Novel spatial scanning method for tokamak spectroscopy

For spectroscopic diagnostics, the problem of optical access to all poloidal regions of the plasma in a large tokamak is severe. One must scan a large angle through a small window in a high magnetic field, often at elevated temperatures. Moving systems have problems in such an environment. A method is described here which uses only a static, curved mirror at the tokamak window. The rest of the elements (field lenses, a small mirror scanner, and the spectrometer) are in another room. The details of a design for the tokamak fusion test reactor (TFTR) are presented. Scan angles of ±40° are easily achieved, the optics of the spectrometer are filled, and the scan angle in the plasma is linearly proportional to the ramp voltage applied to the scanner.