Transport analyses using first-principle turbulence codes and 112-D transport codes usually study radial transport properties between the tokamak plasma magnetic axis and a normalized minor radius around 0.8. In this region, heat transport shows significantly stiff properties resulting in temperature scalelength values (R∕LT) that are relatively independent of the level of the radial heat flux. We have studied experimentally in the tokamak à configuration variable [F. Hofmann et al., Plasma Phys. Controlled Fusion 36, B277 (1994)] the radial electron transport properties of the edge region, close to the last closed flux surface, namely, between ρV=V/Vedge=0.8 to 1. It is shown that electron transport is not stiff in this region and high R∕LTe values (∼20–40) can be attained even for L-mode confinement. We can define a “pedestal” location, already in L-mode regimes, where the transport characteristics change from constant logarithmic gradient, inside ρV = 0.8, to constant gradient between 0.8 and 1.0. In particular, we demonstrate, with well resolved Te and ne profiles, that the confinement improvement with plasma current Ip, with or without auxiliary heating, is due to this non-stiff edge region. This new result is used to explain the significant confinement improvement observed with negative triangularity, which could not be explained by theory to date. Preliminary local gyrokinetic simulations are now consistent with an edge, less stiff, region that is more sensitive to triangularity than further inside. We also show that increasing the electron cyclotron heating power increases the edge temperature inverse scalelength, in contrast to the value in the main plasma region. The dependence of confinement on density in ohmic plasmas is also studied and brings new insight in the understanding of the transition between linear and saturated confinement regimes, as well as of the density limit and appearance of a 2/1 tearing mode. The results presented in this paper provide an important new perspective with regards to radial transport in tokamak plasmas which goes beyond L-mode plasmas and explains some previous puzzling results. It is proposed that understanding the transport properties in this edge non-stiff region will also help in understanding the improved and high confinement edge properties.
A comparative study of X-and 0-mode high field side launch for electron cyclotron heating (ECH) breakdown and startup of tokamak plasmas has been performed. It is found that X-mode power is not absorbed at the cyclotron resonance but uniquely at the upper hybrid resonance, displaced to the low field side of the cyclotron resonance. 0-mode power, however, is absorbed at the cyclotron resonance as well. The displacement of the upper hybrid resonance to the low field side with 0-mode launch is significantly smaller than that with X-mode launch because of the lower densities produced by 0-mode launch at the same microwave power level. The result is a more central and less localized breakdown with 0-mode launch. The breakdown characteristics of X-and 0-mode launch are seen to affect the position of the initial plasma current centroid in the poloidal cross-section. There is a strong correlation between the initial current ramp rate and the initial plasma current position which is most likely due to the dependence of the plasma inductance, toroidal electric field and field line connection lengths on the plasma major radius. 0-mode launch starts the plasma more centrally than X-mode launch and results in higher current ramp rates. X-mode startup occurs further to the low field side where current ramp rates are observed to be poor.
Electron cyclotron resonance heating (ECRH) experiments at 39 GHz with power levels similar to those of Ohmic heating have been conducted in the TCA tokamak in either the X-mode or the O-mode, ECRH was observed to be more efficiently absorbed in low density and low qa discharges. At low density the central electron temperature increased by a factor of two over Ohmic values and the stored energy increased by 40% when up to 80% of the plasma energy was supplied by ECRH. During low density ECRH operation, the electron energy confinement time τEe was the same as during pure Ohmic heating, but at high density τEe degraded with ECRH power. The decrease in the loop voltage required to maintain a constant current during ECRH is a very reproducible quantity and can be attributed in almost all cases to the change in the electron temperature. Sawtooth stabilization occurred during ECRH, with a sawtooth period of up to ten times longer than that before ECRH. This stabilization depends on the ECRH toroidal injection angle and the total input power. High energy ions were observed and their production is attributed, as in earlier experiments, to a three-wave interaction process
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