Table of contents

In this paper, derived from Maxwell and fluid equations of plasmas, unified nonlinear wave equations are used to describe the parametric decay instability (PDI) in magnetized plasmas, and in view of mode-coupling, we can obtain all the possible PDI channels. By solving the nonlinear equations with a mode-coupling method, we obtain the growth rate of the PDI, of which all of the three waves are ordinary mode (O-mode) or extraordinary mode (X-mode) wave. Under the dipole approximation, an explicit formula of the growth rate of the X-mode and the condition of the equilibrium density scale are obtained. According to the existence conditions of three X-mode waves, this kind of instability might exist in ECRH with the second harmonic X-mode wave.

Single-pulse and double-pulse optical emission spectroscopy (OES) analyses were carried out in air by using ultrashort laser pulses at atmospheric pressure. The aim of this work is to use spectroscopic methods to analyze the early phase of laser-induced plasma after the femtosecond laser pulse. The temporal behavior of emission spectra of air plasma has been characterized. In comparison with the single-pulse scheme, the plasma emission obtained in the double-pulse scheme presents a more intense continuum along with several additional ionic lines. As only one line is available in the single-pulse scheme, the plasma temperature measurements were performed using only the relative line-to-continuum intensity ratio method, whereas the relative line-to-line intensity ratio method and the relative line-to-continuum intensity ratio method were used simultaneously to estimate the electron temperature in the double-pulse scheme. The results reveal that the temperature values obtained by the two methods in the double-pulse scheme agree. Moreover, this shows that the relative line-to-continuum intensity ratio method is suitable for early phase of laser-induced plasma diagnostics. The electron number density was estimated using the Stark broadening method. In the early phase of laser-induced plasma, the temporal evolution of the electron number density exhibits a power law decrease with delay time.

Particle-in-cell (PIC) simulation method has been proved to be a good candidate to study the interactions between plasmas and radio-frequency waves. However, for waves in the lower hybrid range of frequencies, a full PIC simulation is not efficient due to its high computational cost. In this work, a gyro-kinetic electron and fully-kinetic ion (GeFi) particle simulation model is applied to study the propagations and mode conversion processes of lower hybrid waves (LHWs) in plasmas. With this method, the computational efficiency of LHW simulations is greatly increased by using a larger grid size and time step. The simulation results in the linear regime are validated by comparison with the linear theory.

Inward energy transport (pinch phenomenon) in the electron channel is observed in HT-7 plasmas using off-axis ion cyclotron resonance frequency (ICRF) heating. Experimental results and power balance transport analysis by TRANSP code are presented in this article. With the aids of GLF23 and Chang-Hinton transport models, which predict energy diffusivity in experimental conditions, the estimated electron pinch velocity is obtained by experimental data and is found reasonably comparable to the results in the previous study, such as Song on Tore Supra. Density scanning shows that the energy convective velocity in the electron channel has a close relation to density scale length, which is qualitatively in agreement with Wang's theoretical prediction. The parametric dependence of electron energy convective velocity on plasma current is still ambiguous and is worthy of future research on EAST.

Using the digisonde data observed at ionospheric station Norilsk (Dip lat. 60°N) in 2006, a statistical study on the characteristics of the ionospheric plasma total absorption of radio waves (IPTAR) was performed. In the winter and some months of equinox, the IPTAR mainly occurred in the nighttime and the highest occurrence rate could be up to 90%. In the summer, the occurrence was relatively low and the differences between nighttime and daytime occurrence reduced. The total duration of IPTAR seemed longer around the winter than that around the summer. The occurrence of IPTAR events ascended as the Kp index increased. The frequent precipitation of energetic particles into the ionospheric plasma in the auroral belt may be the main cause of the IPTAR events.

