The DiPOLE Laser on the Helmholtz Beamline at XFEL

X-rays are a form of electromagnetic radiation with wavelengths shorter than the distance between atoms in a solid, thus they can be used to 'view' matter on atomic dimensions. Over the past few years there has been a revolution in x-ray science: ultra-short pulses of laser-like x-rays can now be produced durations less than a tenth of a trillionth of a second, which is also the sort of time it takes for atoms to move back and forth as they vibrate within a solid. This ultra-bright X-ray laser thus allows us to make stroboscopic 'movies' of matter without motional blurring. The first x-ray laser to be built was in the US, at Stanford, using a 50-year old accelerator. The European version, under construction in Hamburg, is being built from scratch, and as such is based on novel superconducting magnet technology that means it will produce x-ray pulses at a rate several 100 times faster than that of the US system - producing another leap forward in technology. The proposal here is a request for equipment for a UK consortium of 10 leading Universities to help build one of the diagnostic end-stations on this European X-ray Free-Electron Laser (XFEL). The equipment is a very intense optical laser to go alongside the XFEL , allowing matter to first be irradiated by the intense optical beam, and then probed with the unique x-ray beam. This optical/x-ray combination will allow a whole range of different types of research to be performed. For example, when a sample is irradiated with intense optical light, the surface is heated to such high temperatures that a plasma forms. This plasma expands into the vacuum (the experiments are all performed without air), and the reaction force compresses the rest of the target to high pressures - greater than those found at the centre of Jupiter. These conditions exist for about a billionth of a second, before the target falls apart, but in that short time the XFEL (accurately synchronized to the optical laser) scatters from the atoms in the target, and the recorded signal shows their arrangement. In this way, we can discover the conditions that occur at the centre of the giant planets in our own solar system, and also start to explore the types of material that may exist inside the numerous exoplanets that have been discovered (now close to 1000 have been confirmed). This optical/x-ray laser combination makes possible many other types of experiments - for example the x-ray laser itself can heat a solid to several million degrees (it is sobering to realize that these sort of conditions - say a gram per centimeter cubed, and 2 million degrees, are exactly those predicted to exist half way to the centre of the sun). Furthermore, the optical laser can be configured with other lasers to produce very intense light - so intense that electrons within the electric field of the light are accelerated themselves to such high velocities that their mass is altered by Einstein's relativistic equations. As the electrons are flung back and forth, they experience huge accelerations, and it has been predicted that x-rays scattering from them, produced by the XFEL, will allow models of quantum gravity to be explored in the laboratory. These high power lasers can also be used to accelerate particles (electrons or protons) to very high energies, making compact acclerators - but some of the mechanisms involved are not fully understood - mainly because we cannot 'see' inside the target where the particles are produced. The X-ray laser will allow such probing of the target, and thus the aim is to make better compact accelerators that could be used either for fundamental research, or in medical applications, such as the treatment of cancer. It can thus be seen that the experiments that this XEL machine, in combination with the optical laser requested here, is very wide ranging, with implications across a spectrum of disciplines where UK scientists have considerable leadership and expertise.

This project will have a number of important impacts across knowledge, people, society, and the economy. On the economic front, the delivery of this DiPOLE system will be the second such high repetition rate laser to be supplied, by the CLF at RAL. The first system was sold to the ELI project for £10.4M, and there is a growing interest in the technology from a number of industries, both manufacturing and defence. Indeed, the DiPOLE system forms a key project within the STFC CALTRA research centre, and as this system is refined and developed, we expect further direct economic returns to the Harwell campus..

In terms of people - and in particular the users of this equipment, although the UK has significant expertise in high power optical laser technology, as yet we have almost no young scientists trained in using 4th generation light sources, where the technology and skills required cross the boundaries between high power laser and synchrotron research. Indeed, at present there are probably only 2 or 3 young postdocs in the UK capable of mounting a 4th generation light source experiment. Given the high scientific impact of XFEL, the training of a generation of young researchers in 4th generation light source technologies is of paramount importance.

The longer-term impact of this proposal will clearly be related to the actual research undertaken by the members of the consortium as a whole, and this, we believe, will be far-reaching. Much of the proposed work is in high energy density science, and research in this area area will be of high impact both from a pure point of view, and in its application to such fields as planetary science (both for planets within our solar system as well as exoplanets). Similarly, the work will benefit those working towards the successful prosecution of Inertial Confinement Fusion (ICF). During the initial implosion phase of ICF pellets solid, or greater-than-solid, density matter is compressed to high pressures, and target designs exist where diamond is used as the ablator, and is can remain in the solid phase in the initial phase of the implosion. The impacts to academic beneficiaries will be disseminated via the normal channels of high-quality, high-profile, publications and proceedings, and oral presentations at national and international conferences. We would argue that the UK groups that form this consortium are amongst the top research groups in the UK in extreme conditions research, with past achievements at the highest level in this field. Indeed, those groups already involved in x-ray FEL research at LCLS have already garnered important results, published in Nature, Science, and Physicsal Review Letters, and members of these research groups are also giving invited presentations at high-profile international meetings.

It is also difficult to overstate the far wider scientific and societal impact of this proposal, which provides the first strategic step in a formal engagement of the UK physical science community in revolutionary 4th generation light-source research. Put simply, XFEL will be the premier facility in the world for 4th generation light source research. We firmly believe that such sources, possessing a spectral brightness over a billion times greater than any other x-ray source on the planet, will be truly revolutionary across a wide swathe of research themes. We maintain that the evidence already amassing due to the advent of LCLS indicates that it is no exaggeration to say that 4th generation light sources - i.e. x-ray lasers - will have a scientific (and resultant societal) impact comparable to that of both the separate development of x-rays and lasers themselves. For the UK scientists to generate results that will have impact in this area it is imperative that we re-engage immediately if we are not to lag behind in 'catch-up' mode for many years to come. This consortium effort is the first formal step along this route.