Firing all switches repeatedly and reliably
On September 29, we finally achieved repeatable firing of all attached capacitors. That day, and on September 30 and October 1, we fired all eight attached capacitors in 11 successive shots with only three pre-fires. While last month we thought that we had the switch problem under control, we had to change track. Our basic conclusion, that we needed a higher trigger voltage, was correct, but our method of achieving that did not work well. First, the higher charging voltage (30 kV instead of 20 kV) caused shorting in the trigger heads, destroying three more. We were forced to run with only eight capacitors attached. Second, the trigger pins were still shorting out to the adjacent electrodes, limiting the trigger voltage which actually sets off the spark that fires the switch.
At the same time, the new Lexan insulators on the switch spark plugs continued to break. Dr. Subramanian came up with a new insulator design that might increase their longevity, but required a larger hole in the electrode that the insulator and the trigger pin pass through. We had a single new electrode made up, and it worked well when tested, so we immediately had all the electrodes modified to have larger holes. This then led to our successful firing of all eight attached capacitors within 20 ns of each other, and doing that 11 times in a row at 35 kV on the capacitors. Even with this partial bank, we achieved over 1 MA current, which is very encouraging.
We are now rebuilding the four missing trigger heads to a new and more rugged design, a task that should be completed in October. We are confident that we will then be able to repeatably fire all 12 capacitors together.
Switch reliability allows higher fusion yield
With our new ability to reliably control the number of capacitors firing, and thus to control the current produced by the bank, we can adjust the gas fill pressure in the chamber to optimize yield. As a result, on September 30 and October 1, we were able to get two shots with fusion yields above 10^11 neutrons for the first time since late March.
DPF researchers have long known that it is important to match the time that the pinch occurs with the time that the current from the capacitors peaks. Last month, we plotted the fusion yield for FF-1 against the time of the pinch (see Figure 1). We found that for the shots in March and early April, the yield was tightly correlated with pinch timing, and there was a sharp peak right around the time of greatest current, close to 1.8 microseconds. All the high-yield shots had short pinch times, and none of the lower yield shots did. The same pattern was followed at the higher pressures and currents that we used in September, but the whole curve was shifted upwards by about a factor of 5. This shift implies a good scaling with current to the fifth power.
What was particularly significant was that only shots with the axial field coil turned on had the short pinch times. In shots with the coil turned off (or without the permanent magnetic field that was induced in the vacuum chamber in March), shots that should have had short pinch times did not pinch at all. In our most recent shots, which have been with no axial field, we observed the same phenomena, with pinches at a little more than 2.0 microseconds alternating with non-pinching shots. As seen below in Figure 1, the new shots follow the same trend as the earlier ones.
Figure 1. Neutron yield vs. the time from the start of the pulse to the pinch. The neutron count is shown as the logarithm of the neutron count in thousands. A level of 2 corresponds to 10^11 total neutrons. Small blue dots are shots at 10 torr fill pressure in late March and early April, showing a close fit to the empirical scaling line. The other symbols are from shots in August and September at 18-24 torr. The large red squares are the most recent shots at the end of September. The higher pressure shots follow the same trend line as the 10-torr shots, but with higher yield, implying a scaling of about I^5.
It appears that the correct axial field allows a pinch to occur at a time very close to the peak current, while this is not possible without the field. In our experiments this month, we intend to vary the axial field together with the gas pressure to achieve pinching at a time very close to that of peak current. We expect to avoid the “hiccupping” shock waves observed in the lower yielding shots (described in last month’s report) and move up to a yield approaching or exceeding 10^12 neutrons (over 1 joule of fusion energy).
Highest ion energy observed, over 100 keV
In both of the shots with yields over 10^11 neutrons, we observed average ion energies over 100 keV for the first time with FF-1, getting us into the range useful for pB11 fusion. In shot 93002, we measured an average ion energy of 143 keV, and in shot 100102, we measured 108 keV (equivalent to 1.6 billion degrees K and 1.2 billion degrees K respectively). The important thing is that we are confining ions of this energy for tens of ns. If we can achieve the same conditions with pB11 as we did with D in shot 93002, we would expect a pB11 fusion yield of about 0.5 J.
ICCD photos show 150-micron plasmoid, sequence of events in pinch
We are getting high-quality ICCD pictures, thanks to Fred Van Roessel’s work. The main improvement from early images comes from a slight shift in the angle of view so we are not looking straight horizontally, but rather up at the sheath from a slight angle, which allows us to see things a lot better. While we did not plan it that way, these four images end up as a sequence:
Shot 9-09-10-03, 0.845 microsec before pinch
Shot 9-09-10-02, 0.225 microsec before pinch
Shot 9-15-10-07, magnified plasmoid at the pinch. We see the plasmoid on axis, which is about 150 microns across. The small dots are individual pixels, and do not represent actual fluctuations in intensity.
Shot 9-09-10-05, 0.285 microsec after pinch
A new video of our successful shots was skillfully made by Focus Fusion Society Executive director Rezwan Razani and edited by FFS volunteer Derek Shannon and was posted on YouTube, where it seems very popular. It has been viewed over 5,000 times, putting it in the top 6% of YouTube videos.
Derek Shannon has begun to volunteer his time as consultant to LPP here in Middlesex, NJ, as well as volunteering with FFS. Derek has done volunteer work for FFS for several years from California, and his increased level of help is very welcome.