HTPD 2018

A Beam Emission Spectroscopy system is being developed and deployed at the HL-2A tokamak to measure local low wavenumber (k_⊥ ρ_I<1) density fluctuations by observing Doppler-shifted emission from a 50 kV deuterium heating neutral beam. High spatial resolution (∆r≤1cm,∆z≤1.5cm) measurements are obtained with a 2 MHz sampling rate. An f/1.6, f=280 mm radiation-resistant in-vacuum objective lens couples with an off-axis field lens to image the beam to a curved surface with an appropriate angle for coupling to optical fibers. A 2x3 bundle of 1-mm diameter, 10-meter fibers conveys light for each spatial channel to detector arrays. A collimating lens passes light to a 1-nm bandwidth (659.2-660.2 nm, Doppler-shifted D-alpha emission) interference filter. An aspheric lens focuses light to a 2.65 mm square photodiode. High frequency, high-gain, low-noise preamplifiers sample the light intensity at a Nyquist frequency of 1 MHz with a high S/N ratio. A first set of 16 detector channels (12 plasma observing channels, configured in a 6(radial) x 2(poloidal) array) were installed and tested at HL-2A, covering the radial zone r/a=0.7~1.05. The frequency and wavenumber spectra have been measured during various plasma conditions, and primary measurements will be presented.