Development of a combined interferometer using millimeter wave solid state source and a far infrared laser on ENN’s XuanLong-50 (EXL-50)

Abstract

A millimeter wave solid state source - far infrared laser combined interferometer system (MFCI) consists of a three-channel 890 GHz hydrogen cyanide (HCN) laser interferometer and a three-channel 340 GHz solid state source interferometer (SSI) is developed for real-time line-integrated electron density feedback and electron density profile of the EXL-50 spherical tokamak device. The interferometer system is a Mach-Zehnder type, with all probe-channels measured vertically, covering the plasma magnetic axis to the outermost closed magnetic plane. HCN laser interferometer uses an HCN laser with a wavelength of 337 µm as the light source, and uses a rotating grating to modulate a 100 kHz beat frequency signal, giving a temporal resolution of 10 μs. The SSI uses two independent 340 GHz solid-state diode source as the light source, the frequency of the two sources is adjustable, the temporal resolution of SSI can reach 1 μs by setting the frequency difference of the two lasers at 1 MHz. The main optical path of the two interferometers is compactly installed on a set of double-layer optical platform directly below EXL-50. Dual optical path design using corner cube reflectors avoids the large support structures. Collinear the probe-beams of two wavelengths, then the phase error caused by vibration can be compensated. At present, the phase noise of the HCN Interferometer is 0.08 rad，corresponding to a line-integrated electron density of 0.88×1017m-2, one channel of line-integrated electron density was obtained by the MFCI system, and the highest density measured is about 0.7×1019m-2.

Electron density measurement by the three boundary channels of HCOOH laser interferometer on the HL-3 tokamak

“…On HL-3, three corner-cube-reflectors (CCR) which are made of copper with gold-plating are used for the boundary interferometry channels. CCR consists of three mutually perpendicular plane mirrors [15,16]. Due to optic characteristic of the CCR, the reflected wave can propagate along the same path of incident wave by controlling the beam waist at the central area of CCR.…”

Development of a dual HCN laser interferometer on a small tokamak device

“…Far-infrared (FIR) laser interferometry is a precise method for determining the electron density in plasma by measuring the phase perturbation of the probe wave [1]. The hydrogen cyanide (HCN) laser interferometer is a widely used heterodyne interferometer based on the Mach-Zehnder interferometer structure for electron density diagnostic systems in various tokamak devices [2][3][4]. The temporal resolution of the measurement increases as the beat frequency of the interferometer system increases [5].…”