The THOMSON SOURCE AT SPARC_LAB C. Vaccarezza (Resp. Naz.), M.P. Anania (Ass. Ric.), M. Bellaveglia (Art. 23), M. Cestelli Guidi (Art. 23), D. Di Giovenale (Art. 23) G. Di Pirro, A. Drago, M. Ferrario, A. Gallo, G. Gatti, A. Ghigo, A. Marcelli, E. Pace, L. Palumbo (Ass.), F. Villa (Ass. Ric.). Participant institutions: other INFN sections (Mi, RM1, RM2, Ba, Ca, Pi, Ts, Fe, Le, Fi, Na, LNS), ENEA-Frascat In the 2013 the SL-Thomson project has seen the completion of the hardware installation by means of the interaction region finalization and the vacuum connection between the two electron and laser transfer lines. More over the first steps of the electron beam commissioning have been performed in a few days dedicated shifts between September and October 2013 The SL-Thomson project consists in a monochromatic source of ultra-fast X-ray pulses by Thomson back-scattering (TS) between the SPARC photoinjector high-quality electron beam and the 200 TW laser FLAME laser pulse, see Fig.1. The key points of this configuration are the flexibility and the potential compactness with respect to conventional synchrotron sources. Fig.1 The SPARC-LAB Thomson Source layout The source is meant to provide X-rays ranging between 20-500 kev able to produce, for example, suitable radiation for medical imaging at the lower energies and the possibility to investigate the inner quantum nature of the Compton process in the higher part of the spectrum. The Thomson system To bring the two beams in collision a 30 m long double dogleg electron beamline has been constructed that extracts the electron beam at the exit of the the linac and brings it in the outer bay of the SPARC hall (Fig. 2), while the laser pulse coming from the FLAME building enters the hall behind a protective concrete labyrinth and propagates for around 20 m under vacuum (10-6 Torr) up to the Interaction Point, Fig. 3. The two beams parameter table is reported below:
IP Fig. 2 Pictures of the electron beam line Fig. 3: Pictures of the photon beam line: the protection wall for the photon pipe insertion (left above), a mirror chamber(left below), the photon beamline inside the SPARC hall.
The Interaction Chamber A special care has been devoted to the interaction chamber design that in the most compact arrangement provides the housing for both the electron and photon beam focusing systems and for the diagnostic and vacuum pumping as well. In Fig. 4 the CAD 3D design is reported that shows the two beams propagation directions together with the parabolic mirror location (right side) for the focusing of the incoming photons (rightside) while on the leftside the electron beam focusing solenoid is located. In Fig. 5 the Thomson interaction chamber is shown as realized and installed in the beamline. IP solenoid parabolic mirror Fig. 4 CAD design of the Interaction chamber. Fig. 5 The installed Thomson Interaction chamber.
Experimental Results The foreseen first commissoning phase was meant to provide a 200pC electron beam transport at two energies between 80-30 MeV with the final focus obtained by means of the quadrupole triplet upstream the Interaction Point. The first measurements have been performed in the month of September 2013 with a four day shift: the goal was to transport the electron beam bunches up to the interaction chamber. The measurements have been performed using the electron beam diagnostics of the beam line consisting in fluorescent screen mounted on movable supports for the beam imaging and in Beam Position Monitors to record the beam transverse position along the beamline Electron Beam transport and measurements In fig. 6-7 the results of the very first few days of commissioning in September 2013 are reported as the beam pictures grabbed on the Thomson-IP fluorescent screens for two different beam energies: 75 and 50 MeV, while in fig. 8 the beam image on the final dumper screen downstream the IP is reported. The quadrupole focusing has been kept as low as possible in order to minimize the steering effect due to the off-center electron orbit inside the quadrupoles and an rms beam size of σ x =0.50±0.02 mm and σ y =0.28±0.02 mm have been obtained as a first attempt at IP for the 150 and 75 MeV beams with a charge Q=230 pc, while a beam size of σ x =0.18±0.02 mm and σ y =0.17±0.02 mm has been otained in the 50 MeV case. Fig. 6: 75 MeV electron beam on the THMFLG03 screen at the Interaction Point of the SL_Thomson beamline, Q ~ 230 pc, σ x =0.55±0.02 mm and σ y =0.36±0.02 mm.
