Art Carlson wrote:That's why the main question on my mind is what this number means. Is it a prediction or an assumption? Is it for the proof-of-concept experiment they want to build now or is it some kind of fundamental limit? My guess is that it is a near-term goal and there is no reason not to expect Q > 20 once you work out the details. I'll try to find out more.
The paragraph at the top of p.4 of
Quasi-steady Fusion Reactor based on the Pulsed High Density FRC reads
Since the FRC closed poloidal flux, phi_p scales as r_s^2, it can be seen that small s requires small size even at fusion temperatures. Also in past experiments it was found that the more prolate the FRC, the larger the s value before a marked deterioration in confinement was observed. FRCs formed with s/epsilon values less than 0.5 displayed good confinement. Compression will increase the FRC elongation to ~ 20, but the observed threshold based on confinement effectively limits the FRC flux to 75 mWb or less. A flux of 25 mWb is sufficient for a Q ~ 1 fusion burn for the PHD QSFR, and well within the stability and good confinement regime. For higher Q fusion burn, more flux would be required to extend the burn time. For Q > 3 either better confinement scaling must be obtained, or a means for achieving stability at higher flux must be found. It is thought that the kinetic contribution from the helium ash may enhance FRC stability, and allow for higher flux operation. There is reason to believe that the dynamically formed translated FRC has enhanced confinement over the in situ formed FRCs used in the scaling [4]. Significant toroidal flux generation is thought to occur during dynamic formation which has been shown to stabilize rotational modes [5]. Future experiments will tell. However the true value of the FRC based QSFR may not necessarily come from the production of heat from fusion neutrons. Operation as a component test facility for a fusion DEMO, or more notably as a fissile/fusile fuel breeder, a low Q device will be sufficient and possibly even desirable.
That sounds like the answer is that it's a physics limit that won't be solved by simply making the machine a bit bigger or the field a bit stronger. On the other hand, there are also several effects that might enable an end run around this limit. Please excuse me if I remain (guardedly) excited.
On of the features of the polywell that captures the imagination of its followers is that some experiments, even if they are not on the cutting edge, can be done in the garage. Is the same true for FRCs? Mostly you need a big capacitor bank, a vacuum system, and a rugged solenoid. For diagnostics, a few flux loops should be enough to get started. It doesn't sound any worse than a polywell, and you don't have the high voltages to worry about. (Conspiracy warning: I'm planning to hijack this forum and turn it into talk-frc.)