On Jun 26, 2007, at 11:15 AM, Michel Jullian wrote:
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About the low current phenomenon, it occurs to me that a
sufficiently low current ion stream, where the ions would form a
clearly discrete dotted line rather than a continuous-looking
stream, would not expand sideways by self repulsion as we have been
assuming all along. Each ion would just follow the previous one at
comfortable distance, only sigzaging slightly along the line of
maximum field while it collides with neutrals every micron or so...
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It just doesn't make any sense to me. The ions should mutually
repel. If an ion is repelled sideways slightly it lies on a new
field line just as good and equally as "followable" as the one it was
on. The field lines diverge from the tip to the plate. There is no
focusing effect. The principle steering effect, aside from following
the field lines, is momentum. If Bill's estimate of about 10 MPH
speed is right, then momentum is not playing all that much role. Ion
beams can be self focusing, but that requires fairly high beam
currents and relativistic speeds, if I recall correctly, and
diverging field lines make that even more difficult. It may be that
the needles are making a significant focused neutral wind, and the
momentum of that wind carries the ions in a fairly narrow beam for a
while.
Regards,
Horace Heffner
