Graduate Student Seminars
November 3, 2011
|
| | Speaker: | Heather Johns |
| Topic: | Hydrodynamic Analysis of Direct-Drive ICF Implosions at OMEGA
|
| Date: | Thursday, November 3, 2011 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Early hydrodynamic simulations of experiments performed at LLE's OMEGA laser system will be discussed. These 1-d simulations, performed with the radiation hydrodynamics code HYDRA, represent a precursor to benchmarking attempts between 3-d HYDRA results, the experimental spectra, and spectroscopic models. The experimental diagnostics, including 3 DDMMI, allow spatially resolved spectra along 3 lines of sight. A brief overview of the physics of inertial confinement fusion and hydrodynamics will be given as part of this discussion.
| |
|
October 20, 2011
|
| | Speaker: | Dan McEvoy |
| Topic: | The Utility of Precipitation Indices as Indicators of Drought in Nevada
|
| Date: | Thursday, October 20, 2011 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Throughout the western United States snow-dominated watersheds prevail and are critical for water supply for both human and natural environmental demands. Therefore, understanding characteristics of water shortages, such as length and severity, is extremely important to both scientists and the general public. Drought indices provide an assessment of drought conditions using easily accessible input data such as precipitation and temperature. This study examines the effectiveness of two indices, the standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI), to identify drought periods and consider the role of evapotranspiration processes in drought conditions. Whereas the SPI reflects solely precipitation deficit or surplus, the SPEI takes into account the effects of both precipitation and temperature and incorporates a simple water balance. At certain locations large differences were found between SPI and SPEI, which lead to the underlying question of this study: Which index more accurately identifies drought periods of the past? Three steps were taken to draw conclusions from this study: 1) compare SPI and SPEI and determine where and at what time scales the largest differences appear, 2) establish relationships between drought indices and hydrologic and atmospheric variables, and 3) identify historical drought periods and evaluate index performance. The largest discrepancies between indices appear in the southwest United States and more specifically southern California, western Arizona, and southern Nevada. Initial results suggest in arid locations a drought index which incorporates a water balance will prove more useful for drought monitoring applications.
| |
|
October 11, 2011
|
| | Speaker: | Trevor J. Burris-Mog |
| Topic: | Laser Accelerated Protons: The Transition from Fundamental Research to Applications
|
| Date: | Thursday, October 11, 2011 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | The Over the last decade, there has been extensive research regarding the generation of multi-MeV proton beams from the interaction of intense (I > 1018 W/cm2) short-pulse (t [ps]) laser light with solid micron-thick foils. Although this results in high (>1013) per-shot proton yields with small transverse (<0.004 mm mrad) and longitudinal (<104 eV s) emittances, the transition from fundamental research to applications will require control of the 100% energy spreads and large divergences (>20o half-angles) associated with laser acceleration. Results demonstrating efficient capture, collimation and focusing of laser accelerated proton beams from the PHELIX laser system and the ability to control their broad energy range using a pulse power solenoid are presented. Simulations using measured data for the input parameters yields information about the phase-space and transport efficiencies of the captured proton beams. To conclude, results from a feasibility study of a pulse power, compact, achromatic gantry concept for a distant-future laser-driven proton therapy system will be presented. |
|
|
September 29, 2011
|
| | Speaker: | Phillipe Leblanc |
| Topic: | Ionization models for hot electron transport in ultra-intense laser-solid interactions
|
| Date: | Thursday, September 29, 2011 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Having a good model of ionization is important to study the hot electron transport in ultra-intense laser-solid interactions. One of the commonly applied models to calculate ionization in a plasma is the Thomas-Fermi (TF) model. This model is derived from equations of state which in turn assumes the plasma is in an equilibrium state. However, for the timescales of interest for the study of laser-matter interactions, laser created plasmas are highly transients and thus are not in an equilibrium state. The predicted ionization levels by the TF model are based on incorrect assumptions about the plasma conditions. In an effort to improve the accuracy of predicted ionization levels, an impact ionization model is applied to particle-in-cell simulations. This model calculates ionization levels from electron-ion collisional cross-sections and should be better suited to predict ionization levels for non-equilibrium plasmas. A comparison between the TF and Impact model is made for different plasma parameters. Preliminary results indicate the discrepancies between the models to increase as the atomic number (Z) of the target increases. Additionally, the laser intensity is varied to explore how each models reacts in different hot electron energy regimes.
|
|
|
September 22, 2011
|
| | Speaker: | Daniel Papp |
| Topic: | Non-precursor implosion in closely spaced nested wire arrays
|
| Date: | Thursday, September 22, 2011 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Wire array Z-pinches are the most intense and energetic laboratory X-ray sources. In such targets consisting of one to a few hundred wires are driven by MA-magnitude current pulses for a few hundred ns. The most commonly used targets has the wires arranged cylindrically, in a single or double arrays of wires. The latter are referred as nested cylindrical arrays. In most of wire array loads, at the beginning of the implosion, the wire material ablates and jxB forces collect it on the axis of the array. Closely spaced nested arrays were studied at the 1 MA Zebra generator at UNR Nevada Terawatt Facility. These nested cylindrical arrays have a small gap between the inner and outer wires. They were designed so that the outward jxB forces on the inner wires should prevent the formation of the precursor plasma. Closely spaced nested arrays with 6-8 wires imploded without precursor, as observed on shadowgrams and optical streak images. Precursor formation was observed in arrays with 12-16 wires, despite calculated outward ablative forces. Two-frame UV probing was used to resolve details in the ablating plasma. Implosion was found to similar to the cascading implosion in "star" arrays. The keV X-ray yield of closely spaced arrays was found to be 1.5-2.5 times the yield of cylindrical arrays of the same mass.
