Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session PP6: Poster Session VI: Low Temperature Plasmas; Laser Plasma Accelerators and Coherent Sources; C-Mod; Inertial Confinement and Laser-Plasma Interactions |
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Room: Marsalis A/B, 2:00pm - 5:00pm |
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PP6.00001: LOW TEMPERATURE PLASMAS |
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PP6.00002: Measurements of Bohm speeds of positive ions with LIF in Ar/O$_{2}$, electronegative plasmas and comparison to ion acoustic wave phase velocities Young-chul Ghim (Kim), Noah Hershkowitz The negative ion concentration in an electronegative plasma can be determined from the phase velocities of ion acoustic waves. It can also be determined from planar Langmuir probe data. These two approaches often disagree quantitatively. Plasmas with n$_{e} \quad \sim $ 10$^{9}$ cm$^{-3}$, Te $\sim $ 1 eV and the ratio of negative ions to positive ions varying from 0 -- 0.3 are generated in a dc multidipole hot filament discharge for total pressure of Ar/O$_{2}$ from 0.3 -- 1.0 mTorr. Laser induced fluorescence (LIF) measurements of the Ar II excitation transition at 668.614 nm determined metastable ion speed at the sheath edge. An emissive probe is used to determine the potential drop between the sheath edge and the bulk, and the location of the sheath edge. The LIF data for small concentration of negative ions show that positive ions attain the usual Bohm speed of (T$_{e}$/m$_{i})^{1/2}$ at the sheath edge which suggests the presence of a double layer potential structure. Planar Langmuir data were corrected using Sheridan's model$^{2}$. Quantitative agreement of negative ion concentration from two approaches is achieved. $^{2}$ T.E. Sheridan, Phys. Plasmas \textbf{7}, 3084 (2000) [Preview Abstract] |
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PP6.00003: Low-voltage arc discharge device with controllable plasma parameters A.A. Kudryavtsev, S.F. Adams, J. Blessington, V.I. Demidov, E.A. Bogdanov A gas-discharge device which demonstrates control of the plasma parameters by using nonlocal properties of fast electrons [1] is presented. The discharge takes place between an indirectly heated cathode of 1 cm in diameter and an anode of 3 cm in diameter. A special molybdenum diaphragm (the control electrode) with an external diameter of 3 cm and internal diameter of 0.2 cm and thickness of 0.2 cm is placed between cathode and anode. The distance between cathode and diaphragm is 0.8 cm and the distance between the diaphragm and anode is 0.1 cm. A conical electrode (screen) restricts the discharge plasma in the radial direction. Combined experimental and modeling characterization of the device is presented. [1] V.I. Demidov, C.A. DeJoseph, Jr and A.A. Kudryavstev, PRL 95, 215002 (2005). [Preview Abstract] |
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PP6.00004: Helium Ion Emission Spectra in a Helicon Plasma Source Michael Lindon, Earl Scime, Amy Keesee To better understand the poor reproducibility of He II absorption spectroscopy measurements in our helicon source plasmas, we have numerically generated the expected level populations for He II for a wide range of helicon source parameters. The plasma parameters are obtained with a radially scanning Langmuir probe and the level populations generated with the CHIANTI astrophysical spectra code. The CHIANTI code's focus on hydrogen-like ions is ideally suited for these helicon source plasmas. We will present comparison of the experimentally measured emission spectra of the $n $= 5 to $n $= 4 He II transition at 1012.36 nm and the $n $= 4 to $n $= 3 He II transition at 468.57 nm with the CHIANTI code predictions, as well as absorption spectra for the 1012.36 nm line. The effects of trace impurities on the emission spectra at each wavelength will also be reviewed. [Preview Abstract] |
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PP6.00005: Diagnostics and Plasma Wave Modeling of a High Power Helicon Plasma with Magnetic Nozzle Matt Wiebold, C. Mark Denning, John E. Scharer A flowing argon helicon plasma is formed with an axial magnetic field in nozzle or flat configuration, variable up to 1kG in the source region. Experimental upgrades have allowed for operation at high pulsed rf powers (up to 10 kW at 13.56 MHz) and low flow rates and pressures (as low as 10 sccm and 10$^{-5}$ Torr). A five-turn diamagnetic loop is used to measure plasma beta and perpendicular electron temperature during turn-off, and line-averaged electron density is found using 105 GHz microwave interferometry. ANTENA2 and MAXEB plasma wave codes are used to determine wave coupling effects and power deposition fractions for wave propagation with and against the direction of the static magnetic field. [Preview Abstract] |
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PP6.00006: Laser Induced Fluorescence Measurement of Plasma Hole in a Helium Plasma with Argon Impurity Shinji Yoshimura, Atsushi Okamoto, Masayoshi Tanaka A singular vortex with density cavity in its center (plasma hole) has been observed in a magnetized helium plasma. The ion flow velocity field of the plasma hole measured with a directional Langmuir probe shows a monopole vortex with radial flow. In order to measure more precise ion flow velocity field, we have developed a laser induced fluorescence (LIF) Doppler spectroscopy system for the Hyper-I device at the National Institute for Fusion Science, Japan. Since a suitable LIF scheme at visible wavelength is available for argon ions, we employed it to determine the flow velocity field of the plasma hole in a helium plasma in which a small amount of argon gas is introduced as an impurity. An ArII metastable state is excited by a tunable dye laser operating at 611.5nm and the fluorescence decay at 461.0nm is observed by a photomultiplier tube with an optical filter. By sweeping the wavelength of the dye laser, we can obtain information about the Doppler-shifted velocity distribution function of the excited ions. By changing the path of laser beam and the position of collection optics, we can obtain both azimuthal and radial ion flow velocities. [Preview Abstract] |
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PP6.00007: Modelling Ar II spectral emission from the ASTRAL helicon plasma Jorge Munoz Burgos, Robert Boivin, Stuart Loch, Ola Kamar, Connor Ballance, Mitch Pindzola We describe our spectral modeling of ArII emission from the ASTRAL helicon plasma at Auburn University. Collisional-radiative theory is used to model the emitted spectrum, with account being taken for the density and temperature variation along the line of sight. This study has two main aims. Firstly to test the atomic data used in the model and secondly to identify spectral line ratios in the 200 nm - 1000 nm range that could be used as temperature diagnostics. Using the temperature at which Ar II emission starts to be seen we have been able to test recent ionization and recombination data. Using selected spectral lines we were then able to test the importance of the continuum-coupling effects included in the most recent Ar+ electron impact excitation data. Selected spectral line ratios have been identified that show a strong temperature variation and have potential as a temperature diagnostic. [Preview Abstract] |
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PP6.00008: Ducted Alfv\'{e}n Waves in Helicon Plasmas Saeid Houshmandyar, Earl Scime Recent data from \textit{Hinode} spacecraft, the new high resolution solar imaging mission [\textit{Science}, Dec. 2007], has provided strong evidence for the presence of Alfv\'{e}n waves in Sun's corona and coronal loops. Damping of Alfv\'{e}n waves is a strong candidate for explaining the million degree corona. The high density and steep density profile of a typical helicon plasma source makes helicon plasmas a good analog to coronal loop plasma conditions (for example $V_{A-HELIX} /V_{A-Corona} \approx 0.1-1$, $\beta _{HELIX} /\beta _{Corona} \approx 0.1-10)$. Here we present observations of Alfv\'{e}n waves launched via amplitude modulation of the helicon RF antenna in HELIX (Hot hELIcon eXperiment). Plasma parameters include an ion gyro radius to system length ratio of $\rho _i /L\approx 0.01-0.1$, and electron and ion skin depth to system size ratios of $\delta _e /L\approx 0.04-0.4$ and $\delta _i /L\approx 2-100$ (corresponding coronal values for the same parameters are 0.05-0.2, 0.01-0.1 and 1-100, respectively). The waves are excited at sub-cyclotronic frequencies in argon and helium plasmas. Phase velocity and amplitude measurements in the high density region are reported and compared to an Alfv\'{e}n wave model that includes the effect of a strong density gradient. [Preview Abstract] |
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PP6.00009: Flow, flow shear, and related profiles in helicon plasmas Earl Scime, Robert Hardin, Amy Keesee, Costel Biloiu, Xuan Sun Measurements of the three-dimensional ion flow field and the ion temperature in a cross section of a cylindrical, argon, helicon plasma are presented. When combined with radially resolved measurements of the plasma density, electron temperature, neutral density and neutral temperature, the radial profiles of the ion viscosity and ion-neutral momentum transfer rate can be calculated. The ion viscosity and ion-neutral momentum transfer rate profiles are important input parameters for theoretical models of azimuthal flows arising from the nonlinear interaction of drift waves in helicon sources. The experimentally determined magnitudes and radial profiles reported in this work are significantly different than those used in recent theoretical studies. Measurements of the radial flow of argon neutrals and helium neutrals are also presented for a helicon plasma. [Preview Abstract] |
