Development of a yearlong maintenance-free terawatt Ti:Sapphire laser system with a 3D UV-pulse shaping system for THG

• Published in: Quantum electronics (Woodbury) 2007-08, Vol.37 (8), p.697-705
• Main Authors: Tomizawa, H, Dewa, H, Hanaki, H, Matsui, F
• Format: Article
• Language: English
• Subjects: ALGORITHMS; BEAM EMITTANCE; BEAMS; CATHODES; CONTROL; CORUNDUM; ELECTRODES; ELECTROMAGNETIC RADIATION; ELECTRON BEAMS; ELECTRONIC CIRCUITS; ELECTRONIC EQUIPMENT; ENERGY RANGE; FEEDBACK; FREE ELECTRON LASERS; FREQUENCY MIXING; HARMONIC GENERATION; HUMIDITY; IONIZING RADIATIONS; LASERS; LEPTON BEAMS; LIGHT SOURCES; MATHEMATICAL LOGIC; MEV RANGE; MINERALS; MOISTURE; OSCILLATORS; OXIDE MINERALS; PARTICLE ACCELERATORS; PARTICLE BEAMS; PHOTOCATHODES; PULSE CIRCUITS; PULSE SHAPERS; PULSES; QUALITY CONTROL; RADIATION SOURCES; RADIATIONS; RF SYSTEMS; SAPPHIRE; SIGNAL CONDITIONERS; STABILITY; WAVELENGTHS; X RADIATION
• ISSN: 1063-7818; 1468-4799
• DOI: 10.1070/QE2007v037n08ABEH013564

Laser sources that feature a controlled pulse shape and long-term stability are required in a wide range of scientific fields. We developed a maintenance-free 3D-shaped UV-laser system for the photoinjector (RF gun photocathode) of an X-ray SASE free electron laser (FEL). The laser pulse-energy stability was improved to 0.2%-0.3% (rms, 10 pps, 0.4 TW in femtosecond operation) at the fundamental wavelength and to 0.7%-1.4% at the third-harmonic wavelength. This stability was continuously maintained for five months, 24 hours a day. Such improvement reflects an ability to stabilise the laser system in a humidity-controlled clean room. The pulse-energy stability of a mode-locked femtosecond oscillator was continuously held at 0.3% (p-p) for five months, 24 hours a day. In addition, the ideal spatial and temporal profiles of a shot-by-shot single UV-laser pulse are essential to suppress the emittance of the electron-beam pulse generated by the photocathode of the RF gun. We apply a deformable mirror that automatically shapes the spatial UV-laser profile with a feedback routine, based on a genetic algorithm, and a pulse stacker for temporal shaping at the same time. The 3D shape of the laser pulse is spatially top-hat (flattop) and temporally - a square stacked pulse. We apply the Q-scan method to evaluate the emittance of the electron beam generated by a 3D-shaped laser pulse. By using a 3D-shaped laser pulse of diameter 0.8 mm on the cathode and duration 10 ps (FWHM), we obtain a minimum horizontal normalised emittance of 1.4{pi} mm mrad with beam energy of 26 MeV, holding its net charge to a 0.4 nC pulse{sup -1}. At a higher net charge of 1.0 nC pulse{sup -1}, the minimum beam emittance is 2.3{pi} mm mrad with equivalent diameter and a longer pulse duration of 20 ps (FWHM). In this study, we demonstrate 3D shaping [both temporal (1D) and spatial (2D)] short pulse (5-20ps) laser beam as an ideal light source for yearlong stable generation of a low emittance electron beam with a high charge (1-2 nC pulse{sup -1}). Here, we report the principle and development process of our beam-quality control systems. (selected papers reported at the conference 'laser optics 2006')