Abstract
The second operation phase of W7-X (OP1.2) showed the potential of exclusively electron cyclotron resonance heating (ECRH)-sustained plasma operations in stellarators. Employing multi-pass ECRH scenario in the second harmonic O-mode (O2-ECRH), stationary densities of up to 1.4 × 1020 m−3 could be achieved. This scenario also made stationary divertor detachment possible, which is a reactor-relevant scenario for power and particle exhaust. At high densities and with sufficiently high density gradients for an improved ion confinement, the coupling between the electrons and ions was strong enough to bring the ion temperature to values above 3 keV and to the neoclassical limit for some magnetic configurations, thus enabling to test the W7-X neoclassical optimization. The planned enhancement of the ECRH performance will enable to advance towards reactor-relevant beta values and to investigate their stability and confinement of fast particles, which is a priority goal of W7-X.
Highlights
W7-X is an optimized stellarator in which the otherwise strong neoclassical transport is reduced so that high confinement time values can be achieved. In particular this can be done with electron cyclotron resonance heating (ECRH) only, whereby only the electrons are heated directly, while the ion temperature is increased by electron ion collisions
An outstanding result is the stationary operation with pure O2-ECRH at plasma densities of 1.4 × 1020 m−3, which is clearly above the X2 cutoff density as shown in figure 2
The electron-ion coupling was so high that the ion temperature approached the values of the electrons
Summary
W7-X is an optimized stellarator in which the otherwise strong neoclassical transport is reduced so that high confinement time values can be achieved In particular this can be done with electron cyclotron resonance heating (ECRH) only, whereby only the electrons are heated directly, while the ion temperature is increased by electron ion collisions. The superconducting coil system of W7-X generates a magnetic field strength of 2.5 T This requires an ECRH frequency of 140 GHz, which corresponds to the second harmonic resonance of the electrons. The high performance high density scenario must be compatible with a technically reasonable particle and energy exhaust For this purpose, W7-X is equipped with a so-called island divertor, which separates the core plasma from the plasma wall interaction
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