At the PLASMA 2019 conference in Opole, Poland July 15-19, LPPFusion Chief Scientist Eric Lerner presented experimental evidence that our fusion device, FF-2B, has met our initial goal of achieving low-impurity plasma. In other presentations at the conference, and in discussions, it emerged that the purity level achieved by FF-2B is well beyond that reached by the large tokamak experiment JET, which like the LPPFusion device, uses beryllium in its structure. In fact, LPPFusion’s purity results have exceeded those of any major fusion experiment’s published results, including those of the large Chinese tokamak EAST and the new European stellarator, W7-X.
Achieving a high degree of purity in the plasma is key to any approach to fusion energy generation. Impurities, particularly those of heavy elements with many protons in their nuclei (high-z impurities) can greatly increase the resistivity of plasma, and the X-ray radiation from the plasma, thus preventing the confinement of the hot plasma needed for high fusion energy yield. Since the effect of impurities depends on their abundance in the plasma and on the square of the atomic charge, z, (the number of protons in the nucleus), the parameter fz2 is a simple way of measuring the amount of impurities, where f is the fraction of impurity ions in the plasma. Adding up the fz2 contribution of all impurity elements gives the overall measure of impurity.
By this measure, Lerner reported that FF-2B experimental device has reached an fz2 of only 0.08, meaning that impurities change the characteristics of the plasma by only 8%. This is more than five times better than the best reported measure from JET of an fz2 of 0.44. The Chinese tokamak EAST had recently reported a much higher fz2 of around 6, while at the conference the W7-X researchers reported that high impurity levels had limited their machine to only 1.9 keV ion energies (equivalent to a temperature of 20 million degrees K, but a factor of 20 short of the minimum needed to rapidly burn fusion fuel.)
The spectra Lerner presented to
the conference (Fig. 1), representative of many taken in the new experiment,
showed that only deuterium (the fill gas), beryllium and oxygen are present in
the plasma in measurable quantities. The amount of beryllium present was
calculated by measuring the rate of beryllium deposition on the vacuum chamber
windows. By comparing spectra taken through the clean windows with spectra
taken after different numbers of shots, the thickness of the deposits could be
measured, giving the total beryllium in the plasma at each shot (Fig. 2). Since
only 0.35 nm (about three atomic layers) was deposited in each shot, there was
only 70 micrograms of beryllium in the plasma, only 2% of the plasma by mass or
less than 0.5% by number of ions. Taking into account beryllium’s atomic charge
of 4, this leads to the calculated fz2 of 0.08.
SPECTRUM SHOWS LOWEST IMPURITIES
Figure 1. The spectra obtained from FF-2B shots show that only beryllium is the only major impurity in the deuterium plasma. Blue arrows indicate the peaks of beryllium and yellow arrows the peaks of deuterium. No high-z elements are present.
This is a huge improvement compared to the impurity levels obtained with tungsten electrodes—total erosion has been reduced by a factor of 4 by volume, by a factor of 40 by mass and by a factor of hundreds by fz2. The absolute purity level in the plasma before the fusion reactions take place is probably even better than the one we calculated, because much of the erosion comes from the tip of the anode after the plasmoid, where the fusion reactions occur, dissipates.
HOW THE IMPURITY DEPOSITS WERE MEASURED
Figure 2. This graph shows the way we measured the thickness of the beryllium deposits on our vacuum chamber windows. We took the spectrum from one shot (in this case shot 9, July 12) and divided the intensity values by those of the spectrum for an earlier shot (shot 4, June 10). Then we took the logarithm of the ratio—what is shown here. The thickness of the layer is proportional, by a known formula, to the slope of the dotted line fitted to the data. By dividing by the number of shots, we can get the thickness deposited per shot—only 0.35 nm. In addition, the comparison shows that the deuterium lines (yellow arrows) have gotten stronger and the beryllium lines (blue) arrows have gotten weaker, showing that erosion has slowed over the month.
The key point is that LPPFusion’s goal of reducing the impurities to insignificance, defined by a less than 10% effect on plasma parameters, has been achieved. We had set ourselves the goal of demonstrating low impurities in the first 100 shots of the new experiment, and have achieved it in only 75 shots.
Another common way of measuring impurity is with Zeff, which equals (1+fz2)1/2