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June 06, 2011

Lawrenceville Plasma Physics working out solutions to achieve higher yield and fix insulation


1. As Lawrenceville Plasma Physics’s (LPP) research team suspected a few weeks ago, uneven tungsten pins at the inner edge of the cathode plate appear responsible for a markedly asymmetrical current sheath, which in turn led to the formation of the early beam and lower-than-predicted fusion yields.

Continuous knife-edge cathode base, previously produced, is ready for installation in June and expected to bring increase in yield (replaces the uneven pins). Soot contamination also to be improved.



The good news is that this problem may be solved by substituting a continuous metal knife edge for the pins, built into the cathode base plate. Such a continuous knife edge will spread the current much more evenly and be more resistant to developing high spots. This knife edge is already available (left) since it was manufactured as an alternative to the tungsten pin design at the start of the FF-1 project.

The asymmetric firing also created asymmetric wear on the anode, so that too will be replaced. A new spare insulator will also be used, so that no “memory” of the asymmetry should be present in future experiments.

Brzosko related that by switching from the pins to a continuous knife edge—thereby eliminating the asymmetric sheath—the increase in neutron yield was boosted by a factor of five to ten.

Since FF-1 has been producing yields five to ten times below theoretical expectation at 1 MA, we are confident that when we resume firing in July we will achieve our theoretical yield expectations and be back on track on our scaling of yield with current.

LPP Senior Scientist Dr. Murali Subramanian has further suggested that the soot found in the device might be coming from the plastic tube that was used for ground isolation of the metallic fuel tubes. This plastic tube formed the last 10cm of the gas line and was in the vicinity of the pinch zone. Blackening of the plastic tube showed that it had indeed been damaged enough to release soot into the chamber. Such soot would accumulate on the surface of the electrodes. We will detach the plastic tube and replace it with the steel tube as ground isolation is no longer necessary. This may further improve our yield.

2. Observations of the bottom of the vacuum chamber and a steel washer “lightning rod” installed in the drift tube show hundreds of microscopic circles with radii ranging from 50 to 250 microns. Below is one example, with a ~500 micron width wire in frame for scale. The size of the circles indicates the size of the ion beam filaments or microbeams that are emitted by the plasmoid.

This supports our direct ICCD measurement of the plasmoid as 100-200 microns in radius and the filaments in the plasmoid as around 30 microns in radius.

By comparing the time of arrival of the ion beams at the upper Rogowski coil to the time of arrival of the electron beam at the anode, we can get a measurement of the average energy in the beam. In our recent shots, this has been in a range of 260-510 keV.

For a given beam energy, there is a minimum current needed to trap the ions into filaments and allow the beams to stay focused as they move through the plasma. The circle scars show that the microbeams achieved those currents, at least for some of the shots. These minimum currents are in the range (depending on ion energy) of 370-515 kA. Since we expect theoretically that the microbeams will be “on” for half the time and “off” for half the time, the average beam current we expect is then 185-257 kA. What we actually measured with the Rogowski coil were peak currents of 109-193 kA. So these estimates broadly confirm each other and indicate that the highest current shots, but not every shot, were indeed capable of focusing the ions and producing the observed circle marks.

The most powerful of these ion beams are delivering about 100 GW on average and 200 GW at the peak of each microbeam. For comparison, at the pinch, the capacitors are delivering about 60 GW to the electrodes. The difference is being drawn from the energy stored in the magnetic field of the plasmoid. LPP’s research team believes that such intense ion beams should find many spin-off applications. We expect to be investigating these and seeking partners for them in the near future.

3. LPP’s research team, with the able help of LPP consulting scientist John Thompson, has unraveled the likely cause of the insulator breakdown we experienced when firing one capacitor at 45 kV. The upgrade including installation and testing of all 12 new switches should be completed during June and they will resume firing in July. This is dependent on suppliers keeping to their delivery promises.


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