* Implement a free-electron laser (FEL) on the ELETTRA storage ring to produce a source of intense, highly monochromatic, tuneable, spatially coherent radiation.
* Optimise the FEL performance to enable a wide wavelength range to be covered, at least between 350 and 190 nm.
* Demonstrate that the FEL can be operated while still permitting the operation of other synchrotron radiation beamlines.
* Carry out pilot experiments with the FEL beam to demonstrate its potential use.
* Design, construction and testing of the optical cavity unit (task A).
* Production and testing of mirrors (task B).
* Design, construction and testing of undulator and front-end (task C).
* Characterization of electron beam properties (task D).
* Theoretical prediction of FEL performance (task E).
* Commissioning of the FEL (task F).
* Execution of proof-of-principle experiments (task G).
The main activity highlights have been:
* Feb. 29th, 2000: First lasing at 350 nm.
* May 27th 2000: Lasing at 220 nm.
* Sep. 17th 2000: Wide tunability of 30 nm demonstrated around 250 nm.
* Nov. 17th 2000: 180 mW extracted power at 250 nm
* Feb. 6th 2001: Lasing at 190 nm - a world record for the shortest wavelength of a FEL oscillator.
In addition, the first pilot experiments have been carried out using the FEL radiation and one other undulator beamline has successfully operated in parallel with FEL operation. The main objectives of the project have therefore substantially been met. More specifically:
Mirror Chambers: Two sophisticated UHV mirror chambers, each containing up to 3 mirrors that can be remotely interchanged and accurately positioned, were designed and constructed at CLRC-Daresbury Laboratory. Each unit contains 8 drive systems for x, y, z positioning, mirror change, and coarse (stepping motor) and fine (piezo) pitch and yaw motion.
Mirrors: Mirror substrates of both silica and sapphire types have been fabricated under external contract with various suppliers. Substrates were subsequently coated using various types of oxide and fluoride multilayers. Coatings have been successfully produced for different wavelength ranges (350, 250, 220, 190 nm), and for different values of transmission. Novel dual-band coatings were also produced for 220/350 nm operation.
Undulators and Other FEL Hardware: All other FEL hardware items - the two permanent magnet undulators, the electromagnetic modulator magnet and its power supply, the two new bending magnet vacuum chambers and insertion device vacuum chamber, the front-end and back-end of the beamlines - were successfully designed, constructed, tested and installed by Sincrotrone Trieste.
Electron beam tests: A total of 24 shifts in the period May '98 to February '00 were dedicated to the study of ELETTRA performance in the FEL mode as well as to commission the operation of the undulator and modulator magnets, prior to the start of FEL commissioning. Various measurements of bunch length were made, in order to make better predictions of FEL behaviour. Significant time was also devoted to compensation of multibunch instabilities to provide the required stability for FEL operation.
Theory: A number of topics have been studied including parameter optimisation, comparison of measured performance with theory, the use of holes for coupling power out of the optical cavity, and the effect of operating the two undulators in the optical klystron mode but with opposite circular polarization.
FEL Commissioning: As well as testing various different mirror types in order to progress to shorter wavelengths and higher output power, in order to reach a full understanding of the FEL operation on ELETTRA commissioning has also included comprehensive measurements of the following:
- spectral lineshape and pulse length as a function of optical cavity length.
- output power as a function of wavelength, beam current, modulator strength, optical axis alignment.
- polarization of the radiation.
- macro-temporal structure of the radiation by means of a dual-sweep streak camera and photodiodes.
FEL Experiments: Initial experiments using FEL radiation have concentrated on performing photoemission electron microscopy close to the photothreshold. Initial attempts were made on Si interfaces however the laser instability caused space-charge effects and so it was decided to try metal interfaces and look for lateral contrast due to work function variations. On Co/W (110) no contrast was seen at 196 nm. On Pb/W (110) on the other hand lateral contrast was observed of Pb islands on a W surface within a 5 µm x 5 µm observation area.
Funding SchemeLFC - Access to Research Infrastructures