C. Fred Driscoll

Experimental Investigation of Electron‐Acoustic Waves in Electron Plasmas

Electron‐acoustic waves have strong linear Landau damping, but are observed as nonlinear BGK modes in experiments with pure electron plasmas. The waves have phase velocity vφ ≈ 1.3v, in agreement with theory, and the longest wavelength BGK states exhibit only relatively weak damping. Shorter wavelength modes exhibit a strong decay instability which can be experimentally controlled.

Diocotron Instabilities in an Electron Column Induced by a Small Fraction of Transient Positive Ions

It is well known that a small fraction of positive ions can destabilize diocotron modes on electron plasmas. However, the historical (and recent) interpretation of experimental results in terms of 2D (or modified 2D) theories of ion‐induced instabilities is apparently erroneous. Here, we experimentally characterize a strong exponential instability with no threshold, obtaining growth rates orders of magnitude larger than predicted. The positive ion population is maintained either by continuous external injection of ions or by ionization of the background gas within hot electron plasmas. In both cases, the observed exponential growth rate γm is directly proportional to the ion creation rate ν+, i.e., γm = κmν+, with κm ≈ (101–103)/Ne for m0 = 1,2,3. Experimental results also suggest that non‐2D effects, including end confinement fields, are important. This strong instability may have important implications for the anti‐hydrogen creation technique of propelling anti‐protons through trapped e+ clouds.It is well known that a small fraction of positive ions can destabilize diocotron modes on electron plasmas. However, the historical (and recent) interpretation of experimental results in terms of 2D (or modified 2D) theories of ion‐induced instabilities is apparently erroneous. Here, we experimentally characterize a strong exponential instability with no threshold, obtaining growth rates orders of magnitude larger than predicted. The positive ion population is maintained either by continuous external injection of ions or by ionization of the background gas within hot electron plasmas. In both cases, the observed exponential growth rate γm is directly proportional to the ion creation rate ν+, i.e., γm = κmν+, with κm ≈ (101–103)/Ne for m0 = 1,2,3. Experimental results also suggest that non‐2D effects, including end confinement fields, are important. This strong instability may have important implications for the anti‐hydrogen creation technique of propelling anti‐protons through trapped e+ clouds.

Trapped‐Particle‐Mediated Damping and Transport

Weak axial variations in B(z) or φ(z) in Penning‐Malmberg traps cause some particles to be trapped locally. This causes a velocity‐space separatrix between trapped and passing populations, and collisional separatrix diffusion then causes mode damping and asymmetry‐induced transport. This separatrix dissipation scales with collisionality as v1/2, so it dominates in low collisionallity plasmas. The confinement lifetime in the “CamV” apparatus was dominated by a weak magnetic ripple with δB/B ∼ 10−3, and it appears likely that the ubiquitous (L/B)−2 lifetime scalings and other applied asymmetry scalings represent similar TPM effects. TPM transport will limit the containment of large numbers of positrons or ps, since TPM loss rates generally scale as total charge Q2, independent of length.

Thermal excitation of Trivelpiece-Gould modes in a pure electron plasma

Thermally excited plasma modes are observed in trapped, near-thermal-equilibrium pure electron plasmas over a temperature range of 0.05<T<5 eV. The measured thermal emission spectra together with a separate measurement of the wave absorption coefficient uniquely determines the temperature. Alternately, kinetic theory including the antenna geometry and the measured mode damping (i.e. spectral width) gives the plasma impedance, obviating the reflection measurement. This non-destructive temperature diagnostic agrees well with standard diagnostics, and may be useful for expensive species such as anti-matter.

Electron Acoustic Waves in Pure Ion Plasmas

Electron Acoustic Waves (EAWs) are the low frequency branch of electrostatic plasma waves. These waves exist in neutralized plasmas, pure electron plasmas and pure ion plasmas. At small amplitude, EAWs have a phase velocity vph≃1.4 v and their frequencies are in agreement with theory. At moderate amplitudes, waves can be excited over a broad range of frequencies and their phase velocity is in the range of 1.4 v⩽vph⩽2.1 v. This frequency variability comes from the plasma adjusting its velocity distribution so as to make the plasma mode resonant with the drive frequency. These plasma waves can also be excited with a chirped frequency drive resulting in extreme modification of the particle distribution, giving almost undamped waves with (γ/ω∼10−5).

Experimental observation of fluid echoes in a non-neutral plasma

Experimental observation of a nonlinear fluid echo is presented which demonstrates the reversible nature of spatial Landau damping, and that non-neutral plasmas behave as nearly ideal 2D fluids. These experiments are performed on UCSD’s CamV Penning-Malmberg trap with magnetized electron plasmas. An initial mi=2 diocotron wave is excited, and the received wall signal damps away in about 5 wave periods. The density perturbation filaments are observed to wrap up as the wave is spatially Landau damped. An mt=4 “tickler” wave is then excited, and this wave also Landau damps. The echo consists of a spontaneous appearance of a third me=2 wave after the responses to the first two waves have inviscidly damped away. The appearance time of the echo agrees with theory, and data suggests the echo is destroyed at least partly due to saturation.

Trapped‐Particle‐Mediated Collisional Damping of Non‐Axisymmetric 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.

Shear reduction of 2D point vortex diffusion

Theory and simulations establish the effects of shear on the collisional diffusion of a 2D point vortex gas. For finite shear, the diffusion is considerably smaller than previous zero-shear theories predict, scaling inversely with the shear. Surprisingly, changing the sign of the applied shear changes the diffusion by an order of magnitude.

Trapped particle asymmetry modes in non-neutral plasmas

Novel trapped particle asymmetry modes propagate on cylindrical electron columns when axial variations in the wall voltage cause particle trapping. These modes consist of E×B drifts of edge-trapped particles, partially shielded by axial flows of interior untrapped particles. A simple theory model agrees well with the observed frequencies and eigenfunctions; but the strong mode damping is as yet unexplained. These modes may be important in coupling trap asymmetries to particle motions and low frequency E×B drift modes.

Shear-limited test particle diffusion in 2-dimensional plasmas

Measurements of test-particle diffusion in pure ion plasmas show 2D enhancements over the 3D rates, limited by shear in the plasma rotation ωE(r). The diffusion is due to “long-range” ion-ion collisions in the quiescent, steady-state Mg+ plasma. For short plasma length Lp and low shear S≡r∂ωE/∂r, thermal ions bounce axially many times before shear separates them in θ, so the ions move in (r,θ) as bounce averaged “rods” of charge (i.e. 2D point vortices). Experimentally, we vary the number of bounces over the range 0.2⩽Nb⩽10,000. For long plasmas with Nb⩽1, we observe diffusion in quantitative agreement with the 3D theory of long-range E×B drift collisions. For shorter plasmas or lower shear, with Nb>1, we measure diffusion rates enhanced by up to 100×. For exceedingly small she0ar, i.e. Nb⩾1000, we observe diffusion rates consistent with the Taylor-McNamara estimates for a shear-free thermal plasma. Overall, the data shows fair agreement with Dubin’s new theory of 2D diffusion in shear, which predicts an ...