R. Chiche

Non-planar four-mirror optical cavity for high intensity gamma ray flux production by pulsed laser beam Compton scattering off GeV-electrons

As part of the R&D toward the production of high flux of polarised Gamma-rays we have designed and built a non-planar four-mirror optical cavity with a high finesse and operated it at a particle accelerator. We report on the main challenges of such cavity, such as the design of a suitable laser based on fiber technology, the mechanical difficulties of having a high tunability and a high mechanical stability in an accelerator environment and the active stabilization of such cavity by implementing a double feedback loop in a FPGA.

Development of an optical resonator for X-ray production

Inverse Compton Scattering provides a unique way to produce quasi-monochromatic X-rays via the interaction of relativistic electrons with a laser pulse. This process has the advantage of producing a very high flux of X-rays with energies above a few tens of keV for a modest electron beam energy. In addition the output beam divergence is much larger than in classical synchrotron light sources and the X-ray beam is thus easier to manipulate. We present an X-ray source under construction at Paris-Sud University, ThomX. This source has a small footprint of 70m2 compared to synchrotron light sources and uses a 50MeV electron beam that collides at 16.7MHz with a few picosecond pulsed laser beam which power is enhanced at the state of the art. In the nominal design the enhanced average laser-beam power is 600kW and X-rays between 30 and 50 keV are produced with a flux of 1013 photons per second. This energy range as well as the energy-angular dependence due to the Compton back-scattering process are suitable for societal applications like radiotherapy or art history. We use a prototype cavity to perform R&D for the ThomX source. Various diagnostics like real time finesse measurements or active coupling enhancement are under study on this test cavity to increase the achievable stored power. We will present the ongoing developments along with the results we obtained. A very high finesse optical cavity (F > 28000) is used, which is one of the highest finesse cavities in pulsed regime. The numerous challenges and solutions to mitigate the related issues will be presented.

10μJ ultrashort sub-100 fs FCPA synthesizer

We report the generation of 10 μJ, ultrashort 97 fs pulses at 1 MHz by implementing a two-arm spectral coherent combining scheme in a fiber chirped-pulse amplifier (FCPA), allowing both gain-narrowing mitigation and large stretching ratio for energy extraction. Such architecture is able to support the amplification of large-bandwidth (>15 nm) together with high gain factor (>30 dB), allowing the generation of ultrashort sub-100 fs pulses at the output of a FCPA for the first time.

Carrier envelope phase drift of picosecond frequency combs from an ultrahigh finesse fabry-perot cavity

The carrier envelope phase shift of a stacked picosecond pulse train for Compton scattering experiments have been measured to an accuracy of 80 mrad by spectrally resolved interference pattern of a stabilized multiple beam interferometer.