Advances in laser technology over the past 25 years have been impressive, in particular, for the Ytterbium technology where, nowadays, kilowatt-class laser systems are available. This technology also led to the possibility to provide hundreds of kilowatts of laser power by the use of enhancement cavities. We report here on the demonstration of a stable 500 kW average laser power in a high-finesse enhancement cavity. It paves the way toward systems providing laser power in excess of 1 MW and opens the door to a breakthrough in a variety of future applications.

One of the prerequisites for the next generation high luminosity light sources is the availability of the short electron bunches. It also has several applications in other domains, including medical diagnostics and high-resolution imaging. In principle, using photoelectric effect a short electron bunch can initially be generated by illuminating a photocathode with an ultra-short light pulse of appropriate wavelength. Strong EM fields from a RF gun or similar accelerating structures, synchronized with the incoming laser pulses, are then used to accelerate these electron bunches initially up to an energy of tens of MeV. We present our preliminary results on the experimental investigation of two-photon nonlinear photoemission processes for the generation of picosecond, low-charge electron bunches conducted at PHIL photoinjector facility. A comparison of the emission efficiency and bunch characteristics with the single photon emission process is also made.

We report and discuss high-flux generation of circularly polarized γ-rays by means of Compton scattering. The γ-ray beam results from the collision of an external-cavity-enhanced infrared laser beam and a low emittance relativistic electron beam. By operating a non-planar bow-tie high-finesse optical Fabry-Perot cavity coupled to a storage ring, we have recorded a flux of up to (3.5 ± 0.3) × 108 photons per second with a mean measured energy of 24 MeV. The γ-ray flux has been sustained for several hours. In particular, we were able to measure a record value of up to 400 γ-rays per collision in a full bandwidth. Moreover, the impact of Compton scattering on the electron beam dynamics could be observed resulting in a reduction of the electron beam lifetime correlated to the laser power stored in the Fabry-Perot cavity. We demonstrate that the electron beam lifetime provides an independent and consistent determination of the γ-ray flux. Furthermore, a reduction of the γ-ray flux due to intrabeam scattering has clearly been identified. These results, obtained on an accelerator test facility, warrant potential scaling and revealed both expected and yet unobserved effects. They set the baseline for further scaling of the future Compton sources under development around the world.

A new method of laser frequency stabilization using polarization property of an optical cavity is proposed. In a standard Fabry–Perot cavity, the coating layers thickness of cavity mirrors is calculated to obtain the same phase shift for s- and p-wave but a slight detuning from the nominal thickness can produce s- and p-wave phase detuning. As a result, each wave accumulates a different round-trip phase shift and resonates at a different frequency. Using this polarization property, an error signal is generated by a simple setup consisting of a quarter wave-plate rotated at 45°, a polarizing beam splitter and two photodiodes. This method exhibits similar error signal as the Pound–Drever–Hall technique but without need for any frequency modulation. Lock theory and experimental results are presented in this paper.

PHIL [1] is a low energy (E<5 MeV) and high current (1nC/bunch) electron beam accelerator at LAL. It uses a laser beam to extract electron bunch from a copper cathode at a repetition frequency of 5Hz. The stability of the beam charge at PHIL is a key issue for the successful operation of the physic experiences that use the machine. It is also one of the most important specifications of a laser driven high current RF gun. Two Integrated Current Transformers (ICT) and backend electronics are used to monitor the stability of the beam charge at PHIL, with an accuracy of about 1pC [2]. At PHIL, the electron beam charge is quite stable, but we often note a slow charge drift on long duration experiences. Several types of jitter can impact the stability of the beam charge. The fluctuations of the RF power or the RF to laser relative phase drift could have significant influence on the machine stability, due to temperature variations and electronic components overheating. To correct the phase drift, we describe, in this work, a method based on slow analog-digital feedback loop between the RF wave in the gun (3GHz) and the synchronization signal of the laser (75MHz). It allows maintaining the jitter between the laser pulse and the RF wave stable at a very low value (1° of 3GHz). As a result, the electron beam charge will be maintained at a stable level, to meet the requirements of users. ACCELERATOR DESCRIPTION

Carrier-envelope phase (CEP) drift of a pulse train of 2 ps pulses has been measured by a multiple beam interferometer. The round trip time of the interferometer is slightly mistuned from the pulse sequence, leading to spectral interference fringes. We extract the pulse-to-pulse CEP drift from the position of the spectral interference pattern. The length of the interferometer has been actively stabilized to ±10  nm, which sets the ultimate limit on the accuracy of the measurement to 78 mrad, while the CEP-drift (rms) noise of the measurement was 127 mrad (at 800 nm).