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PP6.00010: The HelCat Helicon-Cathode Device at UNM Bricette Cyrin, Christopher Watts, Mark Gilmore, Ralph Kelly, Alan Lynn, Shuangwei Xie, Lincan Yan, Yue Zhang The HelCat helicon-cathode device is a dual-source linear plasma device that has recently begun full operation at the University of New Mexico. HelCat is 4 m long, 50 cm diameter, with axial magnetic field $<$ 2.2 kG. An RF helicon source is at one end of the device, and a thermionic BaO-Ni cathode is at the other end. Discharge characteristics and fluctuations are strongly affected by the grounding scheme at the cathode source end. Thus, in a series of recent experiments we have investigated the effect on the dual source discharge of alternatively floating the anode, cathode, or both. In addition, a movable probe feedthrough utilizing a ball valve type connection is being developed. This movable probe will allow measurement in the azimutha and axial, as well as radial, directions, thereby permitting 2D and 3D mapping of the plasma parameter profiles and fluctuations. [Preview Abstract] |
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PP6.00011: A new high beta plasma device at UCLA Chris Cooper, Walter Gekelman, Patrick Pribyl, Stefanie Stattel, Kimberly DeRose, Troy Carter We recently made a high beta plasma 30 eV and 100G $<$ B$\varphi <$ 250G) on the axis of a toroidal device at UCLA. The highest beta attained, $\beta \approx$ 3, was at the lower field. The vacuum chamber is 30 meters in circumference, 2 meters wide and 3 meters tall. Using a weak vertical field, the 20 cm diameter plasma has made 4 rotations for a length of 120 m. Interestingly, at these plasma parameters the neutral penetration depth is shorter than the radius of the plasma, so we expect the plasma is fully ionized. The cathode used is amorphous Lanthanum Hexaboride (LaB$_{6})$, which has more than 10 times the emission per area of the Barium Oxide cathode now used in the LAPD. Also LaB$_{6}$ cathodes can be run in a hydrogen plasma and are relatively insensitive to oxygen leaks. We have developed a technique to make reliable large cathodes, the one in use is 400 cm$^{2}$. The magnetic field is steady state and the plasma is pulsed at 1 Hz with pulse lengths as long as 120 ms. The plasma is hundreds of parallel Alfv\'{e}n wavelengths long with a Lundquist number of 5000. Some experiments possible on this device are: fully three dimensional magnetic field line reconnection, Alfv\'{e}nic and MHD turbulence, studies of rotational transforms, physics of Alfv\'{e}n waves at high beta in Toroidal geometries, and high beta laser-plasma interactions. [Preview Abstract] |
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PP6.00012: Magnetized Plasma Experiments Using Thermionic- Thermoelectronic Plasma Emitter Eiichirou Kawamori, C.Z. Cheng, Nobuko Fujikawa, Jyun-Yi Lee, Albert Peng We are developing a magnetic mirror device, which is the first magnetized plasma device in Taiwan, to explore basic plasma sciences relevant to fusion, space and astrophysical plasmas. Our research subjects include electromagnetically induced transparency (EIT), Alfven wave physics, and plasma turbulence. A large diameter ($>$ 200 mm) plasma emitter1, which utilizes thermionic- thermoelectronic emission from a mixture of LaB6 (Lanthanum-hexaboride) and beta-eucryptite (lithium type aluminosylicate) powders, is employed as a plasma source because of its production ability of fully ionized plasma and controllability of plasma emission rate. The plasma emitter has been installed recently and investigation of its characteristics will be started. The employment of beta-eucryptite in plasma emitter is the first experimental test because such investigation of beta-eucryptite has previously been used only for Li+-ion source2. Our plan for magnetized plasma experiments and results of the plasma emitter investigation will be presented. 1. K. Saeki, S. Iizuka, N. Sato, and Y. Hatta, Appl. Phys. Lett., 37, 1980, pp. 37-38. 2. M. Ueda, R. R. Silva, R. M. Oliveira, H. Iguchi, J. Fujita and K. Kadota, J. Phys. D: Appl. Phys. 30 1997, pp. 2711--2716. [Preview Abstract] |
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PP6.00013: Characterization of Waves Propagating Parallel and Anti-Parallel Downstream of a High Power Helicon Source James Prager, Tim Ziemba, Race Roberson, Robert Winglee Measurements of the wavelength and phase velocity of waves propagating downstream of a helicon source were found to deviate significantly from the expected values based on a bounded helicon wave and matched more closely with that of a freely propagating whistler wave. The data also revealed significant differences in the measured wave fields between when the antenna was driving waves parallel and anti-parallel to B$_{0}$. These waves are correlated with the ion acceleration detected over a large axial distance, distinguishing this effect from acceleration through a double layer. Radial measurements of the wave fields propagating parallel and anti-parallel show differences in radial confinement of the waves with the profile changing from peaking on axis to maximizing at the edges of the plasma stream. Detailed results of magnetic field measurements taken both axially and radially with 3-axis bdot probes will be presented, and these measurements will be supplemented with plasma density measurements from Langmuir probes and ion energy analyzer results. [Preview Abstract] |
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PP6.00014: Enhanced Output from the High Power Helicon in Association with Macroscopic Currents and Magnetic Field Annihilation Race Roberson, Tim Ziemba, Robert Winglee, James Prager The high power helicon plasma source developed at the University of Washington is capable of transferring tens of kilowatts of power into the plasma with source plasma densities near 2x10$^{20}$ m$^{-3}$. Optical emissions from the plasma show an intense axially peaked central plasma core, typical of a helicon discharge. The intense plasma extends tens of centimeters downstream of the helicon antenna. The helicon operates with a B$_{0}$ magnetic field of .2 to .6 kG on axis that diverges with axial distance from the source. As the plasma moves out from the source, the plasma beta changes from less than unity to greater than unity within two antenna lengths. Beyond this point, the azimuthal currents driven by the helicon wave are sufficient to lead to the cancellation of the base magnetic field, and the external magnetic field leads to a highly collimated beam. The plasma induced delta B is found to be correlated with the measured wave fields from the helicon antenna and are thought to contribute to the pressure driven diamagnetic plasma currents which annihilate the B$_{0}$ field, and definitive dissipation/reflection of the wave energy is observed. The output ion energies are shown to be enhanced under these conditions. [Preview Abstract] |
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PP6.00015: Self-consistent modeling of helicon sources Guangye Chen, B. Breizman, A. Arefiev, C. Lee, R. Bengtson, L. Raja We developed a self-consistent model of helicon discharge. The model takes into account the rf wave excitation, electron heat transfer and ion transport, assuming a given neutral density. As a case study of validating the model, we simulated an early experiment [1] in which a jump in plasma density was observed in a scan of external magnetic field. Our calculation shows that a classical heat transport is unable to sustain the plasma density profile observed in the experiment. Solutions comparable to the experiment are obtained only when extra heat conductivity is used. The radial density profiles and excited wave-lengths are in good agreement with the experiment. In particular, the dual-stable solution found in the simulation supports the observed plasma density jump. The rf-field solver from our simulation code was also used to model a recent experiment at the Univ. of Texas at Austin [2]. The experimentally measured density profile was used to calculate the rf field structure. In comparison with the experimentally measured $B_r(z, r=0)$, the simulated results agree in the field $k_z$ spectra, the field amplitude and phase only when the electron collision frequency is enhanced by a factor of 10. This indicates the need to identify a stronger wave damping mechanism than electron-ion Coulomb collisions. [1] R. W. Boswell, Plasma Phys. Control. Fusion, 26, 1147, 1984 [2] C. Lee, PhD thesis, The Univ. of Texas at Austin, 2008. [Preview Abstract] |
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PP6.00016: Measurements and Modeling of Helicon Wave RF Magnetic Fields Charles Lee, Guangye Chen, Roger Bengtson, Boris Breizman, Alexey Arefiev Traveling-wave characteristics of the RF magnetic field were measured in a helicon discharge using an absolutely calibrated, five-turn, magnetic probe with built-in electrostatic rejection. The discharge is created using a 1-kW RF generator at 13.56 MHz using a left-helical antenna. Axial profile measurements of the $B_r$-component were taken on-axis for the two opposite directions of the static magnetic field. A Fourier transform in the $k_z$-domain of the magnetic field's amplitude reveals a strong asymmetry: there is a strong peak in the spectrum corresponding to a wave propagating in the anti- parallel direction with respect to the static magnetic field. This observation is consistent with the prediction of the Radially Localized Helicon (RLH) [1] theory. The location of the peak in the $k_z$-spectrum is also in good agreement with the RLH dispersion relation. Comparison of the data with an RF- field solver [2] shows significant agreement in the amplitude and phase of the magnetic field and its corresponding $k_z$- spectrum when the electron collision frequency is increased by a factor of ten. [1] B. Breizman and A. Arefiev, Radially Localized Helicon modes in non-uniform plasma, Physical Review Letters, 84:3863, 2000. [2] G. Chen et al., Resonant power absorption in helicon plasma sources, Physics of Plasmas, 13:123507, 2006. [Preview Abstract] |
