Abstract
After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3 × 1019 m−3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.
Highlights
The main objective of the optimized stellarator Wendelstein 7-X (W7-X) is the demonstration of steady-state plasma operation at fusion-relevant plasma parameters, thereby verifying that the stellarator is a viable fusion power plant concept
The main construction phase of W7-X was completed in 2014. It was followed by the commissioning of the superconducting device [3], which was successfully concluded by assessment of a series of careful measurements of the magnetic field, confirming the basic magnetic field topology and demonstrating that potential error fields are within the correction capabilities of the W7-X trim coils [4,5,6]
The main focus of the first operational campaign of W7-X was on the integral commissioning of the basic device together with first plasma operation, the largely trouble-free operation and the fast completion of a comprehensive diag nostic set made it possible to spend a significant fraction of the campaign on physics studies
Summary
The main objective of the optimized stellarator Wendelstein 7-X (W7-X) is the demonstration of steady-state plasma operation at fusion-relevant plasma parameters, thereby verifying that the stellarator is a viable fusion power plant concept. In one of the five torus modules of W7-X (module 4), an increase of the neutral gas pressure (measured with a midplane gas manometer) was observed, associated with an increase of the plasma edge emission (measured with the visible-light cameras) This was followed by a reduction of the electron temperature at constant heating power which preceded the radiation collapse. The longest plasma discharges achieved at moderate heating powers, starting at 1 MW and dropping to 600 kW, lasted up to 6 s, reaching an injected energy of 4 MJ with stationary central temperatures of Te = 5 keV and Ti = 1.5 keV at central densities of several times 1019 m−3. The details of the profile shapes are still under invest igation, which is essential for a refined transport analysis
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