Cathode Spots, Hot Spots for Impurities Release

10th International Conference on Plasma Surface Interactions in Controlled Fusion DevicesLocal overheating, hot spots increase the influx of impurities at divertor target plates. A uniform energy deposition would be optimal. However, most discharges form small cathode spots which provide highly non-uniform energy deposition and cratering of the surface. A plasma in "contact" with a surface assumes a positive potential. The sheath electric field depends on the electron temperature and plasma density, EoC( ne kTg)1''2 . If large enough , enhanced field emission of electrons will begin from small spots on the surface. The emission of electrons and the impact of ions stimulate desorption of neutrals from the surface of the spot. If the plasma potential is larger than the ionization energy, the field emitted electrons will ionize desorbed neutrals. Ions produced a short distance from the electron emitting spot are accelerated back toward the spot. This ion bombardment leads to surface heating of the spot. Calculations of the power deposition show that ion surface heating is initially orders of magnitude larger than joule heating by the field emission current. This ion bombardment of a thin surface layer leads efficiently to further desorption and sputtering of neutrals. The local sheath electric field increases as more ions are produced and this strongly enhances the field emitted electron current. The localized build-up of a higher plasma density above the electron emitting spot naturally leads to pressure and electric field distributions which ignite unipolar arcs. The high current density of the unipolar arc and the associated surface heating by ions provide the explosive like formation of a cathode spot plasm

Observations of enhanced resistivity in the wave front of a laser-produced plasma interacting with a magnetic field

A laser-produced plasma has been used in a new approach to the problem of investigating the mechanisms associated with the turbulent energy dissipation is a collisionless laboratory plasma.Research at the Naval Postgraduate School partially supported by the Naval Air Systems Commands and the Air Force Office of Scientific Research

Unipolar arcing, a basic laser damage mechanism

This report is a reprint of a paper which was presented at the Fourteenth Annual Symposium on Optical Materials for High Power Lasers, November 16-17, 1982, National Bureau of Standards, Boulder, CO.Unipolar arcing has been shown to be the primary plasma-surface interaction process when a laser produced plasma is in contact with a surface. Evidence of unipolar arcing was found on all targets irradiated at atmospheric pressure that also arced in vacuum, stainless steel, titanium, molybdenum, copper, and aluminum. Cratering was observed even for a defocused and low-power laser pulse. The minimum laser power density required for the onset of breakdown on the surface is also sufficient to cause arc damage. Never was there a plasma evident without attendant unipolar arc craters. About 500,000 arc craters per cm have been observed on laser illuminated metal surfaces although no external voltage is applied. Smaller size craters with a density of about 10^/cm^ have been found on higher resistivity materials. The higher resistivity requires the radially inward surface return current to converge to a smaller cathode spot size to achieve sufficient power density to vaporize and ionize the material required for running the unipolar arc. The local increase of the plasma pressure above the cathode spot leads to an electric field configuration which drives the arc current and also facilitates the return current flow to the surface and cathode spot. Unipolar arcing concentrates the available laser-plasma energy towards the cathode spot. Large scale unipolar arcing on metal surfaces increases the coupling of energy from the laser heated plasma into the target. The ejection of a plasma jet from the cathode crater also causes highly localized shock waves to propagate into the target, softening it in the process. Thus, material erosion is much more severe than it would be case for uniform energy deposition over a larger area. This research has wide spread applications. Any situation in which a sufficiently hot surface plasma exists there will be unipolar micro-arcing. The physics relates to other forms of electrical breakdown on surfaces and electrodes.https://archive.org/details/unipolararcingba00sch

Short pulse laser and plasma surface interactions

http://archive.org/details/shortpulselaserp00schwN

Self-generated magnetic fields associated with a laser-produced plasma

The dependence of the self-generated magnetic field associated with a laser-produced plasma on position, time, incident laser power, and nitrogen background pressure has been investigated. The presence of the ambient background is found to influence the generation of the fields during the laser irradiation of the plasma. This influence continues long after laser shutoff and causes the fields to reverse their direction.This work was supported by the Air Force Office of Scientific Research under AFOSR Grant no. MIPR-0004-69 and by the Office of Naval ResearchApproved for public release; distribution is unlimited

Impurity control of tokamaks with in situ metal deposition

The article of record as published may be found at http://dx.doi.org/10.1016/0022-3115(80)90345-1Metal coatings of titanium and chromium of different thickness were deposited in situ in microtor and macrotor tokamaks and tested for impurity control. To improve the microstructure of the metal deposit, Ti and Cr were evaporated at a slow rate of about 20 Å/h during discharge cleaning. A 200 Å fresh metal deposition is required to obtain oxygen free surface layers. The oxygen control by Cr deposits was comparable to the one with Ti coatings. The hydrogen recycling rate increased markedly for Cr deposits. Control measurements show that the problem of hydrogen and deuterium retention in the metal coating during a switchover from one gas to the other can be overcome by short time outgassing to 650 K.US Department of EnergyGrant No. DE-AM03-76SF0001