Spot Plasmas Research Articles

Optical Properties of Arc Spot Plasma for Cleaning Oxide on the Steel Surface

The optical properties have been investigated in situ by optical emission spectroscopy and a high-speed camera for various cathode surfaces in low-pressure arc spot plasma. Electron excitation temperature ( $T_{\mathrm {exc}}$ ) is estimated by using the Fe I lines based on Boltzmann plot. It is found that cathode surfaces affect strongly on the discharge features of spot plasmas. $T_{\mathrm {exc}}$ increases with the arc current for both oxidized and clean surfaces, and $T_{\mathrm {exc}}$ is higher in the case of clean surfaces. Compared with the discharge on oxidized surface, the fluctuation of $T_{\mathrm {exc}}$ along with the intensity of emission lines enhances largely as spots move on a clean surface. The spots on oxidized surface present negligible change in trajectory in observation time, while spots move randomly in the temporal distribution for the clean surface. It is suggested that the presence of oxides on cathode surface strongly influences microexplosion processes on the cathode, and thus improve the stability for the spot plasma.

Resolving hot spot microstructure using x-ray penumbral imaging (invited).

We have developed and fielded x-ray penumbral imaging on the National Ignition Facility in order to enable sub-10 μm resolution imaging of stagnated plasma cores (hot spots) of spherically shock compressed spheres and shell implosion targets. By utilizing circular tungsten and tantalum apertures with diameters ranging from 20 μm to 2 mm, in combination with image plate and gated x-ray detectors as well as imaging magnifications ranging from 4 to 64, we have demonstrated high-resolution imaging of hot spot plasmas at x-ray energies above 5 keV. Here we give an overview of the experimental design criteria involved and demonstrate the most relevant influences on the reconstruction of x-ray penumbral images, as well as mitigation strategies of image degrading effects like over-exposed pixels, artifacts, and photon limited source emission. We describe experimental results showing the advantages of x-ray penumbral imaging over conventional Fraunhofer and photon limited pinhole imaging and showcase how internal hot spot microstructures can be resolved.

Feasibility study on high current ion beam extraction from anode spot plasma for large area ion implantation

In this paper, we have investigated the feasibility of the high current beam extraction from anode spot plasma as an ion source for large area ion implantation. Experiments have been carried out with the ambient plasma produced by inductive coupling with radio-frequency (RF) power of 200 W at the frequency of 13.56 MHz. Anode spot plasmas are generated near the extraction hole of 2 mm in diameter at the center of a bias electrode whose area exposed to the ambient plasma can be changed. It is found that the maximum ion beam current is extracted at the optimum operating pressure at which the area of bias electrode exposed to ambient plasma is fully covered with the anode spot plasma whose size is dominantly determined by the operating pressure for given gas species. It is also observed that the extracted ion beam current increases nonlinearly with the bias power due to the changes in size and shape of the anode spot plasma. With the well-established anode spot plasma operating at the optimum gas pressure, we have successfully extracted high current ion beam of 6.4 mA (204 mA/cm2) at the bias power of 22 W (∼10% of RF power), which is 43 times larger than that extracted from the plasma without anode spot. Based on the experimental results, criteria for electrode design and operating pressure for ion beam extraction from larger extraction aperture are suggested. In addition, the stability of anode spot plasma in the presence of ion beam extraction through an extraction hole is discussed in terms of the particle balance model.

Interaction Between Arc Spot Plasma and Steel Surface in Descaling

Cathode spots of low-pressure arc discharge have been used for cleaning oxides on the steel surface. A phenomenon of two-step plasma scanning on the oxidized steel surfaces is observed, where arc spots first move on the entire surface of the sample with strong evaporation and explosion of oxides. Afterward, there is a brief scanning, and a remaining thin oxide layer is removed while a clean surface of the bulk metal appears. It is believed that the two-step plasma scanning is due to the layered structure of oxides, which results in the self-adjusting of cathode spots on the oxidized surface. The corresponding spot plasmas are investigated by a high-speed camera and an oscilloscope. The voltage waveform and plasma spot images exhibit various characteristics during the two-step scanning process. Compared with the case of Ar arc discharge cleaning, the surface hardness of the sample cleaned with N2 arc spot plasma is obviously enhanced, which should be attributed to the formation of Fe4N during descaling.

Operating conditions for the generation of stable anode spot plasma in front of a positively biased electrode

Stability of an anode spot plasma, which is an additional high density plasma generated in front of a positively biased electrode immersed in ambient plasma, is a critical issue for its utilization to various types of ion sources. In this study, operating conditions for the generation of stable anode spot plasmas are experimentally investigated. Diagnostics of the bias current flowing into the positively biased electrode and the properties of ambient plasma reveal that unstable nature of the anode spot is deeply associated with the reduction of double layer potential between the anode spot plasma and the ambient plasma. It is found that stability of the anode spot plasma can be improved with increasing the ionization rate in ambient plasma so as to compensate the loss of electrons across the double layer or with enlarging the area of the biased electrode to prevent electron accumulation inside the anode spot. The results obtained from the present study give the guideline for operating conditions of anode spot plasmas as an ion source with high brightness.

