A conceptual design for beamed core antimatter propulsion is reported, where electrically charged annihilation products directly generate thrust after being deflected and collimated by a magnetic nozzle. Simulations were carried out using the Geant4 (Geometry and tracking) software toolkit released by the CERN accelerator laboratory for Monte Carlo simulation of the interaction of particles with matter and fields. Geant permits a more sophisticated and comprehensive design and optimization of antimatter engines than the software environment for simulations reported by prior researchers. The main finding is that effective exhaust speeds Ve ~ 0.69c (where c is the speed of light) are feasible for charged pions in beamed core propulsion, a major improvement over the Ve ~ 0.33c estimate based on prior simulations. The improvement resulted from optimization of the geometry and the field configuration of the magnetic nozzle. Moreover, this improved performance is realized using a magnetic field on the order of 10 Telsa at the location of its highest magnitude. Such a field could be produced with today’s technology, whereas prior nozzle designs anticipated and required major advances in this area. The paper also briefly reviews prospects for production of the fuel needed for a beamed core engine.
Technology Review – The maximum speed of a rocket depends on its exhaust velocity, the fraction of mass devoted to fuel and the configuration of the rocket stages. “The latter two factors depend strongly on fine details of engineering and construction, and when considering space propulsion for the distant future, it seems appropriate to defer the study of such specifics,” say Keane and Zhang.
In the past, various physicists have calculated that the pions should travel at over 90 per cent the speed of light but that the nozzle would be only 36 per cent efficient. That translates into an average exhaust velocity of only a third of lightspeed, barely relativistic and somewhat of a disappointment for antimatter propulsion fans.
The new work indicates the pions should travel at 80 per cent the speed of light but that the nozzle would be 85 per cent efficient.
These guys have another surprise up their sleeve. Their nozzle has a magnetic field strength of around 12 Tesla. “Such a field could be produced with today’s technology, whereas prior nozzle designs anticipated and required major advances in this area,” they say.
Long-Term Outlook for Beamed Core Propulsion
The unmatched energy density of antimatter makes it an obvious fuel choice for the ultimate in advanced spacecraft propulsion. Antimatter has been a natural focus for the most futuristic and challenging missions, especially those venturing beyond the solar system. In any scenario where very limited availability of antimatter is not the overriding limitation, the highest performance would be achieved when the antimatter annihilation products generate thrust directly, after being deflected and collimated by an electromagnetic nozzle (the beamed core concept).
The prospect for spacecraft propulsion with antimatter as a fuel crucially depends on whether it ever becomes feasible to accumulate antimatter in macroscopic quantities and store it safely until needed. In 2009, Close published a book titled Antimatter, aimed at the general public. Close assumed that technology for antimatter production will remain static, and argued that it will take 1000 years to make a microgram of antimatter. In contrast, Frisbee made a prediction that the amount of bulk antihydrogen obtainable will grow exponentially, much like the growth of the intensity of beams of antiprotons at accelerators, which has increased about four orders of magnitude per decade since the discovery of the antiproton in Furthermore, production of antimatter for propulsion does not need to rely solely on the
approach used today at accelerator labs, where proton-antiproton pairs are created in matter on-matter collisions – this production method is extremely expensive and has very low energy efficiency, and is the main reason for skepticism by Close and others. An exciting new development was announced to the world in early August 2011 by the PAMELA collaboration: the discovery of large fluxes of antiprotons trapped by the earth’s magnetic field. Such trapping of antimatter by the earth and other planets had been predicted
Following the installation of the Alpha Magnetic Spectrometer on the International Space Station in mid-2011, there will be an enhanced capability in the future to detect, identify and measure charged particles and antiparticles in earth orbit. The recent PAMELA discovery, in which the observed antiproton flux is three orders of magnitude above the antiproton background from cosmic rays, paves the way for possible harvesting of antimatter in space. Theoretical studies suggest that the magnetosphere of much
larger planets like Jupiter would be even better for this purpose. If feasible, harvesting antimatter in space would completely bypass the obstacle of low energy efficiency when an accelerator is used to produce antimatter, and thus could offer a solution to the main difficulties stressed by the skeptics.
Nuclear fusion or any larger power source that can be put into space combined with superconductors will enable antimatter production that can be 100,000 to one million times more effient in terms of cost than earth based systems.
* A $50-100 million system with 200 KW of power using current (or conservatively within four year technology) could produce several micrograms of antimatter each year.
