When ultraintense laser pulses are focussed onto solid targets, the burst of MeV (fast) electrons that is produced is relatively well contained in the target.
This containment is caused by very strong electrostatic fields at the target surfaces which are generated by the charge separation that occurs when the fast electrons try to enter the vacuum.
However these electric fields also act to accelerate protons or ions at the target surface out into the vacuum. In this process protons can be accelerated to multi-MeV energies (up to 50MeV has been reported in the literature).
This is a relatively new way to produce energetic ions. In the future it may find applications ranging from Fast Ignition ICF to the treatment of tumours. It has already been used in the laboratory to probe other laser-matter interactions.
