Dynamics of collisional excitation x-ray lasers

We present absolutely timed spectral measurements, as well as other data, on the exploding-foil Ne-like Se soft--x-ray laser scheme. The data show that amplification occurs near the peak of the optical heating pulse, in agreement with our original theoretical predictions. A recently published alternative scenario predicted that gain develops only after the heating pulse, via recombination in an overionized, rapidly cooling plasma. Since our new data differ so sharply from that prediction, we reexamine the theoretical basis for that alternative scenario.