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.
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