Monday, February 20, 2012

Antimatter-Driven Fusion Propulsion

Every generation has the obligation to free men's minds for a look at new worlds . . . to look out from a higher plateau than the last generation.
- Ellison S. Onizuka

Inertial Confinement Fusion (ICF) is a process where nuclear fusion reactions are initiated by heating and compressing a spherical fuel pellet. In conventional ICF, energy is delivered to the outer layer of the fuel pellet using high energy beams of lasers, electrons or ions. This causes the outer layer of the fuel pellet to explode outwards and produce a reaction force which causes the remainder of the fuel pellet to implode inwards. The implosion process launches a powerful shockwave into the center of the fuel pellet which violently compresses and heats the fuel to the point where nuclear fusion occurs. This drives a thermonuclear burn wave which propagates out from the center of the fuel pellet and consumes it. Within the thermonuclear burn wave, energy is generated when light isotopes such as deuterium and tritium fuse into heavier ones.

A number of studies have been done to investigate the use of conventionally driven ICF as a means of propulsion for space travel. However, conventional ICF drivers are too massive and complex for use as a means of space propulsion. They also tend to require a large amount of recirculating power, waste heat radiators and large lasers or ion-beam particle accelerators. In comparison, antimatter-driven ICF offers zero need for recirculating power, very low antiproton injection energy and a vast reduction in equipment mass and waste heat generators. The energy unleashed from the annihilation of antiprotons with protons is ultimately used to drive an implosion within the spherical fuel pellet and the vast majority of the propulsion energy will be generated from the fusion process itself.

When an antiproton enters normal matter, it slows down by transferring its kinetic energy to nearby electrons in a process known as electron drag. Once it has slowed down sufficiently, the antiproton will displace an outer orbital electron of an atom. The antiproton then cascades towards the ground state via the emission of X-rays and settles into a stable close orbit around the atomic nucleus. From here, the antiproton annihilates with either a proton or neutron from within the nucleus and the annihilation energy released from each annihilation event is about 1.88 GeV.

The annihilation of an antiproton with a proton produces on average 2 neutral pions and 3 charged pions. Each neutral pion decays into 2 energetic gamma ray photons while each charged pion decays into a muon and a neutrino. Subsequently, each muon decays into an electron, an electron-antineutrino and a muon-neutrino. The annihilation of protons by antiprotons generally produces an equal number of positive and negative pions. On the other hand, the annihilation of neutrons by antiprotons produces a greater number of negative pions than positive pions and the proportion of charged pions to neutral pions is also greater.

To describe the process of antimatter-initiated ICF, it is assumed that the fusion fuel is comprised entirely of deuterium and tritium. Both deuterium and tritium are isotopes of hydrogen and the combination of deuterium and tritium as a fusion fuel is simply denoted as DT. The nucleus of a deuterium isotope consists of one proton and one neutron while the nucleus of a tritium isotope consists of one proton and two neutrons. The fusion of one deuterium nucleus with one tritium nucleus produces a single helium-4 nucleus and one neutron.

As shown in the diagram above, the antiproton-driven ICF spherical fuel pellet configuration basically consists of an outer spherical DT ablator shell surrounding a solid spherical DT fuel shell. DT gas fills the volume within the solid DT fuel shell. The DT ablator shell is seeded with uranium-238 (U-238) to enhance the deposition of annihilation energy. The DT ablator shell and the solid DT fuel shell are each made up of a few milligrams of fusion fuel, and the entire antiproton-driven ICF spherical fuel pellet measures only a few millimeters in diameter.

The process begins when a spherical omni-directional beam of antiprotons is directed at the fuel pellet and the kinetic energy of the antiproton beam is precisely controlled so that the antiprotons annihilate only within the DT ablator shell. Almost all of the antiprotons will annihilate with either a proton or neutron of the U-238 nuclei seeded throughout the DT ablator shell. When an antiproton annihilates with a U-238 nucleus, a fraction of the annihilation energy gets transferred to the nucleus through pion interactions. This causes the nucleus to break up violently and produce highly-ionizing short-range nuclear fragments.

Nuclear fragments from the fission of U-238 deposit energy into the DT ablator shell. This causes the shell it to heat up and expand outwards with velocities of several tens of km/s. By the conservation of momentum, the rapid expansion of the DT ablator shell causes the remaining portion of the spherical fuel pellet to violently implode which intensely compresses and heats the DT fuel. The kinetic energy of the DT fuel shell at peak velocity is about a few hundred thousand joules.

When the imploding DT fuel shell stagnates on the DT gas filling the central cavity within the DT fuel shell, all of its kinetic energy will be converted to pressure within the DT gas. At this stage, there will be a high temperature but low density hotspot containing a few percent of the DT fuel mass being created at the center of the DT fuel assemblage. Surrounding the hotspot will be the bulk of the DT fuel which is at a lower temperature but higher density. At this instance, approximately half of the energy will be contained in the central hotspot while the rest of the energy will be compressional energy contained within the surrounding DT fuel.

The shockwave launched by the implosion converges on the central hotspot and violently heats it up to fusion conditions. Thermonuclear fusion begins when the temperature of the hotspot exceeds about a hundred million degrees Centigrade. A fusion chain reaction occurs and drives a thermonuclear detonation burn wave that propagates radially outwards from the hotspot. This thermonuclear burn wave consumes a significant fraction of the entire DT fuel assemblage and generates a fusion yield of a few hundred million joules for each DT fuel pellet.

Credit: David Robinson (2007)

For a spacecraft in reality, this entire process must be repeated at a frequency of a few tens of hertz to provide continuous thrust. Antiproton-driven ICF can generate propulsion exhaust velocities of a few thousand kilometers per second. Each ICF target requires only tens of picograms of antiprotons to initiate nuclear fusion, whereby one gram is a trillion picograms. The estimated mass of antiprotons that is required to initiate and sustain nuclear fusion to accelerate a 100 ton spacecraft to a velocity of 1000 km/s is only a few grams. Such a propulsion system will be useful for the exploration of the outer solar system, Kuiper belt and beyond. At a velocity of 1000 km/s, it only takes 173 days to travel a distance of 100 AU, where one AU is the average Earth-Sun distance. Nevertheless, the capabilities of antiproton-driven ICF still falls short of what is required for traveling to the nearest stars within reasonable durations. Traveling at 1000 km/s, it will take almost 1300 years to get to the nearest star - Proxima Centauri.

To conclude, before antimatter-driven ICF can be utilized as a means of propulsion for interplanetary space travel, there are still numerous technological challengers to overcome. The production, collection, storage, handling and precision beaming of antiprotons will present significant technical challenges. Nonetheless, the potentials and benefits of using antimatter-driven ICF for space propulsion or even terrestrial power generation are enormous.

1. L. John Perkins, et al. (2004), “On the utility of antiprotons as drivers for inertial confinement fusion”, Nuclear Fusion 44 1097 doi:10.1088/0029-5515/44/10/004
2. Schmidt, G., Gerrish, H., Martin, J. J. (1999), “Antimatter Production for Near-term Propulsion Applications”, 1999 Joint Propulsion Conference
3. Gaidos, G., et al. (1998), “Antiproton-catalyzed microfission/fusion propulsion systems for exploration of the outer solar system and beyond”, 1998. AIP Conference Proceedings, Volume 420, pp. 1365-1372