Small nuclear fusion space and energy systems using high efficiency RF heating
Princeton Plasma Physics Laboratory has two NASA grants.
1. High Efficiency RF Heating for Small Nuclear Fusion Rocket Engines
2. Superconducting Coils for Small Nuclear Fusion Rocket Engines
The aim for the fusion drives is to get about 1 kilowatt of power per 2.2 lbs. (1 kilogram) of mass. A 10-megawatt fusion rocket would therefore weigh about 11 tons (10 metric tons). They hope to demonstrate a first nuclear fusion system in 2019 to 2020.
The fusion reactors that Princeton Satellite Systems is developing uses low-frequency radio waves to heat a mix of deuterium and helium-3, and magnetic fields to confine the resulting plasma in a ring. As this plasma rotates in a ring, some of it can spiral out and get directed from the fusion rocket's nozzle for thrust. They could get very high exhaust velocities of up to about 25,000 kilometers per second [55.9 million mph].
The large amounts of thrust this fusion rocket may deliver compared to its mass could enable very fast spacecraft. For instance, whereas round-trip crewed missions to Mars are estimated to take more than two years using current technology, the researchers estimated that six 5-megawatt fusion rockets could accomplish such missions in 310 days. This extra speed would reduce the risks of radiation that astronauts might experience from the sun or deep space, as well as dramatically cut the amount of food, water and other supplies they would need to bring with them.
In addition, the fusion reactors could also help generate ample electricity for scientific instruments and communications devices. For instance, whereas NASA's New Horizons mission took more than nine years to get to Pluto and had little more than 200 watts of power to work with once it arrived, broadcasting about 1,000 bits of data back per second, a 1-megawatt fusion rocket could get a robotic mission to Pluto in four years, supply 2 million watts of power and broadcast more than 1 million bits of data back per second, Paluszek said. Such a mission could also carry a lander to Pluto and power it by beaming down energy
RF heating can theoretically efficiently heat nuclear fusion plasmas. However, radio frequencies will not penetrate deep into the plasma so this limits the potential nuclear fusion system to about 5 to 10 megawatts. This would still be a huge breakthrough.
A 10-megawatt fusion rocket could also deflect an asteroid about 525 feet (160 meters) in diameter coming at Earth, spending about 200 days to travel there and 23 days nudging it off course. Fusion rockets could even enable an interstellar voyage to the nearest star system, Alpha Centauri, although the trip might take 500 to 700 year.
Previous research suggested this kind of fusion rocket in the 1960s, but the designs proposed for them would not stably confine the plasmas, Paluszek said. About 10 years ago, reactor designer Sam Cohen figured out a magnetic-field design "that could make stable plasmas
High power nuclear fusion propulsion systems will require high efficiency radio-frequency heating systems in the MHz range for plasma heating. This proposal is for a novel scalable solid state Class E amplifier using Silicon Carbide switching transistors for plasma heating. This system is potentially 100% efficient compared to 40% for linear amplifiers and can be scaled to any desired size by adding additional segments in parallel. The system includes a novel closed loop feedback control system at the antenna and from the plasma. This eliminates the need for lossy transformers and other non-ideal components. The RF amplifier will be prototyped in Phase I in preparation for a plasma heating experiment in Phase II.
This technology is applicable to all radio-frequency applications for NASA. This includes microwave-heated thrusters, such as Ad Astra's VASIMR. VASIMR is a revolutionary new in-space propulsion system that heats a plasma with two types of microwave radiation. It can provide high thrust and high specific impulse in the 100 kW+ range.
The technology is also applicable to scientific experiments using radio-frequency technology. Current RF applications use tubes or Solid State Power Amplifier (SSPAs). These are typically linear amplifiers and only 40% efficient. Communication systems up to S-band will also be applications of this technology
Laboratory research and aerospace manufacturing using radio-frequency radiation done by NASA will also benefit. It will reduce costs of RF equipment and reduce power consumption.
This technology is applicable to a wide variety of commercial applications. These include:
HF band radars for coastal and over horizon systems (OTH)
Medium (MF) and High Frequency (HF) Radio (ITU Bands 5 and 6)
Communications channels up to S-band
Materials processing
Plasma heating for terrestrial fusion reactors
RF heating for manufacturing
Current RF systems use tubes, such as Traveling Wave Tube Amplifiers (TWTAs) and Solid State Power Amplifiers (SSPAs). These are less efficient than the switching amplifiers in this proposal.
One grant will look at the superconducting coils subsystem, a critical subsystem for the PFRC reactor and Direct Fusion Drive and other fusion and electric propulsion technologies. Their goal will be to design space coils using the latest high temperature superconductors. The coils will be operated at medium temperature, between 20 and 30 K, which eases the cooling requirements and temperature margins compared to 4K low-temperature conductors. This also increases the critical currents providing more margin for neutron radiation damage, possibly reducing shielding. The coils will have highly efficient cooling systems, be low mass and require minimum structural mass. Bath cooling and conduit cooling will be compared. There is likely an optimum operating temperature which minimizes the mass of both the conductors, shielding, and cooling systems. Given the rapid advancement of HTS materials determining the feasibility of such an optimal coil design requires detailed research into the state-of-the-art. Their partner, PPPL, will provide expertise on coil specifications and magnet design. PPPL is the only institution in the world where active research on the physics and technology of small, steady-state fusion devices is being performed. PSS will manage the design process and study closed loop cooling issues. They will design a Phase II experiment to build one or more 2 Tesla coils and potentially integrate them into the existing plasma experiment at PPPL. Our example mission will be a Neptune orbiter which is on the NASA roadmap as a high priority mission and present a challenging on-orbit radiation environment.
A small fusion engine such as Direct Fusion Drive would be useful for almost any deep space mission, as well as inner space missions such as Lagrange points or manned Mars missions. The superconducting coils have applications to scientific payloads as well as other advanced propulsion concepts. For example, the AMS-02 experiment for the ISS had a low-temperature superconducting coil option which was built and tested, but swapped out for a traditional magnet with a longer lifetime when the flight opportunity changed. The VASIMR electric thruster requires superconducting coils. There has been considerable research on using superconducting coils for radiation shielding and they may also be useful for space materials processing and and precision formation flying.
There are many military and civil applications of the engine and the coils. Military space applications include high-power Earth satellites with radar, laser, or communications payloads. There are wider applications including generators for wind turbines, high efficiency motors, particle accelerators, energy storage, and terrestrial fusion reactors. This project would contribute greatly to this wider body of work.