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PP6.00017: Power loss mechanisms in a helicon plasma Daniel F. Berisford, Roger D. Bengtson, L.L. Raja We present a series of experiments designed to quantify the relative importance of power loss through radiation or particle transport in an argon helicon plasma. The experiments were performed on the 6cm diameter, 1kW argon helicon source at the University of Texas at Austin. A set of bolometric probes, located at the plasma edge downstream from the antenna, measures the total heat flux density leaving the plasma radially at several discrete axial locations. A UV photodiode measures the total power loss due only to radiation at similar locations. We present a comparison of the two measurements and a set of preliminary results indicate that radiation and particle loss account for roughly the same power loss. We will also complete the power loss measurements with an infrared camera viewing the heating of the dielectric containment tube near the antenna to give a power balance for the helicon plasma. [Preview Abstract] |
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PP6.00018: Sputtering characteristics of carbon-based materials exposed to H/D/T plasmas --- molecular dynamics simulation study Satoshi Hamaguchi, Masashi Yamashiro Erosion of carbon-based materials facing magnetically confined plasmas is of significant concern for fusion devices. Sputtering properties of graphite and amorphous carbon substrates due to hydrogen (H), deuterium (D), and tritium (T) ion bombardment have been studied at low incident energies using classical molecular dynamics (MD) simulations. MD simulations have been performed in such a way that we allow significant accumulation of incident species up to 1.25 $\times $10$^{17}$/cm$^{2}$. Our simulation results indicate that a high level of H/D/T dose accumulation on the top surface is prerequisite for the formation of relatively large-sized sputtered hydrocarbon species. Significant isotopic dependence of sputtering yields has been also observed after the dose of incident D or T reaches about 10$^{16}$/cm$^{2}$. It has been clearly shown that the sputtering yield can be lower at higher incident energies in the low energy range for D and T injections. These simulation results are consistent with some of earlier published experimental observations of carbon-based material sputtering. For comparison, we shall also present sputtering properties of Si-based materials exposed to chemically reactive plasmas, which have relevant applications for semiconductor processing. [Preview Abstract] |
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PP6.00019: Velocity Distribution and Plasma Density Measurements in an Inductive Microthruster Plume David Peters, Timothy Ziemba, Robert Winglee The plume characterization of an inductive microthruster called $\mu $PIT is presented. Under development at the University of Washington, $\mu $PIT combines elements of PPTs and PITs to efficiently deliver a precise impulse bit. The system uses Teflon as a propellant, similar to a PPT, but provides additional acceleration of the propellant through inductive RF heating of the plasma. This paper details the change in performance when the inductive heating is applied. Additional acceleration of the plasma through the use of a magnetic nozzle is also investigated. A retarding field energy analyzer is used in conjunction with an asymmetric double Langmuir probe to characterize the velocity distribution of the plasma plume. In addition to a total impulse measurement, these data also provide information about the degree of plume divergence and thruster efficiency. These measurements confirm that the antenna continues to ionize and accelerate late-time neutrals after the initial breakdown, which leads to higher energy efficiency and greater specific impulse than PPTs operating at similar energy levels.\footnote{Spanjers, G. G. et al. ``AFRL MicroPPT Development for Small Spacecraft Propulsion,'' AIAA Paper 2002-3974} [Preview Abstract] |
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PP6.00020: Development of a small sized plasma jet by using a high current glow discharge Masayuki Watanabe Various technological applications demand an efficient plasma jet because the plasma jet can generate the high temperature and high-speed plasma flow easily. In this research, a small sized plasma jet by applying a modified pseudo-spark discharge (PSD) has been developed. Since a large number of electrons supplies from the PSD cathode cavity to the plasma discharge, the discharge can keep the glow mode even if the discharge current exceeds the several kilo amperes high. Additionally, an electromagnetic force accelerates the plasma, similar to the MPD Thruster. The size of the plasma jet devise is as follows; the diameter of the plasma jet device is about 20mm, diameters of the cathode and anode holes are 5mm and 10mm. The maximum discharge current is about 6kA and its half period is about 0.1ms on the breakdown voltage of about -1kV. The temperature of the plasma jet was a few eV and the density was in the order of 1019 m-3. This density will depend on the volume inside the cathode cavity. The durability of the electrodes has been tested at the present time. [Preview Abstract] |
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PP6.00021: An experimental study of ion generation and acceleration in a Cylindrical Hall Thruster Martin Griswold, Yevgeny Raitses, Nathaniel Fisch Several interesting effects that were measured in the plume of cylindrical Hall thrusters with mirror and cusp magnetic field topologies [1] such as unusually high ionization efficiency [2] and focusing of high energy ions with the enhancement of the cathode electron emission [3] are thought to be related to the potential structure and ambipolar trapping inside the thruster. A narrow field of view electrostatic energy analyzer and a time of flight tube are used to measure the angular distribution, energy distribution function and charge state of ions in the plasma plume from the thruster in order to determine the location where single and multiply charged ions of different energies are formed. \newline [1] Raitses and Fisch, Physics of Plasmas 8 (2001) 2579 \newline [2] Smirnov, Raitses and Fisch, J. Appl. Phys., 94 (2), 852 (2003) \newline [3] Granstedt, Raitses and Fisch, to appear in J. Appl. Phys. [Preview Abstract] |
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PP6.00022: Development of a 10 mN Class Hall Thruster for Small Satellites in Korea Wonho Choe, Jongsub Lee, Youbong Lim, Mihui Seo, Jongho Seon, Hae June Lee A compact Hall thruster with a thrust of about 10 mN and a specific impulse of about 1500 s is under development for the purpose of controlling or maintaining the orbits of small satellites. The thruster system consists of a Hall thruster head unit, a power processing unit, and a xenon gas feed unit. The target values of the total mass, consumed electrical power, and thrust efficiency of the thruster system are approximately $<$ 5 kg, $<$ 300 W and $>$ 30 {\%}, respectively. Laboratory examination of the thruster performance finds that the thruster meets the design specification. The parametric study of the thruster operation is on-going in order to find optimal operation conditions. The developed thruster is scheduled to be implemented on the STSAT-3 in the year 2010. [Preview Abstract] |
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PP6.00023: Improved Nonambipolar Electron Source Operation with Permanent Magnets Jesse Gudmundson, Noah Hershkowitz The Nonambipolar Electron Source (NES) is a Radio Frequency (rf) plasma-based electron source that does not rely on electron emission at a cathode surface. All electrons are extracted at an electron sheath through a biased ring and all ions are lost radially to a biased Faraday shield. An electromagnet in the original NES has been replaced by a NdFeB permanent magnet array. A portion of the magnet array consists of a ring of radially aligned magnets followed by a ring of axially aligned magnets that produce a peak field of approximately 800 Gauss. Axial magnetic field strength at the extraction ring was increased using an additional ring of axially aligned magnets. Measurement of the magnetic field was in good agreement with field predicted by the FEMM (Finite Element Method Magnetics) code. Optimization of the single turn antenna and biased ring position in the magnetic field will be discussed. At least 15 A of electron current was extracted using a flow rate of 15 sccm Ar at 600 W of rf power at 13.56 MHz. For comparison, the original NES required 1200 W of power to achieve 15 A of extracted current. Compared to the previous coil design, the NdFeB magnets are lighter weight and require no power. [Preview Abstract] |
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PP6.00024: Force measurements and simulations of DBD plasma actuators operated in quiescent air Alan R. Hoskinson, Noah Hershkowitz Recent years have seen significant interest in using dielectric barrier discharge plasmas on surfaces to drive neutral gas flow. Here we have measured the forces exerted by these plasma actuators on atmospheric pressure air using two methods---indirectly via stagnation probe measurements of the induced air velocity and directly via an electronic balance---and compared the results to each other and to 2-dimensional simulations. The measurements and simulations have been repeated for discharge geometries with one or both electrodes insulated and a range of electrode sizes. Data from the two experimental techniques show an exponential increase in the induced force as one of the electrode diameters decreases. The simulations, which couple fluid transport equations to Poisson's equation, show the same general trend. The simulation runs also provide more detail on plasma structure than can currently be observed experimentally. [Preview Abstract] |
