Design and Performance of a Permanent Magnetic Quadrupole for a Low Energy Linear Accelerator Beam Line

Permanent magnets which show the highest magnetic flux density, have been used in constructing an insertable Permanent Magnetic Quadrupole (PMQ). The PMQ is part of an electron irradiation facility for polymer research at the Eindhoven University of Technology. For polymer irradiation that requires a homogeneous dose distribution, the PMQ is inserted and the expanded electron beam will irradiate the target completely. Design criteria of the quadrupole are discussed. The quadrupole geometry has been optimised using CEDRAT finite element software. The influence of mechanical alignment errors (0.15 mm) and variations in permanent magnet properties (0.5%) on the magnetic field have been simulated. Sixteen NdFeB magnets (42x42x10 mm3) have been used to produce a quadrupole with an aperture radius of 50 mm. Before insertion, the magnetic flux density of all magnets has been determined versus magnetic field and temperature. After construction the lens strength of the quadrupole has been determined using the floating wire technique. The flux density has been measured using a Hall probe. Results show a magnetic field gradient that varies less than 0.5% within a radius of 25 mm. Alignment errors have been determined comparing simulation and measurement