The DiPOLE Laser on the Helmholtz Beamline at XFEL

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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.