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PP6.00025: Enhanced ablation of small anodes in a carbon nanotube arc discharge Yevgeny Raitses, Abraham Fetterman, Michael Keidar An atmospheric pressure helium arc discharge is used for carbon nanotube synthesis. The arc discharge operates in an anodic mode with the ablating anode made from a graphite material. For such conditions, models predict the electron-repelling (negative) anode sheath. In the present experiments, the anode ablation rate is investigated as a function of the anode diameter. It is found that anomalously high ablation occurs for small anode diameters ($<$ 0.4 cm). This result is explained by the formation of an electron-attracting (positive) anode sheath leading to increased power losses on small anodes as compared to larger anodes [1]. The suggested mechanism for the positive anode sheath formation is plasma convergence. The increased ablation rate due to this positive sheath could imply a greater yield of carbon nanotube production. [1] A. J. Fetterman, Y. Raitses and M. Keidar, Carbon (2008). [Preview Abstract] |
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PP6.00026: Cold ion atmospheric plasma jets for living tissue treatment Alexey Shashurin, Michael Keidar, Mary Ann Stepp Recently a great attention is attracted to the creation of the cold plasma jets and their interaction with living tissue. The plasma gun operating on helium and equipped with high-voltage resonant transformer is designed. Long nondivergent plasma jets with length more than 5 cm and diameter 1-2 mm are obtained. The measured electrical current in the plasma jet indicates that the plasma jet is discontinuous and represents a series of propagating plasma bundles (two bundles per driving high voltage period) with peak current up to few hundred mA. The exposition of the living tissue (fibroblast cells and PEM cells) to the helium plasma jet causes an immediate detachment of part of the cells from their matrix in the case of direct contact of the jet with cell culture. In addition, it was found that migration velocity inside of the treated region significantly decreases in the case of treatment through the thin layer of the protecting media covering the cell culture. [Preview Abstract] |
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PP6.00027: Optical measurements of dielectric barrier discharges of twisted magnet wires in atmospheric humid air T. Murata, Y. Kikuchi, N. Fukumoto, M. Nagata Dielectric barrier discharges (DBDs) can be generated by a simple electrode configuration in atmospheric pressure, so that it can be used to various applications such as plasma sterilization [1]. We have investigated effects of environmental conditions on DBD in twisted enameled magnet wires. The DBD was generated by an ac sinusoidal voltage application to the twisted pair in a humidity and temperature chamber. Measurements of the DBD were performed by a current sensor and a photomultiplier tube (PMT) in this study. As the result, the discharge inception voltage (DIV) was decreased by increasing the relative humidity. Especially, the DIV was significantly dropped at 95 {\%}RH. The measured DIV could be explained by moisture absorption into the dielectric layer and surface wettability. In addition, the amount of the DBD was also decreased in humid air. Optical measurements of the DBD by a spectrometer are being prepared to know the dominant reaction in the DBD in humid air. [1] M.K. Boudam, et al., J. Phys. D:Appl. Phys. \textbf{39}, 3494 (2006). [Preview Abstract] |
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PP6.00028: ABSTRACT WITHDRAWN |
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PP6.00029: Proof-of-principle measurements using the 300 GHz collective scattering diagnostic Robert Hardin, William Przybysz, Justin Ellis, Earl Scime Experiments designed to provide a proof-of-principle measurement with the WVU 300 GHz collective scattering diagnostic continue. The collective scattering system, capable of measuring wave-numbers ranging from 62 to 89 cm$^{-1}$, is operated in conjunction with a new electrostatic double probe (with a measureable wave-number range up to $\sim $50 cm$^{-1})$. To directly excite finite $k_{\bot }$ electrostatic waves propagating perpendicular to the magnetic field that can be detected with this diagnostic, an internal antenna designed to launch electrostatic ion-cyclotron waves has been built and installed in HELIX. For the magnetic field strength of HELIX, the wave dispersion of such waves is essentially that of an ion-acoustic wave. Wave number measurements for naturally occurring and externally driven waves, using both the scattering system and electrostatic double probe, are presented. [Preview Abstract] |
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PP6.00030: Measurements of X-ray Spectra on an ECR Ion Source B.P. Cluggish, I.N. Bogatu, L. Zhao, J.-S. Kim, R. Pardo, R. Vondrasek, R. Scott FAR-TECH, Inc. is developing an X-ray spectral diagnostic for monitoring electron cyclotron resonance ion sources (ECRIS). In an ECRIS, electrons trapped in a magnetic mirror are heated to high energy by resonant absorption of microwaves. Measurements of the X-ray Bremsstrahlung spectrum provide important information about the electron distribution function, which plays a key role in ionization and production of highly charged ions. To this end, FAR-TECH, Inc. has measured X-ray emission from the ECR-II device in the ATLAS facility at ANL. Our measurements indicate a significant population of electrons with energies in excess of 100 keV in ECR-II. Furthermore, we find that both the intensity and the shape of the observed spectra are highly correlated with measurements of the charge state distribution (CSD) of ions extracted from the ECR-II plasma. X-ray spectra and corresponding CSD data will be presented, as well as analysis of their dependence on device parameters. The results will be compared to simulations of ECR-II using our Generalized ECRIS Model (GEM). [Preview Abstract] |
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PP6.00031: Electrical characteristics and cathode deposit growth in an anodic arc producing carbon nanotubes Michael Keidar, Alexey Shashurin, Yevgeny Raitses Voltage-current (V-I) characteristics of the carbon nanotube producing anodic arc are measured for different gap sizes, anode compositions and background He pressures. It is shown that voltage-current characteristics has V-type shape and with increasing of the gap V-I characteristic shifts to the higher arc voltages, while minimum shifts to higher arc currents. The increasing the metallic catalyst fraction in the anode composition leads to slight decrease in the arc voltage and shifts the minimum position to higher arc currents. Such shape of the voltage current characteristics is explained by superposition of two effects: decreasing of the potential drop in the quasineutral plasma column and increasing of the anode potential drop with arc current. In addition two effects regarding cathode deposit growth in the anodic arc producing carbon nanotubes are reported. First, decreasing of the cathode deposit growth rate with interelectrode gap increase and second, increasing of the cathode deposit diameter with arc current increase. Both effects are explained by invoking the argument that the interelectrode plasma is necessary to trigger the growth of the cathode deposit. [Preview Abstract] |
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PP6.00032: Ultra-long carbon nanotubes for thermal management application V.I. Demidov, A. Shashurin, M. Keidar, S.F. Adams, O. Volotskova Thermal management is becoming one of key enabling technologies leading to mass production of fuel-cell, electric and hybrid vehicles, which must operate at temperatures significantly lower than conventional internal combustion engines. One promising direction to advance thermal management is usage of nanofluids. Nanofluidc are solid-liquid composite materials consisting of nanoparticles suspended in liquid. It was recently demonstrated that the thermal conductivity of an individual SWNT increases with length, thus making SWNTs an ideal structure for thermal control. We have investigated an effect of the magnetic field on SWNT synthesis. Experimental observation lead to the conclusion that yield of SWNTs in condensed products is higher in case when the magnetic field is switched on in comparison to the case without magnetic field suggesting that using the magnetic field it is possible to scale up SWNT production rate. The presence of the magnetic field seemingly doubles the length of SWNTs produced [1]. Additionally, samples from the deposit with the magnetic field have produced some of our longest SWNTs of over 6 microns in length. 1. M. Keidar et al, APL, 92: 043129, 2008 [Preview Abstract] |
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PP6.00033: Feature Profile Evolution in Chlorine Etching of Polysilicon Paul Moroz Chlorine-based plasma etching has been used in the semiconductor industry in integrated-circuit fabrication processes, and has been extensively studied both experimentally (including specially designed beam experiments) and theoretically. Interaction of plasma with material surfaces results in competing mechanisms of etching and deposition, sometimes strongly depending on energy and angular distribution of incoming fluxes of species. We present results of feature profile simulations with a new Monte Carlo simulator written in a C++ environment that has a number of advantages. The case of chlorine etching of polysilicon is convenient for tuning the simulator. All plasma fluxes are modeled as incoming super-particles with energy and angle varied by a random number generator in correspondence with actual fluxes. The code runs in real time with iterations at specified time-step, allows consideration of arbitrary number of solid materials and gas species, convenient input of arbitrary initial geometry of the features, and convenient specification of chemistry models. [Preview Abstract] |
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PP6.00034: LASER PLASMA ACCELERATORS AND COHERENT SOURCES |
