Diode magnetic field influence on radiographic spot size

TL;DR

This paper investigates how magnetic fields influence the radiographic spot size in the DARHT facility, emphasizing the role of canonical momentum and proposing a systematic method to optimize coil tuning for minimal spot size.

Contribution

It quantifies the impact of magnetic fields on spot size and introduces a systematic approach for tuning bucking coils to minimize canonical momentum effects.

Findings

Canonical momentum significantly affects spot size.

Systematic coil tuning reduces spot size variability.

Optimized magnetic field configuration improves radiographic resolution.

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.