Abstract
For stellarators, which need no or only small amounts of current drive, electron-cyclotron-resonance heating (ECRH) is a promising heating method even for the envisaged application in a fusion power plant. Wendelstein 7-X (W7-X) is equipped with a steady-state capable ECRH system, operating at 140 GHz, which corresponds to the 2nd cyclotron harmonic of the electrons at a magnetic field of 2.5 T. Ten gyrotrons are operational and already delivered 7 MW to W7-X plasmas. Combined with pellet injection, the highest triple product (0.68 × 1020 keV m−3 s), observed up to now in stellarators, was achieved (Sunn Pedersen et al 2018 Plasma Phys. Control. Fusion 61 014035). For the first time, W7-X plasmas were sustained by 2nd harmonic O-mode heating, approaching the collisionality regime for which W7-X was optimized. Power deposition scans did not show any indication of electron temperature profile resilience. In low-density, low-power plasmas a compensation of the bootstrap current with electron-cyclotron current drive (ECCD) was demonstrated. Sufficiently strong ECCD close to the plasma centre produced periodic internal plasma-crash events, which coincide with the appearance of low order rationals of the rotational transform.
Highlights
The main objective of the optimized stellarator Wendelstein 7-X (W7-X) is to demonstrate that the underlying magnetic confinement concept fulfils the basic requirements for the development of fusion power plants [1, 2]
The design of W7-X is based on an elaborate optimization procedure which includes improved confinement of the thermal plasma and the fast ions, acceptable plasma equilibrium and stability properties up to 〈β〉 = 5%, and a magnetic field configuration, which is compatible with a resonant magnetic island divertor configuration for controlled heat and plasma exhaust [3, 4]
As the only heating system, available initially, the reliable operation of the electron-cyclotron-resonance heating (ECRH) was a prerequisite for the success of the first two W7-X campaigns
Summary
The main objective of the optimized stellarator Wendelstein 7-X (W7-X) is to demonstrate that the underlying magnetic confinement concept fulfils the basic requirements for the development of fusion power plants [1, 2] For this purpose, the design of W7-X is based on an elaborate optimization procedure which includes improved confinement of the thermal plasma and the fast ions, acceptable plasma equilibrium and stability properties up to 〈β〉 = 5%, and a magnetic field configuration, which is compatible with a resonant magnetic island divertor configuration for controlled heat and plasma exhaust [3, 4]. W7-X is designed for a strayradiation level of 1 MW [16, 17], assuming a maximum of 10% of non-absorbed heating power Another issue concerns microwave diagnostics, which rely on measuring the plasma emission near the ECRH frequency such as electron-cyclotron emission (ECE). A more general overview of the first W7-X results using a divertor configuration is presented in [20]
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