Bruno S. Bauer

The frequency and damping of ion acoustic waves in hydrocarbon (CH) and two‐ion‐species plasmas

The kinetic theory of ion‐acoustic waves in multi‐ion‐species plasmas is discussed. Particular application is made to hydrocarbon (CH) plasmas, which are widely used in laser–plasma experiments. The mode frequencies and Landau damping of the two, dominant, ion‐acoustic modes in CH plasmas are calculated by numerical solution of the kinetic dispersion relation. In addition, some useful results are obtained analytically from expansions of the kinetic dispersion relation and from fluid models. However, these results disagree with the numerical results in domains of particular practical interest. When ion temperatures exceed two‐tenths of the electron temperature, the least damped mode is the one with the smaller phase velocity, and this mode is then found to dominate the ponderomotive response of the CH plasma.

Dependence of stimulated Brillouin scattering on focusing optic F number in long scale‐length plasmas

The onset of stimulated Brillouin scattering (SBS) has been examined by varying the effective F number (lens focal length/incident beam diameter) of the optical system used to irradiate a performed 1 mm long, longitudinally uniform hydrocarbon (CH) plasma. SBS onset occurs at higher average laser intensity for smaller F, in qualitative agreement with theory. Quantitative comparison between theory and experiment, and implications for the National Ignition Facility [Phys. Plasmas 2, 2480 (1995)] are discussed.

Measurements of the angular and temporal structure of second‐harmonic emission from laser‐produced plasmas

We have measured and analyzed the second harmonic emission, both in the plane of the laser electric field and perpendicular to it, at several angles near 135° from the laser wave vector. The experiments used from 1 to 80 J of 1.053 μm light to irradiate carbon–hydrogen (CH) targets with a 550 ps pulse. A random phase plate was used, producing characteristic intensities in the range of 1013–1014 W/cm2. This was sufficient to drive the Ion Acoustic Decay Instability, producing Stokes emission well‐separated from the emission spike at the second harmonic of the laser frequency. The spectral structure of the Stokes emission was qualitatively similar for all intensities and angles of observation. The duration of the signals showed trends anticipated from linear theory. To explain the scaling of the signal strength and spectral width requires nonlinear theory.

Meeting the challenge of detecting ion plasma waves

Ion plasma waves—purely electrostatic ion waves with a wavelength of order of the electron Debye length and frequency of the order of the ion plasma frequency—have long been known in theory but have proven difficult to detect experimentally. The difficulties stemmed from the techniques used to produce the plasma and to drive and detect the waves. In the work reported here, these problems were overcome by using resonant laser scattering to detect ion plasma waves in a multiply ionized, laser‐produced plasma. This nonetheless required careful experimental design to minimize frequency smearing of the scattered signal by plasma gradients. The plasma was extensively characterized, allowing comparison of the theoretical dispersion relation with the wave data. The agreement of these two provides conclusive proof of the detection of ion plasma waves.

X-ray emission from colliding laser plasmas

Colliding Au, CD, and Ti-CR plasmas have been generated by illuminating two opposing foils each with an approximately 100J, 0.5 nsec, 2(omega) Nd-glass laser beam from the Trident laser facility at Los Alamos. The plasmas are being used to study plasma interactions which span the parameter regime from interpenetrating to collisional stagnation. X-ray emission during the laser target interaction and the subsequent collision is used to diagnose the initial plasma conditions and the colliding plasma properties. X-ray instrumentation consists of a 100 ps gated x-ray pinhole imager, a time-integrated bremsstrahlung x-ray spectrograph and a gated x-ray spectrograph used to record isoelectronic spectra from the Ti-Cr plasmas. The imager has obtained multiframe images of the collision and therefore, a measure of the stagnation length which is a function of the ion charge state and density and a strong function of the electon temperature. Other isntrumentation includes a Thomson scattering spectrometer with probe beam, neutron detectors used to monitor the CE coated foil collisions, and an ion spectrometer. We will describe the current status of the experiments and current results with emphasis on the x-ray emission diagnostics. We will also briefly describe the modeling using Lasnex and ISIS, a particle-in-cell code with massless fluid electronics and inter-particle (classical) collisions.

P24 Plasma Physics Summer School 2012 Los Alamos National Laboratory Summer lecture series for students

This report covers the 2012 LANL summer lecture series for students. The lectures were: (1) Tom Intrator, P24 LANL: Kick off, Introduction - What is a plasma; (2) Bruno Bauer, Univ. Nevada-Reno: Derivation of plasma fluid equations; (3) Juan Fernandez, P24 LANL Overview of research being done in p-24; (4) Tom Intrator, P24 LANL: Intro to dynamo, reconnection, shocks; (5) Bill Daughton X-CP6 LANL: Intro to computational particle in cell methods; (6) Kirk Flippo, P24 LANL: High energy density plasmas; (7) Thom Weber, P24 LANL: Energy crisis, fission, fusion, non carbon fuel cycles; (8) Tom Awe, Sandia National Laboratory: Magneto Inertial Fusion; and (9) Yongho Kim, P24 LANL: Industrial technologies.

Studies of Ion Acoustic Decay

In this project, we advanced knowledge of Ion Acoustic Decay on several fronts. In this project, we have developed and demonstrated the capability to perform experimental and theoretical studies of the Ion Acoustic Decay Instability. We have at the same time demonstrated an improved capability to do multichannel spectroscopy and Thomson scattering. We made the first observations of the time-resolved second harmonic emission at several angles simultaneously, and the first observations of the emission both parallel and perpendicular to the electric field of the laser light. We used Thomson scattering to make the first observations of the plasma waves driven by acoustic decay in a warm plasma with long density scale lengths. We also advanced both the linear and the nonlinear theory of this instability. We are thus prepared to perform experiments to address this mechanism as needed for applications.