Experimental study of potential structure in a spherical IEC fusion device

The spherical inertial-electrostatic confinement (SIEC) concept is designed to focus and accelerate ions and electrons radially inward towards the center of a negatively biased, highly transparent spherical grid. The converging ions create a high-density plasma core where a high fusion rate occurs. In addition, under proper conditions, the ion and electron flows create a space-charge induced “double potential” well (a negative potential well nested inside a positive potential well). This structure traps high-energy ions within the virtual anode created by the double potential, providing a high fusion density in the trap volume. The present experiment was designed to verify double potential well formation and trapping by a measurement of the radial birth profile of energetic (3-MeV) protons produced by D-D fusion reactions in a deuterium discharge. This experiment was designed to operate at high perveance (0.4 to 1.4 mA/kV^3/2), where formation of a double well is predicted theoretically. Additional steps to aid well formation included: use of the unique Star mode of operation to obtain ion beam focusing down to -1.6 H the ballistic limit and the incorporation of a second electrically “floating” grid (in addition to the focusing/accelerating cathode grid) to reduce the ion radial energy spread to <10%. The existence of the potential well was then demonstrated by measurement of a two-peak radial D-D proton source rate profile. A capillary proton collimator was developed for the spatial measurement of the escaping protons. This data was then unfolded to obtain the radial proton source rate profile. This profile in turn provided a characterization of the potential-well structure. A two-peak proton-rate density profile was observed at higher perveances, uniquely demonstrating the evolution of a double potential well for perveances >0.34 mA/kV^3/2. As the perveance increased, the depth of the double well also increased. At the maximum perveance studied, 1.38 mA/kV^3/2 (corresponding to 80 mA and 15 kV), the negative potential well depth, corresponding to the measured proton-rate density, was estimated to be 22%-27% of the applied cathode voltage. This represents the first conclusive demonstration of double well formation in an SIEC, since prior measurements by other researchers typically yielded marginal or negative results