Experimental results are presented on the formation and dynamics of a rotating energetic proton ring in a magnetic mirror field. An annular 430 keV proton beam from a magnetically insulated diode is injected through a cusp‐like magnetic field to form a rotating proton beam in a 2 m long solenoidal magnetic field. With 15–400 mT of neutral gas in the 40 cm diam experimental chamber up to 83% of the 350 J total beam energy is in rotation, the axial velocity dispersion is small, and the beam is sharply defined radially with inner and outer radii of 7 and 13 cm, respectively. The beam is 90%–100% axially current neutralized by currents induced in the beam‐generated plasma. Azimuthal plasma currents are observed in air, but not in hydrogen. In hydrogen a ring containing 5×1015protons is formed with sufficiently small axial velocity dispersion so that the protons are confined axially by their own 3% diamagnetic well as they propagate about 1.3 m in an 8 kG solenoidal field. In air, up to 15% of the beam axial energy is inductively coupled to the induced plasma currents, and ≳50% reflections are achieved from a 1.23 mirror ratio magnetic mirror located 2 m from the ion injector. About 1% inductive energy coupling to a wall resistor array was observed in both air and hydrogen. A peak ring diamagnetism of 875 G was observed 50 cm from the injector at a point where the applied field was 11 kG (near the peak of an upstream mirror); this ring contained about 1×1016protons.