Dipole RF Power from Laser Plasmas with no Dipole Moment

Summary form only given. The radio-frequency power radiated from certain laser-target plasmas in a vacuum can be orders of magnitude greater than expected from such sources that have a negligible electric dipole moment. A model combining the Tidman-Stamper circuit model of a laser-target plasma with the theory of radiation from currents immersed in plasmas, however, predicts scaling of electric-dipole power radiated from laser plasmas in agreement with experiments. The model estimates the power, angular distribution, and spectrum of radio-frequency (rf) radiation from laser plasmas produced on solid surfaces in vacuum. The estimates of rf power are compared with experimental data from CO₂-laser interactions with copper targets at incident intensities from about 60 MW/cm² to 40 GW/cm². The model agrees with the limited observations of rf-power scaling with laser power, with the radial and angular dependence of the near-zone fields, and with the correspondence of rf radiation with the formation of a critical-density surface. The model should be valid over a range of laser wavelengths from the ultraviolet to the long- wavelength infrared. The model should also be valid over a range of incident laser intensities that produce plasma electron temperatures of a few eV, just above the plasma ignition threshold, to 1 keV or higher. The laser spot size and pulse duration are key parameters affecting the rf power. The theory suggests that electromagnetic radiation does not escape from the electron current immersed in the plasma plume, which is overdense to rf radiation. But the return current, which flows in the diffuse plasma surrounding the plume, does radiate as a dipole. If a current ring that has zero dipole moment is partly shielded, the unshielded part will radiate dipole power. The net effect is that the poloidal electron current in certain laser plasmas radiates as an electric dipole, instead of a quadrupole.