At low pressures, there is a rotating electron current of about 1 A symmetrical to the axis of the
anode cylinder and perpendicular to the magnetic field (grey area in Fig. 6). Between this electron
space charge and the anode there is a strong electric field and almost all of the full voltage drops
between the space charge and the anode cylinder. For this reason the electrical field gets a strong
radial component. Close to the axis of the cylindrical anode a plasma with equal negative and positive
charges exists. The electron ring current would be completely stable, if no gas molecules were there.
The electrons interact with them in two ways. They hit them with small energy and diffuse out of the
electron space charge or they hit them with higher energy and ionize the molecule. In the latter case
the new electron will be incorporated in the ring current, the ion will be accelerated by the electrical
field and finally reach the cathode. Since both the ionization rate and the diffusion effect (diffusion
coefficient) are proportional to the gas density n, in equilibrium the ring current will be such that the
loss of electrons by diffusion is compensated by the generation due to ionization. This means that the
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ring current will be roughly independent of n and p. For this reason, the outer discharge current will be
proportional to n and p. The fact that the ring current does slightly increase with n has as consequence
that m in Eq. (2) is > 1.