Diagnostic neutral beam injector for active spectroscopy of high beta plasmas

The advanced fast ion-dominated high beta plasma is achieved using multi-MW neutral heating beams. To facilitate the diagnostics of this fast ion-dominated plasma, a high-energy and high-current diagnostic neutral beam (DNB) injector was designed and built by the Budker Institute of Nuclear Physics. The DNB injector made active measurements of ion temperature and rotational velocity possible with the help of charge-exchange spectroscopy for impurity ions and, most importantly, for the main ion (deuterium) component. A DNB energy of 40 keV was chosen to assure low beam attenuation in the plasma and to enable spectroscopic measurements along the entire plasma column. The diameter (level 1/e) of the ballistically focused DNB in the plasma is about 8 cm. To achieve a high temporal resolution, unique methods of beam modulation with a frequency of up to 10 kHz were implemented. The achieved high DNB current of 8 A in atomic hydrogen in combination with the beam modulation enables to obtain an acceptable signal-to-noise ratio of the measured spectra.