4 ms·

I was confused but looked things up so correct me if I'm wrong: * W-7X uses 3T magnets (traditional type-II superconductors), so it has to be big (to give more

I was confused but looked things up so correct me if I'm wrong: * W-7X uses 3T magnets (traditional type-II superconductors), so it has to be big (to give more room for plasma instability to not hit the walls?) * ARC can be smaller because it uses 21T HTS magnets, so the plasma can be more tightly confined. * LTS magnets have an abrupt phase transition. Imperfections have resistance, which causes heat, which can quench the magnet if current gets too high. (?) * HTS magnets for - reasons? - don't quench like this. The magnet just gets hot and slowly degrades. * HTS magnets are really brittle and finicky to work with, so winding them without causing imperfections is hard. Fusion aside, wouldn't 21T magnets be valuable in a lot of areas? Like couldn't you do really high res MRIs with such a magnet? I don't see why there's not more commercial interest in such a general technology.

willis936 5y ago
* W7-X has a large major radius because the scaling laws work out favorably for large machines but there have been very few large machines built to continue pushing out the domain of the empirical scaling laws. Afaik there is not a strong relationship between minor radius and risk of disruption. One of the selling points of stellarators is that they can be operated without risk of disruption, but W7-X has done campaigns that push its operation to disrupt. A machine with dimensions similar to W7-X could be made with higher field magnets. * Yes, higher field means larger machines are not needed to hit the same performance metrics, pushing down the minimum size (and cost) of a reactor. REBCO (HTS material of choice) is quite an expensive material to procure and work, so even modest sized research devices are more expensive than Uncle Sam is willing to gift right now. Private investment into SPARC is changing my opinion about what the future of fusion research will look like. * The details of working HTS into confinement coils is an active area of discussion. HTS coils will be operated at liquid helium temperatures but won't stop superconducting until liquid nitrogen temperatures. The critical current limit decreases as temperature increases, so the risk of quench is still there when operated at near-boundary conditions. One thing that is discussed is whether to insulate the HTS tape or not. Conventionally, coils have insulation between the windings. In the case of superconductors it isn't strictly necessary. Superconductors are embedded in large copper jackets. When the core is superconducting the copper acts as a virtual open. When the core quenches the coil turns into a one turn copper ring and can dump all of the stored energy through the leads quickly without boiling cryogenics because the inductance is much lower. High field magnets are useful, just not to many areas. MRI is the singular commercial application and they are more interested in lower cost (ie being able to run a superconducting magnet without liquid helium) than they are a higher field. NMR is the other big (but actually quite small) application for high field magnets. There are commercially available NMRs today with fields of 50 T.

willis936 5y ago

* W7-X has a large major radius because the scaling laws work out favorably for large machines but there have been very few large machines built to continue pushing out the domain of the empirical scaling laws. Afaik there is not a strong relationship between minor radius and risk of disruption. One of the selling points of stellarators is that they can be operated without risk of disruption, but W7-X has done campaigns that push its operation to disrupt. A machine with dimensions similar to W7-X could be made with higher field magnets. * Yes, higher field means larger machines are not needed to hit the same performance metrics, pushing down the minimum size (and cost) of a reactor. REBCO (HTS material of choice) is quite an expensive material to procure and work, so even modest sized research devices are more expensive than Uncle Sam is willing to gift right now. Private investment into SPARC is changing my opinion about what the future of fusion research will look like. * The details of working HTS into confinement coils is an active area of discussion. HTS coils will be operated at liquid helium temperatures but won't stop superconducting until liquid nitrogen temperatures. The critical current limit decreases as temperature increases, so the risk of quench is still there when operated at near-boundary conditions. One thing that is discussed is whether to insulate the HTS tape or not. Conventionally, coils have insulation between the windings. In the case of superconductors it isn't strictly necessary. Superconductors are embedded in large copper jackets. When the core is superconducting the copper acts as a virtual open. When the core quenches the coil turns into a one turn copper ring and can dump all of the stored energy through the leads quickly without boiling cryogenics because the inductance is much lower. High field magnets are useful, just not to many areas. MRI is the singular commercial application and they are more interested in lower cost (ie being able to run a superconducting magnet without liquid helium) than they are a higher field. NMR is the other big (but actually quite small) application for high field magnets. There are commercially available NMRs today with fields of 50 T.