OBSERVATIONS OF VIRTUAL CATHODE FORMATION AND ELECTRON-DENSITY PROFILE MEASUREMENTS IN AN INTENSE ION-BEAM DIODE

A recently developed high pulse rate intense ion beam diode has been e xploited to measure the relative density profile of electrons in the i on diode's acceleration gap as a function of time. This diode was magn etically insulated and it produced 100 keV Ar+ beams at up to 4 A/cm(2 ) using an active anode plasma ion source. The diode also had a backfi ll of 10(-2) Torr of He that was collisionally excited by energetic el ectrons in the acceleration gap. When the excited He atoms radiatively decayed, the emitted light was measured and used to deduce the relati ve electron density profile with space and time resolution. The profil e was peaked toward the center of the gap and dropped off significantl y toward both the anode and the cathode. The absence of electron densi ty near the physical cathode is evidence for magnetic field profile mo dification from the diamagnetic drift of the electrons in the diode. W ith a macroscopic electric field of order 100 kV/cm, the electron curr ent takes many tens of nanoseconds, from the moment the cathode begins to emit, to reach the Child-Langmuir current. The extent to which thi s limit is exceeded depends strongly on the profile of the insulating magnetic field and appears to be dominated by the dynamics of the elec tron flow in the diode gap. In contrast, the delay between when electr ons are first observed in the gap spectroscopically and when the ion b eam is first formed is independent of the magnetic field profile. This delay is about the time that it takes an argon ion to transit the acc eleration gap, suggesting that ion inertia is the rate limiting factor . (C) 1998 American Institute of Physics.