Demonstrating improved confinement of energetic ions is one of the key goals of the Wendelstein 7-X (W7-X) stellarator. In the past campaigns, measuring confined fast ions has proven to be challenging. Future deuterium campaigns would open up the option of using fusion-produced neutrons to indirectly observe confined fast ions. There are two neutron populations: 2.45 MeV neutrons from thermonuclear and beam-target fusion, and 14.1 MeV neutrons from DT reactions between tritium fusion products and bulk deuterium. The 14.1 MeV neutron signal can be measured using a scintillating fiber neutron detector, whereas the overall neutron rate is monitored by common radiation safety detectors, for instance fission chambers. The fusion rates are dependent on the slowing-down distribution of the deuterium and tritium ions, which in turn depend on the magnetic configuration via fast ion orbits. In this work, we investigate the effect of magnetic configuration on neutron production rates in W7-X. The neutral beam injection, beam and triton slowing-down distributions, and the fusion reactivity are simulated with the ASCOT suite of codes. The results indicate that the magnetic configuration has only a small effect on the production of 2.45 MeV neutrons from DD fusion and, particularly, on the 14.1 MeV neutron production rates. Despite triton losses of up to 50 %, the amount of 14.1 MeV neutrons produced might be sufficient for a time-resolved detection using a scintillating fiber detector, although only in high-performance discharges.

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.

A Doppler reflectometer system has recently been installed in the stellarator TJ-II. The system is optimized for the Q-band (33-50 GHz) and the high-curvature plasmas produced in TJ-II. The launch angle of the microwave beam can be controlled by a steerable mirror to obtain angles between +/-20 degrees enabling the measurement of perpendicular wave numbers in the range of 3-15 cm(-1). The available angular range allows for comparisons between positive and negative values and additionally for calibration of the system. Localization and k(perpendicular)-estimation is done via the three-dimensional ray/beam-tracing code TRUBA. First measured spectra and radial profiles of the perpendicular velocity of plasma density fluctuations are presented.

A finite-difference time-domain code is used to obtain the full-wave solution of the O-X mode conversion process for typical parameters of the TJ-II stellarator in a cylindrical geometry. This reduction of the complicated stellarator geometry to a cylindrical geometry is chosen since the conversion process occurs only over a limited radial plasma volume. In the calculations, Gaussian antenna beams are studied with the option of different beam waists in the poloidal and toroidal direction. Optimum conversion efficiency is found if the wavefront of the incident antenna beam is matched to the local curvature of the O-X conversion layer. Finally, the code is used to calculate the complete O-X-B conversion process into a Bernstein wave.