Unipolar arcing, a basic laser damage mechanism
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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.
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.
NPS Report NumberNPS-61-83-008
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