Session JO8: Waves, Oscillations, and Nonlinear Phenomena

Shear flow instability in a partially ionized plasma sheath around a fast moving vehicle

A shear flow instability in a partially ionized compressible plasma sheath around a fast moving vehicle is described and analyzed for different types of flow profiles. In compressible plasma flow with velocity shear instability occurs for any velocity profile, not only for profiles with an inflection point. Second order differential equation for the electrostatic potential of the ion acoustic waves thus excited is derived and solved numerically using the shooting code with the imposed outgoing wave boundary conditions. In contrast to our earlier study we have appropriately included in our analysis the presence of electron and ion collisions with neutrals as well as electron - ion collisions. It is shown that the density of the neutrals has an important influence on the growth and decay of the instability.   

Ion-Acoustic Wave Instability Driven by Laser-Driven Return Currents

Thomson-scattering measurements of the amplitude and frequency of ion-acoustic waves show an instability when the ion-wave damping is reduced. Experimental results from the OMEGA Laser use simultaneous measurements of the electron-plasma wave and ion-acoustic wave features to characterize the plasma (Te, Ti, Z, Ne) and to directly probe the amplitude of the ion-acoustic waves. The ion Landau damping was varied by changing the target material: CH, V, Ag, Au. The amplitude of the plasma wave increased as the ion Landau damping was reduced and became unstable for ZTe/Ti $>$ 50. As the waves grow to wave-breaking amplitudes, their frequency shifts, and turbulence is expected. These results confirm the speculation that heat-flux--driven ion-acoustic fluctuations exist in laser-produced plasmas, which was previously invoked to increase the collision rate and account for anomalous absorption.\footnote{S. H. Glenzer \textit{et al}., Phys. Rev. Lett. \textbf{88}, 235002 (2002)} This work was supported by Laser Basic Science and the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Diffraction Limits on the Convective Stimulated Raman Scatter Threshold

Laser interaction with inhomogeneous plasma typically exhibits a ``Raman gap'': experiments reveal that scattered light covers a range of frequencies which at the high end corresponds to SRS from~low electron densities where the combination of reduced coupling and increased Landau damping provide a cutoff at roughly the laser frequency $\omega _{0}$, but SRS is \textit{not} observed in a frequency interval that (qualitatively) abuts $\omega _{0}$/2, where scattered light is again observed in a narrow frequency interval which corresponds to scattering from electron densities approximately in the range 0.20 to 0.25 of critical. Several 1D mechanisms, linear and nonlinear, have been proposed to explain this Raman gap. We have found that diffractive effects on light scattered from a wide (beam width$>>$speckle width) speckled laser beam tend to increase the linear convective SRS intensity threshold, I$_{T}$, with increase of density. This may lead to a temperature and density regime in which I$_{T}$ increases with density even though the 1D gain rate, $\sim $ 1 / \textit{$\nu $} $_{Landau}$, increases.   

Newly Discovered Parametric Instabilities Excited by High Power Radio Waves in the Ionosphere

A powerful electromagnetic wave can decay into a large number of low frequency electrostatic waves and a scattered electromagnetic wave by generalized stimulated Brillouin scatter (GSBS). The generalization occurs in the F-layer ionosphere because of the presence of the magnetic field supporting a large number of plasma waves not present in an unmagnetized plasma. Stimulated Brillouin scatter excites the ion acoustic mode. In addition, GSBS can excite slow MHD, Alfven, fast MHD, ion cyclotron, whistler, lower hybrid, ion Bernstein waves. The first detection of this process during ionospheric modification with high power radio waves was demonstrated using the HAARP transmitter in Alaska in 2009. Subsequent experiments have provided additional verification of the GSBS process with quantitative measurements of the scattered electromagnetic waves with low frequency offsets from the pump wave. Relative to ground-based laboratory experiments with laser plasma interactions, the ionospheric HF wave interactions experiments are more completely diagnosed into terms of understanding the basic decay process of the magnetized plasma. Applications of the GSBS observations included remote sensing of the plasma state and launching propagating wave modes.   

Nonlinear Theory of Mode Conversion at Plasma Frequency

The mode conversion occurs when two waves of different physical properties, for example, longitudinal versus transverse, have the same frequency and wave number. The electromagnetic (EM) wave can transfer energy to the electrostatic (ES) wave through mode conversion process near the cut-off layer. The best mode conversion efficiency reaches 50\% at the proper incident angle and the plasma density gradient in linear theory. Due to the large amplitude wave during laser heating, the physics can be highly nonlinear. The sinusoidal incident electromagnetic wave can generate high harmonics and dc component. Moreover, in the vicinity of the cut-off and the mode conversion layers, we find, from the numerical code in the finite difference time domain simulation, the DC magnetic field and the localized plasma flow. The frequency mismatch is compensated by the large wave amplitude, and the mode conversion also occurs readily beyond the linear theory would. The converted longitudinal wave is tantamount to the electron density wave. It can modulate the equilibrium density, and results in the density bubble. As a consequence, ions may be accelerated to 4-5Gev at the present-day laser powers.   

