Comment by willis936
4 days ago
Tokamaks need the central solenoid to have a current ramp, so at some point you run out of voltage. You can turn that way down, but you get less plasma performance. You're traditionally limited by heat rejection capabilities of the vacuum vessel.
These are science machines to learn about plasma and increase performance of future machines. A real reactor involves a lot of engineering to handle the heat rejection problem (and turn it into a revenue stream if you're clever). In terms of the pulsed nature: not really a problem if you keep the duty cycle high enough and maintain sufficient buffers in your coolant to keep the turbines happily turning away.
I learned recently that another limit to plasma duration is contamination. As fusion occurs and high energy particles that escape magnetic confinement blast the toroid wall, ions of metal get mixed into the plasma and degrade performance.
I've seen photos of what the inside of experimental tokamaks look like after many cycles. Metal is eroded away and deposited around the chamber in interesting patterns. Unfortunately a image search isn't surfacing the images I have in mind.
Does that mean it's impossible to have steady-state operation? (And are stellarators different here?)
Impossible with just the basic inductive principle of tokamaks, yes. Some years back I learned that you can keep the current going with microwaves, not sure about recent progress. You can also approximate steady state by reversing polarity of the warp - ehm - magnetic field regularly.
Yes, stellarators are different.
The current ramp in the central solenoid is used to set the plasma rotation direction. Theoretically this is more like Alternating Current, since there is not a fundamental reason the central solenoid couldn't ramp back down (and then proceed to ramp up in the opposite polarization). The existing plasma would need to be cooled and removed first, or some similar mechanism of stabilizing the torus again in the opposite direction.
I look at it as a large optimization problem at this point. Each part of the machine is workable but not yet sufficiently optimized to achieve profitable operation.