Electromagnetic Emission from Laser Wakefields in Magnetized Underdense Plasmas

A wakefield driven by a short intense laser pulse in a perpendicularly magnetized underdense plasma is studied analytically and numerically for both weakly relativistic and highly relativistic situations. Owing to the DC magnetic field, a transverse component of the electric fields associated with the wakefield appears, while the longitudinal wave is not greatly affected by the magnetic field up to 22 Tesla. Moreover, the scaling law of the transverse field versus the longitudinal field is derived. One-dimensional particle-in-cell simulation results confirm the analytical results. Wakefield transmission through the plasma-vacuum boundary, where electromagnetic emission into vacuum occurs, is also investigated numerically. These results are useful for the generation of terahertz radiation and the diagnosis of laser wakefields.     

Mode-Coupling Analysis of Parametric Decay Instability in Magnetized Plasmas

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.     

Steady-State Plasmas in KT5D Magnetized Torus

Steady-state plasma generated by electron cyclotron resonance （ECR） wave in the KT5D magnetized torus was studied using a fast high-resolution camera and Langmuir probes. It was found that both the discharge patterns taken by the camera and the plasma parameters measured by the probes were very sensitive to the working gas pressure and the magnetic configuration of the torus both without and with vertical fields. There existed fast vertical motion of the plasma. Tentative discussion is presented about the observed phenomena such as the bright resonance layer at a high gas pressure and the wave absorption mechanism at a low pressure. Further explanations should be found.     

Effect of ion stress on properties of magnetized plasma sheath

In the plasma sheath,there is a significant gradient in ion velocity,resulting in strong stress on ions treated as a fluid.This aspect has often been neglected in previous sheath studies.This study is based on the Braginskii plasma transport theory and establishes a 1D3V sheath fluid model that takes into account the ion stress effect.Under the assumption that ions undergo both electric and diamagnetic drift in the presheath region,self-consistent boundary conditions,including the ion Bohm velocity,are derived based on the property of the Sagdeev pseudopotential.Furthermore,assuming that the electron velocity at the wall follows a truncated Maxwell distribution,the wall floating potential is calculated,leading to a more accurate sheath thickness estimation.The results show that ion stress significantly reduces the sheath thickness,enhances ion Bohm velocity,wall floating potential,and ion flux at the wall.It hinders the acceleration of ions within the sheath,leading to notable alterations in the particle density profiles within the sheath.Further research indicates that in ion stress,bulk viscous stress has the greatest impact on sheath properties.     

Characteristics of Langmuir Probe in Low Temperature, Weakly Magnetized Plasmas

The systematic Langmuir probe measurements for a weakly magnetized plasma have been carried out in the Linear Magnetized Plasma Device for different magnetic fields. By comparing the ion current density of probes with different sizes, the sheath thickness can be evaluated. It is found that while the ratio of cylindrical probe‘s dimension to ion Larmor radius is not more than 2, the model of probe for non-magnetized plasma is still applicable.     

Observation of Low Frequency Instability in a Magnetized Plasma Column

Detailed analysis of the low frequency instability is performed in a linear magnetized steady state plasma device. Identification and modification of the instability are presented.     

Plasma Density Influence on the Properties of a Plasma Filled Rod Pinch Diode

The rod pinch diode is perfect as a source of accelerators for flash X-ray radiography by virtue of a small and stable spot.But it is not suitable for intensive current drivers because of high diode impendence of 40~60Ω.However,by employing pre-filled plasma into diode prior to the driving current,the diode impendence can be efectively reduced.Plasma density plays an important role in this process,especially for sheath formation and space charge current in the diode.Analysis and simulation results show that a proper range of plasma density could be 1015~1016cm-3.     

Stimulating Effects of Seed Treatment by Magnetized Plasma on Tomato Growth and Yield

Tomato seeds （Lycopersicon esculentum L. Mill. cv. zhongshu No. 6） were treated by magnetized plasma before being sown to investigate its effect on the growth and yield of tomatoes. Biochemical analysis showed that dehydrogenase activity increased with the increase of the current but decreased when the current was higher than 1.5 A. The activities of peroxidase （POD） isoenzyme changed in the same pattern. There was no difference in germination percentage between treatments and control, which were carried out in laboratory conditions. However, significant （c~ = 0.01） difference was observed in germination percentage in the pot experiment. In the pot experiment, the sprouting rate for the treatment with a 1.5 A current was 32.75%, whereas the untreated was only 4.75% on the eleventh day. Germination time is more than one day earlier than the control. The 1.5 A treatment increased the tomato yield by 20.7%.     