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PP6.00035: Charged particle acceleration in dense plasma channels I.Y. Dodin, N.J. Fisch Particle acceleration in plasmas with up to solid-state densities is revisited, with account for multiple scattering that results in collisional losses of energy and pitch-angle diffusion. The latter leads to particle escaping from the driving field; thus channeling is required for efficient acceleration and, in turn, brings in additional radiative dissipation. We derive the channeled particle distribution and reduced nonlinear equations for the oscillation amplitude and the particle energy. The maximum energy gain, as limited by dissipation, is described by three different scalings depending on the channel parameters. [Preview Abstract] |
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PP6.00036: Capillary plasma waveguide to guide an ultrashort laser pulse Hiromitsu Terauchi, Jin-xiang Bai, Takeshi Higashiguchi, Noboru Yugami Optical guiding of an ultrashort and intense laser pulse with long interaction length is important in many applications such as laser-driven plasma accelerators, high-order harmonic radiation, and x-ray lasers. To overcome this limitation, a waveguide which has a refractive index profile like an optical fiber is required [1]. We have developed a plasma waveguide using a capillary discharge- produced plasma with a length of 1.5 cm. Electron densities and its temperatures were observed to be the order of $10^{17}$ cm$^ {-3}$ and a few eV using a laser interferometer and a spectrometer coupled with an ICCD camera, respectively. The width of a plasma waveguide was 200-300 $\mu$m (FWHM). We have demonstrated the guiding of a laser pulse over length, which corresponded to 10 times the Rayleigh length. As a result, the accelerated electrons with the energy of 1 MeV was observed with the blue-shift of the optical pulse at the same time. [1] A.J.Gonsalves et al., Phys. Rev. Lett. {\bf 98}, 025002 (2007). [Preview Abstract] |
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PP6.00037: Simulation and calculation of particle trapping using a quasistatic 2D simulation code Sepehr Morshed, Thomas Antonsen, Chengkun Huang, Warren Mori In LWFA schemes the laser pulse must propagate several centimeters and maintain its coherence over this distance, which corresponds to many Rayleigh lengths. These Wakefields and their effect on the laser can be simulated in quasistatic approximation [1, 2]. In this approximation the assumption is that the driver (laser) does not change shape during the time it takes for it to pass by a plasma particle. As a result the particles that are trapped and moving with near-luminal velocity can not be treated with this approximation. Here we have modified the 2D code WAKE with an alternate algorithm so that when a plasma particle gains sufficient energy from wakefields it is promoted to beam particle status which later on may become trapped in the wakefields of laser. Similar implementations have been made in the 3D code QUICKPIC [2]. We also have done comparison between WAKE and results from 200 TW laser simulations using OSIRIS [3]. These changes in WAKE will give users a tool that can be used on a desk top machine to simulate GeV acceleration.\\[0pt] [1] P. Mora and T. M. Antonsen Jr., Phys Plasma 4, 217 (1997)\\[0pt] [2] C. Huang et al. Comp Phys. 217 (2006)\\[0pt] [3] W. Lu et al. PRST, Accelerators and Beams 10, 061301 (2007) [Preview Abstract] |
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PP6.00038: 10GeV laser-plasma accelerator stage with full-PIC 3D simulations in boosted frames Samuel Martins, Luis Silva, Ricardo Fonseca, Wei Lu, Warren Mori We address full PIC simulations of the next generation of Laser Wakefield Accelerators with energy gains $>$ 10GeV. The distances involved in these numerical experiments are very demanding in terms of computational resources and are not yet possible to (easily) accomplish. Following the work on simulations of particle beam-plasma interaction scenarios in optimized Lorentz frames by J.-L. Vay (PRL 98, 130405), the Lorentz transformation for a boosted frame was implemented in osiris 2.0, leading to a dramatic change in the computational resources required to model LWFA. The critical implementation details will be presented, and the main difficulties discussed. Quantitative comparisons between lab/boost frame results with Osiris and QuickPIC will be given. Finally, a 3D PIC simulation of a $>$10GeV accelerator stage will be presented, including a discussion on radiation emission. [Preview Abstract] |
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PP6.00039: Plasma Wakefield Acceleration Simulations with Multiple Electron Bunches Efthymios Kallos, Patric Muggli, Vitaly Yakimenko, Karl Kusche, Jangho Park, Marcus Babzien, Adam Lichtl In the multibunch plasma wakefield accelerator, a train of electron bunches is utilized to excite a high gradient wakefield in a plasma which can be sampled by a trailing short witness bunch. We show that for five drive bunches with 150 pC total charge which can be generated in the Accelerator Test Facility of the Brookhaven National Lab, a wakefield of 140 MV/m can be generated if the plasma density is matched to the bunch train period. In addition, the possibility of ramping the charge per bunch in order to achieve high transformer ratios ($>$5) is examined, a scenario that is of great interest for a future afterburner collider. The work was supported by the US Department of Energy. [Preview Abstract] |
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PP6.00040: Electron Acceleration Using Hollow Fiber with Table Top Terawatt Laser Yoshio Mizuta, Kiminori Kondo, Chen Zhenglin, Takashi Nakabayashi, Nobuhiko Nakanii, Ryosuke Kodama, Kunioki Mima, Kazuo Tanaka A table top laser system can produce over 10TW laser pulse in present. If we focus these optical pulses to $\mu$m size, the focused intensity can be relativistic for electrons. In such a high field, the electron plasma wave (EPW) can be excited. This EPW is attractive for the accelerating field for charged particles. However, the effective interaction length is only a few hundred micro meters. For GeV electron acceleration, this short interaction length should be overcome. A simple hollow fiber is used for obtaining a longer acceleration distance. The spot diameter and the length of focused area should be 10$\mu$m and 10mm, respectively. We used the hollow fiber which has 20$\mu$m inner diameter and 10mm length. To fill the fiber with enough density atoms with keeping a good vacuum condition, the differential pumping system was used. Presently, we have succeeded in injecting $\mu$J femtosecond pulses into the fine hollow fiber in the atmosphere. In this propagation, a strong self phase modulation occurred to generate a white light. Our plan for coming experiment and possible limiting factors will be discussed. [Preview Abstract] |
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PP6.00041: Positron Injection and Acceleration on the Wake Driven by an Electron Beam in a Foil and Gas Plasma Xiaodong Wang, Patric Muggli, Tom Katsouleas, Mark Hogan, Chen Joshi, Warren Mori A novel approach for generating and accelerating positron bunches in a plasma wake is proposed and modeled. Positrons are produced by a double pulse electron beam colliding with a thin foil target embedded within the plasma. A region suitable for both accelerating and focusing positrons in the plasma wakefield excited by the drive electron beam is created directly after the blowout bubble. Prospects for using this scheme to test positron generation and acceleration in a plasma wake of an electron beam are discussed and modeled with Monte Carlo and 3D PIC simulation codes. For available parameters, a large number of positrons (10$^{7} \sim 10^{8}$) are trapped, and accelerated to $\sim $5~GeV over 1 meter with a relatively narrow energy spread (6{\%}$\sim $16{\%}) and a low emittance. These simulation results will be presented. Work supported by US Dept. of Energy. [Preview Abstract] |
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PP6.00042: Applications of the nonlinear plasma wakefield theory in the blowout regime Wei Lu, Michail Tzoufras, Chengkun Huang, Miaomiao Zhou, Weiming An, Warren Mori In plasma based acceleration for electrons, the blowout regime turns out to be of very importance due to its ability to provide an ideal accelerating and focusing structure and its ability to support large amount of charge (e.g., nC). A theoretical model has been successfully developed to describe this highly nonlinear regime [1]. Based on this model, many important aspects of the blowout regime can be accurately addressed. Here the solutions for four different problems in the blowout regime will be presented, including the optimum plasma density for maximum wakefield amplitude for given beam parameters, beam loading, the transformer ratio for a linearly ramped electron beam driver (optimizing the transformer ratio), and the electron hosing instability. Full and reduced particle-in-cell simulations will be also presented to justify these theoretical analyses. [1] W. Lu et al, Phys. Rev. Lett. 96, 165002 (2006) [Preview Abstract] |
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PP6.00043: Efficient accelerator afterburner design based on plasma wakefield acceleration Chengkun Huang, I. Blumenfeld, C.E. Clayton, F.-J. Decker, M.J. Hogan, R. Iverson, C. Joshi, T. Katsouleas, N. Kirby, W. Lu, K.A. Marsh, W.B. Mori, P. Muggli, R. Siemann, D. Walz, R. Ischebeck, M. Tzoufras Recent plasma wakefield acceleration (PWFA) experiment using short ($\sim$100fs), high peak current ($>$10KA) electron beam as wakefield driver has demonstrated sustained acceleration gradient of $\sim$50GeV/m over 85 cm. The rapid progress of PWFA experiments has attracted interests regarding the possibility of making an ``afterburner'' for a linear collider. In the ``afterburner'' concept, electron acceleration is achieved by placing a trailing electron beam into the wakefield (either by beam splitting or external injection) to extract energy deposited in the plasma wave wake. Several important aspects of the ``afterburner'' design in the blow-out regime, such as wakefield generation, efficient beam loading and hosing instability have been investigated theoretically. These relevant physics will have great impact on the beam quality of a possible ``afterburner'' design. A multi-stage ``afterburner'' design with 25GeV energy gain in each stage is explored numerically with a 3D quasi-static code QuickPIC. Parameters are suggested for a 0.5 TeV PWFA afterburner with this design and simulation result will be presented. [Preview Abstract] |
