Fast electrons on a wire

The advent of ultra-high-intensity lasers has opened up the possibility of producing high-quality electron, proton, X-ray and ion beams in facilities that are much smaller and less costly than a typical particle accelerator or synchrotron radiation source. The ability to generate intense electron and ion beams in particular could hold the key to the so-called fast-ignition approach to laser-driven thermonuclear fusion (M. Tabak, et al. Phys. Plasmas 1, 1626–1634; 1994), in which a target of hydrogen isotopes is first compressed by an array of laser beams, and then ignited by a single tightly focused higher-intensity beam that generates beams of fast electrons (or ions) within the resulting plasma. But when a laser is focused onto a simple planar target (with an intensity of around 10¹⁹ W cm⁻² or higher), the MeV electrons produced emerge at a wide divergence angle of around 40°. This limits the ultimate intensity of the hotspot and is detrimental in most applications for which laser-driven beams are being developed.

A number of designs have been proposed to try and narrow this divergence of the electrons, but one of the most promising consists of a hollow gold cone with a thin (7 μm diameter) copper wire at its tip (R. Kodama et al., Nature 432, 1005–1008; 2004). The guiding of electrons by the large fields that is generated around a wire enables them to propagate for a distance of several millimetres within a diameter similar to that of the wire. This effectively increases their energy flux by an order of magnitude compared with a cone on its own, and by up to 30 times compared with a planar target. In addition, energy loss in the transverse direction (beam cooling) improves the beam emittance — a figure of merit that characterizes a beam's confinement and momentum spread — while electrons propagate along the wire. But the precise details of this process have proven elusive. In this issue J. S. Green and colleagues (Nature Phys. 3, 853–856; 2007) present an exhaustive experimental and numerical study of the intricacies of the fast electron transport along a wire, and at laser irradiancies one order of magnitude higher than previously reported.

The authors' results demonstrate that when a petawatt laser pulse interacts with a cone-wire target, the heating of the plasma is maximized close to the wire surface.Moreover, their simulations show that the complex field structures that emerge from this interaction (see figure) involves a reversal of the magnetic field inside the wire, which enhances the return current within a thin layer beneath its surface. This finding substantially improves our understanding of the guiding mechanism, and should enable further improvements in the design of cone-wire targets for a host of applications in medicine, materials science, physics and biology in which laser-driven electrons beams are expected to be used.