Improvements in Firing and Instruments

Right now, our most important instruments are the Time-Of-Flight (TOF) detectors that measure both neutrons and X-rays emitted by the plasmoid.  These detectors look at narrow beams of radiation that pass through the experimental room’s walls through 1” tubes.  Previously, alignment problems prevented us from seeing both signals clearly.  This month, we found that the detectors’ view of the plasmoid was partially blocked, making us think we were producing fewer neutrons and less X-rays than we were.  We’ve now fixed the alignment and expect to improve it still further in the next month after the arrival of a surveyor’s transit, a type of telescope ideally suited for alignments.

This realignment brought the TOF’s measurements of neutron flux into closer agreement with the measurements of total neutrons performed by the silver activation detector.  This agreement gives us more confidence in both measurements.

Using this new improvement in instrumentation, the LPP team has found that so far, over the range of currents from 500-800 kA, the neutron yield is following an I6 power scaling, exactly what our theory has predicted and considerably better than the I4 scaling obtained by most other researchers.

Equally important, the two TOFs working together have produced more evidence that we are already duplicating the high ion energies achieved with higher currents in the Texas experiments.  As the neutrons travel to the detector, they spread out, due to their different energies, which reflect the energies of the ions whose collisions produced the neutrons.  The more they spread out, the greater the ion energies.  Our measurements show that in our best shots, ion energies are in the range of 40-60 keV (the equivalent of 0.4-0.6 billion degrees K).  An example shot, 032510-07, is shown in the figure.

Figure 1: The spread of neutrons as they move shows their spread in velocity and thus the spread in energy of the ions colliding to produce them.  Here, the Near TOF (green line) at 11 meters from the plasmoid has a narrower spread in neutron arrival times than the Far TOF (purple line) at 17 meters.  Calculations based on these measurements indicate that in this shot, the average ion energy was around 57 keV (630 million degrees K).

Comparing how much X-ray energy is received at the two TOFs can give a measure of the average electron energy.  For relatively low energy X-rays, the air between the plasmoid and the detector filters them, so the ratio at each instant of the X-ray signal at the two detectors can be a measure of average X-ray energy.  This measurement is a bit more complicated, but indicates that peak electron energies are around 30 keV.

A third major new observation comes from a more low-tech measurement device—a depth gauge.  Accurate measurements of the depth of the hole in the central electrode, the anode, show that on average, the electron beam from each pinch vaporizes about 18 microns of copper.  This may not seem like a lot, but to do this, the electron beam must carry about 0.5 kJ of energy and the plasmoid must have about 1 kJ of energy, nearly half that stored in the magnetic field of the device.  So, this is evidence that a substantial part of the total energy available is being concentrated in the plasmoids and transferred to the beams.