Laser pulses at relativistic intensities (more than 1018 W/cm2) create fast electrons which penetrate and exit the target. In case of a solid target - generally a foil- the target rear surface develops a cloud of electrons. These electrons are usually referred to as an electron sheath, and their huge electric field in the order of 10 billions of volts per cm accelerates ions away from the surface. The accelerated ions are normally surface contaminations, and the dominating species are protons, which are provided by adhered water or carbon-hydrides. This acceleration is perpendicular to the rear surface of the foil and is called target normal sheath acceleration (TNSA). Figure 1 shows a simplified sketch of the process; ref.: Cowan 2004.
Laser generated proton beams show a variety of interesting properties:
Image of a laser generated proton beam created at Lawrence Livermore National Laboratory with the Petawatt installation in 1999.
Image of radiochromic film showing the imprint of a proton beam, which has been shaped by grooves on the target rear surface. This proves the capability of shaping as well as the high beam quality (low emittance).
Examples of radiography achieved with laser generated proton beams: The left portion shows a radiograph of an ICF capsule. The right part shows an assembly with thin plastic film made visible behind a tantalum cover. The lines on the lower section are 60µm copper wires (performed at the 100TW facility of LULI in Palaiseau, France).
Possible scenario for proton driven fast ignition: Protons are generated in a production target (a), which is protected from the hohlraum (d) radiation by two gold shields (b,c), ; ref.: Geissel 2005.