|
|
|
Daniel Papp
|
April 7, 2011
|
| | Speaker: | Dambar Air |
| Topic: | Many-body Calculation of Energy Resonance in Be-like carbon
|
| Date: | Thursday, April 7, 2011 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | The dielectronic recombination was described by applying the configuration interaction method together with the many body perturbation theory (CI+MBPT). In this work an advanced method is used in which CI+MBPT approach combine with the complex rotation method(CRM). The energy positions and widths of DR resonances of J=1 odd symmetry were calculated by using CI+MBPT+CRM method. This method found a good agreement with NIST recommended values.
|
|
|
March 10, 2011
|
| | Speaker: | Mike Weller |
| Topic: | Radiative Properties of Mixed Nested Cylindrical Wire Arrays on Zebra at UNR
|
| Date: | Thursday, March 10, 2011 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Our existing theoretical and experimental tools were applied to study mixed nested cylindrical wire arrays (NCWA) performed on the Zebra generator at UNR. In particular, experimental results of mixed brass (70% Cu, 30% Zn) and Al (5056, 5% Mg) NCWAs are compared and analyzed. The inner and outer arrays were approximately the same mass (~30 micro g/cm for both brass and Al), such that Al was either all in the outer array or inner array, and the same with brass. Consequently, radiative properties of K-shell Al and Mg ions and L-shell Cu and Zn ions are compared as functions of the placements of the brass and Al wires on the inner and outer arrays. Results show that the placement of brass or Al, whether inside or out, dramatically changes the intensity of the x-ray emission of L-shell Cu to K-shell Al (by a factor of ~5, i.e. Cu L-shell and Al K-shell are more intense when on outside compared to when inside) and the overall energy yield (~17 kJ when brass is outside, compared to ~14 kJ when brass is inside). A full diagnostic set which included more than ten different beam-lines was implemented. Identical loads were fielded to allow the timing of time-gated pinhole and x-ray spectrometers to be shifted to get a more complete understanding of the evolution of plasma parameters over the x-ray pulse, which showcases the importance of time-gated diagnostics, in particular time-gated spectroscopy. In addition, these results are compared with mixed Mo and Al NCWAs from previous experiments. The importance of the study of NCWAs with different wire materials is discussed.
|
|
|
December 2, 2010
|
| | Speaker: | Vijay Singh |
| Topic: | Inelastic collisions of CaH molecules at cryogenic temperatures
|
| Date: | Thursday, December 2, 2010 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | We are interested in using doublet-sigma molecules to investigate the role of electon spin in cold chemical reactions. As a first step towards this goal, we are creating polarized samples of CaH molecules and studying their collisional properties. We create CaH molecules by laser ablation of a solid CaH2 target and cool them by helium buffer gas cooling. To study the polarization changing collisions of CaH molecules with He atoms, we spin polarize CaH by optical pumping. We measure the spin depolarization rate by setting the return to equilibrium. At 4 Kelvin, our measurement shows a depolarization rate coe?cient of 10-11 cm3 s-1, which is very large compared to the measured spin depolarization rate at 0.4 Kelvin. This large depolarization rate is likely due to excited rotational states. Progress towards measuring this rate coe?cient as a function of temperature will be discussed.
|
|
|
Vijay Singh
|
November 18, 2010
|
| | Speaker: | Rishi Pandit |
| Topic: | Radiation Damping
|
| Date: | Thursday, November 18, 2010 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Laser intensity is getting stronger and stronger. Usually, electron motion is influenced by electromagnetic field and it is influenced to a small extent by radiation damping. We studied the radiation damping effect in the super intense laser regime. We solve the Dirac equation of motion for a single particle interacting a strong laser field. When laser intensity is greater than 1022 w/cm2, radiation damping is become very large and important to include the particle motion.
|
|
|
Rishi Pandit
|
October 21, 2010
|
| | Speaker: | David Martinez |
| Topic: | X-ray yield from pinch-on-target interaction
|
| Date: | Thursday, October 21, 2010 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | The wire array z-pinch is an efficient x-ray source which has been proposed for use in indirect drive inertial confinement fusion schemes. Extensive research has focused on methods to enhance the x-ray yield in a z-pinch. In recent experiments, performed at the Nevada Terawatt Facility, it was observed that a wire added on axis as a target for the implosion increased the total x-ray production of the pinch. This effect was strongly accentuated when a conical wire array was used. To study the effect the center wire had on the z-pinch x-ray emission, an experiment compared conical and cylindrical wire arrays with different size and material center targets. The x-ray emission's dependence on the target diameter will be presented.
|
|
|
David Martinez
|
October 14, 2010
|
| | Speaker: | Kiran Baral |
| Topic: | MElectron-Impact Ionization of Multiply Charged Ions
|
| Date: | Thursday, October 14, 2010 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | A crossed-beams apparatus based on the "animated beam" technique is used at the University of Nevada, Reno to measure absolute electron-impact ionization cross sections of multiply charged ions. Experimental results are presented as a function of electron energy for single ionization of Se2+ and Se3+.The data suggest that inner-shell excitation followed by autoionization dominates the single- ionization cross sections for these ions.
|
|
|
Kiran Baral
|
October 7, 2010
|
| | Speaker: | Kevin Yates |
| Topic: | Magnetized Target Fusion and the FRC
|
| Date: | Thursday, October 7, 2010 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | A description is given for the scientific basis for Magnetized Target Fusion (MTF). This qualitatively different approach to fusion is being explored at Los Alamos National Laboratory and Air Force Research Laboratory at Kirkland Air Force Base. MTF is in an intermediate plasma regime between magnetic and inertial fusion energy. MTF is attractive due to its low cost allowing its development to occur on a faster time scale. MTF uses a field-reversed configuration as its target plasma and is surrounded by an imploding cylindrical metal liner for compression. The kinetic energy of the imploding liner does work on the magnetized DT fuel, bringing it to a temperature relevant for fusion reactions. The magnetic field lines are compressed during the implosion increasing the magnetic field strength to approximately 500 Tesla. This magnetic field is used to suppress thermal conduction losses allowing for fusion burn.
