cyclotron electromagnet - 1 Tesla achieved
Posted: Sun May 13, 2018 4:47 pm
Well, the embarrassment of a naked pole in our cyclotron magnet is finally over - the second coil is wound and installed. Winding 900 feet of 1/4" ACR copper tubing isn't fun, but far worse is sliding 900 feet of fiberglass sleeving over it, a couple of inches at a time (it was truly a family affair). There's more info in general on the coil construction here: https://mullinscyclotron.weebly.com/magnet-coils.html, but below are some pics for this second coil and the finished electromagnet.
Unlike the first coil, I embedded two K-type thermocouples in the center of the winding: After two weeks of evening and weekend sleeving and winding, the coil was done - 900 feet, 18" diameter, around 80 pounds, 0.5 ohms, 269 turns: The pole and magnet top piece weigh around 280 pounds, so time to pull out the Harbor Freight shop crane: That round plate resting on the bottom coil will be used to hold the upper coil onto the top piece - that's 0.190" aluminum plate.
After a few false starts, my son and I ended up resting the coil across two saw horses (same two used to wind it), and lowering the pole piece down, so it pushes out the wooden form in the center of the coil:
After it's in place, the coil is attached with 1" aluminum square tubing and 1/4" threaded rod. Now it can be picked up as one unit: Next, the top piece, along with pole and coil are lowered on the frame, and the coil is wired up. With our normal 1.42" gap between poles, it's reading around 820 mT at full power: Each coil is around 0.5 ohms at room temperature, so the set of both coils is just around 1 ohm. Full power is around 55V at 55A: By inserting a 0.5" pole piece extension (8" diameter, 1018 CR steel), the gap is reduced to 0.92" With that smaller gap, I got just over 1 Tesla: On paper, I should get around 1 Tesla with the 1.42" gap, at 55A. With one coil, I was getting around 470 mT, so doubling that would be 940 mT. Instead, I'm getting around 820 mT, so the "efficiency" (ratio of actual vs predicted field strength) is around 90% at half power, and 80% at full power (3 kW).
820 mT is just barely enough to get past 160 keV, or p-B11 fusion. I was hoping for closer to 900-950 mT, to give some margin. Otherwise the protons at the desired energy will be very close to the pole edge, and approaching the edge of the chamber itself. I may have to replace the power supply with something beefier to provide some margin.
Total coil inductance is difficult to measure directly. My LCR meter only goes to 100 Hz, not low enough to get the "DC" inductance. Readings at 100, 120, and 1000 Hz are 34.3, 32.9, and 14.6 mH, respectively. A (very) rough L/R time constant measurement going from zero to 12 amps gives around 80 mH.
Next step for the magnet: either cooling, or bulking up the power supply. Will be a while though, now it's time to circle back to the cyclotron itself to make further improvements.
Unlike the first coil, I embedded two K-type thermocouples in the center of the winding: After two weeks of evening and weekend sleeving and winding, the coil was done - 900 feet, 18" diameter, around 80 pounds, 0.5 ohms, 269 turns: The pole and magnet top piece weigh around 280 pounds, so time to pull out the Harbor Freight shop crane: That round plate resting on the bottom coil will be used to hold the upper coil onto the top piece - that's 0.190" aluminum plate.
After a few false starts, my son and I ended up resting the coil across two saw horses (same two used to wind it), and lowering the pole piece down, so it pushes out the wooden form in the center of the coil:
After it's in place, the coil is attached with 1" aluminum square tubing and 1/4" threaded rod. Now it can be picked up as one unit: Next, the top piece, along with pole and coil are lowered on the frame, and the coil is wired up. With our normal 1.42" gap between poles, it's reading around 820 mT at full power: Each coil is around 0.5 ohms at room temperature, so the set of both coils is just around 1 ohm. Full power is around 55V at 55A: By inserting a 0.5" pole piece extension (8" diameter, 1018 CR steel), the gap is reduced to 0.92" With that smaller gap, I got just over 1 Tesla: On paper, I should get around 1 Tesla with the 1.42" gap, at 55A. With one coil, I was getting around 470 mT, so doubling that would be 940 mT. Instead, I'm getting around 820 mT, so the "efficiency" (ratio of actual vs predicted field strength) is around 90% at half power, and 80% at full power (3 kW).
820 mT is just barely enough to get past 160 keV, or p-B11 fusion. I was hoping for closer to 900-950 mT, to give some margin. Otherwise the protons at the desired energy will be very close to the pole edge, and approaching the edge of the chamber itself. I may have to replace the power supply with something beefier to provide some margin.
Total coil inductance is difficult to measure directly. My LCR meter only goes to 100 Hz, not low enough to get the "DC" inductance. Readings at 100, 120, and 1000 Hz are 34.3, 32.9, and 14.6 mH, respectively. A (very) rough L/R time constant measurement going from zero to 12 amps gives around 80 mH.
Next step for the magnet: either cooling, or bulking up the power supply. Will be a while though, now it's time to circle back to the cyclotron itself to make further improvements.