Analysis of Tungsten Data Prepares New ExperimentsAugust 29, 2018
Right before shutting down to dissemble, the research team took some final ICCD images of the early stages of the pulse, taking images both earlier and later than the series obtained in April. What the images showed is a somewhat more complex history of filament formation and destruction. In the first 75 ns or so, 16 filaments form opposite the 16 cathode vanes. This is where the small preionization currents have created regions of free electrons that channel the currents. But the initial filaments rapidly disperse, heated by the resistance of the heavy tungsten impurities. Then, as the current sheath moves faster, around 300 ns into the pulse, hydrodynamic forces form a new set of smaller, but still evenly-spaced filaments. This is basically the same process that forms vortices in the wake of any rapidly moving object like a car or plane. Unfortunately these filaments also heat up rapidly and collide with each other. As they collide, their magnetic fields cancel. This change in magnetic field creates electric fields that accelerate particles and rapidly heat the plasma, creating a flash of light that over-exposed the final images. The whole sequence of images has been arranged into a video (see Figure 2). These images tell us that the preionization works in creating symmetric filaments.
Figure 2 In this sequence of ICCD images from FF-1 shots, larger filaments form very early in the pulse, dissipate (making the image dimmer) and then smaller filaments form. Finally the smaller filaments collide, heating the plasma, emitting large amounts of light and saturating the images.
In addition, we can calculate from the over-exposed images the minimum amount of light emitted by the colliding filaments, about 160 kW/cm2. The dimensions of the filaments also give us an estimate of their temperature, around 10eV or 100,000 K. Putting these numbers together, we find that the filaments are emitting 30 times as much light as a pure D plasma at this temperature could emit. This shows that it is the tungsten impurities that are emitting the light. Even though the tungsten is mostly at a low ionization state, it has so many electrons that it can radiate far more energy than either deuterium or the nitrogen we sometimes mix in.
Since both the emissivity and electrical resistivity of the plasma increase in the same way with impurities, the fact that the emission is so dominated by the impurities is more evidence that the resistance is impurity- dominated as well. This confirms the LPPF teams’ theories of why the filaments blow up: they are heated by the impurities’ high resistance. It also increases confidence that this will not occur with the beryllium electrodes. Beryllium, with only four electric charges per nucleus, has a very small effect on resistance.
Disassembly of the electrodes and vacuum chamber also provided other encouraging data. To distinguish between metallic tungsten and tungsten-oxide erosion, LPPF Chief Scientist Eric Lerner measured the resistivity of the material deposited on the vacuum-chamber windows and on the anode itself. Since tungsten is an excellent electrical conductor and tungsten-oxide is a good insulator, the high resistances found showed clearly that the vast majority of the deposits, more than 98%, were tungsten oxide. This again confirmed LPPF’s hypothesis that erosion from the electrodes was fragile tungsten oxide, which breaks up at 500 C, not tough tungsten, which melts at 3,422 C.
This is again good news, since beryllium is much more heat resistant than tungsten oxide, so is unlikely to erode when exposed to energy levels that only break up the oxide. Beryllium oxide forms only in extremely thin layers, so is not expected to be a problem.