Laser-driven particle beams are being pursued by many users of the Central Laser Facility because of the number of applications that this accelerator mechanism can lend itself to, given that the acceleration occurs over a region of a few millionths of a metre. The beams mentioned in this news article, of either energetic electrons or protons, are of particular interest as they could also be used as ignitor beams in the fast-ignition route towards laser-driven fusion energy (link opens in a new window). Improving their energy content and controlling their divergence is vital in working towards this application.
When an ultra-intense laser pulse interacts with matter in a highly ionised state (plasma), a burst of very energetic (~ MeV energy) electrons are accelerated into the solid target. Under single shot irradiance, this beam of ‘fast’ electrons will diverge as it travels ballistically through the solid target, effectively lowering the energy density of the beam.
However, by optimising the ratio of intensities (at 0.1: 1 for Pulse1: Pulse2), as well as the timing of arrival between the two pulses (at 4- 6 picoseconds), the divergence of the fast electron beam was reduced, thus maintaining the high current density of the laser-driven electron beam. Simulations show that a magnetic field generated by the first pulse of fast electrons travelling through the target, leads to a pinching (or focussing) effect that lowers the divergence of the fast electron beam driven by the second, main laser pulse.
Two Vulcan Petawatt pulses have also been demonstrated to benefit laser-driven proton acceleration, although the physical mechanism is slightly different, as detailed in a separate PRL article (link opens in a new window). Here protons are accelerated from the target when some of the fast electrons, accelerated during a laser-plasma interaction, leave the solid target, generating an immensely strong electric field (TV/m). When the double-pulse technique for proton acceleration is employed, an increase in the energy converted from the laser to the proton beam is observed. This enhancement is the result of preferential conditions for energy transfer during the second pulse interaction, which are set up by the first pulse of electrons and protons. (See here (PDF - link opens in a new window) for more).
The work was carried out by a team of researchers from Imperial College, the Central Laser Facility and the universities of Oxford, Strathclyde and York, CELIA and LULI laboratories in France and GoLP, Portugal. The work forms part of the European High Power laser Energy Research facility (HiPER) (link opens in a new window) preparatory project and was funded by STFC (link opens in a new window).
