Browsing by Subject "plasma"

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Asymmetry-driven structure formation in pair plasmas
(2009-12) Mahajan, S. M.; Shatashvili, N. L.; Berezhiani, V. I.; Mahajan, S. M.
The nonlinear propagation of electromagnetic waves in pair plasmas, in which the electrostatic potential plays a very important but subdominant role of a "binding glue" is investigated. Several mechanisms for structure formation are investigated, in particular, the "asymmetry" in the initial temperatures of the constituent species. It is shown that the temperature asymmetry leads to a (localizing) nonlinearity that is qualitatively different from the ones originating in ambient mass or density difference. The temperature-asymmetry-driven focusing-defocusing nonlinearity supports stable localized wave structures in 1-3 dimensions, which, for certain parameters, may have flat-top shapes.
Characterization of a 50kW Inductively Coupled Plasma Torch for Testing of Ablative Thermal Protection Materials Using Non-Air Gases
(2018-05) Cha, Han All; Clemens, Noel
Thermal protection systems have been a major area of study since the advent of space flight, but recent efforts towards crewed spaceflight missions have placed a new importance on the development of such systems. The 50 kW Inductively Coupled Plasma (ICP) Torch Facility at The University of Texas at Austin allows for rapid testing of high-temperature aerospace materials essential to the development of thermal protection systems in planetary re-entry applications. This ICP Torch Facility has been previously characterized using air as the test gas. However, planets of interest for future exploration have atmospheric compositions that differ from air, so testing heat shield materials in the presence of other gases is critical. To address this disparity between tested and actual environment, the current work characterizes the torch using various combinations of argon, CO2, and N2 by determining its operational range at various power settings, mass flow rates, and mixtures these gases. At each setting, the cold-wall heat flux is also measured to determine the range the torch is able to provide. Measurements indicate that using pure Ar gives the torch the largest operating range with regard to power setting and gas injection mass flow rate, and mixing argon into other gases drastically increases the stable operating range compared to the pure gas. Pure CO2 does not form a stable plasma discharge, but a mixture of 50% argon and 50% CO2 (by mass) provides stable operation up to 40 slpm total gas flow rate with a maximum heat flux of 98 W/cm2. Smaller percentages of CO2 allow the cold-wall heat flux to be increased to 110 W/cm2. Pure N2 forms a stable plasma discharge, but the operating range is very limited, providing stable operation up to 20 slpm total gas flow rate with a maximum heat flux of 110 W/cm2.
Creating White Dwarf Photospheres In The Laboratory
(2010-08) Falcon, R. E.; Rochau, G. A.; Bailey, J. E.; Ellis, J. L.; Montgomery, M. H.; Winget, D. E.; Gomez, M. R.; Leeper, R. J.; Falcon, Ross E.; Winget, D. E.; Montgomery, M. H.
We present a preliminary report from the laboratory astrophysics experiments to create macroscopic (similar to 19 cm(3)) hydrogen-plasmas with white dwarf (WD) photospheric conditions (i.e., temperature, electron density). These experiments, performed at the Z Pulsed Power Facility at Sandia National Laboratories, will serve as benchmarks for fundamental atomic line profile measurements in emission and absorption; they are targeted to address the discrepancy between theory and observation of WD photospheres - cooler photospheres in particular.
The Design, Fabrication, and Testing of a Five Megajoule Homopolar Motor-Generator
(1974-11) Weldon, W.F.; Driga, M.D; Woodson, H.H; Rylander, H.G.
The current and future generations of controlled thermonuclear fusion experiments require large amounts of pulsed energy for heating and confinement of plasma. Kinetic energy storage with direct conversion to electrical power (i.e. homopolar machines) seems to be the most economically attractive solution for meeting these requirements. The University of Texas at Austin has a program intended to develop a design technology for homopolar machines to meet a broad spectrum of performance requirements in terms of stored energy and discharge times. The Energy Storage Group at the University of Texas at Austin has in the past ten months designed, fabricated, assembled and begun a thorough testing program on a second generation homopolar machine with a storage capacity of five megajoules. This machine, using room temperature field coils, solid electrical brushes, and hydrostatic bearings has been designed to deliver 42 volt pulses at current levels in excess of 150,000 amperes. The machine has been designed as a laboratory device with extremely stiff bearings, variable brush area as well as variable brush contact force, variable field strength for pulse shaping, and minicomputer controlled data acquisition, real time signature analysis and on line experiment control. A continuing program studying discharge characteristics, brush and rotor dynamics, machine losses, and system efficiencies is already underway and is currently funded through June, 1975. Funding for the project has been provided by the Atomic Energy Commission, the Texas Atomic Energy Research Foundation, and the Electric Power Research Institute.