An RF compensated cylindrical Langmuir probe system has been developed and used to characterize an RF capacitive two temperature plasma discharge in a stochastic mode. The novelty of the work presented here is the use of the driven electrode (cathode) without ground shield. Measurements of the electron energy distribution function (EEDF) and plasma parameters were achieved under the following conditions: 50 W of RF power and 5×10⁻² mbar of argon pressure. The probe measurements are performed at 3 cm above the electrode and the probe was shifted radially (r direction) from the center (r = 0 cm) of the inter-electrodes region towards the chamber wall (R = 10.75 cm). The results show that the EEDF is bi-Maxwellian and its shape remains the same through the scanned region. The farther the probe from the central region, the lower the EEDF maximum. The plasma density is observed to decrease according to a Gaussian profile along the radial direction and falls to 50% of its maximum when close to the cathode edge (r = 5.5 cm). At the same time the effective electron temperature remains constant for r>4 cm and increases for r≥4 cm. The high-temperature and low-temperature electrons' densities and temperatures are also discussed in the article.

Secondary arc discharge is a complicated physical phenomenon and one of the key fundamental issues associated with ultra high voltage (UHV) half-wavelength transmission lines (HWTL). With the establishment of a physical simulation platform for the HWTLs, experiments were carried out regarding the motion and extinction characteristics of secondary arcs. The cathode arc root and the anode arc root were found to show an obvious polarity effect while the arc column was moving in a spiral, due to their different motion mechanisms. The extinction behavior was also recorded and experiments were designed with different compensation conditions. Results show that the arcing time can be greatly reduced if there exists an electrical compensation network. The research provides fundamentals for understanding the physics involved, especially the motion and extinction mechanisms of the secondary arcs.

Dynamic characteristics of discharge particles are described within the framework of a two-dimensional photoionization-hydrodynamic numerical model for the discharge process of SF₆-N₂-CO₂ gas mixtures at atmospheric pressure, under a uniform DC applied field. The finite difference—flux corrected transport (FD-FCT) algorithm is used in the numerical implementation for improving the accuracy and efficiency. Then the tempo-spatial distributions of the gap—space electric field and electron velocity are calculated from the microscopic mechanism, and the dynamic behaviors of charged particles are obtained in detail. Meanwhile, the tempo—spatial critical point of the avalanche-to-streamer in this model is discovered, and several microscopic parameters are also investigated. The results showed that the entire gap discharge process can be divided into two phases of avalanche and streamer according to Raether-Meek criterion; the electron density within the discharge channel is lower compared to that of positive and negative ions; space charge effect is a dominant factor for the distortion of spatial electric field, making the discharge channel expand toward both electrodes faster; photoionization provides seed electrons for a secondary electron avalanche, promoting the formation and development speed of the streamer.

In electrical devices poured by epoxy resin, there are a lot of interfaces between epoxy resin and other solid dielectrics, i.e. solid-solid interfaces. Experiments were carried out to study the flashover characteristics of two typical solid-solid interfaces (epoxy-ceramic and epoxy-PMMA) under steep high-voltage impulse for different electrode systems (coaxial electrodes and finger electrodes) and different types of epoxy resin (neat epoxy resin, polyether modified epoxy resin and polyurethane modified epoxy resin). Results showed that, the flashover of solid-solid interface is similar to the breakdown of solid dielectric, and there are unrecoverable carbonated tracks after flashover. Under the same distance of electrodes, the electric stress of coaxial electrodes is lower than that of finger electrodes; and after the flashover, there are more severe breakdown and larger enhanced surface conductivity at interface for coaxial electrodes, as compared with the case of finger electrode. The dielectric properties are also discussed.

Using a plexiglas plate model, the performance of peristaltic flow acceleration induced by multiple DBD (dielectric barrier discharge) plasma actuators was studied based on PIV (particle image velocimetry). The asynchronous and the duty cycle pulsed actuation modes were proposed and tested. The velocity fields induced by multiple DBD plasma actuators with different phase angles and duty cycle ratios were acquired and the momentum transfer characteristics of the flow field were discussed. Consequently, the mechanism of the peristalsis-acceleration multiple DBD plasma actuation was analyzed. The results show that the peristaltic flow acceleration effect of multiple plasma actuators occurs mainly in paraelectric direction, and the mechanism of peristaltic flow acceleration is ejection pushing effect rather than injection pumping effect. The asynchronous and the duty cycle pulsed actuation modes can, with energy consumption increase of merely 10%, achieve 65% and 42% increase of downstream velocity, and thus are promising in velocity improvement and energy saving.

