magnet design

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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EricF
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magnet design

Post by EricF »

Couple of questions on the nature of magnets with regard to the wiffleball effect. I'm still on my first run through of Forrest Mims' basic electronics book so my knowledgebase is still somewhat low.

- In some of the pictures I have seen of the early design polywell magnets, they were straight copper strands wound into spools, fitted inside the steel casing. Why use straight strands for the spool of wire instead of coils?

- From reading Feynman, is the B-field Bussard mentions simply the magnetic field generated by the magnets? Is the strength and size of the B field mainly dependent on the voltage, current, or overall wattage being used?

Say for the same amount of wattage, would a higher voltage and lower current result in a stronger magnetic field or vice versa? Or do you get the same magnetic field for the same wattage, regardless of how the current and voltage balance out?

blaisepascal
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Re: magnet design

Post by blaisepascal »

EricF wrote:Couple of questions on the nature of magnets with regard to the wiffleball effect. I'm still on my first run through of Forrest Mims' basic electronics book so my knowledgebase is still somewhat low.

- In some of the pictures I have seen of the early design polywell magnets, they were straight copper strands wound into spools, fitted inside the steel casing. Why use straight strands for the spool of wire instead of coils?
I don't understand what you mean by "straight copper strands wound into spools". Could you tell us where to find these pictures so we can at least see what you are seeing?
- From reading Feynman, is the B-field Bussard mentions simply the magnetic field generated by the magnets? Is the strength and size of the B field mainly dependent on the voltage, current, or overall wattage being used?
The B-field is the magnetic field generated by, well, everything in the system that would generate a magnetic field, which includes both the magnets and the contained plasma. B is simply a standard abbreviation for "magnetic".

Magnetic fields are solely dependent on currents and geometry. Voltage and wattage have noting directly to do with the field strength and size.

The factors which go into solenoid strength are the current in the coil, the number of turns, the radius of the solenoid, and it's length. The current and turns appear in the formulas as a product, so 10A through 1 turn gives the same field strength as 1A through 10 turns, all else being equal. It is usually easier to get high fields by using lots of turns than it is using high current.
Say for the same amount of wattage, would a higher voltage and lower current result in a stronger magnetic field or vice versa? Or do you get the same magnetic field for the same wattage, regardless of how the current and voltage balance out?
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Lower current means weaker field, higher current means stronger field -- assuming the geometry allows.

For practical magnet design, however, there are always trade-offs. The voltage across the coil is dependent on both the current and the resistance. More turns means lower current, but higher resistance. The trade-off is usually worth it, since P=I^2R, so if you get your 10000Amp-turns with 100 turns at 100A (and resistance is 0.1Ohm/turn) you are using 10Ohm * 10000A^2 = 1MW of power, but if you use 1000 turns at 10A, you are using 1000Ohm*100A^2 = 100KW. Of course, these figures are illustrative and not representative of real magnets.

EricF
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Re: magnet design

Post by EricF »

ok, that makes sense. thanks dude :D

here's an example of what I mean by a spool. I was thinking of a spool of fishing line as the only other way I could imagine the wire being wound together to form the magnet.

Image
blaisepascal wrote:I don't understand what you mean by "straight copper strands wound into spools". Could you tell us where to find these pictures so we can at least see what you are seeing?

blaisepascal
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Re: magnet design

Post by blaisepascal »

EricF wrote:ok, that makes sense. thanks dude :D

here's an example of what I mean by a spool. I was thinking of a spool of fishing line as the only other way I could imagine the wire being wound together to form the magnet.
What I see there is a cutaway section of a polywell reactor. There are 6 coils (the 6 metal donuts) one of which is cut open to show the copper turns within the coil. Surrounding it is a grid also made of copper, which looks like a bunch of straight copper wires forming a loose screen. Surrounding that is the solid metal vacuum chamber.

The straight copper wires aren't part of the magnetic field system. Only the curved wires within the coils are.

EricF
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Re: magnet design

Post by EricF »

blaisepascal wrote:
EricF wrote:ok, that makes sense. thanks dude :D

here's an example of what I mean by a spool. I was thinking of a spool of fishing line as the only other way I could imagine the wire being wound together to form the magnet.
What I see there is a cutaway section of a polywell reactor. There are 6 coils (the 6 metal donuts) one of which is cut open to show the copper turns within the coil. Surrounding it is a grid also made of copper, which looks like a bunch of straight copper wires forming a loose screen. Surrounding that is the solid metal vacuum chamber.

The straight copper wires aren't part of the magnetic field system. Only the curved wires within the coils are.
the copper turns are what I was referring to as 'straight wire spools', since the wires themselves are just round copper wire. My initial curiousity was why the wire itself isnt coiled like a spring before being wound (something I saw in an electronics book explaining transformers), but your explanation above already answered that :D

chrismb
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Re: magnet design

Post by chrismb »

blaisepascal wrote:More turns means lower current, but higher resistance. The trade-off is usually worth it, since P=I^2R, so if you get your 10000Amp-turns with 100 turns at 100A (and resistance is 0.1Ohm/turn) you are using 10Ohm * 10000A^2 = 1MW of power, but if you use 1000 turns at 10A, you are using 1000Ohm*100A^2 = 100KW. Of course, these figures are illustrative and not representative of real magnets.
On the whole, you will end up putting about the same amount of power into a coil whether you use a lot of small-diameter wire or thick wire, 'cos the resistance goes up linearly as the wire cross-section changes but the field generated also goes up linearly with more coils.

Most power coils, motors, &c., end up using a number that is a compromise according to how stiff the wire is for bending. The thicker the wire, the less enamelling/surface insulation you need (both because of the surface area:volume ratio, plus that fewer coils mean lower resistance means smaller differential voltages in the coils).

Look up the equations and wire resistances/cross-sections in wiki, do a few calcs, and you'll see what I mean soon enough, I am sure.

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