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.

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The d + d -d + p + n breakup reaction was studied over a range of bombarding energies Ed = 14-36 MeV. The proton and deuteron were detected at angle pairs chosen to either emphasize quasifree scattering (QFS) or the n-p final-state interaction (FSI). It was found that by using a smooth cutoff for the deuteron wave function, a modified plane-wave impulse-approximation(PODIA) theory could fit the magnitude and shape of the observed QFS peaks. To fit the QFS plus FSI data only triplet FSI was needed in contrast to some earlier results that indicated a strong singlet FSI. The singlet FSI contribution to the cross sections was estimated to be less than 1% in all cases studied.

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