The d.c. properties of an alkali plasma diode

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Abstract
The theoretical d.c. current-voltage characteristics of a `collisionless' alkali plasma diode are derived. It is assumed that electrons and ions are created only at the two incandescent plane electrodes, the latter being at the same temperature. The electrons and ions are assumed to leave the electrodes with half-Maxwellian distributions of velocities. Some ions are trapped in the potential well between the electrodes. It is found that the potential distribution is associated with a double sheath in front of the negative electrode, a plasma region of constant potential and an electron sheath at the positive electrode. The theoretical (I, V) characteristics are compared with experimental results obtained with a caesium plasma. At low applied voltages good agreement is obtained between theory and experiment; at higher voltages (where oscillations occur) the d.c. current falls to a lower value. Possible mechanisms for this drop are described.