**5. Theoretical background: the problems of paired-particle generation and storage**

### **5.1 Origin of the idea of photon propulsion & theoretical follow-on**

Eugen Sanger, a Bohemian-Austrian-German-French aeronautical engineer and scientist, is generally credited with proposing the idea of a photon-propelled spacecraft in the late 1950s [10]. He even proposed gamma rays as a source of propulsion for rockets. At that time, laser technology was barely in its infancy, and positrons were mainly understood as cosmic rays that arrive from outer space in small numbers, or from rare radioactive decay. No mechanism for generating large quantities of positrons was conceivable.

But the idea of anti-matter annihilation as a potential propulsion source of energy for space travel persisted. The attraction of the energies generated by matter/anti-matter annihilation reactions, combined with the low mass of positrons and electrons, in light of Tsiolkovsky's equation, was too powerful. Furthermore, over the ensuing decades, laser technology, positron generation (even in low numbers, like in PET scanners), fission reactors, and magnetic containment vessels (such as for fusion experiments) were developed.

By the late 1990s and early 2000s, anti-matter propulsion became a topic of increased research and discussion [11–13]. In the United States, at least two programs were undertaken to further matter/anti-matter propulsion. At Embry-Riddle Aeronautical University (ERAU), the Hyperion Project, led by Darrel Smith and Jonathan Webb, sought to advance this form of propulsion. At the conclusion of the project, in 2007, Smith and Webb concluded that the "current state of the art technology is lacking in the areas of positron production and storage techniques for these concepts to be realized any time in the near future" [14]. The prior year, the United States' National Aeronautics and Space Administration (NASA) concluded a project at its Institute for Advanced Concepts (NIAC) exploring the possibility of anti-matter-propelled spaceships. The NIAC team, like the team at ERAU, concluded that the "technical challenge of positron production," and the challenge of "storing enough positrons in a small space," persisted as obstacles for achieving matter/anti-matter propulsion [15].

### **5.2 Key laboratory discoveries**

The difficulties with positron generation and storage did not last long.

First, in 2008, Hui Chen and others on her team at LLNL demonstrated that large quantities of positrons could be produced by high-energy, short-burst lasers striking *Matter/Anti-Matter Propulsion DOI:http://dx.doi.org/10.5772/intechopen.110310*

high-Z (i.e., high-atomic-mass) targets [16]. Then, a few years later (beginning in 2015), the storage of electrons and positrons in an optimized dipole stellarator, by Eve Stenson and others on her team at the MPIPP, was demonstrated [3].

With these two discoveries, the primary obstacles identified by NASA and the Hyperion Project as impediments to matter/anti-matter propulsion had been overcome, at least in the laboratory. Based on these discoveries, the feasibility of matter/ anti-matter propulsion for generating relativistic speeds in space was hypothesized, along with concepts for systems that could utilize Chen's and Stenson's discoveries to make matter/anti-matter propulsion practicable [17, 18].
