Prospects for a Free Electron Laser at the FemtoMAX Beamline

Abstract

A Free Electron Laser (FEL), is a device that generates extremely coherent and brilliant radiation using electrons accelerated to relativistic velocities in a particle accelerator. The process utilises similar equipment to what is used at a synchrotron source like MAX IV, but in addition there is an interaction between the relativistic electron beam and the generated radiation to achieve an exponential growth in radiation intensity. The produced radiation can potentially be several orders of magnitude more brilliant than what is possible at a synchrotron source like MAX IV, which also produces brilliant and coherent radiation in the form of X-rays. This is mainly done in the two storage rings at the facility, but also at a beamline called FemtoMAX. FemtoMAX lies in the Short Pulse Facility (SPF), and is connected directly to the linear accelerator (linac), that also injects electrons into the storage rings.

When MAX IV was built, there was an idea that the linac could also be used to drive an FEL. The prospective FEL at FemtoMAX would be a so-called Self-Amplified Spontaneous Emission (SASE) FEL, similar to other major FELs like European XFEL or LCLS. In such FELs, the radiation is generated with a single pass of electrons through undulators which make the electrons emit radiation. FemtoMAX also generates radiation with an undulator driven by electrons from a linac, but what makes the difference to an FEL is that in the latter the wavelength and electron energy are tuned so that the electrons will emit radiation coherently, which causes a self-amplifying effect, and an exponential growth in radiation intensity. This exponential growth is characterised by a distance over which the intensity increases by a factor of e, the gain length, which depends on a number of parameters of the electron beam. Most importantly for this thesis, it decreases with decreasing electron energy. At the 3 GeV electron energy where FemtoMAX currently operates, the exponential growth requires a considerably longer undulator length than what is available at FemtoMAX. Lowering the energy of the linac would shorten the gain length, and could make the exponential growth long enough to generate FEL radiation. By simulating the FEL process using parameters from FemtoMAX, the prospective FEL at FemtoMAX was investigated. The results show that by lowering the electron energy to 2 GeV or lower, there is a significant lasing and exponential growth of the radiation. At 1.6 GeV, there is even saturation of the FEL process. The conclusion is that there is a real possibility to implement such an FEL at FemtoMAX, provided the energy of the linac can be lowered.

Popular Abstract

The research field of lasers is an incredibly exciting one. If nothing else, this becomes apparent when one looks at the Nobel prizes in physics from recent years, of which several were awarded for improvements to laser technology. These advancements have created entirely new fields of research, like the attosecond research which allows scientists to view the movement of electrons inside the atom, which was awarded the Nobel prize this year. More than that, lasers are used for many things, from measuring distances and mapping out cities to creating miniature stars by igniting fusion in balls of hydrogen.
Bearing all these amazing applications in mind, we will now consider how one can advance laser technology even further. Lasers create light by making electrons in atoms lose energy and create a photon in the process. By a method called stimulated emission, these photons cause other electrons to emit photons in phase with each other.
One of the remaining limitations with lasers is that the wavelength of the laser cannot be arbitrarily short- ened. Shorter wavelength corresponds to the laser photons having higher energy, which in a traditional laser would require that the electrons had higher energies in the atoms. A short wavelength laser would have several interesting new properties. One of the most interesting types of short wavelength light is X-rays. These are well-known for their many uses in medical applications, but they are also highly interesting in various scientific measurements. For instance, the storage ring at MAX IV in Lund generates high intensity X-ray beams which enable scientists to look inside objects and materials to better understand them.
If we bring these two interesting concepts together, we get an X-ray laser, which combines the useful research properties of both lasers and X-rays. As mentioned before though, it is not entirely easy to make an X-ray laser. The problem is that electrons in atoms cannot have high enough energies to create photons with X-ray energies. If they had that much energy, they would simply fly away from the atom, so they cannot create any laser light.
In order to reach the energies required for creating X-rays, particle accelerators are used. By accelerating electrons to nearly the speed of light, and then letting them create photons by making them move in a zig-zag motion, the created light will be X-rays. In order to get laser light however, one must control this process carefully so that all the light is in phase.
A machine that does this is called a Free Electron Laser (FEL), and a few such machines already exist. Given the capabilities of these, the interest in FELs is large in both academia and industry. Therefore, having one in Lund would be very useful. The purpose of my project is to see if one could implement an FEL using the existing equipment at MAX IV, which would be a cheap way of improving the capabilities of the facility. To test this, I used parameters from the beamline FemtoMAX at MAX IV to simulate the FEL. By changing some things about the electrons from the accelerator, the radiation from the FEL would also change. By analysing these changes, I could find the optimal settings for the FEL, and see if it was possible to implement an X-ray laser at MAX IV.

• author: Holmberg, Johan
• supervisor: Francesca Curbis; Sverker Werin
• organization: Synchrotron Radiation Research; Department of Physics
• course: FYSK03 20232
• year: 2024
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Free Electron Lasers, FEL, synchrotron radiation, MAX IV, FemtoMAX, GENESIS 1.3, XFEL, X-rays, soft X-rays
• language: English
• id: 9148406
• date added to LUP: 2024-02-13 12:47:07
• date last changed: 2024-02-13 12:47:07