Laser plasma physicss
Counter-propagating laser beams

• Short, intense laser pulses now reach well into the the "strong field" regime, which means the electrons they encounter are accelerated to relativistic energy.
• When a pulse picks up an electron and accelerates it to high energy, it radiates a few gamma-rays, but otherwise just rides along with the pulse.
• However, if a counter-propagating pulse comes along, the electron is seriously disturbed, and emits many more gamma-rays, which, in their turn, create electron-positron pairs in the laser field.
• Here are two animations showing an electron trajectory in two linearly polarised pulses (both with electric field in the x-direction, propagating along the z-axis) each with an intensity of 8.E23 W/sq cm
• The first shows the full trajectory (upper panel gives x vs z and the electric field as a function of z, lower shows log to base 10 of the Lorentz factor, the QED parameter "eta" and the number of secondary pairs created as functions of time.
• Initially the electron is between the pulses, and coasts to the right at constant Lorentz factor (=10).
• Then it is picked up and accelerated (to a Lorentz factor of a few hundred) by the leftwards moving pulse, but no pairs are produced.
• When the second pulse hits, the energy starts to vary rapidly, "eta" increases dramatically and pairs start to be created.
• After a few oscillations, the electron settles down, the pulses separate and the electron coasts off towards the edge of the system (shown as dashed lines at x=+/- 2*pi ).
• The zoomed animation shows a blow up of the interaction region. Here one can see that the electron eventually finds a position where the electric field has a node (i.e., almost vanishes), which happens at z=(2n+1)pi/2, shown by the vertical lines for n=-2...1. Once it gets there, pair creation ceases. For every electron that executes such a trajectory, on average two pairs are produced. They will also be accelerated, of course, and one can guess at the result...
• Experiments to test this have been proposed for the VULCAN upgrade at the CLF, Rutherford Labs, UK.
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