An ion Doppler spectrometer instrument for ion temperature and flow measurements on SSPX^a)

A high-resolution ion Doppler spectrometer (IDS) has been installed on the sustained spheromak plasma experiment to measure ion temperatures and plasma flow. The system is composed of a $1m$ focal length Czerny–Turner spectrometer with a diffraction grating line density of $2400lines∕mm$⁠, which allows for first order spectra between 300 and $600nm$⁠. A 16-channel photomultiplier tube detection assembly combined with output coupling optics provides a spectral resolution of $0.0126nm$/channel. We calculate in some detail the mapping of curved slit images onto the linear detector array elements. This is important in determining the wavelength resolution and setting the optimum vertical extent of the slit. Also, because of the small wavelength window of the IDS, a miniature fiber-optic survey spectrometer sensitive to a wavelength range $200–1100nm$ and having a resolution of $0.2nm$ is used to obtain a time-integrated spectrum for each shot to verify specific impurity line radiation. Several measurements validate the systems operation. Doppler broadening of $CIII$ $464.72nm$ line in the plasma shows time-resolved ion temperatures up to $250eV$ for hydrogen discharges, which is consistent with neutral particle energy analyzer measurements. Flow measurements show a sub-Alfvénic plasma flow ranging from $5to45km∕s$ for helium discharges.