Simulations for Future X-ray Free electron Lasers

When a beam of fast, high energy electrons are injected into an undulating magnetic field (often called a 'wiggler'), they are forced to oscillate perpendicular to their direction of propagation and to emit light at the electron oscillation frequency. As this mixture of electrons and light propagates along the wiggler, the electrons begin to 'bunch' at the same wavelength as the light and act in unison to generate high brightness coherent light. When this happens it is called a Free Electron Laser (FEL). When the electrons are accelerated to speeds just below the speed of light, the electrons can emit X-ray light. This has a very short wavelength and can be used to make images of very small objects such as atoms. If the X-rays can be made into very short pulses, they can also take images of atoms without blurring - just like using the flash on a camera in a dark room.

Most computer codes that simulate this FEL interaction make simplifications in the process which allows faster computation times. However, these simplifications mean that some information about the process is lost. This lost information is necessary if one wants to simulate e.g. very short light pulse generation. This proposal includes the lost information in a computer simulation code 'PUFFIN' which allows new methods to be investigated to improve the quality of the light emitted by the FEL. In addition, we will connect up PUFFIN with other simulation codes that allow the full FEL to be modelled from the start of the electron acceleration through to their exit at the end of the FEL. These 'start-to-end' simulations are important as they can allow different electron accelerators to be tested as drivers of the FEL, and can model their different characteristics. One such accelerator of current interest is the plasma accelerator which can be much smaller than current Radio-Frequency accelerators used to drive FELs. Use of plasma accelerators would significantly reduce the cost of FELs and make then more accessible to a wider group of scientists. PUFFIN is useful as it can model electron beams from plasma accelerators much better than other simulation codes.

Keeping the extra information contained in the PUFFIN simulations, and linking it up with other simulation codes, results in a powerful FEL simulator that can model effects such as very short pulse generation and plasma accelerator drivers of FELs. This ability opens up many new areas for research to improve the light output from FELs.

With these improvements would come the ability to investigate new areas of science that have until now been closed to us. These areas differ hugely, from observing how viruses and potential new drugs penetrate the membranes of living cells to creating conditions in the laboratory similar to those at the centre of Jupiter and Saturn.

The improvements to simulating the FEL process using PUFFIN have the potential to have a real and large impact on such fundamental scientific knowledge. Furthermore, this fundamental knowledge can play a crucial role in developing new products and processes that will help economies, society and the environment.

The development of the Free Electron Laser modelling software PUFFIN and its linking to other accelerator simulation software, will enable researchers to explore new methods to greatly improve FEL output quality. Light-source facility providers and their users will benefit from these improvements. This benefits scientific researchers and some commercial interests in the shorter term, with potentially longer term societal and economic benefits.
We see three main beneficiaries:

1) Light source facility providers
With the first generation of FEL light sources now on-line for users, there is significant research being conducted by the PI's group and others, towards improving their output qualities. Those facilities able to provide such improved output will inevitably attract the high research impact users who are able to utilise the output for world-first experiments. With no short wavelength FEL facility in the UK, the opportunity exists for the UK to skip the first generation FEL facilities and to invest in a facility with these improved output qualities, or perhaps with the reduced costs that a plasma accelerator driven FEL may offer. The combination of new methods for achieving such results, together with the advanced FEL modelling capability detailed in this proposal, can direct and inform the underpinning experiments and later designs that would enable the UK to build such a second generation FEL facility.

2) Users of light sources (both for research and industrial uses)
The current users of short wavelength FEL facilities are multifarious. The research proposed here would help enable X-ray FEL source development which would impact at least two of the EPSRC 'Physics Grand Challenges': Emergence and Physics Far From Equilibrium (creation and properties of warm dense matter), Understanding the Physics of Life (3D imaging of in-vivo biological samples at the atomic and molecular scales and Synthetic Biology) and several themes including: Digital Economy (understanding magnetisation at the electronic level); Energy (underlying dynamical processes in inertial confinement fusion); . Healthcare Technologies (understanding functional processes at cell membranes leads to basis for the development of new medicines); and Manufacturing the Future (improved silicon wafer lithography and Catalysis). The FEL is an instrument that enables the exploration of matter and its functioning, at a ubiquitous fundamental level that has previously been inaccessible. This ability has a significant potential impact in diverse EPSRC research infrastructure. The development of the software tools proposed here will improve FEL output, so opening up further and digging deeper into, existing fields of investigation.

An FEL in an industrial context will be a significantly different beast from that in a research environment. Their design will require exhaustive simulation studies that examine the statistical nature of the output due to plant fluctuations. Some aspects of the FEL interaction for such facilities may not be modelled correctly by current simulation codes and will require modelling by codes such as Puffin to confirm or otherwise, that output remains within specification. The PI is currently beginning a commercially confidential collaboration with a large industrial company with a multi-billion pound turnover, to investigate such issues.

3) Beneficiaries of light source research
Given the broad range of science that FELs cover, the potential benefits to wider society can be significant. Examples of potential societal and economic impact range from energy generation by internal confinement fusion to the development of new pharmaceuticals. In 10-50 years, new manufacturing methods, products, pharmaceuticals and the way in which some aspects of society functions, will undoubtedly be able to be traced back to discoveries made as a result of research conducted using short-wavelength Free Electron Lasers.