Liverpool Accelerator Physics Group International Linear Collider ILC

Liverpool Accelerator Physics Group International Linear Collider (ILC) R&D The he. Li. Cal collaboration has members from Daresbury Laboratory, Rutherford Appleton Laboratory, DESY, Durham and Liverpool. One of the key components of the ILC design is the positron source. It will have to produce of order 1014 positrons per second, with the nominal ILC bunch structure of 2820 bunches per pulse and 5 pulses per second. As a part of the he. Li. Cal collaboration and the EUROTe. V project, Liverpool contributes to the R&D for an undulator-based positron source. Schematic of undulator-based positron source. The Positron Source In this design the ILC electron beam is passed through a helical undulator of length approximately 100 m (see panel below left) producing synchrotron radiation with a typical energy of approximately 10 Me. V which collides 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 Ge. V and passing through a damping ring. The resulting positron beam is injected into the main accelerator where it is accelerated to the required energy (nominally 250 Ge. V) before passing through the beam delivery system and finally being brought into collision with the opposing electron beam at the interaction point. Possible ILC layout. Photons(≈ 10 Me. V ) Electrons (150 Ge. V to 250 Ge. V) Helical Undulator (≈ 200 m) Photon Collimator Conversion Target (0. 4 X 0 Ti) Polarised Positrons (≈ 5 Me. V) 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 emission of intense circularlypolarised synchrotron radiation on axis. The he. Li. Cal collaboration has developed two undulator prototype modules using different technologies: superconducting and permanent magnet. The superconducting module prototype. Pair-Production Target Liverpool heads the EUROTe. V-funded project to develop a pair-production target as part of a high-intensity polarised positron source, and works in collaboration with the Stanford Linear Accelerator Centre (SLAC) and Lawrence Livermore National Laboratory (LLNL) in the US on the development of a watercooled rotating wheel target design. The wheel consists of a titanium alloy (Ti 6%Al-4%V) disc 0. 4 radiation lengths thick and with a radius of 1 m which rotates at approximately 1000 rpm. A conceptual design for the target is shown below. Capture Optics The permanent magnet module prototype (built at Liverpool) shown in two halves. Positron beam pipe Target wheel Photon beam pipe The superconducting undulator module consists of an aluminium former into which has been machined two interleaved helical grooves with a period of 14 mm. Superconducting (Nb. Ti) 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 below. Further prototypes are currently under construction. Motor Vacuum feedthrough The permanent magnet undulator module consists of trapezoids of Nd. Fe. B magnets arranged to form rings with a dipole field on axis (illustrated below). Successive rings forming the undulator are rotated with respect to each other to give the necessary field. Field measurements for this prototype are ongoing. Photons incident on the target (e. g. from the synchrotron radiation produced in an undulator) produce electromagnetic showers of electrons, positrons and photons. The simulation on the left shows a square of the target with photons incident from the left side. The green lines show photons which have passed through the target. The (relatively few) red and blue lines show electrons and positrons. If the incident photons are circularly-polarised, then the outgoing positrons will tend to be longitudinally spinpolarised. Approximately 20 k. W of heat is expected to be deposited in the target during operation at the ILC. The heat will be dissipated by the water-cooling system whilst the rotation of the wheel will prevent any one spot on the target from overheating. Studies of heating, radiation damage, neutron activation and remotehandling systems are all on-going, and Liverpool will shortly begin constructing target prototypes. Spin Transport Polarised beams allow the structure of particle interactions to be probed more precisely than possible with unpolarised beams. The ILC baseline design specifies that the electron beam should be at least 80% spin-polarised. There is also a strong physics case favouring the use of a spin-polarised positron beam with a polaristion of approximately 60%. This degree of polarisation can be achieved by the helical undulator positron source described in the panel to the left. As part of the PPARC-funded LC-ABD (Accelerator Beam Delivery) project, Liverpool heads a group developing computer simulations that track the evolution of polarised beams as they travel through the ILC from the sources to the beam dumps. Damping ring simulations After production, the electrons and positrons pass through damping rings containing wiggler magnets which act to radiatively ‘cool’ the beams. The simulation to the left is an example of a calculation used to estimate how much of the beam polarisation is lost through radiative spin diffusion as the beam circulates in a ring. Bunch-bunch depolarisation As the bunches of electrons and positrons approach each other in the interaction region, their Coulomb fields perturb the spin orientation of the individual electrons and positrons. This depolarising effect is shown below for two different set of possible ILC beam parameters. In each case a bunch of electrons starts with 100% spin-polarisation which then evolves as the electrons approach a bunch of positrons. TESLA parameter s PINIT=1. 0 Before Interaction Spread in Polarisation During Interaction After Interaction low Q parameter s P =1. 0 INIT Before Interaction The Cockcroft Institute The University of Liverpool is the lead organisation in the newly formed national centre for accelerator science - the Cockcroft Institute. Liverpool’s partners in the Cockroft Institute are the Universities of Lancaster and Manchester, Daresbury Laboratory (CCLRC) and the North West Development Agency (NWDA).