Electrical, gasdynamic, and radiative properties of planar surface discharges

Abstract

An investigation was undertaken of the electrical, gasdynamic, and radiative properties of surface discharges intended as intense optical pump sources for lasers. Large-area (≂20–300 cm2), short-pulse (∼1 μs) planar surface discharges were produced across various ceramic and polymer substrates and operated in all of the rare gases, N2, and air at absolute pressures up to 4 atm. Parametric studies of the electrical circuitry and discharge dimensions demonstrate the importance of proper source-driver impedance matching to short-pulse operation and efficient electrical-to-optical energy conversion. Characteristic times for establishment of a conductive discharge gap in the heavy rare gases are consistent with breakdown by the single-stroke leader mechanism. The electrical strength of the gap following discharge remains low for a time which is much greater than the current duration. The delay in recovery is believed to be controlled by continued substrate ablation occurring after the discharge rather than to mechanisms related to plasma expansion or recombination. A shock wave is launched in a direction perpendicular to the dielectric surface while the plasma remains tightly pressed against the substrate. Measured wave velocities can be correlated with the theory of a Chapman–Jouguet detonation despite the fact that the physical processes in a surface discharge depart significantly from those in an optical detonation. Planar surface discharges radiate as Lambertian sources characterized by effective brightness temperatures ≂10 000–20 000 K (visible wavelengths) for specific input energies of ≂1–4 J/cm2. The plasma is not opaque and does not radiate as an ideal blackbody, but shows considerable structure due to line emission from neutral and singly ionized species originating from the gas atmosphere and vaporized substrate material. Line transitions from excited substrate species can greatly enhance the UV radiative output, especially for discharges across perovskite ceramic substrates. Radiated energy for the rare gases is almost directly proportional to the atomic weight of the gas. Open-shutter photography, performed both perpendicular and parallel to the discharge plane, is used to develop criteria for obtaining uniform, coalesced discharges.