Polarized Positron 2011

The following sections are included:

• PREFACE

• CONTENTS

With the LHC a new era of measurements at the energy frontier has started, and exciting new discoveries are expected. However, also measurements at the precision frontier will be necessary to fully understand the underlying physics model. The programme for the e⁺e⁻ collider projects ILC and CLIC is focused on precision tests of the Standard Model and new physics beyond it at the TeV scale. Polarized positron beams play a crucial role in these analyses. Here, the advantages as well as the requirements using also polarized positron beams for measurements at e⁺e⁻ colliders are discussed.

In this paper, we presented an overview of the positron source for International Linear Collider (ILC). We started with description of the positron source configuration for ILC RDR baseline and SB2009 baseline followed by the status of critical components of ILC positron source R&D. We also presented some parameters of positron source for both RDR and SB2009 baseline.

The parameter set for the International Linear Collider (ILC) positron source given in the Technical Design Report (TDR) is more complicated than that presented in the ILC Reference Design Report (RDR). Studies to define and to optimize the parameters for different scenarios of center-of-mass energies have been performed at both Argonne National Lab (ANL) and Deutsches Elektronen Synchrotron (DESY)/Hamburg University. Results from both institutes agree well and are presented in this paper.

To achieve the extremely high luminosity for colliding electron-positron beams at the future International Linear Collider¹ (ILC) an undulator-based source with about 230 meters helical undulator and a thin titanium-alloy target rim rotated with tangential velocity of about 100 meters per second are foreseen. The very high density of heat deposited in the target has to be analyzed carefully. The energy deposited by the photon beam in the target has been calculated in FLUKA. The resulting stress in the target material after one bunch train has been simulated in ANSYS.

We are analyzing the ILC positron source for the best polarization and efficiency. We represent the arguments why K-factor (K = eHλ_u / 2πmc²) should be ≤0.45. Lower K-factor allows reduction the total power of radiation and dominance the first harmonic in radiation. As the first harmonic can be well collimated, hence more monochromatic, the energy separation of the secondary positrons/electrons at high edge of spectrum can be done easily. As a result ~70% polarization looks feasible for ~170 m undulatorat 150-500 GeV and K factor ≤0.4.

The ILC positron system uses novel helical undulators to create a powerful photon beam from the main electron beam. This beam is passed through a titanium target to convert it into electron-positron pairs. The target is constructed as a 1 m diameter wheel spinning at 2000 RPM to smear the 1 ms ILC pulse train over 10 cm. A pulsed flux concentrating magnet is used to increase the positron capture efficiency. It is cooled to liquid nitrogen temperatures to maximize the flatness of the magnetic field over the 1 ms ILC pulse train. We report on prototypiog effort on this system.

The ILC baseline design for the positron source is based on radiation from a helical undulator to produce positrons in a thin target. Since the photon beam created in the helical undulator is circularly polarized, the generated positron beam is longitudinally polarized. Using a photon collimator upstream the positron target the positron polarization can be enhanced. However, the photon beam intensity yields a huge thermal load in the collimator material. In this paper the thermal load and heat dissipation in the photon collimator is discussed and design solutions are suggested.

Lithium Lens is a key element of FERMILAB proton conversion system in use for many years. We are analyzing the ILC positron source equipped with a scaled version of Lithium lens. Usage of liquid Lithium allows efficient cooling of Lithium container and entrance/exit windows. For the temperature just ~80°C higher, than the temperature of boiling water, the system for circulation of liquid Lithium is a compact and reliable. Overall efficiency of 1.5 secondary positrons per each initial electron passing the undulator is feasible with a compact Lithium lens. Axially symmetric motion of liquid Li does not perturb the field quality required for minimization of emittance of the secondary positrons/electrons polarized longitudinally.

In order to achieve the physics goals of future Linear Collidcrs, it is important that electron and positron beams are polarized. The baseline design at the International Linear Collider (ILC) foresees an e+ source based on helical undulator. Such a source provides high luminosity and polarizations. The positron source planned for ILC is based on a helical undulator system and can deliver a positron polarization of 60%. To ensure that no significant polarization is lost during the transport of the e- and e+ beams from the source to the interaction region, precise spin tracking has to be included in all transport elements which can contribute to a loss of polarization, i.e. the initial accelerating structures, the damping rings, the spin rotators, the main linac and the beam delivery system. In particular, the dynamics of the polarized positron beam is required to be investigated. In the talk recent results of positron spin tracking simulation at the source are presented. The positron yield and polarization are also discussed depending on the geometry of source elements.

