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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