1 Introduction

David Neely completed a BSc Honours degree in Physics at Queen’s University Belfast (QUB). He undertook a final-year undergraduate project on flashlamp pumped lasers with Prof. Ciaran Lewis in 1986, subsequently starting a PhD in 1987 at QUB under Ciaran on the generation of soft X-ray lasers. David’s PhD studies culminated in 1991 with the use of two opposed slab targets to compensate for refraction and increase the gain length product, enabling saturation using neon-like germanium at 23 nm with Vulcan at the Central Laser Facility (CLF). While writing up his PhD thesis he was employed as a Research Assistant at QUB in teaching and supporting the undergraduate research laboratories. In 1993 David moved to the Rutherford Appleton Laboratory to take up a position on the Vulcan laser and in 2005 took charge of the CLF’s Experimental Science Group. He was Head of High Power Laser Science from 2010 to 2012 and was awarded a Science and Technology Facilities Council (STFC) Research Fellowship in 2012.

David naturally built up an incredible network of colleagues and friends throughout the world. He carried out experiments on lasers throughout Europe, Asia and the USA, contributing to experiments at the Lawrence Livermore National Laboratory and the Brookhaven National Laboratory, for example. He held a Visiting Mitsuyuki Abe Chair position at the Proton Medical Research Center, Japan Atomic Energy Agency, giving him an even greater international profile.

He was appointed a Visiting Professor at the University of Strathclyde from 2008 to 2020 and worked very closely with Prof. Paul McKenna. This enabled him to co-supervise PhD students and enhance his research portfolio. He formed close relationships with all his students, which would be lasting, well after they left to take up jobs elsewhere, many at national and international laser facilities.

Fig. 1. David Neely (right) in discussion with Leonida Gizzi (Istituto Nazionale di Ottica, Pisa, Italy) at the HPLSE symposium, Suzhou, China, 2018.

下载图片 查看所有图片

In the weeks before his untimely death on 27 August 2020, David was working on his contribution to a manuscript on the history of high-power laser research and development in the UK, for publication in HPLSE^[1^]. This was one of nearly 400 publications that David co-authored throughout his career. His work was mainly in the field of high-power laser–plasma interactions and the applications of the resulting beams of energetic particles and radiation, and he published extensively in this area. In striving to develop the field, he worked on new instrumentation (including detectors and diagnostics) and methodologies (e.g., plasma optics and targetry). As a result, David has published a rich body of high-quality work across a broad range of topics linked to the field.

This Editorial is in memory of David and his scientific achievements. We have brought together the paper that he was working on at the time of his death and other papers that he published in the journal.

2 Scientific highlights through publications in HPLSE

The first three of David’s HPLSE papers that we highlight provide an overview of ultra-high-power laser systems and laser science. This trio of papers began in 2014 with a publication on pulse fidelity in petawatt-class laser systems^[2^]. The paper summarized the development on the UK’s three ultra-high-power laser facilities. These facilities represent the two main classes of petawatt facilities: the mixed OPCPA/Nd:glass high-energy systems of Orion at the Atomic Weapons Establishment; and Vulcan and the ultra-short-pulse Ti:sapphire system of Astra Gemini, both at the CLF at the STFC’s Rutherford Appleton Laboratory. Many of the techniques used to enhance and control the pulse generation and delivery to target of ultra-high-power pulses have been pioneered on these facilities.

In compiling the paper, an annex was intended to summarize the current status of ultra-high-power lasers throughout the world. As this annex grew, it became significantly longer than the paper itself and so the decision was made to split this out and produce a separate comprehensive review, ‘Petawatt Class Lasers Worldwide’, which was published in 2015^[3^]. This has become an important review paper for researchers in the field and has attracted over 300 citations to date. These include citation by the Nobel Committee in Physics in its 2018 scientific background paper ‘Groundbreaking Inventions in Laser Physics’^[4^] for the award to Profs. Donna Strickland and Gerard Mourou for the invention of the technique of chirped pulse amplification^[5^], which was shared with Prof. Arthur Ashkin for his work on optical tweezers and their application to biological systems.