|
|
|
Kevin Yates
|
September 30, 2010
| Speaker: | Nagendra Aryal |
| Topic: | Photoionization Cross - Section Measurements for Xe@C60+ Ions
|
| Date: | Thursday, September 30, 2010 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Experimental evidence for confinement resonances associated with photoabsorption by a Xe atom trapped inside a C60 cage is presented. The well-known 4d giant resonance in photoionization of Xe atom has been predicted theoretically to be split into multiple peaks when Xe atom is encaged inside a C60 cage due to the interference of photoelectron waves from Xe atom as they are reflected by the C60 cage. The experiment was conducted at the Advanced Light Source, Lawrence Berkeley National Laboratory, CA using the ion-photon merged-beams endstation. Measurements were performed in the photon energy range 60eV - 150eV and this phenomenon was observed in the cross section for double photoionization of Xe@C60+ yielding Xe@C583+ product ions.
|
|
|
Nagendra Aryal
|
September 23, 2010
| Speaker: | Daniel Papp |
| Topic: | Introduction to the physics of wire array Z-pinches
|
| Date: | Thursday, September 23, 2010 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Fast wire array Z-pinches are the most intense laboratory X-ray sources, and have the potential for inertial confinement fusion. In such an experiment, electric current of MA size is driven through a load of thin wires, with a total mass of around 1mg in a few hundred of ns. Experiments on the Z machine at Sandia National Lab achieved 350TW power and 2.7MJ energy with a 26MA drive current. The implosion of such loads have four stages. Wire initiation, ablation of plasma rom the wires, implosion of wire cores and stagnation of the pinch on the central axis. The dynamics of the ablation and implosion phase is strongly dependent on the wire array configuration. The pressure equilibrium of a stagnating pinch can be described by the Bennett equation. The collapse of the pinch is caused by the growth of the m=0 and m=1 mode magnetic Rayleigh-Taylor instabilities. Wire array Z-pinch radiation consist of atomic lines superimposed on a continuous soft X-ray Bremsstrahlung background. Kinetic energy and Spitzer resistive heating only accounts for a fraction of the total radiated energy. Several possible mechanisms were proposed in literature to explain this discrepancy.
|
|
|
Daniel Papp
|
March 26, 2010
| Speaker: | Sandra Stein |
| Topic: | Kelvin-Helmholtz Instability in a Sheared Flow Actuated by a Magnetic Field
|
| Date: | Thursday, March 26, 2010 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | The Kelvin-Helmholtz instability can lead to plasma transport across a magnetic field; one example is the solar wind transport across the earth's magnetopause in the magnetotail. In an experiment done at the Nevada Terawatt Facility, we observed the Kelvin-Helmholtz instability in a laser produced plasma that interacted with an external magnetic field. This instability is evidenced by the presence of evenly spaced vortices on the plasma-field boundary. Due to the interaction with the external magnetic field, a velocity gradient perpendicular to the plasma velocity forms at this boundary. The presence of vortices in a region of sheared flow is characteristic of the development of the Kelvin-Helmholtz instability. The observed structure and its growth rate indicate that large ion Larmor radius effects contribute to its formation. Discussions of the mechanism producing the sheared flow and the resulting instability will be presented.
Work supported by DOE/NNSA grant DE-FC52-06NA27616
|
March 18, 2010
| Speaker: | Mahmoud Ahmed |
| Topic: | Green Lasers
|
| Date: | Thursday, March 18, 2010 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Semiconductor lasers are built as a thin layer of indium gallium nitride (InGaN) sandwiched between two layers of GaN,and. By applying a suitable voltage, the electrons and holes recombine driven by the electric field annihilating one another and generating photons. The wavelength of this light depends on the indium content of the active layer-more leads to longer wavelengths and thus greener light. But the more indium in these layers, the more that indium is likely to pool into small "islands" during manufacture. The islands can alter the light's wavelength- an unacceptable flaw in a laser. These problems are solved by starting with a thin wafer of pure, crystalline GaN that has been sliced along a larger crystal's m-plane and then polished giving rise to a new bunch of application to these multi-coloured and highly efficient lasers.
|
September 10, 2009
| Speaker: | Kenneth Williamson |
| Topic: | The Life and Times of a Plasma Formation
|
| Date: | Thursday, September 10, 2009 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | High energy-density (HED) plasmas have many applications as soft x-ray radiators that can be used to drive fusion reactions, to study the effects of radiation on materials, to study the physics of stellar interiors, or to study basic plasma science. The wire array z pinch is an effective method for producing and studying HED plasmas in a labortory setting. It is essentially an array of wires strung between the anode and cathode of a pulsed-power generator. The fast-rising current heats and ionizes the wires while the Lorentz force 'pinches' the plasma on the 'Z' axis. Many types of wire materials and configurations are used to produce various plasmas of interest, but the topic of this presentation is the double-planar wire array configuration. The break from symmetry produces unexpected, long-lived, off-axis plasma formations before the bulk implosion. Analysis of the life and times of these formations will be presented along with a discussion of energy balance, radiation symmetry, and equilibrium.
|
September 3, 2009
| Speaker: | Glenn Osbourne |
| Topic: | Comparative Analysis of Implosions of Tungsten Single and Double Planar Wire Arrays Produced on the 1-MA Generator at UNR
|
| Date: | Thursday, September 4, 2009 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Analysis of the results of tungsten single and double planar wire array (PWA) experiments performed on the UNR 1MA generator "Zebra" is presented. Uniform as well as combined PWAs with Al wires arrays were investigated and compared. Previous spectroscopic work focused on analysis of time-integrated spatially resolved (TISR) stagnation phase data, while this presentation continues and expands that study to include the early implosion plasma stage and more data from recent shots. A full diagnostic set was utilized, including a pulse laser shadowgraphy system, a gated fast intensified CCD (ICCD) camera, an x-ray time-gated pinhole camera, a TISR spectrometer, and a time-gated spatially integrated (TGSI) spectrometer. Non-LTE kinetic modeling is used to describe spectroscopic data for combined PWAs in particular. Wire Ablation Dynamics Model is employed to model the implosion and interpret shadowgraphy data. Opacity effects in plasmas from uniform and combined PWAs with Al wires are discussed and compared. The advantage of using the tracer Al wires is emphasized. Applications to current fusion research are also taken into consideration.