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PP6.00044: Laser wakefield simulation using a speed-of-light frame envelope model Benjamin Cowan, David Bruhwiler, Peter Messmer, Kevin Paul, Cameron Geddes, Eric Esarey, Estelle Cormier-Michel Simulation of laser wakefield accelerator (LWFA) experiments is computationally highly intensive due to the disparate length scales involved. Current experiments extend hundreds of laser wavelengths transversely and many thousands in the propagation direction, making explicit PIC simulations enormously expensive and requiring massively parallel execution in 3D. We can substantially improve the performance of laser wakefield simulations by modeling the envelope modulation of the laser field rather than the field itself. This allows for much coarser grids, since we need only resolve the plasma wavelength and not the laser wavelength, and therefore larger timesteps. Thus an envelope model can result in savings of several orders of magnitude in computational resources. By propagating the laser envelope in a Galilean frame moving at the speed of light, dispersive errors can be avoided and simulations over long distances become possible. Here we describe the model and its implementation, and show simulations of laser wakefield phenomena such as channel propagation, self-focusing, wakefield generation, and downramp injection using the model. [Preview Abstract] |
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PP6.00045: Multi-cm Long Magnetically Controlled Optical Plasma Channel Bradley Pollock, Dustin Froula, George Tynan, Laurent Divol, Paul Davis, John Palastro, Dwight Price, Siegfried Glenzer Experimental results from the Jupiter Laser Facility at the Lawrence Livermore National Laboratory which show multi-cm long plasma channels with minimum plasma densities below 5x10$^{17}$ cm$^{-3}$ are presented. These results are obtained using an external magnetic field ($<$5 Tesla) to limit the radial heat flux from a pre-forming laser beam. The resulting increased plasma pressure gradient produces a parabolic density gradient which is shown to be tunable by changing the external magnetic field strength. These results are compared with 3-D resistive MHD modeling. For these channel conditions, quasi-static kinetic simulations show that 90{\%} of the energy in a 150 TW short pulse beam is guided over 5 cm and predict electron energy gains of 3 GeV from Laser Wakefield Acceleration. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and was partially funded by the Laboratory Directed Research and Development Program under project tracking code 06-ERD-056. [Preview Abstract] |
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PP6.00046: Hybrid fluid-PIC simulations of injected electron beams in laser wakefield accelerators David Bruhwiler, John Cary, Ben Cowan, Cameron Geddes, Estelle Cormier-Michel, Eric Esarey Laser wakefield accelerators (LWFA) have accelerated $\sim nC$ electron bunches to $\sim GeV$ energies over $cm$ scale distances, via self-trapping of plasma electrons. Self-trapping cannot be tolerated in staged LWFA modules for high-energy physics applications. The \% level energy spread of self-trapped electron bunches is an order of magnitude too large for light source applications. Both of these difficulties could be resolved via external injection of a low-emittance electron bunch into a quasilinear LWFA, for which the dimensionless laser amplitude is in the range $0.5 < a_0 < 2$. It is challenging for an electromagnetic PIC simulation to model such systems without artificial emittance growth in the beam. An improved cold, relativistic fluid model is used in the parallel VORPAL framework to simulate the electron plasma with no particle noise, while PIC is used for the beam. The importance of high-order particle shapes, current smoothing and other techniques is also discussed. [Preview Abstract] |
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PP6.00047: Controlled injection and multi-GeV LWFA stages for laser wakefield applications C.G.R. Geddes, E. Cormier-Michel, E. Esarey, A.J. Gonsalves, D. Panasenko, G.R. Plateau, K. Nakamura, C.B. Schroeder, Cs. Toth, W.P. Leemans, D.L. Bruhwiler, B. Cowan, K. Paul, P. Mullowney, J.R. Cary Collider and light source applications of LWFAs will require staging of controlled injection with multi-GeV accelerator modules, which must be optimized for efficient acceleration of both electrons and positrons with minimum emittance degradation. As an injector, control of particle trapping in a laser wakefield accelerator using plasma density gradients experimentally produced stable electron bunches with 0.17 MeV/c FWHM momentum spread and central momenta stable at 0.76 +- 0.2 MeV/c, and with repeatable charge and pointing. Simulations further show that the short bunch length and observed high laser transmission allow the bunches to be post accelerated if the jet is followed by a long uniform plasma, and indicate that this produces beams with 0.2 MeV/c class momentum spread at high energies. Analytic scalings together with simulations have then been used to evaluate stage designs for upcoming PW-class experiments, and for eventual collider modules. These include linear and nonlinear wake structures, laser pulse and plasma density shaping, and beam loading. Results of stage optimization and tools for accurately modeling emittance will be discussed. [Preview Abstract] |
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PP6.00048: Measurement of the Effective Length of Laser-Plasma Channels by Guided Microwave Backscattering Mark Gilmore, Brian Stoltzfus, Mark Savage, Alan Lynn Laser triggered gas switches are critical components in many pulsed power driven systems, such as ZR at Sandia National Laboratories. Timing jitter is of concern is such systems, where power flow from multiple modules must be switched to a load simultaneously. Laser triggered gas switches utilize a laser-produced plasma channel (LPPC) to initiate breakdown between electrodes biased to $\sim $ 80{\%} of breakdown voltage. The effective length of the LPPC is an important parameter affecting the breakdown timing. Backscattering of microwaves inside a waveguide by an LPPC, introduced by focusing the trigger laser through holes in the broad wall, has been used to characterize effective length of the channel. Simulations indicate that the backscattering is sensitive to the LPPC conductor length both inside and outside the waveguide. A quarter wavelength stub has therefore been introduced outside of the waveguide, to short circuit the LPPC conductor to the waveguide wall, while still allowing laser access. Theoretical, computational, and initial plasma channel experimental results, as well as comparisons with other diagnostics, are presented. [Preview Abstract] |
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PP6.00049: Investigation of Stimulated Raman Scattering Using Short-Pulse Diffraction Limited Laser Beam near the Instability Threshold J.L. Kline, D.S. Montgomery, L. Yin, K.A. Flippo, B.J. Albright, R.P. Johnson, T. Shimada, H.A. Rose, C. Rousseaux, V. Tassin, S.D. Baton, F. Amiranoff, R.A. Hardin Short pulse laser plasma interaction experiments using diffraction limited beams provide an excellent platform to investigate the fundamental physics of Stimulated Raman (SRS) and Stimulated Brillouin (SBS) Scattering. Detailed understanding of these laser plasma instabilities impacts the current inertial confinement fusion ignition designs and could potentially impact fast ignition when higher energy lasers are used with longer pulse durations ( $>$ 1 kJ and $>$ 1 ps). Using short laser pulses, experiments can be modeled over the entire interaction time of the laser using PIC codes to validate our understanding. Experiments have been conducted at the Trident laser and the LULI to investigate SRS near the threshold of the instability using 527 and 1064 nm laser light respectively with 1.5 -- 3 ps pulses. In the case of both experiments, the interaction beam was focused into a pre-ionized He gasjet plasma. Measurements of the reflectivity as a function of intensity and k?$_{D}$ were completed at the Trident laser. At LULI, a 300 fs Thomson scattering probe is used to directly measure the density fluctuations of the driven electron plasma and ion acoustic waves. Details of the experimental results will be presented. [Preview Abstract] |
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PP6.00050: Numerical modeling of mutual interaction between multi-color laser beams in plasmas S. Austin Yi, Serguei Kalmykov, Gennady Shvets The propagation, electromagnetic cascading, and mutual interaction of weakly-relativistic multi-color laser beams in tenuous plasmas is investigated numerically. We model the laser propagation in the paraxial approximation, taking into account local beam-beam interaction through both the density perturbations due to the plasma wake and the relativistic mass corrections. Long laser beams are assumed, so that the density perturbations are due to the instantaneous plasma response to the periodic ponderomotive force. The resulting set of coupled nonlinear wave equations governs the evolution of the laser cascade envelopes. We solve these equations numerically using several pseudospectral methods. This model is used to investigate the mutual interaction of multi-color laser beams propagating non-collinearly in plasmas. It is demonstrated that laser beams with difference frequency slightly above (below) the plasma frequency mutually repel (attract). We also use this model to investigate the collinear propagation of the electromagnetic cascade, and the enhancement or suppression of relativistic self-focusing. These results are in good agreement with fully relativistic PIC simulations from the code WAKE. [Preview Abstract] |
