Development of a compact fiber interferometer on HFRC

A compact fiber interferometer system was developed on the HUST Field Reversed Configuration (HFRC) for plasma electron density measurements. The system employs a Mach–Zehnder configuration using a 1550 nm continuous-wave laser as the light source and includes seven probe chords arranged within the θ-pinch coils in the HFRC plasma formation region. A 40 MHz acousto-optic modulator is adopted for heterodyne detection. Its main body is enclosed in a 6 mm thick aluminum shielding box, with 1550 nm optimized single-mode fibers and high-precision fiber couplers (1525–1575 nm working bandwidth, greater than 90% coupling efficiency, single-mode compatibility) mounted on a quartz tube, occupying less than 0.5 m² for compactness and portability. HFRC experiments demonstrate that it captures plasma dynamics: poloidal density follows coil current trends, axial channels track density decay from 1.577×10²¹ m⁻² to 6.485×10²⁰ m⁻² and migration velocity reduction. The phase resolution of the interferometers is within 0.044 rad.