Xiwei Hu

An 11 cm long atmospheric pressure cold plasma plume for applications of plasma medicine

In this letter, a room temperature atmospheric pressure plasma jet device is reported. The high voltage electrode of the device is covered by a quartz tube with one end closed. The device, which is driven by a kilohertz ac power supply, is capable of generating a plasma plume up to 11cm long in the surrounding room air. The rotational and vibrational temperatures of the plasma plume are 300 and 2300K, respectively. A simple electrical model shows that, when the plasma plume is contacted with a human, the voltage drop on the human is less than 66V for applied voltage of 5kV (rms).

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.

Reflection of a wave from a thin plasma layer attached to a metal plate by finite-difference time-domain analysis

The reflection of an electromagnetic wave in a thin plasma layer attached to a metal plate at high pressure is investigated with the finite-difference time-domain method. The effects of the plasma thickness, the plasma density distribution function, the collision frequency between electrons and neutrals and the frequency of incident wave on the reflection coefficient of the electromagnetic wave are discussed. Numerical results indicate that the reflection coefficient of the wave depends on its frequency, the plasma thickness, the plasma density distribution and the collision frequency. The reflection coefficient is low only at the low band of the calculated frequency for different plasma distribution functions if the plasma layer is very thin, e.g. 10 mm. Plasmas with an excess of 20 mm for a high collision frequency such as 103 GHz are capable of absorbing microwave radiation over a wider frequency range for different plasma distributions.

Attenuation and propagation of a scattered electromagnetic wave in two-dimensional atmospheric pressure plasma

Finite-difference-time-domain arithmetic is applied to simulate the propagation of an electromagnetic (EM) wave in a two-dimensional atmospheric pressure plasma (APP) and a metal layer with strong electron-neutral collisions. The dependences of the EM wave attenuation on the parameters of the APP are provided. The two-dimensional numerical results indicate that when the profile of the electron density is given, the attenuation of an EM wave in APP is strongly affected by (a) the polarization mode (TM mode or TE mode), (b) the incident angle of the EM wave, (c) the EM wave frequency, (d) the width of the plasma layer, and (e) the collision frequency between electrons and neutrals. In this paper, the behaviour of the propagation of an EM wave inside the plasma layer is explained by the principle of wave interference. The relationship between the attenuation property and the above-mentioned parameters is also studied.

Attenuation of wave in a thin plasma layer by finite-difference time-domain analysis

The attenuation of the electromagnetic wave in a thin plasma layer at high pressure is investigated with finite-difference time-domain method. The effects of the plasma thickness, plasma density distribution function, collision frequency between electron and neutrals, and the frequency of incident wave on the attenuation of the electromagnetic wave are discussed. Numerical results indicate that the phase shift is sensitive to plasma distributions, and the attenuation of wave depends on its frequency, the plasma thickness, plasma density distribution, and collision frequency. In the case of a thin plasma layer, the attenuation of wave is strong only at the low band of frequency for the different distribution functions with a certain collision frequency. Plasmas with a certain thickness for high collision frequency are capable of absorbing microwave radiation over a wider frequency range for the different plasma distributions.

Locking of Tearing Modes by the Error Field

The locking of tearing modes by the error field is studied by nonlinear numerical modeling. The threshold of mode locking for J-TEXT tokamak plasmas is found.

The propagation of a microwave in an atmospheric pressure plasma layer: one- and two-dimensional numerical solutions

Abstract The propagation of a microwave in an atmospheric pressure plasma (APP) layer is described numerically with an integral–differential wave equation in one dimension (normal incident) case and with the Finite Difference Time Domain (FDTD) method in two dimension (oblique incident) case. When the microwave passes through the APP layer, its amplitude and phase of the wave electric field are obviously modulated by both the electron density and the collisions between the electrons and neutrals. The dependencies of the passed wave behaviors (i.e. the phase shift, the reflectivity, the transmissivity and absorptivity) on the APP layer characteristics (width, electron density, and collision frequency) and microwave characteristics (incident angle and polarization) are presented. The Appletons Equation can be derived from the Wentzel–Kramers–Brillouin (WKB) solution of the integral–differential wave equation and is compared with the one-dimensional numerical solution.

Calculation of the Poloidal Magnetic Field Configuration for the J-TEXT Tokamak

Two approximate analytical methods are used to calculate and analyze the poloidal magnetic field configuration for the J-TEXT tokamak, an iron core tokamak. The methods are based on a spool model and an infinite length model, respectively. By comparing the calculated results with the experimental data, the two methods are modified by the least square method. The results given by the modified method based on the spool model shows good agreements with the experimental data. The modified method based on the spool model replaces the similar function in a general equilibrium code, EFIT. The displacements of the centriod of the plasma current in the J-TEXT tokamak reconstructed by the revised EFIT are coincident with the results measured by the soft-X ray array and magnetic probes.

The instabilities induced by electrostatic fields and gradients in a plasma shock front

The linear instabilities induced by the electrostatic fields and gradients of equilibrium parameters in a plasma shock front are analyzed for the plasma shock structure. Small perturbations as well as the steady-state shock structure are described by a set of coupled two-fluid and Poisson equations. The dispersion relations are obtained at high frequency in two cases, in which the wave vectors are, respectively, parallel and perpendicular to the shock propagation direction. The imaginary parts of the frequency (growth rates) of the instabilities are dependent on the fields and the gradients.

The Effect of Plasma Density Profile on Two-Plasmon Decay in Tokamak Plasma

Electron cyclotron extraordinary-wave decay into two upper hybrid waves in an inhomogeneous magnetized plasma is studied theoretically and numerically. The analytic expressions for the local coupling constant and convective amplification factor of two-plasmon decay (TPD) are obtained. It shows that the TPD threshold intensity is very high (about 15 MW) with the linear density profile in a tokamak plasma. However, within the nonmonotonicity density profile, the power threshold of TPD can significantly be reduced more than one order of magnitude (about 1.2MW). The comparison of the numerical and simulation results of the local coupling constant shows that they agree well with each other.