Observation of enhanced emission of the O VIII Hα line in a recombining laser-produced plasma

We have observed enhanced emission of the O VIII Hα line at 121 eV from a recombining plasma produced by laser heating of a thin Formvar foil with two opposed cylindrically focused (0.3×1.4 cm) 5-TW laser beams. The time-dependent intensity of the Hα line increases by nearly an order of magnitude as the plasma length is increased from 0.27 to 1.27 cm. This increase in intensity is greater than that expected from a medium exhibiting no gain. A comparison of the measured time history of the emission with that of non-local thermodynamic equilibrium time-dependent calculations suggests that a small-signal gain ∼0.5 cm−1 is obtained.