Once again, Rick is playing his cards close to his chest, and doesn't actually say what the experimental results are. Nor does he say what the diagnostic methods were, much less let us look directly at the data. Still, it is hard to read this quote without getting the strong impression that he is claiming to have results that are massively better than what I have been predicting. Although I try to avoid data-less speculation, it might be profitable to consider what he might have measured or might be able to measure.rnebel wrote:Loss fraction = (summation (pi*rl**2))/(4*pi*R**2) where rl is the electron gyroradius and R is the coil radius. The summation is a summation over each of the point cusps. If you calculate rl from one of the coil faces, then there are "effectively" ~ 10 point cusps (fields are larger in the corners than the faces). The factor that your observed confinement exceeds this model is then lumped together as the cusp recycle factor.
All we have to go on directly is this:
That is, they measured density with interferometry, and didn't measure it with Thomson scattering. What else would be needed to get a complete picture?rnebel wrote:Consequently, we have done density interferometry on the WB-7. We chose this diagnostic for the WB-7 because we knew through previous experience that we could get it operational in a few months (unlike Thomson scattering which by our experience takes more than a man-year of effort and requires a laser which was outside of our budget) and density is always the major issue with electrostatic confinement.
- Interferometric tomography of the ball: A single channel only gives you the product of the density and the radius. If you have several channels, you can separate these and possibly learn a bit about shape and/or density profile. The access is probably rather limited, so it will be hard to use this diagnostic to its full potential.
- Interferometry of the cusps: If you have a sheet of plasma coming out of a line cusp, interferometry will give you the product of the density and the thickness. If you have a beam of plasma coming out of a point cusp, I believe that interferometry can give you the product of the density and the cross-sectional area (although I have never seen interferometry used this way). It should also be possible to use much of the same setup for microwave scattering to determine the size of the plasma beam independently of the density (maybe).
- Magnetic field probes and/or flux loops: It will be hard to interpret these measurements unambiguously because of the complex geometry, but they are easy to make and access a fundamental quantity, so they should be milked for what they're worth.
- Langmuir probes: Despite all the limitations of Langmuir probes, which I know as well as anyone, I don't think you can get around using them. The set-up is relatively easy and cheap, and they open a window on density, temperature, electron energy distribution, potentials, and flows. They can be used in both the main ball and the cusp plasmas.
- Thermography and/or bolometry: This is another diagnostic that is relatively easy, but has some tricky bits. Ideally, you can determine the flow of power to the solid surfaces with good spatial resolution. If it works well, this is the best indication you can get of the power loss both through the cusps and to the magrid.
- Spectroscopy: Easy to do but hard to interpret. It tells you at least what your impurities are, but can also give you some hints on temperature, flows, and even non-thermal distributions.
- Thomson scattering: I know Rick decided not to use this as a density diagnostic, but it is easier to use as a temperature diagnostic (because you only need a relative calibration of the sensitivity of the channels, not an absolute calibration). The energy of the particles is so important here, that you have to measure it somehow.
- Ion beam spectroscopy: This is less conventional but might work well in a polywell. If you shoot a high-energy ion beam in through a cusp, it should penetrate to the center, slowing down and speeding up according to the local electrical potentials. You might also be able to send a beam across the field. You can measure the velocity spectroscopically by using the Doppler shift. You might also consider detecting neutrals formed by charge exchange.
(Another question that needs clarification is whether Rick is talking about electron losses, ion losses, or energy losses. I assume the last, since that is what matters at the end of the day, although it is a bit harder to measure.