Electron Temperature Gradient Mode Transport
(2008-05) Horton, W.; Kim, J. H.; Hoang, G. T.; Park, H.; Kaye, S. M.; LeBlanc, B. P.; Horton, W.; Kim, J. H.
Anomalous electron thermal losses plays a central role in the history of the controlled fusion program being the first and most persistent form of anomalous transport across all toroidal magnetic confinement devices. In the past decade the fusion program has made analysis and simulations of electron transport a high priority with the result of a clearer understanding of the phenomenon, yet still incomplete. Electron thermal transport driven by the electron temperature gradient is examined in detail from theory, simulation and power balance studies in tokamaks with strong auxiliary heating.
Emittance Control In Laser Wakefield Accelerator
(2001-05) Cheshkov, S.; Tajima, T.; Chiu, C.; Breitling, F.; Cheshkov, S.; Tajima, T.; Chiu, C.; Breitling, F.
In this paper we summarize our recent effort and results in theoretical study of the emittance issues of multistaged Laser Wakefield Accelerator (LWFA) in TeV energy range, In such an energy regime the luminosity and therefore the emittance requirements become very stringent and tantamount to the success or failure of such an accelerator. The system of such a machine is very sensitive to jitters due to misalignment between the beam and the wakefield. In particular, the effect of jitters in the presence of a strong focusing wakefield and initial longitudinal phase space spread of the beam leads to severe transverse emittance degradation of the beam. To improve the emittance we introduce several methods: a mitigated wakefield focusing by working with a plasma channel, an approximately synchronous acceleration in a superunit setup, the >horn> model based on exactly synchronous acceleration achieved through plasma density variation and lastly an algorithm based on minimization of the final beam emittance to actively control the stage displacement of such an accelerator.
Fast-electron-relaxation measurement for laser-solid interaction at relativistic laser intensities
(2007-11) Chen, H.; Shepherd, R.; Chung, H. K.; Kemp, A.; Hansen, S. B.; Wilks, S. C.; Ping, Y.; Widmann, K.; Fournier, K. B.; Dyer, G.; Faenov, A.; Pikuz, T.; Beiersdorfer, P.; Dyer, G.
We present measurements of the fast-electron-relaxation time in short-pulse (0.5 ps) laser-solid interactions for laser intensities of 10(17), 10(18), and 10(19) W/cm(2), using a picosecond time-resolved x-ray spectrometer and a time-integrated electron spectrometer. We find that the laser coupling to hot electrons increases as the laser intensity becomes relativistic, and that the thermalization of fast electrons occurs over time scales on the order of 10 ps at all laser intensities. The experimental data are analyzed using a combination of models that include K alpha generation, collisional coupling, and plasma expansion.
Formation Of Optical Bullets In Laser-Driven Plasma Bubble Accelerators
(2010-11) Dong, P.; Reed, S. A.; Yi, S. A.; Kalmykov, S.; Shvets, G.; Matlis, N. H.; McGuffey, C.; Bulanov, S. S.; Chvykov, V.; Kalintchenko, G.; Krushelnick, K.; Maksimchuk, A.; Matsuoka, T.; Thomas, A. G. R.; Yanovsky, V.; Downer, M. C.; Dong, P.; Reed, S. A.; Downer, M. C.
Electron density bubbles generated in plasma of density n(e) similar to 10(19)/cm(3) are shown to reshape copropagating probe pulses into optical >bullets.> The bullets, reconstructed by frequency-domain interferometric techniques, are used to visualize bubble formation independently of relativistic electron generation.
Frequency Domain Tomography Of Evolving Laser-Plasma Accelerator Structures
(2009-11) Dong, P.; Reed, S.; Kalmykov, S.; Shvets, G.; Downer, M.; Dong, Peng; Reed, Stephen; Kalmykov, Serguei; Shvets, Gennady; Downer, Mike
Frequency Domain Holography (FDH), a technique for visualizing quasistatic objects propagating near the speed of light, has produced >snapshots> of laser wakefields,(1) but they are averaged over structural variations that occur during propagation through the plasma medium. Here we explore via simulations a generalization of FDH --- that we call Frequency Domain Tomography (FDT) --- that can potentially record a time sequence of quasistatic snapshots, like the frames of a movie, of the wake structure as it propagates through the plasma. FDT utilizes a several probe-reference pulse pairs that propagate obliquely to the drive pulse and wakefield, along with tomographic reconstruction algorithms similar to those used in medical CAT scans.
