MHD instability fluctuation measurement on the HL-2A tokamak by using the HCN laser interferometer

The multichannel HCN laser interferometer has been routinely used to measure the electron density (n (e) ) on the HL-2A tokamak for several years. In order to explore its capability for qualitatively evaluating the Magnetohydrodynamic (MHD) fluctuation in plasmas from the probing signal, we implemented two important modifications on the hardware of channel #3 (r = -10.5 cm): (1) application of the high-response Schottky diode waveguide detector, and (2) utilization of a preamplifier with a broad pass-band of up to 500 kHz so as not to filter the fluctuation component mixed with the intermediate frequency (IF). Many MHD fluctuation phenomena, such as tearing modes (TMs), fishbones, long lived modes (LLMs), have been successfully detected by using the modified channel #3 in the HL-2A experimental campaign, and the results were quite coincident with those derived from the magnetic probe and the soft X-ray systems. We propose to apply this novel approach to more channels of the HCN laser interferometer.