FF-1’s repeatability improves six-fold, with fusion yield staying within 3%
FF-1 has taken a long step toward demonstrating the level of repeatable firing needed for a fusion generator. On November 2, FF-1 fired five shots in a row, under the same conditions, with fusion yield varying by only plus or minus 2.6% from an average of 0.9x1011 neutrons. While dense plasma focus (DPF) devices preceding FF-1 have had a reputation for large shot-to-shot variability, a fusion generator (as well as most other applications) requires repeatable functioning. In May 2011, LPP reported that our research team had succeeded in stabilizing FF-1’s output to within a range of plus or minus 15%. The latest, tighter stability of function represents a six-fold improvement over the May results and achieves approximately the range of reliability that would be required in a generator pulsing many times per second.
The greater repeatability, we believe, is due to our tighter control of asymmetries in the device, including the centering of the electrodes (see more below). But the axial field coil (AFC), a magnetic coil which imposes a small field along the axis, probably contributes as well. The coil’s field of only 2 gauss, just 6 times the earth’s magnetic field in the vertical direction, imparts spin to the plasma which is greatly amplified while the current sheath moves down the electrodes. As with a spinning bullet, the spin can stabilize the sheath, making the output more repeatable. The sequence of five highly similar shots ended when the team deliberately changed the axial field to 4 gauss. While this is suggestive of a positive effect, more data will be needed to unequivocally demonstrate the role of the AFC.
Automation of data collection allows more shots, faster progress
Changes to our data collection software have allowed us to reduce the time we take between shots, thus speeding our research progress. Until this month, it took several minutes to download the data from all 24 channels on our six oscilloscopes. This significantly extended the time it took us to prepare to fire the next shot. Thanks to work by our IT contractor, Ivana Karamitsos, our software was revised to reduce the data collection time to under a minute. With more shots per day, we’ll make faster progress. Coming soon—faster data analysis, too!
Latest FF-1 tweaking aims to eliminate small tilts
In LPP’s continuing effort to improve the symmetry of FF-1’s electrodes, we made modifications to the central o-ring and insulating Mylar sheets to eliminate a small tilt we had detected in the alignment of the electrodes. The alignment of the electrodes is critical in producing a symmetric current sheath, which in turn is needed to get the highest compression of the plasmoid where the fusion reactions occur. There is only a 15 mil (thousandths of an inch) clearance between the insulator and the cathode, or outer electrode, so this distance must be kept constant to within one mil around the whole circumference of the insulator. While previous efforts have accurately centered the insulator at its base, any slight tilt in the large steel plate holding the anode can create a misalignment when the insulator passes close to the cathode.
LPP’s team did detect such a tilt. The cause was a small error in the dimensions of a rubber o-ring that surrounds the anode and provides a vacuum seal to prevent air from entering the vacuum chamber. Because the ring was too thick, it did not compress enough under the weight of the upper steel plate (which is attached to the anode) to rest securely on the Mylar plastic insulating layers and the lower steel plate (which is attached to the cathode). Instead, the o-ring carried the main weight and allowed the steel to tilt slightly. A reduction in the o-ring thickness as well as adding more lead weights to increase the compression will cure this problem, and the solution was tested by assembling the electrodes, insulator, and plates on our work table. After a breakdown earlier in November, improvements are underway to protect the inner assembly to electrical breakdown at the 120 kV levels that we expect to reach during future shots. LPP’s team expects that the elimination of the tilt will improve symmetry and increase fusion yield.