Speaker
Dr
Vladimir Mirnov
(Department of Physics, University of Wisconsin-Madison, Madison, WI 53706, USA)
Description
Several major laser diagnostics are under development for measurement of plasma density, electron temperature, magnetic field and current density in ITER: toroidal interferometer/polarimeter (TIP), poloidal polarimeter (PoPola) and Thomson scattering systems (TS). Each of them is based on the electron response to laser light propagating through the plasma. We examine the effects of relativistic electron thermal motion on the refractive indices and polarization of high-frequency electromagnetic waves (specifically laser light, both directed and scattered). Two different topics are covered: interferometry/polarimetry (I/P) and polarization of Thomson scattered light, unified by the importance of relativistic (quadratic in $v_{Te}/c$) electron kinetic effects.
For I/P applications, rigorous analytical results are obtained perturbatively by expansion in powers of the small parameter $\tau = T_{e}/m_{e}c^{2}$, where $T_{e}$ is electron temperature and $m_{e}$ is electron rest mass. There are two physically different sources of linear in $\tau$ thermal corrections which are comparable in magnitude but contribute with opposite signs: non-relativistic (NR) Doppler-like effects and the relativistic electron mass dependence on the velocity. The relativistic mass effect reduces the magnitude of the NR correction for the Cotton-Mouton effect and changes the signs of the corrections to the interferometric phase and Faraday rotation angle. Experimental validation of the analytical models has been made by analyzing data of more than 1200 pulses collected from high-$T_{e}$ JET discharges. Good agreement with the full relativistic model was demonstrated, and disagreement with the cold plasma and NR models. Based on this validation the relativistic analytical expressions are included in the error analysis and design projects of the ITER TIP and PoPola systems. The linear in $\tau$ model has been recently extended from Maxwellian to a more general class of anisotropic electron distributions to account for distortions caused by equilibrium current, ECRH and RF current drive effects.
The polarization properties of incoherent Thomson scattered light are being examined as a method of $T_{e}$ measurement relevant to ITER operational regimes. The theory is based on Stokes vector transformation and Mueller matrices formalism. The general approach is subdivided into frequency-integrated and frequency-resolved applications. For each of them, the exact relativistic solutions are presented in the form of Mueller matrix elements averaged over the relativistic Maxwellian distribution function. These exact analytic calculations are performed without any approximations for the full range of incident polarizations, scattering angles, and electron thermal motion from non-relativistic to ultra-relativistic. Newly obtained results for the frequency-resolved case have been verified by comparison with the numerical code developed by L. Giudicotti and co-authors. The results are in a good agreement (< 0.01% deviations) verifying both calculations. The precise analytic expressions provide output much more rapidly than relativistic kinetic numerical codes. The speed will be critical to the development of real time feedback control of burning plasmas.
This work was supported by the U. S. Department of Energy
Primary author
Dr
Vladimir Mirnov
(Department of Physics, University of Wisconsin-Madison, Madison, WI 53706, USA)
Co-author
Dr
Daniel Den Hartog
(Department of Physics, University of Wisconsin-Madison, Madison, WI 53706, USA)
