Slow antihydrogen (H) is produced within a Penning trap that is located within a quadrupole Ioffe trap, the latter intended to ultimately confine extremely cold, ground-state H[over ] atoms. Observed H[over ] atoms in this configuration resolve a debate about whether positrons and antiprotons can be brought together to form atoms within the divergent magnetic fields of a quadrupole Ioffe trap. The number of detected H atoms actually increases when a 400 mK Ioffe trap is turned on.
Measurements of the radial, azimuthal, and field-aligned mode structures of interchange instabilities excited by energetic electrons confined by a magnetic dipole are presented. The mode structures are determined using a correlation analysis of movable high-impedance floating potential probes located at various positions within the plasma. The hot electron population, produced by electron cyclotron resonance heating, becomes unstable to hot electron interchange ͑HEI͒ instabilities which saturate nonlinearly with a complex and time-varying frequency spectrum. Although the mode frequencies vary dramatically, it is found that the mode structures do not evolve significantly in time, being determined by the azimuthal mode numbers. These measurements are compared to a self-consistent nonlinear particle simulation of the HEI mode in dipole geometry. Upon appropriate adjustment of the boundary conditions, the simulation reproduces the measured radial and azimuthal structures at large amplitudes.
Nonlinear frequency sweeping of unstable waves in a laboratory plasma is suppressed upon application of rf fields. Frequency sweeping is driven by a population of energetic electrons trapped in a magnetic dipole field that excite drift-resonant potential fluctuations and create coherent structures in phase space. Self-consistent numerical simulation reproduces the suppression and suggests an explanation due to rf scattering of energetic electrons that destroys the phase-space structures.
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