I have an interesting question that's been racking my brain. What is the capicatance of a fusor at a given vaccum? I know the higher the vaccum the better the dielectric. Considering the fusor is basically two pieces of metal seperated by a vaccum there had to be a rating. Could the fusor be used as a big capacitor? I don't know for what, but I think it would be interesting. What value of energy would it store and can the capacitance of a fursor effect it's performance? I would go on and on with more questions but it' bed time....
Lee
Fusor Capacitance
Re: Fusor Capacitance
Generally speaking, vacuums aren't great energy storers. As a matter of fact, the system of dielectric constants is based on a hard vacuum, which is assigned a value of 1. Air at 1atm basically has a dielectric constant of 1 as well.
Three factors basically determine the capacitance of a simple two-plate capacitor: surface area of the plates, thickness of the dielectric, and the dielectric constant of the dielectric.
For that same two plate cap, those factors are related by the following formula:
C = (0.224 * K * A / d)
where C is capacitance in picofarads, K is the dielectric constant, A is the area of the plates, and d is the distance between the plates.
Consider a 6" spherical two grid fusor with a 6 wire, 1" diameter inner grid using .025" diameter grid wires.
The distance between the inner and outer grid is 2.5",
The crossectional surface area of the inner grid is approx 0.46 sq in.
(the outer grid would have a larger surface area, but we must use the plate of smallest surface area the larger plate can't couple to area that ain't there)
The dielectric constant of the vacuum is about 1, so now we can work our formula to get a general idea:
C = (0.224 * 1 * 0.46 / 2.5)
C= about 0.26pf.
This value seems like a pretty reasonable approximation, and shows that very little energy is stored capacitively in the fusor. As to whether or not a fusor's C might affect its performance in some way, it might, but there are tons of other things that seem to have greater effect.
Adam Parker
Three factors basically determine the capacitance of a simple two-plate capacitor: surface area of the plates, thickness of the dielectric, and the dielectric constant of the dielectric.
For that same two plate cap, those factors are related by the following formula:
C = (0.224 * K * A / d)
where C is capacitance in picofarads, K is the dielectric constant, A is the area of the plates, and d is the distance between the plates.
Consider a 6" spherical two grid fusor with a 6 wire, 1" diameter inner grid using .025" diameter grid wires.
The distance between the inner and outer grid is 2.5",
The crossectional surface area of the inner grid is approx 0.46 sq in.
(the outer grid would have a larger surface area, but we must use the plate of smallest surface area the larger plate can't couple to area that ain't there)
The dielectric constant of the vacuum is about 1, so now we can work our formula to get a general idea:
C = (0.224 * 1 * 0.46 / 2.5)
C= about 0.26pf.
This value seems like a pretty reasonable approximation, and shows that very little energy is stored capacitively in the fusor. As to whether or not a fusor's C might affect its performance in some way, it might, but there are tons of other things that seem to have greater effect.
Adam Parker
Re: Fusor Capacitance
Thank-you for your insight and calculations. The fusor is a very interesting little device. It's amazing how many scientific principles apply. I guess it would take a little experimentation to discover them all and of course to figure the effects of it's small C rating...
Lee
Lee
-
Richard Hull
- Moderator
- Posts: 15027
- Joined: Fri Jun 15, 2001 9:44 am
- Real name: Richard Hull
Re: Fusor Capacitance
Actually, the capacitance is not related to the wire cross section or area. The capacitance is a bit more due to the effective "sheet" capacitance of the inner sphere. For calculation, the area of the inner sphere would be used. The net capacitance is a few PF due to this larger area. Energy is stored, but it would never be a significant amount (microjoules).
Richard Hull
Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
-
guest
Re: Fusor Capacitance
It might be interesting to use the capacitance to figure out an optimum frequency for radio pumping; That is to see what effects setting up a standing wave system within the fusor cavity would have. As I've yet not begun building a fusor, seems I have some work to do...
