Multichannel submillimeter interferometer/polarimeter for TdeV

An interferometer/polarimeter has been developed and tested for operation on the TdeV tokamak. The system operates at 393.6 μm on 7 channels equally spaced over the plasma diameter. The interferometer has operated routinely for six years and is used for density feedback control on TdeV, with very little supervision and maintenance. A sensitivity of 2.5×10−16 m−2 and a response time of 5 μs enables small density perturbations (Δn≥0.005n̄ e ) and fast transients to be resolved. With the appropriate choice of assumptions and boundary conditions, the system can supply credible electron density profiles together with their temporal evolution, using as isocontours the flux surfaces obtained from equilibrium calculations. When operated as a polarimeter, the system now routinely provides current‐density profile measurements as well. Optical cross‐talk between channels, identified as the main obstacle to the extension of the interferometer to polarimetry, was mastered using optical isolation techniques. The system can now resolve, on each chord, a Faraday rotation of 1.5 mrad, limited by vibration, cross‐talk and deflection. Using a parametric approach, poloidal field and current‐density profiles, as well as the axial safety factor and internal inductance of the discharge, are made available after each shot. The apparatus will be used for current‐transport, MHD‐equilibrium and current‐drive studies on TdeV.