Focused proton beams for fast ignition

Article Written by Adam Higginson, a Postdoctoral Researcher in Farhat Beg's Lab

Featured in Laserlab Forum: Lasers for Fusion Energy   |   Issue 26, January 2019 

Researchers from British Laserlab-Europe partners the University of Strathclyde and the Central Laser Facility (CLF) have used the petawatt-class Orion laser system in the UK to produce tightly focused beams of protons, using novel shaped targets, to assess physics relevant to Inertial Confinement Fusion and to develop laserdriven proton heating for high energy density physics.

High-power laser pulses are capable of accelerating protons to tens of MeV energies, in durations of the order of the laser pulse (tens of femtoseconds to several picoseconds) at the source. This high-dose, rapid burst of radiation is capable of isochorically heating matter to exotic states that, until recently, have been extremely difficult to generate.

The team of researchers performed an experiment on the UK’s Orion laser as part of the AWE’s academic access programme, where they employed novel target designs involving hemispherical targets to produce a high-flux, focused proton beam. A combination of gold hemispherical target foils and conical attachments produce a focused proton beam by focusing field lines on the target rear, in addition to strong transverse electric fields on the cone walls; the latter pushing the already focusing protons into an even tighter beam.

The experiment resulted in a clear demonstration of proton focusing using an open-tipped cone. This is observed at 21 MeV in the featured image. The beam appears annular at higher energies, due to ‘over-focusing’, which results from the higher transverse fields surrounding the cone at early times, when the highest energy protons are accelerated and traverse the cone structure.

With their successful proof-of-principle experiment, the international collaboration, which also included researchers from UC San Diego (USA) and TU Darmstadt (Germany), plans on developing this work, by further studying the focusing capabilities and designing target geometries that make the beam suitable as a tool for proton fast ignition and to superheat samples for material studies.