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Lockheed Martin Corp said on Wednesday it had made a technological breakthrough in developing a power source based on nuclear fusion, and the first reactors, small enough to fit on the back of a truck, could be ready in a decade.

Is positive news definitely, but when it comes to fusion energy being skeptic is the wisest course of action. On the back of a truck basically means that the energy density of the device is such that the surface energy of a (steady state) plasma is beyond currents materials limits. It is probably a pulsed (ignitor-type) device, which are prone to instabilities. That could therefore refer to their breakthrough claim, of which they hardly say anything though. Lets see

Since there were already a lot of false alarms on the field, one should definitively be careful about it. However not a single detail about its structure or how it works in this new... Here http://www.fusenet.eu/node/400 you can find a bit more of information about it.

A new method of separating nuclear isotopes that exploits the slight differences in their electronic energy levels has been developed by physicists in the US. The energy-efficient separator was used to create isotopically pure lithium-7, which is used in some nuclear reactors. Good news for any future nuclear powered space missions perhaps? It could potentially replace the current (and much less energy efficient) methods that were developed in the 1950's

An exhaustive overview of all possible advanced rocket concepts, eg.. "As an example, consider a photon rocket with its launching mass, say, 1000 ton moving with a constant acceleration a =0.1 g=0.98 m/s2. The flux of photons with E γ=0.5 MeV needed to produce this acceleration is ~1027/s, which corresponds to the efflux power of 1014 W and the rate of annihilation events N'a~5×1026 s−1 [47]. This annihilation rate in ambiplasma l -l ann corresponds to the value of current ~108 A and linear density N ~2×1018 m−1 thus any hope for non-relativistic relative velocity of electrons and positrons in ambiplasma is groundless." And also, even if it would work, then one of the major issues is going to be heat dispersal: "For example, if the temperature of radiator is chosen T=1500 K, the emitting area should be not less than 1000 m2 for Pb=1 GW, not less than 1 km2 for Pb=1 TW, and ~100 km2 for Pb=100 TW, assuming ε=0.5 and δ=0.2. Lower temperature would require even larger radiator area to maintain the outer temperature of the engine section stable for a given thermal power of the reactor."

The problem is not achieving a high energy density, that we can already do with nuclear fission, the question however is how to confine or harness this power with relatively high efficiency, low waste heat and at not too crazy specific mass. I see magnetic confinement as a possibility, yet still decades away and also an all-or-nothing method as we cannot easily scale this up from a test experiment to a full-scale system. It might be possible to extract power from such a plasma, but definitely well below breakeven so an additional power supply is needed. The fission fragments circumvent these issues by a more brute force approach, thereby wasting a lot of energy for sure but at the end probably providing more ISP and thrust.

Sure. However, the annihilation based photon rocket concept unifies almost all relevant drawbacks if we speak about solar system scales, making itself obsolete...it is just an academic testcase.

NASA and the Department of Energy's National Nuclear Security Administration (NNSA) have successfully demonstrated a new nuclear reactor power system that could enable long-duration crewed missions to the Moon, Mars and destinations beyond. Main page for kilopower project: https://www.nasa.gov/directorates/spacetech/kilopower