In preparing for FF-1’s new experiments, the key objective is to eliminate as much as possible erosion (vaporization) of the electrodes into the plasma, which creates impurities and interferes with achieving high density. The new tungsten electrodes will cut way down on several main mechanisms for this vaporization, but the LPPFusion team identified a mechanism that the tungsten by itself won’t stop – runaway electrons. Now, experiments performed by LPP Fusion collaborators at the Plasma Physics Research Center in Tehran have shown that simply increasing the pressure of the gas in the vacuum chamber can stop the runaways, as theoretical calculations had predicted. This gives assurance that this source of erosion too can be stopped in the new FF-1 tests.
Runaway electrons occur at the start of the current pulse, when high electric fields break down the resistance of the neutral gas, stripping electrons off of atoms and creating a plasma that can carry the current. The high fields accelerate some electrons to such high energy that they smash into the anode, the inner electrode, vaporizing a ring of material, as described in the LPP Fusion Feb. 27 report. Increased pressure should slow the electrons down with more collisions, thus preventing the vaporization.
That’s exactly what the PPRC team found. When they ran their small 2 kJ plasma focus device with 0.2 torr of nitrogen, heavy erosion of the anode occurred. But when they lifted the pressure to 1 torr, the erosion was too small to be measured. These results both confirm the hypothesis that runaway electrons are doing the erosion and show that they can be stopped.
In the meantime, the FF-1 team has implemented a second way to stop the runaways with pre-ionization. Here a small pulse of electricity smoothes the way for the much larger flow, getting rid of the high fields that lead to runaways. So we’ll have defense in depth with two complementary methods when experiments re-start.