New records on Wendelstein 7-X

I thought the expectation was that actually-operating fusion plants would operate in pulses rather than continuously, but I could be misremembering.

Toroidal reactors have to operate in pulses. Stellarators can be operated in steady-state (although sometimes they are pulsed to achieve higher energy).

But don't you need to "refuel" now and then?

W7x has a pellet injection system now.

This is shared in the better article here: https://www.ipp.mpg.de/5532945/w7x

> During the record-setting experiment, about 90 frozen hydrogen pellets, each about a millimeter in size, were injected over 43 seconds, while powerful microwaves simultaneously heated the plasma. Precise coordination between heating and pellet injection was crucial to achieve the optimal balance between heating power and fuel supply.

Tokamaks can also be operated in steady-state, at least theoretically. The reason a tokamak is pulsed is due to the fact the toroidal current is driven inductively, so there is a limit to how long you can keep increasing the current in the central solenoid. However there are other methods, for example, neutral beam injection and electron cyclotron current drive. You can even exploit the bootstrap current (self-generated by collisional processes in the plasma) to obtain a near 100% non-inductive toroidal plasma (this is called "advanced tokamak" regime).

Anyway, the older generation of devices was pulsed for engineering reasons (like non-superconducting coils getting too hot). The current generation of device is solving most of these and is limited by MHD instabilities alone (neoclassical tearing modes, mostly), if we can get active control mechanism working, then will be finally approach the long-pulse or steady-state regime.

Refueling is not why tokamaks are pulsed.

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.

Neat, thanks