X-ray-laser-related investigations

Stig Borgström, Ernst Fill^*, Anders Persson, Sven-Göran Pettersson, Tomas Starczewski, Jürgen Steingruber^*, Sune Svanberg and Claes-Göran Wahlström,

^*Visiting scientist

The research programme in Lund related to X-ray lasers started about two years ago, at the time of the inauguration of the new terawatt laser. Shortly afterwards, we were also accepted as one of ten members of the European Network for X-ray lasers. The Lund terawatt laser, with its ultrashort pulse duration, high peak power and high repetition rate, may be an interesting pump source for a "table-top X-ray laser". The interest within the scientific community in this kind of pump source is great, because it is much cheaper, more widespread and has a much higher repetition rate than the few existing, enormous X-ray laser establishments in the world. However, the successful realisation of a table-top X-ray laser requires considerable efforts, since a large number of requirements must be simultaneously fulfilled.

Our activities have been divided into studies with solid targets and with gas targets. In the case of solid targets, we have explored the possibility of focusing the terawatt laser into a plasma, preformed by means of an additional laser. However, we found that the present laser wavelength (800 nm) is too long for sufficient penetration into the plasma. The experiments will be repeated, utilising a newly acquired frequency-doubling crystal, and interesting results are expected. An alternative approach, with solid targets, is to focus the terawatt laser along a line on the target. Experiments are underway together with Dr Ulf Litzén at the Division of Atomic Spectroscopy here in Lund. Preliminary spectroscopic results from a point focus revealed production of very highly ionised species (see section A4).

In our studies with gas targets, we have a fruitful cooperation with the Max-Planck-Institut für Quantenoptik in Garching, Germany. Using optical-field ionisation (OFI) we have investigated a number of gases [A9, A10]. We have found evidence for gain in four-times-ionised nitrogen on, e.g., the transition 3d-5f (Fig. A3). Also, using our new soft-X-ray streak camera we have observed a peculiar time behaviour of the emission from He⁺ [52]. These experiments will continue with improved methods and with other gases.

Fig. A3. Spectra obtained through optical-field-ionisation of nitrogen. The spectra were recorded in the direction of the laser beam and in the perpendicular direction, respectively.

At the 4th International Colloquium on X-Ray Lasers, held in the USA, we were given the honour of organising the next meeting. The 5th meeting in this series will thus be held in Lund in 1996.