Lecture Notes in Physics

8 H. Schwoerer
Fig. 2.1. If the focal spot size of the sunlight lens is 0.1 mm 2 , the intensity there
would be 10 20 W/cm 2 . This value can be reached with state-of-the-art high-intensity
laser systems, admittedly, only for 10 −12 s
In the laboratory however, they can be produced only in a controlled way in
high-intensity laser plasmas.
Before we describe the fundamental interaction between such intense light
fields with matter, we quickly have to characterize the light fields themselves
in the following Sect. 2.2. In Sect. 2.3, we will introduce the basic mechanisms
of laser–matter interaction at relativistic intensities, starting from the free
electron in a strong electromagnetic wave all the way to the forced wakefield
or bubble acceleration. Section 2.4 covers the generation of Bremsstrahlung
in the multi-MeV range, and the final Sect. 2.5 describes the fundamentals of
proton and ion acceleration with intense laser pulses.
2.2 The Most Intense Light Fields
The intensity of a laser pulse is given by the pulse energy E divided by the
pulse duration τ and the size of the focal area A. In order to reach relativistic
intensities, E has to be large whereas τ and A must be as small as possible.
For technical and financial reasons basically two combinations of these parameters
exist in real lasers: a high-energy version and an ultrashort version.
The high-energy laser systems typically deliver pulse energies of hundreds to
thousand Joules within pretty short pulses below 1 ps. The ultrashort laser
systems concentrate their pulse energy in the range of 1 J within much less
than 100 fs. Because of better focusability in the latter case, both types are
able to generate the same maximum intensity. Another difference between the
two types of lasers is the rate of shots. Thermal effects within the laser typically
limit the high-energy systems to one shot within half an hour whereas
the ultrashort systems can operate around 10 Hz. Because of the high investments
and operational costs of a high-energy laser system, these are run by
national laboratories like the Rutherford Appleton Laboratory in the United