As testimony to the rapid growth of the field of petawatt laser–plasma science, this paper was followed in 2019 with a presentation of petawatt- and exawatt-class lasers worldwide^[6^]. This review paper built on the earlier review paper ‘Petawatt Class Lasers Worldwide’ but was much more comprehensive, providing a discussion of the history of ultra-high-power lasers and a look forward to which techniques and research activities were emerging to produce the next generation of lasers.

In parallel with his interests in developing high-power lasers, David made leading contributions to a large body of work on various aspects of the interaction of high-power laser pulses with gaseous and solid density plasma. He introduced a number of novel ideas to the development of laser-driven ion acceleration and contributed to studies that used ion acceleration to diagnose the physics of fast electron generation and transport in dense plasma, which is directly relevant to the development of advanced ignition schemes for inertial fusion. An example of the latter is that the investigation into the effect of the electronic sheath field, responsible for ion acceleration, on fast electron dynamics within foil targets is reported in Ref. [7]. In that paper, it is shown that energetic electrons that do not overcome the electrostatic potential formed at the target rear are reflected and reflux within the foil. The temporal characteristics of this population of refluxing electrons and those that escape the target foil are investigated and correlated with the magnitude and spatial and temporal evolution of the electrostatic fields. This work is one of many that David led on this topic.

David is well known for introducing new innovations into the field of laser–plasma science and one such innovation was the use of ultrathin foil targets for ion acceleration. This was a direct result of David challenging the Target Fabrication group at the CLF to produce the thinnest foils possible to enhance the electrostatic fields responsible for ion acceleration at the rear of the foil. In addition to investigating ion acceleration from ultrathin foils, David contributed to numerous recent studies into laser pulse propagation and collective electron dynamics in ultrathin foils undergoing expansion and becoming relativistically transparent to the intense laser light. In an example of this work, Ref. [8] reports on an experimental and numerical investigation into the collective response of electrons in an ultrathin foil target irradiated by an ultraintense laser pulse. It is shown that when the target thickness is decreased such that it becomes relativistically transparent to the laser light early in the interaction, diffraction of the transmitted laser light occurs through a so-called ‘relativistic plasma aperture’, inducing structure in the spatial-intensity profile of the beam of accelerated electrons. It is also shown that the electron beam profile can be modified by variation of the target thickness and degree of ellipticity in the laser polarization.

A related study^[9^] reports the role of magnetic field evolution in filamentary structures that form within the beam of protons accelerated during the interaction of an intense laser pulse with an ultrathin foil target. Such behaviour is shown to be dependent on the formation time of quasi-static magnetic field structures throughout the target volume and the extent of the rear surface proton expansion over the same period. By controlling the intensity profile of the laser drive, via the use of two temporally separated pulses, both the initial rear surface proton expansion and magnetic field formation time can be varied, resulting in modification to the degree of filamentary structure present within the laser-driven proton beam. In separate work, David used dual laser pulses to drive proton acceleration and demonstrated enhanced energy transfer using this approach.

David’s other work on novel targets includes mass-limited targets to limit lateral energy loss in thin foils and structured targets to enhance laser energy absorption by electrons or to generate point-like sources of energetic radiation. He contributed to the study published in Ref. [10], in which diffraction and speckle patterns measured in the spatial-intensity profile of laser light reflected during the interaction with a microstructured target are investigated experimentally, and the potential to apply this as a diagnostic of laser focus and plasma temperature is explored. Using modelling, the possibility of applying this as a diagnostic of the evolving plasma surface is discussed. In another example involving innovative targets, David led a study^[11^] into the use of constrained targets to enhance laser-driven Bremsstrahlung emission for industrial radiography applications. By limiting the lateral dimensions of the target, X-ray generation was confined to a localized region, enabling imaging with enhanced resolution and contrast, compared to the use of unconstrained foil targets. The sheath field formed around the constrained target was also shown to enhance electron refluxing within the target, driving a brighter source of X-rays. David was keen to develop novel laser-driven X-ray sources for applications such as non-destructive imaging.

