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Ion internal transport barriers (iITBs) are first observed in neutral beam injection (NBI) heated plasmas at the HL-2A tokamak. The position of the barrier foot, in the stationary state, coincides with the q  =  1 surface within its uncertainty of measurement. iITBs can develop more easily at the beginning of NBI heating. Also, iITBs are unstable for the sawtooth plasma. Simulations reveal that the thermal diffusivity of ions (χ i) inside the barrier can be as low as the neoclassical level. It is observed that the flow shear in the stationary iITB state reaches the level required for suppressing the ion temperature gradient mode instability, which indicates the important role of flow shear in sustaining the iITB.

The trapped electron modes (TEMs) are numerically investigated in toroidal magnetized hydrogen, deuterium and tritium plasmas, taking into account the effects of impurity ions such as carbon, oxygen, helium, tungsten and others with positive and negative density gradients with the rigorous integral eigenmode equation. The effects of impurity ions on TEMs are investigated in detail. It is shown that impurity ions have substantially-destabilizing (stabilizing) effects on TEMs in isotope plasmas for(< 0), opposite to the case of ion temperature gradient (ITG) driven modes. Detailed analyses of the isotope mass dependence for TEM turbulences in hydrogenic isotope plasmas with and without impurities are performed. The relations between the maximum growth rate of the TEMs with respect to the poloidal wave number and the ion mass number are given in the presence of the impurity ions. The results demonstrate that the maximum growth rates scale as γ ∝ − M i max 0.5 in pure

Turbulent transport of impurity ions with hollow density profiles (HDPs), which are widely observed in magnetically confined plasmas and desirable for fusion reactor, is self-consistently investigated. A full gyrokinetic description is employed for main and impurity ions. Instead of conventional ion temperature gradient (ITG, including impurity ITG) and trapped electron modes (TEMs), impurity modes (IMs), driven by impurity ion density gradient opposite to that of electrons, are considered. The impurity ion flux induced by IMs is shown to be approximately one order of magnitude higher than that induced by TEMs when both kinds of modes coexist. Main ITG and electron temperature gradient (ETG) are found to reduce influx of impurity ions significantly, resembling temperature screening effect of neoclassical transport of impurity ions. The simulation results such as peaking factor of the HDPs and the effects of main ITG are found in coincidence with the evidence observed in argon injection experiment on HL-2A tokamak. Thus, the IM turbulence is demonstrated to be a plausible mechanism for the transport of impurity ions with HDPs. A strong main ITG, ETG, and a low electron density gradient are expected to be beneficial for sustainment of HDPs of impurity ions and reduction of impurity accumulation in core plasma.

A 32/64-channel charge exchange recombination spectroscopy (CXRS) diagnostic system is developed on the HL-2A tokamak (R = 1.65 m, a = 0.4 m), monitoring plasma ion temperature and toroidal rotation velocity simultaneously. A high throughput spectrometer (F/2.8) and a pitch-controlled fiber bundle enable the temporal resolution of the system up to 400 Hz. The observation geometry and an optimized optic system enable the highest radial resolution up to ∼1 cm at the plasma edge. The CXRS system monitors the carbon line emission (C VI, n = 8-7, 529.06 nm) whose Doppler broadening and Doppler shift provide ion temperature and plasma rotation velocity during the neutral beam injection. The composite CX spectral data are analyzed by the atomic data and analysis structure charge exchange spectroscopy fitting (ADAS CXSFIT) code. First experimental results are shown for the case of HL-2A plasmas with sawtooth oscillations, electron cyclotron resonance heating, and edge transport barrier during the high-confinement mode (H-mode).

The comprehensive study of ubiquitous modes (UMs) was performed by means of gyrokinetic simulation, employing the gyrokinetic equations for drift waves in the frequency regime of v ti ω/k || v te in tokamak plasmas. The results show that the UMs are mostly in the short wave length regime of1, with ρ s = 2T e /m i /Ω i being the average ion gyroradius, and Ω i the ion gyrofrequency. It was demonstrated that a fluid-like instability may occur when b i ≈ 1 5 (n/n eT ) 3 , η i ≈ 0 and ŝ = 0.2. The results suggest that both trapped electrons and electron/ion density gradient are involved in the driving of UMs. The ion temperature gradient has a significant impact on UM instability. A lower electron temperature gradient, higher ion temperature gradient, adequate fraction of trapped electrons, and limited magnetic shear (ŝ 1 is optimum) are required for UMs to be unstable. The mode structure is highly localized. It was also indicated that the UMs are usually inevitable in tokamak plasmas and may contribute greatly to plasma transport in specific parameter regimes.

Systematic calibration experiment of Langmuir probe sheath potential coefficient Λ, which is a critical coefficient for estimating plasma sheath potential, has been carried out in the HL-2A tokamak deuterium plasmas. The electron energy probability function (EEPF) shows that electron outside last-closed-flux-surface (LCFS) is Maxwell distribution, but inside LCFS it changes to bi-Maxwell. Two kinds of plasma potential measuring method and three kinds Λ estmating method were compared. It is found that the estimated Λ coefficient is in the region of 2-3 outside LCFS and then increases to ~5 inside LCFS due to the high temperature electron effect. Fortunately, the results show that the commonly used value Λ = 2.8 is still available to calculate plasma potential when we use the overestimated electron temperature measured by three-tip probe in bi-Maxwell case. Further analysis indicated this value should be corrected. Or it may lead to a error when we calculate the the electric field E r and its shear dE r /dr. The corrected value monotonically increased from ~2.2 to ~2.9 while Langmuir probe moved from 40 mm outside LCFS to 20 mm inside LCFS.

Isotope effects on instabilities driven by ion temperature gradient (ITG) and impurities in tokamak plasmas in the presence of tungsten ions are numerically studied. It is revealed that the tungsten ions significantly modify the isotope scaling of the maximum growth rates of the instabilities with respect to the main or effective ion mass number or Meff [=(1  −  fz)Mi  +  fzMz] with (=Znz/ne) being impurity charge concentration. The most reasonable scaling is deduced as with , for ITG driven modes, whilst holds with , for tungsten impurity modes, in significant contrast with the case of light or intermediate impurities where the scaling is and for ITG and impurity modes, respectively. These results suggest that existence of tungsten impurity would enhance (weaken) the isotope effect of instability driven by ITG (tungsten impurity ions), which is beneficial (harmful) for improvement of confinement when hydrogen isotopes are used in plasmas. The results might also provide hints on studies of particle and energy transports and discharge performance, particularly, in ITER-like wall machines.