|
April 30, 2009
| Speaker: | Mei-Ju Lu |
| Topic: | Atomic Coherence Experiments in a Pure Nuclear Spin System
|
| Date: | Thursday, April 30, 2009 at 1:00 pm |
| Location: | LP 208 |
| Abstract: | Spin coherence is widely used in many atomic physics experiments such as atomic magnetometers and quantum memory. Having long spin coherence times and large numbers of atoms are of essential importance for those experiments. However, it is often difficult to maintain long spin coherence times while having large atomic numbers because of inelastic collisions.We propose using a pure nuclear spin system which is expected to have much better inelastic collisional behaviors than alkali atoms and less dephasing problems due to stray magnetic fields. In this seminar, I will present the details of how we use laser ablation and buffer-gas cooling to generate large atomic numbers and atomic density of ytterbium atoms at 5 Kelvin. I will also discuss the slow light experiment by using electromagnetically induced transparency (EIT) in Yb-173 atoms as well as the measurements of spin depolarization (T1) and spin decoherence (T2).
|
April 30, 2009
| Speaker: | Brian Chrisman |
| Topic: | Study of Ultra-Intense Laser Produced Plasmas via Computer Simulation
|
| Date: | Thursday, April 30, 2009 at 1:00 pm |
| Location: | LP 208 |
| Abstract: | Recent advances in the development of intense short pulse lasers have led to exciting progress in high energy density physics (HEDP). As an example, a several _m thin foil that is irradiated by a 100 TW, sub-picosecond laser pulse reaches keV (1 keV _ 11,000,000 C) temperatures at solid density. The resultant electron distribution is temporarily far out of equilibrium, featuring two or more widely distinct temperatures. In modeling such extreme plasmas, both kinetic and collisional effects on the energy transport are essential. Of particular difficulty is the large density gradients between the critical density,(the density at which the laser is absorbed), and solid densities exceeding several hundred times the critical density. For a 1 _m wavelength laser pulse, the critical density, nc, is 1021 cm??3. This means that a numerical model needs to describe the laser-plasma interaction in the low density region, as well as fast particle transport in the extremely dense target region where Coulomb collision processes are important for energy transfer. In cone-guided fast ignition inertial confinement fusion experiments, fuel previously compressed by an ablative implosion is ignited by the injection of an intense short laser pulse via a cone embedded within the fuel target. The implosion precondition creates density scales which range over five orders of magnitude from the cone interior to the highly compressed core. A critical issue for this process is whether the hot electrons produced in the interaction are in an energy range conducive to efficient heating of the core. In this work, Particle-in-Cell simulations evaluate the entire cone- guided fast ignition experiment for the first time, including hot electron generation at the cone tip, energy transport to the compressed fuel core, and subsequent collisional core heating. The laser- plasma interaction within the cone target is particularly important, as temperatures of hot electrons generated here are found to be lower than previously expected while overall absorption is influenced
|
April 23, 2009
| Speaker: | Sandrine Gaillard |
| Topic: | PROTON, ELECTRON AND K-ALPHA EMISSION FROM MICRO-SCALE COPPER CONE TARGETS
|
| Date: | Thursday, April 23, 2009 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Fast ignition fusion requires transporting a large amount of energy into the compressed ICF capsule in a short period of time. One method for efficiently transporting laser energy into a compressed fusion capsule is to use a cone situated near or in the capsule, decreasing the distance to the core, improving laser conversion efficiency and concentrating the charged particle energy flow. Results from recent experiments performed at the LANL 200 TW Trident short-pulse laser at ~1020 W/cm2 (80-100 J and ~600 fs) are presented. We have conducted laser-ion acceleration experiments comparing flat foils and new Cu micro-cone targets in three separate geometries (flat-top, funnel, and snub-nose) to elucidate the production of hot electrons and ions in these cones, which have been shown to produce higher proton energies and conversion efficiencies than flat foils at lower laser intensity and energy (~2x1019 W/cm2, 20 J). Data from a Cu K? 2D imaging crystal, an X-ray single hit CCD, proton beam images on RCF film stacks, and an electron/proton spectrometer are presented and compared, showing the importance of not just generating hot electrons, but in the case of ion acceleration, efficiently propagating these hot electrons to the accelerating "tip", where they can then be efficiently converted to ion energy.
|
April 16, 2009
| Speaker: | Michael Weller |
| Topic: | Overview of three research topics: Electron Fluid Dynamical Waves, STM Imaging of GaAs, and The Measurement of Optical Properties of Aerosols
|
| Date: | Thursday, April 16, 2009 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Students in physics often encounter a broad range of research topics. What will be presented here are three main research projects that I have worked on at three different institutions: Arkansas Tech University, The University of Arkansas, and Jet Propulsion Laboratory. The first topic covers Electron Fluid Dynamical Waves, in which a one-dimensional, steady state, electron fluid dynamical model is used to describe ionizing breakdown waves. The second topic covers an attempt to more routinely attain STM imaging for cross sectional study of III-V compound semiconductor heterostructures, in particular GaAs. The third topic covers a method of measuring optical properties of aerosols by measuring the complex forward scattering of the electromagnetic field.
|
April 9, 2009
| Speaker: | Taisuke Nagayama |
| Topic: | Overview of Spectroscopic Electron Temperature and Density Diagnostics of ICF Implosion Cores
|
| Date: | Thursday, April 9, 2009 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Inertial confinement fusion (ICF) is a process to initiate nuclear fusion reactions by heating and compressing a fuel target. The goal of the ICF research is to achieve an efficient fuel burn as known as "ignition." Thus, techniques to study plasma conditions in the core are significant to study what actually happened in ICF experiments, and also to benchmark the hydrodynamics simulation codes. Two different levels of electron temperature and density diagnostics can be achieved by analyzing data recorded by two spectrometers SSCA and MMI. Basic principles will be discussed along with the processing and analysis of the data.
