Speaker
Mr
Oisin McCormack
(RFX - University of Padova)
Description
The depolarisation of the Thomson scattering (TS) radiation, a relativistic effect noticeable only in very high $T_e$ plasmas and never before observed in a fusion machine, is measured for the first time in JET. This depolarisation effect is at the basis of polarimetric TS, an alternative method for the measurement of the plasma electron temperature, suitable for high $T_e$ plasmas such as in ITER. In our test, we employed unused fibres, at the fringe of the JET HRTS system, to detect both polarisation components of the TS radiation in high Te discharges, with measured $T_e$ up to 8 keV. Polarisers perpendicular to the laser light were placed in front of a fibre pair observing a region close to the plasma core, with another adjacent fibre pair used as a control with no polariser. The intensity ratio of the unpolarised component to the full signal is found to have an approximately linear relationship with the electron temperature, as expected from the theory. The measured intensity ratio is consistent with the theory, taking into account sensitivity coefficients of the two measurement channels determined with post-experiment recalibrations and Raman scattering. As polarimetric TS is not dependent on wavelength it can provide significant advantages as an alternative to the conventional TS for measurements at high $T_e$, and may be integrated into a standard TS system.
Primary author
Prof.
Leonardo Giudicotti
(Padova University - Department of Physics and Astronomy)
Co-authors
Dr
Joanne Flanagan
(CCFE Culham Science Centre)
Dr
Mark Kempenaars
(CCFE Culham Science Centre)
Mr
Oisin McCormack
(RFX - University of Padova)
Dr
Roberto Pasqualotto
(Padova University - Department of physics and astronomy)
