I love vague terms like "long periods". Long compared to the Planck length? Geological time? Is the advertised 43 seconds almost there or "off by 17 orders of magnitude?"
I love vague terms like "long periods". Long compared to the Planck length? Geological time? Is the advertised 43 seconds almost there or "off by 17 orders of magnitude?"
A smooth toroidal magnetic field cannot confine plasma. The field at the outer side (further away from axis) are spread more widely and weaker than in the inner side. In a very short time, this will cause ions to drift out of confinement at the outer side. The solution is to produce a twisted, helical field, where the field lines go in circles in both directions of the toroid simultaneously, like the stripes of a candy cane in the bend.
Different reactor designs have different solutions to this. Tokamaks use a solenoid to drive a strong toroidal current in the plasma. This, in turn, causes a poloidal magnetic field, which provides the second half of the field needed for confinement. But this only works when magnetic field of the solenoid coil is varying smoothly over time in a single direction. Eventually, you hit some limit in your ability to do that, at which point you lose your ability to confine the plasma and the pulse ends.
Stellarators do not have this issue. They get the full field geometry needed from their primary field, by twisting it around the toroid in a very complex path. The downside is that they are much more difficult to design and build.