Current distribution measurement in a tokamak by FIR polarimetry (invited)

The poloidal magnetic field distribution in a tokamak can be determined by probing the plasma with an array of FIR laser beams perpendicular to BT and measuring the Faraday rotation ψ of their plane of polarization. A concomitant detection of the phase shifts Δφ provides sufficient information to take the electron density profile properly into account. For the TEXTOR tokamak (R=1.75 m, a=0.5 m, BT =2 T) a nine-channel Véron-type interferometer has been upgraded to allow simultaneous measurements of Δφ and ψ with sensitivities of about 0.01×2π and 0.15° and time resolutions of 0.1 and 3 ms, respectively. The number of channels and their distribution are sufficient to reconstruct the local poloidal field and hence the current density profile with an accuracy of about ±5%–10% at half the plasma radius. For the central current density an accuracy of ±15%–20% is achieved by slowly moving the plasma across the probe beams and thereby multiplying the number of chords especially near the magnetic axis. For rapid qualitative information the current profile is approximated by j0 [1−(r/a)2]p, and the shape parameter p is obtained by real-time conversion of measured signals in an analog computing circuit.