A Study of Mechanical and Magnetic Issues for a Prototype Positron A Study of Mechanical and Magnetic Issues for a Prototype Positron Source TargetSource Target

The positron source for the International Linear Collider (ILC) comprises a helical undulator and a rotating titanium wheel, which can generate unprecedented quantities of polarised positrons.

Daresbury Laboratory has produced a prototype target wheel, the data from which will allow benchmarking and validation of various magnetic and thermal modelling codes.

A design for a titanium target wheel that satisfies the requirements of the ILC baseline positron source has been developed, and benchmarking of magnetic simulations against real data from an engineered prototype is underway. In our simplified model, at 2000 rpm in a realistic field of 1.16T, calculations show that the wheel will be susceptible to maximum eddy current power losses of 3kW. The associated opposing force is 30 N, corresponding to a torque of 17 Nm.

Current results appear to indicate that a gaussian distribution is insufficient to describe the particle flow through the target, which could have a positive bearing on the survivability of the titanium wheel. However before firm conclusions can be drawn, the heat flow and the pressure wave have to be simulated with the full hydrodynamic model.

A possible layout of the ILC

Target Prototype

• The photon beam is incident on the rim of the titanium target wheel. Simulations show that ~8% 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 magnetic and mechanical simulations.

Proposed layout of the ILC positron source

Left: Drawing of ILC target wheel partly immersed in static magnetic field.

J.A. Clarke+, D.G. Clarke, K.P. Davies, A.J. Gallagher

STFC ASTeC Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK

J. Rochford, C. J. Nelson, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK

I.R. Bailey+, L. Jenner+, Department of Physics, University of Liverpool, Oxford St., Liverpool, L69 7ZE, UK

J. Gronberg, L. Hagler, T. Piggott, Lawrence Livermore National Lab, Livermore, CA 94551, USA

S. Hesselbach, Institute of Particle Physics Phenomenology, University of Durham, Durham DH1 3LE, UK

+ Cockcroft Institute, Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK

The eddy currents in the target wheel are calculated in Vector Fields Opera using the ELEKTRA rotational solver. In order to do this, the model has to be simplified from the real 5-spoked prototype (left) to a fully axially symmetric ring (right). The solenoidal coils (red) are approximately equivalent to the pole caps of the dipole magnet being used in the prototype tests.

Above: A typical output from the Opera simulation. The wheel rim is shown in grey and the field strength on the z-axis is shown as a coloured contour plot overlaid. The peak field strength is 1.63T.

Above right: The calculated values for the torque expected at incremental rotation rates, benchmarked against real copper disk measurements performed at SLAC. The good agreement of simulation and experiment gives confidence to its predictions for the titanium wheel which has a corresponding conductivity of 9.26 x 105 S/m.

Right: Simulations for the eddy current power loss in the titanium prototype wheel at various angular velocities and with varying immersion into the magnetic field. As expected, the further the wheel is into the field and the higher the speed of rotation, the greater the power loss.

Left: During the operation of the target wheel at the ILC, the rapid energy deposition from the photon beam will cause pressure shock waves which may damage the target.

Initial simulations by LLNL showed that these stresses would be within acceptable limits but more recent studies at Cornell have shown greater negative pressures associated with the shockwave on the downstream face of the target, assuming a gaussian distribution of energy deposition.

The simulation was cross-checked against a FLUKA simulation at Durham which showed that the energy was deposited in a considerably greater volume than the simple gaussian model predicted.

Right: ILC Target wheel parameters.

* relates to a solid disk design