Observations of virtual cathode formation and electron density profile measurements in an intense ion beam diode
- Laboratory of Plasma Studies, Cornell University, Ithaca, New York14853 (United States)
A recently developed high pulse rate intense ion beam diode has been exploited to measure the relative density profile of electrons in the ion diode{close_quote}s acceleration gap as a function of time. This diode was magnetically insulated and it produced 100 keV Ar{sup +} beams at up to 4A/cm{sup 2} using an active anode plasma ion source. The diode also had a backfill of 10{sup {minus}2} Torr of He that was collisionally excited by energetic electrons in the acceleration gap. When the excited He atoms radiatively decayed, the emitted light was measured and used to deduce the relative electron density profile with space and time resolution. The profile was peaked toward the center of the gap and dropped off significantly toward both the anode and the cathode. The absence of electron density near the physical cathode is evidence for magnetic field profile modification from the diamagnetic drift of the electrons in the diode. With a macroscopic electric field of order 100kV/cm, the electron current takes many tens of nanoseconds, from the moment the cathode begins to emit, to reach the Child{endash}Langmuir current. The extent to which this limit is exceeded depends strongly on the profile of the insulating magnetic field and appears to be dominated by the dynamics of the electron flow in the diode gap. In contrast, the delay between when electrons are first observed in the gap spectroscopically and when the ion beam 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 acceleration gap, suggesting that ion inertia is the rate limiting factor. {copyright} {ital 1998 American Institute of Physics.}
- OSTI ID:
- 627890
- Journal Information:
- Journal of Applied Physics, Journal Name: Journal of Applied Physics Journal Issue: 10 Vol. 83; ISSN JAPIAU; ISSN 0021-8979
- Country of Publication:
- United States
- Language:
- English
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