Evolution of an arched magnetized laboratory plasma in a sheared magnetic field

Arched magnetized structures are a common occurrence in space and laboratory plasmas. Results from a laboratory experiment on spatio-temporal evolution of an arched magnetized plasma ( $\beta \approx 10^{-3}$ , Lundquist number $\approx 10^{4}$ , plasma radius/ion gyroradius $\approx 20$ ) in a sheared magnetic configuration are presented. The experiment is designed to model conditions relevant to the formation and destabilization of similar structures in the solar atmosphere. The magnitude of a nearly horizontal overlying magnetic field was varied to study its effects on the writhe and twist of the arched plasma. In addition, the direction of the guiding magnetic field along the arch was varied to investigate its role in the formation of either forward- or reverse-S shaped plasma structures. The electrical current in the arched plasma was well below the current required to make it kink unstable. A significant increase in the writhe of the arched plasma was observed with larger magnitudes of overlying magnetic field. A forward-S shaped arched plasma was observed for a guiding magnetic field oriented nearly antiparallel to the initial arched plasma current, while the parallel orientation yielded the reverse-S shaped arched plasma.

Influence of Energy Parameters on Plasma Radius, Energy Fraction and Plasma Flushing Efficiency for a Single Discharge in EDM

An electrical discharge forms a crater on the workpiece surface. The crater morphology estimates the performance parameters of the electrical discharge machining process. The energy parameters (gap voltage, discharge current and the pulse on time), the plasma channel radius and the energy fraction coming to the workpiece determine the molten cavity radius and depth. The plasma flushes away a portion of material from the molten cavity forming a crater and resolidification of the remaining molten material forms a recast layer. The plasma flushing efficiency determines the crater’s radius and depth. Few researchers have successfully expressed the plasma radius, energy fraction and plasma flushing efficiency in relation to two of the energy parameters, namely, discharge current and pulse on time but not as a gap voltage function. This work attempted to develop a thermo-physical model to express plasma radius, energy fraction and plasma flushing efficiency as a function of all three energy parameters, such as gap voltage, discharge current and pulse on time. Plasma flushing efficiency was calculated and plasma radius and energy fraction were estimated by inverse finite element method from the measured values of crater radius, crater depth and recast layer thickness. The expressions for plasma radius, energy fraction and plasma flushing efficiency were found out from the regression equations obtained from the designed data set using the Taguchi method. Validation shows that the modeled and experimental values of crater radius, crater depth, and recast layer thickness agree well.

Effect of machining polarity on single discharge characteristics in micro-EDM

In micro electrical discharge machining (micro-EDM), polarity effect is attributed to the difference in energy distribution into the anode and cathode. Understanding the effect of machining polarity on energy distribution bears significance in predicting and controlling machining performances. Single discharge experiments were conducted in this study, to explore single discharge characteristics. The plasma radius and energy distribution were calculated by combining the crater size and the electro-thermal model. The results show that the influence of discharge current on crater depth-to-diameter ratio (H/D) is not significant with positive polarity. The plasma radius, fraction of energy transferred to workpiece, and crater size are greater for micro-EDM with negative polarity than positive polarity.

ECCD operations in the second experimental campaign at W7-X

In the Wendelstein 7-X stellarator, up to 7MW of power are delivered to the plasma by an electron cyclotron resonance heating system consisting of ten 140 GHz gyrotrons [1]. Due to the flexible front steering mirror of each beam line, the power deposition can be varied over the whole plasma radius and is optionally combinable with additional current drive. This flexibility, together with small toroidal currents in the stellarator, makes W7-X a perfect testbed for electron cyclotron current drive (ECCD) experiments, which have been successfully accomplished during the first two experimental campaigns OP1.1 and OP1.2a. Long discharges (lasting up to 30s) have been performed in OP1.2a, thus allowing the study of the current drive time evolution and the possibility to compensate the bootstrap current. ECCD efficiency has been studied using different power deposition profiles combined with a variation of the injection angles in relation to the magnetic field. During ECCD experiments, saw-tooth-like oscillations have been observed. Depending on the driven current density, ECCD can significantly modify the rotational transform (iota) profile, which can locally reach low order rational, thus triggering plasma instabilities. Different current density profiles have been tested, in order to try to understand the main trigger parameter for the instabilities. In particular, effects caused by current density gradient have been investigated producing both co- and counter-current drive at different radial positions: the total current drive is negligible, but a strong current gradient arises by driving currents in opposite directions. In this work an overview of ECCD operations in OP1.2a is given and first results, comparing different diagnostics, are presented. An initial 1-D model, coupled with the ray tracer TRAVIS, is developed, in order to have an estimation of current diffusion times and the radial position where a low order rational crosses the disturbed iota profile.