In keeping with the theme of secondary radiation generation, David also made important contributions to the development of terahertz (THz) radiation sources driven by intense laser–solid interactions. He published two papers on this topic in HPLSE^[12^,13^]. In an early work in 2014^[10^], the effects of target pre-heating and expansion on THz radiation production from intense laser–solid interactions were first explored. The total energy of the THz radiation is found to decrease by approximately a factor of two when preheating the target compared to a cold target reference, and is attributed to an increase in the scale length of the preformed plasma in the region in which the THz radiation is generated. The results show the importance of controlling the pre-plasma scale length for THz production, and this is a theme that is reflected in other work that David contributed to on the role of pre-plasma in ion acceleration. In related work on THz generation, published in 2019^[13^], backward-directed THz radiation from intense picosecond laser–solid interactions was investigated, using a multichannel calorimeter system jointly introduced by David. The paper reports on the dependency of THz energy and spectrum on the drive laser pulse energy, target thickness and pre-plasma scale length. It shows that a large-scale pre-plasma can enhance the high-frequency component backward-directed THz radiation.

Those who knew David understood his passion for solving challenging problems that limited progress in experimental science. One such issue is that of laser-generated electromagnetic pulses (EMP), which can damage electronics, particularly in detectors and computers positioned near the target interaction chamber. EMP emission is caused by the acceleration of hot electrons inside the target, which produces radiation across a wide band from DC to THz frequencies. In 2018, David published a paper^[14^], involving researchers from five institutions, in which it was demonstrated experimentally how EMP can be readily and effectively reduced. Using novel targets, many of which were introduced by David, it was demonstrated that target stalk geometry, material composition, geodesic path length and foil surface area can all play a significant role in the reduction of EMP. A combination of electromagnetic wave and 3D particle-in-cell simulations helped in understanding the effects of stalk geometry on EMP, providing an opportunity for comparison with existing charge separation models. This work is a significant achievement and one in which David had a leading role.

In a follow-up study, published in 2020^[15^], David brought together a collaboration of researchers from 21 institutions across Europe and China to review EMP generation, detection and mitigation in the interaction of high-power, high-energy laser pulses with different types of solid targets. The resulting review paper includes new experimental data obtained independently at several international laboratories and analyses the mechanisms of electromagnetic field generation as a function of the intensity and the spectral range of emissions they produce. The work focuses on the GHz frequency domain, which is the most damaging for electronics, but also discusses the physics of electromagnetic emissions in other spectral domains, in particular THz and MHz. Theoretical models and numerical simulations are compared with the results of experimental measurements to provide new insight into the underlying physical processes and provide a basis for developing techniques to mitigate the EMP threat and to harness electromagnetic emissions. This review paper provides an important reference work on EMP in laser–plasma interactions and in itself reflects David’s legacy in the field.

David was keen to push the boundaries of what is possible in laser–plasma science. He strongly promoted the development of multi-petawatt lasers and the new topics that these would enable. In preparation for future experiments on the next generation of high-power laser facilities, David contributed to a number of numerical studies into the new physics that may be enabled. Ref. [16] provides an example work in which approaches involving colliding laser pulses on existing petawatt laser facilities are discussed as a route to providing insight into the strong-field quantum electrodynamics (QED) physics that may be explored at multi-petawatt-scale facilities. This includes radiation reaction physics and Breit–Wheeler pair production. David’s untimely passing means that he will not see this new chapter in high-power laser–plasma science. His legacy is that many of the innovations he has introduced in the field will be applied to enable experiments in this new-intensity regime, and many of the scientists he helped to train will be performing those experiments.

3 Summary

David was much admired and respected. He worked tirelessly to develop high-power laser–plasma science and to educate new scientists in the field. His passion and enthusiasm for his science and for mentoring younger scientists was infectious. He will be deeply missed by many in the community. As a member of the Editorial Board for HPLSE, we are sure that David would be pleased to see this Editorial with a summary of the works he published in it.