J. P. Appruzese and his colleagues at the Naval Research Laboratory noted in their presentation at ICOPS that several experiments with pinching machines had achieved ion energies above 10-20 keV, with the highest being the 200 keV previously reported from the big Z-machine at Sandia National Laboratory. Appruzese also mentioned LPP’s results with the DPF as a further example. He pointed out that such high ion energies could be used for fusion with advanced fuels—in his example, D-He3.
Appruzese suggested that these high ion energies might be caused by turbulent heating. Such heating of the ions occurs when frictional forces within plasmas get large enough to disrupt the smooth flow of the plasma, leading to energy dissipation and heating, just as turbulence in ordinary fluids heat them up.
LPP’s initial analysis of this suggestion seems to indicate that it does not explain our higher-than-expected ion energies with deuterium gas. Turbulent heating increase rapidly with increasing atomic charge on the nucleus, so may be significant for pB11, but does not seem to be quite enough for deuterium, which has only a single charge on its nucleus. However, the idea is an interesting one and this process should be included in our future analyses.
Our assessment, from discussions with other researches at ICOPS, is that the reporting of high ion energies in other experiments has given considerable credibility to our own results.
In addition to these specific results, we had extensive talks with many other researchers in the DPF field. We strengthened our ties with the team working on the Polish DPF, PF-1000, still the world’s most powerful DPF. A young researcher at Imperial College offered to analyze some of our neutron data with his new algorithm that can provide more information about ion energy distribution. We also had some preliminary talks with researchers from Voss Scientific about a possible joint grant application to the National Science Foundation for work on DPF simulation techniques. Overall, we felt that our participation at ICOPS, which included our whole research team as well as our visitors from Kansas Sate University, greatly benefited our Focus Fusion project. We can’t overstate the importance of collaboration within the world-wide DPF community to the success of our efforts.
Lawrenceville Plasma Physics recently presented the results of their latest dense plasma focus experiments at the International Conference on Plasma Science (ICOPS).
The results include ion temperatures of 20-70 keV, record high fusion yield for a given current, and good agreement of the experiments with theory.
The theoretical model predicts that, in the range of peak currents explored so far by FF-1, ion temperatures will increase linearly with current, plasmoid density will scale as the square of current and plasmoid lifetime will scale linearly with current. Since fusion reaction rates go up as the square of the density and approximately the square of the temperature—in this temperature range—the model implies yield scales as current to the seventh power. That is exactly the scaling observed so far, and the absolute number of fusion reactions is just as predicted.
However, not all the results fit theory completely. For three of the four shots where the data is best, the ion energies were well above the predicted value—50-70 keV instead of the predicted value around 20 keV. On the other hand, the value of n^2V—the density squared times the volume—was about ten times less than predicted. So these plasmoids are hotter and either less dense or smaller than predicted. It is expected that newly functioning instruments will help sort this question out in the near future.
Another promising development is that other researchers at ICOPS reported high ion temperatures with pinch-type machines and this has added credibility to LPP’s claims. The highest being 200 keV previously reported from the big Z-machine at Sandia National Laboratory.