Martin–Puplett multichannel far infrared heterodyne interferometer on the Helicity Injected Torus II

Abstract

A novel multichannel implementation of the Martin–Puplett interferometer (MPI) configuration designed for plasma density measurements on the Helicity Injected Torus II (HIT-II) device is described. It is believed that the system represents the first use of a Martin–Puplett interferometer as a plasma density diagnostic in the far infrared (FIR) range and the first MPI system with a frequency-encoded, polarized laser source. The system uses a continuous wave difluoromethane FIR laser (185 μm) with an output power of 70 mW. The MPI replaces a Mach–Zehnder interferometer used on HIT-II while retaining the existing heterodyne scheme which employs a rotating grating wheel that selectively frequency shifts one polarization component of the laser. The advantages of the Martin–Puplett system over the Mach–Zehnder system are (1) the capability to expand to multiple channels, (2) the ability to easily scan through various impact parameters on a shot-to-shot basis, (3) minimization of refractive misalignments due to edge density gradients, and (4) doubling the phase sensitivity using the double pass probe arm. The advantages of the MPI over a modified Michelson system using corner cubes include (1) isolation of the laser source from the detector by the polarizing beam splitter, (2) suppression of spurious reflections at the detector, and (3) a twofold increase in power obtained at the output of a Martin–Puplett interferometer relative to a Michelson interferometer. Two tangentially viewing channels are presently in operation, and each channel is capable of scanning through six impact parameters. This has enabled, for the first time, acquisition of density profile information on both coaxial helicity injection plasmas and inductively driven plasmas on the HIT-II device. Phase sensitivities of 10−2 fringe and bandwidths as high as 250 kHz have been achieved.