Ion acceleration in a moving virtual cathode

When an intense electron beam is injected into an evacuated drift cavity placed in a strong longitudinal magnetic field, a virtual cathode will form if the beam current exceeds the space charge limiting current. If the drift tube is flared, the virtual cathode can form a substantial distance downstream from the injection plane, and will propagate backward toward the injector, depending upon the time dependent diode voltage and current waveforms. In a recent series of experiments a small amount of neutral gas was introduced into the drift tube and acceleration of gas ions was observed to occur, also in a direction backward toward the injection plane. The acceleration occurs over a very narrow pressure range (2-8 × 10
^-4
Torr for H
₂
and D
₂
), and can be entirely suppressed by reducing the magnetic field strength. Using a 2 MeV, 20 kA electron beam nuclear activation of thin (1 mil) anode foils has been observed for a number of different reactions, including C
¹²
(d,n)N
¹³
and Cu
⁶³
(d, 2n)Zn
⁶³
for deuterons, and Ti
⁴⁷
(p,n)V
⁴⁷
and Cu
⁶³
(p,n)Zn
⁶³
for protons.