Study of Electron Heat Pulse Propagation induced by ECRH/on-off on T-10 and LHD

S.V. Neudatchin1,2) S. Inagaki2) , A.Ya. Kislov1), D.A. Kislov1), D.A. Krupin1), S. Kubo2),
S.E. Lysenko1), K. Ohkubo2), Yu.D. Pavlov1), V.I. Poznyak1), T. Shimozuma2), A.V. Sushkov1), M. M. Yakovlev2), K. Ida2), K. Itoh2)

1)Nuclear Fusion Institute, RRC Kurchatov Institute, 129332, Kurchatov sq.1, Moscow, Russia
2)National Institute for Fusion Science , Toki,Gifu 509-5292, Japan

Internal Transport Barrier (ITB) was found at T-10 by means of heat pulse propagation (HPP) and cold pulse propagation (CPP) analysis in target sawteeth-free plasma. Target sawteeth-free plasma was created either by high-field-side off-axis ECRH (140 GHz) or by low-field-side off-axis ECRH (130 GHz) at normalized minor radius r/a ≈ +/- (0.45). Regimes with 0.18-0.22MA/2.3-2.5T, central line-averaged density <n> from 1.4 to 2.7 1019/m3 were studied. Outward HPP was created by switching on-axis ECRH imposed on the background, which was created by off-axis ECRH. Inward CPP was created by turning-off the off-axis ECRH. In both cases, R/LTe =RgradTe/Te rises well (up to 23) above Ohmic value on heat (cold) wave front while dynamic electron heat diffusivity χeHP values are low (0.06-0.25 m2/s). We believe that features of ITB are clearly observed exactly during not-steady-state HPP (CPP) processes. Later, transport increases in both cases. CPP analysis shows that χeCP values increase by ~3 times (from minimum level 0.1 m2/s inside r/a < 0.4) under variation of the resonance position at the same off-axis power or by increase of power level. In all cases stronger redistribution (increase of wideness) of Te(r)/Te(0) profile (normalized to OH level) leads to the increase of χeCP. Probably, χeCP ≈ 0.1 m2/s corresponds to some “optimal” q profile with low shear zone and q(0) slightly above 1. Moreover, the reduction of q during CPP with χeCP≈ 0.3 m2/s brings interruption of diffusive CPP process by improvement of confinement in central region (inside r/a= 0.15). We believe that this improvement represents the influence of some 'optimal' q value, which lies near 1.
Preliminary results of HPP/CPP study at LHD show significant difference between low-density and higher density (<n> > 0.7 10 19/m3 ) discharges with NBI heating. HPP/CPP looks fast at higher density (χeHP order of 5-10 m2/s) while HPP/CPP propagates slowly (with values of χeHP smaller by order of amplitude) in the zone near r/a=0.5 in low-density discharges. The validity of ECE measurements in this region of low-density discharges is under discussion. The study of χeHP variation with plasma parameters is in progress.