Beryllium Decreases Electrode ErosionMay 13, 2020
While the cracks were bad news, our inspection of the beryllium anode also brought good news. The erosion of the electrode near the insulator has markedly decreased with the beryllium electrode as compared with our previous tungsten electrode. This erosion and roughening, seen clearly in Figure 3a of our last tungsten anode caused an asymmetric formation of the current sheath at the very start of the pulse. This, in turn, was a big contribution to an asymmetric compression, and much less density and fusion yield than our goals. The roughened surface acted like thousands of tiny lightning rods, making breakdown easier and faster in some spots and slower in others.
But after nearly 200 shots, almost exactly as many shots as the tungsten anode underwent, the beryllium anode remains smooth and almost mirror-like near the insulator. A small amount of melting has clearly rippled the surface somewhat, as indicated by the broadening of the light reflection as compared to the untouched metal that was protected by the insulator. But the heavy roughening and erosion so evident with the tungsten has disappeared.
Figure 3. The tungsten anode (a) shows heavy erosion and roughening near the insulator, while the beryllium anode (b) shows a much smoother surface. The lower half of each anode was covered by the insulator and so was not eroded at all. The rainbow-like colors are created by thin layers of metal deposited on them.
Why would beryllium, with a boiling point of 2470 ℃, resist erosion better than tungsten, with a boiling point of 5550 ℃? We’re sure the answer is that the tungsten anode had a deep layer of tungsten oxide which we were never able to remove. The tungsten oxide was very fragile and easy to vaporize, disintegrating at 500 ℃, but the layer kept re-forming after each shot. In contrast, beryllium forms a very thin oxide layer, which was vaporized with our first shot, back in June of last year. After that, the bare beryllium metal was able to conduct the heat generated in the initial “breakdown” that forms the plasma without vaporizing.
Eliminating the roughness is a big step forward towards our goal of symmetric plasma breakdown and compression. But we can improve further. We suspect that the small light grey patches on the beryllium anode were formed during that first “blow-off “shot which covered the chamber with a layer of beryllium oxide. The patches may still contribute somewhat to an irregular breakdown. In preparing the next anode, we will be working with the manufacturer to see if we can prevent the formation of even a thin layer of oxides, so that the first shot of the new series can’t create any asymmetries.