Prototype Heterodyne Interferometer for ECLAIR and SSX

We present results from a prototype heterodyne interferometer being built and tested at the SSX facility at Swarthmore. Our plan is to calibrate the new heterodyne system against our Mach-Zehnder system that has been calibrated and in use for decades. The heterodyne interferometer employs an Acoustic Optical Modulator (AOM) to generate a modulated HeNe laser beam at 80 MHz that acts as a carrier frequency. The modulated beam is interfered with a scene beam whose phase has been modulated by the plasma, acting as a local oscillator at up to 10 MHz. The scene beam travels several meters through the plasma and is reflected and re-columnated with a spherical mirror. The outputs are shifted (90^o and 0^o) and mixed with rf electronics to generate the sine and cosine of the signal, and the 80 MHz is filtered out. The heterodyne system is much more compact than our Mach-Zehnder system (which spans the entire machine). All of the optical components fit on a 12x18 inch breadboard. SSX and ECLAIR plasmas are similar, high-density, gun-produced, relaxed MHD states. Typical peak electron density in both is n_e ≈10²² m^-3.