Preionization Tests Prepare for New ShotsApril 6, 2016
The second main source of impurities in previous FF-1 shots has been vaporization of the anode material by runaway electrons. These high-energy electrons occur very early in the pulse, when the plasma has not yet become fully ionized—in other words, when only a few electrons are free to move outside of atoms. The cure for these runaways that the LPPFusion team has proposed is preionization: a very small initial pulse of current smooths the way for the big pulse by producing lots of ionization—many electrons free to move. This is like deliberately producing an electron traffic jam—many electrons moving slowly rather than a few moving so fast that they can vaporize tungsten.
After the successful bake-out, Hassan and Lerner turned to testing the preionization, recording the shots with video cameras. The images showed that the preionization currents were flowing in narrow bands, only about 0.5 mm in radius, opposite each cathode vane. This is good news, because it means ionization levels will be high (good traffic jams) and the main current will also be tightly concentrated, encouraging filament formations, which in turn leads to higher density plasmoids and more fusion reactions. However, they also found that in some conditions, the preionization occurred unevenly, concentrated at some of the cathode vanes but not others. That would lead to asymmetrical main pulses and poor compression of the plasma, thus poor fusion yields. By varying the current supplied to the preionization pulses and the amount of time between them, the team was able to obtain symmetrical preionization, as shown in this video. The video was taken from a window looking up at the electrodes from a little off-axis, which shows almost half of the circle of vanes on the cathode. The team is still studying and optimizing the preionization process.
During the preionization tests, a couple of electrical components failed in the power supply system that feeds current to the capacitors. As a final step in preparing for new shots, the team is replacing these components with more robust ones that can hold off higher voltages.
While these tests were ongoing, Chief Information Officer Ivy Karamitisos and IT associate Jose Varela upgraded the data system that will record, process and back-up data from the shots. They also improved communication links with several instruments, including the Residual Gas Analyzer that measures oxygen and water levels in the chamber, and a new Geiger counter, which will be used to detect short-lived radioactive isotopes produced during shots.