Lecture 2: The origins and development of free-electron lasers in the UK
Prof. Elaine Seddon
My notes from the lecture (if they don’t make sense then it is entirely my fault)
Definition of laser from Encyclopaedia Britannica
A laser is a device that stimulates atoms or molecules to emit light at particular wavelengths and amplifies that light, typically producing a very narrow beam of radiation. The emission generally covers an extremely limited range of visible, infrared, or ultraviolet wavelengths. Many different types of lasers have been developed, with highly varied characteristics. Laser is an acronym for “light amplification by the stimulated emission of radiation.”
A free-electron laser (FEL) is a kind of laser whose amplification/gain medium consists of very-high-speed relativistic electrons moving freely through a periodic magnet array, hence the term free electron. The free-electron laser is tunable and has the widest frequency range of any laser type, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, ultraviolet, and X-ray.
The free-electron laser was invented by John Madey in 1971 at Stanford University. The free-electron laser utilizes technology developed by Hans Motz and his coworkers, who built an undulator at Stanford in 1953, using the wiggler magnetic configuration which is one component of a free electron laser. Madey used a 43 MeV electron beam and 5 m long wiggler to amplify a signal.
To create a FEL, a beam of electrons is accelerated to almost the speed of light. The beam passes through a periodic arrangement of magnets with alternating poles across the beam path, which creates a side to side magnetic field. The direction of the beam is called the longitudinal direction, while the direction across the beam path is called transverse. This array of magnets is called an undulator or a wiggler, because due to the Lorentz force of the field it forces the electrons in the beam to wiggle transversely, traveling along a sinusoidal path about the axis of the undulator.
The transverse acceleration of the electrons across this path results in the release of photons (synchrotron radiation), which are monochromatic but still incoherent, because the electromagnetic waves from randomly distributed electrons interfere constructively and destructively in time. The resulting radiation power scales linearly with the number of electrons. Mirrors at each end of the undulator create an optical cavity, causing the radiation to form standing waves, or alternately an external excitation laser is provided. The synchrotron radiation becomes sufficiently strong that the transverse electric field of the radiation beam interacts with the transverse electron current created by the sinusoidal wiggling motion, causing some electrons to gain and others to lose energy to the optical field via the ponderomotive force.
This energy modulation evolves into electron density (current) modulations with a period of one optical wavelength. The electrons are thus longitudinally clumped into microbunches, separated by one optical wavelength along the axis. Whereas an undulator alone would cause the electrons to radiate independently (incoherently), the radiation emitted by the bunched electrons is in phase, and the fields add together coherently.
The radiation intensity grows, causing additional microbunching of the electrons, which continue to radiate in phase with each other. This process continues until the electrons are completely microbunched and the radiation reaches a saturated power several orders of magnitude higher than that of the undulator radiation.
The wavelength of the radiation emitted can be readily tuned by adjusting the energy of the electron beam or the magnetic-field strength of the undulators.
The fraction, ƒ, of the electrons in the “micro-bunch” radiate coherently with intensity proportional to Ne2 (where Ne is the number of electrons)
Short wavelength free-electron lasers (FELs) offer sub-picosecond pulses with full transverse coherence and are fourth generation light sources.
Long X-ray mirrors (4/5m) are required for photon beam transport and shaping in the beamlines at free-electron lasers (FELs). The material is chosen to reduce oscillation build up.
Most recent developments have concentrated on achievement of much more powerful amplifiers that remove the need for mirrors completely.
Classical picture – X-ray free-electron lasers, Authors: Feldhaus, J.; Arthur, J.; Hastings, J. B. http://iopscience.iop.org/article/10.1088/0953-4075/38/9/023/meta https://digital.library.unt.edu/ark:/67531/metadc883674/
John Madey on the free-electron laser https://www.youtube.com/watch?v=tORTVMVOGYo
John M. J. Madey (1943 – 5 July 2016) was a professor of Physics at the University of Hawaii at Manoa, a former director of the Free Electron Laser Laboratory at Duke University, and formerly a professor (research) at Stanford University. He is best known for his development of the free-electron laser (FEL) at Stanford University in the 1970s. He worked out the quantum theory of how the FEL worked.
The first Felix project at Daresbury was started in 1979. The NINA linac was used
The initial proposal was rejected but was found to have benefits
The UK free electron laser http://iopscience.iop.org/article/10.1088/0741-3335/27/12A/006
THE UK FREE ELECTRON LASER M. Poole, S. Smith
Evolution of Free Electron Lasers https://accelconf.web.cern.ch/accelconf/f06/TALKS/MOAAU02_TALK.PDF
The UK FEL 1982-1986 – showed gain but didn’t lase. The longer term benefits were FEL expertise and the current FELIX project
Currently, the FELs operating in EU are three, FERMI, FLASH and FLASH II, operating in the soft X-ray range and two are under commissioning, SwissFEL and EuroXFEL, which will operate in the hard X-ray scale. While most of the worldwide existing FELs use conventional normal conducting 3 GHz S-band linacs, others use newer designs based on 6 GHz C-band technology, increasing the accelerating gradient with an overall reduction of the linac length and cost.
The European X-Ray Free-Electron Laser Facility (European XFEL) is an X-ray research laser facility commissioned during 2017. The first laser pulses were produced in May 2017 and the facility started user operation in September 2017.
The Oxford Free Electron Laser Project https://ieeexplore.ieee.org/document/716029
Many false starts:
– FELIX at Daresbury (1980 – not funded)
– UK-FEL in Glasgow (Built 1983-1986 – not operated)
– Oxford FEL (1990 – not funded)
– FELIX – Nieuwegein, Netherlands (Operational 1991 – present) re-used UK-FEL expertise and undulator http://accelconf.web.cern.ch/accelconf/erl2015/talks/mopcth009_talk.pdf
The long term benefits of the false starts included energy recovery
CLARA (Compact Linear Accelerator for Research and Applications) is a proposed FEL test facility at Daresbury Laboratory, a major upgrade to the existing RF photoinjector test facility. Started in 2012. Its target is lasing
The 4GLS was a proposed 4th Generation Light Source, based at the Daresbury Laboratory in Cheshire, England, intended to combine energy recovery linac (ERL) and free electron laser technologies to provide synchronised sources of synchrotron radiation and free electron laser radiation covering the terahertz (THz) to soft X-ray regimes.
In early 2008 the Science and Technology Facilities Council decided not to proceed with the 4GLS.
First lasing of the ALICE IR-FEL was achieved on October 23rd 2010, making it the first FEL to operate in the UK, and the first FEL based on an ERL accelerator in Europe. First lasing was achieved at 27.5 MeV electron beam energy and 8 μm radiation wavelength.
Lasers for the New Light Source (NLS). This was halted in 2010 due to lack of money