Picosecond imaging of low-density plasmas by electron deflectometry.

We have imaged optical-field ionized plasmas with electron densities as low as 10(13) cm(-3) on a picosecond timescale using ultrashort electron pulses. Electric fields generated by the separation of charges are imprinted on a 20 keV probe electron pulse and reveal a cloud of electrons expanding away from a positively charged plasma core. Our method allows for a direct measurement of the electron energy required to escape the plasma and the total charge. Simulations reproduce the main features of the experiment and allow determination of the energy of the electrons.

Effective traveling-wave excitation below the speed of light.

We demonstrate that effective traveling-wave excitation of high-gain amplifiers requires velocities that are remarkably slower than the velocity of light. Experiments with a femtosecond-laser-pumped molecular hydrogen laser exhibit pronounced enhancement of the intensity if an excitation velocity that is slower than the velocity of light is employed. These results are directly scalable to shorter wavelengths, paving the way for a more effective pump setup for x-ray lasers.

X rays from optical-field ionized plasmas at low density.

We present results of an experiment in which x rays from an optical-field ionized plasma are generated under well-controlled conditions in a low-pressure gas cell. In this way high-density effects such as electron heating, collisional ionization, and ionization defocusing are avoided. Using N(2) as the medium, we show that many features of the soft-x-ray emission follow theoretical predictions. In particular, higher x-ray intensity is observed on most lines for circularly than for linearly polarized light. However, several Li-like lines show anomalously strong emission for linear polarization. Mechanisms that may be responsible for this effect are discussed.

Self-photopumped neonlike x-ray laser.

We propose a new x-ray laser mechanism that uses radiation from the strongest 3d ? 2p Ne-like resonance line in an optically thick plasma to radiatively drive population from the Ne-like ground state to the 3d state, which then lases to two 3p states. Collisional mixing of the 3p states with nearby 3s and 3d states depopulates the lower laser states. Modeling is presented for this mechanism in Ne-like Ar, and in experiments we observe one potential 3d ? 3p lasing transition at 45.1 nm in Ne-like Ar.

Two-dimensional near-field images of the neonlike germanium soft-x-ray laser.

We present what are believed to be the first two-dimensional near-field images of the neonlike germanium x-ray laser obtained with multilayer imaging mirrors. The Asterix iodine laser, with a low-intensity prepulse 5.23 ns before the main pulse, was used to irradiate germanium slab targets. We observe a large difference in the spatial dependence of the J = 0-1 and J = 2-1 lines of germanium, with the J = 2-1 emission peaking farther away from the target surface. The prepulse level is also observed to have a significant effect on the spatial dependence of the germanium laser lines. A great deal of structure is observed in the near-field images, particularly in the J = 0-1 emission.

High-gain x-ray lasing at 11.1 nm in sodiumlike copper driven by a 20-J, 2-ps Nd:glass laser.

Evidence of high gain pumped by recombination has been observed in the 5g-4f transition at 11.1 nm in sodiumlike copper ions with use of a 20-J 2-ps Nd:glass laser system. The time- and space-integrated gain coefficient was 8.8 +/- 1.4 cm(-1), indicating a single-transit amplification of ~60 times. This experiment has shown that 2 ps is the optimum pulse duration to drive the sodiumlike copper recombination x-ray lasing at 11.1 nm.