Fusion Ignition Experiment (IGNITEX)
(1988-04) Carrera, R.; Driga, M. D; Gully, J. H; Hallock, G; Hertel, N; Hsieh, K. T; Walls, W. A; Weldon, W. F; Woodson, H. H; Wu, A. Y; Montalvo, E; Ordonez, C; et. al.
Fusion Ignition Experiment (IGNITEX)
(1987-10) Carrera, R.; Driga, M. D; Gully, J. H; Hsieh, K. T; Montalvo, E; Ordonez, C; Rosenbluth, M. N; Walls, W. A; Weldon, W. F; Woodson, H. H.
A Gravitational Redshift Determination Of The Mean Mass Of White Dwarfs: DBA And DB Stars
(2012-10) Falcon, Ross E.; Winget, D. E.; Montgomery, Michael H.; Williams, Kurtis A.; Falcon, Ross E.; Winget, D. E.; Montgomery, Michael H.
We measure apparent velocities (nu(app)) of absorption lines for 36 white dwarfs (WDs) with helium-dominated atmospheres-16 dbAs and 20 dbs-using optical spectra taken for the European Southern Observatory SN Ia progenitor survey. We find a difference of 6.9 +/- 6.9 kms(-1) in the average apparent velocity of the H alpha lines versus that of the He I 5876 angstrom lines for our dbAs. This is a measure of the blueshift of this He line due to pressure effects. By using this as a correction, we extend the gravitational redshift method employed by Falcon et al. to use the apparent velocity of the He I 5876 angstrom line and conduct the first gravitational redshift investigation of a group of WDs without visible hydrogen lines. We use biweight estimators to find an average apparent velocity, (BI), (and hence average gravitational redshift, (BI)) for our WDs; from that we derive an average mass, < M >(BI). For the dbAs, we find (BI) = 40.8 +/- 4.7 kms(-1) and derive < M >(BI) = 0.71(-0.05)(+0.04) M-circle dot. Though different from of DAs (32.57 km s(-1)) at the 91% confidence level and suggestive of a larger dbA mean mass than that for normal DAs derived using the same method (0.647(-0.014)(+0.013) M-circle dot; Falcon et al.), we do not claim this as a stringent detection. Rather, we emphasize that the difference between (BI) of the dbAs and of normal DAs is no larger than 9.2 kms(-1), at the 95% confidence level; this corresponds to roughly 0.10 M-circle dot. For the dbs, we find (BI) = 42.9 +/- 8.49 km s(-1) after applying the blueshift correction and determine < M >(BI) = 0.74(-0.09)(+0.08) M-circle dot. The difference between (BI) of the dbs and of DAs is <= 11.5 kms(-1) (similar to 0.12 M-circle dot), at the 95% confidence level. The gravitational redshift method indicates much larger mean masses than the spectroscopic determinations of the same sample by Voss et al. Given the small sample sizes, it is possible that systematic uncertainties are skewing our results due to the potential of kinematic substructures that may not average out. We estimate this to be unlikely, but a larger sample size is necessary to rule out these systematics.
Laboratory Measurements Of White Dwarf Photospheric Spectral Lines: H Beta
(2015-06) Falcon, Ross E.; Rochau, G. A.; Bailey, J. E.; Gomez, Thomas A.; Montgomery, Michael H.; Winget, D. E.; Nagayama, T.; Falcon, Ross E.; Gomez, Thomas A.; Montgomery, Michael H.; Winget, D. E.
We spectroscopically measure multiple hydrogen Balmer line profiles from laboratory plasmas to investigate the theoretical line profiles used in white dwarf (WD) atmosphere models. X-ray radiation produced at the Z Pulsed Power Facility at Sandia National Laboratories initiates plasma formation in a hydrogen-filled gas cell, replicating WD photospheric conditions. Here we present time-resolved measurements of H beta and fit this line using different theoretical line profiles to diagnose electron density, n(e), and n = 2 level population, n2. Aided by synthetic tests, we characterize the validity of our diagnostic method for this experimental platform. During a single experiment, we infer a continuous range of electron densities increasing from n(e) similar to 4 to similar to 30 x 10(16) cm(-3) throughout a 120-ns evolution of our plasma. Also, we observe n(2) to be initially elevated with respect to local thermodynamic equilibrium (LTE); it then equilibrates within similar to 55 ns to become consistent with LTE. This supports our electrontemperature determination of T-e similar to 1.3 eV (similar to 15,000 K) after this time. At n(e) greater than or similar to 10(17) cm(-3), we find that computer-simulation-based line-profile calculations provide better fits (lower reduced chi(2)) than the line profiles currently used in the WD astronomy community. The inferred conditions, however, are in good quantitative agreement. This work establishes an experimental foundation for the future investigation of relative shapes and strengths between different hydrogen Balmer lines.
