Direct drive nuclear fusion propulsion

We covered Princeton Satellite Systems nuclear fusion space propulsion work last year. Here is some new work and designs.

A paper explores the use of a rocket engine based on nuclear fusion to rendezvous with and move an asteroid. The engine is based on a 5 MW Direct Fusion Drive (DFD) and is presented in the context of a conceptual spacecraft. The transfer orbit to the asteroid is developed along with the strategy for moving the asteroid. The bene t of using the DFD is that it can apply moderate variable thrust with high exhaust velocity, enabling it to reach asteroids more quickly and impart more delta-v than traditional propulsion methods.

The paper is organized as follows. They first provide an overview of the DFD design, with a brief discussion of the fundamental physics behind the technology. The reader is referred to previous publications for a more in-depth presentation. We then examine the threats posed by asteroids of different size, considering their relative likelihood and risk. Next, we discuss the overall mission design. We begin by describing the spacecraft design and concept of operations, followed by a deflection maneuver strategy that is based on achieving a desired relative state in b-plane” of the encounter. We then present an analysis of the deflection capability for an asteroid half the size of Apophis, and conclude with an example deflection maneuver.

The Direct Fusion Drive (DFD) is comprised of multiple innovations that together yield a safer, more compact, and lighter-weight engine that directly produces a high exhaust velocity and medium thrust, and in addition produces electrical power. The fi eld-reversed con guration (FRC) allows for magnetic con finement with a simpler, more natural geometry for propulsion than, for instance, a tokamak. The increased safety is due to the choice of an aneutronic fuel, D {3He. The plasma is heated by an odd-parity rotating magnetic fi eld (RMFo), which is predicted to promote better energy confinement, hence allow smaller, more stable engines. Other advantages include a small start-up system and a variable thrust propulsion system for more flexible mission designs

Many physics challenges remain before the RMFo-heated FRC can be developed into a practical reactor. The predictions of excellent energy con finement, stability, effi cient electron and ion heating, and current drive to fusion-relevant temperatures must be validated. Substantial progress has occurred in the first three areas. In 2010 and 2012, TriAlpha Energy Corp reported near-classical energy con finement time in their reactor. The direct drive reactor needs energy con finement time only 20% as long as the classical. In 2007, an RMFo heated FRC achieved stable plasma durations 3,000 times longer than predicted by MHD theory. By 2012 that record was extended to over 100,000 times longer. Finally, theoretical studies by Glasser, Landsman, and Cohen indicate that RMFo will be able to heat plasma electrons and ions to fusion relevant temperatures. These are promising starts, but much research is needed at higher plasma temperatures and densities and with burning, i.e., fusing, plasmas.

RF Heating can reduce the size of direct drive fusion reactor to about 0.5 meters in diameter. It would have about 10 megawatts of power.

Modular Aneutronic Fusion Engine This paper uses an engine of a moderate power,1 MW, and specific impulse of at least 10000 seconds.

A compact aneutronic fusion engine will enable more challenging exploration missions in the solar system. This engine uses a deuterium-helium-3 reaction to produce fusion energy by employing a novel field-reversed magnetic field configuration (FRC). The FRC has a simple linear solenoidal coil configuration yet generates higher plasma pressures for a given magnetic field than other designs. Waste heat generated from bremsstrahlung and synchrotron radiation is recycled to maintain the fusion temperature. The charged reaction products, augmented by additional propellant, are exhausted through a magnetic nozzle. As an example, we present a mission to deploy the James Webb Space Telescope from LEO to an L2 halo orbit using a one MW compact aneutronic fusion rocket engine. The engine produces 20 N of thrust with an exhaust velocity of 55 km/s and has a specific power of 0.77 kW/kg.

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