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

An intense electron beam in a solenoidal axial magnetic field rotates azimu thally, and generates a diamagnetic axial field that counteracts the applie d field. However, the magnetic flux within the beam pipe is conserved over sufficiently short times, and this induced diamagnetic field must lead to a n 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 tra nsverse beam size. However, if the axial magnetic field has sinusoidal comp onents, measurement of the diamagnetic field at two axial positions yields both the beam's rms transverse size and the beam's fourth radial moment. Co mparison of these moments can give a figure of merit of the beam's density uniformity. (C) 1999 American Institute of Physics. [S0034-6748(99)00708-X] .