Beamline description

 

Overview and working principle

The standard process leading to CHG using FELs is based on the frequency up-conversion of a high-power laser pulse (the so-called seed laser) using a relativistic electron bunch as gain medium. The process is based on the layout modulator - dispersive section - radiator and the layout employed is sketched here:

The laser pulse is focused into a first undulator, called modulator so as to overlap (both in space and time) an incoming electron bunch. The interaction between the seed laser and the electron beam modulates the electron beam energy. Then, the beam passes through a magnetic chicane (referred in the following as the dispersive section) where the energy modulation is converted into a spatial modulation of longitudinal electron density into micro-bunches, with a periodicity equal to the seed wavelength and its higher order harmonics.

At the exit of the dispersive section, the beam is injected into a second undulator, called radiator, which is tuned at one of the seed harmonics. The micro-bunched electrons radiate coherently and the extracted power is proportional to the square of number of modulated electrons. If the radiator is sufficiently long, the amplification continues until saturation is reached. The radiation inherits both the properties of coherence and the temporal duration of the seed. 

Starting from a UV seed pulse permits the generation of coherent radiation in the VUV and soft X-ray ranges, offering the possibility to overcome the ionization potential of many materials and chemical species.

 


Beamline Layout

The experimental setup for CHG at Elettra is shown in the previous figure. Although on a reduced scale, the scheme is equivalent to the layout designed for seeded single-pass FELs, like FERMI@Elettra. The main difference with LINAC-based FELs is that in this case the electron beam is re-circulated in the storage ring.
The beamline is composed of three main blocks: the optical klystron (the suite of modulator, dispersive section and radiator), the seed laser and timing back-end station and the front-end station. Both the back and front-end stations are used for the detection/diagnostics of the CHG pulses, and to focus the harmonic light into an experiment end-station. 




 

Optical Klystron

The two APPLE-II type permanent magnet undulators, together with an electromagnetic chicane, constitute modulator (Elettra ID 1.1), radiator (Elettra ID device 1.2) and dispersive section of the optical klystron. Wavelength and polarization of the undulators are independently tunable.

The Apple II type helical undulator is a pure permanent magnet structure, composed of four arrays, as shown in the Figure. The arrangement of blocks is such that there are four blocks per period. By moving two opposing magnet arrays with respect to the other two longitudinally (a phase shift), the strengths of the vertical and horizontal magnetic field components can be varied, and hence the polarization of the radiation produced.

The advantage of such a device is that the radiation can be polarised vertically, horizontally, and circularly by moving the arrays, which provide a horizontal field, as well as a vertical one, purely from magnet blocks above and below the electron beam.

The SR-FEL beamline shares the insertion device with the Nanospectroscopy beamline. During the Nanospectroscopy operations the dispersive section is used as a phase-shifter, which enables the two undulators to be properly phased, thus effectively doubling the undulator length and the useful flux.


Seed laser system

The laser system consists of an SAM mode-locked fiber laser oscillator and a regenerative amplifier. The passive mode-locked oscillator (FemtoFiber pro IR, Toptica Photonics) can generate pulses as short as 20 fs, that for this experiment has been set to about 12 nm FWHM at 780 nm, in order to match the bandwidth required by the regenerative amplifier (Legend HE, Coherent Inc.). The repetition rate of the oscillator is 80 MHz in order to allow synchronization.

The amplifier, in which the active medium is a sapphire crystal (Al2O3), doped with titanium ions (Ti3+), provides pulses of up to 2.5 mJ energy and about 100 fs (FWHM) duration; it has a repetition rate from 1 Hz to 1 kHz. The amplifier is also equipped with second and third harmonic generation stages in BBO crystals, in order to generate radiation at 390 nm (with 0.8 mJ of energy per pulse) and at 260 nm (with 0.3 mJ per pulse): in this way it is possible to perform the seeding at those wavelengths.

The laser system is located in the FEL back-end station, in a temperature-controlled room so as to minimize the thermal drifts of the optical alignment.

Top: SAM mode-locked fiber laser oscillator; Bottom: Ti:Sa regenerative amplifier.




Experimental chambers

Experimental activities using the radiation produced from the storage ring FEL are performed in different experimental chambers:

IRMA reflectometer

Condensed matter experiment



Contacts: Maurizio Sacchi (CNRS-SOLEIL (Fr)) and Carlo Spezzani (Elettra)

TOF electron and ion spectroscopy

Gas phase experiment



Contacts: Marcello Coreno (CNR-Elettra) and Marek Stankiewicz (UK and Pl)
Last Updated on Wednesday, 12 December 2012 13:43