Zhanhui Wang

Overview of the recent research on the J-TEXT tokamak

The experimental research over last two years on the J-TEXT tokamak is summarized and presented in the paper. The high-performance polarimeter-interferometer developed on J-TEXT, aiming to measure electron density and Faraday angle simultaneously, has time response up to 1 µs, phase resolution <0.1° and spatial resolution ~3 cm. Such high resolution permits investigations of fast equilibrium dynamics as well as magnetic and density perturbations associated with magnetohydrodynamic instabilities. Particle transport due to the sawtooth crashes is analysed. The sawteeth only partially flatten the core density profile and recovery between crashes implies an inward pinch velocity extending to the centre. The resonant magnetic perturbation (RMP) system on J-TEXT can generate a rotating helical field perturbation with a maximum rotation frequency up to 6 kHz, and dominant resonant modes of m/n = 2/1, 3/1 or 1/1. It is found that tearing modes can be easily locked and then rotate together with a rotating RMP. The effects of RMPs on plasma flows and fluctuations are studied with Langmuir probe arrays at the plasma edge. The toroidal velocity increases and the radial electric field decreases with RMP coil current when the RMP current is no more than 5 kA. When the RMP current reaches 6 kA, the toroidal velocity profile becomes flattened near the last closed flux surface. The geodesic acoustic mode is damped in most of the edge region, while the low frequency zonal flow is damped inside the islands, but increases at its boundary.

The 7-channel FIR HCN interferometer on J-TEXT tokamak

A seven-channel far-infrared hydrogen cyanide (HCN) laser interferometer has been established aiming to provide the line integrated plasma density for the J-TEXT experimental scenarios. A continuous wave glow discharge HCN laser designed with a cavity length 3.4 m is used as the laser source with a wavelength of 337 μm and an output power up to 100 mW. The system is configured as a Mach-Zehnder type interferometer. Phase modulation is achieved by a rotating grating, with a modulation frequency of 10 kHz which corresponds to the temporal resolution of 0.1 ms. The beat signal is detected by TGS detector. The phase shift induced by the plasma is derived by the comparator with a phase sensitivity of 0.06 fringe. The experimental results measured by the J-TEXT interferometer are presented in details. In addition, the inversed electron density profile done by a conventional approach is also given. The kinematic viscosity of dimethyl silicone and vibration control is key issues for the system performance. The laser power stability under different kinematic viscosity of silicone oil is presented. A visible improvement of measured result on vibration reduction is shown in the paper.

Progress of the in-building J-TEXT FIR three-wave polarimeter

A Far-infrared (FIR) polarimeter has been designed and is under development on J-TEXT tokamak. J-TEXT polarimeter is proposed to measure internal plasma magnetic field and electron density simultaneously, with a temporal resolution of ~ 1 μs and spatial resolution up to 1.5 cm. After earlier conceptual design, extended work has been done for the system set-up and optimization. The configuration of J-TEXT polarimeter has been improved for better performance, and is more compatible to adapt to limited port space. Three FIR lasers at 432 μm are already prepared and tested, which are capable to maintain more than 30 mW/cavity output for 8 hours without manual control. The output characteristics of beam expansion optics employed for the multichannel arrangement are measured and show a good agreement with expectation. A bench test is carried out to assure the feasibility and reliability of the system. Phase resolution of < 1 mrad is obtained, which meets the requirement of measurement. Vibration tests on the mechanical supporting structure are conducted to verify and improve the system environment. Vibration reduction is achieved by optimizing mechanical structures, and the preliminary results show a decrease on vibration amplitude of up to two orders. The first result of J-TEXT polarimeter is expected to be obtained during the next experimental campaign of J-TEXT tokamak.

First results of the J-TEXT high-resolution 3-wave polarimeter-interferometer system

A far-infrared laser polarimeter-interferometer system based on Three-wave technique has been established on the J-TEXT tokamak. The system determines Faraday angle by measuring phase difference between two collinear, counter-rotating, circularly polarized laser beams, and acquires line-integrated electron density simultaneously by phase comparison between the two beams and a third local oscilate (LO) beam. Three seperately pumped HCOOH lasers at 432 ?m are adopted as sources, suppling more than 100 mW power output in sum. Parabolic mirrors are used to expand probe beams to 450 mm wide, covering ~ 80% of plasma cross section, which allows profile measurement with high spatial resolution. First experimental results of the polarimeter-interferometer have been obtained. 12 chords (3 cm chord spacing) simultaneous polarimetric and interferometric measurements are achieved, with phase resolution up to 0.1? at bandwidth of 50 kHz. With the high temporal and phase resolution, perturbations associated with the sawtooth cycle and MHD activity have been observed.

