Spatio-Temporal Control of Ionization and Electron Dynamics in Laser Plasmas

Colorado School Of Mines, Golden CO

Investigators

Abstract

This project will explore how a specially-shaped intense laser beam can control the movement of electrons in a plasma, a gas of electrons and charged atoms. A beam of light can put pressure on things to force it to move. Normally, this force is too small to see, but if a laser beam is bright enough and the objects have a small weight, it is possible to see these effects. In this project, we will look at how controlling the shape of a pulse of laser light can make it easier for this force to get electrons moving fast. We will also look at how these laser pulses can make strong waves in a plasma, where the electrons and ions become separated by another laser pulse. Our pulses of light are so short (a fraction of a trillionth of a second) that the ions left behind don't have time to move. The electrons accelerate in the laser pulse like a surfer on a big water wave, where she chooses just the right angle to ride the wave. We hope to show that this can lead to a new way to make beams of electrons traveling near the speed of light. The work should also help us understand how we can use these laser pulses for making small channels and for eye surgery. It will also show us how to convert these pulses to other colors of light. The research aims to explore using spatio-temporally structured intense laser pulses to control both ionization and electron dynamics in laser-produced plasmas. By controlling the transverse and angular distribution of the frequency components of the beam, we can control the spatio-temporal structure of a pulse. Important for this project, the angular spatial chirp results in an intensity envelope that is tilted relative to the direction of propagation. This gives us control of the transverse group velocity, which can range from super- to sub-luminal. We will use this control to affect the dynamics of electrons and waves in the plasma. In the free electron regime we will test our calculations that predict that with the proper pulse front tilt angle, electrons can be captured and accelerated to the side by the ponderomotive force of the beam. Such a configuration could ultimately be useful for direct acceleration of electrons to the MeV range, useful on their own or as an optical injector for wakefield accelerators. In the underdense and overdense plasma regime, the tilted pulse fronts can enhance the generation of waves in the plasma. In the latter case, we will perform a series of experiments to explore coupling of ultrafast pulses to surface plasmon waves. Our projects closely couple experiment, theory, and computational modeling. The experiments will be performed at CSM with our kHz repetition rate Ti:sapphire amplifier. Computational modeling will be performed with the open source 3D EM PIC program Epoch and with the finite-element Comsol Multiphysics platform.