Accelerator Science and Technology Centre astec.ac.uk

1. LLNL - draft design On-axis field measurements for first superconducting prototype Section of second superconducting prototype HeLiCal Contribution to the Polarised Positron Source for the International Linear Collider Abstract The baseline design of the positron source for the International Linear Collider (ILC) incorporates a helical undulator and pair-production target in order to generate the unprecedented quantities of positrons required to sustain the intended ILC physics programme. This configuration is challenging but readily achievable by using novel adaptations of existing technologies to avoid problems inherent in conventional positron sources in which the stresses in the target(s) and activation of the target station are both serious problems. In addition, a highly polarized positron beam, essential for realizing the full potential of the ILC, can be produced by a simple upgrade to the baseline design. A major contribution to the international design effort is being led by the UK-based HeLiCal collaboration. The collaboration takes responsibility for the design and prototyping of the helical undulator itself, which is a short period device with a small aperture, and also leads development of the start to end simulations of the polarized particles to ensure that high levels of polarization are maintained from the sources, through the beam transport systems and up to the interaction point(s). Members of the collaboration are also involved in the EUROTeV-funded research programme to produce a design for a pair-production target which can operate reliably in the high photon flux of the undulator. This paper will provide an update on the work of the collaboration, focusing on the design, construction and testing of components of the polarized positron source, and will also discuss future plans. Positron Source Overview The ILC positron source will have to produce of order 1014 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. In the current design the ILC electron beam is passed through a helical undulator of length approximately 200 m (see panel below left) producing synchrotron radiation with a first harmonic energy of 10 MeV which interacts with a pair-production target (see panel below right). Positrons produced from the target are captured by a tapered magnetic field before being accelerated to 5 GeV and passing through a damping ring. Proposed ILC layout. Schematic of undulator-based positron source. Pair-Production Target Role of Polarised Positrons at ILC • The photon beam is incident on the rim of the 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. • The diameter of the wheel is then determined by the characteristics of the drive motor, the rate of radiation damage to the target, and the required target lifetime. 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 addition, it 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 on the right. 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 • 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. • 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 on axis. DL - draft prototyping design (*Numbers refer to LLNL study of earlier solid-disc design with 220 kW photon beam.) The heLiCal collaboration has developed short undulator prototype modules for the ILC using two different technologies: superconducting and permanent magnet. • Simulations of the activation of the target from neutron production predict dose rates 1 m from the target at a depth of 10 mm of soft tissue to be ~150 mSv / h after 1 week of shutdown compared with ~4000 mSv /h for an equivalent ‘conventional’ positron source • (A. Ushakov, Proceedings of EPAC 2006 conference.) • The EU exposure limit for radiation workers is 20 mSv / year. • A full remote-handling system is required. The concept of a possible vertical remote-handling system for the ILC is illustrated below. Superconducting module prototype. Permanent magnet module prototype. The four short superconducting prototype modules The first superconducting undulator module consists of an aluminium former into which has been machined two interleaved helical grooves with a period of 14 mm. Superconducting (NbTi) wire ribbons are wound into the grooves and current is passed in opposite directions along the two helices to give a design field of 0.8 T on axis. The results of on-axis Hall probe field measurements are shown in the figure on the right. The permanent magnet undulator module consists of trapezoids of NdFeB magnets arranged to form rings with a dipole field on axis. Successive rings forming the undulator were rotated with respect to each other to give the necessary field configuration. The photograph above shows the undulator in two halves. M. Woodward, B. Smith - RAL • The University of Liverpool plans to play a key role in prototyping the target, with a series of prototypes being assembled and tested at Daresbury Laboratory in the period 2007-2010. A. Birch+, J.A. Clarke+, O.B. Malyshev+, D.J. Scott+ CCLRC ASTeC Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK E. Baynham, T. Bradshaw, A. Brummitt, S. Carr, Y. Ivanyushenkov, A. Lintern, J. Rochford CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK I.R. Bailey+, P. Cooke, J.B. Dainton+, L. Jenner+, L. Malysheva+ Department of Physics, University of Liverpool, Oxford St., Liverpool, L69 7ZE, UK D.P. Barber+, P. Schmid DESY-Hamburg, Notkestraße 85, 22607 Hamburg, Germany G.A. Moortgat-Pick+ Institute of Particle Physics Phenomenology, University of Durham, Durham DH1 3LE, UK, and CERN, CH-1211 Genève 23, Switzerland + Cockcroft Institute, Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK The HeLiCal Collaboration I.R. Bailey1,3 ,V. Bharadwaj4, D. Clarke5, P. Cooke3, J.B. Dainton1,3, J. Gronberg2, N. Krumpa5 , D. Mayhall2, T. Piggott2, D.J. Scott1,3, J. Sheppard4, W. Stein2, J. Strachan5, P. Sutcliffe3 1 Cockcroft Institute, Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK. 2 Lawrence Livermore National Laboratory, Livermore, CA 94551, USA. 3 Department of Physics, University of Liverpool, Liverpool, L69 7ZE, UK. 4 Stanford Linear Accelerator Center, PO Box 20450, Stanford, CA 94309, USA. 5 CCLRC Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK. Accelerator Science and Technology Centre www.astec.ac.uk