SLC positron source pulsed flux concentrator

SLC (SLAC Linear Collider) positron beams produced by a very high energy electron beam, impinging on a high Z target, have initially small transverse size but large divergence, a situation ill matched to the following S-band accelerator. The flux concentrator is an adiabatic matching device placed between the target and this accelerator, which trades divergence for size. It produces a magnetic field which rises sharply over less than 5 mm to its peak value, and then falls off adiabatically over 10 cm. It is a 12-turn, 10-cm-long copper coil with a cylindrical outside radius of 4 cm and a conical inside radius growing from 3.5 mm to 2.6 cm. The 0.2-mm gaps between the individual windings were manufactured by electric discharge machining out of one copper block. Excitation current and water cooling are provided by a hollow rectangular conductor brazed to the outside of the coil (also 12 turns).<>