X-mode scattering for the measurement of density fluctuations on TFTR (invited)

Most microwave scattering measurements in tokamaks are done with the polarization of the incident radiation approximately parallel to the toroidal field (ordinary mode) and at a frequency which is sufficiently above the plasma frequency ω_pe that refraction effects are small. However, as confinement devices get hotter, drift wave turbulence is expected to occur at longer wavelengths. In order to observe density fluctuations associated with these phenomena with good spatial resolution, the ideal probe wavelength λ_i is near or beyond the ordinary cutoff at λ_i = 2πc/ω_pe. As a result, we have considered the propagation and scattering of the extraordinary (X) mode where the incident polarization is nearly perpendicular to the confining fields and where the cutoff density is significantly higher than in the ordinary mode. We have modeled the propagation of the X mode in a tokamak field geometry and shown that the polarization typically remains perpendicular to the local field along the ray path, simplifying the interpretation of the scattering results. We have calculated the single-particle cross section and give a formula for X-to-X-mode scattering. An apparatus for carrying out these measurements on TFTR at λ_i = 0.5 cm will be described. For B_T=5.0 T, the X-mode cutoff density in this case is 1.5×10¹⁴ cm^-3, so that a major part of the operating density range can be studied by this technique.