4 ms·

> most MRI scanner magnets cannot be turned off like that. For the few that can, it's going to cost >$50,000 just to refill the liquid helium, I now nothing ab

> most MRI scanner magnets cannot be turned off like that. For the few that can, it's going to cost >$50,000 just to refill the liquid helium, I now nothing about MDI so please tell me: why does it need to refuel the helium? Aren't the magnets “just” superconductive electromagnets? Why can't the current powering the magnet be stopped? Edit: thanks everyone for your explanations, I appreciate it.

fivestones 1y ago
For what it’s worth I just learned this from chatgpt. But the magnet is a superconducting electromagnet, and it’s started up by cooling the superconducter to around 4 Kelvin, and then electricity is turned on, and then once it’s going at the desired current, there is some kind of superconducting switch that disconnects the power source. After that no more power is supplied to the magnet—it will run with that same current for years, as long as you keep it cold enough to be super conducting. Apparently the resistance is actually truly zero, not just almost zero. So there is no current to turn off. The current in the magnet is running on its own from when it was first supplied. The only way to stop it is to heat up the superconductor. I thought maybe you could draw it off and use it up somewhere else, but then if I’m remembering right electricity will always take the last of least resistance, so none of it would leave a superconductor. Right? It also makes me wonder, if someone is ever able to build a warm superconductor, how would we ever stop electricity we’ve put into a system like that?
- snypher 1y ago
  There is some small losses, it does not run 'forever'. However the superconducting switch is pretty neat. It's a small section of superconductor between the ends of the coil, but is wrapped in a heating element. When the coils is first powered up the element is heated and the material has a resistance. When the coil is ready the heating element is turned off and the shorted conductor cools down and starts superconducting.
- littlestymaar 1y ago
  > For what it’s worth I just learned this from chatgpt. Why the hell are people doing that over again? Nobody asked you to copy-paste a plausible AI-generated response you have no idea about its accuracy. That some people like do it is depressing, but the fact that you are bragging about that is truly beyond words.
  - fivestones 1y ago
    I wrote 100% of my post myself. There’s no bragging going on. I’m warning people that I learned it from chatgpt so they know it may not be trustworthy. Still in my experience the vast majority of what I learn from ChatGPT/gemini/claude is correct, and in this kind of case where the knowledge is interesting but not something I’m going to need to rely on for anything, the benefits of quickly learning are so high for me that I thought someone else might appreciate it too. I’m sorry if I offended you.
    - littlestymaar 1y ago
      > the vast majority of what I learn from ChatGPT/gemini/claude is correct Roughly 90% according to benchmarks. Which means you're learning 10% bullshit. > the benefits of quickly learning are so high for me that I thought someone else might appreciate it too ChatGPT has been there for two and a half years already, of course everyone on this forum knows about it, you don't have to tell people how cool it is… Rather you can safely assume that if someone asks a question here, and not as a ChatGPT prompt, it means they doesn't want an answer from ChatGPT!
      - fivestones 1y ago
