[DennisP]

Here's the last of a great series of articles on that idea (with links to the earlier articles): https://asiatimes.com/2020/05/meet-the-father-of-the-hydroge...

And a startup hoping to try it, run by the guy who came up with the idea decades ago: https://www.hb11.energy/

There are several groups doing experiments with it, and it seems to be going really well.

There are two lasers. One hits a target that generates a magnetic field; it'd be hard to describe without a picture but see the articles at the first link. Basically the laser blasts electrons off a metal surface, they hit another surface and flow through a coil. For a nanosecond there's a 4000 tesla field. (An MRI machine generates around 3 tesla.)

The second laser is faster and more powerful: 10 petawatts or more, for only a picosecond. That hits the fuel. It's enough to kick off fusion by itself, but the magnetic containment creates an avalanche effect that multiplies output. Then it all blows up, you harvest the energy and cycle in another target.

[moneytide1]

Thanks for the link. It is a much more thorough explanation.

How fast is the fuel used up within the field? Would there be a way to inject the actively fusing reaction with a steady fuel input rate for long term generation (neutron bombardment embrittles superconducting metal containment with the D/T reaction, unlike boron encased in supposed laser induced magnetic field?)

I'd imagine this would occur in a sphere (closed and contained). Tokamak designs aren't spheres, but also closed relying on magnetism to push back against a reaction that is pushing out as fusion occurs:

  To produce thrust - what if it was a half sphere somehow? Propellant implies ejection of something, and a fusion reaction ball is magnetically interactive, with no radioactive material byproduct? What if a fusion thruster harvested some energy from the reaction to  "push" back against an actively fusing pellet feed rate? Could this propel a craft or am I missing something fundamental here?

[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"

[moneytide1]

Formatting error, last paragraph:

To produce thrust - what if it was a half sphere somehow? Propellant implies ejection of something, and a fusion reaction ball is magnetically interactive, with no radioactive material byproduct? What if a fusion thruster harvested some energy from the reaction to "push" back against an actively fusing pellet feed rate? Could this propel a craft or am I missing something fundamental here?