X-ray Lasers and Applications of X-rays

E. Fill, J. Stein, R. Tommasini

1. 10 Hz soft X-ray laser

In spite of recent progress in reducing driver requirements for X-ray lasers the repetition rates of these lasers are still limited to about 10 shots per hour. This is due to the fact that 5 to 10 J of pump energy are needed to generate and heat plasmas with inversion on transitions in the soft X-ray region. We investigate a setup in which transverse and longitudinal pumping are combined to reduce the pumping energy further and to increase the repetition rate to the 10 Hz provided by the ATLAS laser. The experimental arrangement in shown in the figure:

Here part of the fs-ATLAS pulse illuminates the target from above to generate a dense preplasma. To be able to ablate more target material the pulse is made longer by means of a stepped mirror. The main pulse, with fs duration, is focused along the target into the preplasma and provides excitation in a traveling wave arrangement. In first experiments the electron density distribution generated by the transverse prepulse is investigated using the lateral deflection of the beam. In this way an optimization of the many parameters in these experiments (such as prepulse level, prepulse-main pulse delay, target surface condition etc.) is obtained. The next step will be do demonstrate lasing in nickel-like molybdenum at 18.9 nm.

2. Bright ultra-short hard X-ray source

Hard X-rays with fs pulse durations will have applications in many fields, such as materials science, plasma physics, X-ray lasers and in testing of optics for 4th generation light sources. We use relativistic intensities to generate bright ultra-short pulses of Cu Kαradiation. The transition from non-relativistic to relativistic interaction is demonstrated by the appearance of a bright X-ray emitting spot with a small diameter. Simultaneously Cerenkov light generated in a dielectric shows a new hot-electron population accelerated parallel to the laser beam. The upper figure shows a half-shadow image of hard X-rays and the corresponding X-ray spot. The lower figure shows Cerenkov light observed in a BK7 glass sample behind the target using non-relativistic (left) and relativistic (right) intensities. The asymmetry and the dip in the center are clearly visible.

3. Time-resolved X-ray diffraction

We have developed a copper tape target (see photograph) which is easy to adjust, cheap to operate and allows accumulation of up to 10000 shots in a single run. Effects investigated include propagation of shock-waves in semiconductors and photo-induced structural changes in molecular solids. This method will eventually allow observation of molecular motion in photo-excited biological and chemical systems.

Selected recent publications

1. R. Tommasini and E. Fill, Generalized Linford formula and its application to traveling Wave Excitation, J. Phys. IV France 11, Pr2-285 (2001).

2. R. Tommasini and E. Fill, Effective traveling wave excitation below the light speed, Opt. Lett. 26, 689 (2001).

3. R. Tommasini, J- Nilsen and E. Fill Investigations on 10-Hz sub-Joule fs-laser pumped neon- and nickel-like X-ray lasers, SPIE 4505, 85 (2001)

4. E. E. Fill, Analytical theory of pulsed relativistic electron beams entering a vacuum, Phys. Plasmas 8, 4613 (2001).

5. E.E. Fill, G. Pretzler and Th. Schlegel, Photopumping of innershell transitions by relativistic plasmas, J. Phys. IV 11, 217 (2001).

6. E. Fill, J. Bayerl and R. Tommasini, A novel tape target for use with repetitively pulsed lasers, Rev. Sci. Inst. 73, 2190 (2002).