Switch Problem ResolvedSeptember 6, 2010
In late August, we received the higher voltage DC power supply for the trigger, which allowed us to increase the voltage of the HV power supply that charges the trigger from 17 to 20 kV. By increasing the charging voltage of the capacitors from 27 kV to 31 kV, we were able to get the total voltage of the switch (the sum of the charging and trigger voltages) above the 50 kV needed to fire the switches together.
We found the switches would still stop firing even after we had adjusted the spark gap carefully with its micrometer screw top. Measurements showed this was due to the impact of the switch firing causing the spark plugs to move slightly, and a motion of more than one or two thousandths of an inch was enough to cause misfiring. Dr. Subramanian came up with the simple idea of using an Anti-Backlash Stabilization Mechanism (better known as a nut) to stop the screw from moving. This has worked well.
Unfortunately, during the testing of the HV power supply, we inadvertently overstressed one of the trigger heads and it will have to be replaced. We expect to do that in September or October, depending on delivery of the parts. To maintain symmetrical firing, we took an oppositely placed capacitor out of service, so we are firing with 10 switches. On September 1, we were able to fire all 10 switches together. We see a sudden jump within 100 ns to the maximum rate of increase. In contrast, with the old automotive spark plugs, the peak dI/dt (rate of current increase) was achieved after only 300 ns. Such an improvement in simultaneity is essential to the optimal functioning of the DPF.
Testing the Axial Field Coil
We have returned to testing the axial field coil, which we think can control the angular momentum and thus the size of the plasmoid. So far we have fired only a limited number of shots with the coil this month (15 in all), but it is interesting and encouraging that this series has yielded the two highest-yielding shots of the whole month. In both shots, on Sept. 1, the coil was producing a field twice that of the earth’s magnetic field. In the best shot, 9-01-10-14, we observed 0.7 x 10^11 neutrons, twice as much as any shot fired this month without the coil. Since we did not see this same effect with the field at 3 times that of the earth, it appears to be a fairly narrow optimal range. In addition, we are not yet seeing as high yield as we expect to at this current, close to 1 MA. That would be nearly 10 times higher than that observed. However, we think we may understand why this is occurring and what to do about it.
As reported last month, we have observed for some time that the X-ray pulses from the DPF have a three-pulse structure. The third pulse is always the one associated with the fusion reactions. We can tell this because of their timing relative to the neutron pulse. What we have now observed is that while there is a correlation between the size of the third X-ray pulse and the number of neutrons produced, the first pulse can lead to a smaller fusion output. In addition, we have observed that in many of the recent shots, there is a little “hiccup” in the current about 45 ns before the pinch maximum, when the plasmoid forms. This drop indicates absorption of energy in some process. What is particularly interesting is that this drop is simultaneous with the first peak in the X-ray pulse and must be associated with it. However, in the earlier shots back in March, where we achieved our highest fusion yields, there is no such hiccup in the current.
While we need far more data, we have hypothesized a possible theoretical explanation of this data. We believe that the first pulse is created as the filaments of the sheath converge into the pinch and annihilate each other, forming a single filament. The second pulse occurs when the kinks converge to form the plasmoid, and the final one is the heating and compression of the plasmoid itself. If there is insufficient angular momentum, the filaments crash into each other, causing a shock and absorbing much of the energy in the pinch, leaving too little left over for the plasmoid. If there is too much angular momentum, the filaments circle around each other without fully merging. But with just the right amount (the Goldilocks point!), the filaments spiral smoothly into each other, allowing the maximum energy to be transferred to the plasmoid. This would explain the sensitivity of the axial magnetic field adjustment and the lower-than-expected yields. If this hypothesis is valid, we should be able to double the radius of the plasmoids and thus increase their volume and fusion yield by 8-10 fold, through precision adjustment of the axial field. We will be investigating this more in September.