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.

Articles you may be interested inCO2 laser-based dispersion interferometer utilizing orientation-patterned gallium arsenide for plasma density measurements Rev. Sci. Instrum. 84, 093502 (2013); 10.1063/1.4819028 First results from the modular multi-channel dispersion interferometer at the TEXTOR tokamak Rev. Sci. Instrum. 82, 063509 (2011); 10.1063/1.3600896Edge transport and turbulence reduction with lithium coated plasma facing components in the National Spherical Torus Experiment a)A dispersion interferometer based on a continuous-wave CO 2 laser source ͑ = 9.57 m͒ with double plasma passage for measurements of the line-integrated electron density in the TEXTOR tokamak and the GDT linear system has been developed and tested in experiments. A sensitivity of ͗n e l͘ min =2ϫ 10 17 m −2 and a temporal resolution of 1 ms have been achieved. The interferometer does not need any rigid frame for vibration insulation. Its basic components are installed compactly on an optical bench placed on a stable support outside of the torus. The possibility for the development of a multichannel dispersion interferometer for the next generation of fusion devices ͑e.g., W7-X, ITER͒ is discussed.

The design and main characteristics of fourteen-channel dispersion interferometer for plasma profile measurement and control in TEXTOR tokamak are presented. The diagnostic is engineered on the basis of modular concept, the 10.6 µm CO 2 laser source and all optical and mechanical elements of each module are arranged in a compact housing. A set of mirrors and retro-reflectors inside the TEXTOR vacuum vessel provides full coverage of the torus cross-section with twelve vertical and two diagonal

A double-pass dispersion interferometer based on a 9.6-µ m ëé 2 laser with a sensitivity of 〈 n e l 〉 min 1 × 10 13 cm -2 and a temporal resolution of ~50 µ s, designed to measure linear plasma density, is described. A ZnGeP 2 nonlinear crystal is used as the frequency doubler. The main advantages of the interferometer are its compactness and a low sensitivity to vibrations of optical elements. The interferometer requires no special vibration isolation. Its main components are arranged compactly on an optical bench outside the apparatus, except for a window for radiation injection and a retroreflector; these are mounted on the wall of the experimental facility's vacuum chamber. The advantages of the dispersion interferometer have been demonstrated in an experiment with a gas-dynamic trap.

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