|
April 2, 2009
| Speaker: | Chris Plechaty |
| Topic: | The Role of Flute Instabilities in Plasma Penetration of a Magnetic Field
|
| Date: | Thursday, April 2, 2009 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | In both natural and laboratory environments, plasma-magnetic field interactions are common. Under many circumstances, a plasma can penetrate across an externally applied magnetic field. To investigate the role of flute instabilities in the penetration of a plasma across an applied magnetic field, an experiment was performed at the Nevada Terawatt Facility (NTF). In the experiment a plasma produced by laser ablation of a polyethylene target, penetrates an external magnetic field generated by passing a 0.6 MA current (in 200 ns) through a straight cylindrical electrode. The laser produced plasma penetrates due to flute instabilities which form on the interface layer between the plasma and the magnetic field. The results of two experiments, performed in different plasma regimes, will be presented.
|
March 26, 2009
| Speaker: | Kiran Baral |
| Topic: | Overview of Endohedral Fullerenes
|
| Date: | Thursday, March 26, 2009 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | A general review of endohedral fullerenes is presented. Their structures, properties, synthesis and possible applications in various areas are discussed. The photoionization of endohedral fullerenes using the photon-ion merged-beams technique is presented along with a brief introduction of the ALS photon-ion end station.
|
March 6, 2009
| Speaker: | David Martinez |
| Topic: | Overview of Wire Array Z-pinch Instabilities
|
| Date: | Thursday, March 6, 2009 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Wire array Z-pinches are an effective way to generate short intense X-ray pulses for inertial confinement fusion. In order to optimize the X-ray yield and power, it is important to have knowledge of the pinch instabilities which play a profound role in plasma confinement. In the Z-pinch, instabilities form as early as the ablation of the wires and then seed Rayleigh-Taylor instabilities during the implosion of the plasma shell. Lastly, these instabilities manifest themselves as the pressure driven MHD instabilities, such as the "sausage" and "kink" instabilities, that are responsible for the disruption of the pinch plasma column. In this talk I will review the Z-pinch implosion and discuss the prominent instabilities.
|
February 26, 2009
| Speaker: | Rohini Mishra |
| Topic: | Hot electron generation at steep interface in ultra intense laser-solid interaction
|
| Date: | Thursday, February 26, 2009 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | In laser plasma interaction experiments, super intense laser lights >1020 W/cm2 are able to sweep the pre-plasma over short times and compress the density gradient. Hot electron generation and absorption of ultra-intense laser light in the step-like densities, highly overdense plasma is discussed. To understand absorption physics we performed one-dimensional collisional particle simulation (PICLS1D). These simulations are able to simulate hot electrons generation in the laser plasma interaction, and energy transport thorough the cold resistive plasma. For a high intensity normal incident laser light the absorption is mainly due to JXB absorption. We observed the electrons are trapped in the interaction region by the electrostatic potential, and the JXB force drives the high energy electrons inwards from the trapped region with the frequency 2w (twice the laser frequency). Except these 2w electron jets there are also large number of diffusive electrons with lesser energy and with the frequency 3w, 6w. When we increase the target density, the JXB electron jets start to disappear resulting in to the decrease of total absorption. The physics inhibiting the production of 2w frequency electrons discussed. Together with this analytical prediction for 2w and diffusive electrons is predicted which is consistent with the simulation results.
|
November 20, 2008
| Speaker: | Ken Williamson |
| Topic: | Scaling of Radiation Yields of Planar Wire Arrays at the 1MA Zebra Generator
|
| Date: | Thursday, November 20, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Analysis is presented on scaling of radiated x-ray power, energy and implosion timing of single planar wire arrays (SPWA) and double planar wire array (DPWA) of Mo and W with respect to current peak (0.8-1.4 MA), mass and array dimensions at 100ns current pulse. Such scaling investigations are important for understanding the potential of these loads as an ICF radiation source1. These data are used to identify promising directions to pursue with regard to highest x-ray output, smallest load size, and most consistent shot-to-shot performance. It is shown that W SPWA and DPWA total energy yield and peak power increased near-quadratically with current. DPWA scans of inter-planar gaps from 1.5mm to 9mm show an output maximum at 1.5mm with decreasing output for 6mm and 9mm. A DPWA width scan shows that radiation yields decrease slowly as the width is decreased, which may allow for more compact loads without significant sacrifice to the output radiation. A mass scan of several W loads show that the implosion timing increases with mass.
|
November 13, 2008
| Speaker: | Nagendra Aryal |
| Topic: | An Introduction to Fullerenes and Their Photoionization
|
| Date: | Thursday, November 13, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | A brief introduction to the Nobel Prize winning discovery of fullerenes, a new form of carbon molecules, will be discussed with their unique structures, characteristics and possible applications in various areas. Current absolute cross-section measurements for the single photoionization of various fullerene ions using the ion-photon merged-beams endstation at the Advanced Light Source in Berkeley, California will be discussed along with the photo excitation of surface and volume plasmons in fullerenes.
|
November 6, 2008
| Speaker: | Vijay Singh |
| Topic: | Increasing Lifetime of an Air Plasma Channel Using Short Laser Pulses
|
| Date: | Thursday, November 6, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | An experiment in pump-probe configuration was conducted to increase the lifetime of an air plasma channel. The plasma channel was created by focusing a 50ps, 36mJ laser pulse in air. The lifetime of the plasma channel was estimated by monitoring the temporal evolution of the fluorescence of a spectral line at 504.5 nm of N+ transition 3p 3S - 3s 3p°. A four-fold increase in lifetime of the plasma channel was observed by propagating a 6ns, 12mJ laser pulse through the channel at 7ns delay after the picosecond laser pulse. In order to understand the process of increase in lifetime, we characterized our plasma channel using optical emission spectroscopy. Assuming the plasma to be in local thermodynamic equilibrium, plasma temperature of ~ 8.2 eV was calculated from Boltzmann plot of relative intensities of nitrogen lines. The electron density of ~ 1018 cm-3 was estimated using Stark broadening of 649.2 nm line of N+ transition 3d 3D° - 4p 3D. An enhancement in electron density of the plasma channel by a factor of ~ 2 was observed at a 7ns delay of the nanosecond laser pulse relative to the picosecond laser pulse.
