F. Anderegg

Electron acoustic waves in pure ion plasmas

Standing electron acoustic waves (EAWs) are observed in a pure ion plasma. EAWs are slow nonlinear plasma waves; at small amplitude their phase velocities (vph≃1.4v¯ for small kλD) and their frequencies are in agreement with theory. At moderate amplitude, EAW-type plasma waves can be excited over a broad range of frequencies. This frequency variability comes from the plasma adjusting its velocity distribution so as to make the plasma mode resonant with the drive frequency. Wave-coherent laser-induced fluorescence shows the intimate nature of the wave-particle interaction, and how the particle distribution function is modified by the wave driver until the plasma mode is resonant with the driver.

Cyclotron resonance phenomena in a non-neutral multispecies ion plasma

Cyclotron modes of a non‐neutral Mg ion plasma were studied in a long cylindrical Penning–Malmberg trap. Several modes with angular dependence exp(ilθ), l≥1, are observed near the cyclotron frequencies of the various Mg ions. The l=1 modes for the majority species are downshifted from the cyclotron frequencies, with downshifts as large as four times the diocotron frequency. These large shifts are quantitatively explained by a multispecies cold‐plasma theory, including the dependence on the plasma size and composition. These dependencies allow the plasma size and composition to be obtained from the measured mode frequencies. In contrast, the l=1 downshifts for minority species are generally close to twice the diocotron frequency, and remain unexplained. Cyclotron heating of the plasma ions was also observed with a surprising effect of improving the plasma confinement.

A NEW PURE ION PLASMA DEVICE WITH LASER INDUCED FLUORESCENCE DIAGNOSTIC

We describe a new apparatus for magnetic confinement of a pure ion plasma, with laser diagnostics to measure test particle transport across the magnetic field. In addition to the axisymmetric trapping potential, rotating electrostatic wall perturbation is used to counteract the plasma loss processes, giving steady-state ion confinement for weeks. Electronic spin polarization of the ion ground states is used to label the test particles; this spin orientation is controlled by direct optical pumping. The laser-induced fluorescence (LIF) technique is used to nondestructively measure the ion velocity distribution; and an absolute calibration of the charge density is obtained from the LIF measurement of the plasma rotation velocity. Two new technological improvements compatible with ultrahigh vacuum systems have been used: a semirigid Teflon insulated coaxial cable has low microphonic noise, and an antireflective coating is used to reduce reflection of ultraviolet light inside the vacuum chamber.

Shear reduction of collisional transport: Experiments and theory

Experiments and theory on collisional diffusion and viscosity in quiescent single-species plasmas demonstrate enhanced transport in the two-dimensional (2D) bounce-averaged regime, limited by shear in the plasma rotation. For long plasma columns, the measured diffusion agrees quantitatively with recent theories of three-dimensional long-range E×B drift collisions, and is substantially larger than predicted for classical velocity-scattering collisions. For short plasmas, diffusion is observed to be enhanced by Nb, the number of times a thermal particle bounces axially before being separated by shear. Equivalently, recent theory in the 2D bounce-averaged regime shows how diffusion decreases with increasing shear, generalizing the zero-shear perspective which gives Bohm diffusion. Viscosity is similarly enhanced in the 2D regime, but there is presently only qualitative agreement with theory. These results apply to both non-neutral and neutral plasmas, and provide the first rigorous analysis of shear reductio...

Plasma Diagnostics with Spin-Polarized Ions

Abstract Ionic spin polarization by optical pumping is demonstrated as a technique for plasma test-particle diagnostics. “Spin-tagging” is used to observe the magnetized plasma tag-particle transfer function.

Test particle transport from long-range collisions

Enhanced cross-magnetic-field diffusion of test particles in pure ion plasmas has been measured. The ion plasma is contained in a Penning-Malmberg trap for weeks near thermodynamic equilibrium, characterized by rigid rotation and uniform density and temperature. Plasma expansion and loss is suppressed by a “rotating wall” technique, i.e., a weak electrostatic potential rotating faster than the plasma. Test particle transport is then measured even though there is zero net transport, in a regime where neutral collisions are negligible. The observed test particle transport is diffusive, i.e., proportional to the gradient of the test particle concentration. The measured diffusion coefficients scale as nT−1/2B−2 over a range of 40 in density, 50 in temperature, and 5 in magnetic field. This diffusion is about ten times greater than predicted by classical collisional theory, which describes velocity-scattering collisions with impact parameters ρ≲rc. The enhanced transport is thought to be due to non-velocity-sc...

Measurement of correlation-enhanced collision rates using pure ion plasmas

This paper presents the first direct experimental measurements of the Salpeter enhancement of collisions due to particle correlations. The perpendicular-to-parallel collision rate ν⊥∥ is measured in laser-cooled pure ion plasmas, spanning the regimes from weak to strong magnetization, and from weak to strong particle correlations. The abrupt suppression of collisions in the strongly magnetized regime of T≲10−3 eV is observed, mitigated by the Salpeter enhancement when correlation effects become significant. This ν⊥∥ enhancement due to correlations is directly analogous to the enhancement of fusion collisions in hot dense stellar plasmas. The measured collisional enhancement is approximately exp(Γ), where Γ is the correlation parameter, and this is quantitatively consistent with analytical estimates based on thermal equilibrium shielding and correlations.

Cyclotron mode frequencies and resonant absorption in multi-species ion plasmasa)

Cyclotron mode frequencies are studied on trapped rigid-rotor multi-species ion plasmas. Collective effects and radial electric fields shift the mode frequencies away from the “bare” cyclotron frequencies 2πFc(s)≡(qsB/Msc) for each species s. These frequency shifts are measured on the distinct cyclotron modes ( m=0,1, and 2) with cos(mθ) azimuthal dependence. We find that for radially uniform plasmas the frequency shifts corroborate a simple theory expression, in which collective effects enter only through the E × B rotation frequency fE and the species fraction δs. The m = 1 center-of-mass mode is in agreement with a simple “clump” model. Additionally, ultra-cold ion plasmas exhibit centrifugal separation by mass, and additional frequency shifts are observed, in agreement with a more general theory.

Measurement of cross-magnetic-field heat transport due to long-range collisions

Cross-magnetic-field heat transport in a quiescent pure ion plasma is found to be diffusive and to be dominated by long-range “guiding center” collisions. In these long-range collisions, which occur in plasmas with Debye length λD greater than cyclotron radius rc, particles with impact parameters rc<ρ⩽λD exchange parallel kinetic energy only. The resulting thermal diffusivity χL is independent of magnetic field B and plasma density n. The measured thermal diffusivity χ agrees within a factor of 2 with the long-range prediction χL=0.49nvb2λD2 over a range of 1000 in temperature, 50 in density, and 4 in magnetic field. This thermal diffusivity is observed to be up to 100 times larger than classical diffusivity from short-range velocity-scattering collisions. These long-range collisions are typically dominant in unneutralized plasmas, and may also contribute to electron heat transport in neutral plasmas.

Thermally excited fluctuations as a pure electron plasma temperature diagnostic

Thermally excited charge fluctuations in pure electron plasma columns provide a diagnostic for the plasma temperature over a range of 0.05 0.2, so that Landau damping is dominant and well modeled by theory. The third method compares the total (frequency-integrated) number delta N of fluctuating image charges on the wall antenna to a simple thermodynamic calculation. This method works when lambda(D)/R-p > 0.2.