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Time resolved X-ray probing of Matter under Extreme conditions

Periodic Reporting for period 3 - TeX-MEx (Time resolved X-ray probing of Matter under Extreme conditions)

Reporting period: 2019-07-01 to 2020-12-31

The TeX-MEX project aims to use the unique properties of the beams produced by laser wakefield accelerators to study the extremely high pressures, temperatures and photon densities
that can be produced by intense laser interactions. These conditions are so extreme that they are only usually found in astrophysical phenomena such as inside stars and planets, on the surface of quasars and near black holes. These extreme conditions are also produced during laser driven fusion experiments, and will be important in the interactions produced in the next generation of high intensity lasers currently under construction around the world. By studying these extreme conditions in the laboratory we aim to further our understanding of the universe and some of the fundamental processes that occur in applications such as laser driven fusion energy and laser driven particle acceleration.

We are using a technique called laser wakefield acceleration to probe these extreme conditions. Laser Wakefield Accelerators are compact particle accelerators driven by a short pulse, high intensity laser. Using laser pulse which are just 40 fs in duration with a peak power of 200 TW we can produce electron beams with gigaelectronvolt energies in just an accelerator just a centimetre long. These high-energy, femtosecond duration electron beams can be used to probe extreme physics directly, or we can use them to produce X-rays or high energy gamma rays which are well suited to probe different types of extreme physics.
We have made significant progress in developing the broadband X-ray source produced by laser wakefield accelerators for use in a powerful technique called X-ray Absorption Near Edge spectroscopy, XANES. XANES can provide detailed information about the temperature and structure of materials and the ultrashort broadband X-rays that we can produce using a laser wakefield accelerator are well suited to probing matter than is rapidly heated to extremely high temperatures and densities, of the kind that is found deep inside gas giant planets.
During the initial part of the grant we were able to record X-ray absorption spectra of a sample using a relatively low intensity X-ray source, but needed to integrate data over many shots to get a useful signal. Since then we have shown that we can do this in just a single laser shot, this makes it feasible to do studies where we watch how matter at extremely high temperatures (> 10000 ºC) rapidly evolves. We have also used the Gemini laser system to develop a new system for generating X-rays that can perform the heating. This means that we are now poised to make measurement of the rapid evolution of matter under extreme conditions using the dual beams of Gemini: one to produce the laser wakefield accelerators and ultra-short X-ray flashes and one to rapidly heat a sample to thousands of degrees.

We have developed an experiment that uses the electron beam produced by a laser wakefield accelerator to investigate the fundamental processes that occur in extremely high electromagnetic fields (such as might be found on the surface of a quasar). We have seen the first evidence of an electron losing energy when it collides with a very intense laser pulse, a process called radiation reaction and have seen hints that this behaviour can only be modelled using theories based on extreme electromagnetic fields. This work was published in two papers in Physical Review X and has received a lot of attention, with from the research community and outside.

We have successfully completed our first experimental run which aims to study how matter can be created when two photons collide. Our experiment uses our laser wakefield accelerator to produce high energy electrons, which are converted to high energy gamma rays. We then collide the gamma rays with a dense field of X-ray photons which are produced by shooting another high intensity laser at a think metal foil. As the gamma ray photons pass through the dense X-ray field then can collide with the X-ray photons and can produce pairs of electrons positions. Analysis of this run is ongoing so it is too early to say if we have observed photons colliding and creating matter, however all aspects of the experiment worked successfully.
The development of single shot XANES using the X-rays from a laser wakefield accelerato goes significantly beyond the state-of-the-art: previous attempts at this by ourselves and other groups have needed more than 50 shots to get enough signal to make a measurement. The ability to do this is a single shot is what will allow us to develop pump probe experiments where we can observe how rapidly heating matter evolves on picosecond timescales.

Our experiments on radiation reaction are the first experiments to report evidence for this effect in the collision of a high energy electron beam and a high intensity laser. Our experiments are on the edge of being able to tell the difference between predictions made by different models of this effect. In future experiments we aim to collide at high intensity and high electron energy to allow us to make more definitive statements about this fundamental aspect of extreme electromagnetic fields.

Our experiment which studies the collision of high energy photons with dense X-ray fields was a technical success that goes beyond the state of the art. In our first data run, we successfully commissioned all the experimental components and recorded a number of successful collisions. The aim is to see the creation of electrons and positrons due to the collision of two photons for the first time. Data analysis is ongoing.