Visible and near-infrared divertor spectroscopy on the MAST and JET-ILW tokamaks

Abstract

Passive spectroscopy diagnostics play a key role in advancing the physics of plasma exhaust in the edge and divertor regions of tokamaks. Information obtained from spectral line intensities and profile shapes is crucial for estimating plasma parameters, particularly for studies of cold and dense detached plasmas. This work aims to characterise the visible and near-infrared spectral regions with emphasis on the Balmer and Paschen hydrogen series lines, their diagnostic applications and interpretation techniques. Whereas the Balmer series lines are measured routinely, few observations of the Paschen series lines have been carried out in the fusion plasma context. Extending observations to the near-infrared region for more detailed studies of the Paschen series is addressed through diagnostic development with the aim of providing coverage of the visible to near-infrared spectral range (350-1900 nm) along the same optical line-of-sight. An initial spectral survey on MAST using a purpose built diagnostic provides new insight into the spectral features in the near-infrared and confirms the viability of Paschen line observations. Following the proof of concept measurements on MAST, diagnostic development on the JET ITER-like wall mirror-linked divertor spectroscopy system facilitated more refined measurements. The main outputs include first of its kind measurements of the Pa-alpha line and spatially resolved spectral line profile measurements of the Pa-beta line. In the visible range ELM-resolved Balmer and impurity emission profile measurements at high spatial resolution were obtained using a new filtered camera system. A detailed assessment of the diagnostic scope for parameter estimation from both high-n and low-n Balmer and Paschen series lines is presented, underpinned by a parametrised line profile model which captures the relevant broadening and splitting mechanisms, including the Zeeman, Stark and Doppler processes. Interpretation of JET divertor plasma measurements, in combination with synthetic data simulation results, highlights the importance of complementary Balmer and Paschen series measurements for refining parameter estimates in divertor plasmas.