Abstract. This paper provides an empirical test of a hypothesis, which describes the effects of structural mechanisms in genetic programming. In doing so, the paper offers a test problem anticipated by this hypothesis. The problem is tunably difficult, but has this p roperty because tuning is a ccomplished through changes in structure. Content is not involved in tuning. The results support a prediction of the hypothesis -that GP search space is significantly constrained as an outcome of structural mechanisms.

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Many of the studies assessing the capability of the Gasdynamic Mirror (GDM) fusion propulsion system used analyses that ignored the ambipolar potential. The electrostatic potential arises as a result of the fast escape of the electrons due to their small mass. As they escape they leave behind an excess of positive charge which manifests itself as a positive electric potential that slows down the electron escape while speeding up the ions until their respective axial diffusions are equalized. The indirect effect on the ions is that their confinement time is reduced, and to compensate for that, the length must increase, relative to that of zero potential, in order to allow for recovery of an equal amount of fusion power. But as they emerge from the thruster mirror, the ions acquire an added energy equal to the potential, and that manifests itself in increased specific impulse and thrust. We examine in this paper the underlying theory of this effect and evaluate its impact on the GDM propulsion capability. Nomenclature A c = area of plasma core A 0 = mirror area D = axial diffusion coefficient E = electric field E e = electron energy E L = escape energy e = electron charge k = density scale length L = length of plasma * AIAA Associate Fellow ** Graduate student in Aerospace Engineering ln Λ = Coulomb logarithm m = particle mass n = particle density R = plasma mirror ratio T = temperature v = monoenergetic particle velocity z = charge number Γ = velocity-averaged particle flux µ = mobility τ = confinement time ϕ = electrostatic potential ν = collision frequency τ RT = Earth-Mars round trip time d = distance between Earth and Mars g = Earth's gravitational acceleration

With the potential availability of sufficient amounts of antiprotons at reasonable costs in the next decade or so, it is reasonable to examine some currently envisaged propulsion systems that may utilize these particles, and ask what performance capability do they provide concerning certain missions that are contemplated in the same time frame. We focus in this paper on two systems: a gasdynamic mirror (GDM) propulsion system that uses U 238 as a propellant for which the energy is provided by the "at rest" annihilation of antiprotons in U 238 nuclei, and a magnetically insulated inertial confinement fusion (MICF) system where antiprotons are used to initiate fusion reactions in pellets containing fusion fuel. We employ these propulsion devices in two missions where in the interest of conserving the amount of antiprotons used we consider scenarios where thrusting is allowed for a certain period, followed by coasting, then followed by another thrusting period until reaching destination and a return trip following the same approach. One trip is to Mars, and the other to Jupiter in order to simulate a potential JIMO mission. We find that in the fission-based system a round trip to Mars will take 127 days with 2.2 days of thrusting if a gram of antiprotons is used, and a round trip to Jupiter will take 1012 days under the same conditions. The Mars and Jupiter missions can be achieved in 110 days and 780 days respectively with the use of the fusion MICF system for the same amount of antiprotons but requiring about 30 days of thrusting.