Fast-electron generation in long-scale-length plasmas

Long-scale-length (∼400-μm) planar CH plasmas have been generated on OMEGA EP with laser intensities of the order of 1014 W/cm2 and ∼1-mm focal spots to quantify the number and temperature of fast electrons caused by the two-plasmon-decay instability. The main diagnostics were the time-integrated Kα line emission and the hard x-ray bremsstrahlung (HXR) from a molybdenum (Mo) substrate. For the intensity range of 1-7 x 1014 W/cm2, the Mo Kα and HXR energies increased by more than three orders of magnitude. The fast-electron temperature in this range (deduced from the x-ray bremsstrahlung emission) rose from ∼20 keV to ∼90 keV. A Monte Carlo code was used to estimate the total energy (or number) of fast electrons based on these two experimental signatures. The resulting energy in fast electrons as a fraction of the laser energy was found to rise in the same intensity range up to ∼1%.