The performance of a 15-cm-radius applied-magnetic-field ion diode was investigated on the PBFA II accelerator at a power of 23 TW. The power coupling between the accelerator and diode was measured and compared with numerical simulations that show the effects of the electron flow in the MITL. The power coupled to the cathode of the diode was 18 MW. Measurements of the lithium beam generated from an electric-field-emission LiF anode showed a lithium beam power of 9 TW. The lithium beam was ballistically focused in a gas cell filled with 2 torr argon. The resultant focused power density was ∼1.8 TW/cm2 equivalent on a cylindrical target at the centerline of the diode. The focused power was limited by the 20- to 30-mR divergence of the beam caused by the LiF source used and by virtual cathode instabilities in the anode–cathode gap. The ion mode instability in the virtual cathode was studied extensively by measurement of waves in the ion emission pattern from the anode and of the E-P0 correlation between variations in the beam energy and transverse momentum. The instability Played a dominant role in the limitation of the focused lithium power.