Improvement in breakdown uniformity in coaxial vacuum gaps using electrode coatings and external magnetic fields

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Abstract
Vacuum gap breakdown uniformity in coaxial systems at high voltages and currents remains a critical topic for pulsed power applications. This is especially relevant as new drivers for inertial fusion energy are developed, which will generate significantly higher electrical power than present machines. We present experimental investigations performed on the COBRA generator at 1 MA, examining two methods to force uniform breakdown and current density distribution in a coaxial system: cathode surface coatings and the application of external axial magnetic fields. A series of coatings, gold, copper, and graphite (Aerodag-G), were applied to aluminum cathode heads, each introducing distinct work functions and surface morphologies. It was found that copper and gold coatings delayed breakdown initiation relative to bare aluminum, while graphite coatings accelerated plasma formation and produced the first observed fully filled coaxial gap, correlating to a symmetric current density profile. Surface scanning electron microscope imaging and plasma behavior indicated that uniformly spaced, conductive surface microstructures enhanced material-assisted breakdown and rapid gap fill. Separately, external magnetic fields of up to 1.9 T were applied during breakdown, demonstrating that increasing magnetic field strength enhanced azimuthal plasma spread. Optical diagnostics revealed that plasma expansion velocities increased with magnetic field strength and that an axial field induced retrograde motion of breakdown sites, consistent with J x B-driven plasma dynamics. These combined approaches suggest that surface engineering and an applied magnetic field can control breakdown symmetry and current density distribution in coaxial vacuum gaps, offering direct relevance to designs such as MagLIF on Sandia's Z-machine.
Funding Information
  • National Nuclear Security Administration (DE-FC03-02NA00057)
  • Sandia National Laboratories (2004104)
  • National Nuclear Security Administration (DE-NA0004148)

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