Electron-beam source development for magnetic field alignment on MFTF-B

For proper physics operation of the Mirror Fusion Test Facility (MFTF-B) tandem mirror experiment, alignment of the superconducting magnet coils is critical. A narrow, low-energy electron beam is used to trace out the field lines along the axis of the machine. Six crossed-wire detector assemblies are located at various axial positions. The crossed-wire detector is used to locate the electron beam and can trace out flux tubes up to 20 cm in radius. In this paper, the authors describe the design and prototype development of a 1 mA, 10 kV e-beam source used for this diagnostic. Dispenser cathodes are used due to their long lifetime operating at rather high current densities. Experimental results from a prototype gun operating in a 1 kG field are presented. In most cases, the beam diameter was found to match closely the limiting aperture on the e-beam source grid. Also, the effect of secondary-electron emission off the detectors is investigated. Data on the poisoning of dispenser cathodes is also presented