Beyond Pluto: Status of Direct Fusion Drive

Author: Dr. Charles Swanson, senior scientist at Princeton Satellite Systems.

Background: The Direct Fusion Drive (DFD), based on Princeton Plasma Physics Laboratory’s (PPPL’s) Princeton Field-Reversed Configuration (PFRC) experiment and reactor concept, is a concept for a steady-state magnetic confinement fusion reactor designed from the ground up to be compact and portable, particularly as an in-space rocket engine. Princeton Satellite Systems (PSS) has had three NASA grants supporting research into DFD: a NASA NIAC which concluded in May, 2019; a Phase I STTR on the RF heating system; and an ongoing Phase II STTR on superconducting magnets.  We have leveraged these grants into a new ARPA-E effort under their 2018 OPEN solicitation, as part of a new ARPA-E focus on fusion. The DFD is well-suited to be the propulsion system of interstellar precursor and interstellar missions.

Objectives: Our experimental objectives are to demonstrate the plasma physics required to confine and heat plasma to fusion-relevant conditions. Our engineering design objectives are to produce high-fidelity models of the various subsystems required for a DFD-based interstellar spacecraft, including neutron shielding, heat recovery, RF amplifiers, superconducting magnets, optical communications, and in-space startup. Our mission architecture objectives are to determine the trajectories and payloads appropriate for interstellar precursor and interstellar missions.

Methods: To accomplish our experimental objectives, we are performing electron and ion heating experiments on the PPPL PFRC experiment, supported by an ARPA-E grant. We are also applying appropriate plasma physics models to the outstanding physics questions. To accomplish our design objectives, we are performing studies on each of the subsystems to determine suitable near-term technology. To accomplish our mission architecture objectives we are consulting with planetary scientists to determine the required and desired scientific payloads, and performing trajectory analyses of interstellar precursor and interstellar missions.

Results: We will present the current status of DFD design, our ARPA-E electron and ion heating milestones, and the plan and status for our superconducting magnet experiments. We will also present the relevance of DFD to interstellar missions including interstellar precursor, the solar gravitational lens, and a rendezvous mission to Alpha Centauri. A DFD mission to Alpha Centauri would take longer than a beamed mission but enable orbit insertion around an exoplanet and provide substantial power – hundreds of kW – for returning data.

TRL Assessment: We evaluate the TRL of the DFD to be 2.

Near-term milestones:

  • Next-level design of the interstellar DFD spacecraft
  • Demonstrate ion heating in the PFRC-2 experiment
  • Design the PFRC-3 experiment


  • Build a balance-of-plant testbed to demonstrate all subsystems of the DFD, without fusion
  • Build a plasma physics testbed to demonstrate reactor-relevant plasma physics and fusion yield, without thrust production or energy capture
  • Build a terrestrial prototype power plant to provide dispatchable, portable, high-value remote power

Conclusions: The DFD is a near-term steady-state magnetic confinement fusion propulsion technology for interstellar precursor and interstellar missions, which would require a much longer mission time than a beamed-power approach, but enable an orbital insertion at the target system and provide substantial power for interstellar communication.

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