Boosting the efficiency of laser particle acceleration

09 Aug 2013

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The interaction of an ultra-intense laser pulse with a solid target involves the laser acting as a compact electron accelerator, generating copious numbers of high energy (~MeV) electrons travelling at relativistic speeds.  This process of rapid electron displacement can, amongst other things, potentially be exploited to generate ultra-short pulses of X-rays for imaging applications and also results in bright sources of electrons and ions which are being explored for their use in advanced approaches to inertial fusion. Understanding the electron acceleration processes in laser-solid interactions is therefore an important topic in laser plasma physics.

Direct laser acceleration of electrons in the pre-expanded plasma of a solid target involves both the laser field and powerful electrostatic fields established by charge separation. In a PRL article (link opens in a new window) published this week, the CLF’s Alex Robinson and David Neely, together with Alex Arefiev of the University of Texas, show how these electrostatic fields, self-generated by displacing electrons relative to the ions, play a subtle role in reaching high energies.

“This may open up new ways to engineer laser-solid interactions so that the electron acceleration process is more efficient and reaches higher energies.”

 

Dr Alex Robinson
CLF Plasma Physics Group

When the laser field is present simultaneously, the electrostatic field can slow the rate at which an electron undergoing direct laser acceleration goes out of phase with the laser field, allowing it to gain larger amounts of energy than if the electrostatic field wasn’t present. This effect is even observed as breaking the ‘ponderomotive limit’, which is normally seen as a ceiling for the electron energy via direct laser acceleration. Furthermore, this boosting process can happen even when the electrostatic field is too weak to provide much ‘direct’ acceleration of the electron itself.

Simulation results:Trajectory of an electron that goes superponderomotive, on top of electric field (top image) and electron density (bottom image) plots taken at a representative moment in time. Colour of the trajectory indicates different periods of the interaction: red segment is interaction with strongly accelerating longitudinal electric field and light blue segment in the period in which the electron goes superpondermotive. Click on image for larger version.
(Credit: Physical Review Letters 2013)

 

Robinson and Arefiev showed this effect analytically and then proceeded to show the process occurring in fully self-consistent laser-plasma simulations.