        I don’t think you fully grasped what I wrote. I didn’t give you an answer from chatgpt. I’m not telling anyone how cool it is. I’ve been here for years and I’m well aware of the level of exposure to LLMs there is for people on HN . I did give out a warning to people that I had learned something from chatgpt, not from some other source, so they could take it for what it was worth. I’m so tired of seeing people in other forums try to help someone and having someone give them a shit response. One thing I like about HN is the fact that that doesn’t happen much here. I still think what I wrote in response to your question was helpful and thoughtful. Back up a minute and just think it over, please. I’d love to see which benchmarks you’re talking about, and if they apply to all three of those specific LLMs, and which version of their models, and if they differentiate topics. My guess is that in the case of LLM output that is a response to a purely factual question, or a response to a question asking a technological question where I present it with something I know occurs but want to know the means by which it occurs, the amount of hallucination is much less than 10%. And if you have some background in the area you are asking about, you can easily filter out some of what is hallucinated, making the learned material well about 90%. But I’m just guessing based on my experience of asking LLMs a lot of questions about both things I already know quite well and others that I don’t. So, which benchmarks are you talking about?
BenjiWiebe 1y ago
If it's a loop of superconducting material, which seems likely as that's how you prevent losses, then you don't have to supply current so there's no current to stop supplying.
KingMob 1y ago
They require extremely low cooling from the helium to achieve superconductivity. And with superconductivity, by definition, current flows without resistance; it continues even without energy, so turning off the power won't stop it. Nor will it heat up and decay from resistance. Modern MRIs are well-insulated enough to maintain their field without power from days to weeks. The only thing that collapses the field is to warm it up to where superconductivity stops, which can be done slow and expensively, or in an emergency, fast and even more expensively. By venting the supercooled gases in what's called a quench, you can turn it off faster, but the time it needs can depend on the model. It could be 20 seconds, or it could be 2 minutes, which, depending on the emergency, may be insufficient. A quench itself can be dangerous, though usually less so than a patient pinned to the magnet. There's a chance that poor ventilation can flood the room with helium, causing loss of consciousness in seconds. The increase in pressure can also make it impossible to escape if the door's not built for that. You'd have to break a window. On top of which, it's dangerously cold, and the explosive bang can rupture your eardrums.
anonymars 1y ago
From Wikipedia: "Any change to the current through the magnet must be done very slowly, first because electrically the magnet is a large inductor and an abrupt current change will result in a large voltage spike across the windings, and more importantly because fast changes in current can cause eddy currents and mechanical stresses in the windings that can precipitate a quench [...]. So the power supply is usually microprocessor-controlled, programmed to accomplish current changes gradually, in gentle ramps. It usually takes several minutes to energize or de-energize a laboratory-sized magnet." https://en.wikipedia.org/wiki/Superconducting_magnet https://en.wikipedia.org/wiki/Superconducting_magnet
mNovak 1y ago
You actually don't need to actively supply current -- it just keeps going round and round, and you can't "short" it out because the path of least resistance will always be the superconducting winding. You also can't open a switch to stop the current because it's basically a giant inductor, it really wants to keep the magnetic field (and current) constant. Meaning if you suddenly disconnected the winding, it would arc across the gap (continuously, for quite a while until the stored energy was spent). So what they do is vent/boil off the liquid helium which is keeping the magnet cold, such that it's no longer superconducting and the current will die off. You can't reclaim the helium, hence you need a fresh refill to chill down the magnet again.