Interferograms of a cathode spot plasma obtained with a picosecond laser

Cathode spot plasmas as well as balls-moving dense plasma blocks in vacuum arcs-were investigated by laser interferometry and absorption photography with high spatial (1.2 /spl mu/m) and temporal (100 ps) resolution. The discharge was initiated between a needle-type cathode and plane anode with a cathode-anode distance of a few millimeters. The duration of the discharges was about 10 ms and the current up to 100 A. Picosecond interferometry yielded spatial-temporal electron density distributions in the arc phase of the cathode spot. An absolute electron density up to 2/spl times/10/sup 26/ m/sup -3/ and plasma temperature about 1 eV in narrow plasma jets with a diameter about 5 /spl mu/m were estimated.

ELECTRODE SPOT PLASMAS OF ELECTRIC ARCS

ELECTRODE SPOT PLASMAS OF ELECTRIC ARCS

Measurement of cathode spot parameters with pulsed laser diagnostics

The cathode spot formation in air within the first 170 ns was investigated by laser absorption photography and ps-pulse interferometry. The discharge was initiated between electrodes made from Ag or Pd with cathode-anode distance below 300 /spl mu/m, the arc duration was some milliseconds, and the arc current 5-10 A. Picosecond holographic interferometry and momentary absorption photography yielded spatial-temporal density distributions in the ignition phase of the cathode spot. An absolute electron density value on the order of 4/spl times/10/sup 26/ m/sup -3/ has been found. In contrast to vacuum, the cathode spot plasmas broaden little with increasing distance from the cathode, thus narrow plasma channels are observed in the vicinity of the cathode surface having diameters <20 /spl mu/m.

Expansion Model of the Neutral Component in Vacuum Arc Spot Plasmas

AbstractPlasma generated by cathode spots in vacuum arcs expands towards the anode and the walls. In spite of a high degree of ionization (mostly), this plasma contains also a small but noticeable constituent of neutral atoms. Starting from the known analytical solution of a hydrodynamic model describing the plasma ions and electrons, the corresponding motion of neutrals is treated in a similar way. The results show: 1) The neutrals take part in the general plasma expansion; however, their final kinetic energy is lower (by about 50%) than the final energy of the ions. 2) The temperature of the neutrals also is lower than the ion temperature and decreases during expansion. 3) The acceleration of neutrals is caused partly by the pressure gradient, partly by interaction with the ions (friction); the forces diminish rapidly with increasing distance. Finally, the limitations of this approximate solution are discussed.

A revised theoretical model of vacuum arc spot plasmas

The quasi-stationary hemispherical expansion of the cathodic plasma in vacuum arcs can be modeled with hydrodynamic two-fluid equations. In any case, the state of the plasma is determined by the only variable (I/r)/sup 2/5/ (with current I, distance r). In order to avoid some deficiencies of the model (as published) and to investigate more carefully the dependence of the plasma parameters on the arc current, the known analytic solution to the problem is improved by taking into consideration the variability of the Coulomb logarithm and the dependence of the boundary conditions on I. These effects are treated separately. Examples are used to illustrate the new results, with particular emphasis on ion acceleration. The influence of the above factors turns out to be rather unimportant. Quantitatively, they cause some shifts, but no qualitative change of the basic behavior of the plasma is seen. >

Pressure ionization: its role in metal vapour vacuum arc plasmas and ion sources

In most plasmas ionization is brought about by classical ionizing collisions between particles. In metal vapour vacuum arc plasmas and ion sources, however, pressure ionization plays an important role in determining the final ion composition. The authors describe the basic mechanism of pressure ionization as a collective phenomenon in non-ideal plasmas, induced mainly by the many-particle Coulomb interaction and quantum-mechanical exchange. They outline the properties of the cathode spot plasmas of vacuum arc discharges. The path of the cathode material in the electron density-temperature phase diagram is considered: starting from the solid, the matter is heated, forms a liquid and then a supercritical fluid which expands explosively, driven by a very high pressure gradient. The final ion charge state distribution is produced and frozen in the transition from the dense, non-ideal plasma (where pressure ionization is dominant) to the expanded, ideal, non-equilibrium plasma. The final ion charge state distribution depends on the thermophysical properties of the solid and liquid cathode material as well as on the electronic structure of its atoms and ions. Some consequence for the performance of the metal vapour vacuum arc ion source are discussed.