Anode Cracks; Another Design on The WayMay 13, 2020
Just as the LPPFusion research team was about to resume firing in March, we discovered that FF-2B’s anode was cracked. We’ve used the shutdown time, necessitated by both the crack and the coronavirus, to complete the design of our new switches, and to redesign the anode. We’re aiming to resume firing with these crucial new upgrades in the fall. This will allow us to keep to our plan of initiating experiments with hydrogen-boron fuel in 2020.
We discovered the cracks on March 23, when we imaged the anode from a protected window at the bottom of the vacuum chamber. It was clear that we would have to replace the anode but we did not want to open the chamber and completely re-assemble the electrodes. Since we would have to be extremely careful to ensure that no beryllium dust escaped to contaminate the experimental room, we had not planned to do any complete disassembly this year. However, the team came up with a plan to remove just the anode from above, while maintaining a reduced pressure in the chamber, guaranteeing that air would be flowing inwards into the chamber and no dust could escape.
The tricky part was to lift the anode vertically so it would not hit the surrounding ceramic insulator and crack that, too. There is only a 1 mm gap between the two parts. But Research Scientist Dr. Syed Hassan worked out a way to lift the anode with lab jacks and a level to guide a supporting rod. During the delicate operation on March 30, which we recorded, one jack collapsed. Fortunately, Dr. Hassan’s many lab skills include a quick reaction time, so he seized the supporting rod in time to prevent any damage to the insulator. (Fig.1) With Chief Scientist Eric Lerner assisting, Dr. Hassan successfully removed the anode and substituted an older steel plate with an O-ring as a temporary seal.
Fig. 1 LPPFusion Research Scientist Dr. Syed Hassan, fully protected against any release of beryllium dust, signals to Chief Scientist Eric Lerner the successful removal of the cracked anode (dimly visible under steel plate attached to steel beam).
Fig. 2 Side view of anode shows one of two cracks extending down the sides. Top view shows two cracks, top and bottom extending out of the damage area on the inner lip of the anode.
Finding the Cause
Inspection showed two cracks running the entire length of the anode shaft (Fig.2). Initially, we suspected this might be due to mechanical stresses caused by the attachment of the upper vacuum chamber. But we saw no cracks on the base of the anode, where the upper chamber is attached. In addition, CAD simulations performed by LPPFusion Mechanical Engineer Rudy Fritsch showed that the stresses we had created were small on the base of the anode and negligible on the shaft. But if mechanical stress was not a problem, what could have concentrated stress to create these two cracks? The damage seemed to have originated from 100-micron wide cuts in the inner lip of the anode.
At this point, LPPFusion CIO Ivy Karamitsos pointed out that, since we had been so concerned for a long time about asymmetries in the current sheath, why couldn’t these same asymmetries have led to the cracks? This seemed a good idea to look at. Lerner’s calculations showed that a plasma filament with 100-micron diameter could vaporize the beryllium if it carried a current of more than 60 kA. A symmetric set of filaments would carry only 20 kA apiece, so would not cause the observed damage. But in a very asymmetric set of filaments, one filament could easily carry three times the average current.
This hypothesis explained how the cracks originated in small areas at the hollow end, or mouth, of the anode, where the current is at its most concentrated, and then propagated mechanically toward the anode base. The mild melting on the rest of the inner anode lip was consistent with what would be expected from an un-filamented sheath. There was also independent evidence for an asymmetric discharge. The deposits of beryllium on our two opposing windows were very different in shape and amount.
But what caused such asymmetric filaments and when did they occur? Since we don’t have any images of the anode taken from the bottom window during last fall’s firing, we don’t know for certain when the cracks occurred. We will in the future have a procedure for taking images after each day’s firing. The deposits on the windows did give us a clue. The thickness can be measured from the spectrum, which was taken with nearly every shot. This data implied that erosion increased between the end of October and mid-December, a period with 30 shots.
During this period, there were three likely causes, which probably had to occur in some combination to cause the damage. First there were several prefires, in which one switch alone fired, causing major asymmetries in current. Second, we had deliberately turned off the pre-ionization as a test. Pre-ionization smoothes the initiation of the current, and turning it off could have increased asymmetry. Finally, several of the shots were at low pressure, which also can lead to an uneven breakdown. In a couple of shots on October 29 all three conditions occurred simultaneously.
Prevention and Beyond—New Switches
Fortunately, we are in a good position to prevent such cracks going forward. We have made major design changes to our new switches that will likely eliminate prefires. We will avoid in future shots turning off the preionization and firing at low pressures. In addition, the new anode we will be getting can be strengthened with design changes and annealing, a process of controlled heating to release strains caused during the machining of a part.
We expect that it will take about three to four months to replace the beryllium anode and we’ve already contacted potential suppliers. To avoid a major delay in our work, we will simultaneously be getting our new switches made and installed. We are already soliciting bids on their manufacture. The new switches will not only eliminate prefires, they will also allow us to have much less down time for maintenance. Even more important they will increase the amount of current the device produces which will increase fusion yield. When we resume firing in the early fall, we believe we will have a device that can overcome the remaining hurdles to high fusion yield. We’ll have more details on our plans in the next report (coming soon).