Abstract
For successful operation of Wendelstein 7-X (W7-X) control of plasma impurity content and fuel recycling is required. This can be achieved by using wall conditioning methods. During the first divertor operation campaign (OP1.2a) of W7-X glow discharge conditioning (GDC), weekly in hydrogen and daily in helium for impurity and hydrogen removal respectively, was used in the absence of the magnetic field. He electron cyclotron resonance heating (ECRH) discharges were applied for density control in hydrogen plasmas during experimental days. The optimization of GDC and He ECRH wall conditioning on W7-X are presented. Solutions for glow discharge ignition problems are examined. The suitable He – GDC parameters, i.e. anode current and neutral gas pressure, are defined to keep the balance between maximum possible hydrogen removal rate and minimum plasma – facing component (PFC) erosion. Sequences of short He – ECRH pulses, so-called pulse trains, has been successfully implemented. The effect of pulse train main parameter variation such as gas prefill, input power, pulse length, duty cycle is described. The efficiency of single He recovery discharges and pulse trains are compared. The results of this work show significant improvement of wall cleaning efficiency.
Highlights
The main aim of the superconducting stellarator Wendelstein 7-X (W7-X), located at the Max-Planck-Institute in Greifswald, is to demonstrate the viability of optimized stellarators as potential fusion reactor
Due to the high probability of a radiative collapse during this type of recovery discharges and, sometimes, insufficient result the discharge is often repeated one or two times. These facts lead to the increase of total time for preparation, execution and post – discharge procedures which can exceed time spent on pulse train performance by factor of 1.5–2. Considering He pulse train with the pulse length of 1.5 s, duty cycle of 30 s, input power of 2.1 MW and gas prefill of 15 ms is the most effective electron cyclotron resonance heating (ECRH) wall conditioning tool which has been developed in OP1.2a
In the first divertor campaign on W7-X different wall conditioning techniques were found to be optimized by a systematic change of characterizing parameters
Summary
The main aim of the superconducting stellarator W7-X, located at the Max-Planck-Institute in Greifswald, is to demonstrate the viability of optimized stellarators as potential fusion reactor. The available conditioning techniques in OP1.2a were baking, glow discharge cleaning (GDC) and electron cyclotron resonance heating (ECRH) conditioning. The combination of these techniques turned out to be essential to get to good wall conditions. During OP1.2a, weekly performed H2 – GDC could reduce the amount of impurities produced by wall components erosion, the chemical formation and leaks He – GDC was carried out daily to desaturate the wall from hydrogen before the beginning of the experimental session [14] due to its availability only between experimental sessions when W7-X superconducting magnets are not powered. This work is interdependent with the paper dedicated to overview of wall conditioning studies throughout OP1.2a on W7-X [12]
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