The behavior of runaway current in massive gas injection fast shutdown plasmas in J-TEXT

Runaway currents following disruptions have an important effect on the first wall in current tokamaks and will be more severe in next generation tokamaks. The behavior of runaway currents in massive gas injection (MGI) induced disruptions have been investigated in the J-TEXT tokamak. The cold front induced by the gas jet penetrates helically along field lines, preferentially toward the high field side and stops at a location near the q = 2 surface before the disruption. When the cold front reaches the q = 2 surface it initiates magnetohydrodynamic activities and results in disruption. It is found that the MGI of He or Ne results in runaway free shutdown in a large range of gas injections. Mixture injection of He and Ar (90% He and 10%Ar) consistently results in runaway free shutdown. A moderate amount of Ar injection could produce significant runaway current. The maximum runaway energy in the runaway plateau is estimated using a simplified model which neglects the drag forces and other energy loss mechanisms. The maximum runaway energy increases with decreasing runaway current. Imaging of the runaway beam using a soft x-ray array during the runaway current plateau indicates that the runaway beam is located in the center of the plasma. Resonant magnetic perturbation (RMP) is applied to reduce the runaway current successfully during the disruption phase in a small scale tokamak, J-TEXT. When the runaway current builds up, the application of RMP cannot decouple the runaway beam due to the lower sensitivity of the energetic runaway electrons to the magnetic perturbation.

Enhancement of runaway production by resonant magnetic perturbation on J-TEXT

The suppression of runaways following disruptions is key for the safe operation of ITER. The massive gas injection (MGI) has been developed to mitigate heat loads, electromagnetic forces and runaway electrons (REs) during disruptions. However, MGI may not completely prevent the generation of REs during disruptions on ITER. Resonant magnetic perturbation (RMP) has been applied to suppress runaway generation during disruptions on several machines. It was found that strong RMP results in the enhancement of runaway production instead of runaway suppression on J-TEXT. The runaway current was about 50% pre-disruption plasma current in argon induced reference disruptions. With moderate RMP, the runway current decreased to below 30% pre-disruption plasma current. The runaway current plateaus reach 80% of the pre-disruptive current when strong RMP was applied. Strong RMP may induce large size magnetic islands that could confine more runaway seed during disruptions. This has important implications for runaway suppression on large machines.

Investigation of molecular penetration depth variation with SMBI fluxes

We study the molecular penetration depth variation with the SMBI fluxes. The molecular transport process and the penetration depth during SMBI with various injection velocities and densities are simulated and compared. It is found that the penetration depth of molecules strongly depends on the radial convective transport of SMBI and it increases with the increase of the injection velocity. The penetration depth does not vary much once the SMBI injection density is larger than a critical value due to the dramatic increase of the dissociation rate on the fueling path. An effective way to improve the SMBI penetration depth has been predicted, which is SMBI with a large radial injection velocity and a lower molecule injection density than the critical density.

Simulations of the effects of density and temperature profile on SMBI penetration depth based on the HL-2A tokamak configuration

Using the trans-neut module of the BOUT++ code, we study how the fueling penetration depth of supersonic molecular beam injection (SMBI) is affected by plasma density and temperature profiles. The plasma densities and temperatures in L-mode are initialized to be a set of linear profiles with different core plasma densities and temperatures. The plasma profiles are relaxed to a set of steady states with different core plasma densities or temperatures. For a fixed gradient, the steady profiles are characterized by the core plasma density and temperature. The SMBI is investigated based on the final steady profiles with different core plasma densities or temperatures. The simulated results suggest that the SMB injection will be blocked by dense core plasma and high-temperature plasma. Once the core plasma density is set to be () it produces a deeper penetration depth. When is increased from 1.4 to 3.9 at intervals of 0.8, keeping a constant core temperature of = 725 eV at the radial position of , the penetration depth gradually decreases. Meanwhile, when the density is fixed at and the core plasma temperature is set to 365 eV, the penetration depth increases. The penetration depth decreases as is increased from 365 eV to 2759 eV. Sufficiently large or causes most of the injected molecules to stay in the scrape-off-layer (SOL) region, lowering the fueling efficiency.

Simulations of fast component and slow component of SMBI on HL-2A tokamak*

It is very important to improve the penetration depth and fueling efficiency of supersonic molecular beam injection (SMBI) especially for the next generation fusion devices such as ITER. Two components, a fast component (FC) and a slow component (SC), have been observed in the HL-2A SMBI experiments for several years, and the FC can penetrate much more deeply than the common SMBIs which draws a great deal of attention for a better fueling method. It is the first time to the FC and SC of SMBI have been simulated and interpreted in theory and simulation in this paper with the trans-neut module of the BOUT++ code. The simulation results of the FC and SC are clear and distinguishable in the same way as the observation in experiment. For the major mechanism of the FC and SC, it is found that although the difference in the injection velocity has some effect on the penetration depth difference between the FC and SC, it is mainly caused by the self-blocking effect of the first ionized SMB. We also discuss the influence of the initial plasma density on the FC and SC, and the variation of the SC penetration depth with its injection velocity.

Observations of zonal flows in electrode biasing experiments on the Joint Texas Experimental tokamak

Zonal flows (ZFs) are observed during the electrode biasing (EB) high confinement mode (H-mode) using Langmuir probe arrays on the edge of J-TEXT tokamak. The long-distance correlation characteristics of floating potentials and interactions with turbulence are studied. During positive biasing H-mode, either the geodesic acoustic mode or low frequency ZF increases. Strong suppression of radial transport by ZFs is found in the low frequency region. The components of the radial particle flux without and with EB are compared in the frequency domain. The interaction between ZFs and ambient turbulence is also discussed. The results show that the rate of ZFs shear is comparable with that of E×B shear, suggesting that ZFs could be the trigger of the biasing H-mode.