Game Changing Direct Drive Fusion Propulsion Progress

Direct Fusion Drive, is a unique fusion engine concept based on the Princeton Field-Reversed Configuration (PFRC) fusion reactor under development at the Princeton Plasma Physics Laboratory. The truly game-changing levels of thrust and power in a modestly sized package could integrate with our current launch infrastructure while radically expanding the science capability of these missions. NIAC grants require that a technology be studied in the context of a specific mission. The mission context is the delivery of a Pluto orbiter with a lander, which cannot be done with any other technology. Direct Fusion Drive (DFD) provides moderate thrust to allow for reasonable transit times to Pluto while delivering substantial mass to orbit: 1000 kg delivered in four years using 5 N constant thrust.

Since DFD provides power as well as propulsion in one integrated device, it will also provide as much as 1 MW of useful electrical power to the payloads upon arrival. This enables high-bandwidth optical communication, powering of the lander from orbit, and radically expanded options for instrument design.

The Princeton Field-Reversed Configuration (PFRC), which is a steady-state fusion reactor concept with heating via rotating magnetic fields. A thrust model of the reactor is presented that is based on a fluid code modeling the exchange of energy in the plasma surrounding the reactor.

The Direct Fusion Drive concept is an extension of ongoing fusion research at Princeton Plasma Physics Laboratory dating to 2002. The Princeton Field-Reversed Configuration machine (PFRC) employs a unique radio frequency (RF) plasma heating method invented by Dr. Samuel Cohen. Odd-parity heating was first theorized in 2000 and demonstrated in the 4 cm radius PFRC-1 experiment in 2006. Experiments are ongoing with the second-generation machine, PFRC-2, which has a plasma radius of 8 cm. Studies of electron heating in PFRC-2 have surpassed theoretical predictions, recently reaching 500 eV with pulse lengths of 300 ms, and experiments to measure ion heating with input power up to 200 kW are ongoing.

When scaled up to achieve fusion parameters, PFRC would result in a 4-8 meter long, 1.5 meter diameter reactor producing 1 to 10 MW. This reactor would be uniquely small and clean among all fusion reactor concepts, producing remarkably low levels of damaging neutrons.

The plasma heating method uses a unique configuration of the radio antenna. Attempts have been made to heat FRC plasmas with RF before, but always with single-loop antenna that resulted in a near-FRC plasma but with open field lines. They call this even-parity heating due to the symmetry of the induced magnetic field. Open field lines give the plasma an opportunity to escape and reduce confinement time. In contrast, the PFRC antenna are figure-8s. Two pairs operate 90 degrees out of phase on adjacent sides of the plasma. An antenna (wrapped in orange Kapton tape) is clearly visible on the side of PFRC-2 . This results in so-called odd-parity heating – the magnetic field on one side of each figure-8 is in the opposite direction as the other side – and closed field lines in the generated FRC. Closed field lines keep the plasma trapped as it is heated. The oscillation of the currents in the RF antenna result in a rotating magnetic field, RMF, with about 0.3% of the strength of the axial magnetic field.

Reduction in neutrons is achieved through multiple channels. First and foremost, our choice of Helium-3 (3He) and deuterium (D) fuels results an aneutronic primary reaction. However, there are still D-D side reactions, the proportion of which is governed by the fusion cross-section at the relevant ion temperature.

The D-D side reactions are divided equally into two, a D-D reaction producing a 2.45 MeV neutron and a D-D reaction producing tritium (T). This tritium can then fuse with deuterium to produce a high energy 14.7 MeV neutron.

The PFRC is specifically designed to produced the absolute minimum number of neutrons from these side reactions.

1. the small size of the reactor results in a favorable ratio of surface area to plasma volume. reducing the wall load compared to larger machines.
2. they adjust the operating fuel ratio of 3He:D to 3:1, sacrificing some power density for lower radiation.
3. the reactor is designed to rapidily eliminate the tritium produced by the D-D side reactions, preventing any D-T reactions from occurring. This means that the only neutrons produced are those with energy of 2.45 MeV.
4. A reduction in neutrons may occur due to preferential heating of the 3He over the D by the rotating magnetic fields; at the correct frequency, they estimate that the 3He may reach a temperature of 140 keV while the D is only at 70 keV. This would result in another (CHECK) 10 fold reduction in neutrons. The final result of these design features is that only 1%, or perhaps even only 0.5% or less, of the fusion power is in 2.45 MeV neutrons, and the power in 14.7 MeV neutrons is effectively zero.

Initial analysis of the Pluto mission relied on simple Lambert trajectories that assumed relatively short acceleration and deceleration phases at the beginning and end of the trajectory. The power level is an input, along with a specific power and thrust efficiency, which then determines the Isp given a thrust level. This simple analysis indicated that assuming a fairly conservative specific power of 0.7 kW/kg, a mission was possible under 8000 kg using two 1 MW engines. A single SpaceX vehicle or Delta Heavy could launch the mission.

The mission would deliver 1000 kg to Pluto and then provide about 1MW of electrical power on orbit. The mission parameters assuming a 74 km/s transfer, which is possible for a launch date in 2036.

A burn time of 292 days corresponds to a required thrust of 16.2 N and an Isp of 12,554 seconds, assuming an overall thrust efficiency of 0.5.

Critical subsystems
include the superconducting coils, space radiators, cryogenic systems, high-power optical transmission systems, and thermal conversion. Available data suggests that research in all these areas has made tremendous
progress recently and we have not identified any roadblocks.

They received a Phase II NIAC grant in May, 2017 to continue this work, and also two NASA STTRs relating specifically to the RF subsystem and superconducting coil subsystem. We will be further refining our models and designs to create a roadmap for building the first DFD prototype. Ongoing work at PPPL on proving ion heating using the RMFo method in PFRC-2 will need to be followed by a larger machine with superconducting coils that can heat the plasma sufficiently to demonstrate fusion. We are actively searching for funding mechanisms to continue this research.

A compact 1 MW space power plant would be truly game-changing for multiple applications. In Earth orbit, it enables high-power payloads like radar as well as nearly unlimited orbit maneuverability.

In cislunar space, DFD could transform the envisioned Deep Space Gateway while also powering human bases on the lunar surface. Robotic missions to asteroids, Jupiter and its moons, and any other deep space destination become more faster, cheaper, and can return orders of magnitude more science. There are many missions that can be accomplished now with a small amount of helium-3 from terrestrial sources, and enormous reserves are available on the moon for future missions.

Under IR&D, they are developing Space Rapid Transit (SRT), a two stage to orbit launch vehicle that takes off and lands horizontally. SRT has an air-breathing first stage and an LH2/LO2 propelled second stage.


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