Virtual cathode ion acceleration behind the anode foil of a pinched-beam ion diode

In a cylindrical pinched-beam ion diode¹ the electron beam focuses to a tight spot on the axis at the center of the anode and passes through the thin anode foil into a beam dump region behind the anode foil. The beam dump usually consists of a cylindrical anode-can with roughly the same radius R as the diode and a depth L. Because of energy deposition from the intense electron beam, the interior surfaces of the anode-can are expected to be space-charge-limited emitters. Therefore the electron space charge in the anode-can will draw ions off these surfaces. There is evidence from nuclear activation which suggests that ions exist in the anode-can with energies that exceed those associated with the diode voltage.²