Abstract
In the Wendelstein 7-X stellarator, up to 7MW of power are delivered to the plasma by an electron cyclotron resonance heating system consisting of ten 140 GHz gyrotrons [1]. Due to the flexible front steering mirror of each beam line, the power deposition can be varied over the whole plasma radius and is optionally combinable with additional current drive. This flexibility, together with small toroidal currents in the stellarator, makes W7-X a perfect testbed for electron cyclotron current drive (ECCD) experiments, which have been successfully accomplished during the first two experimental campaigns OP1.1 and OP1.2a. Long discharges (lasting up to 30s) have been performed in OP1.2a, thus allowing the study of the current drive time evolution and the possibility to compensate the bootstrap current. ECCD efficiency has been studied using different power deposition profiles combined with a variation of the injection angles in relation to the magnetic field. During ECCD experiments, saw-tooth-like oscillations have been observed. Depending on the driven current density, ECCD can significantly modify the rotational transform (iota) profile, which can locally reach low order rational, thus triggering plasma instabilities. Different current density profiles have been tested, in order to try to understand the main trigger parameter for the instabilities. In particular, effects caused by current density gradient have been investigated producing both co- and counter-current drive at different radial positions: the total current drive is negligible, but a strong current gradient arises by driving currents in opposite directions. In this work an overview of ECCD operations in OP1.2a is given and first results, comparing different diagnostics, are presented. An initial 1-D model, coupled with the ray tracer TRAVIS, is developed, in order to have an estimation of current diffusion times and the radial position where a low order rational crosses the disturbed iota profile.
Highlights
The superconductive optimized stellarator Wendelstein 7-X (W7-X) [1] generates the rotational transform by means of five-period external coils, toroidal current is not needed to confine the plasma
During the first two operation campaigns (OP1.1 and OP1.2a) plasma start-up and heating have been obtained using Electron Cyclotron Resonance Heating (ECRH), with up to 10 gyrotrons at 140 GHz, able to operate both in O2 and X2 mode, for a maximum delivered power of 7MW [2]
Since the current drive is generally run between 10% and 30% of the effective radius, during lowbeta and relatively low density, the bootstrap current in this region is not expected to provide a relevant contribution to rotational transform modification, effects of Electron Cyclotron Current Drive (ECCD) and of the relative shielding current only will be considered
Summary
The superconductive optimized stellarator Wendelstein 7-X (W7-X) [1] generates the rotational transform by means of five-period external coils, toroidal current is not needed to confine the plasma. During the first two operation campaigns (OP1.1 and OP1.2a) plasma start-up and heating have been obtained using Electron Cyclotron Resonance Heating (ECRH), with up to 10 gyrotrons at 140 GHz, able to operate both in O2 and X2 mode, for a maximum delivered power of 7MW [2]. If oblique injection takes place, Electron Cyclotron Current Drive (ECCD) is generated [3]; the generation of ECCD is well localised too and it can be used to locally modify the plasma rotational transform and shear or to compensate bootstrap current. During ECCD experiments in abovementioned experimental campaigns, fast collapses of electron temperature have been observed [1] [4] to present strong similarity with well-known saw-tooth- [3] [5]crashes in tokamaks [6]. The 1-D model for current evolution, under development, will be presented
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