I serve on the External Council of NIAC, NASA's Innovative Advanced Concepts program, an organization that funds and encourages innovative ideas that are applicable to the US space program.  NIAC had a meeting this June in Washington , DC , and there I heard a presentation describing an innovation that I've awaited for many years: the promise of a rocket that is propelled by the energy of nuclear fusion.  Chemical rockets, the workhorses of our present space program, produce too little energy per kilogram of fuel and have exhaust velocities and specific impulse values that are too small.  Elaborate and very expensive aerospace engineering efforts are necessary to get around the inadequacies of chemical rockets.   The high energies and high exhaust velocities provided by fusion for space propulsion are exciting prospects.  In particular, fusion rockets should have specific impulse values (how much push you get per mass of fuel) that are 10 to 100 times larger than those of chemical rockets.  Amazingly, there is now a line of development that may make fusion rockets possible.  To begin this discussion, I want to review a few basic ideas about nuclear fusion.

The energy source that heats most stars is the nuclear fusion of hydrogen.  For our Sun, it is a multi-step process involving the strong and weak interactions that starts with four protons (symbol p, the nucleus of a hydrogen atom) and finishes with the production of helium-4 (⁴He, a mass-4 helium nucleus containing two neutrons and two protons), along with some neutrinos, positrons, and gamma rays.  In the deep interior of the Sun the temperature and pressure are high enough to fuse two protons to deuterium (d, mass-2 hydrogen nucleus containing one neutron and one proton), to fuse deuterium and a proton to helium-3 (³He, mass-3 helium nucleus containing one neutron and two protons), and to fuse two helium-3 nuclei to one helium-4 (⁴He, two protons and two neutrons) plus two free protons.  This is the so-called p-p fusion chain that dominates fusion processes in all stars having the mass of our Sun or smaller.

Fusion machines on Earth like the large and expensive ITER tokomak machine being developed in France must operate at lower temperatures and pressures than those in a star and must attempt to produce usable energy from controlled fusion using a simpler single-step process that fuses deuterium and tritium (t, mass-3 hydrogen nucleus containing two neutrons and one proton) into a ⁴He and a neutron (reaction: d+t → ⁴He+n).  Much of the energy from this process is given to the neutron with a kinetic energy of 14.1 million electron volts (14.1 MeV).  This neutron steals valuable energy and presents hazards for both radiation-sensitive materials and for human operators.  Usually the fast neutrons must be moderated with massive shielding and captured in a lithium blanket to produce additional energy.  The break-even temperature, the temperature to which the d+t plasma (ionized atoms) must be heated in order to produce more fusion energy than was required to create it, is 13.6 thousand electron volts (13.6 keV).  Fusion machines are designed to contain such plasmas in a strong magnetic field while the plasma is heated, fusion occurs, and energy extracted.

The problem with applying such fusion-machine designs to fusion-driven space propulsion is that they are typically very large and expensive, they have not yet produced any useful fusion energy, they have no obvious exhaust ejection path for propulsion, and they would produce large quantities of 14.1 MeV neutrons in close proximity to payloads and crew, with their neutrons removing energy needed for propulsion and requiring many tons of shielding material.  It is apparent that d+t fusion, while possibly useful for power applications on Earth, is inappropriate for space applications.  Clearly, a different approach is needed.

Such a new approach may be the direct fusion drive rocket (DFDR) design of the Princeton Plasma Physics Laboratory, which has provided the basis for a new NIAC Phase I grant to Stephanie Thomas of Princeton Satellite Systems.  At the NIAC meeting I attended, she discussed a mission to Pluto using a DFDR drive.  The DFDR geometry is a cylinder that uses the so-called "field-reversed configuration".  This means that the loading of the plasma in the system employs a trick.  A plasma is initially created within a magnetic field from pinch coils at the two ends of a cylindrical polycarbonate vacuum vessel.  This field is aligned in along the axis of the cylinder and causes the ions to orbit around the lines of magnetic flux, preventing them from moving away from the axis of the cylinder and sustaining the field.  Then the current in the pinch coils is abruptly reversed, applying a new external magnetic field in the opposite direction.  This abrupt change compresses and heats the plasma and creates a layered and increased magnetic field, with flux lines in the inner region in one direction and those in the outer region in the opposite direction.  The outer magnetic field lines attempt to expand outward but are confined within the cylinder by a set of passive warm-superconducting "flux conserver" pancake coils spaced along the length of the cylinder.  These coils develop induced currents of thousands of amperes when field lines attempt to cross them, keeping the magnetic flux lines within the cylinder.  The trapped ions execute figure-8 orbits at the boundary between the inner and outer fields.

The DFDR is designed to fuse deuterium and helium-3 to form a proton and a helium-4 nucleus (reaction: d+³He → p+⁴He), liberating 18.3 MeV of usable charged-particle kinetic energy in each fusion reaction.  Note that neutrons are not produced in this primary reaction.  However, because deuterium is present in the plasma, the fusion of two deuterium nuclei to tritium and a proton or to helium-3 and a neutron with a kinetic energy of 2.45 MeV (reactions: d+d → t+p or d+d → ³He+n) are both secondary reactions that will also occur in the heated plasma.  Further, if tritium from the first reaction is allowed to accumulate, one could again face the problem of dealing with 14.1 MeV neutrons from d+t fusion.  Fortunately, the DFDR design includes mechanisms for suppressing d+d reactions by selective ³He heating and for moving tritium from the plasma into the exhaust stream as it is produced.

