My (possibly wrong) hypothesis is that the field is so low that the ions only emanate from the highest field point of the tip, so they just follow the particular field line which starts from there and ends at the nearest point of the plate, and that there is so much space between successive ions that they don't repel much laterally. We don't really know the value of the current, so for what we know there might be enough distance between one ion and the next for space charge effects to be negligible compared to electrode charge effects.
The derivation of the classical ~60° fanning out (Warburg law) space charge limited current distribution in point to plane corona is based on a continuous space charge model. This model obviously breaks down somewhere between an ion saturated gap (space charge in the way -> fanning out) and one ion at a time in the gap (no space charge -> shortest route), so a significant narrowing of the ion path distribution when current tends to zero may not be totally nonsensical. Michel ----- Original Message ----- From: "Horace Heffner" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Tuesday, June 26, 2007 10:23 PM Subject: Re: [Vo]:Air threads > > On Jun 26, 2007, at 11:15 AM, Michel Jullian wrote: > > >> ---------- >> 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... >> ---------- > > 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 > > > >