|
October 30, 2008
| Speaker: | Kyle Beloy |
| Topic: | Microwave clock based on atoms in an engineered optical lattice
|
| Date: | Thursday, October 30, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | We propose a new class of atomic microwave clocks based on the hyperfine transitions in the ground state of aluminum or gallium atoms trapped in optical lattices. For these elements magic wavelengths exist at which both levels of the hyperfine doublet are shifted at the same rate by the lattice laser field, cancelling its effect on the clock transition. The accuracy of the proposed microwave lattice clock is competitive to that of the state-of-the-art primary frequency standard.
|
October 23, 2008
| Speaker: | Glenn Osborne |
| Topic: | Diagnostic of Charge Balance in High-Temperature Tungsten Plasmas Using LLNL EBIT
|
| Date: | Thursday, October 23, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Diagnostic of high-temperature M-shell W plasmas is challenging because of contribution of numerous ionization stages in a relatively narrow x-ray spectral region. A method using LLNL EBIT data generated at different electron beam energies has been established for the identification of prominent spectral features and for the determination of charge balance in x-ray M-shell W spectra between 3.5 and 8.5 Ĺ. This diagnostic procedure was tested with results from Z-pinch plasmas produced on the 1 MA pulse power generator Zebra at UNR.
|
October 9, 2008
| Speaker: | Michael Bakeman |
| Topic: | Magnetic Characterization and Design of an Undulator-based Electron Beam Diagnostic
|
| Date: | Thursday, October 16, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | The LOASIS Laser Wakefield Accelerator (LWFA) has achieved quasi-mono-energetic electron beams with energies up to 1 GeV. These beams offer the potential for use with insertion devices such as wigglers and undulators to create tabletop XUV and x-ray free-electron laser (FEL) sources. To achieve a high quality light source producing high brightness radiation requires an electron beam with low energy spread and low emittance. Here we discuss the use of an undulator to generate XUV radiation to characterize in a single shot the electron beam energy spread and emittance with high precision.
|
October 9, 2008
| Speaker: | David Martinez |
| Topic: | Analysis of Conical Wire Array Z-Pinch Stability with a Center Wire
|
| Date: | Thursday, October 9, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | The presence of sheared flow has been conjectured to stabilize z-pinch instabilities. Its effect has been investigated theoretically and computationally. However, experimental testing in the high energy density regime is still lacking and, generally, dedicated experiments are rare. In a z-pinch relevant current-carrying configuration, plasma flows can be generated using conical wire arrays. Supersonic and super-Alfvénic streams of plasma ablated from the wire surfaces are accelerated towards the array axis in a direction perpendicular to the wires. At the axis, the kinetic energy of the radial motion is thermalized. The axial momentum is preserved, leading to the formation of an axial plasma flow. The implosion of a conical wire array normally produces a current carrying, unstable, non-uniform emitting column on the axis. Placing an additional wire on the axis of the array introduces a velocity shear (radial gradient of the axial velocity) in the flow. A second important role of the central wire is to carry current early on. After it turns into plasma, the instabilities are afforded time to grow before the flow fully develops. Therefore, adding a wire on the axis of a conical wire array provides a simple way of investigating the effects of sheared flows upon the z-pinch stability. Based on this concept, an experiment was developed and performed at the Nevada Terawatt Facility on the 1 MA Zebra pulsed power generator. Results show that a wire on the axis of a conical wire array stabilizes the precursor and the pinch. To identify the stabilization mechanism, the study included equivalent cylindrical wire arrays. This comparison indicated that a thick (~50 µm) central wire is sufficient to stabilize the kink mode in both arrays. In contrast, a thin (~10 µm) central wire produced a stable pinch only in the conical configuration, indicating that the sheared flow is effective. Additional experimental tests have been designed to further verify this observation.
|
September 25, 2008
| Speaker: | Madhu Gyawali |
| Topic: | Aerosol Optics of Summer 2008 California Wildfires: Comparison with a 'Normal' Month
|
| Date: | Thursday, September 25, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Hundreds of wildfires in Northern California were sparked by lightning during the summer of 2008, resulting in downwind smoke for the months of June and July associated with the flaming and smoldering stages of the fires. These fires are consistent with a growing trend towards increasing biomass burning worldwide. Climate impacts from the smoke depend critically on the smoke amount and aerosol optical properties. We report comparison of aerosol optics measurements in Reno Nevada made during the very smoky summer month of July with the relatively clean, average month of August. Photoacoustic instruments equipped with integrating nephelometers were used to measure aerosol light scattering and absorption at wavelengths of 405 nm, and 870 nm. Total aerosol optical depth was measured with a sun photometer operating at 430nm, 470nm, 530nm, 660nm, 870nm and 950nm. These measurements document the intensity of the smoke optical impacts downwind. They are processed further to reveal a strong variation of the aerosol light absorption on wavelength, indicating the presence of light absorbing organic carbon and perhaps wavelength dependent absorption caused by black carbon particles coated with organic and inorganic particulate matter. On the day with most smoke in Reno (July 10, 2008), Angstrom coefficients for absorption as high as 3.6 were found for wavelengths of 405 nm and 870 nm, with the corresponding single scattering albedo near 0.92 at 405 nm. Aerosol optical depths of 3.5 were found for 430 nm on July 10th from the sun photometer measurements. A roughly fourfold increase in aerosol optical quantities was observed between the months of July and August 2008, attesting to the large average effects of biomass aerosols from the California wildfires. The 'normal' month of August exhibits surprisingly low Angstrom exponents for aerosol light absorption at mid day when the single scattering albedo is highest, likely as a consequence of the wavelength dependence of aerosol light absorption by particles coated with non absorbing organic and inorganic matter.