Molecular dynamics investigation on tin

Laser-produced tin (Sn) plasma has been considered as one of main candidates for extreme ultraviolet (EUV) light source used in EUV lithography. In order to increase conversion efficiency and to mitigate energetic ions and neutral from laser-produced Sn plasma, fundamental investigation is necessary. Theoretical study by means of hydrodynamic simulation is expected to guide us to optimize Sn target and pumping conditions. Equation of state of materials is an inevitable ingredient of all hydrodynamic simulations. However, the early stages of the laser-matter interaction process, equation of state at phase transition, for example, liquid-vapor transition, and the ejection of particles are remain unexplored. The aim of this research is to report the simulated properties of Sn over wide physical conditions by means of Materials Studio code and a 3D homemade molecular dynamics code developed for this purpose. Results have shown transient effects on the phase transitions. The simulation results are compared to experimental data obtained by pulsed laser ablation of Sn. Velocity distributions of evaporated particles from the Sn are discussed as a function of laser fluence. Also, the equation of state has been tabulated in warm dense region.   

Inverse Faraday Effect Revisited

The inverse Faraday effect is usually associated with circularly polarized laser beams. However, it was recently shown that it can also occur for linearly polarized radiation [1]. The quasi-static axial magnetic field by a laser beam propagating in plasma can be calculated by considering both the spin and the orbital angular momenta of the laser pulse. A net spin is present when the radiation is circularly polarized and a net orbital angular momentum is present if there is any deviation from perfect rotational symmetry. This orbital angular momentum has recently been discussed in the plasma context [2], and can give an additional contribution to the axial magnetic field, thus enhancing or reducing the inverse Faraday effect. As a result, this effect that is usually attributed to circular polarization can also be excited by linearly polarized radiation, if the incident laser propagates in a Laguerre-Gauss mode carrying a finite amount of orbital angular momentum.\\[4pt] [1] S. ALi, J.R. Davies and J.T. Mendon\c{c}a, \textit{Phys. Rev. Lett.}, {\bf 105}, 035001 (2010).\\[0pt] [2] J. T. Mendon\c{c}a, B. Thid\'{e}, and H. Then, \textit{Phys. Rev. Lett.} \textbf{102}, 185005 (2009).   

Effect of Electron Temperature Fluctuations on the Anomalous Particle Flux inferred by Electrostatic Triple Probes

Plasma anomalous transport severely reduces the economical attractiveness of any possible fusion energy reactor based on magnetically confined thermonuclear plasma. Understanding the major mechanisms of this transport, mainly due to the anomalous particles losses, is vital to ameliorate the potential of such reactor, and plasma edge is a key area for this research. We reported here data of a 4-pin triple probe at TCABR tokamak [R=0.615m, a=0.18m, B$_{T}$=1.15T, I$_{p}\le $120kA, n$_{e}$(bar)$\le $4x10$^{19}$m$^{-3}$, T$_{e}$(0)$\le $600eV, T$_{i}$(0)$\le $400eV, 100ms, circular limiter]. Plasma density (n$_{e})$, potential (V$_{p})$, electron temperature (T$_{e})$, and respectively fluctuations, all were simultaneously measured or inferred with high spatial($\sim $3mm) and temporal (1$\mu $s) resolution. Corrections in the fluctuation driven particle flux($\Gamma )$ via the poloidal electrical field (E$_{\theta })$ and n$_{e}$ are used: real geometry of the tips; V$_{p}$ (instead of floating potential) between the two tips for inferring E$_{\theta }$; a correction on n$_{e}$ due to the finite electrical sheath formed at the probe ion collecting area via an analytical formula based on the Hutchinson model for collisionless plasma. The role of T$_{e}$ fluctuations in $\Gamma $ is analyzed and the results are correlated with the dynamic of the global plasma parameters on discharges under auxiliary heating via RF injection (4MHz, 30kW, Alfv\'{e}n Wave scheme) in which confinement improvement has been observed.   

First Results from the Neutral Particle Analyzer at the Madison Symmetric Torus RFP

A neutral particle analyzer has been used at the Madison Symmetric Torus to (MST) to study T$_{i}$ with Neutral Beam Injection (NBI). The Compact Neutral Particle Analyzer (CNPA), formerly on the Sustained Spheromak Physics Experiment (SSPX) with Hydrogen plasmas, has been modified and calibrated for MST's deuterium plasmas. The new calibration has measured the flux of D$^{0}$ atoms emitted by the plasma which strip the neutrals in a stripping cell with Helium gas (10$^{-2}$ Torr). The ions are focused by two permanent magnets into 25 channels with an energy range from .34 -- 5.2 keV. From the channels in the keV range, T$_{eff}$ (T$_{i})$ has been measured to be around 600 eV during sawtooth events and 200 eV in between events. Further T$_{i}$ studies will compare on and off neutral beam shots.   

Applied Magnetic Field Design for the FRC Compression Heating Experiment (FRCHX) at AFRL

Detailed calculations of the dynamics of the formation, guide, and mirror applied magnetic fields were conducted using a commercially available generalized finite element solver. As part of the integrated FRC compression heating experiment (FRCHX), an applied magnetic field forms, translates and finally captures the FRC in the liner region sufficiently long to enable compression. Large single turn coils are used in the formation region, and detailed information on the magnetic field greatly enhances the fidelity of 2-D magnetohydrodynamic simulations using MACH2. Solenoidal coils produce the necessary magnetic field for translation and capture of the FRC prior to liner implosion. Since the liner implosion is underway before the FRC is injected, the magnetic flux that diffuses into the liner is compressed, and the calculations must account for the liner motion. Design iterations were performed using the detailed magnetic field solver with MACH2 to achieve both the coil design and operating parameters which resulted in the highest likelihood of FRC capture prior to compression. This work is funded by the U.S. Department of Energy Office of Fusion Energy Sciences.