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
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
R. Tommasini and E.
Fill, Generalized Linford formula and its application to
traveling Wave Excitation, J. Phys. IV France
11, Pr2-285 (2001).
R. Tommasini and E. Fill, Effective traveling wave
excitation below the light speed, Opt. Lett. 26, 689
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
E. E. Fill,
Analytical theory of pulsed relativistic electron beams entering
a vacuum, Phys. Plasmas 8, 4613 (2001).
E.E. Fill, G.
Pretzler and Th. Schlegel, Photopumping of innershell transitions
by relativistic plasmas, J. Phys. IV 11, 217 (2001).
E. Fill, J. Bayerl
and R. Tommasini, A novel tape target for use with repetitively
pulsed lasers, Rev. Sci. Inst. 73, 2190 (2002).