First electron density fluctuation measurement via the HCOOH laser far-forward collective scattering on HL-2A tokamak

Based on the fact that the scattered power is proportional to the square of the electron density fluctuation amplitude, the four-channel formic acid (HCOOH, λ=432.5 um) laser interferometer has been firstly upgraded to measure the far-forward collective scattering (FCS) from the electron density fluctuation (ne) on HL-2A tokamak in 2014–15 experimental campaign. The HCOOH-FCS system provides the line-integrated measurement of fluctuations covering the wavenumber range: k⊥ <1.6 cm−1 (perpendicular to the magnetic field). A lot of electron density fluctuations have been successfully measured by the HCOOH-FCS. In especial, the high-frequency density fluctuation (up to 500 kHz) induced by the core-localized Alfvenic eigenmode (AE) has been observed by the innermost scattering chord. The HCOOH-FCS system will be extremely helpful for the research of energetic particles instability on HL-2A.