Plasma Heating due to Cyclic Diffusion across a Separatrix.

We observe plasma heating due to collisional diffusion across a separatrix when a magnesium ion column in a Penning-Malmberg trap is cyclically pushed back and forth across a partial trapping barrier. The barrier is an externally applied axisymmetric "squeeze" potential, which creates a velocity separatrix between trapped and passing particles. Weak ion-ion collisions then cause separatrix crossings, leading to irreversible heating. The heating rate scales as the square root of the oscillation rate times the collision frequency and thus can be dominant for low-collisionality plasmas. The particle velocity distribution function is measured with coherent laser induced fluorescence and shows passing and trapped particles having an out-of-phase response to the forced plasma oscillations.

Trapped Particle Effects in the Parametric Instability of Near-Acoustic Plasma Waves.

Quantitative experiments on the parametric decay instability of near-acoustic plasma waves provide strong evidence that trapped particles reduce the instability threshold below fluid models. At low temperatures, the broad characteristics of the parametric instability are determined by the frequency detuning of the pump and daughter wave, and the wave-wave coupling strength, surprisingly consistent with cold fluid, three-wave theories. However, at higher temperatures, trapped particle effects dominate, and the pump wave becomes unstable at half the threshold pump wave amplitude with similar exponential growth rates as for a cold plasma.

First Test of Long-Range Collisional Drag via Plasma Wave Damping.

This paper presents the first experimental confirmation of a new theory predicting enhanced drag due to long-range collisions in a magnetized plasma. The experiments measure damping of Langmuir waves in a multispecies pure ion plasma, which is dominated by interspecies collisional drag in certain regimes. The measured damping rates in these regimes exceed classical predictions of collisional drag damping by as much as an order of magnitude, but agree with the new theory.

Space charge frequency shifts of the cyclotron modes in multi-species ion plasmas.

Shifts of the cyclotron frequency away from the "bare" cyclotron frequency are observed to be proportional to the total ion density through the E × B rotation frequency, and to the relative concentration of each ion species, in quantitative agreement with analytic theory. These shifts are measured at small excitation amplitudes on the typical center-of-mass m = 1 mode, and also on cyclotron modes with m = 0 and m = 2 azimuthal dependence. The frequency spacing between these modes is proportional to the rotation frequency of the ion cloud, which is controlled and measured using a "rotating wall" and laser-induced fluorescence. These cylindrical ion plasmas consist of Mg(+) isotopes, with H3 O (+) and O2 (+) impurities. It is observed that the shift in the m = 1 cyclotron frequency is larger for the minority species (25)Mg(+) and (26)Mg(+), than for the majority species (24)Mg(+). A simple center-of-mass model is presented, which is in quantitative agreement with these results. It is also shown that this model interprets and expands the intensity dependent calibration equation, (M/q) = A/f + B/f (2) + CI/f (2).

Measurement of correlation-enhanced collision rates.

We measure the perpendicular-to-parallel collision rate nu perpendicular parallel in laser-cooled magnetized ion plasmas, spanning the uncorrelated to correlated regimes. In correlated regimes, we measure collision rates consistent with the "Salpeter correlation enhancement" of roughly exp(Gamma), for correlation parameters Gamma less, similar 4. This enhancement also applies to fusion in dense plasmas such as stars.

Wave-particle interactions in electron acoustic waves in pure ion plasmas.

Electron acoustic waves (EAW) with a phase velocity less than twice the plasma thermal velocity are observed on pure ion plasma columns. At low excitation amplitudes, the EAW frequencies agree with theory, but at moderate excitation the EAW is more frequency variable than typical Langmuir waves, and at large excitations resonance is observed over a broad range. Laser induced fluorescence measurements of the wave-coherent ion velocity distribution show phase reversals and wave-particle trapping plateaus at +/-vph, as expected, and corroborate the unusual role of kinetic pressure in the EAW.

Measurement of Landau damping and the evolution to a BGK equilibrium.

Linear Landau damping and nonlinear wave-particle trapping oscillations are observed with standing plasma waves in a trapped pure electron plasma. For low wave amplitudes, the measured linear damping rate agrees quantitatively with linear Landau damping theory. At larger amplitudes, the wave initially damps at the Landau rate, then regrows and oscillates, approaching a steady state, as predicted by O'Neil in 1965 [Phys. Fluids 8, 2255 (1965)]]. This BGK equilibrium is observed to decay slowly due to external dissipation.

Thermally excited modes in a pure electron plasma.

Thermally excited plasma modes are observed in near-thermal-equilibrium pure electron plasmas over a temperature range of 0.05