In addition to validating sufficiently high X-ray energies for X-Scan, February saw other major milestones for LPP, including:
- New spark plug manufactured—It’s big!
- New radioactive isotope diagnostics set up
- New photos of the pulse—in true color
First sample of new spark plugs produced
Unified Machine & Design has delivered the first sample of our new, much larger spark plugs that will be used in the upgrade of the switching system to reach 45 kV (see Fig. 2). The switch plates are currently on order, and we expect to test the first sample of the new switch sometime in March.
Figure 2. Our spark plugs: original on left, current design in middle, and new design on right.
New radioactive isotope diagnostics set up
Thanks to a suggestion by Dr. Subramanian concerning possible medical radioisotope applications, FF-1 now has a new diagnostic instrument—a Geiger counter—to monitor short-lived radioactive elements. This new instrument will help us to measure the ion energy in the plasmoid and, possibly, in the ion beam.
When small amounts of nitrogen are introduced into the fill gas, high energy deuterons can collide with the nitrogen to produce radioactive oxygen-15, with a half-life of only 2 minutes. The oxygen-15 emits a positron which is annihilated by an electron, producing two 511-keV gamma rays. These can penetrate the vacuum chamber wall and be counted by our new digital Geiger counter.
Since the d-N reaction occurs at higher energies than the d-d reaction, the number of oxygen-15 nuclei produced is a second, independent measure of how hot the plasmoid is. Although we expect 1,000 times fewer d-N reaction than d-d reactions, on larger shots, we do expect to see a measurable signal.
FF-1 shows its true color—violet!
LPP has now taken pictures with an ordinary video camera of the FF-1 pinch and its aftermath, thanks to an initiative by Derek Shannon. Video frames taken every 50 ms (see Fig. 3 and 4) show the violet flash of the pinch, the color expected from extremely hot plasma, and, interestingly, also show the fading glow of the still white-hot plasma 50 ms later. Aside from giving us real color photos of the pinch, these images show approximately how slowly the plasma cools off. The relatively slow cooling is good news for several reasons, including making it easier for a future generator to keep the boron in a gaseous sate. However, the conditions in a generator, with much denser gas, are quite different from our current conditions, so extrapolation must be done with care.
You can watch a full movie similar to the one from which these frames were taken here at the Focus Fusion Society website. We will be making more videos in the future, including with neutral density filters that will avoid the over-exposure at the time of the pinch.
Figures 3 and 4. Two successive 50 ms frames taken from a video of FF-1 at the exact time of a shot.