Youwen Sun

Toroidal modeling of plasma response and resonant magnetic perturbation field penetration

The penetration dynamics of the resonant magnetic perturbation (RMP) field is sim- ulated in the full toroidal geometry, under realistic plasma conditions in MAST experiments. The physics associated with several aspects of the RMP penetration - the plasma response and rotational screening, the resonant and non-resonant torques and the toroidal momentum balance - are highlighted. In particular, the plasma response is found to significantly amplify the non-resonant component of the RMP field for some of the MAST plasmas. A fast rotating plasma, in response to static external magnetic fields, experiences a more distributed electro- magnetic torque due to the resonance with continuum waves in the plasma. At fast plasma flow (such as for the MAST plasma), the electromagnetic torque is normally dominant over the neoclassical toroidal viscous (NTV) torque. However, at sufficiently slow plasma flow, the NTV torque can play a significant role in the toroidal momentum balance, thanks to the precession drift resonance enhanced, so called superbanana plateau regime.

Temperature-dependent photovoltaic effects in the manganite-based heterojunction

Strong temperature-dependent photovoltaic effects have been observed in the heterojunction composed of a La0.32Pr0.35Ca0.33MnO3 (LPCM) film and a 0.5 wt % Nb-doped SrTiO3 substrate. The photovoltage shows a monotonic increase with the decrease of temperature, and its relative change can be as large as ∼7000% for a modest light intensity of 20 mW (wavelength=632nm) when cooled from room temperature down to 17 K. The synchronous variation of photovoltage and the magnetization of LPCM indicates the magnetic origin of the temperature dependence of the photovoltaic effect. It is suggested that the temperature affects the photovoltaic effect by modifying the magnetic order, then the band structure of LPCM. An enhancement of the photovoltaic effect under strong light illumination is also observed, which is probably a result of illumination-induced change of the band structure of LPCM.

Toroidal modeling of penetration of the resonant magnetic perturbation field

A toroidal, quasi-linear model is proposed to study the penetration dynamics of the resonant magnetic perturbation (RMP) field into the plasma. The model couples the linear, fluid plasma response to a toroidal momentum balance equation, which includes torques induced by both fluid electromagnetic force and by (kinetic) neoclassical toroidal viscous (NTV) force. The numerical results for a test toroidal equilibrium quantify the effects of various physical parameters on the field penetration and on the plasma rotation braking. The neoclassical toroidal viscous torque plays a dominant role in certain region of the plasma, for the RMP penetration problem considered in this work.

Observation of heat transfer across x point of the islands during sawtooth crash on the HT-7 tokamak

It is observed that a large heat flow abruptly transfers across the x point (which can be located at both the high field side and the low field side) of the 1/1 or 2/2 magnetic islands during a sawtooth crash on the HT-7 tokamak by using tomography of the high-resolution soft-x-ray emission together with the singular value decomposition technique. Although the poloidally asymmetric heat flow contributes an m = 1 component of perturbation in the signals, the heat flow is obviously different from the growth of the m = 1 magnetic island. Furthermore, it is also shown that the large heat flow or the displacement of the hot core occurs only after the crash. Hence, the large displacement of the hot core is just a result of the crash rather than the cause. The heat flow from both the x points of the m = 2 island is also observed. This observation demonstrates that a purely fast reconnection of the m = 1 magnetic island is not responsible for the crash, but that it may be due to the rapid energy and particle diffusion in the stochastic region.

Crossover from negative to positive magnetoresistance in La0.7Ce0.3MnO3-SrTiO3-Nb heterojunctions

A crossover from negative to positive magnetoresistance (MR) is observed in the heterojunction composed of a La0.7Ce0.3MnO3 film and a 0.5wt.% Nb-doped SrTiO3 substrate. The temperature and bias current dependences of MR sign in the junction are investigated carefully. It is found that the positive MR occurs when temperature or applied bias current is increased to a higher value. The relation between MR and an external magnetic field is also found to be temperature dependent. We attribute the appearance of the positive MR at higher temperatures to the tunneling transport mechanism. These results are helpful in configuring artificial devices using manganite-based heterojunctions.

