II Encontro Ibérico de Espectroscopia, Setembro de 2002

	X-ray spectroscopy diagnostics in the tokamak TCV, in the frame of a collaboration between CFN and CRPP
	T. I. Madeira, P. Amorim, B.P. Duval e C.A.F. Varandas
	Resumo
	A collaboration between Centro de Fusão Nuclear and Centre de Recherches en Physique des Plasmas have been established since 1996, in the field of X-ray spectroscopy. The first diagnostic installed on the tokamak TCV (CRPP, Lausanne) in the frame of this collaboration was a Soft X-ray Pulse Height Analysis (PHA) spectrometer to measure the electron temperature in the plasma core and to survey the plasma impurity content, from the soft X-ray plasma emissions. Individual photons are detected by a nitrogen cooled germanium diode and ranked according to their energy. The resulting histogram corresponds to the SXR energy spectrum. Spectral lines when observed, which correspond to characteristic electron transitions between atomic energy levels, indicate the presence of impurities and permit their identification. Plasma temperature is deduced from the bremsstrahlung and recombination radiation spectra, which do not require an absolute intensity calibration, assuming a maxwellian distribution function. The original associated analogue pulse conditioning electronics is now being replaced by a digital processing unit (DSP) in order to enhance the PHA spectroscopic capabilities. In the frame of the same collaboration, a Rotating Crystal Spectrometer, which is a long term loan apparatus from Princeton Plasma Physics Laboratory to Centro de Fusão Nuclear, will be used on TCV to determine ion temperatures and study the impurities ionisation states. The basic principle of operation of this spectrometer is based on Bragg's Law : , where ë is the wavelength of the illuminating radiation, è is the angle made by the radiation path to the crystal planes, d is the distance between atomic planes in the crystal and n is the order of reflection. A photon that is not in a narrow energy band of satisfying Bragg's Law is not diffracted, so the device acts as a spectrometer passing only a particular wavelength and its higher orders. The crystal can be either set at a particular angle, in which case the output signal represents the time history of the line emission (monochromatic mode), or it may be rotated so that different angles are continuously sampled and the spectrometer sweeps an entire spectral range.