High flux polarized gamma rays production: first measurements with a four-mirror cavity at the ATF

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 $4\times10^6 \gamma$/s with an average energy of about 24 MeV was generated. Several upgrades currently in progress are also described

THE FOUR-MIRROR LASER STACKING CAVITY FOR POLARIZED GAMMA-RAY/POSITRON GENERATION

Abstract A non planar four mirror cavity has been designed and constructed to demonstrate the production of high gamma ray fluxes from Compton scattering of laser and electron beams at ATF. A pulsed laser is amplified using the recent technology of Yb-doped photonic cristal fibres. Seeding the high finesse four-mirror cavity with this amplified laser beam will allow reaching average powers between 0.1MW and 1MW

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

Coupled structural and magnetic properties of ferric fluoride nanostructures part I: a Metropolis atomistic study

A modified Metropolis atomistic simulation is proposed to model the structure of grain boundaries (GBs) and interfaces in ionic nanostructured systems and is applied to the magnetically interesting case of iron trifluoride (FeF3). We chose long-range interatomic potentials adjusted on experimental results, and adapted a previously established Monte Carlo scheme consisting of various modifications of the simulated annealing/ Metropolis algorithm. Atomic structures of twisted and tilted GBs as a function of the relative disorientation of the grains have been achieved yielding close to experimentally measured properties. This approach takes into account the structure of the grains far from the interface in order to constrain the relative orientation of the grains, without any periodic boundary conditions. One concludes that a long-range coulombic falloff of the interatomic potentials is necessary to obtain GB structures presenting a correct local topology but with a smooth transition from crystalline to amorphous states. The structural features are finally discussed in terms of topological aspects and local magnetic structure.Comment: submitted to JMM

Production of gamma rays by pulsed laser beam Compton scattering off GeV-electrons using a non-planar four-mirror optical cavity

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). 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 $\pm$ 0.2 gamma rays per bunch crossing ($\sim3\times10^6$ gammas per second) during the commissioning

Inhomogeneous States in a Small Magnetic Disk with Single-Ion Surface Anisotropy

We investigate analytically and numerically the ground and metastable states for easy-plane Heisenberg magnets with single-ion surface anisotropy and disk geometry. The configurations with two half-vortices at the opposite points of the border are shown to be preferable for strong anisotropy. We propose a simple analytical description of the spin configurations for all values of a surface anisotropy. The effects of lattice pinning leads to appearance of a set of metastable configurations.Comment: 10 pages, 7 figures; submitted to Phys. Rev.

The ThomX project status

Work supported by the French Agence Nationale de la recherche as part of the program EQUIPEX under reference ANR-10-EQPX-51, the Ile de France region, CNRS-IN2P3 and Université Paris Sud XI - http://accelconf.web.cern.ch/AccelConf/IPAC2014/papers/wepro052.pdfA collaboration of seven research institutes and an industry has been set up for the ThomX project, a compact Compton Backscattering Source (CBS) based in Orsay - France. After a period of study and definition of the machine performance, a full description of all the systems has been provided. The infrastructure work has been started and the main systems are in the call for tender phase. In this paper we will illustrate the definitive machine parameters and components characteristics. We will also update the results of the different technical and experimental activities on optical resonators, RF power supplies and on the electron gun

Simulation énergétique des trajectoires d'électrons rapides à travers la matière

A novel simulation for the transfer function of high energy electrons with initial energy $E_0$ ($\rm 5~keV \leq E_0 \leq 15~keV$) based in an energy analysis of each stage of the diffusion, is presented for CEMS (Conversion Electron Mössbauer Spectroscopy). The numerical results are in good agreement with ICEMS (Integral Conversion Electron Mössbauer Spectroscopy) experimental data and with the results of previous simulations.Une nouvelle simulation de la fonction de transfert des électrons d'énergie initiale $E_0$ ($\rm 5~keV \leq E_0 \leq 15~keV$) basée sur une analyse énergétique de chaque étape de la diffusion est proposée pour le CEMS (Conversion Electron Mössbauer Spectroscopy). Les résultats numériques obtenus, sont en bon accord avec les expériences obtenues en ICEMS (Integral Conversion Electron Mössbauer Spectroscopy) et avec les résultats des simulations précédentes

Magnetic properties of maghemite nanoparticles: a heisenberg - monte carlo study

En este trabajo se investigan las propiedades magnéticas de nanopartículas de maghemita g -Fe 2 O 3 utilizando el método de Monte Carlo-Metropolis sobre la base de un modelo de Heisenberg clásico tridimensional con anisotropía magnetocristalina. La estructura espinela ha sido simulada en forma realista con condiciones de frontera libres para tener en cuenta el efecto de la superficie en una nanopartícula de diámetro 3.34 nm. También se han tenido en cuenta las diferentes interacciones de superintercambio competitivas entre iones de Fe 3+ incluyendo sitios tetraédricos y octaédricos. Los resultados revelan una marcada disminución de la temperatura de Curie de la nanopartícula considerada respecto a aquella de una maghemita en bulk, como consecuencia del menor número de coordinación promedio. Finalmente se presenta y discute el efecto de la anisotropía de superficie sobre la configuración magnética de los espines en el límite cuando la temperatura tiende a cero

An atomic scale Monte Carlo study of exchange bias in homogeneous/inhomogeneous core/shell Fe3O4/CoO nanoparticles

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