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PP6.00051: All-optical control of nonlinear self-focusing of laser beams in plasma beat wave accelerators S. Kalmykov, S. Austin Yi, G. Shvets The nonlinear focusing of a bi-color laser beam in tenuous plasmas can be all-optically enhanced or suppressed depending whether the beat wave frequency $\Omega$ is below or above the electron Langmuir frequency $\omega_p$. The driven electron density perturbation produces a co-moving index grating, which is focusing if $\Omega<\omega_p$ and de-focusing otherwise. Self-consistent guiding of a mildly over-critical long (many plasma periods) laser beam can be all-optically initiated by mixing with a second, much weaker, beam shifted in frequency by $\Omega>\omega_p$. The guiding effect initially owes to the de-focusing properties of the laser beat wave-driven 3D electron density perturbation. Electromagnetic cascading and resonant self-modulation contribute to the guiding process at propagation distances over one Rayleigh length. Acceleration in the non-resonant plasma beat wave yields quasi-monoenergetic bunched electron beams with the energy over a hundred MeV. In the case of $\Omega<\omega_p$, acceleration efficiency is generally higher because of nonlinear focusing enhanced by the plasma wave excitation. To achieve quasi-monoenergetic acceleration in this regime, electrons should be injected in the plasma wake at a distance from the plasma boundary. [Preview Abstract] |
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PP6.00052: Nonlinear transport velocity and evolution of intense short-pulse lasers in underdense plasma Carl Schroeder, Eric Esarey, Wim Leemans, Bradley Shadwick The nonlinear transport velocity of a relativistically-intense, short-pulse laser propagating in a cold underdense plasma is investigated. The transport velocity of the laser intensity in the plasma determines the phase velocity of a plasma wave driven by the short-pulse laser. Expressions for the plasma wave phase velocity dependence on the relativistic laser intensity are derived in the adiabatic and quasi-static regimes. The relativistic laser evolution (energy depletion, frequency shifting, and intensity steepening) is also examined. In a laser-plasma-based accelerator, the plasma wave phase velocity excited by the laser pulse determines the dephasing length of the plasma accelerating structure, and therefore the energy gain of the accelerated charged particle beam. [Preview Abstract] |
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PP6.00053: Performance Analysis of Quantum cascaded Lasers Mohamed S. El-Tokhy, Imbaby I. Mahmoud, M.B. El-Mashade Improving the performance of the QCL through block diagram as well as mathematical models is the main scope of this paper. In order to enhance the performance of the proposed device, the mathematical model parameters are used in a reliable manner in such a way that the optimum behavior was achieved. These parameters play the central role in specifying the optical characteristics of the considered laser source. Since threshold current density J$_{th}$, represents a limiting factor in the QCLs operation, it is of importance to reduce its value and in the same time to have a large amount of radiated power, where increasing the amount of radiated power and decreasing the threshold current density represent the main hopping process that can be predicted from the behavior of quantum laser devices. It was found that there is a good agreement between the calculated values from our mathematical model and those obtained with VisSim and experimental results. These demonstrate the strength of implementation of both mathematical and block diagram models. [Preview Abstract] |
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PP6.00054: The computer simulation of laser proton acceleration for hadron therapy Vladimir Lykov, Grigory Baydin The ions acceleration by intensive ultra-short laser pulses has interest in views of them possible applications for proton radiography, production of medical isotopes and hadron therapy. The 3D relativistic PIC-code \textit{LegoLPI} is developed at RFNC-VNIITF for modeling of intensive laser interaction with plasma. The \textit{LegoLPI}-code simulations were carried out to find the optimal conditions for generation of proton beams with parameters necessary for hadrons therapy. The performed simulations show that optimal for it may be two-layer foil of aluminum and polyethylene with thickness 100 nm and 50 nm accordingly. The maximum efficiency of laser energy transformation into 200 MeV protons is achieved on irradiating these foils by 30 fs laser pulse with intensity about 2$\cdot $10$^{22}$ W/cm$^{2}$. The conclusion is made that lasers with peak power about 0.5-1PW and average power 0.5-1 kW are needed for generation of proton beams with parameters necessary for proton therapy. [Preview Abstract] |
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PP6.00055: Radiation processes in laser-wakefield accelerators using particle tracking in PIC simulations Joana Martins, Fabio Peano, Samuel Martins, Ricardo Fonseca, Lu\'Is Silva The detailed dynamics of relativistic particles in LWFA can be followed with PIC simulations but resolving short-wavelength radiation, e.g. from betatron oscillations, in 3D simulations can be computationally very demanding. We employ particle tracking in OSIRIS simulations, combined with a post-processing radiation diagnostic, to evaluate the features of the radiation mechanisms of accelerated electron in blowout regime of the LWFA. Radiated power and spectra illustrate the signature of self-injection and allows us to determine the main features of the collimated radiation beam due to the betatron motion in the blowout region. A study of the angular dependence of the radiated power is also presented and compared with theoretical models. This analysis also allows for the direct calculation of the radiation losses of the self-injected bunch. Results and discussion are presented for both 1 GeV and 10 GeV laser wakefield accelerator stages. [Preview Abstract] |
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PP6.00056: The Effect of the Self-Magnetic Field on the Current Limit in a Laser Wakefield Accelerator (LWFA) Ling-Chieh Tai, Peng Zhang, Wee-Shing Koh, Lay-Kee Ang, Shih-Hung Chen The challenge for the development of LWFA is achieving a nC- and GeV-level electron beam. Previous experiments [1] produced a GeV electron beam with total charges 30 pC, which is much less than the theoretical prediction [2]. The discrepancy might be due to the space charge and self-magnetic fields in the electron beam. The electrostatic diode model [3] for the space-charge-limited current has been developed and verified by the simulation. In order to understand the effect of the self-magnetic field on a relativistic electron beam, particle-in-cell simulations with electrostatic and electromagnetic models are performed, respectively. The simulation results can help the development of a new theoretical model based on a parabolic potential profile, which can be applied on more precise predictions of the limited currents in LWFA. 1. W. Leemans et al., Nature Phys. 2, 696 (2006). 2. W. Lu et al., Phys. Rev. ST Accel. Beams 10, 061301 (2007). 3. L. K. Ang et al., Phys. Rev. Lett. 98, 164802 (2007). [Preview Abstract] |
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PP6.00057: Laser Acceleration of Monoenergetic Protons in Moving Double Layer from Thin Foil V.K. Tripathi, C.S. Liu, X. Shao We present analytic theory of laser acceleration of monoenergetic protons by irradiation on a thin foil, reported by Yan et al., 2008 in simulations. The ponderomotive force pushes the electrons forward, leaving ions behind until the space charge field balances the ponderomotive force at distance $\Delta _s \approx a_0 (n_c /n_0 )\lambda _L /\pi $, where $a_0 $ and $\lambda _L $ are the normalized laser amplitude and wavelength, $n_c $ is the critical density and $n_0 $is the plasma density. For the target thickness $D=\Delta _s $, the electron sheath piled up at the rear surface, is detached from the bulk ions and moves into vacuum, carrying with it the protons contained in the sheath width $\sim c/\omega _p $, where $\omega _p $ is the plasma frequency. These protons are trapped by the self field of the dense electron sheath and are collectively accelerated as a double layer by the laser ponderomotive force, giving proton energy $\approx $ 200 MeV at $a_0 =5$, $n_c /n_0 =10$ and pulse length 90 fs. [Preview Abstract] |
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PP6.00058: Optical diagnostics on capillary waveguides for extension of laser wakefield acceleration lengths C. McGuffey, T. Matsuoka, M. Levin, P. Rousseau, V. Chvykov, G. Kalintchenko, V. Yanovsky, A. Zigler, A. Maksimchuk, K. Krushelnick The electron beams produced from laser wakefield acceleration (LWFA) have been proposed for next generation electron accelerators and can be used for applications in imaging, microwave generation, and x-ray generation. The diffraction length achievable for LWFA can be extended by guiding the focused laser pulse using external guiding structures. This length is a fundamental limitation on the acceleration length and therefore maximum attainable electron energy. Capillary waveguides are used to setup guiding structure by creating a discharge plasma along the direction of laser propagation with varying plasma density radially. Two different waveguide schemes are being explored on the Hercules laser system (Ti:Sapphire, 30 fs, recently upgraded to 300 TW) - a purely ablative capillary discharge and a gas filled capillary discharge. The two schemes provide a range of differing characteristics such as lifetime, ionization state, and complexity. Diagnostics indicate that high intensity can be sustained over a few centimeters, with acceptable laser depletion. [Preview Abstract] |
