Performance of the first neutral beam injector at the Wendelstein 7-X stellarator

The neutral beam injection (NBI) system at Wendelstein 7-X (W7-X) was operated for the first time in 2018. Detailed calorimetric measurements were carried out to accurately determine the energy flow within the NBI system and the injected power into the plasma. Of the electric energy put into the system (89± 6) % can be accounted for. The time-averaged injected NBI power is assessed to be Pinj ≈ (3.1 ± 0.8) MW for hydrogen injection with two positive ion sources at the maximum acceleration voltage of 54 kV. This corresponds to a conversion efficiency of the initially generated ion beam power to the injected neutral beam power of about 33%. The ionization in a plasma with an ion and electron temperature of Ti ≈ Te ≈ 1.3 keV and an electron density of ne ≈ 4 ·1019 m−3 is calorimetrically determined to be about 90%. The injected power is validated against beam emission spectroscopy applying Fast-Ion Dα analysis (FIDASIM). The inferred injected power leads to an energy confinement time of around τE ≈ 150 ms for pure NBI heating plasma experiments at W7-X with these heating and plasma parameters.