High intensity fs-laser pulses can deliver focused intensities
in the region of 1016–1019
W/cm2. If the laser pulse is focused onto a
solid or gaseous material, a plasma is created. The electrons,
as well as the ions are accelerated in the strong laser
field up to energies in the range of keV to several MeV.
The interaction of the high energy particles with cold
material, that is, the solid target yield of intense X-ray
emission, K-shell—as well as bremsstrahlung-radiation.
The K-shell emission from layered targets is a
useful indicator of the production efficiency, energy distribution,
and transport of hot electrons produced in fs-laser plasmas.
For the diagnosis of laser plasma interaction and its application
as an intense X-ray source, the spatial, temporal and spectral
distribution of K-shell X rays is of fundamental
importance. Focusing crystal spectrographs can be used
to obtain a single shot X-ray spectra of laser plasmas
produced by table top fs-lasers. With a spatial- and spectral-focusing
spectrograph based on a toroidally bent crystal, the emission
region of the hot plasma and Kα-radiation
can be determined. Recording the spectra online by a frontside
illuminated charge-coupled device (CCD) allows alignment
of the crystal spectrograph, as well as the laser beam
focusing leading to different X-ray source sizes. Using
a controlled fs-prepulse, an increase in Kα
radiation could be observed with the diagnostic.Measurements of calibrated high resolution spectra are compared
with particle-in-cell (PIC) calculations of the laser absorption
and hot electron production postprocessed by a Monte–Carlo
(MC) transport model of electron stopping and Kα X-ray
generation.
Influence of hot electrons on the spectra of iron plasma irradiated by femtosecond laser pulses with 1021 W/cm2 intensities