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.

A new multi-channel far infrared (FIR) laser interferometer was built up and applied to HL-2A. The unique feature of real-time heterodyne interferometer is the combination of high power radiation source (300 mW), lower noise room temperature detector (noise temperature below 6000 K) with good spatial resolution of 7 cm. Various parameters are optimized for maximum laser output power. Zero crossings of the signals are counted with field programmable gate array (FPGA) digital circuitry yielding the resolution of 1/1000 fringe. The newly measured results including density fluctuation are also presented.

Investigations on density limit have been performed under a variety of discharge conditions on HL-1M, which include hydrogen isotope, siliconized wall coating and a variety of fuelling methods (gas puff, pellet and supersonic molecular beam (SMB) injection). Detailed analysis shows that the HL-1M density limit is a disruptive limit and related to first wall recirculating and fuelling methods. The destruction of the balance between radiation and input power is the main reason for the density disruption.

A preliminary experiment for triggering a plasma current quench by means of injection of high_Z impurities has been performed on the HL_1M before the major disruption.Using injection of impurity with higher charge of the nuclei allows u s to increase the radiation cooling on a 3?ms time scale and the electronic tem perature loss contract is 80% before current terminates.It can be a simple and p otential approach to decrease significantly the plasma thermal energy and the ma gnetic field energy before a disruption and to a safe plasma termination.

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