Symmetry Solutions

[markus7]

Henning wrote:

EDIT: I wonder if such an arrangement of 16 “spark plug plasma jets”, in addition to improving current sheath symmetry, might make it possible to eliminate using the main capacitor switches to trigger pulses. That is, the 16 outer cathode rods would, while running, be connected to the capacitors, and the “spark plug plasma jets” would take over the function of the vacuum switches, making them redundant. The function of the vacuum switches would be part of the fusor unit.

Generally a good idea, but I think the electrodes will shorten before the target voltage is reached, i.e. distance between cathode and anode are too short to hold back the sparks, as we try to reduce the diameter of the DPF and increase the amperage of the pulse (which goes along with the voltage). Anyone an idea how to reduce voltage and increase amperage? Well, by reducing inductance, which is somewhat one of the main goals…

Yes, after I posted, it occurred to me that glow or even spark discharges prior to reaching desired anode potential voltage could be a problem even for cold test hardware – single shot hardware as is now being tested. A real fusor running at a few hundred hertz would doubtless be so hot that glow and spark leakage would be entirely unacceptable. So I expect we would be stuck with the external vacuum switches even if it was found to be useful to install internal spark plug plasma generators to improve symmetry in the initial current sheath.

But that brings up a question. Could the cold shot tests of LPP’s fusor be providing misleading data? I invite you to comment on my other post “Would pre-heating LPP’s test chamber produce higher quality results?”

Of course, my interest in this second question is driven in part by the idea that the existing cold shots may be providing pessimistic results and neutron production will go up with a pre-heated fusor. (Maybe just heated by a tubular electric heater embedded in the cathode base plate and let copper’s conductance take care of heat distribution.)

Regarding your question about how we might increase shot amperage at a given voltage (mainly by reducing inductances?), I don’t know enough about the details of the existing setup to usefully comment. Is there any existing descriptive material (maybe even with present estimates of inductances) you could point me to?

[TimS]

markus7 wrote: Could you explain what you mean by “add a slight inductance at the base of the main electrodes”?

“Inductance is the property of an electrical circuit causing voltage to be generated proportional to the rate of change in current in a circuit.”

A current divided in N paths may not divide evenly if the paths are not symmetrical. However, by adding an equal impedance to each path, the current will divide more evenly. An series inductance is an impedance at high frequencies. This is a bad idea for the main cathodes for the reason stated by the previous poster- for a maximum current per voltage the cathode inductance should be reduced.

However, if it is desired to use the tungsten pins instead of the knife edge, it might help to add inductance to the pins. The main current is through the main cathodes, not the pins, so decreasing pin current would not be as bad. Inductance in the pin effectively filters out the higher frequences of current change through the pin, so that it would take time for a current path to build up from the first firing pin to the anode, and by then the current might have started from the other pins. The inductance of each pin could be increased by bending it into a spiral.

[markus7]

TimS wrote:

Could you explain what you mean by “add a slight inductance at the base of the main electrodes”?

“Inductance is the property of an electrical circuit causing voltage to be generated proportional to the rate of change in current in a circuit.”

A current divided in N paths may not divide evenly if the paths are not symmetrical. However, by adding an equal impedance to each path, the current will divide more evenly. An series inductance is an impedance at high frequencies. This is a bad idea for the main cathodes for the reason stated by the previous poster- for a maximum current per voltage the cathode inductance should be reduced.

However, if it is desired to use the tungsten pins instead of the knife edge, it might help to add inductance to the pins. The main current is through the main cathodes, not the pins, so decreasing pin current would not be as bad. Inductance in the pin effectively filters out the higher frequences of current change through the pin, so that it would take time for a current path to build up from the first firing pin to the anode, and by then the current might have started from the other pins. The inductance of each pin could be increased by bending it into a spiral.

Oh yes. I see. Even if a fair number of turns had to be added, it sounds like an idea worth exploring. It is not difficult to imagine, say 16 pins whose bases were little free standing, open coils.

[willit]

would it be worth investigating a pre ignition system for each pin by a small plasma current path that is pre initiated before the shot?
rather than hoping that each rod successfully establishes a filament, create the filament. the main shot would follow the small plasma line to each rod ensuring symmetry, it may in fact relieve the hat insulator from being subject to high currents along the sides.
it would also be useful in turning on or off a number of rods to find a suitable combination. a high voltage flyback transformer for each pin could easily control a small streamer with a small insulated electrode close to the center electrode. just a thought.

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