Abstract
During the first operational phase (OP 1.1) of Wendelstein 7-X (W7-X) electron cyclotron resonance heating (ECRH) was the exclusive heating method and provided plasma start-up, wall conditioning, heating and current drive. Six gyrotrons were commissioned for OP1.1 and used in parallel for plasma operation with a power of up to 4.3 MW. During standard X2-heating the spatially localized power deposition with high power density allowed controlling the radial profiles of the electron temperature and the rotational transform. Even though W7-X was not fully equipped with first wall tiles and operated with a graphite limiter instead of a divertor, electron densities of n e > 3·1019 m-3 could be achieved at electron temperatures of several keV and ion temperatures above 2 keV. These plasma parameters allowed the first demonstration of a multipath O2-heating scenario, which is envisaged for safe operation near the X-cutoff-density of 1.2·1020 m-3 after full commissioning of the ECRH system in the next operation phase OP1.2.
Highlights
On 10th December 2015 the first plasma was created in Wendelstein 7-X (W7-X)
The confinement of a stellarator is only given by the external field coils, electron cyclotron current drive (ECCD) can be used at W7-X to compensate a finite bootstrap current of up to tens of kilo-Ampéres in some magnetic field configurations, which can cause a shift of the strike line on the divertor
The steady state operating magnetic field prevents any re-conditioning of the wall by glow discharges, requiring the development of adapted electron cyclotron resonance heating (ECRH) wall conditioning scenarios
Summary
On 10th December 2015 the first plasma was created in Wendelstein 7-X (W7-X). It is the world’s largest optimized stellarator (R = 5.5 m, a = 0.5 m) with 3-D shaped superconducting modular coils and a five-fold symmetry. W7-X aims to achieve reactor relevant plasma parameters in quasi steady state operation [1]. For this reason, the device will be equipped with a 10 MW electron cyclotron resonance heating (ECRH) system allowing at least 30 minutes continuous operation. The steady state operating magnetic field prevents any re-conditioning of the wall by glow discharges, requiring the development of adapted ECRH wall conditioning scenarios. Instead of the foreseen 10 divertor elements, a graphite limiter was installed on the high field side (HFS) in each bean plane of the five modules preventing a direct contact of the plasma with the unprotected regions of the plasma vessel [3]. The flexible coil system of W7-X was used to operate a magnetic field configuration at a lower rotational transform developing
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