Search

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. [ABSTRACT FROM AUTHOR]

A phase tracking method based on Hilbert transform algorithm is applied to the nonlinear wave-wave coupling analysis on HL-2A tokamak. Synthetic signal analysis is given to show the principle of phase analysis for the detection of nonlinear coupling. If the phase difference between two coherent modes is synchronized with the phase of a third mode, these three modes are nonlinearly coupled, vice versa. The time evolution of the phase of a coherent mode could be computed with Hilbert transform for experimental data, and a phase tracking flowchart is summarized. On HL-2A tokamak, the nonlinear coupling among two Alfvén modes (AMs) and a tearing mode (TM) has been confirmed with bicoherence analysis (Shi et al 2017 Phys. Plasmas 24 042509). With Hilbert transform, it is clearly observed that the phase difference between two AMs Δθ12 is synchronized with the phase of the TM θTM. The synchronization is confirmed with normalized cross-correlation. An alternative to check this synchronization is to observe the histogram of the phase difference Δθ12 − θTM. [ABSTRACT FROM AUTHOR]

Poloidal symmetric low frequency oscillations at the plasma edge were observed during the low-intermediate-high transitions on the HL-2A tokamak. The limit cycle oscillation (LCO) frequency gradually reduces from 3.0 kHz to 1.5 kHz approaching the H mode. Meanwhile a transition of phase shift between the turbulence and the radial electric field was identified. In the initial phase, the turbulence grows first, followed by the localized electric field. In the later phase, the electric field leads turbulence, and the magnitude of the Reynolds stress gradient is out of phase with the LCO flow. The increasing magnitudes of the magnetic fluctuation may be correlated with the I-H transition. In addition, the divertor heat flux is also significantly modulated by the LCOs. [ABSTRACT FROM AUTHOR]

High- β p scenario addresses needs for an attractive tokamak fusion reactor design, where β p is the normalized plasma poloidal beta. High- β p experiments have been performed during recent years in the HL-2A tokamak. The high-performance region is realized by the integration of internal and external transport barriers which are dubbed as ITBs and ETBs, namely, double transport barriers (DTBs), with the high-power NBI heating. Generally, the ITB forms and becomes strong after the NBI injection on HL-2A. Subsequently, the edge ion temperature and toroidal rotation increase, as a result that the ETB or pedestal creates and L-H transition occurs. A high- β p (β p ∼ 3) scenario is obtained with I p ∼ 110 kA and q 95 ∼ 5 , and another high- β p (β p ∼ 2.7) scenario is also realized with I p ∼ 160 kA and q 95 ∼ 4.2. The two high- β p scenarios are both characterized by a large-radius ITB and an ETB. However, the two scenarios are different. For the former there are strong large-radius ITB and weak ETB on radial ion temperature profiles, but for the later there are weak large-radius ITB and strong ETB. Although two high- β p scenarios on HL-2A have been accessed, however they are both transient. In addition, MHD instabilities in high- β p scenarios are also present for a high fusion gain and reactor relevant high fusion-density plasma. • The high-performance region has been realized by the integration of ITB and ETB in the HL-2A tokamak. • High- β p scenarios are characterized by a large-radius ITB and an ETB. • There are many abundant MHD instabilities in high- β p high confinement plasmas. • Experiments to enhance β p will contribute to future plasma operation, such as ITER and CFETR. [ABSTRACT FROM AUTHOR]