Progress with Pre-Pulses

As Lerner reported to the DZP 2023 conference, in May our new switches allowed LPPFusion’s FF-2B device to reach a peak current of 1.5 MA (million amps).  Since our device is powered by a capacitor bank storing only 60 kJ, our energy efficiency, producing the most current for the least energy, is now better than that of any other operational DPF device. (For comparison, 60 kJ is the caloric energy delivered by eating three pistachio nuts. Eating pistachios does give you a boost—but not 1.5 MA’s worth!)

However, we also encountered a problem. To increase the density of our plasma—which increases fusion yield—we have increased the density and pressure of our fill gas, deuterium, to 40 torr (5% of atmospheric pressure). We efficiently transferred energy to the higher-pressure gas (see Fig.1). But that higher pressure makes the gas more difficult to “break down”—to convert into a plasma that carries electricity. Harder breakdown makes for a less symmetric breakdown, and less symmetry means less compression and less final density in the fusion-producing plasmoid. To  make breakdown easier, Lerner decided to greatly increase the current we use to pre-ionize the gas right before each shot. Instead of a 60-microamp current, creating what is called a “dark discharge” we would go to 2 A current creating what researchers call an “anomalous glow discharge” or AGD.

Fig. 1 | lpp fusion

Figure 1 The blue curve shows clearly the increase in current achieved in FF-2B as compared with FF-1 (red curve with old switches and tungsten electrodes). The big drop in current is a measure of the efficient transfer of energy from the circuit to the plasmoid. However, the much slowed current drop in the blue curve as compared with our record yield shot back in 2016 shows that compression was  unsymmetrical, leading to lower fusion yield. This is what we are now trying to fix.

This upgrade required a switching circuit to release the prepulse of higher current. This circuit turned out to be more complicated than we thought. A circuit that Lerner designed did not work, nor did two circuits designed by a contractor, Timothy Klein. But in July a fourth circuit designed by Jon Williams, an electronics expert with local supplier Greenbrook Electronics did work.

Before we test the new pre-pulse system with our main bank, we are doing preliminary tests with the trigger pulse that triggers our spark gap switches. When we use the trigger pulse without charging our big capacitors, we get a much smaller discharge that we can repeat rapidly for testing. Instead of a 40 kV, 1.5 MA main pulse, a whopping 60 GW for the 2 microseconds that our machine fires, the trigger gives us a 6 kV, 30 kA pulse, still a respectable 180 MW of power for 0.4 microseconds. We can use the trigger pulse to see how symmetric the breakdown is, taking an image with an ordinary camera from a window at the bottom of the vacuum chamber. We can’t use this window with the main shots as it would get coated swiftly with a thin layer of beryllium from the anode.

Our immediate goal is to get a symmetrical breakdown at 40 torr pressure like the trigger shot on the left in Fig. 2 , which we got at 24 torr pressure, not like the one at the right, which was obtained without any pre-pulse at 40 torr. Once we optimize the pre-pulse with the trigger-tests, we’ll try it out later this month with deuterium fill gas and the main bank firing.

Fig. 2 | lpp fusion

Fig. 2 Our trigger tests allow us to image the breakdown with an ordinary camera pointed upwards towards the electrodes. Here the anode is the black central circle, that cathode vanes are on the outside circle and the breakdown region is between the insulator and anode. At 24 torr (left) we get good symmetry, but not at 40 torr(right). We are testing the enhanced pre-pulse to get high symmetry at high pressure and mass density.

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