[Mechanik]

asymmetric_implosion wrote: Tin activates as it is used in activation counters. Argon might be an option as it is pretty immune to neutrons

I would be very interested to see a link to an example of an activation counter based on tin. You might be thinking of indium – that has one of the highest cross-sections to thermal neutrons there is. Indium and silver are the usual materials used in activation counters. I can’t see anyone would have any reason at all to use tin instead, when indium and silver are conveniently available.

There is no element with as many stable isotopes as tin – nine stable isotopes in all, ten if you also count the ‘essentially-stable’ Sn-124.

Argon 40 (the commonest of the two naturally occurring isotopes) activates to the beta emitting Argon 41 with a half-life of 100 minutes or so, which makes its specific radioactivity non-trivial. It has a cross-section of around 1 barn to thermal neutrons, which is not as big as some cross-sections (like indium) but that makes it far from being immune to activation.

If you really want a gas to do your cooling that is as immune to neutron activation as possible, then the choice should be either oxygen (smallest neutron cross section of all the elements = 0.2 mbarns) or helium (which is a bit higher, 7 mbarns, but won’t practically activate in any case, though I guess you might end up with a bit of Li 6 from double-captures), the latter having good thermal conduction properties. I guess helium is your gas of choice, which brings the tread back, full circle!!!…

[Mechanik]

asymmetric_implosion wrote: Li is the only option but you have to worry about the neutron activation and all that goes with it.

In practical terms, Li does not activate. In practice, activated Li should all end up as 4He, 3H, or 9Be.

I’d guess the 9Be content is no big deal, until it comes to recycle the lithium coolant, and in fact the Be-9 and Li-7 reactions will chuck yet more neutrons out (the 9Be is a neutron ‘multiplier’), and the total additional neutron capture reactions should increase the total heating of the blanket (the follow-on heating of a neutron irradiated Li blanket would not be an insignificant fraction of the total heat output of a ‘real’ fusion reactor)

The 4He and T are gases and, presumably, should be readily collectable out of a liquid!?

There is one element that looks like a good neutron-resistant material that might be used as a liquid coolant (though I have never heard anyone else suggest it), and that is tin. For an ‘average’ atom in a given quantity of naturally occurring tin, it can repetitively absorb some 4 neutrons or so before it will become an activated isotope. But, then, why would you bother if liquid Li generates more heat for itself under neutron bombardment, along with some very useful tritium.

[Mechanik]

I’ve been trying to make sense of what is actually arcing to what, but the September report doesn’t seem to provide the detail.

Can we see a schematic section, with the locations of where the spark occurs and the electrical potential of the surrounding parts? Also, it would be good if you can overlay onto that the lines of magnetic field you expect during the pulse. I say that, because I have a magnetic trap experiment and above a critical voltage I actually get ‘arcs’ between parts of the outer magnetic yoke that are at the same potential. I say they are, because they are electrically connected and I am not even exceeding ‘mA’, let alone ‘MA’!!!

It seems to me that if you have a strong enough convex magnetic field at a point where the (crossed-field) electric fields meet some breakdown criterion, then what will happen is that electrons can become trapped, irrespective of the currents or voltages at the ends of the arc itself. These trapped electrons then cause further ionisation. The ions escape (due to their bigger gyro radius) whilst this apparent ‘arc’ is struck and sustained indefinitely yet seemingly without an actual electrical discharge driving it.

This then becomes self-sustaining, because the excited electrons flow down the magnetic field line, intersect whatever it intersects with, and causes sputtering that then sustains the crossed-field convex magnetic effect.

It’s just a suggestion; the solution, or in-fact the issue, may be to re-design the magnetics of the locality. It may not be the electrical properties of the steel plate that are the issue, but its magnetic behaviour. (Whether Austenitic or not, it will contain some degree of Martensitic content.)

In regards plating, to achieve maximum conductivity I expect you will need to layer your plating, such as a tin layer first, then gold flash over nickel on top of this. If we’re talking low volts and high current, then migration through the layers should be insignificant. I can ask a specialist metallurgist friend what the best recommendation is, if you are sure it is gold you want to end up with on the top. What is the exact grade of stainless? (PM me, if it is proprietary information.)

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