Neutral temperature measurements in ADITYA-U Tokamak

Presence of neutral atoms plays a crucial role in affecting the edge dynamics inside a tokamak plasma (a magnetically confined fusion device) [1-4]. To understand the dynamics of neutrals in the edge region, spectral line emission from the neutrals is routinely monitored using visible spectroscopy in ADITYA-U tokamak.

Temperature measurement of neutral atoms & several charged ions has been performed using Doppler’s broadening. However, the presence of high magnetic field inside a tokamak over-estimates the neutral temperature due to Zeeman splitting. A 1m long Czerny-Turner (multi-tract spectrometer), coupled with a fast sCMOS camera has been used to collect the spectral data. Several optical fibers are installed on the top as well as the radial port of ADITYA-U tokamak, to accomodate simultaneous collection of the spectral line emission coming from both directions: parallel and perpendicular to the magnetic field B.

Interestingly, in the presence of high magnetic field (B ~ 1T) in ADITYA-U tokamak, dopplers broadening over-estimates the measured neutral temperature due to the presence of Zeeman splittig. Experimental observation suggests that the measurement of H-alpha signal, performed parallel to the magnetic field consists of two Zeeman components, namely σ+ & σ – component), however the measurement done perpendicular to the magnetic field shows the presence of all the three Zeeman components i.e. σ+, σ – & ℼ-component. The presence of these Zeeman component causes over-estimation of neutral temperature in ADITYA-U.

Apart from the Zeeman splitting, it has been found that the hydrogen neutrals are produced via various different mechanisms in ADITYA-U, e.g. recombination process, passive charge exchange, etc. These processes are responsible for heating the hydrogen neutrals to different temperatures. In ADITYA-U, hydrogen neutrals have three different temperature componenta, namely: cold, warm and hot component. Temperature estimation after incorporating these three components as well as Zeeman correction gives the correct temperature. Estimated temperature after these treatments suggests that the temperature of cold, warm and hot component is ~ 0.5eV, 2-3eV and 15-20eV respectively. A detailed measurement technique will be discussed in this presentation.