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As an incandescent bulb goes pop, it releases a burst of hard thermal X-rays, consistent with temperatures of millions of degrees. The tungsten filament develops a neck because one point is hotter than the others. The filament evaporates at that point forcing the current through the ever-decreasing remaining
volume. In the last few nanoseconds, the filament at the neck goes straight from solid to non-conductive plasma so quickly that none of the tungsten has time to move significantly. This gives a tiny volume that gets heated to mad temperatures.
The plasma expands, and neutralizes itself. The expanding atoms eventually become separated enough for the electric field to trigger an avalanche discharge. This gives the pop we hear and most of the flash we see. There is a video by the Slo-Mo Guys that shows some of this happening.
Laser fusion experiments struggled to get hundreds of joules of energy into a sub-millimeter target. You can get millions of joules in a pulse from a Marx generator into a wire with nanosecond rise times with a Blumlein tube. We have the right energies and the right temperatures for fusion. Can we bring everything together? I think we can. We can't have a 3D implosion but we can perhaps do it with 2D + 1D...
Instead of a wire, we use a 1mm tube with 0.1 mm internal bore. It does not have to be made out of tungsten as it is not going to be an incandescent lamp, but something with a high density and a very fine microstructure should do. We cool it and fill it with a liquid deuterium-tritium mixture (density about 0.25 g/cc, then seal it. We connect this up to the pulsed power supply.
The electrical pulse may be millions of volts and millions of amps running for a fraction of a nanosecond. If we had a small pulse, the current would start flowing along the surface and gradually penetrate, generating a toroidal magnetic field and a back EMF. Here the current takes the material straight to a non-conducting plasma, so the conducting surface moves in. The plasma can move away at the surface, and the reaction generates an inward-moving shockwave. The combination of electrical pulse and shockwave will have focused 10x by the time it reaches the inner wall of the tube.
The tube implodes. It is unlikely the whole tube was smooth to microns, so it will probably close at the centre in some places marginally before others. Let us suppose some parts close, while others remain 10 microns open. If we get that far we will have compressed the DT mixture by 10x. We now have a thin void that is closed at either end and bigger in the middle. The converging shock waves will drive the lighter DT mixture much like the shock waves in an armour piercing shell focus a cone of material into a narrow jet, only we have two narrow jets of 10x compressed Deuterium-Tritium heading for each other.
This is an experiment. It is not intended to generate power. But the explosion will generate a big electrical pulse as the electrons outrun the protons, and you may be able to recycle some of that.
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DT: deuterium and tritium? |
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Yep. D-T = deuterium and tritium, the easiest fusion option. D-D is second. The options are listed in the Wikipedia article on Fusion Power. |
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This is above my physics-grade, but it's beautifully imagined. |
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I used to work with pulsed power, so the power supply bits are all based on real stuff. I did work making targets for high-power lasers, so I know a bit about the problems getting enough joules of light onto something to something the size of a fusion target, even if that is not what I did. The exploding wire physics is known, and the shockwaves are used in detonators. The numbers all seem to fit. The only bit I have qualms about is using a dense metal to squeeze much lighter liquid hydrogen, as the geometry risks breaking up with Rayleigh-Taylor instabilities, but people have worked with that in other devices. I was never able to get anyone interested in actually trying it in the eighties, so here I am... |
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I wikipediad Blumlein and discovered he was a clever bloke who sadly died a heroic death [+]. But yes, Rayleigh-Taylor instability can be a bummer. |
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Are the x-rays actually the result of superheated tungsten radiating as a black body, or merely from fast electrons striking a tungsten target, exactly like a conventional x-ray tube? |
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Light bulb filaments are tightly coiled, forming an inductor. When energized inductors explode, they generate intense electromagnetic fields, which could easily supply the force needed to accelerate electrons. |
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The pulsed power dumps enough energy into an exploding wire to turn it into a plasma before it can move significantly. This has been proposed as an alternative to railgun for propelling projectiles. This gives the expected blackbody spectrum for the bulk of the material, but you also get much hotter spots that can be captured on streak spectrographs. |
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You get a more complicated shape with a coiled filament - as in the Slo-Mo Guys video - but the microscopic behaviour is much the same as the straight wire. At these timescales, the magnetic field of the local currents and the back EMF as the field tries to penetrate a conductor dominates. |
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You don't really get any free electrons until the avalanche breakdown of the plasma in the second flash, by which time my experiment would have finished. |
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