From: Paul
Date: Sat, 23 Jun 2001 15:00:45 +0100
Subject: Re: [TSSP] Some Considerations
Malcolm wrote: > any msmt must be "central" to the resonator axis to avoid distorting > the electric fields Would also distort the field when on the axis of the coil. If removed to a distance, then the lead-out wire would add its distortion instead. We will have to accept some field distortion somewhere. > ...may be checked for disturbance by F and Q msmts before and after. > I don't see why doing such checks at low levels would be a problem as > long as there is no corona n the probe at normal running voltages. Agreed. A couple of small signal measurements would provide sufficient data to model the probes' effect. By incorporating the probe into the model in this way, we make its presence 'official'. However, if we want to use the topvolts probe to confirm that a particular toroid breaks out at the voltage predicted by Terry's program, then the probe itself must not introduce a local field gradient anywhere exceeding the max surface gradient of the toroid. Hmm. The advantage, if any, to a axial probe (either above the coil, or inside it) would be the relative ease at which the modified field can be calculated if necessary. Dealing with brush discharge impedance measurements would presumably be much easier because the properties of the discharge would be dominated by the local field around the discharge point and the probe would be just an extra constant RC load. > Top voltage vs breakout ... single shot mode ... The predicted breakout voltage, derived on the basis of 26kV/cm surface field, would apply only to the first shot of a run (and so would be independent of BPS), so I agree that single shot measurements are necessary. Given a satisfactory result from the single shots, it would be interesting to measure by how much the breakout voltage is reduced on subsequent shots of a run. Presumably after sufficient shots, the breakout voltage will have fallen as low as it's going to go, which means that a given topload is characterised by two numbers - the initial breakout voltage, and a lower, 'continuous run' breakout voltage. Associated with this latter figure would be a max streamer length. Digressing slightly, has anyone ever illuminated a breaking-out topload with a projector lamp, casting the light onto a screen. Streamers would be rendered totally invisible but the hot, low density pathways which accumulate over a series of bangs might be rendered visible as shadows on the screen. Marc wrote: > how does a poly tube filled with salt water or a conducting > fluid and graphite plugs in the ends sound? Sounds like a water divider. I made one of these a couple of weeks back, but I've not got around to trying it yet. Used de-ionised water in mine - a 1.5 metre by 1/4" I.D. polyethylene tube, with 1/4" jack plugs a tight fit in each end to form electrodes. It's DC resistance is around 50 Meg but I've yet to measure the AC response. 50 Meg is quite a low resistance to hook onto the top of a TC, but if it is stable enough then, as Marco says: > we can measure it with a KNOWN load and then "model out" its effect. Malcolm wrote: > It seems to me that this is a chicken and egg situation. We want to > verify models so incorporating a model as part of the verification > process seems self-defeating. We can manage so long as the topvolts probe is stable and linear, ie no breakout from the probe itself as the voltage is turned up. This is because we are directly measuring the topvolts, rather than deducing it from say, a base current measurement. If all we had was a base current reading, then we could not deduce the top volts without already knowing the load impedance - that's the chicken and egg loop - a loop which is broken by going for the topvolts directly. I think that to strive for a non-perturbative topvolts probe would raise problems in trying to accurately register the minute trickle of current through the probe. At the other extreme, a divider which draws too much current from the coil top (either reactive or lossy) may prevent the coil from reaching the topload breakout voltage that we want to measure. The output Z of the coil is Q/(2.pi.f.Cee) which for a 100kHz coil with a Q of 200 and a Cee of 40pF (a fairly large coil, eg 10" by 30") then Zout is around 8 Meg. If we could aim for a probe resistance of at least 10 times this value, then we could obtain 10% accuracy even if we didn't bother to 'model out' the probe load. Malcolm wrote: > I'd run with the capacitive divider idea as being cleaner than a > resistive chain (which would also have some capacitance). Question > then is: how much additional capacitive loading can we tolerate and > still claim a good measurement? >From the modeling point of view, I think we can accomodate some quite large probe capacitances. The potential difficulty comes when the charge stored by the probe becomes a noticeable fraction of the total and therefore would begin to have an effect on streamer formation. In connection with this, Greg Leyh wrote, (forwarded by Bert): > The capacitor stack that I placed in my coil was not for measurement > purposes, but rather to see if increasing the top capacitance improves > the arc performance. Would be interesting to know the outcome of that experiment. The idea that Greg proposes, ie a capacitive divider formed by the toroid and some kind of pickup plate suspended above it, is basically a suspended field probe as was suggested earlier. Would only work with toroids wide enough so that the pickup plate cannot see the coil charge. To summarise my views: a) We can accomodate a modeled-out probe resistance of resistance greater than say, 10 times the output resistance, and 1/10th the total topload capacitance, without any worries, even if it is a little non- linear. We can probably take account of a probe which applies a heaver load than this, providing it is linear, stable, and does not prevent the topload reaching breakout. b) Capacitive dividers seem to be high up on everyones list of suggestions. Maybe a different approach is required depending on whether we're measuring the breakout voltage of a certain toroid, or the voltage behind a CW brush discharge. The E-field pickup of the toroid field is one extreme kind of capacitive divider, but its calibration may be affected by streamer formation in a way that a remotely placed divider column would not be. c) For determining whether Terry's program will correctly predict top- load breakout voltages, I think a calibrated accuracy of 5% would be enough to give a good confidence in the predictions. For determining brush discharge impedances, the important thing is linearity, ie freedom from internal corona loss within the divider, since the impedance is arrived at by considering the departure from linearity of the top volts as the power is raised. Cheers All, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.