Using the induced axial magnetic field to measure the root mean square beam size and beam density uniformity of an electron beam in an induction linac

Abstract

An intense electron beam in a solenoidal axial magnetic field rotates azimuthally, and generates a diamagnetic axial field that counteracts the applied field. However, the magnetic flux within the beam pipe is conserved over sufficiently short times, and this induced diamagnetic field must lead to an increase in the axial field outside the beam, which is measurable with a loop or an optical Faraday rotator. If the applied axial magnetic field is uniform across the beam pipe, the measurement directly leads to the rms transverse beam size. However, if the axial magnetic field has sinusoidal components, measurement of the diamagnetic field at two axial positions yields both the beam’s rms transverse size and the beam’s fourth radial moment. Comparison of these moments can give a figure of merit of the beam’s density uniformity.