Microbeam scars bolster 100-micron radius of plasmoids

Posted by Rezwan on Jun 04, 2011 at 01:59 PM
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Observations of the bottom of the vacuum chamber and the steel washer “lightning rod” installed in the drift tube show hundreds of microscopic circles with radii ranging from 50 to 250 microns. At right 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. As many other DPF researchers have found, the beams are made up of many microbeams, and their size is a good indication of the size of the accelerating region. 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.

The circles tell us even more. In a given magnetic field, the radii of ion orbits are dependent on the ion energy—more energy means larger orbits. The fact that the circles are relatively narrow, open circles indicates that each microbeam has a relatively narrow range of energy—an observation other DPF searchers have made.

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.