[en] The stability of a low-pressure discharqe with a closed drift of magnetized electrons in a cylindrical geometry (as applied to instruments of the type of direct and inverted magnetrons) is discussed. It is shown that with an increase in pressure above a certain critical value the discharge becomes unstable with respect to long-wave perturbations with an oscillation period greatly exceeding the ion time-of-flight. The effect of the geometry on the instabilitY criterion is determined and properties of excited oscillations are considered. The calculation results agree with experimental data and explain the cause of the breakaway and rearrangement of the discharqe, which have been observed in indicated instruments