Changes in plasma potential and turbulent particle flux in the core plasma measured by heavy ion beam probe during L–H transitions in the TJ-II stellarator

Abstract

Plasma potential and turbulent particle flux ΓE×B in the plasma core were measured for the first time during L–H/H–L transitions in the TJ-II heliac using a heavy ion beam probe. The L–H transition is characterized by a simultaneous potential drop ∼−100 V over the whole plasma radius, formation of density pedestal and a strong radial electric field Er between −100 to −150 V cm−1 in the edge (0.8 < ρ < 0.9), strong suppression of the broadband density and potential oscillations and of ΓE×B in the edge and core plasma.

Frequency and plasma condition dependent spatial structure of low frequency global potential oscillations in the TJ-II stellarator

“…Under the presence of Zonal Flows and during the L-H transition, the radial particle flux is strongly reduced in TJ-II, as reported in previous works [125,195]. Considering the evolution of ZF-like fluctuations and the mean radial electric fields during the L-H transition, it is…”

H-mode transition in the TJ-II stellarator plasmas

“…The HIBP diagnostic allows these type of measurements not only in the plasma periphery but also in the core region. In TJ-II, HIBP measurements have shown a reduction in the plasma turbulence and associated turbulent flux not only in the plasma edge region but also in the plasma core at ρ∼0.5 [13]. The changes in the turbulent flux are detected in the two plasma regions simultaneously within a time resolution of ∼1 ms.…”

“…The question, however, is how the radial electric field is driven and what are the related temporal and spatial scales that can impact the transition and whether these scales end up in different routes to reach the transition conditions. Besides, the strongest reduction in the turbulent transport at the transition has been detected at the barrier region where the maximum E×B flow shear develops, but in some cases, a reduction in the turbulent transport is found in a wider radial range, with weak or even zero flow shear, which points to other mechanisms beyond the turbulence suppression by local sheared flows [13].…”

High Voltage Test Bench for Heavy Ion Beam Probe Diagnostics on T-15md Tokamak

“…HIBP is a unique tool to directly measure plasma potential in magnetically confined plasmas [4]. Measurements of plasma potential allow us to study radial electric field [5] and its coupling on transport processes including the transition to H-mode [6]. This coupling has been studied during recent years, but it still presents an open question of modern plasma physics.…”