Experimental study of potential structure in a spherical IEC fusion device

Abstract

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 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/sup 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 0.34 mA/kV/sup 3/2/. As the perveance increased, the depth of the double well also increased. At the maximum perveance studied, 1.38 mA/kV/sup 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.