Diode magnetic field influence on radiographic spot size

Abstract

The Dual-Axis Radiography for Hydrodynamic Testing (DARHT) facility at Los Alamos was developed for flash radiography of large hydrodynamic experiments.Both axes now routinely produce radiographic source spot sizes having full-width at half-maximum (FWHM) less than 1 mm. The beam canonical angular momentum contributes to spot size, and it is minimized by using a shielded source of electrons.An ideal shielded source creates the beam in a region where the axial magnetic field is zero, thus the canonical momentum is also zero, since the beam is born with no mechanical angular momentum.It then follows from Busch's theorem that the canonical angular momentum is minimized at the target, at least in principal. In the DARHT accelerators, the axial magnetic field at the cathode is minimized by using a solenoid with reverse polarity to cancel out whatever solenoidal beam transport field exists there. This is imperfect in practice, because of radial variation of the total field across the cathode surface, solenoid misalignments, and long-term variability of solenoid fields for given currents. This article quantifies the relative importance of canonical momentum in determining the focal spot, and establishes a systematic methodology for tuning the bucking coils for minimum spot size.