Fusion by 'compressing' tokomak plasma (CT, spheromak, dynomak etc.)

Question

Question

How thoroughly has the idea of 'compressing' the torus of plasma in a tokamak been considered/experimented?

I'm looking for references to scientific papers.

Motivation

Tokamaks and stellarators are popular magnetic confinement reactors. Field Reversed Confinement (FRC) devices including the Theta-pinch and spheromak have been considered. The wikipedia article Compact toroid is a stub, but is probably the closest thing to what I'm describing.

I'm curious if net-positive FRC can be achieved by 'compressing' the plasma in a tokamak. This would involve a small or nonexistent inner column, which is a fundamental change from typical designs.

Apparently the 'Dynomak' was made at 1/10th commercial scale, with its "inductive drive" being predicted to be orders of magnitude more efficient than RF or neutral beam heating (not achieved in experiment).

It's intuitive that compressing an existing plasma torus heats it more efficiently -- if you have strong enough electromagnets. The dubious startup Helion claims numbers for prototypes and goals between 4T and 40T; perhaps 40T is out of reach with today's technology.

Here's a table of some links I read and quotes of why compression of tokamak plasmoid is infeasible.

#	link	quote
1	Can X-rays compress plasma for fusion?	"Confinement of plasma using photons has been considered on several occasions" ... "using RF to create 'plugs' at the end of linear pinch systems" ... "high-energy RF, not X-rays, and try to push the ions back"
2	Why is spherical magnetic confinement not used instead of tokamaks in nuclear fusion?	"spherical geometry... will leak plasma at at least one point"
3	How to make a small tokamak?	"Creating smaller reactors would be very good, but the confinement of plasma particles gets worse as the plasma gets smaller"
4	Is there a "pinch effect" in a Tokamak?	ZETA was a "stabilized pinch" design, but it turns out that did indeed help with the large-scale pinch, but did nothing to the small-scale "microinstabilities" ... "keep the ratio of external magnetic force to current below a certain threshold"
5	Can fusional nuclear compression theoretically be achieved with a unidirectional compressive force?	"there's no way the fuel mass will ever compress down" ... "fuel is deposited as a very thin layer on the inside of a denser "pusher" material so that it is collapsing into an empty center with so little material that the R-T instability is minimized"
6	Is the Dynomak concept physically sound?	"imposed-dynamo current drive (IDCD)" ... "the fluctuations of the magnetic field induced by IDCD might eventually spoil the confinement of the plasma"
7	Rayleigh–Taylor_instability	"instability... when the lighter fluid is pushing the heavier fluid" ... "instabilities in plasma fusion reactors"
8	Dynomak	"copper wall to conserve and direct the magnetic flux that is injected into the machine. This wall butts up against the plasma, creating the possibility of high conduction losses" ... "injection of magnetic helicity... break the magnetic flux surfaces... dynomaks' heating mechanism does not work as efficiently" ... "complex chamber geometry"

It seems that the Dynomak overcame the Hairy Ball problem in 2, but not 4 or 5.

Perhaps 6 (which links to a paper I haven't yet read) is most damning: fundamentally using magnets to create CT clobbers confinement. Is it true that the 'IDCD' means the Dynomak doesn't use neutral plasma? Are there alternatives to 'IDCD' that are expected to 'behave better'?

Post-Question

Most links seem to indicate that further research is necessary. Where is the frontier of research?

Would, for instance, further simulations with ALCON or FOCUS or some fugly fortran code be interesting, perhaps to calculate the strength of magnetic field required to achieve xxx plasma density or $\beta$ or whatnot? Surely it's not that difficult to shrink some sphere of magnetic flux and see what happens.

Lot's of links are provided in FRC; I'm looking for 'wisdom of the crowd' with this question before I digest a lot of literature.

---

Funny stuff

The spheromak wikipedia article mentions using "such plasmas as 'bullets' to fire at incoming warheads", as in literally shooting small fusion reactor spheromaks out of railguns.

Answer 1

How thoroughly has the idea of 'compressing' the torus of plasma in a tokamak been considered/experimented?

The entire tokamak concept came about due to attempts to compress toroidal plasmas. Every attempt ended in failure as a never-ending stream of new instabilities arose every time the last one was solved. ZETA was the straw the broke the camel's back - every measurement device they had said the plasma was stable, but this was simply because the new instability they generated was microscopic and none of their systems could see it, in contrast to the way the earlier kink instabilities could be easily seen with the naked eye.

While most simply gave up on pinch, Soviet researchers came up with the "safety factor", which limited the amount of pinch you could apply for any given layout and external field. This suggested greatly increasing the external field while reducing the pinch current would do the trick - and it did. However, the resulting pinch was orders of magnitude lower than in ZETA, which led to the requirement for external heating and all the resulting complexity.

The idea of RF plugs has been introduced on occasion, but it is difficult to arrange in the case of a toroid. The only practical schemes I have found are in mirror machines, where they are proposed as a way to plug the end losses, which are small annular areas. I am not aware of any machine that has actually used this approach, as better concepts came along quite rapidly, especially in the 1970s.

Ultimately the goal is not really to "make some fusion" but "make fusion in a practical device", and it is for this reason that many of the alternative devices fell away over time. While adding more and more stabilizing concept to the tokamak is certainly possible, the question is whether or not those systems can be realistically added to the layout of a production machine, in a form where they provide more stabilization than simply increasing the field from the magnets. To date the answer is always "no", increasing the magnetic field is something we still have a lot of headroom to explore, like CFS and TE, and if that doesn't work the relatively marginal improvements available in additional systems is almost certainly not going to help build a practical machine.

As to the dynomak, it's basically dead. I'm not sure why, but I cannot find any evidence of recent "real" work on a prototype machine.