Intense Sub-Femtosecond Optical Radiation from Relativistic Plasmas

University Of Maryland, College Park, College Park MD

Investigators

Abstract

This work will explore the shortest duration optical flashes ever generated, produced by accelerating electrons from rest to almost the speed of light over a distance less than one-hundredth the thickness of a human hair. The accelerating charged particles are produced by the interaction of low energy (but very high intensity) laser pulses with dense jets of hydrogen gas. The hydrogen gas has all of its electrons stripped off by the laser pulse, forming a plasma, and radiation pressure from the laser pulse then pushes the free electrons out of the way. This produces an electrostatic disturbance that moves as a wave in the plasma (a 'plasma wave') and is so strong that it can accelerate its own electrons from rest to nearly the speed of light in the form of directed beams. These beams of accelerated electrons are useful for medical and scientific imaging. One of their intriguing, exciting, and potentially useful byproducts is radiation flash emission, which takes place over just one half cycle of the radiation emission itself, making it of sub-femtosecond duration (less than 1/1000000000000000 second long). Such a light source, in addition to its fascinating internal relativistic dynamics, can act as the fastest-ever optical strobe source for capturing images, for example, of electrons in mid-flight in their orbits in an atom. This project will develop and characterize a new source of intense, coherent, sub-femtosecond pulses (or flashes) in the optical range. These pulses arise directly from wave breaking of relativistic plasma waves and are only indirectly driven by a pump laser field. The radiation flash appears to arise purely from classical acceleration of charged particles exposed to the enormous electric and magnetic fields inside relativistic plasma waves. Their spectral and coherence characteristics are strongly affected by the plasma density, which sets the group velocity of the plasma waves, the spatial scale of the radiating bunches, and the laser energy required for relativistic self-focusing of the initiating laser pulse. Such flashes are consistent with unidirectional acceleration of electrons from rest to nearly the speed of light. The project will consist of detailed measurements of flash coherence properties, temporal pulse structure, and exploration of timing control of this radiation. Accompanying the experiments will be extensive 2D and 3D particle-in-cell simulations. A new 1 kHz, 10 mJ laser system will enable experiments at a much higher repetition rate, improving shot-to-shot reproducibility and allowing high data collection rates, both of which lead to better signal-to-noise and more precise measurements.