[DennisP]

The magnetic field disappears in a nanosecond, the fuel pellet gets used up, and it explodes, destroying the coil that generated the field. So you just send in another target assembly and fire the laser again, every second or two.

There's nothing wrong with a pulsed system like that. Lots of fusion designs are pulsed. A gasoline generator with an internal combustion engine is a pulsed system too.

Add a magnetic nozzle and you could definitely turn this into a rocket. Thrust would be low but efficiency very high, so it'd be useless for launch but great for long-distance travel.

[pfdietz]

There is a very general problem with pulsed systems for fusion. The issue is that plasma-facing surfaces are confronted with extreme instantaneous power levels. The depth to which heat can diffuse is proportional to (pulse length)^(-1/2). A nanosecond pulse will deposit heat in a tenth of a micron thickness, or less.

This forces any fusion reactor that uses pulses to have a sacrificial ablative layer on these surfaces that must be renewed (and to deal with the forces from the explosive vaporization of this thin layer). This is problematic if the reactor also requires high vacuum. The scheme for p-11B fusion that this subthread was talking about, for example, has been presented with a direct conversion scheme that uses a megavolt level vacuum capacity. Imagine what happens to such a capacitor when its surfaces flash superheated vapor.

[DennisP]

Interesting. But if the direct conversion works, then the magnetic field removes most of their kinetic energy from the charged particles before they get to the walls. All it gets is the x-rays. There has to be some radius where that's no longer a problem. Each pulse in this design would be about 300 kWh; offhand I don't know what percentage is x-rays.

If it's too hard to maintain vacuum, then reverting to a plain ol' thermal cooling could be a backup plan.

[pfdietz]

A magnetic field leaves the kinetic energy of a charged particle unchanged.

The electric field is supposed to reduce the energy of the alpha particles, but (1) the alphas from p-11B are not monoenergetic, and (2) what is keeping the electrons (that are inevitably liberated in the extremely energetic explosion of the target, the impact of the alphas with the collecting electrode, and photoelectric emission from all surfaces exposed to photons from the plasma) from shorting the whole thing out?

The scheme does not make any sense.

[pfdietz]

"capacitor"