Summary: Experiments this month gave further confirmation of our basic theoretical model of the DPF. ICCD images showed the pinched filament kinking into a plasmoid and gave us clearer estimates of the plasmoid radius and density. X-ray emission energy continues to increase, giving us more confidence for our spin-off X-ray generator applications. Our replacement trigger heads were completed and tested, making us ready to return to 12-capacitor firing. Aaron Blake and Derek Shannon are joining the LPP team full time.
ICCD images catch filament kinking, give plasmoid size estimates
Figure 1. The axial condensation from shot 101402, 200 ps exposure, 85 ns before pinch peak, 30 kV, 13 torr. Bright filament [C] is about 240 microns in diameter. Note the formation of kink [B] at top of helix.
For the first time, we have captured images of the kinking process that leads from the pinched filament column to the formation of the plasmoid. This process, portrayed in the Focus Fusion Society animation of DPF functioning, is critical to our theoretical understanding of how the plasma in the pinch region gets further concentrated into the donut-like plasmoid. While others have observed plasma columns kinking in different and larger devices, this process has never before, to our knowledge, been directly observed in the DPF.
In shot 101402 (Fig. 1), taken 85 ns before the pinch peak, we can see the kinking process clearly at work. The central bright filament has already wrapped itself into a helical pattern [A], and at the end of the pinch area [B] (top in this image, bottom in the actual device), the twisted filament has started to bend over toward the horizontal, kinking itself like a twisted telephone cord, the first step in forming the plasmoid. The image shows that the angle of the helical cycles tilt more and more towards the end of the pinch region. This is also the time when the current through the anode start to drop rapidly, as energy is transferred into the kink.
Figures 2 and 3. A somewhat earlier image, 92 ns before the pinch in shot 102604, shows the slender pinched filament [D] before it starts to twirl up into a helical form (Fig. 2). At a slightly later time in a different shot, 55 ns before the pinch peak in shot 102603, the kinking has started to form the dense plasmoid [E] at the end of the pinch column (Fig. 3).
Together, these and earlier images give consistent dimensions of about 240 microns for the diameter of the central filaments and for the plasmoid itself. While this is relatively close to the pixel size of the images, 60 microns, it gives us a reasonably good estimate of plasmoid size. We have now installed a new lens which will give us 30-micron resolution in future shots.
X-ray energies continue to increase to hundreds of keV
Shots taken at the end of October have shown very high X-ray energies. The main evidence for this is the comparison between the PMTs (photomultiplier tubes) that are shielded with varying amounts of copper. In at least two shots, there was apparently little loss in energy in the PMT shield with 6 mm of copper relative to the shield with 3 mm. The signals were also very strong compared with an unshielded PMT. Since the ratios do not clearly fit either a Maxwellian (random) distribution of electron energies, nor a mono-energetic beam (one with all the same electron energies), we can’t yet say precisely how hot the electrons producing the X-rays are, but it is clear that the typical X-rays emitted must have energies of around 150 keV, with large fractions at 200-300 keV, above the minimum goals we have set for our spin-off X-ray inspection application. The total amount of X-rays emitted (tens of mJ) are still far short of the goals for this application, and more data will be needed to get a precise measurement of the X-ray spectrum and the electron energy distribution, but we are excited about the progress thus far.
Size measurements provide estimates of density, better comparison with theory
While still tentative pending higher resolution data and X-ray imaging, the size estimates for the plasmoid from the ICCD images allow us to calculate density estimates and get better comparisons with theory and other experiments. If we assume the plasmoid has a volume similar to a 120-micron radius sphere, and that the ion energy we observe is all random energy (not rotational energy), then we can calculate the neutron density and the total energy in the ions of the plasma from the known reactivity of deuterium.
By comparison with some actual shots, our theoretical predictions give a considerably higher total number of ions in the plasmoid but a lower ion energy, so that total energy in the ions is about 1/5 of that predicted. However, these are far from an optimized shots, as they have a pinch time of more than 2 microseconds, which we know from earlier experiments is too long. We expect better approximation to theory with more optimized shots.
