Speakers
Mr
Chris Gowers
(Retired from CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK)Dr
Hans Salzmann
(Retired from MPI f. Plasmaphysik, Garching)Dr
Per Nielsen
(Retired: Conzorzio RFX, Padova Italy)
Description
Calibration, after initial installation, of the proposed LIDAR Thomson Scattering System requires no access to the front end and does not require a foreign gas fill for Raman scattering. As already described (Salzmann et al), the variation of solid angle of collection with scattering position is a simple geometrical variation over the unvignetted region. The additional loss over the vignetted region can easily be estimated and in the case of a small beam dump located between the Be tiles, it is within the specified accuracy of the density. The only calibration needed in the course of the ITER lifetime is the absolute spectral transmission of the front optics.
With time, we expect the transmission of the two front mirrors to deteriorate. The reduction is likely to be worse towards the blue end of the scattered spectrum. By routinely comparing a calculated line integral from the measurements with the line integral measured by interferometry, we will see the first indications of such a drop in transmission. In low temperature plasmas, 3 – 5 keV, the fit will exhibit a variation with scattering position of the chi-square value. In this temperature range, the combined spectrum from the two-wavelength approach offers a means to determine the spectral variation of the transmission loss.
At the outer plasma boundary, the standard resolution of the LIDAR system is not sufficient to determine the edge gradient in an H-mode plasma. However, because of the step like nature of the signal here, it is possible to carry out a deconvolution of the scattered signals, thereby achieving an effective resolution of $\sim 1$ cm in the outer 10 – 20 cm of the plasma.
It is possible to get even better resolution at the edge by both shortening the laser pulse width and using faster detectors. Faster detectors exist with lower quantum efficiency. Combined with a shorter laser pulse width it is possible to have a spatial resolution of only $\sim 3$ cm. Deconvolution at the edge may still be applied in this case, resulting in a resolution of the edge gradient of $\sim 0.5$ cm. This approach will obviously decrease the signal to noise level slightly. In case this is a problem for the central plasma, one may bin the signals at the centre to maintain the required 7 cm resolution. Note please, that the LIDAR design allows changes of lasers, detectors and spectrometer as new technology develops.
Primary author
Dr
Per Nielsen
(Retired: Conzorzio RFX, Padova Italy)
Co-authors
Mr
Chris Gowers
(Retired from CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK)
Dr
Hans Salzmann
(Retired from MPI f. Plasmaphysik, Garching)
