Spatial profile measurement of femtosecond laser - Compton xrays

Abstract A femtosecond X-ray source was developed by Thomson scattering through interaction between a lowemittance picosecond electron beam and a terawatt femtosecond laser light at 90 o configuration. The observed X-ray intensity with peak energy of 2.3 keV and pulse duration of 270 fs rms was typically 1.4x10 4 photons/pulse. The pulse-to-pulse fluctuation of the X-ray intensity was measured to be 25%. The spatial profile of the X-rays was measured with a technique of X-ray imaging on a phosphor screen using an image-intensified CCD camera. The dependence of the X-ray beam profile on the scattering laser polarization was obtained and compared with theoretical analysis. INTRODUCATION A short pulse X-ray source is an important tool for studying the dynamics of the materials in the fundamental time scale. The development of femtosecond laser has made it possible to generate such ultrashort X-ray pulses in femtosecond region by means of 90-degree (90 o ) Thomson scattering with a relativistic ultrashort-pulse electron beam The intensity of the X-rays generated in Thomson scattering is proportional to the densities of both the electron and laser beam. It is important to tightly focus both the beams in the transverse direction to generate high-brightness X-rays. In addition, the small focused beam size should be required to reduce the interaction time in 90 o Thomson scattering for the generation of femtosecond X-ray pulse EXPERIMENTAL ARRANGEMENT The Thomson femtosecond X-ray source was consisted of a picosecond electron source and a tabletop terawatt femtosecond pulse laser An Electron Source The electron beam was produced by a S-band (2856 MHz) photocathode rf gun. The rf gun, which was constructed under the BNL/KEK/SHI collaboration [6], was consisted of two cells: a half cell and a full cell. A copper cathode was located on the side of the half cell. The length of the half cell was designed to be 0.6 times of the full cell length to reduce the beam divergence. At the exit of the rf gun, a single solenoid magnet was mounted for space-charge emittance compensation. The rf gun was driven by an all solid-state LD-pumped Nd:YAG picosecond laser. The laser was consisted of a passive mode-locked oscillator, a regenerative amplifier, a post amplifier and a frequency converter. The oscillator was phase-locked with a frequency of 119 MHz, the 24 th sub-harmonic of the accelerating 2856 MHz rf, by dynamically adjusting the cavity length of the oscillator with a semiconductor saturable absorber mirror controlled by a timing stabilizer. The output of the oscillator was amplified the pulse energy up to 2mJ in the regenerative amplifier and the post amplifier. The amplified laser pulse was frequency quadrupled to 262 nm ultraviolet (UV) light by a pair of frequency conversion crystals. The UV light was injected on the cathode surface at an incident angle of 68 o along the electron beam direction. The electron beam produced from the rf gun was accelerated with a 70 cm long standing-wave linear accelerator (linac) produced with an alternating-periodic structure. The linac is located at a position of 1.2 m from the cathode. The input rf peak power of both the rf gun and the linac was 7.5 MW that was produced with a 15 MW Klystron. The peak electric fields on axis in the rf gun and the linac were approximately 100 and 25 MV/m, respectively. The repetition rate of the operation was 10 Hz in the experiments. The accelerated electron beam was focused at the interaction point for scattering with the laser light by a triplet quadrupole magnet downstream of the linac. The scattered electrons were bended by a 90 o dipole magnet A Terawatt Femtosecond Laser The terawatt femtosecond laser was consisted of a mode-locked Ti:Sapphire laser oscillator, a pulse stretcher, a regenerative amplifier, a multi-pass post amplifier, and a pulse compressor. The oscillator generated 50 fs pulses at the repetition rate of 119 MHz. The frequency of the laser oscillator was phase-locked with the 119 MHz rf by the same method as the driving laser of the rf gun. ___________________________________________