fivestones 1y ago

For what it’s worth I just learned this from chatgpt. But the magnet is a superconducting electromagnet, and it’s started up by cooling the superconducter to around 4 Kelvin, and then electricity is turned on, and then once it’s going at the desired current, there is some kind of superconducting switch that disconnects the power source. After that no more power is supplied to the magnet—it will run with that same current for years, as long as you keep it cold enough to be super conducting. Apparently the resistance is actually truly zero, not just almost zero. So there is no current to turn off. The current in the magnet is running on its own from when it was first supplied. The only way to stop it is to heat up the superconductor. I thought maybe you could draw it off and use it up somewhere else, but then if I’m remembering right electricity will always take the last of least resistance, so none of it would leave a superconductor. Right? It also makes me wonder, if someone is ever able to build a warm superconductor, how would we ever stop electricity we’ve put into a system like that?

snypher 1y ago
There is some small losses, it does not run 'forever'. However the superconducting switch is pretty neat. It's a small section of superconductor between the ends of the coil, but is wrapped in a heating element. When the coils is first powered up the element is heated and the material has a resistance. When the coil is ready the heating element is turned off and the shorted conductor cools down and starts superconducting.
littlestymaar 1y ago
> For what it’s worth I just learned this from chatgpt. Why the hell are people doing that over again? Nobody asked you to copy-paste a plausible AI-generated response you have no idea about its accuracy. That some people like do it is depressing, but the fact that you are bragging about that is truly beyond words.
- fivestones 1y ago
  I wrote 100% of my post myself. There’s no bragging going on. I’m warning people that I learned it from chatgpt so they know it may not be trustworthy. Still in my experience the vast majority of what I learn from ChatGPT/gemini/claude is correct, and in this kind of case where the knowledge is interesting but not something I’m going to need to rely on for anything, the benefits of quickly learning are so high for me that I thought someone else might appreciate it too. I’m sorry if I offended you.
  - littlestymaar 1y ago
    > the vast majority of what I learn from ChatGPT/gemini/claude is correct Roughly 90% according to benchmarks. Which means you're learning 10% bullshit. > the benefits of quickly learning are so high for me that I thought someone else might appreciate it too ChatGPT has been there for two and a half years already, of course everyone on this forum knows about it, you don't have to tell people how cool it is… Rather you can safely assume that if someone asks a question here, and not as a ChatGPT prompt, it means they doesn't want an answer from ChatGPT!
    - fivestones 1y ago
      I don’t think you fully grasped what I wrote. I didn’t give you an answer from chatgpt. I’m not telling anyone how cool it is. I’ve been here for years and I’m well aware of the level of exposure to LLMs there is for people on HN . I did give out a warning to people that I had learned something from chatgpt, not from some other source, so they could take it for what it was worth. I’m so tired of seeing people in other forums try to help someone and having someone give them a shit response. One thing I like about HN is the fact that that doesn’t happen much here. I still think what I wrote in response to your question was helpful and thoughtful. Back up a minute and just think it over, please. I’d love to see which benchmarks you’re talking about, and if they apply to all three of those specific LLMs, and which version of their models, and if they differentiate topics. My guess is that in the case of LLM output that is a response to a purely factual question, or a response to a question asking a technological question where I present it with something I know occurs but want to know the means by which it occurs, the amount of hallucination is much less than 10%. And if you have some background in the area you are asking about, you can easily filter out some of what is hallucinated, making the learned material well about 90%. But I’m just guessing based on my experience of asking LLMs a lot of questions about both things I already know quite well and others that I don’t. So, which benchmarks are you talking about?

BenjiWiebe 1y ago

If it's a loop of superconducting material, which seems likely as that's how you prevent losses, then you don't have to supply current so there's no current to stop supplying.

KingMob 1y ago

They require extremely low cooling from the helium to achieve superconductivity. And with superconductivity, by definition, current flows without resistance; it continues even without energy, so turning off the power won't stop it. Nor will it heat up and decay from resistance. Modern MRIs are well-insulated enough to maintain their field without power from days to weeks. The only thing that collapses the field is to warm it up to where superconductivity stops, which can be done slow and expensively, or in an emergency, fast and even more expensively. By venting the supercooled gases in what's called a quench, you can turn it off faster, but the time it needs can depend on the model. It could be 20 seconds, or it could be 2 minutes, which, depending on the emergency, may be insufficient. A quench itself can be dangerous, though usually less so than a patient pinned to the magnet. There's a chance that poor ventilation can flood the room with helium, causing loss of consciousness in seconds. The increase in pressure can also make it impossible to escape if the door's not built for that. You'd have to break a window. On top of which, it's dangerously cold, and the explosive bang can rupture your eardrums.

anonymars 1y ago

From Wikipedia: "Any change to the current through the magnet must be done very slowly, first because electrically the magnet is a large inductor and an abrupt current change will result in a large voltage spike across the windings, and more importantly because fast changes in current can cause eddy currents and mechanical stresses in the windings that can precipitate a quench [...]. So the power supply is usually microprocessor-controlled, programmed to accomplish current changes gradually, in gentle ramps. It usually takes several minutes to energize or de-energize a laboratory-sized magnet." https://en.wikipedia.org/wiki/Superconducting_magnet https://en.wikipedia.org/wiki/Superconducting_magnet

mNovak 1y ago

You actually don't need to actively supply current -- it just keeps going round and round, and you can't "short" it out because the path of least resistance will always be the superconducting winding. You also can't open a switch to stop the current because it's basically a giant inductor, it really wants to keep the magnetic field (and current) constant. Meaning if you suddenly disconnected the winding, it would arc across the gap (continuously, for quite a while until the stored energy was spent). So what they do is vent/boil off the liquid helium which is keeping the magnet cold, such that it's no longer superconducting and the current will die off. You can't reclaim the helium, hence you need a fresh refill to chill down the magnet again.