An adjustable permanent magnet quadrupole (PMQ) final focus system for low energy experiments

The final focus system for the Thomson X-ray scattering experiment termed PLEIADES (Picosecond Laser-Electron InterAction for Dynamic Evaluation of Structures) at LLNL demands ultrahigh field gradient quadrupoles in order to focus initially small beams to 10-20 μm spot sizes. The scheme present here circumvents limitations due to chromatic aberrations and space-charge effects in this relatively low energy (<100 MeV) system. The final focus scheme is based on an ultrahigh gradient (≤300 T/m) quadrupole which employs the Halbach 16-piece, permanent magnet design. Use of this optimized geometry, NdFeB material, and a small (5 mm) bore allows the desired field gradient, and a few cm focal length, to be achieved. The adjustability of the focusing system is obtained by changing the relative longitudinal positions of subcomponent focusing and defocusing magnets on precision movers. We present the results of RADIA 3D design simulations, and also discuss the results of beam dynamics simulations of the PLEIADES system using the tracking code.