Generally, big resistive lab magnets are made of square copper tubing with coolant running through the center. Attached is a random google image search result of a typical bending magnet for a beamline.D Tibbets wrote:PS: I don't know the limits of copper. When transformer windings, electromagnet windings, etc. are chosen, given a certain quality of copper wire, are the engineers limited more by global heat isues or by local limitations where these defects/ contaminates/ choke points occur?
For a magnet. all you really care about is the number of amp turns at the end of the day; everything else basically works out backwards from there.
For a given number of amp-turns, you get a given resistive waste heat for a given copper alloy (any decent coil will use the electrical grade 101 alloy). This is basically constant; more turns of thinner wire increases the voltage and resistance and reduces the current, fewer turns of larger wire reduces the voltage and resistance and increases the current. It basically washes out. Here the hard limit becomes your power supply which have some bound on reasonable voltages and currents available.
To remove that given waste heat, you need a given mass flow of coolant. You get the same sort of trade off here. More turns of thinner wire increases the pressure needed while fewer turns reduces the pressure needed. Here the hard limit is hitting turbulent/choked flow with long lengths of small pipe diameters. It is not good if your coolant starts to boil before it makes it out the other side of the coil.
This is where pulsed magnets can excel. In that case, heat rejection does not matter. Heat capacity and structural integrity become the main concern. Here, you can use thinner and thinner wire with higher and higher voltages until the inter-winding magnetic forces literally rip the magnet apart.
So overall, you want lots of turns of thin wire so you don't need stupidly high currents and balance that with a reasonable pressure drop to allow non-choked flow coolant of through the coil which pushes you back towards fatter tubes.
Like you said, for a steady-state electromagnet it is all about the conductivity of the material (read: resistive losses); micro-defects are not an issue at all in the real world at macro scales.