I'm sorry I don't have time to respond immediately to all the responses. In particular I would like to talk in numbers concerning the consequences of a super thin current layer. But I'll make a couple quick comments right away. Since Mike Holmes thinks hanelyp raised some interesting points, I will start there.
hanelyp wrote:1. The wiffleball effect of the electron cloud pushing back on the magnetic field is supposed to make the cusps less leaky. More like a pinhole and less like a funnel.
As I understand it, the plasma pressure is what closes the slit down to the size of a gyroradius, and otherwise it would be much worse. I am not calling that effect into question. What I doubt is that the hole is a point and not a slit.
hanelyp wrote:2. Since the magnetic field is used to confine electrons but not ions, an electric field outside the magrid can recirculate electrons, greatly reducing losses.
I think there may be some misconceptions lurking behind these statements, but I have not yet addressed the issue of recirculation. I would like to first settle the question of how big the holes are before we discuss whether it is possible to plug them.
hanelyp wrote:3. Since we're using the magnetic field to confine electrons, particle with a much higher charge to mass ratio than ions, a weaker field can confine the electron to a smaller radius than if standard magnetic confinement was used.
This corresponds to the use of the gyroradius for the slit width, since it is much smaller for electrons than for ions. Or are you trying to say something else?
None of the comments of hanelyp even touch on my central point, indeed my only point so far, that the cusps are lines and not points, so I don't understand what Mike Holmes finds so interesting about them.
Moving on to 93143 (I hope you don't mind if I dispense with the title and last name and just call you 31.),
I think the line cusp issue has been adequately addressed, at least considering that there is currently an experiment running.
The Valencia paper
, page 8 (which I believe was quoted on cosmiclog), mentions the existence of line cusps, and gives some criteria for mitigation in order to keep the trapping factor high. Remember that cusp losses in an open magrid design are not actually losses, since the electrons are attracted back in by the positive magrid and prevented from hitting it by the magnetic field. The goal is simply to maintain a high interior-to-exterior density ratio.
If anyone actually understands what Bussard was trying to say, maybe they can translate it into English (or at least Physics) for me. (What are "line cusp corner spacing flow factors"?) It sounds to me like he is back peddling because he realized it is not possible to obtain point cusps so he is now relying on even better recirculation, but like I said, I don't really know what he is saying.
93143 wrote:Also, Dr. Nebel says that wiffleball mode has been demonstrated experimentally, which I would tend to believe at face value until some good reason for doubting his word comes up. (I don't have the resources to replicate the experiments myself...)
I haven't called into doubt anything that Nebel said, I just asked for more details. If this is supposed to resemble a scientific discourse, then appealing to "pumpkin papers" isn't going to get us anywhere.
As for believing in a scale up to a power reactor, ITER is predicted to produce 30 times as much fusion power as JET, and I am pretty sure they will land within a factor of 2 of that. If WB-7 works as predicted, what is the scale factor to a power reactor? A million? That's a long row to hoe. Purely empirical extrapolation over more than a factor of 3 or 4 is very iffy. That is one of the reasons I am pressing to understand the basic physics of the concept. Even if the toy experiments work in accordance to somebody's model, the model could still be wrong and/or physical understanding can identify show-stoppers waiting in the wings.