|
September 18, 2008
| Speaker: | Nicholas Ouart |
| Topic: | Analysis of Compact Cylindrical Wire Arrays Implosions with Brass and also by Alternating Brass and Al wires on the 1-MA COBRA Generator |
| Date: | Thursday, September 18, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Implosions from compact cylindrical wire arrays (CCWA) with mid-Z and low-Z wires were carried out on the 1-MA COBRA generator at Cornell University. In particular, the CCWA used either Brass 310 (70% Cu, 30% Zn) or Al 5056 (95% Al, 5% Mg) wires, as well as combination of them arranged in an alternating pattern. This study will focus mainly on the spectroscopy of the loads containing brass by applying the non-LTE kinetic models of Cu and Zn to account for the L-shell radiation. The resulting plasma parameters, electron density and temperature, will be discussed and compared. Simulations with the Wire Ablation Dynamics Model to analyze the differences in implosion dynamics of the uniform and alternating CCWA will be discussed.
|
September 11, 2008
| Speaker: | Heather Johns |
| Topic: | Inertial Confinement Fusion Implosions |
| Date: | Thursday, September 11, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Inertial confinement fusion (ICF) is an experimental method in which the plasma is confined only by the inertia of the initial implosion, driven by either lasers or particle beams. A project involving time-resolved data from direct-drive implosions at OMEGA is given as an example of scientific ICF research. The state of the compressed Ti-doped shell is determined using a model of Ti K-shell line absorption spectra, developed to create synthetic spectra at known electron temperatures (Te) and electron number densities (Ne). Synthetic spectra which compare to the data well therefore reveal the Te, Ne state of the compressed shell at the time the spectrum was produced.
|
May 1, 2008
| Speaker: | Guoxun Tian |
| Topic: | Oxygen Measurement using Photoacoustics |
| Date: | Thursday, May 1, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Available instrumentation includes innovative optical and acoustic instruments for the real-time quantification of particulate matter (PM) and its optical properties. Instruments have been designed to measure aerosol extinction, dots scattering and absorption components, and are being used for ambient measurements in air quality studies. We use oxygen as a sample and built a photo-acoustic detection system to measure the absorption of oxygen by using frequency and amplitude modulation of a laser at the same time. Compared with the theoretical spectrum, measurements using this system match very well.
|
April 24, 2008
| Speaker: | Sudarshan Dhugana |
| Topic: | Electrons in an External Oscillator Potential |
| Date: | Thursday, April 24, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | The problem of the Schrödinger equation for one electron in harmonic oscillator potential is exactly solvable. The same problem for two electrons (interacting with Coulomb potentials) can have an approximate analytical solution. For large separation distance between the two electrons, the corresponding potential is cast in the form of a parabolic potential and hence the problem can be solved exactly using quantum mechanics. The consequence arising due to the presence of two electrons will be discussed. A small separation distance between the two electrons leads to the anharmonic oscillator problem which may be solved by the perturbation method. The three-dimensional Schrödinger equation for three electrons in a simple harmonic confinement potential can be decoupled into three pairs of problems, provided the expectation value of the center of mass vector R is small compared with the average distance between the electrons. The solutions for different correlation limits will be discussed.
|
April 17, 2008
| Speaker: | Mei-Ju Lu |
| Topic: | Measurements of Spin-Relaxation Collisions for Cold Atoms with Orbital Angular Momentum |
| Date: | Thursday, April 17, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Atoms with nonzero orbital angular momentum have highly anisotropic interactions resulting in strong coupling between their internal quantum state and their motion. While the collisional properties of these atoms have been studied theoretically for decades, there is little experimental data at cold temperatures. We use helium buffer gas cooling, optical pumping, and laser spectroscopy to measure spin-relaxation collisions in anisotropically-interacting atoms. We also use electromagnetically-induced-transparency to measure spin-decoherence collisions. The presentation will include experimental methods and results for Ti-He inelastic collisions.
|
April 3, 2008
| Speaker: | Marin Djendjinovic |
| Topic: | Volumetric Properties of Solvated Protein: A Molecular Dynamics Study |
| Date: | Thursday, April 3, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Proteins are large organic molecules whose building blocks, amino acids, are arranged in a linear chain and joined together by peptide bonds. A functioning protein is solvated inside of a living cell, and has its amino acids arranged in a 3D conformation, called native state. A change in environment (such as change in pressure, temperature, a solvent pH value or other) can cause protein to "unfold" to a linear chain (denatured state). In denatured state, protein cannot perform its biological function. This transition, which is a reversible thermodynamic process, is yet to be fully understood. It has been shown, however, that the understanding of volumetric properties of solvated proteins in the native state, namely the coefficient of thermal expansion and compressibility, is crucial to the understanding of the transition between the two states. In this presentation,a recent computer simulation study of the compressibility of the BPTI protein will be discussed.
|
March 20, 2008
| Speaker: | Rajan K. Chakrabarty |
| Topic: | Aerosol Classification by Morphology using Electrostatic Charge |
| Date: | Thursday, March 20, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | A novel technique to select fractal-like aerosol agglomerates based on their morphology using electrostatic classifiers will be presented. Given a population of agglomerates with similar mobility diameters, the technique distinguishes agglomerate morphology based on the net charge carried by the agglomerate. This talk will discuss the successful application of this technique to flame-soot agglomerates where singly and doubly net-charged particles corresponding to mobility diameters of 220 and 460 nm, respectively, were size selected using electrostatic classifiers and shown to have different morphologies. The flexibility and simplicity of this technique does not limit its application to aerosols, but makes it an attractive candidate for performing particle shape selection of different types of nano and micro materials.
