A novel multi-channel quadrature Doppler backward scattering reflectometer on the HL-2A tokamak

Abstract

A novel 16-channel fixed frequency Doppler backward scattering (DBS) reflectometer system has been developed on the HL-2A tokamak. This system is based on the filter-based feedback loop microwave source (FFLMS) technique, which has lower phase noise and lower power variation compared with present tunable frequency generation and comb frequency array generation techniques [J. C. Hillesheim et al. Rev. Sci. Instrum. 80, 083507 (2009) and W. A. Peebles et al. Rev. Sci. Instrum. 81, 10D902 (2010)]. The 16-channel DBS system is comprised of four × four-frequency microwave transmitters and direct quadrature demodulation receivers. The working frequencies are 17-24 GHz and 31-38 GHz with the frequency interval of 1 GHz. They are designed to measure the localized intermediate wave-number (kρ ∼ 1-2, k ∼ 2-9 cm) density fluctuations and the poloidal rotation velocity profile of turbulence. The details of the system design and laboratory tests are presented. Preliminary results of Doppler spectra measured by the multi-channel DBS reflectometer systems are obtained. The plasma rotation and turbulence distribution during supersonic molecular beam injection are analyzed.

Progress of microwave diagnostics development on the HL-2A tokamak

“…Three frequency modulation continuous wave (FMCW) reflectometers and two amplitude modulation (AM) reflectometers are developed to measure the density profiles from low field side in the horizontal plane [31][32][33][34]. Two sweeping DBS reflectometers and four by four fixed-frequency DBS reflectometers are developed to measure the profiles of the plasma rotation and turbulence [15,35,36].…”

“…The multiple-simultaneouschannel DBS system provides significant flexibility for experimental observations, especially for the fast variation phenomena, like L-H transition, ELMs and nonlinear interaction of turbulence. To measure the density fluctuation and poloidal rotation profile simultaneously, four four-frequency (16-fixed frequencies) DBS systems have been developed on the HL-2A tokamak [15]. The unique feature of the multichannel DBS system is the development of the filter-based feedback loop microwave source (FFLMS) technique.…”

“…With the characteristic of local measurement, good accessibility, and without external perturbation to plasmas, more and more attentions have been paid to microwave diagnostics by scientists, and some new techniques have been developed and applied to the hardware development, signal processing and data interpretation in recent twenty years. So far, significant progresses have been made, especially on ECEI, microwave imaging reflectometry (MIR), multi-channel DBS reflectometer and high temporal resolution frequency modulation (FM) reflectometers [12][13][14][15][16][17][18][19].…”

Design of the cross-polarization scattering diagnostic on the HL-2A tokamak

“…Doppler backscattering (DBS) diagnostic. For DBS, a tilt angle of the microwave beam is launched relative to the normal of the cutoff layer and backscattered radiation is detected [14,15]. The CPS diagnostic was originally demonstrated on Tore Supra tokamak [16], where the effect of magnetic fluctuations on electron heat transport was studied.…”

Beam model of Doppler backscattering

“…Here, k ⊥ is the wavenumber of turbulent fluctuations perpendicular to the magnetic field, and ρ i is the ion gyroradius. Consequently, it is a widely used diagnostic for both tokamaks and stellarators [8,9,10,11,12,13,14,15,16,17,18]. Moreover, since DBS is a microwave diagnostic, it is one of the few diagnostics that can be used in burning plasma because they are resilient to high neutron fluxes [19,20].…”