The machine described in this document is an advanced Source of up to 20 MeV Gamma Rays based on Compton back-scattering, i.e. collision of an intense high power laser beam and a high brightness electron beam with maximum kinetic energy of about 720 MeV. Fully equipped with collimation and characterization systems, in order to generate, form and fully measure the physical characteristics of the produced Gamma Ray beam. The quality, i.e. phase space density, of the two colliding beams will be such that the emitted Gamma ray beam is characterized by energy tunability, spectral density, bandwidth, polarization, divergence and brilliance compatible with the requested performances of the ELI-NP user facility, to be built in Romania as the Nuclear Physics oriented Pillar of the European Extreme Light Infrastructure. This document illustrates the Technical Design finally produced by the EuroGammaS Collaboration, after a thorough investigation of the machine expected performances within the constraints imposed by the ELI-NP tender for the Gamma Beam System (ELI-NP-GBS), in terms of available budget, deadlines for machine completion and performance achievement, compatibility with lay-out and characteristics of the planned civil engineering.

Dans le cadre de la recherche et du développement de sources de rayons gamma polarisés, une cavité Fabry-Perot (FP) non planaire et un système laser de forte puissance moyenne ont été développés et utilisés sur un accélérateur de particules afin de produire un rayonnement gamma par diffusion Compton inverse. La conception du système laser et de la cavité FP est présentée ainsi que les méthodes de synchronisation nécessaires.

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.

We report on the first demonstration, to the best of our knowledge, of the locking of a Fabry-Perot cavity with a finesse of 28,000 in the pulsed regime. The system is based on a stable picosecond oscillator, an ultrastable cavity with high-reflection mirrors, and an all-numerical feedback system that allows efficient and independent control of the repetition rate and the pulse to pulse carrier-to-envelop phase drift (CEP). We show that the carrier to envelop phase can have a dramatic effect even for pulses with hundreds of cycles. Moreover, we have succeeded in unambiguously measuring the CEP of a 2 ps pulse train. Finally, we discuss the potential of our findings to reach the MW average power level stored in an external cavity enhancement architecture.

We report on the first demonstration of the locking of a 28000 finesse cavity in picosecond regime. Besides, we have unambiguously measured the CEP drift of 2 ps pulses and characterized its effect on cavity locking.

LAL is now equiped with its own platform for photoinjectors tests and Research and Developement, named PHIL (PHotoInjectors at LAL). This facility has two main purposes: push the limits of the photoinjectors performances working on both the design and the associated technology and provide a low energy (MeV) short pulses (ps) electron beam for the interested users. Another very important goal of this machine will be to provide an opportunity to form accelerator physics students, working in a high technology environment. To achieve this goal a test line was realised equipped with an RF source, magnets and beam diagnostics. In this article we will describe the PHIL beamline and its characteristics together with the description of the first two photoinjector realised in LAL and tested: the ALPHAX and the PHIN RF Guns.

A bandwidth-independent, linear and scalable method for carrier-envelope phase drift measurement demonstrated. Our experiments reveal that carrier-envelope phase drift of a picosecond pulse train can be directly obtained from the spectrally resolved interference pattern of a length-stabilized multiple-beam interferometer. The retrieved phase from the pattern correlates well with the strongly CEP-sensitive coupling signal between the frequency combs of the picosecond oscillator and an ultra-high finesse Fabry-Perot interferometer. Our results can lead to the generation of a robust CEP-stabilized seed pulse train for high resolution comb spectroscopy as well as to compact Compton X-ray and gamma-ray sources

The PHIL accelerator is in operation since November 2009. Its main goals are the R&D on photo-injectors and to provide electron beam to users. We report on the experimental characterization of the electron beam produced by a 3 GHz 2,5 cells RadioFrequency (RF) gun and operation with a Magnesium photo-cathode.

As part of the positron source R&D for future e+−e− colliders and Compton based compact light sources, a high finesse non-planar four-mirror Fabry-Perot cavity has recently been installed at the ATF (KEK, Tsukuba, Japan) [1]. The first measurements of the gamma ray flux produced with a such cavity using a pulsed laser is presented here. We demonstrate the production of a flux of 2.7 ± 0.2 gamma rays per bunch crossing ( ∼ 3 × 106 gammas per second) during the commissioning.

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.

The next generation of e+/e- colliders will require a very intense flux of gamma rays to allow high current polarized positrons to be produced. This can be achieved by converting polarized high energy photons in polarized pairs into a target. In that context, an optical system consisting of a laser and a four-mirror passive Fabry-Perot cavity has recently been installed at the Accelerator Test Facility (ATF) at KEK to produce a high flux of polarized gamma rays by inverse Compton scattering. In this contribution, we describe the experimental system and present preliminary results. An ultra-stable four-mirror non planar geometry has been implemented to ensure the polarization of the gamma rays produced. A fiber amplifier is used to inject about 10W in the high finesse cavity with a gain of 1000. A digital feedback system is used to keep the cavity at the length required for the optimal power enhancement. Preliminary measurements show that a flux of about 4×10^6 γ/s with an average energy of about 24 MeV was generated. Several upgrades currently in progress are also described.