Searching for the short pulse time—resistance counts
As reported last month, optimal fusion yields seem to require short times to pinch, so that the current is still rising when the pinch process begins. On first consideration, it seemed that getting to that short pulse time would be a simple matter of reducing the fill pressure in the vacuum chamber. For a given current, the velocity of the current sheath should rise as the pressure falls, making the time to pinch proportional to the square root of the fill pressure. However during shots taken this month, as pressure went down, the pinch time did not drop by anything near this much. Instead at lower fill pressures, the peak current also went down. Fred van Roessel suggested that perhaps resistance was increasing, and proposed measuring this resistance by looking at the decay of the current or voltage waveforms. After the pinch, the current sloshes back and forth into and out of the capacitors, decaying at a rate that is proportional to the product of resistance and capacitance. By measuring this decay rate, we found that indeed resistance did increase with lower fill pressure, thus explaining our results. Overall this is an optimistic conclusion, since as we move to higher currents with more capacitors on line and higher voltage, we will use higher fill pressure and thus resistance will decrease.
One limit to these conclusions is that by measuring the decay rate, we are measuring the resistance of the plasma after the pinch, while the run-down occurs before the pinch, so might have a different resistance. However, the qualitative conclusions should remain the same.
Trigger heads successfully replaced, but more work needed on spark plug longevity
The new trigger heads, with a new design by Mr. Van Roessel, were completed on schedule in October with the help of new LPP employee Derek Shannon and Dr. Subramanian, and tested in FF-1, showing that the trigger operated with the new heads in place. This gets us ready to return to firing with all 12 capacitors in November.
When the trigger fires the switches, we are continuing to get all eight switches that are now connected to fire together. At a capacitor charge of 35 kV, this has now happened 23 times in a row with no switch misfiring, making for a reliability of 99.5%. However, we were not yet able to resolve the problem noted last month of spark plug longevity. Indeed, with the new design of the insulator, we are getting much more frequent breakage, preventing us from having a large number of shots. Based on drawings prepared by Dr. Subramanian, plasma physics graduate student James Robinson at the University of Warwick, UK, is modeling the electric fields that lead to this breakdown, while we concurrently have new, more rugged designs being manufactured.
Swarthmore’s Michael Brown prepares generally favorable report on LPP effort
Dr. Michael Brown, Professor of Physics at Swarthmore University, was asked by the Abell Foundation to prepare a progress report on LPP’s project. The review was very thorough and the report is quite favorable: “I was generally very impressed with the operation at LLP,” Brown wrote in his summary. “The group has clearly made significant progress in the past year. The operation is very professional and the procedures they follow are scientific. I feel that this work is worthy of support.”
Dr. Brown’s basic concern was with the indisputable fact that our current results are several orders of magnitude away from demonstrating the scientific feasibility of net energy. We addressed that gap in our comments on the report. Dr. Brown made several useful recommendations concerning better statistics, determining the isotropy of the neutron emissions, better graphical presentation, and better marshalling of evidence. We are taking these recommendations seriously and should be able to implement them quickly.
LPP is pleased to announce that Aaron Blake, our long-time Secretary-Treasurer and Vice President, will be joining the company full-time as Chief Financial Officer at LPP’s Middlesex facility on Feb. 1. He has made large contributions to the company both in the technical and business realms. He contributed the axial field coil idea, has made substantial investment himself to LPP, and has drafted or edited many of our proposals and business plans. He has an MBA and has participated in two other start-up ventures.
LPP is also pleased to announce the appointment of Derek Shannon as Director of Business Development. Mr. Shannon has been volunteering part-time for LPP for several years and full-time for the last month and a half. He introduced the Abell Foundation to our work. He will devote part of his time to enhancing LPP’s capital drives and part to helping out as needed in the laboratory. Mr. Shannon has BS and MS degrees in Geobiology from Caltech and the University of Southern California, respectively, and a Certificate in Technology Commercialization from USC's Marshall School of Business.