We note, however, that ³He is an expensive fuel, mainly a small byproduct derived from helium-rich natural gas wells.  Its use in fusion requires a considerably higher plasma temperature for fusion.  While the break-even temperature for d+t fusion is 13.6 keV, the break-even temperature for d+³He is 58 keV, more than three times higher.  Thus, an effective method for heating the plasma to this high temperature is a key element of the DFDR design.

This heating is accomplished by applying to the plasma an "odd-parity rotating magnetic field" produced by four rectangular figure-eight shaped antenna coils surrounding the cylinder above, below, and on the sides.  The drive currents in the four antenna coils are sequenced so that the resulting magnetic field is always perpendicular to the cylinder axis and rotates at a drive frequency in the MHz region.    The "odd parity" description means that the front and rear parts of the antenna coils have oppositely circulating currents and produce magnetic field that point in opposite directions.  This drive field oscillates near the cyclotron resonance frequency of ³He and therefore selectively transfers more heat to ³He ions than to d ions.  In particular, ³He ions are heated to about 100 keV while d ions are heated to only 50 keV.  This differential heating enhances ³He+d fusion with respect to d+d fusion by a large factor, minimizing the production of neutrons and tritium.

The use of the rotating magnetic field brings another advantage: following fusion ignition of the plasma, the same antennas can be used to directly extract electric power from the plasma ions through induction.  The result is a fusion rocket that also directly generates its own electric power, a very significant advantage for space missions that venture far from the Sun, where solar panels are not efficient.

As illustrations of the utility of the DFDR, Princeton Satellite Systems has designed and described in some detail a spectrum of space missions that might use the fusion rocket technology.  These include: (1) the transport of the James Webb Space Telescope from low-earth orbit to the desired Lagrange-2 halo orbit by using a 1 megawatt (1 MW) DFSR; (2) a Pluto Orbiter/Lander sample-return mission, the subject of the NIAC grant mentioned above, using a 2 MW DFDR, (3) the deflection of an Apophis-type asteroid away from an Earth-collision orbit using a reusable "space tug" with a 5 MW DFDR engine; and (4) a manned mission to Mars with a 46 day one-way travel time using a cluster of eight DFDRs generating power totaling 160 MW and delivering 2,000 newtons of thrust.   All of these missions would be cheaper and faster than similar missions using chemical rockets.  Clearly a mature DFDR technology would bring a "game changing" renaissance to our space program.

Therefore, the questions are: how soon will the research on fusion rockets be completed, and when can we begin to use them?   The present incarnation of this technology, the PRFC-2, is only a prototype unit that has demonstrated ion heating to a few kV with a 15 KW rotating magnetic field for a duration of 100 ms. The system is reported to be behaving as predicted by theoretical modeling and shows good confinement time and no unanticipated instabilities.  The warm-superconductor flux conserver coils are behaving as expected and can sustain induced currents of up to 3,100 amperes.

However, there is a long way to go from this test configuration to a usable fusion rocket.  To achieve fusion temperatures, the rotating magnetic field drive must deliver around 200 KW.  The detailed design of the magnetic nozzle that shapes and directs the fusion products in the exhaust is a work in progress.  The expectations are that the work will ultimately result in fusion engines capable of producing power of 1 to 20 megawatts, thrusts of 5 to 250 newtons, and specific impulses of 5,000 to 50,000 seconds.

The timeline provided by Princeton Satellite Systems and the Princeton Plasma Physics Laboratory shows the planned PFRC-3 test unit demonstrating fusion around 2019, the PFRC-4 unit demonstrating operational thrust and energy production on the ground around 2024, the first robotic space mission being operational in about 2030, and a Mars Orbital Mission around 2038.  This is probably an optimistic schedule, and it critically depends on whether the Department of Energy and NASA will provide funding that supports these developments.

All I can say is that we really need fusion rocket technology, the DFDR is the most promising fusion rocket design I have seen, and I hope the timeline can be maintained.

John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.

SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106. His new novel, Fermi's Question may be coming soon.

Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .

References:

Modular Aneutronic Fusion Engine, IAC-12,C4,7-C3.5,10
http://w3.pppl.gov/ppst/docs/razin2012iac.pdf

Direct Fusion Drive Rocket for Asteroid Deflection, J. B. Mueller, et al.
 http://yosefrazin.weebly.com/uploads/1/5/7/7/15770718/iepc-2013-296.pdf

Direct Fusion Drive for a Human Mars Orbital Mission, M. Paluszek, et al.,
http://bp.pppl.gov/pub_report/2014/PPPL-5064.pdf

Fusion-Enabled Pluto Orbiter and Lander
https://www.nasa.gov/feature/fusion-enabled-pluto-orbiter-and-lander

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