Trapped-particle asymmetry modes in single-species plasmas

Chaotic neoclassical separatrix dissipation in parametric drift-wave decay

Experiments and theory characterize a parametric decay instability between plasma drift waves when the nonlinear coupling is modified by an electrostatic barrier. Novel mode coupling terms representing enhanced dissipation and mode phase shifts are caused by chaotic separatrix crossings on the wave-ruffled separatrix. Experimental determination of these coupling terms is in broad agreement with new chaotic neoclassical transport analyses.

Resonant drift-wave coupling modified by nonlinear separatrix dissipation

Resonant drift-wave coupling experiments characterize a new dissipative coupling term caused by a trapping separatrix. The system is a cylindrical pure-electron plasma with an axial trapping separatrix generated by an applied theta-symmetric wall voltage. The resonant decay of m_{theta}=2 diocotron modes into m_{theta}=1 trapped-particle diocotron modes is measured and compared to parametric mode coupling theory. Experiments quantify the traditional nonlinear mode coupling term, plus a new separatrix-generated dissipative coupling term which is not yet characterized theoretically.

Trapped-particle-mediated collisional damping of nonaxisymmetric plasma waves

Weak axial variations in magnetic or electric confinement fields in pure electron plasmas cause slow electrons to be trapped locally, and collisional diffusion across the trapping separatrix then causes surprisingly large trapped-particle-mediated (TPM) damping and transport effects. Here we characterize TPM damping of m theta not equal to 0, m(z) = +/-1 Trivelpiece-Gould plasma modes in large-amplitude long-lived Bernstein-Greene-Kruskal states. The TPM damping gives gammaBGK/omega approximately 10(-4) and seems to dominate in regimes of weak interparticle collisions.

Three-dimensional annihilation imaging of trapped antiprotons

We demonstrate three-dimensional imaging of antiprotons in a Penning trap, by reconstructing annihilation vertices from the trajectories of the charged annihilation products. The unique capability of antiparticle imaging has allowed, for the first time, the observation of the spatial distribution of the particle loss in a Penning trap. The radial loss of antiprotons on the trap wall is localized to small spots, strongly breaking the azimuthal symmetry expected for an ideal trap. Our observations have important implications for detection of antihydrogen annihilations.

Damping of the trapped-particle diocotron mode

The damping mechanism of a recently discovered trapped-particle mode is identified as collisional velocity scattering of marginally trapped particles. The mode exists on non-neutral plasma columns that are partially divided by an electrostatic potential. This damping mechanism is similar to that responsible for damping of the dissipative trapped-ion mode. The damping rate is calculated using a Fokker-Planck analysis and agrees with measurement to within 50%. Also, an experimental signature confirms a causal relation between scattering of marginally trapped particles and damping.

Trapped-particle modes and asymmetry-induced transport in single-species plasmas

A new asymmetry-induced transport mechanism in pure electron plasmas is shown to be proportional to the damping rate of the corresponding trapped-particle mode, with simple scalings for all other parameters. This transport occurs when axial particle trapping exists due to variations in the electric or magnetic confinement fields. This new transport is strong for even weak unintentional trapping (deltaB/B approximately 10(-3)), and may be prevalent in transport experiments with magnetic or electrostatic theta asymmetries.

Decay of the diocotron rotation and transport in a new low-density asymmetry-dominated regime

The decay of the diocotron rotation was studied in a new regime in which trap asymmetries dominate. Decay within a few diocotron periods was observed, sometimes orders of magnitude faster than predicted by the traditional "rotational pumping" theory. The decay does not conserve angular momentum, and is strongest for small, low-density columns. The new regime appears when "magnetron-like" rotation from the end confinement fields becomes dominant, and appears to be associated with errors in these fields. Transition to decay dominated by rotational pumping was observed for larger and denser columns. The asymmetry-dominated transport was also studied, and found to depend linearly on the line density (and not the density) over nearly 4 orders of magnitude.