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Post by n2maniac on Oct 6, 2017 3:43:27 GMT
I've found some isues with the skin effect argument. Primarily, the skin effect occurs in high frequency AC systems. With pulse times in the range of 10s to 100s of milliseconds (effectively meaning switching on and off again during firing with a "frequency" somewhere around 100 to 10 herz) I'm not sure if the skin effect is that much of an issue. Beyond that, the skin effect is highly dependant on the geometry of the current flow. With railgun armatures shaped as they are, the skin effect may be significantly weakened. Look up velocity skin effect, there is a whole research area on this for railguns. There is a good diagram on the second page here: repositories.lib.utexas.edu/bitstream/handle/2152/30836/PR_86.pdf?sequence=1Effectively, the magnetic field doesn't diffuse into the rails so quickly (this is the skin effect) and problems arise at the armature-rail interface.
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Post by omnipotentvoid on Oct 17, 2017 6:33:33 GMT
Well, that just further demonstrates how bad penny shaped armatures are. Still, most of my railguns use superconducting rails, does the skin effect still apply to them?
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Post by n2maniac on Oct 17, 2017 17:57:55 GMT
Well, that just further demonstrates how bad penny shaped armatures are. Still, most of my railguns use superconducting rails, does the skin effect still apply to them? Given that the papers I found attempt to correct this by making the rails more resistive on the surface, very likely.
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Post by omnipotentvoid on Oct 18, 2017 12:57:08 GMT
Seeing as the argument for more resistivity comes from the dispersion time of a magnetic field, I have difficulty seeing how the argument would translate well (if at all) to superconductors, considering that they expell all magnetic fields (ignoring for a moment the added complexety of type 2 superconductors).
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Post by n2maniac on Oct 19, 2017 0:01:53 GMT
Seeing as the argument for more resistivity comes from the dispersion time of a magnetic field, I have difficulty seeing how the argument would translate well (if at all) to superconductors, considering that they expell all magnetic fields (ignoring for a moment the added complexety of type 2 superconductors). That is exactly how it translates. Current will flow closer to the surface than on copper. It should make the problem worse.
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Post by omnipotentvoid on Oct 19, 2017 4:48:20 GMT
That makes little sense to me. As far as I’m aware, the diffusion of a magnetic field in a superconductor with no resistance would either be infinite, considering the argument in question, or as close to instant as it gets in physics, because the field is expelled as fast as light speed allows. That is if magnetic fields can even exist inside a superconductor. To put it simply: I imagine that applying logic from normal conductors to superconductors is going to work about as well as applying the logic of normal fluids to superfluids.
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Post by n2maniac on Oct 21, 2017 2:55:13 GMT
That makes little sense to me. As far as I’m aware, the diffusion of a magnetic field in a superconductor with no resistance would either be infinite, considering the argument in question, or as close to instant as it gets in physics, because the field is expelled as fast as light speed allows. That is if magnetic fields can even exist inside a superconductor. To put it simply: I imagine that applying logic from normal conductors to superconductors is going to work about as well as applying the logic of normal fluids to superfluids. Superconductors resist magnetic fields penetrating them. By the same token, all current wants to flow at the surface (lest an internal magnetic field would be created). This is the same thing as the skin effect, but the timespan is infinite. Something I forgot that may make superconductivity a moot point: what is the magnetic field of the railgun? Superconductors refuse to function in high enough magnetic fields, which will be hit around 1-100 MA.
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Post by omnipotentvoid on Oct 23, 2017 7:14:44 GMT
I’m well aware of the fact that a superconductor cannot withstand a magnetic field beyond a certain point. However I still have problems with the implications of the skin effect in regards to superconductors. The problem I have is this: the skin effect shows that the current flows primarily in a layer on the surface with a depth inversely proportionate to the conductivity. Since conductivity is achieved through moving electrons. The smallest possible depth is thus the width of an electron. This is a non 0, non infinitesimal width, meaning that the conductivity is finite. This obviously isn’t the case in superconductors. This is seen (as far as I’m aware) in the AC skin effect in superconductors, where it is observed when the excitation and thermal energy are enough to break some of the cooper pairs, causing the superconductor to act like a mix of classical and superconductor, thus allowing for a skin effect.
I could, of course, still be wrong, however to convince me that the skin effect is relevant here I need some reason why infinite conductivity still allows for a non infinitesimal skin n depth or why the the projectile traveling along the rails would break up cooper pairs. Otherwise, simply saying the classical model still applies, when that statement is very obviously not trivial, isn’t a good argument.
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Post by matterbeam on Oct 23, 2017 9:05:16 GMT
I’m well aware of the fact that a superconductor cannot withstand a magnetic field beyond a certain point. However I still have problems with the implications of the skin effect in regards to superconductors. The problem I have is this: the skin effect shows that the current flows primarily in a layer on the surface with a depth inversely proportionate to the conductivity. Since conductivity is achieved through moving electrons. The smallest possible depth is thus the width of an electron. This is a non 0, non infinitesimal width, meaning that the conductivity is finite. This obviously isn’t the case in superconductors. This is seen (as far as I’m aware) in the AC skin effect in superconductors, where it is observed when the excitation and thermal energy are enough to break some of the cooper pairs, causing the superconductor to act like a mix of classical and superconductor, thus allowing for a skin effect. I could, of course, still be wrong, however to convince me that the skin effect is relevant here I need some reason why infinite conductivity still allows for a non infinitesimal skin n depth or why the the projectile traveling along the rails would break up cooper pairs. Otherwise, simply saying the classical model still applies, when that statement is very obviously not trivial, isn’t a good argument. Does this mean that acceleration inside a coilgun, using induction forces, cannot be applied to superconducting projectiles?
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Post by omnipotentvoid on Oct 23, 2017 14:32:26 GMT
Does this mean that acceleration inside a coilgun, using induction forces, cannot be applied to superconducting projectiles? I don't quite understand the question. Induction still works on superconductors and the current inside a superconductor will change if the external magnetic field changes in order to maintain the fact that the magnetic field doesn't penetrate the surface. So changing the field in a coilgun will accelerate the projectile, the exact way that happens may be different.
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