Collective Thomson scattering by means of Pulsed Spectropolarimetry, an advanced diagnostic for TAE’s Copernicus FRC device

Pulsed polarimetry[1] is a LIDAR technique which uses the intensity and state of polarization of the emission to yield local measurements of density, n_e(s), and magnetic field, B_||(s), along the pulse trajectory in a magnetized plasma. With the recent generation of multi-mJ ultrashort pulsed THz sources in the 0.2-2THz range, the Pulsed Polarimetry technique has become applicable to all MFE plasmas with densities of 0.3-1x10²⁰m^-3 and confining magnetic fields of 0.1-2T. The longest wavelengths are enabling for spectral measurements of ion density fluctuations using Collective Thomson scattering (CTS) to yield spatially distributed measurements of ion temperature, T_i(s) and fast ion dynamics. As devices, such as TAE’s Copernicus device, approach fusion parameters, the diagnostic mission becomes one of control and optimization. Pulsed polarimetry is well suited for field reversed configuration (FRC) equilibria by addressing key physics issues through the direct measurement of the field reversed magnetic profile with an inference of the radial current density profile using Ampere’s law. With the extension of pulsed polarimetry to distributed spectral measurements, pressure profiles can be determined as well. A novel heterodyne CTS Pulsed Spectropolarimeter design will be discussed for the Copernicus FRC equilibria.

[1] R J Smith, “Nonperturbative measurement of the local magnetic field using pulsed polarimetry for fusion reactor conditions (invited)^a)”, Rev. Sci. Instru., 79, 10E703(2008)