In a Penning discharge in a magnetic mirror field with an axial mirror ratioBmax/Bmin = 40, a burning voltage as high as 20 kV with a current of a few amps is observed. Normally, the gas is hydrogen in the pressure range10−4‐10−3Torr. The classical mean free path is longer than or comparable to the dimensions of the device. From calorimetric measurements on the cathodes, the plasma density and ion energy in the cathode regions are estimated to a few times1010cm−3and a few kiloelectron volts, respectively. Magnetic loop measurements in the anode region indicate a plasma energy density in that region of1012‐1013eV/cm3. Neutral particles, emitted perpendicular to the magnetic field, with energies exceeding 4 keV, are observed with a scintillator. Measurements on a split anode show oscillations in the frequency range 3‐20 kHz. The aim of the experiment is to investigate the potential distribution in this type of low‐density plasma, in particular the electric field component parallel to the magnetic field, and to determine typical energies of the plasma particles.