Continued testing of LPPFusion’s dual switches have demonstrated that they can increase current when firing at the same time, a key goal of using the new switches. The LPPFusion research team has also reduced the undesirable negative pulse from the switches by a factor of three, a big step towards reliable firing. But parts for our latest design were delayed and so important tests still lie ahead in the coming weeks.
While the team had expected to receive parts for a new Teflon-Kapton armored switch design in October, the parts did not actually arrive until well into November, so there has not yet been time to test them. But the team made good use of the two-switch testbed while awaiting the new parts.
First, on LPPFusion Research Scientist Dr. Syed Hassan’s suggestion, the team put the old, larger single switch on the testbed to use as a control for the new switches. Surprisingly, the old switch generated the early negative pulse of electricity that had affected the firing of the new dual switches. This was puzzling, as in previous experiments on the FF-2B fusion experimental device, the negative pulse had only been produced by the new switches, not by the old ones. Since the correct firing of the switches allows current to flow from the positively-charged capacitor, the team had long identified the negative pulse as a sure symptom of misfiring switches.
To try to more closely replicate the conditions that the large switches originally functioned under, Dr. Hassan installed the old resistors in the trigger head. The trigger head transmits the negative trigger pulse from a trigger generator to the switch’s spark plug to fire the device (fig 1). We found that the negative pulse was smaller in these shots by a substantial factor of three, dropping from a range of 12-16 kV to a range of 4-6 kV What was better was that when we put the dual switches on, the much smaller negative pulse was repeated (fig. 2). This was good news!
It was also somewhat unexpected, as in previous shots with the dual switches attached to FF-2B, a faster-rising trigger pulse, allowed by the new “low-inductance” resistors, led to faster and somewhat more reliable firing of the switches. (Inductance is a measure of how much magnetic field energy a device generates for a given current. More inductance slows devices down.) But in the new tests, the slower-rising trigger current for the older, high-inductance, resistors actually caused a faster breakdown of the switch. (Breakdown is the point at which gas starts conducting current) Faster breakdown means the switch passes the positive current vertically through the switch gas before the undesirable horizontal breakdown, which carries the negative pulse, has time to develop (Fig.3). Why this apparently paradoxical behavior occurs needs more study.
Figure 1. The newer “low-inductance” resistor (top left inside pink plastic) was replaced by the old resistor (top right inside black plastic and tape) in the trigger head that feeds current to the spark plug. Normally, the resistors are hidden within the PVD trigger head housing (bottom).
Figure 2. Switch Voltage Comparison: The 16 kV negative pulse typical of earlier performance of the dual switches (blue line, test shot 1 September 24) has been reduced in more recent shots to 4-6 kV (orange line, test shot 1 November 8). Changes in the oscillations are also due to the different resistors in the trigger head but will be studied further in upcoming tests.
Figure 3. Inside The New Switch: (Repeated from October 5 report) Horizontal breakdown between the top electrode (arrow) and the central pin (beige) prevents proper switch functioning. The intended breakdown is between the central pin and the bottom electrode (lower copper-colored strip) which releases the charge in the capacitor. Current then rapidly jumps over to connect the top and bottom electrodes to complete the circuit.
The placement of the old resistors in the trigger head allowed us to fire the dual switches simultaneously (within 20 ns) of each other for five shots on Nov. 8. In turn, this made it possible for the LPPFusion team to measure how much improvement in current-production is possible with the dual switches. By measuring the time to peak current (rise-time) for the dual switches, for the old switch and for a single new switch LPPFusion Chief Scientist Eric Lerner was able to calculate the inductance for each set-up. He used two independent methods of calculations: one just using the differences in rise-times among the shots and a second using the calculated inductance of the rest of the circuit—mainly of the cables connecting the switches to the dump chamber.
Both methods arrived at the same answer—the dual switches have only two-thirds the inductance of the old single switch. Combined with other known reductions in inductance in the plates that connect the switches to the rest of the FF-2B device, the new switches can increase overall current in the machine by 30%. This is an important gain—but is only achievable if all 16 switches can fire reliably together.
More testing is still needed to achieve that reliable firing. For 16 switches to fire together even 4 times in a row, misfires have to occur less than 1.5% of the time for each switch. It will take 32 shots with a single dual-switch pair to demonstrate that. We now have a few alternate designs to try out, starting early December. We’ll be using an upgraded testbed, with closer imitation of conditions on the FF-2B device.