Silver inner grid

For posts specifically relating to fusor design, construction, and operation.
kvedera
Posts: 13
Joined: Sun Feb 03, 2013 8:13 pm
Real name:

Silver inner grid

Post by kvedera » Tue Feb 05, 2013 1:35 am

Hello, does anyone know whether or not an inner grid made of silver would work? This is for a DD reactor. I can't think of any drawbacks, but if anyone else can, I'd like to know.
Thanks a lot.

Tyler Christensen
Site Admin
Posts: 551
Joined: Tue Feb 24, 2009 2:08 am
Real name:

Re: Silver inner grid

Post by Tyler Christensen » Tue Feb 05, 2013 1:37 am

It has a low melting temperature. Wouldn't be able to sustain anywhere close to the wattage that a Tungsten grid could without melting.

kvedera
Posts: 13
Joined: Sun Feb 03, 2013 8:13 pm
Real name:

Re: Silver inner grid

Post by kvedera » Tue Feb 05, 2013 1:51 am

Ah, so it definitely would not work?
Tyler Christensen wrote:
> It has a low melting temperature. Wouldn't be able to sustain anywhere close to the wattage that a Tungsten grid could without melting.

User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

Re: Silver inner grid

Post by Carl Willis » Tue Feb 05, 2013 2:37 am

It probably would melt apart before you could measure any neutrons. There seems to be no rationale (unless you happen to have a spool of it and mere availability is driving the decision) to use it when tungsten wire is easy to obtain, and even stainless steel is serviceable.

-Carl
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

kvedera
Posts: 13
Joined: Sun Feb 03, 2013 8:13 pm
Real name:

Re: Silver inner grid

Post by kvedera » Tue Feb 05, 2013 2:42 am

Wait, you can use stainless steel?! But its conductivity is terrible! And the reason I wanted to use silver was because of its excellent conductivity. I guess I could use stainless steel if it works OK.

Tyler Christensen
Site Admin
Posts: 551
Joined: Tue Feb 24, 2009 2:08 am
Real name:

Re: Silver inner grid

Post by Tyler Christensen » Tue Feb 05, 2013 2:51 am

The conductivity of the grid is irrelevant so long as it is some conductive metal. It's carrying milliamps, so V=IR will show that the drop is negligible compared to the HV potential.

Dan Tibbets
Posts: 578
Joined: Thu Apr 17, 2008 5:29 am
Real name:

Re: Silver inner grid

Post by Dan Tibbets » Wed Feb 06, 2013 3:21 am

As mentioned any fair conductivity is adequate. With Ohms Law A=V/R. With house voltage of ~ 110 volts the wire heating due to Ohmic (resistive) heating may be one unit. At a Fusor voltage of 11,000 Volts the heating will be 100 times less at the same current flow. This is why long range electrical power transmission is generally done at high voltages, less energy is lost through heating the wires. The heating of a thin wire at ~ 10,000 volts and several dozen milliamps would be minimal. The wire will not get very hot as even radiation transfer of heat from the wire could probably keep up- no gas convection is needed- thus high vacuums may not change the picture much. What heats the wires is the bombardment by the high speed ions which are an integral part of how Fusors work and cannot be avoided. To control this bombardment heating you use high melting point wire, actively cool the wire, and/or try to magnetically shield the wire. The magnetic shielding could reduce hot ion impacts but would not effect hot neutrals impact, and these hot (high speed)neutrals are unavoidable in normal Fusors. They come mostly (I think) from high speed ions that have recombined with electrons to make a high speed neutral atom. Hot neutrals might also come from up scattered neutrals. What percentage of the neutrals have high velocity is uncertain, though I suspect it is relatively low, but the total neutrals may outnumber the ions by a factor of 10-100 in a normal glow discharge Fusor.

Dan Tibbets

User avatar
Rich Feldman
Posts: 1333
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Silver inner grid

Post by Rich Feldman » Wed Feb 06, 2013 2:35 pm

> As mentioned any fair conductivity is adequate. With Ohms Law A=V/R. With house voltage of ~ 110 volts the wire heating due to Ohmic (resistive) heating may be one unit. At a Fusor voltage of 11,000 Volts the heating will be 100 times less at the same current flow.
Wrong! I know you're trying to help, but it does not help if you don't get it yourself, or are too tired to think & write right. The heating will be the same at the same current. It has nothing to do with the voltage between the heated wire and other wires.

> This is why long range electrical power transmission is generally done at high voltages, less energy is lost through heating the wires.
Yes, but. In your previous example, a wire at 11,000 volts (with respect to the return wire) could carry the same POWER as it would at 110 volts,
with 1/100 of the current, and 10,000 times less heating of the wire.

> The heating of a thin wire at ~ 10,000 volts and several dozen milliamps would be minimal. ...
Yes, no different from several dozen mV and several dozen mA. What counts is the voltage drop due to the wire's own resistance.
All models are wrong; some models are useful. -- George Box

George Dowell
Posts: 157
Joined: Fri May 09, 2003 2:30 pm
Real name:

Re: Silver inner grid

Post by George Dowell » Thu Feb 07, 2013 11:21 pm

Rich have you given any thought to the SIZE of the wire in the grid, in terms of Voltage gradient?

George Dowell

User avatar
Rich Feldman
Posts: 1333
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Silver inner grid

Post by Rich Feldman » Fri Feb 08, 2013 12:08 am

George Dowell wrote:
> Rich have you given any thought to the SIZE of the wire in the grid, in terms of Voltage gradient?

Well, in another thread, Chris Trent gave a link to what passes for a design rule document at Shapeways, for their Stainless Steel material. The ordinary minimum section thickness is 3 mm. You can go down to 1.5 mm if it's kept short and closely supported by thicker material.

Putting numbers on the point first made by Tyler in this thread:
A fusor-grid-shaped part with wires that thick, of any plausible metal, would have much less than 1 ohm of resistance between any two points, even at its melting point.
So at fusor currents (order of 10 mA), the voltage gradient will be negligible (as Tyler said) compared to the fusor HV fields.

Dan was right in saying that the ohmic heating would be negligible. I think it would be a challenge to make that term reach 1 mW. (e.g. 100 mA through 100 milliohms, giving a drop of 10 millivolts).

p.s. The Shapeways material data sheet does not give the electrical resistivity, but it does give the Young's modulus (21 MSI), yield strength (66 KSI), and elongation (2.3%).
All models are wrong; some models are useful. -- George Box

Post Reply