Investigation of ultrashort soft x-ray pulses.
Ultrashort pulses from the visible to the x-rays.
table-top lasers of subpicosecond pulse durations can provide high intensities.
The availability of table-top short pulse systems in the recent years gave new
impetus for small-scale research in the field of x-ray generation and
spectroscopy in laser-produced plasmas. Recent developments of high-power
lasers of ~100 fs duration will lead to bright laser plasma XUV sources of
extremely short duration, too. There is a great demand for short and coherent
x-ray pulses. The possible applications of these new XUV sources are x-ray
microscopy, lithography, holography, investigation of biological specimen etc. The
commonly used lasers are solid state devices based on chirped pulse
amplification (CPA). An alternative is gas discharge excimer lasers, which
allow direct amplification therefore pedestal-free-short pulses. Here the use
of short-pulsed high-power excimer laser systems for this purpose is presented.
As a consequence of the high focusability the short wavelength of these pulses,
intensities as high as 1020 W/cm2 can easily be obtained
with modest laser systems.
plasma experiments have been carried out by our group of the Department of
Plasma Physics of the KFKI-Research Institute for Particle and Nuclear Physics using
the KrF laser system of the HILL Laboratory of the Department of Experimental
Physics of the University of Szeged in a close collaboration. The high intensity hybrid
excimer-dye laser system generates hot plasmas, and excites nonlinear interactions. Our aim is
the generation of ultrashort EUV (10-150 nm) and soft x-rays (0.3-10nm), and
also seeking their applications.
possibilities of generating coherent ultrashort XUV pulses, high harmonics
generation is one of the most straightforward way, because the requirements are
not as stringent for x-ray lasers. High harmonics can be generated in gases
using modest laser intensities and from laser plasma interactions at high
intensities. We intend to follow both ways. Our main goal is to obtain high conversion
efficiency harmonics. Harmonics from plasmas are generally more effective, than
that from gases, reaching conversion up to 10-4. However using resonances of
the gaseous atoms the latter can be even more effective, but the high
conversion is limited to resonant structures.
pulses from the K
radiation of different materials can be obtained using the nonlinear
interaction which accelerate electrons to high speed. Fast electrons may penetrate deeply into the
solid target matter knocking out K-electrons of the still cold matter. This
vacancy results in the generation of Ka radiation of
pulse durations even shorter than that of the laser pulse. Besides Ka
generation hot electrons have a heating effect as well, they heat up the target
matter isochorically, thus generating K- and L-shell radiation of the hot dense
matter which have typically some ps duration. We are planning to determine the
conditions of these two processes and to find the border line between these
In order to study
these processes we can use the spectroscopical tools of our laboratory which
cover the full spectral range from the visible radiation to the 2 keV x rays.
The upgraded laser system of the University of Szeged is the main tool of the investigations.
Technical assistance is availbale in the workshop of the university, and also
in the workshop of the KFKI-Research Institute for Particle and Nuclear
Physics. Besides the researchers participating in this proposal the
collaboration is to be continued with the University of Debrecen (E. Takács,
Cs. Szabó) on x-ray spectroscopy, David Salzmann (Soreq NRC, Israel) on
simulations, with the WAT university Warsaw (R. Rakowski, H. Fiedorowicz) on
gas jet targets and with G. Holland of the NRL, USA on the x-ray detectors.
The three main topics of our investigations are summarized below.
Resonant generation of harmonics in gases
One of the most exciting application
of ulrashort laser pulses is the generation of high harmonics in gases. These
experiments made it possible in different laboratories to generate coherent radiation
up to the keV photon energy, of durations in the attosecond range. Harmonic
generation in gases is known to produce odd-integer harmonics. It is also
possible to generate harmonics by ultrashort UV pulses using ions as a
nonlinear medium as well. One of the reasons is that ionized atoms can be
generated by UV lasers by keeping the temperature low. In one of the first
efforts KrF laser produced high-harmonics using preionized rare-gas-like
alkaline ions. In the case the power of the generated harmonics does not
follow the classical perturbative exponential decrease with the harmonic orders
any more, but instead above a certain harmonic order a plateau range appears,
where the harmonic power remains almost constant. Thus the conversion to a
certain harmonics is – in general - low. This however was drastically changed
in a recent experiment , where 3-photon-absorption resonance in the
degenerate 4-wave mixing scheme in Ar yielded a conversion efficiency of ~1%
for the third harmonics of the KrF laser, corresponding to 100m
J energy for the 82.8 nm VUV radiation.
Our aim is to optimize this scheme. While maintaining this conversion
efficiency we plan to obtain more energy when upgrading our laser system from
the present 10-15 mJ/pulse to 50-60 mJ. This could lead to a coherent radiation
source in the VUV of mJ energy. We try to find other resonances to obtain
similarly efficient VUV sources for even shorter wavelengths. The upgrade our
laser system to this energy is planned to be realized during 2005. The last
year a gas jet target has been developed for these experiment, which was
characterized by x-ray shadowgraphy. This work is done in the frame of the Marie-Curie Network
of the EC (project leader in Hungary: S. Szatmári).
High harmonics from laser plasmas
Similarly to gas-harmonics, in our laboratory KrF laser is also used to
obtain harmonics in laser plasmas .
Our investigations on this type of harmonics generation have been supported by
several OTKA contracts. In the frame of the last contract - to be finished in
2005 - we have shown harmonics generation up to 62 nm with the experimental
demonstration of the appearance of critical surface rippling above 1016
W/cm2 intensity. The increase of laser energy should lead to a
similar increase of the intensity as well, thus harmonics up to at least 50 nm
are expected according to the particle in cell (PIC) simulations.
A further advantage of the increase of laser energy is the availability
of larger number of photons. In our previous efforts the laser intensity was
increased by stronger focusing. As the total energy remained constant, the
number of VUV photons was not increased significantly either, leading to
detection problems. With the increase of laser energy a significant improvement
in the signal to noise ratio is expected for the same intensity.
Isochoric heating and Ka
generation by hot electrons
In laser plasma interactions the
high intensity laser field initiates nonlinear interactions.
The strong ponderomotive force in the plasma
corona accelerates the electrons to high energies; a significant part of the
energy is transferred to fast electrons. The laser deposits its energy within
the skin layer of 5-10nm thickness. This very thin corona layer expands rapidly
already during the duration of the laser pulse. The layer below the corona is
heated by electron heat conduction similarly to the laser-plasma interactions
using longer pulses. Hot electrons however penetrate deeper into the target and
heat a region whose thickness is determined by their mean free path. For our
conditions the typical energy of the electrons is 10 keV and the thickness of
the heated layer exceeds several hundred nanometers. This thick layer remains
in the dense state for 1-2 ps before significant expansion developes. This
isochoric heating produces several 100 eV temperatures in the laboratory.
Matter in this extreme state can be found in the interior of stars and is
therefore of interest for astrophysics. In earth-based experiments such plasmas
are achievable in the compressed core of inertial confinement fusion pellets,
whose realization requires large, high energy laser facilities firing a few
shots per day only. The attractiveness of using fs-lasers for generating such
plasmas is connected to their high repetition rates (10Hz or more) and their low
cost. This opens a unique possibility to systematically study the complex
behavior of dense hot plasmas. Line radiation of this matter can be studied
spectroscopically, investigating the line broadening and line shift of these
pulses of picosecond duration. It is
expected that KrF lasers are appropriate for isochoric heating, as the short
wavelength and several hundred femtosecond pulse duration can result in high
temperatures in the depth of the target.
On the other hand multilayer targets
can be used to generate Ka
radiation of short pulse duration from the depth of the still cold
matter. The multilayer targets can be made of thin boron and aluminium layers
deposited on a quartz substrate with the boron layer facing the laser beam. Fast
electrons generated in the boron layer penetrate through the boron and the Al
layers into the quartz substrate. Throughout their motion thay knock out
K-electrons from the Al and Si atoms. These atoms with a resulting vacancy in
their K-shell, emit Ka
photons that emerge from the target and are measured using an x-ray
spectrometer. Besides generating XUV pulses of shorter duration than the laser
pulse, the ratio of the Al Ka
line (1468 eV) and the Si Ka
line (1720 eV) is a good measure of the temperature of the hot
electrons. David Salzmann of Soreq NRC carried out a numerical simulation for
our conditions giving a good basis for the planning of our experiments.
It can clearly be seen that both
effects use the same fast electrons, the difference between cold matter Ka
generation and isochoric heating lies in the characteristic of the
multilayer targets; in the depth of the matter to be investigated. Therefore we
plan to investigate both effects. Varying target configuration we intend to
determine the conditions where isochoric heating and Ka
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