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Fusion energy for space propulsion

Opening the Solar System to human colonization

Fusion energy is, in principle, the only conceivable source of energy for rapid, efficient, rocket space travel to Mars, the outer planets, and nearby stars, if the payloads are heavy and/or designed to carry humans. From fuel selection (deuterium-tritium, deuterium-helium 3, proton-boron, matter-antimatter, etc.) to confinement concepts (magnetic, inertial, magneto-inertial, etc.) and conversion methods (direct propulsion, electric conversion, etc.), fusion reactor concepts are not sufficiently developed to determine the detailed technical requirements and to establish the full spectrum of challenges that must be addressed.

Compared to fusion energy for terrestrial electrical power, the physics/engineering requirements for space power and propulsion may be different, and more or less difficult. For example, the quality of vacuum is a major consideration in terrestrial fusion concepts whereas it is a minor consideration for space-based fusion concepts. The necessary effort for investigating the feasibility of fusion energy for space power and propulsion could be estimated by comparing the effort required to develop fusion energy with that required to develop a new technology for operation in space. The Dept. of Energy Office of Fusion Energy Sciences defines five stages of development for fusion energy technology, which can be compared to NASA's ten Technology Readiness Levels (TRLs) for space technology:

 

1.   Concept Exploration (CE)

CE research programs are typically at <$5M/year and involves the investigation of basic characteristics. Experiments cover a smaller range of plasma parameters (e.g., at <1 keV) and have fewer controls and diagnostics than a PoP level experiment. However, sufficient diagnostics are required to carry out high quality, scientific investigations.

  • Examples: university-scale experiments.
  • Time: Takes about 5-10 years.
  • Cost: <$5M/yr, for a total of <$50M.
  • Milestone: favourable basic characteristics ?
  • Approx. equivalent: NASA TRL 1-2 (Basic princples observed and reported; Technology concept and/or application formulated).

 

2.   Proof-of-principle (POP)

POP research is the lowest cost program ($5M to $30M/year) to develop an integrated understanding of the basic science of a concept. Well-diagnosed and controlled experiments are large enough to cover a fairly wide range of plasma parameters, with temperatures of a few kiloelectron volts, and some dimensionless parameters in the power plant range.

  • Examples: large university or national laboratory-scale experiments; MAST, NSTX-U, MST, Alcator C-Mod…
  • Time: Takes another 10-15 years, for a total of 15-25 years.
  • Cost: $5M-30M/yr, for a total of $50M-450M.
  • Milestone: favourable scaling of parameters and integrated understanding ?
  • Approx. equivalent: NASA TRL 3-4 (Analytical and experimental critical function and/or characteristic proof of concept; Component and/or breadboard validation in laboratory environment).

 

3.   Performance Extension (PE)

PE research programs explore the physics of the concept at or near fusion-relevant regimes. Experiments have a very large range of parameters and temperatures >5 keV, with most dimensionless parameters in the power plant range. Diagnostics and controls are extensive.

  • Examples: large national or international-scale experiments; JET, DIII-D, JT-60…
  • Time: Takes another 15-20 years, for a total of 25-45 years.
  • Cost: $30M-300M/yr, for a total of $450M-$4.5B.
  • Milestone: favourable physics at relevant regimes ?
  • Approx. equivalent: NASA TRL 5-6 (Component and/or breadboard validation in relevant environment; System/sub-system model or prototype demonstration in relevant environment).

 

4.    Fusion Energy Development (FED)

FED programs develops the technical basis for advancing the concept to the power plant level in the full fusion environment. It includes ignition devices, integrated fusion test systems, and neutron sources.

  • Examples: very large national or international-scale experiments; ITER.
  • Time: Takes another 15-20 years, for a total of 60 years approximately to get here.
  • Cost: $300M-$3B/yr, for a total of $4.5B-$45B.
  • Milestone: achieved breakeven and ignition ? Favourable technical scaling ?
  • Approx. equivalent: NASA TRL 7-8 (System prototype demonstration in relevant environment; actual system completed and "flight qualified" through test and demonstration).

 

5.    Demonstration power plant (DEMO)

DEMO is constructed and operated to convince electric power producers, industry, and the public that fusion is ready for commercialization.

  • Examples: none yet.
  • Time: Takes another 15-20 years probably, for a total of 80 years approximately from concept-to-commercialization.
  • Cost: $300M-3B/yr, for a total of $8B-100B (order of magnitude).
  • Milestone: commercially viable electricity production ?
  • Approx. equivalent: NASA TRL 9 (Actual system flight proven through successful mission operations).

 

The effort required to demonstrate the viability of fusion energy for terrestrial power is considerable, and historically proceeds approximately by orders of magnitude across each stage of development. The level of effort required to demonstrate the viability of fusion energy for space propulsion is still an open question, but significant overlap between terrestrial fusion and plasma physics research programs allow for their concept exploration.

 

Collaboration with Icarus Interstellar Project

The Icarus Interstellar project aims to provide a blueprint for the feasibility of a robotic interstellar mission based on fusion energy concepts. It is an updated study of the 1970s Project Daedalus by the British Interplanetary Society. Our group provides occasional advice and expertise.