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
- 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
(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
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.