Mesures et caractérisation du halo du faisceau de l'accélérateur ATF au KEK

par Renjun Yang

Projet de thèse en Physique des accélérateurs

Sous la direction de Philip Bambade.

Thèses en préparation à Paris Saclay , dans le cadre de Particules, hadrons, énergie et noyaux: Instrumentation, Imagerie, Cosmos et Simulation , en partenariat avec LAL - Laboratoire de l'Accélérateur Linéaire (laboratoire) et de Université Paris-Sud (établissement de préparation de la thèse) depuis le 14-09-2015 .

  • Résumé

    Le projet ATF2 est une plateforme expérimentale unique dans son genre pour le R&D et pour la formation des nouvelles générations de physiciens et d'ingénieurs en sciences des accélérateurs. Son exploitation est prévue jusqu'au démarrage du futur collisionneur linéaire. Les programmes prioritaires sont la réduction de la taille verticale en-deça de 37 nm, la stabilisation du faisceau au point focal et la maîtrise de faisceaux de forte intensité.

  • Titre traduit

    Diagnostics and characterization of beam halo at KEK Accelerator Test Facility

  • Résumé

    The ILC (International Linear Collider) and CLIC (Compact Linear Collider) are two high energy electron positron colliders planned in the next decade [1,2] to complement the Large Hadron Collider (LHC) presently operating at CERN. Achieving the very high specified luminosities will require maintaining stably focused beams with nanometer transverse sizes at the collision point. For this purpose, ultra-low emittance beams must first be provided, through radiation damping of the particle phase space, in special storage rings similar to 3 rd generation synchrotron light facilities. After acceleration, the beam sizes must be reduced by another factor of about 50 at the collision point. This is achieved via a new « final focus » concept, providing the needed optical demagnification through a sophisticated scheme with local control of the chromatic and geometric aberrations up to 3 rd order [3]. The Accelerator Test Facility (ATF) is an international accelerator R&D complex based at KEK, operating an electron damping ring with transverse emittances reaching unprecedently low values of less than 2 nm and 10 pm in the horizontal and vertical planes, respectively [4]. In the past few years, ATF2, a low energy prototype of the final focus system for future linear colliders, has been added, using the extracted ultra-low emittance beam from ATF as input [5]. The main goals of both ATF and ATF2 are developing and validating the state of the art instrumentation and experimental beam handling techniques needed for future linear colliders. The specific goals of the ATF2 project are to (1) produce and maintain over time a stable beam with transverse size smaller than 40 nanometers and (2) demonstrate 1-2 nanometer beam position stability at the collision point usingbunch to bunch feedback. Two teams from IN2P3 laboratories participate in ATF2 within a community of American, Asian and European specialists. A major issue in ATF2 and in linear colliders, as well as in many other accelerator facilities for high energy physics, is controling the beam halo before the collision point. Beam halo consists of tails extending far beyond the Gaussian core of the beam. Halo can be generated during the acceleration process, through wakefields and so-called « dark current » emission, as well as in the damping ring, via non-linearities, or through multiple Coulomb scattering of particles within bunches, scattering off the residual gas molecules in the vacuum chamber, and even scattering off photons from the black body thermal radiation present in the environment. From the experience at the Stanford Linear Collider (SLC) in the nineties and from more recent measurements at ATF, typically 10 -3 of the total bunch charge can populate the halo. When tail particles reach the vacuum chamber and start showering in the material, large numbers of secondary particles are produced. The place where tail particles are the most likely to be intercepted are in the last focusing quadrupole magnets, just before the collision point. In a linear collider, such particle losses will be unacceptable near the collision point, as the produced secondary particles would have devastating effects on the experiments. For this reason, special collimation sections are planned far upstream in the system to clean up the beam halo. The design of these sections uses assumptions and experience from the SLC concerning the population and propagation of halo particles. At ATF2, there are at present no collimators for the beam halo, although physical apertures of the vacuum chamber at various locations along the beam line will intercept some of it. Dedicated collimators are however now being prepared within the collaboration. The main tool to measure the beam size at the focal point of ATF2 is based on setting up an interference pattern between two laser beams and detecting the Compton scattered γ photon rate while the beam is scanned accross the interference fringes. From the modulation in the γ photon rate, the beam size could be extracted with a resolution as small as 20 nm [6]. This tool is however very sensitive to bremstrahlung photons emitted when halo particles are intercepted in the last quadrupole magnets and in the vacuum chamber after the collision point. Although specific collimation has been installed to shield the solid angle of the γ photon detector against such bremstrahlung photons, this background prevents the use of the largest horizontal and vertical demagnifications factors available in the optics. Recently, vertical beam sizes of  45 nm are routinely produced at low charge [7]. An alternative technique to measure the rate of Compton scatters during the interaction of the beam with the interference fringes is to detect the recoil electrons. Since these electrons have up to 2.23% lower energy compared to other beam particles, detection behind a large 20° bending magnet used after the collision point can be considered. The visibility in the presence of the beam halo was checked in simulation. While the halo is clearly dominating, by installing an extra focusing quadrupole between the collision point and the bending magnet, the halo can be focused enough to enable the edge and about half the Compton spectrum to be clearly measured. A new diamond sensors with four strips has been installed for this purpose in the vacuum chamber, near the beam, using a movable stage to scan the horizontal dimention [8]. A second one is planned in 2015 for scans in the vertical dimensions. The radiation that would have to be tolerated from backscattered neutrons and from the intercepted halo itself was estimated using a preliminary GEANT4 simulation, showing that the maximum yearly dose would be less than 25 kGy. This would be acceptable for scCVC diamonds. The electronics and signal processing chain are relatively simple given the large signal and low bunch frequency (1.5 to 6 Hz). However, the very large dynamic range implies special care to enable collecting the largest charges without biases at the higher end of the dynamic range. Shielding against high frequency electromagnetic pickup induced by the passage of the beam is also an issue in the lower end of the dynamic range. In parallel, an identical device has been prepared to characterize the beam distribution at the exit of the PHIL facility at LAL, where the main aim is to provide a diagnostic capable of probing very low intensity beams suitable for detector R&D activities. PHIL is a facility at LAL for photo- cathode R&D with very short pulses [9]. Electron beams with 3-5 MeV, 10-500 pC, 7 ps FWHM bunches are produced daily, including for users doing other R&D. Recently, the high powerLASERIX team has joinded and is planning to send low power sub-picosecond pulses on the PHIL photo-cathode. A novel multi-photonic beam production mechanism is also being investigated at PHIL, to drastically reduce the beam charge, by reducing the photon energy on the photo-cathode below the threshold for extracting electrons, requiring hence coincident photons. The diamond sensor to be installed will be essential for tuning and operating in this multi-photon production mode. Summarising, the main goals of the project to which the student will contribute are: o Measure and characterise the beam halo for different optical magnifications and parameters at ATF2 and PHIL beam lines, respectively at KEK and LAL o Simulate beam halo generation, propagation and experimental setups o Detect the recoil electron spectrum for linear (first order) Compton scattering o Participate in R&D towards new diamond sensors with improved performance