Experimental investigation of m/n = 1/1 and high-order harmonic modes during the sawtooth oscillation in a low β tokamak plasma

Sawtooth oscillations were investigated using an electron cyclotron emission imaging diagnostic technique on the HT-7 tokamak. High-order harmonic modes are observed in sawtooth precursors. They cause sharp pressure points, leading to the occurrence of reconnection events at more than one place, and which are not preferential on the low field side of the q ~ 1 radius at low density. The reconnection proceeds in two stages. In the first stage, a weak reconnection happens which is characterized by a slow emergence of a small amount of heat and particles through the finite openings. Subsequently, in the second stage, most of the remaining heat and particles in the core escape outward. The observations at low density indicate that high-order harmonic modes are responsible for the secondary reconnection stage, which has seldom been mentioned in reconnection model. The characteristics of the modes are then described by investigating a large number of sawteeth oscillations with different densities and qa values. It is found that when the density increases above a threshold at qa = 3.3, the m/n = 1/1 mode amplitude significantly exceeds its harmonics. The threshold decreases as qa increases.

ELM control with RMP: Plasma response models and the role of edge peeling response

Resonant magnetic perturbations (RMP) have extensively been demonstrated as a plausible technique for mitigating or suppressing large edge localized modes (ELMs). Associated with this is a substantial amount of theory and modelling efforts during recent years. Various models describing the plasma response to the RMP fields have been proposed in the literature, and are briefly reviewed in this work. Despite their simplicity, linear response models can provide alternative criteria, than the vacuum field based criteria, for guiding the choice of the coil configurations to achieve the best control of ELMs. The role of the edge peeling response to the RMP fields is illustrated as a key indicator for the ELM mitigation in low collisionality plasmas, in various tokamak devices.

Toroidal modeling of interaction between resistive wall mode and plasma flow

The non-linear interplay between the resistive wall mode (RWM) and the toroidal plasma flow is numerically investigated in a full toroidal geometry, by simultaneously solving the initial value problems for the n = 1 RWM and the n = 0 toroidal force balance equation. Here, n is the toroidal mode number. The neoclassical toroidal viscous torque is identified as the major momentum sink that brakes the toroidal plasma flow during the non-linear evolution of the RWM. This holds for a mode that is initially either unstable or stable. For an initially stable RWM, the braking of the flow, and hence the eventual growth of the mode, depends critically on the initial perturbation amplitude.

Excitation of (2,1) neoclassical tearing modes by mode coupling with (1,1) internal mode in EAST

Neoclassical tearing modes (NTM) are observed in discharges with auxiliary heating LH+ICRF and LH only during H-mode in EAST. The m/n?=?2/1 NTM is triggered by strongly coupling with an m/n?=?1/1 internal mode. Here, LH and ICRF are the abbreviations of lower hybrid resonance heating and ion cyclotron resonance frequency heating, respectively. The mode number of the NTM is m/n?=?2/1, where m is the poloidal mode number and n is the toroidal mode number. Just before the triggering of NTMs, an m/n?=?1/1 internal mode appears in the soft x-ray emission at plasma centre when the intensity of hard x-ray (IHX) reaches a critical value. The mode, characterized by frequency chirping in the spectrum, may be related to suprathermal electrons produced by LH. The saturated magnetic island width wsat of the NTM is strongly correlated with poloidal ?p. Normalized ?N,onset and the magnetic island critical width wcrit increase with electron temperature Te.

Observation of the m = 1 mode during the ramp phase of the sawtooth oscillations in LHCD plasmas on the HT-7 tokamak

A new oscillation with m = 1 and n = 1, where m and n are the poloidal and toroidal mode numbers, respectively, has been observed with lower hybrid current drive (LHCD) on the HT-7 tokamak. This mode appears during the sawtooth ramp, saturates, damps and finally disappears before the subsequent sawtooth collapse and before the appearance of the precursor oscillations. The necessary condition for its observation is that the sawtooth period exceeds 6–7 ms and that the LHCD power exceeds the threshold value of about 90 kW. We propose that the mode may be destabilized by the current density gradient, the evolution of which is determined by the suprathermal electrons generated by the LHCD.