Polarmetric phase imaging for plasma current profile measurement

摘要

Detailed knowledge of the structure of the poloidal magnetic field in tokamak plasmas is essential to an understanding of the startup phase, disruptions and general tokamak confinement problems. A non-perturbing technique for obtaining a measure of the magnetic field is via Faraday rotation of the polarization of a probe beam. The choice of probing wavelength is a compromise between maximizing the expected Faraday rotation angle (αλ²) whilst minimizing refractive bending due to plasma density gradients (αλ²) and elliptization of probe, beam due to plasma birefringence (Voight effect αλ³). The far infrared region of the spectrum is most useful in this application, though for most tokamak plasmas, maximum rotation angles are ≳20° so that some care must be taken to ensure an accurate measurement is obtained.