Z. C. Yang

Spatiotemporal characterization of zonal flows with multi-channel correlation Doppler reflectometers in the HL-2A Tokamak

The oscillations of poloidal plasma flows induced by radially sheared zonal flows are investigated by newly developed correlation Doppler reflectometers in the HL-2A tokamak. The non-disturbing diagnostic allows one to routinely measure the rotation velocity of turbulence, and hence the radial electric field fluctuations. With correlation Doppler reflectometers, a three-dimensional spatial structure of geodesic acoustic mode (GAM) is surveyed, including the symmetric feature of poloidal and toroidal Er fluctuations, the dependence of GAM frequency on radial temperature and the radial propagation of GAMs. The co-existence of low-frequency zonal flow and GAM is presented. The temporal behaviors of GAM during ramp-up experiments of plasma current and electron density are studied, which reveal the underlying damping mechanisms for the GAM oscillation level.

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.

Observation of the double e-fishbone instability in HL-2A ECRH/ECCD plasmas

Two m/n = 1/1 kink modes excited by energetic electrons (called double e-fishbone) have been observed near the q = 1 flux surfaces in the HL-2A discharges. The negative magnetic central shear configuration was achieved with localized electron cyclotron resonance heating and electron cyclotron current drive during plasma current ramp-up. The features of the modes have been first shown by advanced 2D electron cyclotron emission imaging (ECEI) system. From ECEI, two m/n = 1/1 modes propagating in the opposite directions poloidally have been clearly observed. These modes can be found only in low density discharge, and their frequencies are close to the precessional frequency of the trapped energetic electrons. More interestingly, the thermal energy transfer between the two modes was revealed by this new diagnostic, which is found to be related to the nonlinear interaction of the two modes and local electron thermal transport.

Experimental observation of turbulence transition and a critical gradient threshold for trapped electron mode in tokamak plasmas

In HL-2A and J-TEXT ohmic confinement regimes, an electrostatic turbulence with quasi-coherent characteristics in spectra of density fluctuations was observed by multi-channel microwave reflectometers. These quasi-coherent modes (QCMs) were detectable in a large plasma region ( r / a ∼ 0.3 − 0.8). The characteristic frequencies of QCMs were in the range of 30–140 kHz. The mode is rotated in the electron diamagnetic direction. In the plasmas with QCMs, trapped electron mode (TEM) was predicted to be unstable by gyrokinetic simulations. The combined experimental results show that the TEM is survived in the linear ohmic confinement regime of plasmas. The quasi-coherent TEM was replaced by broad-band fluctuations when the plasma transits from linear to saturated ohmic confinement regime. The observation was strongly related to the turbulence transition from TEM to ion temperature gradient mode. A critical gradient threshold for TEM excitation in electron temperature gradient was directly found. The effect of ...

Excitation of edge plasma instabilities and their role in pedestal saturation in the HL-2A tokamak

In HL-2A, the characteristics of the edge plasma instabilities and their effects on the dynamical evolution of the pedestal in H-mode plasmas have been investigated. In the edge pedestal region with steep pressure gradient, a quasi-coherent mode (QCM) has been observed in density fluctuations with a frequency range of 50–100 kHz. It appears during the edge localized mode (ELM)-free period after the L–H transition and prior to the first ELM. A threshold in the pedestal density gradient has been identified for the excitation of this mode. The QCM can also be observed during inter-ELM periods. It is excited early in the inter-ELM period, and disappears when the ELM onset starts. The radial wave-number of the mode is estimated with two radially separated reflectometers. It shows that the mode is radially propagating inward. The poloidal wave number estimated with the Langmuir probes is k θ ~ 0.43 cm−1. The mode propagates poloidally in the electron diamagnetic direction in the plasma frame. The toroidal mode number, deduced from Mirnov signals, is n ~ 7. The corresponding poloidal mode number is m ~ 21 according to the local safety factor value. The analysis for the dynamical evolution of the pedestal during the ELM cycle clearly shows that the mode is excited before the ELM onset. During and after the ELM crash, the mode disappears. It suggests that the QCM is driven by the pedestal density gradient, and the mode in return regulates the pedestal density evolution.

