You would then run a heat exchanger from the hot lithium to create steam to then turn a turbine and make electricity.

Examples of fusion fuels whose main reaction produces only charged products are deuterium helium-3 and proton boron-11. These reactions however require higher temperatures and better confinement characteristics.

The reason why deuterium tritium is the major focus of most (though certainly not all) research is that it has the highest reactivity at the lowest temperature compared to other fuels. Unfortunately it produces a high energy neutron which makes the conversion to electricity more complex.

There's just the matter of getting the temperature up to 10x of an ordinary D-T fusion plasma. Non-equilibrium reactors like the polywell try to bypass the problem entirely, but (to my knowledge) it's very hard to maintain a non-thermal state.

I used to work with under a guy who worked on ITER, they were all aware of that part of the problem. It just wasn't part of the research they were engaged in.

True for any field you are not an expert in.

This sounds simple, but actually shows one of the biggest problems with fusion. The wall can handle only a certain power/area before the engineering becomes too difficult. And because the wall has to be outside the plasma, the ratio of the wall area to plasma volume will be quite low. In contrast, in a fission reactor, the "wall" is the walls of the fuel rods, which are only about 1 cm in diameter. There is lots of surface area through which to convey the heat.

The square/cube law comes into play here. Fusion reactors will have terrible volumetric power density, compared to fission reactors (or, even worse, internal combustion engines like gas turbines.) As a result, fusion reactors will be massive, expensive things, compared to the competing source of heat.

The figures for existing reactor proposals illustrate this. The volumetric power density of a PWR primary reactor vessel is ~20 MW/m^3. In constrast, the power density of the ARC proposal from MIT (counting the volume of the reactor + blanket + magnet shield + magnets + support structure) is around 0.5 MW/m^3.

If someone breathlessly hypes some fusion reactor design, ask them what its volumetric power density is.

For example: the plasma facing surfaces on ITER are backed by a CuCrZr alloy with high thermal conductivity. The amount of direct thermal power this innermost wall can withstand, from the plasma and photons, is proportional to this thermal conductivity, since stress there is proportional to the thermal gradient.

But in an actual reactor, designed to operate for long periods of time, this alloy cannot be used, as it becomes too activated. Instead, DEMO is going to use a kind of reduced activation steel (Eurofer 97) in which the thermal conductivity is an order of magnitude lower. This is one of the things pushing DEMO far outside the range of potentially competitive cost.

It seems to me that a thick (~ cm at least) flowing liquid first wall is probably the way to go for achieving high power density. The liquid has its own problems though, such as corrosion compatibility with the underlying structural material, the need to have low vapor pressure at high temperature, the possibility of splashing, and MHD effects if the liquid is conductive.

They seem to have taken to hear some lessons learned from previous experiments (cannot assume lossless compression, must pay attention to RT instabilities, must think about vaporization of first-wall).

One downside to a fully liquid blanket: the peak temperature is limited by the vapor pressure of the first wall, which in turn is limited by the impurity tolerance of the plasma. In a non-imploding device, it would be possible to run the majority of the blanket at a higher temperature & separate that from the cooler first-wall liquid. Of course, this puts some solid material closer to the neutrons, so YMMV.

https://arpa-e.energy.gov/sites/default/files/ALPHA_TURCHI.p... https://arpa-e.energy.gov/sites/default/files/06_TURCHI.pdf

Radiation is a function of temperature (to the 4th power) and surface area. It seems even harder to channel the heat from the plasma to the engine this way.

So how would you do either of these 'more so'?

Note: I am not a physicist and has no idea if it really works. But I guess when it comes to Nuclear Fusion no one really does.

[1] https://www.intelligentliving.co/future-fusion-radiationless...