Effects of Sputtering of and Radiation by Aluminum on Magnetized Target Fusion Plasmas

We estimate numerically the rate of radiation by aluminum impurities for parameters relevant to magnetized target fusion (MTF) plasmas. We demonstrate that the coronal equilibrium is appropriate for expected MTF plasma parameters. Using the coronal equilibrium, we estimate the power radiated per impurity ion is 0.25–0.5 × 10⁻¹⁶ MW for temperatures and densities relevant to present plasma parameters taken from the FRX-L experiment at Los Alamos National Laboratory and is approximately 75.0 × 10⁻¹⁶ MW for temperatures and densities relevant to anticipated MTF plasmas. We calculate the sputtering rate of aluminum by thermal deuterium and tritium plasma ions is a few percent assuming an impact angle of 45°. Finally, we estimate that with aluminum impurity levels of a few percent, the impurity radiation power density would be approximately 25 kW/cm³ for FRX-L conditions and 2.5 GW/cm³ for anticipated conditions in a MTF plasma. While we have assumed a sputtering model of impurity generation, the results for the power density apply for impurity levels of a few percent, regardless of the generation mechanism.     

Characteristics of a Sheath with Secondary Electron Emission in the Double Walls of a Hall Thruster

In order to investigate the effects of secondary electrons, which are emitted from the wall, on the performance of a thruster, a one-dimensional fluid model of the plasma sheath in double walls is applied to study the characteristics of a magnetized sheath. The effects of secondary electron emission (SEE) coefficients and trapping coefficients, as well as magnetic field, on the structure of the plasma sheath are investigated. The results show that sheath potential and wall potential rise with the increment of SEE coefficient and trapping coefficient which results in a reduced sheath thickness. In addition, magnetic field strength will influence the sheath potential distributions.     

Spectroscopic Studies on Impurity Transport of Core and Edge Plasmas in LHD

Spectroscopic diagnostics have been extensively developed for studies of impurity and neutral particle transports at core and edge plasmas in LHD. Diagnostics of core plasmas are similar to a tokamak case, i.e., Zeff from visible bremsstrahlung, K-x-ray measurements from xray spectroscopy using Si（Li） detectors and a compact crystal spectrometer, and high-Z impurity diagnostics from VUV spectroscopy using a flat-field EUV spectrometer. A combination of impurity pellet injection and visible bremsstrahlung is an active tool for determination of the diffusion coeffici＇ent D and convective velocity V. Using this tool the spatial structures of D and V are obtained and discussed with a neoclassical effect. On the other hand, the spectroscopic method for edge diagnostics is considerably different from the tokamak case because of the existence of a thick ergodic layer in addition to the z-points necessarily included into the diagnostic chord view. In order to break this negative situation, Zeeman and polarization spectroscopy are adopted to LHD edge plasmas. As a result, 2-dimensional emission contours of HeI and Ha are successfully obtained. Laser absorption spectroscopy is tried to measure hydrogen neutrals directly. Radial profiles of edge impurities are also measured with a mirror-assembled 3 m VUV spectrometer. Recent results of and progress in LHD spectroscopy are briefly reviewed.     

Methods of Power Balance Analysis and Estimation of Current Density Profile for LHCD and IBW Discharges in HT-7 Tokamak

In this paper a simple code has been developed to analyze power balance and qualitatively evaluate current profiles for discharges with lower hybrid current drive （LHCD） and ion Bernstein wave （IBW） heating in the HT-7 tokamak. Electron and ion thermal diffusivity, profiles of the bootstrap current density and total plasma current density can be estimated by this code using the experimental data. This code offers an easy and reasonable means to understand plasma transport in HT-7.     

