3D Magnetic Reconnection in SSX: Heterodyne Interferometry

We present results from a novel Helium-Neon (HeNe) Heterodyne Interferometer measuring line-averaged plasma density at the Swarthmore Spheromak Experiment (SSX). The Heterodyne Interferometer uses an Acousto-Optic Modulator (AOM) to introduce an 80 MHz modulated signal into the system, and uses Radio Frequency (RF) electronics to interfere the modulated reference beam and scene beam and extract phase information. By using RF techniques, the Heterodyne system improves on our traditional HeNe Mach-Zehnder Interferometer in its ease of set-up and space efficiency; the beams may be different lengths without compromising phase information. Here we detail the design of our Heterodyne Interferometer and present line-averaged plasma density measurements. We'll test the interferometer on the SSX device, and ultimately use it on the ECLAIR device at Helicity Space. In SSX we merge hot (T_i ≅ 65 eV), highly magnetized (B ≅ 0.4 T), high Reynolds number (R_m ≅ 1000) relaxed helical plasma plumes. Heterodyne measurements are corroborated against simultaneous Mach-Zehnder measurements, with typical peak density measurements of n_i ≅ 10²²m^-3.