# Thomson source

The SPARC_LAB Thomson source is a compact X-ray source based on the Thomson backscattering process.

The electron beam energy ranges between 30 and 150 MeV, the electrons collide head-on with the Ti:Sapphire FLAME laser pulse the energy of which ranges between 1 and 5 J with pulse lengths in the 25 fs–10 ps range, this provides an X-ray energy tunability in the range of 20–500 keV, with the further capability to generate strongly non-linear phenomena and to drive diffusion processes due to multiple and plural scattering effects. A 20 m double dogleg carries the electron beam output from the photo-injector down to the Thomson Interaction Point where the FLAME laser pulse is brought by a 20 m in vacuum optical transfer line.

Since the first planned experiment with the Thomson radiation was the X-ray imaging of mammography phantoms with a phase contrast technique, the source parameters of the electron and laser beams have been so far optimized to obtain the required flux of photons with moderate (20%) monochromaticity according to the simulation results and are reported in the table below. Other possible applications are in identification of fissile materials, crystallography and microdensitometry 3D for cultural heritage.

The focusing in the interaction point is performed in the final straight section using a quadrupole magnet triplet and a solenoid, with a maximum field B=1.1 T, close to the IP. At 30 MeV the minimum obtained spot size for the electron beam was around 60-80 μm.

The required laser focal spot has been obtained with the use of the adaptive optic placed inside the compressor. It focuses the beam in a 10 μm diameter (FWHM) spot at the interaction point.

The Thomson scattering experiment needs an extremely precise synchronization between electron bunch and laser pulse. The electrons and the photons collide well inside the waist region of the laser beam of final focus provided that the relative time of arrival jitter at the IP between the two beams is <500 fs rms. The synchronous arrival of electrons and photons at the IP is obtained by locking the oscillators of the photo-cathode laser and interaction laser systems, and the phase of the RF accelerating fields to a common Reference Master Oscillator (RMO). The RMO is a low phase noise (60 fs rms integrated in the 10 Hz–10 MHz range) microwave oscillator tuned at the Linac main frequency 2856 MHz. So, the expected relative jitter of the arrival time at IP of electron bunch and laser pulse is < 100 fs rms.

The x-ray detector we selected is a scintillator crystal coupled with a photomultiplier tube (PMT) located at 450 cm downstream the Thomson IP. The crystal used is a CsI(Tl) of size (20×20×2) mm3, coupled with a light-guide to a PMT Hamamatsu, mod. R329-02. In addition to the PMT described, the beamline is equipped with a set of Si PIN diode detectors, previously calibrated with monochromatic synchrotron light, located at 200 and 300 cm from IP respectively, that together with an X-ray imager and techniques specifically developed, allow a full characterization of the X-ray source in terms of flux, energy distribution, spatial distribution and beam stability.

With our commissioning setup the expected number of photons in the 20% bandwidth is $1.4 \cdot 10^6$ photons/shot while after the two month of commissioning our measured photon flux is about $10^4$ photons/shot. With the available hardware the applied acceleration/deceleration scheme worked well enough to produce a low energy spread electron beam at 30 MeV, even though resulting in a strong machine imperfections/stability sensitivity for the electron beam.

The optimization plan foresees a better control of the electron trajectory at the IP to avoid unrecoverable off-axis emission of the Thomson radiation and too high background contribution to the X-ray detectors signal. An interaction setup upgrade is also under study, coming to a non-zero angle collision to make it easier the electron and laser pulse trajectory control removing the on-axis counter propagation that limit the room availability for both beams diagnostic.

 Electron beam Energy 30-150 MeV Energy spread <0.1% Charge 100-800 pC Emittance 1-3 mm mrad Laser beam Wavelength 800 nm Pulse energy 1-5 J Pulse length 6 ps Spot size 10 Repetition rate 10 Hz X-ray beam Photon energy 20-22 keV Photons per shot $10^9$ Source rms radius 10 ps Bandwidth 10-20%

Commissioning parameters.

Publications Highlights:

• “Energy distribution measurement of narrow-band ultrashort x-ray beams via K-edge filters subtraction”, P. Cardarelli et al., J. App. Phys. vol. 112 (2012) pag. 074908, doi: 10.1063/1.4757027.
• “Electron Linac design to drive bright Compton back-scattering gamma-ray sources”, A. Bacci et al., J. App. Phys. vol. 113 (2013) pag. 194508, doi: 10.1063/1.4805071.
• “The SPARC_LAB Thomson source”, C. Vaccarezza et al., Nucl. Instr. Meth. Phys. Res. A vol. 829 (2016) pag. 237, doi: 10.1016/j.nima.2016.01.089

Contact person: C. Vaccarezza, cristina.vaccarezza@lnf.infn.it, tel. (+39) 069403-2517