Feasibility of a Laboratory X-Ray Laser Pumped by Ultrashort UV Laser Pulses

In order to allow widespread application of soft X-ray lasers there is a strong effort worldwide to use as small as possible pump lasers for plasma production. Short pulse lasers (-c~ 1 ps), particularly in the UV, have attracted much interest, since extremely high intensities (up to 10 ~8 W/cm 2) can be achieved with a relatively high repetition rate. In this article we discuss their merit for soft X-ray laser pumping and possible solutions to the specific problems, for instance pulse front distortion, nonlinear absorption in window materials, plasma formation by short laser pulses and the relatively low total pump energy. PACS: 42.55V (X-ray laser) Laser-produced plasmas still appear to be the only suitable medium for laser action below 250 A. To generate population inversion on ionic transitions with an energy of more than 50 eV a high plasma temperature is necessary, which until now has been achieved by long-pulse high energy pump lasers. Unfortunately, these lasers are huge single-shot devices, and the overall efficiency is very low. Despite the fundamental limitations regarding, e.g., the efficiency, considerable effort is being expended worldwide to realize an X-ray laser as a small laboratory device. There is little doubt that widespread applications will only become possible if the requirements concerning the pump laser dimensions can be relaxed and the repetition rate can be increased to, say, 1 Hz. A promising approach to high-intensity target irradiation employs the extremely high optical power nowadays available with ps or even sub-ps laser pulses, particularly in the ultraviolet [1-5]. However, present short-pulse (< 10 ps) laser systems produce at most a few Joules of optical energy compared to the mostly more than 100 J of pump energy employed in successful X-ray laser experiments so far [6]. As a conse* Present address: Central Laser Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OXl l 0QX, UK ** Permanent address: JATE University, Research Group on Laser Physics of the Hungarian Academy of Sciences, D6m t6r 9, H-6720 Szeged, Hungary quence of the small pulse energy, an efficient focussing geometry is required. In particular, a travelling-wave pump arrangement may be advantageous, as has already been pointed out early [7]. Whereas effective multi-photon ionization in collisionless plasmas [8] and high-order frequency multiplication down to 146 A [9] have been demonstrated by ps UV pulses, no successful X-ray laser experiment has been reported using them as pump pulses. A few years ago the Laser Physics Group at the Max-Planck-Institut ffir biophysikalische Chemic, G6ttingen, started to develop a short pulse UV laser system on a laboratory scale for potential application to X-ray laser pumping. In this paper we summarize the results that have been obtained concerning short pulse generation, amplification, and target experiments. Pulse propagation in optical materials has been studied in detail and will be discussed together with the consequences for suitable target geometries. The final section will be about laser-plasma interaction of ultrashort UV laser pulses and possibly applicable X-ray laser pumping schemes. 1. Short Pulse Generation For the generation of sub-ps UV pulses we employ a hybrid excimer-dye laser system, of which the essential part is a short (80 gin) distributed feedback dye laser 266 M. Steyer et al.