Development of a multi-channel visible bremsstrahlung measurement system for the effective charge in the versatile experiment spherical torus (VEST).

A diagnostic system for measuring the effective charge in the versatile experiment spherical torus (VEST) has been developed. The system utilizes a toroidal array to observe the plasma radius on the low magnetic field side, providing a spatially resolved Zeff. The target wavelength of visible bremsstrahlung (VB) was carefully selected to avoid contamination by line emissions. The detector signal was calibrated using a halogen light source and an integrating sphere to obtain an absolute value of the radiative power from each chord. The local emissivity profile was reconstructed from the line-integrated VB emission using the Abel inversion method. Reconstruction tests were performed on various shapes of phantom profiles to effectively reconstruct the local emissivity from the measurements. We found that the initial measurements of the multi-channel VB system were consistent with the results of other independent measurements, supporting the validity of the new measurements. Finally, we obtained the initial result of Zeff in the VEST.