|
March 13, 2008
| Speaker: | Kiattichart Chartkunchand |
| Topic: | Laser Photodetached Electron Spectroscopy of Negative Ion Beams |
| Date: | Thursday, March 13, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Laser Photodetached Electron Spectroscopy (LPES) is an important technique in the study of negative ions. Here a beam of negative ions produced by a cesium-sputter source is crossed perpendicularly with monochromatic laser light, ejecting the excess electron in a process known as photodetachment. The photodetached electrons, or photoelectrons, are then measured by an electron energy spectrometer to produce a photoelectron kinetic energy spectrum. Along with providing measurements of the electron affinity of the neutral species which makes up the negative ion, the LPES technique also facilitates the measurement of photodetachment differential cross sections through angular distribution measurements of the ejected photoelectrons. Use of the LPES technique on both atomic and molecular negative ion beams, with particular emphasis on the hydrogen molecular anion, will be discussed.
|
March 6, 2008
| Speaker: | Madhu Gyawali |
| Topic: | “NASA-A TRAIN” Coordinated Satellite Measurements of The Earth's Atmosphere |
| Date: | Thursday, March 6, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | The “NASA-A Train” constellation of seven Earth–orbiting satellites remotely sense clouds, aerosols, water vapor, and trace gases such as CO2, NO2, and O3. The satellites orbit the earth sun–synchronously at an altitude of 705 km with Aqua in the lead, followed in order by CALIPSO, CloudSat, PARASOL, and Aura (two more satellites OCO and Glory will join the constellation at the end of 2008). With Aqua in the lead and Aura at the tail, this formation has been termed the afternoon constellation or A-Train, and all satellites cross the equator within a few minutes around 1:30 pm local time (ascending node). These satellites employ revolutionary measurement methods to probe the Earth's atmosphere, and will improve our weather and climate forecasts. Data from these satellites can be used together to obtain comprehensive information about atmospheric processes. This talk will give an overview of the active and passive remote- sensing measurement techniques used on satellite as well as description of the importance of the atmospheric properties being measured. A discussion will be presented on the importance of synergistic measurements of the satellite suite.
|
February 28, 2008
| Speaker: | Tasha Goodrich |
| Topic: | A Semi-Analytic Liner Implosion Model for Flux Compression on ATLAS |
| Date: | Thursday, February 28, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | A flux compression experiment is being designed for the Atlas pulsed power facility. The purpose is to investigate generation of megagauss fields with liner technology in the geometry needed for compression of a stabilized diffuse z pinch. To survey possible parameters quickly and conveniently, a semi-analytic model has been developed that computes liner motion under the assumption that the liner remains cylindrically symmetric during the implosion and the metal of the liner is incompressible. Thus the liner thickness increases during implosion in a predictable way to conserve liner mass. Equations are derived for the time variation of liner position and circuit current including the effect of back pressure from the compressed flux. The model allows using realistic Atlas circuit parameters. The equations are integrated using the Matlab program and a standard Runge Kutta method. Recently the model has been extended to account for a shunt resistor and the resulting time-dependent current that would be generated inside the liner. The important advantage of a shunt resistor is that an auxiliary power supply is not needed to generate the seed flux which liner motion will compress. By tapping the power of Atlas to generate the seed flux, the incremental cost of a flux compression experiment is minimized. The selection of shunt material and dimensions must consider both the heating of the shunt and the amount of trapped flux, which along with the liner kinetic energy determines the final level of compressed magnetic field. Initial results suggest that readily available materials (a steel shunt and an aluminum liner) and properly chosen dimensions give a workable combination that generates magnetic field of several megagauss.
|
February 21, 2008
| Speaker: | Nicholas Ouart |
| Topic: | Measurements of plasma conditions in precursor plasmas on the 1-MA Zebra generator |
| Date: | Thursday, February 21, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Precursor plasmas have been observed on many z-pinch experiments at various facilities, including low current (~1 MA, MAGPIE, Zebra, COBRA) and high current (>15 MA, Z) facilities. The impact of the precursor on stagnated plasmas and inertial confinement fusion targets is still under evaluation. Experiments have been performed on Zebra to study precursor plasmas with copper cylindrical wire arrays in collaboration with Sandia National Labs. Significant precursor radiation at photon energies >1 keV was observed on filtered PCDs. Precursor temperatures and densities have been obtained from modeling of the Cu L-shell emission recorded from a time-gated x-ray spectrometer. The precursor plasma temperatures are consistently >250 eV, which is higher than previously observed.
|
February 14, 2008
| Speaker: | Essam Yasin |
| Topic: | Plasma Micro and Macro Instabilities |
| Date: | Thursday, February 14, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | Plasma instabilities are of great importance, especially in controlled fusion research. This seminar will focus on the classification of plasma instabilities, how they develop and what are their roles in studying plasma. The seminar will focus on the two-stream instability and its importance, particularly in beam physics.
|
February 7, 2008
| Speaker: | Brian Chrisman |
| Topic: | Intensity Scaling of Hot Electron Coupling in Cone-Guided Fast Ignition |
| Date: | Thursday, February 7, 2008 at 4:00 pm |
| Location: | LP 208 |
| Abstract: | A critical issue for the fast ignition of inertial fusion targets, where compressed fuel is ignited by injection of an intense short laser pulse, is whether the hot electrons produced in the interaction are in an energy range conducive to efficient heating of the core. This work presents the first comprehensive two dimensional kinetic simulation of the cone-guided approach to fast ignition. Simulation results predict the hot electron temperature to be much lower than previous expected, which indicates the possibility to use them for optimum core heating. Furthermore the roles of collisional versus kinetic processes for transport and as heating mechanisms of the core plasmas are clarified. The core heating efficiency scales linearly with intensity at ranges of 1019 - 1020 W/cm2, but falls below this range due to deflection of hot electrons in strong magnetic filaments behind the cone target.
|
