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Over the past several years, high-βN experiments have been carried out on HL-2A. The high-βN is realized using double transport barriers (DTBs) with hybrid scenarios. A stationary high-βN (>2) scenario was obtained by pure neutral-beam injection (NBI) heating. Transient high performance was also achieved, corresponding to βN≥3, ne/neG∼0.6, H98∼1.5, fbs∼30%, q95∼4.0, and G∼0.4. The high-βN scenario was successfully modeled using integrated simulation codes, that is, the one modeling framework for integrated tasks (OMFIT). In high-βN 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-βN discharges, it is observed that the NTMs with m/n=3/2 limit the growth of the plasma energy and decrease βN. 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-βN on HL-2A suggests that core-edge interplay is key to the plasma confinement enhancement mechanism. Experiments to enhance βN will contribute to future plasma operation, such as international thermonuclear experimental reactor .

The dispersion relation for high toroidal mode number n beta-induced Alfvén eigenmodes (BAEs) excited by magnetically trapped energetic ions generated with ion-cyclotron resonance heating via precessional resonance in low magnetic shear (s) tokamak plasma is investigated analytically and numerically. The dynamics of the energetic particles (EPs) is treated non-perturbatively and finite drift orbit width (FOW) effects are taken into account. It is found that, depending on the plasma parameters, the most unstable mode can be either the BAE mode or the energetic particle mode (EPM). Both modes can resonate with the trapped-particles' magnetic precessional drifts and become unstable. The mode frequencies and growth rates depend strongly on the EP parameters. FOW can stabilize high-n BAE modes by reducing the wave-particle interaction. Magnetic shear has an important effect on the growth rate of the modes. For BAE, the growth rate presents the trend of first increase and following decrease with the increase of s. The peak positions of the mode growth rate move towards small s with increasing EPs' density. Besides, the BAE growth rate increases with n for small kϑρLE, and decreases for kϑρLE > 0.34. On the other hand, the EPM becomes stable for sufficiently large s and enters into the second stability regime. [ABSTRACT FROM AUTHOR]

The strong fishbone mode (FB) and long-lived mode (LLM) have been observed during neutral beam injection (NBI) on the HL-2A tokamak. The FB and LLM can transit between each other. The LLM is identified as an internal kink mode (IKM) with the mode structure obtained using a newly developed electron cyclotron emission radiometer imaging (ECEI) system. The frequency of the LLM (fLLM) is higher than the toroidal rotation frequency (ft) near the q = 1 surface (r ∼ 10 cm). Experimental results show that the LLM is likely to be excited at a higher line-averaged electron density (n̄e) than that of the FB when the NBI power is fixed. It is found that the FB and its harmonic as seed magnetic islands can trigger tearing modes (TMs). The mode numbers for the low-frequency and high-frequency TMs are m/n = 2/1 and 3/2, respectively. By further investigation, it is found that there is an m/n = 1/1 IKM coexisting at the same time and with the same frequency as the m/n = 2/1 TM, and the m = 1 mode structure of the IKM in the radial cross section is obtained by the Bayesian tomography method utilizing Soft X-ray arrays. The nonlinear coupling conditions are satisfied among the two TMs and IKM. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A space-resolved grazing-incidence flat-field extreme ultraviolet (EUV) spectrometer has been developed in the HL-2A tokamak to measure vertical impurity emission profiles with simultaneous spectral, temporal, and spatial resolution. The spectrometer working in the wavelength range of 30-500 Å has been equipped with a gold-coated varied-line-spacing holographic grating with cur2 vature of 5606 mm and a back illuminated charge-coupled device with size of 6.6 x 26.6 mm (255 x 1024 pixels). A lower half of the HL-2A plasma with averaged minor radius of 40 cm is observed when the spectrometer with horizontal dispersion is placed at a distance of 7.5 m away from the plasma center. An excellent spatial resolution of 12 mm is achieved when a space-resolved slit with vertical width of 0.5 mm is adopted. The radial profiles of intrinsic impurities in several ionization stages have been measured with high throughput and extremely low stray light. [ABSTRACT FROM AUTHOR]

