Nuclear fusion

This is one of the most speculative potential applications for APM.
The unreliability of the speculations presented herein must not be confused with the solid foundations of APM.

Types of fusion

Things we would desire for fusion energy productiona re among others:

• envirounmental friendliness
• low cost
• relatively small size and low weight

In how far may atomically precise technology be able to help in these regards?

Magnetic enclosure

For a basic introduction please consult the Wikipedia article about "magnetic confinement fusion" [1]

Field strength and plant size

The limit in achievable magnetic field strength has led to the construction giant prototype plants (see ITER). But tokamak devices can be scaled down greatly by raising the magnetic field that confines the plasma. Newer magnet technology (high temperature super conductors HTSLs) is promising some degree of notable size reduction (see company "Tokamak Energy").

It is yet unknown what kind of fields will be achievable once advanced APM gives us much more control over matter and lets us mechanosynthesize super-exotic thermodynamically metastable superconductor compounds.

It may not be unreasonable to assume that large margins for improvements are still possible but at this point this is highly speculative since it still critically depends on fundamentally unpredictable discoveries in science. It's hard to project how the performance of superconductors will further increase. This limits the range of possible serious predictions severely. (See: non mechanical technology path).

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The penetrative power of some kinds of radiation may pose limits on attempts to scale down magnetic confinement fusion devices. See section "radiation damage" for more details.

Power fluxes

• APT is great at dealing with very high levels of power density (see: Thermal energy transport).
  High thermal power densities require refractory materials.

With high magnetic fields and high plasma densities power fluxes become very high. (Potentially higher than atmospheric reentry of a space-shuttle) On the other hand smaller devices feature much more wall surface area per enclosed volume.

Note that a significant amount of energy does not get deposited directly onto the chamber walls but into the volume behind mainly due to neutron radiation.

Mechanical stability

Extremely high magnetic fields cause extremely high mechanical forces. Both structural frame parts and structural superconductor carrier strips need to be very sturdy. (Structural superconductor carrier strips: Note that most of the cables do not carry the current but provide the necessary mechanical strength)

Wile even with the next generation of HTSL high field tokamaks steel can provide sufficient strength with even higher fields this might not be the case anymore.

Abundant elemental metals on the other hand are by far not strong enough. There are exotic metal alloys that are a quite a bit stronger than what is used now but they contain elements that are rather rare and thus way too expensive to use in excessive quantities.

Thus one will want to go to cheap metamaterials that are based on strong gemstones like e.g. diamond (C), moissanite (SiC), colorless sapphire (Al₂O₂), ...
Those are hopefully strong enough. For all we know at the moment there cannot be anything much stronger than those.

By switching we incur a pretty severe problem though. Gemstones are much more susceptible to ionizing radiation than metals. Broken covalent bonds in gemstones are not always self healing like the electron-gas of metals can be.

There may be a workaround. See section "radiation damage" for more details.

One can speculate whether or not one could reduce the overall weight of a reactor far enough to make it suitable for application in spaceships. (investigation needed) Currently (2016...2018) it still pretty impossible to reach anything near light enough.

Radiation damage

APT is bad at dealing with radiation. (See: radiation damage) And some parts of the radiation (especially neutron radiation) are very hard to shield.

Radiation threatens the structural integrity of the frame and perhaps more critically it could lead to a breakdown of super-conduction (quenching) due to an accumulation of faults in the superconductor material.

One could try to treat the cause or the symptoms.

Treating the cause:

One could try to target fusion reactions that do produce almost no neutrons (are mostly aneutronic). But it probably will be really desirable to archive the capability to run solely on the most abundant hydrogen isotope (protium ¹H) (and other exotic fuel combinations).

Shielding is only possible in a limited sense. It cannot be influenced too much by the choice of materials.

Treating the symptoms

A solution to the rapid material degradation problem may be continuous active repair. A system that is continuously exchanging degraded structural microcomponents with new ones. Very unlike the normal use case where you want to avoid putting things together atom by atom every time anew and do microcomponent recycling instead. Here the massive inefficiency really is necessary and overcompensated by the reactors energy output anyway.

Designing such a pretty complex self repairing system would pose massive design effort. It should not be expected as one of the first products producable by advanced APM.

