Raytrace correction and in situ calibration for collective Thomson scattering diagnostics in the LHD

A fusion reaction produces energetic charged particles in plasmas for self-burning. The confinement of charged particles determines the performance of thermonuclear reactors. Collective Thomson scattering (CTS) diagnostic is one of the candidates to study the transport of MeV charged particles. In the large helical device (LHD), we have developed the CTS diagnostic using a millimeter wave of 77 GHz, 154 GHz, and 300 GHz. A heterodyne receiver detects scattered radiation at an overlap location between a probing beam and a receiving beam. The receiver is usually calibrated absolutely by using a liquid nitrogen source or a blackbody source. The radiation intensities are quite low compared with an intensity level of electron cyclotron emission (ECE) under an actual measurement condition. Thus, we have combined ECE and an electron temperature measured by an incoherent Thomson scattering. In addition, a millimeter wave of 77 GHz starts refraction even though the electron density is far below the cutoff density. The tendency is more significant when the lines of sight for the probing and the receiving beams are oblique to the magnetic field due to a density gradient. Introducing a ray trace code corrects the error of measurement locations for the refraction, the Doppler effect, and the relativistic effect. The calibration is applied to an off-timing of the modulated probing beam for subtracting the background ECE when performing the CTS diagnostic. This novel method for bulk and fast ion diagnostics is powerful and valuable under reactor-relevant experiments.