The potential on the grid is irrelevant

[Art Carlson]

Since somebody asked me a couple days ago to make I sketch of what I thought the spacial structure of the electric potential looked like, I've been trying to tap all the pieces of my model into place.

What do you make of Joel G. Rogers' PIC Simulation of Polywell?

First, it's very strange to simulate a single point in parameter space and then use somebody's idea of scaling to extrapolate to reactor conditions. Why not simulate a second point and test the scaling yourself. Better still is 3 points to separate the B scaling from the R scaling.

Second, and more serious, is what happened to Gauss's Law? I am worried about those fans of electrons without ions. I would be very interested in knowing how big the charge per unit area is, but I can't figure it out. The information might be there, but it is hard to tease it out of the plots and statements. Have any of you been able to figure it out?

Related to this, take a look at the plot of the potential on p. 9 and compare it with the plot of the electron density on p. 8, concentrating on one of the corners of the tank, whose walls are held at 0 V. If there are electrons sitting out there on the diagonal and no ions, why doesn't the potential drop below zero?

Let's say this is the first presentation of a young post-doc just getting started on a new project. Once he gets the wrinkles ironed out - peer review would help that process - he might have an interesting tool to begin investigating polywell physics. (Too bad, though, that there is no data to which he can compare his results. OK, maybe one measurement by Krall of the electron loss time in the mirror regime.)

[alexjrgreen]

Why not simulate a second point and test the scaling yoyurself. Better still is 3 points to separate the B scaling from the R scaling.

His presentation from last year (Steady State Polywell Fusion Device Designed Using 2D Simulation) says that he's using OOPIC-Pro, for which a research license costs $997.00.

[TallDave]

Why not simulate a second point and test the scaling yourself.

I wondered about that too. Why simulate at 9W output and extrapolate to a reactor at constant B of 10T?

I'm guessing the answer is that as the radius gets bigger, you have a lot more particles to simulate so it's easier to work with small R.

[D Tibbets]

...What do you make of Joel G. Rogers' PIC Simulation of Polywell?

Something new. There seems to be a lot of arbitrary assumptions, though it is interesting that there seems to be little if any assumption of ion convergence, yet a radius of only 1.3 meters seems to be enough for breakeven with D-D fusion. I don't know why 15,000 volts was chosen. It is the steepest portion of the crossection graph for D-D, but way below the peak. Why is that important?

Dan Tibbets

[Solo]

I've got a student license for OOPIC-Pro, it was only $50 but it was only good for a year though. It's not exactly the best tool for what we're trying to study. It doesn't allow the plasma to modify a background magnetic field if you chose to add one, so we can't find anything about diamagnetic effects.
I tried to duplicate Joel Rogers' results, but I found that the ion and electron count would stay pretty close together, and the potential well evaporated.

@Art Carlson: I agree with the title of this thread. Dolan's electrostatically plugged cusp machines relied on limiters and very strong cusp fields to keep the cusp plasma nonneutral. We don't have that for polywell, so we can basically ignore electrostatic confinement in the cusps: it'll be mirror confinement. The potential on the grid is irrelevant to confinement. (Now, if you let it float, it might reduce the energy loss associated with particle losses to it; Nebel said they've got significant heating on the coil braces.)