  Vacuum arc thruster performance in a magnetic nozzle configuration is
experimentally characterized. Measurements are performed on a miniature coaxial
thruster with an anode inner diameter of $1.8$ mm. The magnetic field B is
produced by a single air coil, $18$ mm in diameter. Direct measurement of
thrust, mass consumption and arc current are performed. To obtain statistically
viable results $\approx 6000$ arc pulses are analyzed at each operational
point. Cathode mass erosion is measured using laser profilometry. To sustain
thruster operation over several measurement cycles, an active cathode feeding
system is used. For $0< B \leq 0.2$ T performance increase over the
non-magnetic case is observed with the best thrust to arc power ratio $T/P
\approx 9$ $\mu$N/W obtained at $B \approx 0.2$ T. A parametric model is
provided that captures the performance enhancement based on beam collimation
and acceleration by the magnetic nozzle. For $B > 0.2$ T the arc discharge is
shown to be suppressed nullifying any additional gains by the nozzle effect.