*A one gigawatt power system inside of an earth orbiting superconducting traps could produce 95 milligrams of antimatter per year. Antimatter could be used to enable super high performance space ships.
The baseline concept calls for using conventional high temperature superconductors to form two pairs of RF coils that have a radius of 100 m and weigh just 7000 kg combined. A 5000 kg nuclear or solar power system provides the 200 kW required to operate systems and compensate for dissipative losses in the plasma. The magnetic field induced by plasma motion driven by the RF coils is used to first concentrate the incoming antiprotons and then to trap them. Based on the Earth antiproton flux, the system would be capable of collecting 25 nanograms per day and storing up to 110 nanograms of it in the central region between the coils. The system is more than five orders of magnitude more cost effective than Earth based antiproton sources for space-based applications.
The system could collect antimatter at the rate of 8.6 micrograms per year. It only stores 110 nanograms so the stored antimatter would need to be shifted every few days to more permanent storage.
Technology Development Requirements
The collection and use of antimatter produced naturally in the space environment requires four fundamental advances: an understanding of the natural distribution of antimatter, a highly efficient collector, a stable storage medium, and a mechanism to induce thrust. The proposed collection system does not require the development of any fundamentally new technology to make it work.
However, the demonstration of key technologies and significant improvements in several areas would improve the risk weighted economic feasibility. To this end, we have identified the following technologies that need to be demonstrated at a TRL level of seven (7) or above.
Technologies in order of importance
• Compact mass spectrometer placed in highly eccentric orbit. In situ measurements of antimatter fluxes in the Earth’s radiation belt and around the Jovian planets have not been made. The models developed as part of this program should be verified by direct experimental evidence before significant resources are committed to implementing a full system. Current orbital missions do not have the spatial and/or property coverage to characterize the relevant environment. A compact mass spectrometer capable of differentiating protons, antiprotons, electrons, and positrons should be developed and flown in a highly eccentric orbit with an apogee of at least six Earth radii (6 RE) to completely characterize the antiproton and positron environment. Such a system will also contribute greatly to radiation belt knowledge and the interaction between the magnetosphere and the Sun.
• Large-scale demonstration of a plasma magnet. The technology is a critical path item that appears to provide the only mass-efficient system capable of collecting significant quantities of antiprotons. The RF generation equipment and its integration with large-scale coils in the space environment need to be demonstrated.
• Low mass, high strength, long strand, ultra-high current loops. Though the plasma magnet significantly reduces the need for high current wires, RF coils would still benefit from higher current densities. High temperature superconductors with current densities much greater than 10^10 A/m2 at 90K will enable far more compact and mass-efficient systems.
• Radiation tolerant in-orbit power source. The particle collection system is required to operate in a high radiation environment. Though the magnetic field will shield the system from much of the incoming flux, a radiation tolerant power source is necessary to generate the initial current before the field is fully established. The intrinsic energy contained in the field dictates that a high power source be available in order to charge the system in a reasonable time. A space-qualified nuclear reactor with a power output of at least 100 kWe is desirable.
• Antiproton catalyzed fission/fusion engine. Nanograms to micrograms of antiprotons do not have enough intrinsic energy to propel a spacecraft to high velocities when exclusively using the annihilation products. Instead, most concepts rely on using antiprotons to induce fission reactions. The antiprotons catalyze nuclear reactions in sub-critical fissile material to propel the vehicle by leveraging the nuclear material in a safe and controllable manner.
• Passive cooling systems. Reduced-mass multi-layer thermal blankets for passive temperature control of large structures with Tmax < 90K at 1 AU will improve the overall mass efficiency and reduce requirements on the high temperature superconductors wires used. • Affordable lift. Reducing the cost to orbit with new affordable heavy lift options, though not strictly required, will improve overall feasibility.
The ‘base design’ consisted of a 4000 ton model planned for ground launch from Jackass Flats, Nevada. Each 0.15 kt of TNT (600 MJ) (sea-level yield) blast would add 30 mph (50 km/h, 13 m/s) to the craft’s velocity. A graphite based oil would be sprayed on the pusher plate before each explosion to prevent ablation of the surface. To reach low Earth orbit (300 mi), this sequence would have to be repeated about 800 times, like an atomic pogo stick.
Most of the three tons of each of the “super” Orion’s propulsion units would be inert material such as polyethylene, or boron salts, used to transmit the force of the propulsion unit’s detonation to the Orion’s pusher plate, and absorb neutrons to minimize fallout.