Experimental study of potential structure in a spherical IEC fusion device

  • Gu Y
  • Miley G
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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/kV3/2), 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/kV3/2. As the perveance increased, the depth of
the double well also increased. At the maximum perveance studied, 1.38
mA/kV3/2 (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

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  • Yibin Gu

  • George H. Miley

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