The spherical inertial-electrostatic confinement (SIEC) concept is designed
to focus and accelerate ions and electrons radially inward towards the cen
ter of a negatively biased, highly transparent spherical grid. The convergi
ng 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 s
pace-charge induced "double potential" well(a negative potential well neste
d 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 v
erify double potential well formation and trapping by a measurement of the
radial birth profile of energetic (3-MeV) protons produced by D-D fusion re
actions in a deuterium discharge. This experiment was designed to operate a
t 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 similar to 1.6 H the ballistic limit and the incorporation of a second
electrically "floating" grid tin addition to the focusing/accelerating cath
ode 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 radi
al D-D proton source rate profile, A capillary proton collimator was develo
ped 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-pea
k proton-rate density profile was observed at higher perveances, uniquely d
emonstrating 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 in
creased. At the maximum perveance studied, 1.38 mA/kV(3/2) (corresponding t
o 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 c
athode voltage, This represents the first conclusive demonstration of doubl
e well formation in an SIEC, since prior measurements by other researchers
typically yielded marginal or negative results.