A 1 m normal incidence vacuum ultraviolet (VUV) spectrometer has been developed for spatial distribution measurement of edge impurity line emission in the wavelength range of 300–3200 Å on HL-2A tokamak. A vertical profile of the impurity line emission is measured with a space-resolved slit placed between an entrance slit and a grating of the spectrometer. Two concave 1200 grooves/mm gratings blazed at 800 and 1500 Å are set on a rotatable holder in the spectrometer, which gives wavelength dispersion of 0.12 mm/Å. A back-illuminated charge-coupled device is used as a detector with an image size of 6.7×26.6 mm2 (26×26 μm2/pixel). An excellent spatial resolution of 2 mm is obtained with good spectral resolution of 0.15 Å when the space-resolved slit of 50 μm in width is used. The space-resolved spectra thus provide three radial profiles of emission line intensity, ion temperature, and poloidal rotation. The electron temperature can be measured by the intensity ratio, e.g., CIII 2s2-2s3p (386 Å)/2s2-2s2p (977 Å). The sensitivity of the spectrometer is calibrated in situ by using the VUV bremsstrahlung continuum radiation emitted from the tokamak plasma. A good performance of the spectrometer system for the edge impurity and temperature profile measurements is presented with results on Ohmic and H-mode discharges. [ABSTRACT FROM AUTHOR]

The experimental results of low pressure supersonic molecular beam injection (SMBI) fuelling on the HL-2A closed divertor indicate that during the period of pulsed SMBI the power density convected at the target plate surfaces was 0.4 times of that before or after the beam injection. An empirical scaling law used for the SMBI penetration depth for the HL-2A plasma was obtained. The cluster jet injection (CJI) is a new fuelling method which is based on and developed from the experiments of SMBI in the HL-1M tokamak. The hydrogen clusters are produced at liquid nitrogen temperature in a supersonic adiabatic expansion of moderate backing pressure gases into vacuum through a Laval nozzle and are measured by Rayleigh scattering. The measurement results have shown that the averaged cluster size of as large as hundreds of atoms was found at the backing pressures of more than 0.1 MPa. Multifold diagnostics gave coincidental evidence that when there was hydrogen CJI in the HL-2A plasma, a great deal of particles from the jet were deposited at a terminal area rather than uniformly ablated along the injecting path. SMB with clusters, which are like micro-pellets, will be of benefit for deeper fuelling, and its injection behaviour was somewhat similar to that of pellet injection. Both the particle penetration depth and the fuelling efficiency of the CJI were distinctly better than that of the normal SMBI under similar discharge operation. During hydrogen CJI or high-pressure SMBI, a combination of collision and radiative stopping forced the runaway electrons to cool down to thermal velocity due to such a massive fuelling. [ABSTRACT FROM AUTHOR]

A novel design of three-step Langmuir probe (TSLP) array has been developed for zonal flow (ZF) studies in the HL-2A tokamak. Three TSLP arrays are used to determine three-dimensional (3D) features of the ZFs. Two TSLP arrays are located in the same poloidal plane at a 6.5 cm separation, while the third TSLP array is separated toroidally by 80 cm. A fourth TSLP array driven by pneumatically reciprocating system is applied for boundary parameter profile measurements within 8 cm. The TSLP structure is described in detail. The measured 3D properties of the geodesic acoustic mode (GAM) ZFs are described and the poloidal and toroidal mode numbers (m<2,n∼0) are simultaneously determined in the HL-2A tokamak for the first time. The radial wave vector at the GAM frequency is estimated as 2.6±0.2 cm-1, corresponding to radial scale length of 2.4 cm. [ABSTRACT FROM AUTHOR]

A microwave imaging reflectometry system has been developed to visualize the density fluctuations on the HL-2A tokamak. This system is characterized by a quasi-optical system, a four frequency microwave transmitter, and a microwave quadrature receiver system with a 3D adjustable U-shaped horn antenna array, that generate 8 (poloidal) × 4 (radial) × 2 (toroidal) = 64 channel images of density fluctuations. Simulations and laboratory tests of the optical system have been conducted. The test results are in good agreement with the simulations.