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Without applying at least one of the two approaches the inherent penetrative strength of neutron radiation may pose a fundamental hindrance for scale-down. The requirement to absorb most of the neutron radiation before it reaches coils and cryo-system then creates a natural lower end size limit for magnetic enclosure fusion. But in many cases this might be ok for non mobile power plants, where ease of long term operation is of more importance than size and mass.

Albeit non atomically precise in nature molten salt blankets are probably a pretty good idea since the crystal structures of liquids do not incur any radiation damage since they have none. For the the vessels they are confined in its probably desirable to use gemstone based metamaterials. They may not carry high mechanical loads but unlike metals they do not corrode.

Inertial fusion

(wiki-TODO: treat the following topics in more detail)

• macroscopic vibration damping
• neutral particle carriage acceleration
• highly symmetric enclosement (thermal and quantum mechanical uncertainty)
• low reflectivity of hydrogen - minimal isolating plasma shell thickness (severe!)
• fast cavity cleanout
• fast radiation seals
• carriage particle accelerators
• small scale Laser particle accelerators

Electrostatic fusion

This is astoundingly easy to achieve.
In fact it has been done repeatedly by several DIY projects.
The big caveat is that it can't get anywhere remotely near breakeven Q>1.

Protons or deuterons or whatever positive ions one uses hit the acceleration-wire-cage-wires too soon,
or one gets three body interactions that start to spread the precise energy from acceleration out to a more thermal distribution
making nigh all of the ions eventually gain so much energy that they hit the vessels walls long before doing a fusion event.

There are questionable attempts to fix it by adding magnetic fields like making the cage into coils (but not going to magnetic confinement scales).
(TODO: IIRC There is a clear issue why these approaches are very likely doomed to fail, related to Liouville theorem, to remember and explain here.)

Cold fusion

Sadly this does not work.
Assuming a naive press-together-approach here.
No cheating with a particle accelerator or nuclear radiation sources (which is hot particles).
There are some approaches that do that and still call themselves cold fusion, I won't count them here.

When pressing together two atoms with large directed mechanical force,
as will be possible with advanced mechanisms for mechanosynthesis,
then long before the nuclei get anywhere "near" each other one of the strong covalent bonds
that holds in place the atoms with the to-fuse-nuclei within
bends away to the side (or breaks). => Fail.
Adding a "catalytic" metal in the process won't change anyting on thios outcome.
Note that "near" is meant in a relative sense.
I.e. the ratio of the size of the nuclei to nucleus-to-nucleus-distance.

Beyond that due to the spacial confinement to the tip atom via chemical bonds the zero-point-energy of the nucleus is so big that
even near absolute zero of temperature the positional quantum delocalization of the nuclei should be much bigger than their own sizes.
So even if one could overcome the electrostatic repulsion with a pick-n-place approach (which is impossible as explained above) one would not get the nuclei to contact directly but only have as a low density probability cloud overlap.
While that overlap could well suffice the pre-condition is still what is highly prohibitive.
In any collapsed state snapshot of the two nuclei their positions they will well evade each other.
With a remnant barrier still way too big for tunneling.

It is perhaps vaguely similar thing to how future advanced neutral helium microscopy won't ever be able to image through gaps
that are smaller than helium atoms despite the matter wavelength being way smaller than helium atoms.
As in any collapsed state the helium atoms they do not fit through that gap.
Assuming the barrier to be too for tunneling.

Highly speculative one try one hit fusion

Warning! you are moving into more speculative areas.

• forum discussion

General notes

• Thermal throughput bottleneck thermal energy transmission
• self repair of thermal and radiation damage
• isotope sorting (e.g. tuning fork method) & closed loop nuclear waste recycling
• usage for spacecraft propulsion possible? - earth or space only?
• Implications of Liouville's theorem or "why nuclear mechanosynthesis don't work" - detour over thermal step unavoidable
• surface power/(heat flor) density limit - capsule based thermal energy transport (asymmetric figure eight loop in tokamaks?) may move it further down to more tacklable values.
• consistent high temperature stable designs SiC (H-passivation?)

Related

• APM and nuclear technology
• Spaceflight with gem-gum-tec
• sci-nanotech.com forum: Highly speculative discussion about "shoot-and-hit" fusion

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For replacing micro-parts before radiation damage amasses to the point of micro-systems gunking up:

• Gem-gum waste dissolution
• Atomically precise disassembly