Design of far-infrared three-wave polarimeter-interferometer system for the J-TEXT tokamak

A multichannel three-wave far-infrared polarimeter-interferometer will be constructed on the J-TEXT tokamak (R=1.05 m, a=0.27 m, B(T)≤3 T, and I(P)≤350 kA) for current density profile and electron density profile measurements. The system will adopt the three-wave polarimeter configuration which was first introduced on RTP. Three 432.5 μm HCOOH lasers pumped by three CO(2) lasers separately will be adopted, which could generate high output power, nearly 50 mW at each cavity. Two of them will be counter-rotating circularly polarized to probe the Faraday angle, while the third laser will be used as a local oscillator to get the phase shift caused by electron density. Excellent port access (600×76 mm(2)) and high laser power would promise a profile measurement across the whole plasma section with good signal quality. A high-speed digital phase comparator with a few (∼2) μs temporal resolution will be developed, so that fast changes of current or density profile could be measured. Six channels will be installed in the first stage.