Septum Dump Quadrupole Measurements and Analysis

Abstract

The purpose of the septum dump quadrupole (SDQ) magnet is to expand the beam size on the septum dump of the DARHT Axis 2 downstream transport. This is necessary in order to minimize the heating of the carbon beam dump and suppress desorption of ions. Initially, ions are generated through stimulated desorption and ionization of the monolayers of water vapor stuck to the beam dump surface. These ions propagate upstream at high speeds and act as an electrostatic focusing lens by reducing the beam space charge. As the beam size on the dump decreases, the dump is sufficiently heated so that a surface plasma is formed and much greater quantities of ions are generated through space charge limited emission. The interaction of these ions with the beam causes significant disruption of the beam. Because the SDQ focuses the beam in one plane and defocuses the beam in the orthogonal plane, it is critical that the beam in the focusing plane be over-focused so that the beam size in this plane is large on the beam stop. Thissets a minimum field at which the SDQ must operate. Recent magnetic measurements of the SDQ show that the actual field is about 15% lower than the design field. This note investigates the possible causes for this discrepancy and a path forward.