Ionic Conductivity of Highly Ionized Plasmas

When a cloud of highly ionized gas ejected by a plasma shock tube is made to travel across a constant magnetic field, an electromotive force is produced in the plasma in a direction normal to both the field and the plasma path. Using two probes facing one another this electromotive force has been measured with an oscillograph. Its maximum value was found to be proportional to the field and the probe separation. By taking the maximum probe potential for different values of the external resistance between the probes, the lowest value of the "resistivity of the plasma" as measured by a current entering and leaving it was obtained. The resistivity has been shown to be independent of the magnetic field, the collecting area, the separation and surface state of the probes. All experiments were made in hydrogen at a gas pressure between 0.5 and 5 mm Hg with a nearly critically damped current pulse of order 10⁴ amperes lasting for about 6-8 {μ}sec and fields 2000 gauss.The plasma resistivity between the probes was found to be of the order 1 ohm cm at a gas pressure of a few mm of Hg. This is about 100 times larger than the electronic resistivity of a fully ionized gas for currents circulating internally. The degree of ionization is thought to be high enough for interaction between charged particles to predominate. The measured values of the plasma resistivity agree with the results obtained from theory based on ionic conduction in the plasma which is here the necessary prerequisite for maintaining the electric neutrality of the moving plasma.From the measured probe voltage the flow velocity of the plasma was derived. Its variation with gas pressure agrees with shock wave theory.