  The low-frequency instability of a cylindrical poorly magnetized plasma with
an inward-directed radial electric field is studied changing the gas pressure
and the ion cyclotron frequency. The unstable frequency always decreases when
the gas pressure is increased indicating collisional effects. At a fixed
pressure, the unstable frequency increases with the magnetic field when the
B-field is low and decreases at larger magnetic field strength. We find that
the transition between these two regimes is obtained when the ion cyclotron
frequency equals the ion-neutrals collision frequency. This is in agreement
with the theory of the slow-ion drift instability induced by the collisional
slowing of the electric ion drift (A. Simon, Phys. Fluids 6, 382, (1963).