Electro-thermal-based finite element simulation and experimental validation of material removal in static gap single-spark die-sinking electro-discharge machining process

Existing single spark models are subjected to too simplistic assumptions such as uniform or point heat source, constant plasma radius, invariable materials properties and constant surface temperature during discharge making them far from reality. In this study, more realistic assumptions including Gaussian type distribution of spark heat flux, temperature dependent materials properties, latent heat of melting and expanding plasma channel with pulse current and time have been made to establish a comprehensive modeling platform. The ABAQUS FEM software has been used to simulate the mechanism of crater formation due to a single discharge. The non-uniform thermal flux was programmed through the DFLUX subroutine. The simulation results show that the temperature of work piece decreases as the discharge time increases while the volume of melted and evaporated material increases. A specially designed single spark experimental set-up was developed in laboratory to carry out a few single spark tests for verification purposes. The obtained craters morphologies were examined by optical microscopy and scanning profilometer. It has been shown that the present approach outperforms other previously developed thermal models with respect to cavity outline and size possessing the maximum confirmation errors of 18.1% and 14.1% in predicting crater radius and depth, respectively. Parametric analysis reveals that the melting boundary moves onward by increasing discharge current, whereas it moves back prolonging discharge time. Finally, a closer proximity to experimental material removal rates than those predicted by analytical approach has been recognized which confirms its more precise generalization capabilities towards the real state EDM process.

The Effect of Energy Leakage Probability on Burn Propagation in an Optically Thick Fusion Plasma

In an optically thick plasma, the mean free path of bremsstrahlung photons is smaller than the plasma radius, and radiation can be treated as a photon gas in thermal equilibrium. In these conditions, the black body radiation spectrum exceeds the number of hot photons, and reabsorption processes such as inverse bremsstrahlung radiation and inverse Compton scattering become important. It has been shown that a dense fusion plasma like the one being used in ICF method is initially optically thick. When the fuel pellet is burning, the temperature of its electrons rises (approximately greater than 90 KeV), and the pellet becomes rapidly optically thin. In this paper, we have shown that the energy leakage probability makes electron temperature remain low (approximately smaller than 55 KeV), and as a result the fuel pellet remains optically thick during burning.

Research on Output Power of Plasma for PCVD Synthesizing Silica Glass

Super purity silica glass was synthesized by Plasma Chemical Vapor Deposition (PCVD) method, during which inductively coupled plasma was used as the heat source. Influence of structure factor of tube and working gas on output power were discussed. The structure factor of tube was confirmed by analyzing the relation between structure factor of tube and coupling efficiency. With the increase of coupling efficiency, output power and plasma radius grow bigger. Super purity silica glass can be successfully synthesized when air was used working gas, during which input power was 100kW and output power reached to 40kW.

Turbulent transport in magnetic confinement: how to avoid it

From recent tokamak research, there is considerable experimental evidence that superthermal ions slow down and diffuse classically in the presence of turbulent fluctuations that cause anomalous transport of thermal ions. Further more, research on field-reversed configurations at Los Alamos is consistent with the view that kinetic effects suppress instability growth when the ratio of plasma radius to ion orbital radius is small; turbulence is enhanced and confinement degrades when this ratio increases. Motivated by these experiments, we consider a plasma consisting of large-orbit non-adiabatic ions and adiabatic electrons. For such a plasma, it is possible that the anomalous transport characteristic of tokamaks can be avoided and a compact reactor design becomes viable.