Recent Progress of the HL-2A Multi-Channel HCOOH Laser Interferometer/Polarimeter

Abstract

A multichannel methanoic acid (HCOOH, λ = 432.5 µm) laser interferometer/polarimeter is being developed from the previous eight-channel hydrogen cyanide (HCN, λ=337 µm) laser interferometer in the HL-2A tokamak. A conventional Michelson-type interometer is used for the electron density measurement, and a Dodel-Kunz-type polarimeter is used for the Faraday rotation effect measurement, respectively. Each HCOOH laser can produce a linearly polarized radiation at a power lever of ∼30 mW, and a power stability <10% in 50 min. A beam waist (diameter d0 ≈12.0 mm, about 200 mm away from the outlet) is finally determined through a chopping modulation technique. The latest optical layout of the interferometer/polarimeter has been finished, and the hardware data processing system based on the fast Fourier transform phasecomparator technique is being explored. In order to demonstrate the feasibility of the diagnostic scheme, two associated bench simulation experiments were carried out in the laboratory, in which the plasma was simulated by a piece of polytetrafluoroethene plate, and the Faraday rotation effect was simulated by a rotating half-wave plate. Simulation results agreed well with the initial experimental conditions. At present, the HCOOH laser interferometer/polarimeter system is being assembled on HL-2A, and is planned to be applied in the 2014-2015 experimental campaign.

Alfvénic ion temperature gradient activities in a weak magnetic shear plasma

“…The HL-2A plasma is almost a circular cross-section equilibrium although it corresponds to a divertor configuration in all during the discharges. The electron density was detected using a multi-channel HCOOH laser interferometer [12]. The polodial mode number m is measured using the electron cyclotron emission imaging (ECEI) signals by the spatial two-point correlation method [13].…”

“…The experiments discussed here are performed in deuterium plasmas with plasma current I p 150-170 kA, toroidal field B t 1.32-1.40 T, and an edge safety factor q a 4.2-4.8 on HL-2A, which has the major/minor radius R 0 /a = 1.65 m/0.4 m. The HL-2A plasma is almost a circular cross-section equilibrium although it corresponds to a divertor configuration in all during the discharges. The electron density was detected using a multi-channel HCOOH laser interferometer [12]. The polodial mode number m is measured using the electron cyclotron emission imaging (ECEI) signals by the spatial two-point correlation method [13].…”

Compact phase comparison system for the synthetic HCOOH laser diagnostic system in HL-2A

“…In contrast to some diagnostic systems such as electrostatic probe system [8], microwave reflectometer [9,10], far-infrared (FIR) laser can transit through the plasma core without contaminating the plasma [11]. By using different measurement schemes, it can diagnose density profiles, density fluctuations, or plasma current density distribution.…”

Progress of the synthetic HCOOH laser diagnostic system on HL-2A tokamak

“…Far-infrared (FIR) laser-based Polarimeter/Interferometer is the most accepted diagnostic used to measure the line-integrated electron density and Faraday rotation angle (as well as their fluctuations), and then achieve the current density profile after equilibrium reconstruction [5][6][7][8]. Limited by HL-2A spatial structure, FIR laser Polarimeter/Interferometer was designed to be a combination of a four-channel Michelson-type Interferometer below the plasma middle-plane (Z = −24.5, −17.5, −10.5, −3.5 cm) and a four-channel Faraday-effect Polarimeter above the plasma middleplane (Z = 3.5, 10.5, 17.5, 24.5 cm) [9,10]. Here, Z expresses the vertical distance relative to the geometry center of vacuum vessel.…”