The guide field for a low inductance (55×10−9H) air‐core plasma betatron is constructed by placing flux‐concentrators within the interior of a single‐turn solenoid. The betatron field coil is energized by the discharge of a 75 &mgr;F, 20 kV capacitor bank. The quarter period of the field‐coil capacitor bank system is 4 &mgr;sec and the peak magnetic and electric fields at the equilibrium orbit (r0=4.83 cm) are ≈3250 G and 60 V/cm, respectively. Techniques used to make accurate (±0.1%) measurements of transient magnetic field distributions are also described.