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 experiment 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—up to 8 shots a day. In addition, the improvements in our imaging capabilities have given us valuable insights into the plasma focus functioning.
Once firing resumed in January, LPPF Research Physicist Dr. Syed Hassan re-aligned the optical path to our ultra-fast (0.2 ns exposure time) 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 cycled in the animation below.
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.”
The money to fund these beryllium experiments will come from our Wefudner campaign, now past the half-way point. Over $570,000 raised, but there are still 230 spots available for far-sighted investors. Jump out of the stock market and into fusion!
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