Residual energy and its effect on gain in a Lyman-α laser

Abstract

To examine prospects for gain in a Lyman-a recombination laser driven by a high-intensity, shortpulse laser, we calculate the residual energy in both hydrogen and helium during recombination after the ionizing pulse. The expected gain as a function of residual energy and density is then separately evaluated. The residual energy calculation includes above-threshold ionization (ATI) in the presence of a background plasma, as well as inverse-bremsstrahlung heating. At electron densities over 10" cm ' but below critical density, the plasma reduces the ATI energy by approximately a factor of 2, but without a previously reported dependence on the pulse width. Inverse-bremsstrahlung heating can be significant, but is not dominant for the parameters considered. Detailed recombination-laser gain calculations were performed for the Ly-a transitions of both H and He, using Stark profiles to represent the laser line cross section. To obtain gain of near 2 cm lasting at least a few ps, the H plasma temperature must be less than 3.5 eV and electron density between 4X 10" and 4X10" cm; for He, the temperature must be less than 15 eV and the electron density between 2X 10'8 and 2X10' cm '. Our calculations indicate that these conditions can be satisfied for H, if the driving laser intensity is above 4X 10' Wcm, and for He, if the laser intensity is above 1.7 X 10' W cm and the wavelength is below 0.6 pm.PACS number(s): 42.55. Vc, 32.80.Rm, 52.40.Nk, 52.50.Jm

Measurement of Velocity Distributions and Recombination Kinetics in Tunnel-Ionized Helium Plasmas

“…Timedependent fluorescence spectra allow us to assess whether such a cascade proceeds rapidly. In this context note that the temporal evolution of 1s-2p line emission is indicative of the time scale on which appreciable population funnels into the n = 2 state; this state is the upper level of a possible resonance line laser in Hett [9]. Experimentally, we find that the 1s-2p transition dominates the recombination spectrum at times later than 400 ps after the laser pulse.…”

“…2 (bottom) (curves a, b, and c, respectively). While the distributions are observed to thermalize at longer times, it is significant that the distribution does not thermalize in the -10 ps time scale necessary for peak gain in a HeII 1s -2p recombination laser [9]. We note that thermalization is observed to occur in -200 ps, in marked disagreement with the electron-electron collision time of -1 ps [12] for a Maxwellian distribution at our experimental parameters (Z = 2, kT = 20 eV, 10' atoms/cm ).…”

“…While a least-squares fit by a Maxwellian distribution (kT = 20 eV, average energy = 30 eV) accurately predicts the average energy, it does not accurately reproduce the measured distribution of electron energies. While this result may not be surprising (the quasistatic model [2] does not predict a Maxwellian distribution) it is significant because calculation of gain in recombination laser proposals typically assumes a Maxwellian distribution [1,9]. The quasistatic model accurately predicts the average energy in the distribution of Fig.…”

“…Calculations for gain in recombination lasers typically assume a Maxwellian distribution of electrons at a temperature estimated from the quasistatic model [9]. Using this assumption, we model our time-dependent recombination spectra with the time-dependent recombination kinetics code FLY [13].…”

Electron residual energy due to stochastic heating in field-ionized plasma

“…The net energy that transfers to the electrons in the presence of laser field by the different heating mechanisms is known as electron residual energy. Recently, the electron residual energy in optical field-ionized plasmas has extensively been elaborated by many theoretical and experimental physicists [11][12][13][14][15][16][17][18]. The residual momentum and energy of electron 3 are analytically investigated as a function of gas and laser parameters.…”