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.
The impact of supersonic molecular beam injection (SMBI) fuelling on the plasma turbulence and shear flows, which form a self-consistent state, has been studied in the HL-2A tokamak. The SMBI flattens the shear layer by the collisional flow damping and increases the turbulence intensity. It is found that the SMBI can enhance the non-linear regulation dynamics and acts as an external tool for turbulence quench via stimulating two dynamic processes. The SMBI-stimulated non-linear interactions facilitate the L-H transition at the marginal heating power for accessing the H-mode. The effective reduction of the H-mode power threshold by SMBI is demonstrated. The result suggests that the SMBI could be a general tool for realization of controllable L-H transition via reducing the H-mode power threshold in future devices.
The control of the edge localized mode (ELM) in tokamaks has been intensively investigated with different techniques. Recently, ELM control with impurity seeding has also raised to be a hot topic. This paper reports the effect of the laser blow-off impurity seeding on the pedestal turbulence and ELMs. It has been found that the seeded impurity could have dual impacts on pedestal instabilities, depending on the quantity of the injected impurity. The impurity enhances the pedestal turbulence and further mitigates the ELMs when its quantity exceeds a lower threshold. Instead, the impurities suppress the pedestal turbulence and suppress ELMs when the quantity of the injected particles reaches a second higher threshold. It has also been observed that the ion toroidal velocity increases due to the impurity seeding while the collision rate is increasing. In addition, the impurities reduce the velocity shear rate by changing the toroidal velocity term rather than by changing the pressure gradient term as observed in ELM mitigation with LHCD. The reduction of the E × B velocity shear rate enhances the pedestal turbulence possibly through the turbulence spectral shift process during ELM mitigation.
Effect of impurity seeding on plasma global confinement has been investigated in H-mode plasmas of the HL-2A tokamak. Metal and gas impurities can be externally seeded by laser blow-off (LBO) and supersonic molecular beam injection (SMBI) systems, respectively. Using the LBO system to seed aluminium impurities into H-mode plasmas, it is observed that the ELM frequency after the impurity seeding is reduced by about 50%. The plasma stored energy is enhanced. The corresponding energy loss caused by each ELM increases with the decrease of ELM frequency. Besides, the neon and argon gas impurities have been seeded into H-mode plasmas by SMBI. The ELM frequency decreases to 0.3–0.5 times lower than that before the SMBI. The prolonged inter-ELM periods allow the plasma to build a higher pedestal density. It is observed that the energy confinement of the H-mode plasma is improved by the edge-deposited impurities, which is mainly attributed to the enhancement of plasma ion temperature. Both the edge and core ion temperatures are increased by 20%–40% after the impurity seeding. The quasi-linear simulations predict that the ion heat flux induced by ion temperature gradient mode is deceased in the present of impurity. The result suggests that the seeded impurity could reduce the edge ion thermal transport, resulting in the formation a higher edge ion temperature, which is a boundary condition for further increasing the core temperature through the profile stiffness.
Turbulent fluctuations within a quasi-coherent frequency range (peak frequency of ∼130 kHz, ∆f ≈ tens of kHz) are modulated by the rotation of low frequency tearing modes in the core of HL-2A ohmic plasmas. The quasi-coherent modes (QCMs) emerge outside the island boundary as the island O-point passes by in the large island cases (W > 4.5 cm), where the local electron temperature profile is steepened. Statistical analysis shows that for the QCM excitation, a threshold value of temperature gradient is identified and the QCM is solely observed in low density discharges, consistent with the linear ohmic confinement regime. These experimental evidence suggests that the observed QCMs are driven by the trapped electron mode, in agreement with linear stability calculations. Cross-correlation analysis reveals that the QCMs have long-range poloidal correlations and radially propagate outwards. Besides, the bispectral analysis indicates: (i) there exists non-linear coupling between the tearing mode and micro-turbulence, most significantly in the QCM frequency range; (ii) the QCMs nonlinearly couple by themselves to excite the second harmonic, whereas no non-linear interaction is observed between the QCMs and ambient turbulence.
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