-
guest
RF matching networks and such...Re: Fusor Capacitance
Direct observations-
Frequency power & loading perturb distribution of RF plasma sheath. Variable inductive & capacitive loading alters standing wave ratio, w/ more or less power ionizing gas. Across a fairly wide frequency range, at similar SWR and input power, uniformity of plasma can vary. Uniform dicharge can be obtained from even a crude device.
view.php?site=fusor&bn;=fusor_images&ke ... 36339&last; (BROKEN LINK)
With some adjustment, significant plasma nonuniformity can be observed.
Cheers
R
Frequency power & loading perturb distribution of RF plasma sheath. Variable inductive & capacitive loading alters standing wave ratio, w/ more or less power ionizing gas. Across a fairly wide frequency range, at similar SWR and input power, uniformity of plasma can vary. Uniform dicharge can be obtained from even a crude device.
view.php?site=fusor&bn;=fusor_images&ke ... 36339&last; (BROKEN LINK)
With some adjustment, significant plasma nonuniformity can be observed.
Cheers
R
Re: RF matching networks and such...Re: Fusor Capacitance
Capacitance of the Fusor, by which we must mean the capacitance between the inner wire cage, and the inside of the spherical outer shell will be a fairly complicated calculation, but why not simply measure it using a Digital voltmeter with the capacitance measuremnt function.
I am quite certain Richard Hull has correctly estimated the capacitance as just a fraction of pF.... which might possibly be beyond the range of the digital meter. But you could us a Capacitance bridge for more significant figure.
As I recall, the capacitance of an isolated spherical shell is 4pi x Epsilon x r , wher r is the radius of the shell. The fusor shell is only about 3-4 radii away, but the inner electrode is not a shell, only a cage. The projected area of the wire cage will be approximately equal to 2pi x r x wire dia. for each closed loop, which is quite a bit smaller than an entire spherical surface.
Probably not enough to be bothered about for energy storage.
As to resonant frequency, one needs to calculate the inductance of the wire cage in the larger spherical shell,,, also not an easy job, but surely in the micro to sub micro Hy range, making resonant freqs rather high.
Dave Cooper
I am quite certain Richard Hull has correctly estimated the capacitance as just a fraction of pF.... which might possibly be beyond the range of the digital meter. But you could us a Capacitance bridge for more significant figure.
As I recall, the capacitance of an isolated spherical shell is 4pi x Epsilon x r , wher r is the radius of the shell. The fusor shell is only about 3-4 radii away, but the inner electrode is not a shell, only a cage. The projected area of the wire cage will be approximately equal to 2pi x r x wire dia. for each closed loop, which is quite a bit smaller than an entire spherical surface.
Probably not enough to be bothered about for energy storage.
As to resonant frequency, one needs to calculate the inductance of the wire cage in the larger spherical shell,,, also not an easy job, but surely in the micro to sub micro Hy range, making resonant freqs rather high.
Dave Cooper
-
Richard Hull
- Moderator
- Posts: 15027
- Joined: Fri Jun 15, 2001 9:44 am
- Real name: Richard Hull
Re: Fusor Capacitance
If one re-reads my original post above, I noted that the capacitance would be a few PF. Perhaps, even over 10pf. The wires on the inner grid really don't matter electrostatically, it is a closed perfectly spherical sheet of metal for Capacitance measurments at DC up to many megahertz. The surface area of the inner grid as if it were a ball is used in simple capacitance calculations.
There is virtually no energy storage capacibility in the capacitance of the device, as it is so small.
Likewise, there is no real inductance of the inner grid wires unless you are in the microwave region.(wavelength comparable to the wire openings.)
If in the many gigahertz range, the inner grid is a nightmare of multi-interactive tuned and sub-tuned circuits.
One of Farnsowrths most interesting inner spehrical grids was assembled from a plethora of doughnut shapes flat rings of equal size. I don't think it went anywhere though.
Richard Hull
There is virtually no energy storage capacibility in the capacitance of the device, as it is so small.
Likewise, there is no real inductance of the inner grid wires unless you are in the microwave region.(wavelength comparable to the wire openings.)
If in the many gigahertz range, the inner grid is a nightmare of multi-interactive tuned and sub-tuned circuits.
One of Farnsowrths most interesting inner spehrical grids was assembled from a plethora of doughnut shapes flat rings of equal size. I don't think it went anywhere though.
Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment