From: Bert Hickman
Date: Fri, 01 Mar 2002 07:41:09 -0600
Subject: Re: [TSSP] Ion Cloud Loading
Paul wrote:> > Greg wrote: > > I suspect that it's the additional capacitance of the streamer > > channels themselves which are actually responsible for any > > measured tuning shifts. > > And I'm guessing that Bert would concur. The streamer channels, > rather than a cloud of ionised gas molecules, would be the > major source of an 'upheld' charge. Yes. The top terminal is connected to what could (grossly) be called a lossy capacitance. The hot, arc-like, leader can be modeled (as a first approximation) as a resistive stick projecting outward, possessing its own additional incremental inductance and capacitance. A fine spray of much poorer conducting streamers fans outwards from the tip of the leader, serving to transfer charges to or from a larger region outwards from the leader tip once localized E-Fields become strong enough to initiate electrical breakdown at the leader tip. These are the little discharges that last only for nanoseconds or tens of nanoseconds. As breakdowns occur in the streamer region, charge then transfers into/out of the surrounding "space charge" region. The change in distribution of charge immediately after a discharge has the effect of reducing the E-field stress that initiated it, choking off (temporarily at least), further charge transfers to that particular region. However, similar breakdowns/charge transfers occur to other nearby regions if the potential at the leader tip can be sufficiently maintained (i.e., terminal voltage maintained or increasing and leader channel kept hot via Joule heating). The overall displacement current at the root of the leader is the sum of the individual dq/dt's coming from countless short-lived streamers as they very quickly dissipate major differences in charge between the leader tip and the surrounding region. Additional displacement current are also driven by terminal voltage changes charging/discharging the leader's isotropic capacitance. In Greg's coil we can see excellent evidence of VHF oscillations as the top turns on the resonator and topload/leader capacitance was shock excited by current spikes during leader propagation. On a very gross time scale (1/Fo), the leader-streamer system should sort of "look" like a lossy capacitor, with much energy being lost in initiating and maintaining the conduction processes that allow us to inject/remove charge from regions remote to the top terminal. Unlike a normal capacitor, this model relies upon dielectric breakdown to function properly... > > Oh, and while we're putting kV of DC into the coil base, would > this give a nice and accurate method for estimating the total Cdc > of the secondary? Charge to a few kV, disconnect the Bertan, and > then time the voltage decay as it leaks back to ground via a > known shunt resistance? Sounds like a good idea! > -- > Paul Nicholson, > -- -- Bert Hickman Stoneridge Engineering Email: bert.hickman@aquila.net Web Site: http://www.teslamania.com
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.