Breaking Records and Window

Brokenwindow | lpp fusion

With all switches firing and central components cleaned, realigned, and in some cases even resurfaced, Focus Fusion-1 (FoFu-1) has pushed the frontier of DPF functioning to record pressures of fill gas. This is a prerequisite for achieving high fusion yields. The yield increases with the plasma density in the tiny plasmoid where fusion is produced, and for a given type of gas, this density is proportional to the fill gas pressure. While no other DPF has achieved fusion reactions at fill pressures above 30 Torr, and FoFu-1 had previously only done this once, on Sept. 12, LPP’s device achieved fusion reaction at these high pressures in 10 shots, including several times at 44 Torr and a single shot over 75 Torr.

Since atmospheric pressure is around 760 Torr, this is fusion at roughly 10% of atmospheric pressure, truly putting the “dense” in dense plasma focus. For comparison, a tokamak fusion machine generally operates at just one thousandth of a single Torr. The shock wave from a blast of fusion at 75 Torr was too much for this glass window (pictured at right), but its quartz replacement should be able to take the pressure.

Equally important, the first day’s firing showed a very tight correlation between the height of the voltage spike that occurs at the time of the pinch, when the plasma is compressed into the plasmoid, and the amount of fusion energy produced (see fig. 1 below). When this pinch and compression occur, the voltage spike is a measure of the energy being transferred from FoFu-1’s capacitors into the plasmoid.


Figure 1. FoFu-1’s fusion yield, measured in billions of neutrons produced, is tightly correlated with the height of the voltage spike (see fig. 2) in new shots performed after the tungsten pins on the cathode plate were aligned (blue line on left). Those shots were at the record gas filling pressure of 42-44 Torr and capacitor charge of 34 kV. The slope shows that fusion yield scales with spike voltage to the 2.73 power. By comparison, many shots with unaligned pins produced a correlation with much more scatter that levels off at high pinch height (green line on right).

This correlation, which continued on the second day of firing, is significant for two reasons. Its straightness on the log-log plot shows that fusion yield is increasing steadily almost with the cube of pinch height. These act like an arrow on a map, pointing to what the best yields at this current are likely to be. With the largest typical voltage spikes at 50 kV, fusion yields should be over 1 joule (about 1012 neutrons), exactly what our theory projects. The agreement of our theoretical projection with the extrapolation of the experimental curve gives us increased confidence in both. Second, this tightness of the correlation implies a more repeatable operation of FoFu-1 with its newly realigned tungsten pins (see below for details on the latest refurbishing). Of course, these preliminary results must be confirmed with more shots, but they are encouraging.

Graph2 sept2011 | lpp fusion

Figure 2. In this plot for shot 091511-05 at 44 Torr, the initial voltage spike from the capacitor bank is on the left, while the voltage spike on the right is from the pinch transferring the energy into the plasmoid, where the fusion reactions take place. The height of the pinch spike above the trend of falling voltage in this case is 9.6 kV, making this a small pinch. In a “big pinch” the spike rises as much as 50

In all these shots, the early beam, which had previously reduced yields by interfering with symmetrical compression of the plasma, was completely absent, further supporting that the misalignment of the cathode plate tungsten pins was indeed the cause of this persistent problem.

The shakedown period since completing the FoFu-1 switch upgrades illustrates how routine experimental glitches can be sorted out—with a little determination. Initially, we found that FoFu-1 was not pinching at all—no fusion. Opening the chamber up, we discovered that some grease from the machine shop had accidently been left in tiny screw holes intended to increase the rigidity of the electrodes. When the plasma hit the grease, it spread hydrocarbons all over the machine. When these landed on the insulator, next to where the current sheath first forms, the heavy hydrocarbons contaminated the filaments, slowing them down unevenly and destroying the symmetry necessary for good compression and the high densities needed for fusion reactions.

In addition, we found that the new copper knife-edge was being seriously eroded by the current. The knife edge is where the current starts to flow through the plasma and must be sharp to build up the high electric fields needed to strip the first electrons off the gas, creating the current-carrying plasma. We had thought the copper knife edge would work better than the uneven tungsten pins, but we had not counted on how rapidly the intense current eroded the copper, which has a much lower evaporation temperature than tungsten. So we had the tungsten pins evened out and tried them again.

In the meantime, time was passing, and we wanted new results for the Warsaw conference presentation—but still had no pinches. On a hunch, team member Derek Shannon moved the fill pressure up to first 50, then 60, and then 75 Torr (when the window broke). Such high pressures would slow the plasma sheath, potentially reducing the maximum spread introduced by initial asymmetry. FoFu-1 then generated small pinches at the record high pressures, but results were still far from satisfactory.

After another disassembly and inspection, team member Aaron Blake noticed that there was arcing between the copper cathode plate and the steel plate where it was attached. When arcing occurs due to poor contact between the two metal conductors, sparks form that can eat away at the metal, making the arcing worse. Blake saw that the steel plate had been eaten away and measured the change in surface level as 10-20 mils (thousandths of an inch). Via Skype, he explained the problem to Lerner, already in Warsaw, and together with Shannon, the three agreed that the copper and steel plates needed to be machined smooth again so that they would form a good contact.

Based on the newly smoothed contact and aligned pins, FoFu-1 started producing frequent, if still small, pinches in the new high pressure regime. Repairs were over for the time being, and physics exploration resumed. Twists and turns remain the norm for both plasma filaments and the road to discovery!









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