We propose the conventional positron source driven by a several-GeV electron beam for ILC. Thermal load of the positron production target was a risk of the conventional positron source. To cure it, we employ a 300 Hz electron linac to create positrons with stretched pulse length. In ILC, the bunch timing structures and pulse timing structures can be diffecent in the positron source, in the DR, and in the main linac. We have some flexibility to choose timing structures in positron source and we use it for time stretching. ILC requires about 2600 bunches in a train in the main linac which pulse length is 1 ms. In the conventional source, about 130 positron bunches are created by each pulse of the 300Hz linac. Then 2600 bunches are created in 63 ms. We optimized parameters such as drive beam energy, beam size on the target, and target thickness to maximize the capture efficiency and to mitigate the target thermal load. A slow rotating tungsten disk is employed as positron production target.

We construct a new detector to monitor γ yields in bunch by bunch basis for the Laser Compton experiment at the KEK ATF which is capable to separate γ rays in 5.6ns spacing multi-bunch operation of the KEK ATF. In this article we report a result of measurement of multi bunch γ ray detection for the first time at the KEK ATF.

A new kind of positron source is presented. It uses the intense radiation emitted by channeled electrons in axially oriented crystals to generate positrons in a granular amorphous converter. A distance of 2 meters between the crystal radiator and the amorphous converter is used to put a bending magnet which sweeps off the charged particles allowing only to the photons to hit the converter. The simulation results on photon production, and positron generation are reported. A particular attention is brought to the energy deposition and Peak Energy Deposition Density (PEDD) in the targets. Emphasis is put on the problems of heating, cooling and on different methods and devices to handle the power dissipation in the targets. An application for the case of ILC (International Linear Collider) is worked out.

Short reviews of Compton e⁺ sources R&D for ILC are given. Especially, the Compton sources using Ring and ERL are discussed according to the scheme of truly 300Hz conventional e+ source with the change of the positron generation scheme from nonpolarized to highly polarized e⁺. As we have proposed the Ring Compton source since 2005, the improvement of the design is described. The technologies for X-ray and Gamma-ray source based on the inverse-Compton scattering have been improved by recent fiber laser and high finesse optical cavity developments.

Proposed is an enhancement of laser radiative cooling by utilizing laser pulses of small spatial and temporal dimensions, which interact only with a fraction of an electron bunch circulating in a storage ring. The dynamics of such electron bunch when laser photons scatter off the electrons at a collision point placed in a section with nonzero dispersion is studied. In this case of ‘asymmetric cooling’, the stationary energy spread is much smaller than under conditions of regular scattering where the laser spot size is larger than the electron beam; and the synchrotron oscillations are damped faster. Results of extensive simulations are presented for the performance optimization of Compton gamma-ray sources and damping rings.

BEPCII- an upgrade project of the Beijing Electron Positron Collider (BEPC) is a factory type of e⁺e⁻ collider. The fundamental requirements for its injector 1inac are the beam energy of 1.89GeV for on-energy injection and a 40mA positron beam current at the linac end with a low beam emittance of 1.6μm and a low energy spread of ±0.5% so as to guarantee a higher injection rate (≥50mA/min) to the storage ring. Since the positron flux is proportional to the primary electron beam power on the target, we will increase the electron gun current from 4A to 10A by using a new electmn gun system and increase the primary electron energy from 120MeV to 240MeV. The positron souree itself is an extremely important system for producing more positrons, including a positron converter target chamber, a 12kA flux modulator, the 7-m focusing module with DC power supplies and the support. The new positron production linac from the electron gun to the positron source has been installed into the tunnel. In what follows, we will emphasize the positron source design, manufacture and tests.

Recently, good X-ray pulses have been generated successfully via Thomson scattering between the 47MeV electron and 800nm TW laser at Tsinghua Thomson scattering X-ray source. In this paper, we report the preliminarily results of the experiments.