Observation of Spherical Focus in an Electron Penning Trap

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We produce simultaneously dense and well-confined nonneutral plasmas by spherical focusing. A small (3 mm radius) Penning trap has low-energy electrons injected at a single pole of the sphere. Precisely when the trap parameters are adjusted to produce a spherical well, the system self-organizes into a spherical state through a bootstrapping mechanism which produces a hysteresis. Additional confirmation of the dense spherical focus is provided by electrons scattered by the central core. Core densities up to 35 times the Brillouin density have been inferred from the data.