We report experimental studies demonstrating a controlled transition to fully developed broadband turbulence in an argon helicon plasma in a linear plasma device. We show the detailed dynamics during the transition from nonlinearly coupled but distinct eigenmodes at low magnetic fields to fully developed broadband turbulence at larger magnetic fields. As the magnetic field (B) is increased from B ∼ 40 mT, initially we observe slow smooth changes in the dynamics of the system (to B ∼ 140 mT), followed by a sharp transition (within ∼10 mT) to centrally peaked narrow density profiles, strong edge potential gradients and a pronounced bright, well-defined plasma core. At low magnetic fields, the plasma is dominated by drift waves. As the magnetic field is increased, a strong potential gradient at the edge introduces an E × B shear-driven instability. At the transition, another mode with signatures of a rotation-induced Rayleigh-Taylor instability appears at the central plasma region. Concurrently we also find large axial velocities in the plasma core. For larger magnetic fields, all the instabilities co-exist, leading to rich plasma dynamics and fully developed broadband turbulence at B ∼ 240 mT.

Measurements of positron-molecule binding energies are made for molecules with large permanent dipole moments (>2.7  D), by studying vibrational-Feshbach-mediated annihilation resonances as a function of incident positron energy. The binding energies are relatively large (e.g., ≥90  meV) as compared to those for similar sized molecules studied previously and analogous weakly bound electron-molecule (negative ion) states. Comparisons with existing theoretical predictions are discussed.

Energy-resolved studies of positron-molecule collisions exhibit vibrational Feshbach resonances in annihilation, thus providing evidence that positrons can bind to these species. The downshifts of the observed resonances from the positions of the vibrational modes provides a measure of the positron-molecule binding energies, which range from 1 to 300 meV. Reported here are annihilation spectra and binding energies for a wider range of chemical species than studied previously, including aldehydes, ketones, formates, acetates, and nitriles. While the measured binding energies show an approximate correlation with molecular dipole polarizability and permanent dipole moment, other effects are important for dipole moments 2.0 D. For these compounds, it appears that localization of the positron wave function near a portion of the molecule leads to enhanced binding and an increased dependence on both the molecular dipole moment and the electron-positron correlations. The relationship of these results to theoretical calculations is discussed.

Helicon plasmas are typically associated with a core, a radially localized central area of strong ion light emission. Here, we investigate the role of electrostatic instabilities that lead to the formation of the classic blue core. We show that helicon plasma can also occur without the distinct core. In these conditions, the plasma is dominated by low-frequency resistive drift wave (RDW) instabilities propagating in the electron diamagnetic drift direction. When the intense sharp core is present, a new global equilibrium state is achieved where three radially separated plasma instabilities exist simultaneously. The density gradient driven RDWs separate the plasma radially into an edge region and a core region. The edge is dominated by strong, turbulent, shear-driven instabilities, while the core shows very coherent high azimuthal mode number fluctuations propagating in the ion diamagnetic drift direction and associated with enhanced ion emission. The particle flux is directed outward for small radii and inward for large radii, thus forming a radial particle transport barrier. The radial extent of the inner mode and radial location of the particle transport barrier is the same as the radius of the blue core. This new equilibrium, with the three coexisting radially separated plasma instabilities, leads to the formation of a very strong enhanced blue core. For a range of operating parameters, just prior to the blue core formation, the system undergoes incomplete intermittent transitions between the two equilibrium states, leading to the visual perception of a broad less intense helicon core. This is the first time that the development of the helicon core is shown to be associated with changes in radial transport driven by inherent low-frequency plasma instabilities.

Abstract. Results are presented for positron binding to a selection of molecules containing the hydroxyl functional group. These molecules, which span in the range of carbon atoms from 1 (methanol) to 4 (1-butanol), have moderate permanent dipole moments ranging from about 1.4 to 2.4 D. The dependence of the binding energy on the magnitude of the molecular dipole polarizability and static dipole moment is studied. An effect that appears to be due to the localization of the bound positron is discussed. Contents

Publisher's Note: Interplay between permanent dipole moments and polarizability in positron-molecule binding [Phys. Rev. A 85, 022709 (2012)]

We report experimental observation of ion heating and subsequent development of a prominent ion temperature gradient in the core of a linear magnetized plasma device, and the controlled shear de-correlation experiment. Simultaneously, we also observe the development of strong sheared flows at the edge of the device. Both the ion temperature and the azimuthal velocity profiles are quite flat at low magnetic fields. As the magnetic field is increased, the core ion temperature increases, producing centrally peaked ion temperature profiles and therefore strong radial gradients in the ion temperature. Similarly, we observe the development of large azimuthal flows at the edge, with increasing magnetic field, leading to strong radially sheared plasma flows. The ion velocities and temperatures are derived from laser induced fluorescence measurements of Doppler resolved velocity distribution functions of argon ions. These features are consistent with the previous observations of simultaneously existing radially separated multiple plasma instabilities that exhibit complex plasma dynamics in a very simple plasma system. The ion temperature gradients in the core and the radially sheared azimuthal velocities at the edge point to mechanisms that can drive the multiple plasma instabilities, that were reported earlier. Published by AIP Publishing.

Comparisons of the plasma ion flow speed measurements from Mach probes and laser induced fluorescence were performed in the Controlled Shear Decorrelation Experiment. We show the presence of the probe causes a low density geometric shadow downstream of the probe that affects the current density collected by the probe in collisional plasmas if the ion-neutral mean free path is shorter than the probe shadow length, Lg = w2 Vdrift/D⊥, resulting in erroneous Mach numbers. We then present a simple correction term that provides the corrected Mach number from probe data when the sound speed, ion-neutral mean free path, and perpendicular diffusion coefficient of the plasma are known. The probe shadow effect must be taken into account whenever the ion-neutral mean free path is on the order of the probe shadow length in linear devices and the open-field line region of fusion devices.