1. Status of the HELICAL contribution to the polarised positron source for the International Linear Collider The positron source for the International Linear Collider (ILC) is a helical undulator-based design, which can generate unprecedented quantities of polarised positrons. The heLiCal collaboration takes responsibility for the design and prototyping of the superconducting helical undulator, which is a highly demanding short period device with very small aperture, and is producing a prototype target wheel which will allow validation of various magnetic and thermal modelling codes. The HeLiCal collaboration is making an active contribution to the ILC undulator-based positron source design, particularly through the design and prototyping of the helical undulator itself, assessing its impact on the main electron beam and simulating depolarisation effects from start to end. A possible layout of the ILC The ILC positron source will have to produce of order 10 14 positrons per second, with a nominal bunch structure of 2625 bunches per pulse and 5 pulses per second, and a bunch duration of 1 ps. The undulator-based positron source has been chosen as the baseline technology for the positron source because it offers the lowest-risk alternative for producing the required number of positrons for the ILC. In the current design the ILC electron beam is passed through a helical undulator of length approximately 200 m producing synchrotron radiation with a first harmonic energy of 10 MeV which interacts with a pair- production target. Positrons produced from the target are captured by a tapered magnetic field before being accelerated to 5 GeV and passing through a damping ring. The helical undulator design has the strong advantage of producing the positrons in a longitudinally polarized state. Higher levels of polarization can be achieved by lengthening the undulator and collimating the synchrotron radiation. Having both the electron and positron beams polarized is essential for maximising the physics reach of the ILC; an example of the role of polarized positrons for determining the quantum numbers of supersymmetric particles is shown below. More details of the physics case for polarized positrons and all aspects of the positron source can be found at: http://www.ippp.dur.ac.uk/~gudrid/source/ Helical Undulator Insertion Device Magnets or current elements are used to generate a spatially rotating magnetic dipole field along the major axis of the undulator. Charged particles entering the undulator describe helical trajectories in the field. This leads to the emission of intense circularly polarised synchrotron radiation along the axis of the undulator. Superconducting technology has been selected for the ILC positron source as it offers high field quality and easily tunable field strength. Five short superconducting undulator prototypes with a length of 300 mm have already been built. All prototypes have successfully demonstrated their full design field levels. Full scale prototype will have a 4 m cryostat containing two ~2 m undulators. After magnetic testing the full scale prototype will undergo electron beam transport tests. Bellows will be used to join the warm and cold regions between undulator sections. It has been shown that an unshielded vacuum bellows design does not create significant wakefield kicks. Target Prototype The photon beam is incident on the rim of the titanium target wheel. Simulations show that ~10% of the beam energy will be absorbed by the target (< 30 kW). A rotating target design has been adopted to reduce the photon beam power density. The rate at which the target can be cooled determines the required angular velocity to the target rim. Target prototype will be rotated in magnetic field at Daresbury Lab to validate simulations. Cross section of 4m undulator prototype Measured B-field on axis in short prototype 1 Left: 4 short undulator prototypes Right: Generation of circularly polarised photons in a helical field Structure of undulator windings Proposed layout of the ILC positron source Left: Drawing of ILC target wheel partly immersed in static magnetic field. Below: ILC Target wheel parameters Above: Unshielded bellows design. Left: Results of undulator prototypes: A period of 10mm and field of 1.05T is desirable The heLiCal collaboration A. Birch +, J.A. Clarke +, O.B. Malyshev +, D.J. Scott +, B.J.A. Shepherd + STFC ASTeC Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK E. Baynham, T. Bradshaw, A. Brummitt, S. Carr, Y. Ivanyushenkov, A. Lintern, J. Rochford STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK I.R. Bailey +, P. Cooke, J.B. Dainton +, K. Hock +, L. Jenner +, L. Malysheva +, L. Zang + Department of Physics, University of Liverpool, Oxford St., Liverpool, L69 7ZE, UK D.P. Barber + DESY-Hamburg, Notkestraße 85, 22607 Hamburg, Germany G.A. Moortgat-Pick + Institute of Particle Physics Phenomenology, University of Durham, Durham DH1 3LE, UK A. Hartin John Adams Institute, Oxford University Physics, Oxford, OX1 3RH, UK. + Cockcroft Institute, Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK Above: Simulation of the eddy currents induced in the wheel by B-field Below: Power dissipated in eddy currents and torque