Localized temperature and velocity measurements of C VI ions using active CXRS spectroscopy in the TJ-II stellarator

Introduction An active charge-exchange recombination spectroscopy (CXRS) diagnostic, based around a diagnostic neutral beam injector (DNBI) and a bidirectional (two vertical opposing views) multi-channel spectroscopic system, has been set-up on the TJ-II, a four-period [1], low magnetic shear, stellarator. It provides 5 ms long pulses, up to two per discharge, of neutral hydrogen accelerated to 30 keV with equivalent current of 3.3 equ. A. The bidirectional diagnostic is designed to measure Doppler shifts and widths of the C VI line at λ = 529.06 nm in up to 3 arrays of 12 channels with ~1 cm spatial resolution across the plasma minor radius [2]. The light dispersion element is a Holospec spectrograph with three 100 μm curved entrance slits (these are curved to compensate for its short focal length) and a transmission grating sandwiched between two BK7 prisms. It provides a focal-plane dispersion of ~11.5 A/mm at 529 nm. Moreover, a narrow bandpass filter centred on 529 nm prevents spectral overlapping from the multiple fibre arrays at the image plane. The set-up includes a high-efficiency back-illuminated CCD camera and fast mechanical shutter ( 4.5 ms time window). With on-chip binning, multiple spectra can be collected during discharges ( 300 ms). Finally, fibre alignment was performed by illuminating each fibre bundle with a bright light source and observing, through an unused viewport, the orientation and location of the resultant bright spots with respect to markings on the inside of the opposing vacuum flange. In this way, the sightlines through plasmas can be determined using a cross-sectional machine drawing and magnetic configuration maps. Finally, when compared with the neutral beam geometry the normalized radius, ρ, corresponding to each beam/line-of-sight interaction volume is found.