LWFA With Low Energy Raman Seeded Pulses
(2004-12) Fomyts'kyi, M.; Chiu, C.; Downer, M.; Grigsby, F.; Fomyts’kyi, Mykhailo; Chiu, Charles; Downer, Michael; Grigsby, Franklin
Analytical and numerical calculations of plasma wakefield excitation and particle acceleration by Raman seeded laser pulse in self-modulation regime are presented. We derive energy threshold for self-modulation of diffraction-limited pulses. The parameter range where the Raman seeded amplitude plays an important role is investigated. We show that the seeded amplitude provides a coherent control mechanism for the phase of the wakefield wave. We show that with the use of Raman seed self-modulated wakefield acceleration is achievable for the pulses of intensities much lower than those typically used in the experiments. In particular, our 2D particle-in-cell simulations show that 30 mJ pulse combined with Raman seeded pulse, which is 1% in intensity of the main pulse is capable of generating similar to1 nC of relativistic electrons.
Mechanistic Study Of Plasma Damage Of Low k Dielectric Surfaces
(2007-10) Bao, J. J.; Shi, H. L.; Liu, J. J.; Huang, H.; Ho, P. S.; Goodner, M. D.; Moinpour, M.; Kloster, G. M.; Bao, J. J.; Shi, H. L.
Plasma damage to low k dielectric materials was investigated from a mechanistic point of view. Low k dielectric films were treated by plasma Ar, O-2, N-2/H-2, N-2 and H-2 in a standard RIE chamber and the damage was characterized by Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS), X-Ray Reflectivity (XRR), Fourier Transform Infrared Spectroscopy (FTIR) and Contact Angle measurements. Both carbon depletion and surface densification were observed on the top surface of damaged low k materials while the bulk remained largely unaffected. Plasma damage was found to be a complicated phenomenon involving both chemical and physical effects, depending on chemical reactivity and the energy and mass of the plasma species. A downstream hybrid plasma source with separate ions and atomic radicals was employed to study their respective roles in the plasma damage process. Ions were found to play a more important role in the plasma damage process. The dielectric constant of low k materials can increase up to 20% due to plasma damage and we attributed this to the removal of the methyl group making the low k surface hydrophilic. Annealing was generally effective in mitigating moisture uptake to restore the k value but the recovery was less complete for higher energy plasmas. Quantum chemistry calculation confirmed that physisorbed water in low k materials induces the largest increase of dipole moments in comparison with changes of surface bonding configurations, and is primarily responsible for the dielectric constant increase.
Multi-GeV Electron Generation Using Texas Petawatt Laser
(2010-11) Wang, X.; Du, D.; Yi, S. A.; Kalmykov, S.; D'Avignon, E.; Fazel, N.; Zagdzaj, R.; Reed, S.; Dong, P.; Henderson, W.; Dyer, G.; Bernstein, A.; Gaul, E.; Martinez, M.; Shvets, G.; Ditmire, T.; Downer, M.; Wang, X.; Du, D.; Yi, S. A.; Kalmykov, S.; D'Avignon, E.; Fazel, N.; Zagdzaj, R.; Reed, S.; Dong, P.; Henderson, W.; Dyer, G.; Bernstein, A.; Gaul, E.; Martinez, M.; Shvets, G.; Ditmire, T.; Downer, M.
We present simulation results and experimental setup for multi-GeV electron generation by a laser plasma wake field accelerator (LWFA) driven by the Texas Petawatt (TPW) laser. Simulations show that, in plasma of density n(e) = 2 - 4 x cm(-3), the TPW laser pulse (1.1 PW, 170 fs) can self-guide over 5 Rayleigh ranges, while electrons self-injected into the LWFA can accelerate up to 7 GeV. Optical diagnostic methods employed to observe the laser beam self-guiding, electron trapping and plasma bubble formation and evolution are discussed. Electron beam diagnostics, including optical transition radiation (OTR) and electron gamma ray shower (EGS) generation, are discussed as well.
On Hamiltonian And Action Principle Formulations Of Plasma Dynamics
(2009-11) Morrison, P. J.; Morrison, P.J.
