Ray-tracing analysis for combined Doppler backscattering and cross-polarization scattering diagnostic on the HL-2M tokamak.

To measure plasma density and magnetic fluctuations on the HL-2M tokamak simultaneously, a new diagnostic system combining Doppler backscattering (DBS) and cross-polarization scattering (CPS) is under development. It is essential to understand the propagation of injected and scattered rays to support the electronic/quasi-optical design and subsequent interpretation of the detected signals of the multi-channel DBS/CPS measurements. Thus, ray-tracing analysis with the axisymmetric ray-tracing code BORAY has been performed to estimate the scattering location and wavenumbers of the density and magnetic fluctuations. In addition, the influence of accordance between toroidal and poloidal launch angles is investigated. The received DBS/CPS signal quality can be improved by matching the parallel wavenumber in the direction of magnetic field lines.

Recent advances in high- ß N experiments and magnetohydrodynamic instabilities with hybrid scenarios in the HL-2A Tokamak.

Over the past several years, high- ß N experiments have been carried out on HL-2A. The high- ß N is realized using double transport barriers (DTBs) with hybrid scenarios. A stationary high- ß N ( > 2 ) scenario was obtained by pure neutral-beam injection (NBI) heating. Transient high performance was also achieved, corresponding to ß N ≥ 3 , n e / n e G ∼ 0.6 , H 98 ∼ 1.5 , f b s ∼ 30 % , q 95 ∼ 4.0 , and G ∼ 0.4 . The high- ß N scenario was successfully modeled using integrated simulation codes, that is, the one modeling framework for integrated tasks (OMFIT). In high- ß N plasmas, magnetohydrodynamic (MHD) instabilities are abundant, including low-frequency global MHD oscillation with n = 1, high-frequency coherent mode (HCM) at the edge, and neoclassical tearing mode (NTM) and Alfvénic modes in the core. In some high- ß N discharges, it is observed that the NTMs with m / n = 3 / 2 limit the growth of the plasma energy and decrease ß N . The low-n global MHD oscillation is consistent with the coupling of destabilized internal (m/n = 1/1) and external (m/n = 3/1 or 4/1) modes, and plays a crucial role in triggering the onset of ELMs. Achieving high- ß N on HL-2A suggests that core-edge interplay is key to the plasma confinement enhancement mechanism. Experiments to enhance ß N will contribute to future plasma operation, such as international thermonuclear experimental reactor .

A multiplexer-based multi-channel microwave Doppler backward scattering reflectometer on the HL-2A tokamak.

The Doppler backward scattering (DBS) reflectometer has become a well-established and versatile diagnostic technique for the measurement of density fluctuations and flows in magnetically confined fusion experiments. In this work, a novel multiple fixed-frequency array source with a multiplexer technique is developed and applied in the multi-channel DBS system. The details of the system design and laboratory tests are presented. Preliminary results of Doppler shift frequency spectra measured by the multi-channel DBS reflectometer systems are obtained. Characteristics of plasma rotation and turbulence before and after supersonic molecular beam injection are analyzed.

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

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 (k⊥ρ ∼ 1-2, k⊥ ∼ 2-9 cm-1) 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.