Experimental Studies of Microwave Reflection and Attenuation by Plasmas Produced by Burning Chemicals in Atmosphere

A series of chemicals are designed and prepared. With the method of thermodynamics, the average electron densities of the plasmas generated by burning chemicals are calculated. The reflection and attenuation of the microwaves, in a frequency band of 2 GHz to 15 GHz, by the plasma are measured. The results of measurements indicate that the plasma can absorb the energies of the microwaves in a broad band and reflect them faintly. Moreover, theoretical discussion reveals that the electron-neutral collision is the major factor that results in the absorption in the wide band. By using Appleton equations, average collision frequencies and electron densities are calculated from the attenuations of microwaves.     

Computing Open-Loop Optimal Control of the q-Profile in Ramp-Up Tokamak Plasmas Using the Minimal-Surface Theory

The q-profile control problem in the ramp-up phase of plasma discharges is consid- ered in this work. The magnetic diffusion partial differential equation (PDE) models the dynamics of the poloidal magnetic flux profile, which is used in this work to formulate a PDE-constrained op- timization problem under a quasi-static assumption. The minimum surface theory and constrained numeric optimization are then applied to achieve suboptimal solutions. Since the transient dy- namics is pre-given by the minimum surface theory, then this method can dramatically accelerate the solution process. In order to be robust under external uncertainties in real implementations, PID (proportional-integral-derivative) controllers are used to force the actuators to follow the computational input trajectories. It has the potential to implement in real-time for long time discharges by combining this method with the magnetic equilibrium update.     

Characteristics of C2 Radical in CHF3 and CF4 Electron Cyclotron Resonance Plasmas Studied by Optical Emission Spectroscopy

The characteristics of CHF3 and CF4 electron cyclotron resonance (ECR) plasma have been studied by optical emission spectroscopy (OES) and Langmuir probe. It is found that C2 radical is one of main compositions in both of the two plasmas. We investigated the relative concentration of C2 radical as a function of F (H) radical and ion density. The formation mechanism of C2 radical is analyzed.     

Measurements of the Plasma Current Density and Q-Profiles in IR-T1 Tokamak

The purposes of this paper are determination of the current density and safety factor profiles, J(r) and q(r), in IR-T1 tokamak. For these purposes, a diamagnetic loop with its compensation coil, and also an array of magnetic probes were designed, constructed, and installed on outer surface of the IR-T1 tokamak chamber, and then the poloidal beta and poloidal and radial magnetic fields measured. Moreover, a few approximate values of the internal inductance for the different possible profiles of the plasma current density are also calculated. From the results, current density and q-profiles obtained.     

The Effect of Ion Current Density on Target Etching in Radio Frequency-Magnetron Sputtering Process

The effect of ion current density of argon plasma on target sputtering in magnetron sputtering process was investigated. Using home-made ion probe with computer-based data acquisition system, the ion current density as functions of discharge power, gas pressure and positions was measured. A double-hump shape was found in ion current density curve after the analysis of the effects of power and pressure. The data demonstrate that ion current density increases with the increase in gas pressure in spite of slightly at the double-hump site, sharply at wave-trough and side positions. Simultaneously, the ion current density increases upon increase in power. Es- pecially, the ion current density steeply increases at the double-hump site. The highest energy of the secondary electrons arising from Larmor precession was found at the double-hump position, which results in high ion density. The target was etched seriously at the double-hump position due to the high ion density there. The data indicates that the increase in power can lead to a high sputtering speed rate.     

The Properties of the Rectangular Arc Ion Plating with Magnetic Filtering Shutter

A major obstacle to the broad application of cathodic arc plasma deposition is the presence of macroparticles. In this paper, the properties of the large rectangular arc ion plating with a magnetic filtering shutter system to filter macroparticles are studied. It is proposed that the macroparticles in the plasma beam are effectively removed with the magnetic filtering shutter system, and the quality of the deposited films is improved.     

Report on the Fifth Symposium on Current Trends in International Fusion Research

The Fifth Symposium on Current Trends in International Fusion Research was held on 24–29 March 2003 in Washington, USA in co-operation with the International Atomic Energy Agency (IAEA). Well-known scientists in their field of interest in fusion research were invited to present review papers. The presentations covered a broad range of fusion topics, including inertial confinement fusion, electrostatic confinement, various magnetic confinement schemes, and hybrid schemes. Each session chairman composed a session report, which John Pucadyil used to create this Report on the Fifth Symposium.