A free electron laser (FEL) is a source of electromagnetic radiation with exceptional qualities, making it an extremely useful tool for science. The MaRS group is leading a project to demonstrate a free electron laser at the ALICE (Accelerators and Lasers In Combined Experiments) facility at the UK’s STFC Daresbury Laboratory.
First lasing (link opens in a new window) 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. Further work is continuing to characterise the FEL performance and output. Continuous wavelength tuning (link opens in a new window) between 5.7-8 μm (through varying the undulator gap) has been demonstrated - this is one of the key properties of free electron lasers. Further details of the output properties are given below.
The IR-FEL is being used for a select programme of scientific experiments; recently it has been used to collect images of oesophageal cancer cells (link opens in a new window).
Panoramic photograph of the ALICE free electron laser
The ALICE FEL is driven by relativistic electrons delivered from the ALICE Energy Recovery Linac (ERL) accelerator. The electron beam is passed through an array of alternate polarity magnets, which cause the electrons to oscillate transversely and emit infra-red radiation. This infra-red radiation field is contained within a highly reflective optical cavity. The interaction of the infra-red radiation field and the electron beam leads to an exponential increase of the intensity of the radiation field. In this process the electron beam develops a longitudinal bunching which stimulates the emission of sub-picosecond pulses of coherent infra-red radiation with multi-megawatt peak power.
Schematic diagram of an oscillator free electron laser
The achieved output of the ALICE IR-FEL is summarised in the table below. Further work is ongoing to optimise the performance.
| Parameter | Value |
| Wavelength range | 5-9µm |
| Pulse length* | ~1ps FWHM |
| Time structure | Macropulses@ 10Hz
Micropulses @ 16.25MHz
Macropulse length → up to ~1380 micropulses per macropulse |
| Output power and energy † | Direct from FEL | After vacuum window (~70% transmission efficiency) | In diagnostics room (assuming 25% transmission efficiency‡) |
| Average power | 45mW | 32mW | 8mW |
| Average power within macropulse | 53W | 37W | 9.2W |
| Peak power | 3.6MW | 2.5MW | 0.6MW |
| Pulse energy | 3.3µJ | 2.3µJ | 0.6µJ |
| | |
| FWHM bandwidth | 0.9-1.8% |
* Pulse length has not yet been measured directly but has been inferred from spectral measurements and direct measurements of the electron bunch length.
† The given values are based on the maximum recorded average power values measured after the vacuum window, which were obtained at 8µm wavelength.
‡ Transmission efficiency of ~25% has been measured at 8µm, with ~35% at 6µm.
From the MaRS group, Jim Clarke, David Dunning, Mark Surman and Neil Thompson are members of the FEL commissioning team, and Ben Shepherd carried out measurements of the undulator.
