On the cover of HPL: ‘Proton probing of laser-driven EM pulses travelling in helical coils‘, by H. Ahmed, S. Kar, A.L. Giesecke, D. Doria, G. Nersisyan, O. Willi, C.L.S. Lewis and M. Borghesi

All-optical approaches to particle acceleration have attracted very significant interest in the scientific community over the past two decades, in light of the potential significant reductions in accelerators’ cost and footprint, which could result from the extremely high accelerating gradients that can be sustained in a laser-produced plasma. Particularly important progress towards the production of particle beams of quality comparable to conventional accelerators has been obtained using wakefield techniques for electron acceleration. There has also been a very significant experimental effort in laser-driven ion acceleration, mainly employing the so called Target Normal Sheath Acceleration (TNSA) mechanism, where ions are accelerated from the surface of laser-irradiated foils. The ion beams resulting from this mechanism possess some unique properties, but also present limitations in terms of energy, energy spread and beam divergence, which have severely hindered their applicative use.

An article recently published in Nature Communication (S. Kar et al. Nat. Commun, 7, 10792 (2016)) sets out the details of an experiment showing how the ions driven by the laser, out of a foil target, can be accelerated further by means of a coil device attached to the back surface of the foil. Not only does the coil boost the energy of the ions, but it also has the intrinsic advantage of collimating ions within a narrow range of energy. Furthermore, by carefully arranging coils and targets in sequence, one can devise a multi-stage accelerator with dynamic beam collimation and energy selection. Dr. Satya Kar of Queen’s University Belfast (UK) said: “This development sets a cornerstone for a next-generation of extremely compact and cost-effective particle accelerators, which complements the current drive for miniaturization in advanced accelerator technology.”

The coil works by transporting an ultra-short electromagnetic (EM) pulse along its helical path, while the laser-driven ions propagate along the coil axis. The radial component of the electric field generated by the EM pulse is strong enough to constrain the protons near the axis of the coil, while the longitudinal component of the electric field accelerates the guided ions. As reported in the Nature Communication publication, the proof-of-principle experiment employing a university scale laser showed efficient post-acceleration of the transiting protons at a rate of 500 MeV/m, which is well beyond what can be sustained by conventional accelerator technologies.

The success of this scheme depends hugely on our understanding of the EM pulse and its propagation along the coil, which was studied in the article by Hamad et al. published in High Power Laser Science and Engineering (HPL, Vol. 5, e4). As shown in Fig. 1 below, and in the image on the cover page, the propagation of the EM pulses in a helical coil was studied in situ by probing the coil both transversely and longitudinally using a self-probing technique employing laser driven protons [H. Hamad et al., Nucl. Instrum. Methods A, 82, 172 (2016)]. The paper in HPL describes in detail the results obtained in an experiment performed by researchers from Queen’s University Belfast and Heinrich-Heine-Universität, Dusseldorf, Germany. In particular, the temporal profile of the EM pulse travelling along the helical coil was characterised from the transverse probing, and found to be similar to what was previously measured in a planar wire geometry. On the other hand, the longitudinal probing of the coil elucidated the effect of the ultra-short burst nature of the EM pulse – i.e. the energy dependent reduction of the proton beam divergence. By doubling the length of the coil, the focusing field was applied for a longer time, which resulted in a tightly focussed proton beam. These results aid underpinning the underlying mechanism of selective guiding ions by the helical coil targets, and are highly beneficial for further development of the technique.