Fig. 7: 50 MeV electron beam on the THMFLG03 screen at the Interaction Point of the SL_Thomson beamline, Q ~ 230 pc, σ x =0.18±0.02 mm and σ y =0.17±0.02 mm. Fig.8: 75 MeV electron beam on the THMFLG04 screen about two meter downstream the final dumper, Q ~ 230 pc, σ x =1.60±0.02 mm and σ y =0.34±0.02 mm Laser Pulse transport @IP The laser beam transfer line to the interaction region is composed by a series of high reflectivity mirrors inserted in a vacuum pipe 50 m long see fig. 3. The mirrors, 8 inches diameter, are supported by motorized gimbal mounts in order to assure the alignment up to to the off-axis parabola that focuses the laser pulse on the electron beam. The design of the line has been performed with ZEMAX optical code to simulate the effect of the misalignment of the mirrors on the final spot. The FLAME laser beam is extracted from the FLAME target area and guided up to the Thomson IP by means of an under vacuum beamline: the vacuum of the photon beam line is a the level of 10-6 Torr suitable for the transport of the compressed laser pulse ( 10fs length) as needed for the plasma acceleration experiment. A concrete wall has been realized in order to stop any radiation draft from the FLAME area towards the SPARC bunker, and to allow people entering in the SPARC hall during the FLAME laser operation and viceversa. The synchronization system The Thomson scattering experiment needs an extremely precise synchronization between electron bunch and laser pulse. The relative time of arrival jitter of the two beams is fundamental to obtain a repeatable and efficient interaction. The electrons and photons have to be synchronized with a relative jitter < 500 fs RMS. This is obtained with a standard electrical distribution of the reference signal, already present at SPARC. A Reference Master Oscillator (RMO) with a good phase noise performance (60fs RMS, measured from 10Hz to 10MHz from the carrier) is used to lock the subsystems with different feedback loops. They can be divided in two general types: slow (bandwidth <10 Hz) and fast (10 Hz to some MHz bandwidth). The formers are used typically to compensate slow drifts caused by thermal elongation of cables and are implemented by means of high resolution stepper motors. The others are designed to compensate the high frequency noise suffered by the systems that is normally due to mechanical vibrations or electrical noise in the RF circuits or power amplifiers (klystron tubes and driver amplifiers). For the photocathode laser oscillator we have enhanced the performance of the PLL modifying the loop error amplifier, providing a RMO relative phase noise jitter <50fs RMS. We also added a new electronic board to choose the relative delay to the FLAME laser oscillator pulses (already locked to the RMO) to ensure the longitudinal superposition of the electron beam and the photon pulse at the interaction point.
Pubblicazioni: Petrillo, V.; Bacci, A.; Curatolo, C.; Maroli, C., Serafini.L.; Rossi A.R. 'Time evolution analysis of the electron distribution in Thomson/Compton back-scattering' JOURNAL OF APPLIED PHYSICS Volume: 114 Issue: 4 Article Number: 043104 Published: JUL 28 2013 Bacci, A.; Alesini, D.; Antici, P.; et al. 'Electron Linac design to drive bright Compton back-scattering gamma-ray sources' JOURNAL OF APPLIED PHYSICS Volume: 113 Issue: 19 Article Number: 194508 Published: MAY 21 2013 Maroli, C.; Petrillo, V.; Tomassini, P.; Serafini L. 'Nonlinear effects in Thomson backscattering' PHYSICAL REVIEW SPECIAL TOPICS-ACCELERATORS AND BEAMS Volume: 16 Issue: 3 Article Number: 030706 Published: MAR 27 2013 Petrillo, V.; Chaikovska, I.; Ronsivalle, C.; Rossi, A.R., Serafini L.; Vaccarezza C. 'Phase space distribution of an electron beam emerging from Compton/Thomson back-scattering by an intense laser pulse' Europhysics Letters Volume: 101 Issue: 1 Article Number: 10008 Published: JAN 2013