Hello,
I watched this presentation. I made some notes on it. I did some research on what I did not know and wrote it up. I am looking for corrections, insight and feedback. I have also left some questions I would like to answer.
Starting on slide 4: “How plasma is confined”
I see two principals here:
1. A better vacuum helps performance. Air in the chamber will cool off the plasma.
2. Hotter plasma helps performance. He mentions three methods for heating:
a. Radiofrequency heating – this is like microwaving the plasma. Once it is hotter than 16 eV, the deuterium breakings into (+) and (-). The Lockheed effort may use this.
b. Induction heating – I assume he means when a magnetic field is switched back and forth, causing gas to heat up and ionize. Though it is hard to see the difference between this and RF heating.
c. Hot particle injection – this is what it sounds like.
A couple questions:
1. What is the electron energy inside the device? Is it different from the ion temperatures? Any data on this?
2. Why does he not mention heating ions using electric fields? Isn’t this the fundamental principal of fusors, polywells and all IEC devices?
3. I am unclear on the difference between inductive heating and RF heating. They sound very similar; maybe someone can explain this to me?
Slide 5: “Confinement is hampered by anomalous transport”
The goal in magnetic confinement is to hold in the plasma. However, in all cases, the plasma is never contained nearly as long as predicted (10x or 100x short time frames). The reason given is
Anomalous transport. As far as I can tell this is a blanket term for all the physics caused by instabilities and it gets worse as machines get bigger. There are lots of papers on anomalous transport, especially in the tokamak world.
Slide 6: “4 ways to stop anomalous transport”
Anomalous transport (AKA plasma instabilities) is suppressed by:
1. Velocity shear – this is too vague. What does he mean? Does he mean differing rates of flow within plasma? Does he mean differing directions of flow within plasma? Does he mean higher velocities of flow within plasma? Is a plasma “swirling” around the outside, going to suppress instabilities?
2. Steep plasma gradients.
3. Small surface to area volume ratios.
4. Sharp field gradients.
Slide 12: “Problems with fusors”
Four problems with fusors:
1.Energy loss as material hits the cage and is conducted away.
2.In big fusors, cage heats up and burns off.
3.Electrons are lost when they bounce off one another or the electric field and hit the wall.
4.Ions thermalize in the core and/or mantle. I take Mr. Baker to mean the same core, plateau, mantle and edge model that Tim Thorson worked out. This is from page 27 of his thesis. If so, I have tried to mark out where thermalization is taking place.
Slide 13: “Variables which control the polywell”
I think this slide deserves more emphasis, for a number of reasons:
1. Many skeptics argue that the plasma will not have this much fine structure, AKA specific “edge” and “core” properties. IDK - I do not think there is hard data on this. The closest data I have seen was Joe Khachans October paper, which basically measures electron trapping at low beta.
This data shows some edge and core behavior, but it is not conclusive. To measure this, I think we will need Thompson laser scattering of a polywell. You shine a laser into the plasma, read the reflections and get maps of the densities. Wisconsin recently (October) added this to their HOMER fusor.
2. I am not sure this is a complete variable list. We need a good list of the input parameters and dimensionless numbers. Here are some I like:
Picking dimensionless numbers is an art form. Once you pick good ones, you can make sense of your data, guide experiments better and map out modes of operations with fewer simulations. If I had cash, I would do this for the polywell. Here is a description of their list.
He mentions a couple points. First, that edge density determines overall cloud stability. Second that edge electrons motion “resists” the applied field.
Skeptics would pounce on this. Though, there is a physical mechanism and evidence from magnetic mirrors of this;
we do not have strong published data showing this in polywells. This forms the basis of the “whiffle ball” confinement concept. He cannot make that statement unless he has the data to back it up.
Slide 16: “Problems we have found”
This is the meat of the presentation. Here is what he mentions:
1. Ion injection is a problem. Dr. Alex Klein, who worked for Bussard listed ion injection as a major problem. You need to control: (1) location, (2) energy, (3) scattering and (4) vacuum. Joel may have circumvented this issue with his “differentially pumped ion source” which is a mechanical solution that is mentioned later on but is not explained. They even provide a picture of this device.
Does anyone have a better picture, where you can make out these labels?
2. They mention two acoustic instabilities: Weibel and Diocotron instabilities. I am weak on instabilities; if someone can explain these to me I would appreciate it. Here is my understanding of Weibel.
This instability also occurs in a counter streaming case. Some questions:
a. Does this occur in (+) beams and (-) beams? Does it occur in “packets” of moving neutral plasmas?
b. What is the difference between the counter streaming case and the uniform beam case?
c. What are the factors that lead to it or stop it? Beam speed? Beam density? Beam diameter? Background environment? I see that it can be created with a beam is hit with an off-axis electromagnetic perturbations.
d. Where does this apply in the polywell? In the lecture I watched a beam was sent into neutral plasma and a counter stream formed against it. Though the polywell plasma is very (-) I think this would happen in a polywell.
The Diocotron instability is created when two sheets of plasma move past one another. Here are some cases:
Are all these cases true? Can this be mitigated? Where would this be an issue inside the polywell?
3. Mr. Baker says that the magnetic fields must be “conformal” to the metal surfaces, both at startup and at the end. The field cannot cross a metal surface after the plasma pushes the B field around. It sounds like they are working hard on this.
4. Loss of convergence inside the device. They give a few reasons:
a. Lensing or defocusing of plasma at the edge.
What is the physical mechanism of “lensing”?
b. Ion collisions cooling down the ion population.
c. Electric and magnetic heating of the ions at the edge.