A novel design is proposed for highly sensitive surface-plasmon-resonance sensors. The sensor is based on a microstructured optical fiber with two layers of annular-shaped holes. A gold layer is deposited on the inner surface of the second hole-layer, in which the holes have several micrometers thickness in size, facilitating analyte infiltration and metal layer deposition. In the first layer of holes, the sector-ring-shaped arms, used as supporting strips, are utilized to tune the resonance depth of the sensor. Numerical results indicate that the sensor operation wavelength can be tuned across the C+L-band. The spectral sensitivity of 1.0 · 10⁴ nm · RIU⁻¹ order of magnitude and a detection limit of 1.0 · 10⁻⁴ RIU order are demonstrated over a wide range of analyte refractive index from 1.320 to 1.335.

Based on the transmission line code (TLCODE), a circuit model is developed here for analyses of main switches in the high pulsed-power facilities. With the structure of the ZR main switch as an example, a circuit model topology of the switch is proposed, and in particular, calculation methods of the dynamic inductance and resistance of the switching arc are described. Moreover, a set of closed equations used for calculations of various node voltages are theoretically derived and numerically discretized. Based on these discrete equations and the Matlab program, a simulation procedure is established for analyses of the ZR main switch. Voltages and currents at different key points are obtained, and comparisons are made with those of a PSpice L-C model. The comparison results show that these two models are perfectly in accord with each other with discrepancy less than 0.1%, which verifies the effectiveness of the TLCODE model to a certain extent.

The KTX device is a reversed field pinch (RFP) device currently under construction. Its maximum plasma current is designed as 1 MA with a discharge time longer than 100 ms. Its major radius is 1.4 m and its minor radius is 0.55 m. One of the most important problems in the magnet system design is how to reduce the TF magnetic field ripple and error field. A new wedge-shaped TF coil is put forward for the KTX device and its electromagnetic properties are compared with those of rectangular-shaped TF coils. The error field B_n/B_t of wedge-shaped TF coils with 6.4 degrees is about 6% as compared with 8% in the case of a rectangular-shaped TF coil. Besides, the wedge-shaped TF coils have a lower magnetic field ripple at the edge of the plasma region, which is smaller than 7.5% at R=1.83 m and 2% at R=1.07 m. This means that the tokamak operation mode may be feasible for this device when the plasma area becomes smaller, because the maximum ripple in the plasma area of the tokamak model is always required to be smaller than 0.4%. Detailed analysis of the results shows that the structure of the wedged-shape TF coil is reliable. It can serve as a reference for TF coil design of small aspect ratio RFPs or similar torus devices.

In order to investigate the effect of aperture geometry on deposition mitigation, stainless steel (SS) first mirrors (FMs) were fixed on the holders of protective aperture geometry with different depth-diameter ratios (DDRs) and exposed in the deposition dominated environment of EAST. A baffle was used during the wall conditioning. The surface properties and reflectivity of the FMs were characterized before and after exposure. It is shown that using aperture geometry and a baffle can effectively mitigate the impurities deposition. The degradation of the surface and specular reflectivity of the FMs is reduced with the increase of DDRs in the range of 0 to 2. The main contaminated elements in a low-Z and high-Z mixed wall materials environment were still carbon and oxygen.

The superconducting outsert of the 40 T hybrid-magnet in High Magnetic Field Laboratory (HMFL) of Chinese Academy of Sciences (CAS) requires a highly stabilized power supply. In this paper, two kinds of power supply design are briefly presented and both advantages and disadvantages are analyzed. In order to overcome the drawbacks of switching power supply, a series regulated active filter is adopted and a new design is proposed which ensures cooperative relationship between the feedback control loops of the switching converter and the series regulated active filter. Besides, unlike the traditional switching power supply, which can generate positive voltage only, this new design can also generate negative voltage which is needed in the quench protection for the superconducting magnet. In order to demonstrate the effectiveness of the methodology, a low-power prototype has been accomplished. The simulation and experiment results show that the power supply achieves high precision under the combined action of two feedback control loops. The peak-to-peak amplitude of the output ripple voltage of the prototype is 0.063%, while the peak-to-peak amplitude of the output ripple current is 120 ppm.