L‐H Transition Experiments in TJ‐II

Abstract

In the TJ-II stellarator, L-to H-mode transitions are achieved under NBI heating conditions when operating with lithium coated walls. Several magnetic configurations have been explored in order to study the influence of the rotational transform in the transition. The shear of the negative radial electric field, E r , increases at the L-H transition by an amount that depends on the magnetic configuration and heating power. Magnetic configurations with and without a low order rational surface close to the plasma edge show differences that may be interpreted in terms of local changes in the radial electric field induced by the rational surface that could facilitate the L-H transition. Fluctuation measurements show a reduction in the turbulence level that is strongest at the position of maximum Er shear. High temporal and spatial resolution measurements indicate that turbulence reduction precedes the increase in the mean sheared flow, but is simultaneous with the increase in the low frequency oscillating sheared flow. These observations may be interpreted in terms of turbulence suppression by oscillating flows, the so-called zonal flows.

Scale-selective turbulence reduction in H-mode plasmas in the TJ-II stellarator

“…A possible explanation could be a large-scale flow driven by turbulence, which has recently been observed in several experiments, such as TJ-II [56], NSTX [57], ASDEX Upgrade [58] and DIII-D [59].…”

Influence of Low‐Order Rational Surfaces on the Radial Electric Field of TJ‐II ECH Plasmas

“…In the TJ-II stellarator (B = 1 T, R = 1.5 m, a ≤ 0.22 m), changes in local transport have been detected associated with low order rational surfaces; the lowest values of the effective diffusivity being found in regions around the position of the lowest order rationals [4]. In addition, low order rational surfaces are found to ease transitions to Core Electron Root Confinement (CERC) [5] [6] [7] and to High confinement mode (H-mode) [8] [9]. In the first type of transitions, an increase in the positive radial electric field is measured in the plasma core region [5] whereas in the second one, the radial electric field shear shows an oscillating behavior close to the plasma region where the transport barrier develops [8].…”

23rd IAEA Fusion Energy Conference: summary of sessions EX/C and ICC

“…38. The turbulence induced flow shear is generated causing a reduction in the density fluctuations and a subsequent drop in the flow shear, which is followed by the increase in the density fluctuations [90][91][92]. These results have many of the characteristics of the predicted dynamics of L to H transition models based on turbulence driven flows.…”

“…Experiments on MAST [41], NSTX [39] and TJ-II [90][91][92] have reported no significant change in mean shear in the radial electric field immediately prior to the transition. As shown in figure 35 from results on TJ-II [90,91], there is an abrupt change in the density fluctuation magnitude at the time of the L to H transition time while the shear in the mean radial electric field decreases afterwards. This is an example of an abrupt transition.…”