification of core plasma transport in ECH dominant H-mode discharges have not been closely examined.…”

“…prominent off-axis maxima with large T e gradients on both sides of the peak. In addition, a recent study in the large helical device (LHD) stellarator has observed quasi-steady-state hollow T e profiles when heating with off-axis ECH [6]. These experimental results highlight a change in core plasma transport during the application of off-axis ECH.…”

Prospect for Experimental Investigation of Phase-Space Turbulence in Magnetically Confined Fusion Plasmas

“…However, there are still several observations that cannot be explained by the quasilinear model. Those are, for example, nondiffusive transport [12,13], nonlocal/transient transport [14,15], profile stiffness [16,17], and others. Those transport events are called the transport conundrums in this paper.…”