A general discussion of Hamiltonian and action principle formulations for fluid and plasma models is given. A procedure, based on Hamilton's principle of mechanics but adapted for continua, for the construction of action principles for fluid and kinetic models is given. The transformation from action principles in terms of the Lagrangian variable description to the Eulerian variable description in terms of noncanonical Poisson brackets is described. Two examples are developed: ideal MHD and Braginskii's fluid model with gyroviscosity.
Optical Frequency Domain Visualization Of Electron Beam Driven Plasma Wakefields
(2010-06) Zgadzaj, R.; Downer, M. C.; Muggli, P.; Yakimenko, V.; Kusche, K.; Fedurin, M.; Babzien, M.; Zgadzaj, Rafal; Downer, Michael C.
Bunch driven plasma wakefield accelerators (PWFA), such as the >plasma afterburner,> are a promising emerging method for significantly increasing the energy output of conventional particle accelerators [1]. The study and optimization of this method would benefit from an experimental correlation of the drive bunch parameters and the accelerated particle parameters with the corresponding plasma wave structure. However, the plasma wave structure has not been observed directly so far. We will report ongoing development of a noninvasive optical Frequency Domain Interferometric (FDI) [2] and Holographic (FDH) [3] diagnostics of bunch driven plasma wakes. Both FDI and FDH have been previously demonstrated in the case of laser driven wakes. These techniques employ two laser pulses co-propagating with the drive particle bunch and the trailing plasma wave. One pulse propagates ahead of the drive bunch and serves as a reference, while the second is overlapped with the plasma wave and probes its structure. The multi-shot FDI and single-shot FDH diagnostics permit direct noninvasive observation of longitudinal and transverse structure of the plasma wakes. The experiment is being developed at the 70MeV Linac in the Accelerator Test Facility at Brookhaven National Laboratory to visualize wakes generated by two [4] and multi-bunch [5] drive beams.
The Physics of Crystallization from Globular Cluster White Dwarf Stars in NGC 6397
(2009-03) Winget, D. E.; Kepler, S. O.; Campos, Fabiola; Montgomery, M. H.; Girardi, Leo; Bergeron, P.; Williams, Kurtis; Winget, D. E.; Montgomery, M. H.; Williams, Kurtis
We explore the physics of crystallization in the deep interiors of white dwarf (WD) stars using the color-magnitude diagram and luminosity function constructed from proper- motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with the theory of crystallization of the ions in the interior of WD stars and provide the first empirical evidence that the phase transition is first order: latent heat is released in the process of crystallization as predicted by van Horn. We outline how these data can be used to observationally constrain the value of Gamma = E(Coulomb)/E(thermal) near the onset of crystallization, the central carbon/oxygen abundance, and the importance of phase separation.
Proton acceleration by irradiation of isolated spheres with an intense laser pulse
(2016-09) Ostermayr, T. M.; Haffa, D.; Hilz, P.; Pauw, V.; Allinger, K.; Bamberg, K. U.; Bohl, P.; Bomer, C.; Bolton, P. R.; Deutschmann, F.; Ditmire, T.; Donovan, M. E.; Dyer, G.; Gaul, E.; Gordon, J.; Hegelich, B. M.; Kiefer, D.; Klier, C.; Kreuzer, C.; Martinez, M.; McCary, E.; Meadows, A. R.; Moschuring, N.; Rosch, T.; Ruhl, H.; Spinks, M.; Wagner, C.; Schreiber, J.; Ditmire, T.; Donovan, M. E.; Dyer, G.; Gaul, E.; Gordon, J.; Hegelich, B. M.; Martinez, M.; McCary, E.; Meadows, A. R.; Spinks, M.; Wagner, C.
We report on experiments irradiating isolated plastic spheres with a peak laser intensity of 2-3 x 10(20) W cm(-2). With a laser focal spot size of 10 mu m full width half maximum (FWHM) the sphere diameter was varied between 520 nm and 19.3 mu m. Maximum proton energies of similar to 25 MeV are achieved for targets matching the focal spot size of 10 mu m in diameter or being slightly smaller. For smaller spheres the kinetic energy distributions of protons become nonmonotonic, indicating a change in the accelerating mechanism from ambipolar expansion towards a regime dominated by effects caused by Coulomb repulsion of ions. The energy conversion efficiency from laser energy to proton kinetic energy is optimized when the target diameter matches the laser focal spot size with efficiencies reaching the percent level. The change of proton acceleration efficiency with target size can be attributed to the reduced cross-sectional overlap of subfocus targets with the laser. Reported experimental observations are in line with 3D3V particle in cell simulations. They make use of well-defined targets and point out pathways for future applications and experiments.