Uneven tungsten pins likely cause of asymmetric firing, lower fusion yields.
Continuous knife-edge cathode base, previously produced, is ready for installation in June and expected to bring increase in yield. Soot contamination also to be improved.
As LPP’s research team suspected a few weeks ago, uneven tungsten pins at the inner edge of the cathode plate appear responsible for a markedly asymmetrical current sheath, which in turn led to the formation of the early beam and lower-than-predicted fusion yields. Measurements of the pins from photographs (below) showed that uneven wear and loosening of the pins due to vibration led to uneven pin heights. In particular, two pins were about 0.5 mm higher than those around them. Since they were closer to the end of the insulator, current would naturally start to flow from these high points.
Supporting evidence for this explanation was found in the shadows created by the plasma on the cathode base and on the brass collar around the cathode rods. As the plasma swept past the cathode rods, it cleaned the copper, but left dark areas in the regions shadowed by the rods. By tracing back the shadows with taut string, LPP CFO Aaron Blake was able to show that the plasma blast originated not symmetrically, but preferentially from the two high pins (below). A similar pattern of shadows on the collar, which is one-third of the way down the rods, showed that the asymmetry persisted as the sheath moved down the electrodes.
The asymmetry meant that one part of the sheath was leading the others. When the earlier part of the sheath reached the end of the anode, it would also compress first, so that the resulting sheath was thick, and the asymmetry made it more difficult for filaments to form to compress the sheath. As explained in previous reports, the thick sheath meant that kinking and the formation of a beam occurred before the plasmoid could compress fully. This would then lead to density about ten times lower than expected and similarly lower fusion yields.
Jan Brzosko, who had been a colleague of DPF pioneers Winston Bostick and Vittorio Nardi, confirmed in a phone call with LPP’s lead scientist Eric Lerner that indeed, the Stevens Institute team had tested the pins and found that one pin always fired first, leading to asymmetrical sheaths and lower yields. (Unfortunately, this result was not published and this question was not asked six months ago!)
The good news is that this problem may be solved by substituting a continuous metal knife edge for the pins, built into the cathode base plate. Such a continuous knife edge will spread the current much more evenly and be more resistant to developing high spots. This knife edge is already available (below) since it was manufactured as an alternative to the tungsten pin design at the start of the FF-1 project.
The asymmetric firing also created asymmetric wear on the anode, so that too will be replaced. A new spare insulator will also be used, so that no “memory” of the asymmetry should be present in future experiments.
Brzosko related that by switching from the pins to a continuous knife edge—thereby eliminating the asymmetric sheath—the increase in neutron yield was boosted by a factor of five to ten. Since FF-1 has been producing yields five to ten times below theoretical expectation at 1 MA, we are confident that when we resume firing in July we will achieve our theoretical yield expectations and be back on track on our scaling of yield with current.
LPP Senior Scientist Dr. Murali Subramanian has further suggested that the soot found in the device might be coming from the plastic tube that was used for ground isolation of the metallic fuel tubes. This plastic tube formed the last 10 cm of the gas line and was in the vicinity of the pinch zone. Blackening of the plastic tube showed that it had indeed been damaged enough to release soot into the chamber. Such soot would accumulate on the surface of the electrodes. We will detach the plastic tube and replace it with the steel tube as ground isolation is no longer necessary. This may further improve our yield.