Synchronous oscillation prior to disruption caused by kink modes in HL-2A tokamak plasmas

A class of evident MHD activities prior to major disruption has been observed during recent radiation induced disruptions of the HL-2A tokamak discharges. It can be named SOD, synchronous oscillations prior to disruption, characterized by synchronous oscillation of electron cyclotron emission (ECE), core soft x-ray, Mirnov coil, and radiation signals at the divertor plate. The SOD activity is mostly observed in a parametric regime where the poloidal beta is low enough before disruption, typically corresponding to those radiation-induced disruptions. It has been found that the m/n = 2/1 mode is dominant during the SODs, and consequently it is the drop of the mode frequency and the final mode locking that lead to thermal quench. The mode frequency before the mode locking corresponds to the toroidal rotation frequency of the edge plasma. It is also found that during SODs, the location of the q = 2 surface is moving outward, and most of the plasma current is enclosed within the surface. This demonstrates that the current channel lies inside the rational surface during SOD, and thus the resistive kink mode is unstable. Further analysis of the electron temperature perturbation structure shows that the plasma is indeed dominated by the resistive kink mode, with kink-like perturbation in the core plasma region. It suggests that it is the nonlinear growth of the m/n = 2/1 resistive kink mode and its higher order harmonics, rather than the spontaneous overlapping of multiple neighboring islands, that ultimately triggered the disruption.

Note: Upgrade of electron cyclotron emission imaging system and preliminary results on HL-2A tokamak.

The electron cyclotron emission imaging system on the HL-2A tokamak has been upgraded to 24 (poloidally) × 16 (radially) channels based on the previous 24 × 8 array. The measurement region can be flexibly shifted due to the independence of the two local oscillator sources, and the field of view can be adjusted easily by changing the position of the zoom lenses. The temporal resolution is about 2.5 μs and the achievable spatial resolution is 1 cm. After laboratory calibration, it was installed on HL-2A tokamak in 2014, and the local 2D mode structures of MHD activities were obtained for the first time.

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.

Pedestal dynamics across low to high confinement regime in the HL-2A tokamak

Interactions among pedestal shear flows, turbulence, and the formation of the edge transport barrier have been studied in H-mode plasmas of the HL-2A tokamak by multi-channel Doppler reflectometry with high spatiotemporal resolution. Geodesic acoustic mode (GAM) has been observed during the L-I-H transition. It has been observed that the plasma transits into the I-phase when the mean E × B shear flow reaches a critical value. The bi-spectrum analysis has shown that there is a strong interaction between GAM and limit cycle oscillation (LCO), and the energy transfer is from GAM to LCO, suggesting that GAM can assist the L-I transition. The regulation of the edge turbulence by LCOs helps to build the steep pedestal and initialize the confinement improvement of the plasma. It has been found that the mean E × B shear flow is further increased just before the I-H transition, accompanied by the turbulence suppression, leading to the edge transport reduction and the pedestal formation. It has been demonstrated th...

Formation and evolution of quasi-interchange convection cell on the HL-2A tokamak

Formation and evolution of quasi-interchange convection cell have been observed for the first time by an electron cyclotron emission imaging (ECEI) system on the HL-2A tokamak. The instability with mode numbers of m/n = –1/–1 and a frequency of 2–4 kHz propagates in the electron diamagnetic drift direction. The mode downgrades the inner stored energy WE and enhances the divertor Hα signal, which indicates a degradation of plasma confinement. The ECEI reveals that the colder core temperature fluctuation distorts to a crescent shape, while a bubble originates from hot fluctuation gradually and draws into the concave side of the crescent. A finger-like structure can be observed during the convection process, and it presents a typical feature of the quasi-interchange model. The radial structure of the interchange mode obtained from electron cyclotron emission is quite narrow at the high field side but relatively wider at the low field side. Further analysis suggests that the discharge parameters are favorable for the excitation of interchange mode, and the stability criterion is violated in the core region. Moreover, the experimental frequency agrees with that given by the linear dispersion relation of interchange instability. The large grow rate suggests that the mode grows on a very short time scale.Formation and evolution of quasi-interchange convection cell have been observed for the first time by an electron cyclotron emission imaging (ECEI) system on the HL-2A tokamak. The instability with mode numbers of m/n = –1/–1 and a frequency of 2–4 kHz propagates in the electron diamagnetic drift direction. The mode downgrades the inner stored energy WE and enhances the divertor Hα signal, which indicates a degradation of plasma confinement. The ECEI reveals that the colder core temperature fluctuation distorts to a crescent shape, while a bubble originates from hot fluctuation gradually and draws into the concave side of the crescent. A finger-like structure can be observed during the convection process, and it presents a typical feature of the quasi-interchange model. The radial structure of the interchange mode obtained from electron cyclotron emission is quite narrow at the high field side but relatively wider at the low field side. Further analysis suggests that the discharge parameters are favorable...