Spectroscopic gas puff imaging diagnostic for the measurement of two-dimensional electron density and temperature distributions

A novel spectroscopic gas puff imaging (SGPI) diagnostic is presented for the purpose of simultaneous two-dimensional measurements of the electron density (ne) and temperature (Te). The based on the He line-ratio technique, this diagnostic achieves a spatiotemporal resolution in the edge plasma of the TJ-II stellarator of a few mm and down to 5 µs. A high-speed intensified camera records a scene that combines images of the plasma volume where helium is injected using three lenses. Each lens is fitted with a band-pass filter for one of the emission lines of He-I (667 nm, 706 nm and 728 nm). The electron density and temperature are deduced from the ratios of intensities using a collisional radiative model. The location of the helium injector was optimised to align the line(s) of sight of the diagnostic with the magnetic field within the volume of the injected thermal neutral cloud of helium. This specific alignment serves to minimise any artefacts caused by line integration, thereby enhancing spatial resolution. A series of experiments were conducted under varying plasma heating conditions—electron cyclotron resonance and neutral beam injection—to cross-validate the electron density and temperature profiles obtained with the novel SGPI diagnostic against Thomson scattering and the supersonic helium beam. Furthermore, the use of the diagnostic for the purpose of turbulent fluctuation analysis is explored, and preliminary results are presented.