Ion beam divergence due to plasma heating in the Ampfion diode

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Abstract
In the Ampfion diode, an ion beam is extracted from an anode plasma that is being pushed away from the cathode by a rising magnetic field. There is concern that ion heating in the anode plasma sheath may result in a significant ion beam divergence. To investigate this possibility, a comparison of Z pinches, θ pinches, and Ampfion is made. Ampfion sheath scenarios are examined and a simple argument is presented that indicates that an anomalous resistivity should develop. An upper bound estimate of the ion divergence is calculated, and examples are given for PBFA-II parameters. For a small radius diode with a high density anode plasma, it is shown that ion beam divergence due to plasma heating should not be a problem. For a large radius diode with a low density anode plasma, it is shown that ion beam divergence due to plasma heating is a problem, but that an Ampfion filtering mechanism should select a cool portion of the heated ions and help to alleviate this problem.
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