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 H2 and D2), 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 C¹² (d,n)N¹³ and Cu⁶³ (d, 2n)Zn⁶³ for deuterons, and Ti⁴⁷ (p,n)V⁴⁷ and Cu⁶³ (p,n)Zn⁶³ for protons.