Guiding of intense pulses in fully ionized hydrogen plasma waveguides from a cluster jet

Summary form only given. The first demonstration of guiding of intense laser pulses in a preformed plasma channel involved the use of a Bessel beam pulse (produced by an axicon) to generate an extended line focus, producing a long plasma channel in either in backfill gases or in a gas jet, using non-clustered gases. As the plasma evolves and expands in the radial direction, it develops the profile required for wave guiding a few nanoseconds later. This method was found to have some limitations. Channel generation through efficient breakdown and inverse bremsstrahlung heating occurs only with resultant electron densities in excess of /spl sim/10/sup 19/ cm/sup -3/, which is too high for some applications. Also, both in backfill and gas jet realizations, these channels had a significant taper (over /spl sim/0.5-1 mm) at the entrance which impeded the injection coupling of the high-intensity pulse. Since the mid-nineties, clusters have been known to absorb laser energy very efficiently. This is primarily due to the solid-like density of an individual cluster. A hydrocode has been used to calculate the complex polarizability of the cluster as it evolves. At early times, when the cluster is mostly supercritical, the real part of the ensemble polarizability is positive, and its radial profile is concave. In this regime the laser pulse can undergo self-focusing. The evolution of the polarizability was measured using spectral interferometry, and these experiments confirmed the general predictions of the model. Self-focusing was also demonstrated in a different set of experiments. Since a clustered gas is expected to guide a laser pulse under appropriate conditions, a plasma channel can be generated which is much longer than the Rayleigh range of the focused beam. The channel can therefore be produced by end-pumping, greatly simplifying the experimental arrangement compared to the axicon case, We have constructed a pump probe experiment using a 1 cm long cluster jet in whic- a pump pulse self-guides and creates a /spl sim/1 cm plasma channel into which an intense co-propagating probe pulse is injected. This has been done for both argon and hydrogen cluster jets.