Formation of hot plasma in strong magnetic fields during the breakdown of high-pressure gases in short gaps was studied experimentally. The breakdown occurs as a result of interaction of monoenergetic electron beam with dense plasma (n ∼ 6·1022 m−3). As a result of the spark channel formation and its explosive expansion inside the gap, the magnetic field in the anode and cathode regions becomes more intense, while between the electrodes it becomes weaker due to the expansion of the channel plasma. Thus, the axially symmetrical magnetic-mirror-type system is formed, which is similar to the magnetic trap. Formation of this system leads to the reduction of energy losses accompanied by a simultaneous increase in the plasma thermal energy, that is, to a considerable increase in its conductivity and temperature.