Tungsten Experiments Conclude with New Insights, Upgrades in Instruments and EquipmentFebruary 2, 2018
After some delays due to the need to replace and upgrade elderly equipment, the LPPF research team is now concluding the experiments with tungsten electrodes. While the experiments originally were planned to be completed in the fall of last year, the failure of the main roughing vacuum pump and the trigger head stopped operations for two months. However, the new upgraded equipment has allowed the team to fire shots more quickly. In addition, the improvements in our imaging capabilities have given us valuable insights into the plasma focus functioning.
The trigger head provides the spark to the main trigger switch. This switch in turn generates the spark for the 8 switches on the capacitors that allow the current to flow to the electrodes. The original trigger head had functioned for 8 years, so was due to fail. The upgraded trigger head provides a nearly three times larger spark, so provides a much more reliable triggering sequence. The roughing pump was of an old, piston pump design, and was replaced with a new scroll pump, which squeezes the air out between two rapidly rotating spiral vanes. Its greater power allows faster pump downs after each shot, so less time between shots.
With the two upgrades the LPPF team has been able to fire as many as 8 shots in a day and is close to the 25 shots per week that are planned for the upcoming experiments with beryllium electrodes. Once LPPF has the crowdfunding money in hand, we also intend to purchase spares of critical parts so that future breakdowns won’t lead to long delays awaiting replacements.
Once firing resumed in January, LPPF Research Physicist Dr. Syed Hassan re-aligned the optical path to our ultra-fast ICCD camera to obtain close-ups of the plasmoids through our large quartz window. Using this new alignment, the team obtained a sequence of images that provide the clearest picture yet of the evolution of the plasma as the dense plasmoid forms. (The plasmoid is where the fusion reactions take place.) The images were taken from different shots, with the sequence determined by the difference in time between the time the image was taken and the time the first X-ray pulse was observed. The six image sequence is shown in Figure 1 and is cycled in the animation.
Fig. 1 Animation of 6 ICCD camera images (0.2 ns exposure time) of pinch in FF-1 device.
The first two images (-24ns and -15ns) show the pinch region, where the electric current converges, first forming then moving away from the anode. (These images are inverted for easier viewing—in the device the anode actually points downwards.) At 0 ns, a strong beam of ions and electrons is generated and a first, strong X-ray pulse is emitted from the heated electrons. The subtle rings below the glowing blob in this contrast-enhanced image show that the plasma is undergoing what is called a “sausage” instability, in which the radius of the tube of current rapidly changes along its length. This instability causes rapid changes in magnetic field, which in turns cause a large electric field accelerating the electron and ion beams. This sausage instability is an undesirable one because it leads to a large loss of energy before the plasma is dense enough to produce many fusion reactions.
In frame 4 (12ns), the kink instability starts to twist the current path up into the dense plasmoid. The helical current path is visible in the lower half of this contrast-enhanced image. By 25 ns after the X-ray pulse, the current has twisted up into the tight, dense plasmoid in frame 6, about 200 microns in radius, which is continuing to move away from the anode. At this point the fusion reactions are at a peak and a second X-ray pulse and beam pair are emitted.
This sequence shows how FF-1 is functioning in the presence of continuing tungsten impurities that prevent the early formation of current filaments. They will be used as a comparison to those obtained with the beryllium electrodes, without any heavy-metal impurities. “With no heavy metal impurities, we expect that we will have current filaments during pinch formation. A tighter pinch will make the kinking instability speed up, so there won’t be time for the sausage instability to form first,” explains Lerner. “That will eliminate the loss of energy in the initial beam pulses and lead to much higher densities and more fusion yield.”
Analysis of the data from FF-1’s many instruments confirm that the shorter 10-cm anode is transferring energy into the pinch as efficiently as the 14-cm anode did in 2016 experiments. The new test matched the highest values of the old ones in total energy transferred to the pinch—over 10 kJ—as well as in X-ray energy emitted and in calculated plasma density. This is good news, as the beryllium electrodes are also 10 cm long. Lerner’s calculations indicate that with low impurities, the shorter electrode length will lead to a higher current and thus higher fusion yield.
However, the experiments in 2017 and this past month did not achieve the goal of reducing the tungsten impurities sufficiently to create the current filaments, which would have led to much higher plasma densities and fusion yields. As pointed out back in LPPF’s December 7, 2016 report, the filaments would survive only if tungsten impurities were reduced five-fold from 2016 levels to below 4% by mass. Despite microwave cleaning, the best values obtained in the current experiments were around 6% by mass, above the critical threshold required. Without greater density, no greater yields could be obtained either.
Fortunately, the oxides that have impaired the tungsten results will have little or no effect on the upcoming beryllium experiments. First, beryllium oxide is far more heat resistant than tungsten oxide. But more importantly, the very light beryllium nuclei, with only 4 positive charges, will have enormously less effect on the plasma than the tungsten nuclei with their 74 charges. The effect of impurities scales with the square of the electrical charge, so each beryllium ion has 300 times less effect.
Despite the continuing oxygen problems, the tungsten experiments that began in 2016 did lead to the publication of new world record results, as detailed in the next news item.