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PP6.00059: Electron Heating by Plasma Waves E.G. Evstatiev, B.A. Shadwick Lagrangian fluid plasma simulations show that when a test electron moves with the plasma wave, its phase space trajectory appears to be random. The average momentum of a collection of test electrons changes with the plasma wave, but their momentum dispersion is very small or zero. Similar simulations are done with a multiwave plasma model.\footnote{T.~M.~O'Neil, J.~H.~Winfrey, and J.~H.~Malmberg, Phys. Fuids {\bf 14}, 1204 (1971); E.~G.~Evstatiev, P.~J.~Morrison, and W.~Horton, Phys. Plasmas {\bf 12}, 072108 (2005).} When the initial electron velocity is low compared with the phase velocity of the plasma wave, similar results to the fluid simulations are observed. When the electron velocity is close to the phase velocity of the plasma wave and wave-particle resonance becomes important, the momentum dispersion of the electrons is no longer small and indicates electron heating and trapping. These results may help understand the significance of an initial electron velocity spread (temperature) for electron trapping in laser-plasma experiments as well as the effects of numerical particle heating (such as in particle-in-cell simulations) in laser-plasma simulations. [Preview Abstract] |
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PP6.00060: Optical Diagnostics of Laser Plasma Wakefield Experiments using 200 Terawatt Pulses F.J. Dollar, Y. Horowitz, C. McGuffey, C. Huntington, T. Matsuoka, S. Bulanov, V. Chvykov, G. Kalintchenko, P. Rousseau, V. Yanovsky, R.P. Drake, A. Maksimchuk, K. Krushelnick Understanding the electron injection mechanism in laser wakefield acceleration (LWFA) is a key to realizing GeV class electron acceleration. To study the interaction of 30 fs laser pulses with power up to 200 TW at the Hercules laser facility with LWFA plasmas various optical diagnostics were employed. The transverse optical probe has been utilized for shadowgraphy and interferometry in order to study laser propagation. The probe was also used as a faraday rotation diagnostic to measure self generated magnetic fields due to electron beam current. The magnetic fields measurements reveal the insight into the electron injection mechanism. [Preview Abstract] |
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PP6.00061: Measuring the bunch duration of ultra-short, mono-energetic electron beams with coherent transition radiation in the infrared region William Schumaker, Jean-Rapha\"{e}l Marqu\`{e}s, Nicolas Bourgeois, Sandrine Dobosz, Tiberio Ceccotti, Pascal D'Olveira, Fabrice Reau, Arnaud Andre, Philippe Martin The duration of electron bunches created by laser wakefield accelerators (LWFA) can be effectively determined by measuring the coherent transition radiation (CTR) generated at the plasma-vacuum interface. Our research focuses on measuring ultra-short electron bunch durations down to 5 fs (1.5 $\mu$m) generated from a 25 fs, 70 TW Ti:Sapphire laser focused on a gas jet target with a 7.5 $\mu$m spot size. To characterize CTR in the infrared region, we will alter the gas jet pressure, position, and size, correlating our results with other diagnostics such as electron spectrometry, interferometry, and x-ray detection. [Preview Abstract] |
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PP6.00062: Development of surface plasmon based x-ray detector Yuichi Kunieda, Tetsuya Kawachi, Noboru Hasegawa, Keisuke Nagashima We propose a novel ultra-fast x-ray detector by use of the evanescent field assisted by surface plasmon resonance (SPR). Under the SPR condition driven by the short laser pulse, the plasmons in the metal surface are excited followed by emitting free electrons in vacuum, which are accerelated by the gradient of the strength of the evanescent electric field. Under the condition that an additional x-ray pulse is temporally synchronized with the driving pulse of SPR, the kinetic energy of the emitted electrons originated from the evanescent field may be shifted due to the effect of the photoionization with the incident x-ray. This scheme has potential to determine the duration of the x-ray pulse by monitoring the energy shift as a function of the timing of the driving pulse of SPR. The temporal resolution is comparable to the decay time of the plasmon excitation, which is around 50 fs. In the presentation, we report the preliminary experiment and show the prospect of this scheme as the ultra-fast x-ray detector. [Preview Abstract] |
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PP6.00063: Modeling Non-LTE Oxygen-like Multiplet Dynamics in Intense Laser-produced Xenon Plasmas K.G. Whitney, Tz. B. Petrova, J. Davis An often used approximation employed to simplify the problem of modeling the L- and M-shell ionization dynamics of moderate to high atomic number plasmas is to lump the states within each $n\ell$ multiplet of each ionization stage by assuming the multiplet substates are in LTE with respect to one another. In plasmas created by intense laser pulses ($10^20$ W/cm$^{-2}$) irradiating a gas of xenon clusters, this assumption breaks down. A diagnostic for this breakdown is the appearance of a strongly amplified x-ray line at 2.9 \AA. In this talk, we study the subpopulation dynamics in the O-like ionization stage of Xe where significant amounts of population can be stored in excited states. The non-LTE behavior of the following states is calculated: the ground states, the $\Delta n=0$, and the $2p^33\ell$ or $2p^23\ell$ excited states of O-like Xe, and used to determine the impact on lumped state excitation and ionization rates and on the possibility of generating a population inversion between the substates of the n=2 and n=3 states. In particular, the reduction of the lumped state Einstein decay rates of the $n=3$ states as a function of ion density is calculated.\footnote{K. G. Whitney, et. al., J. Phys. B, \bf{40}, 2747 (2007).} [Preview Abstract] |
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PP6.00064: Quasitransient regimes of backward Raman amplification of intense x-ray pulses Vladimir Malkin, Nathaniel Fisch The backward Raman amplification (BRA) of laser pulses is considered under conditions when important features of the transient BRA survive, while BRA is noticeably affected by damping of the Langmuir wave mediating energy transfer from the pump to the pumped pulse. These quasitransient BRA regimes appear to be relevant to possible principle-of-proof experiments on BRA of intense x-ray laser pulses in plasmas. In particular, such experiments found to be feasible within the parameter range of currently built powerful soft x-ray sources. [Preview Abstract] |
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PP6.00065: Evolution of Molecular Alignment in a Background Plasma Andrew Pearson, Thomas Antonsen We study numerically the behavior of rotational revivals in a molecular gas when subject to the fluctuating electric field of a background plasma. We model a molecule as a rigid rotor and couple it to an electric field using permanent and induced multipole interactions. The evolution of the density matrix for the molecule is calculated for a short, intense laser pulse, followed by a fluctuating electric field. A broad superposition of angular momentum eigenstates of a molecule is created by the laser field, and the result is a set of recurring peaks in the probability density for observing a particular orientation -- the so-called 'rotational revivals.' Experimentally, this effect is manifest as a variation in the refractive index of the gas [1]. The fluctuating field is created using the dressed particle method, and the result is a loss of coherence between the phases of the basis states of the molecule, which causes a decreasing amplitude for subsequent alignment peaks. Modern short-pulse lasers operate with sufficient intensity to make this effect relevant to experiments in molecular alignment. This work was supported by the Department of Energy.\newline [1] Y.-H. Chen et. al., Optics Express Vol. 15, No. 18, 11341 (2007) [Preview Abstract] |
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PP6.00066: Study of nonlinear Landau damping in backward Raman amplifier Nikolai Yampolsky, Nathaniel Fisch A plasma wave mediates laser coupling in a plasma-based resonant backward Raman amplifier/compressor (BRA) for high power amplification of short laser pulses. The resonant nature of amplification requires long lifetime of the plasma wave. However, the plasma wave can be heavily Landau damped in warm plasma. On the other hand, Landau damping can be saturated in the presence of strong plasma wave. First, we develop a simplified model of nonlinear Landau damping which describes this kinetic phenomenon in terms of fluid equations. We verify the simplified fluid model numerically and show that it has about 90\% accuracy within its applicability limits. This model can be applied to the BRA problem and a semi-analytical solution can be found. We find the regime of amplification in which the originally large Landau damping can be significantly reduced during the interaction. In this a regime BRA can occur at higher plasma temperatures than would be calculated from a purely linear theory. This work supported by the NNSA and by DOE Contract DE-AC02-76CH03073. [Preview Abstract] |
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PP6.00067: Neutron production from interactions of high-intensity ultrashort pulse laser with a planar deuterated polyethylene target George Petrov, Jack Davis The neutron production from D(d,n)-$^{3}$He nuclear fusion reactions was studied with a two-dimensional electromagnetic particle-in-cell method combined with a three-dimensional Monte Carlo ion beam-target deposition model. The precursor for nuclear fusion reactions is high-energy (MeV) deuterons generated from a double-layer or uniform deuterated polyethylene target in the ultra-relativistic regime for peak laser intensities between 10$^{19}$ and 10$^{21}$ W/cm$^{2}$. The angular scattering of neutrons is found to be non-isotropic having a significant component in the forward (laser propagation) direction. A neutron yield of 10$^{5}$ - 10$^{7}$ neutrons per Joule laser energy is inferred from simulations. [Preview Abstract] |
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PP6.00068: Measurement of Conical Second Harmonic Generation in Intense Laser Interaction with Plasma Michael Helle, Dmitri Kaganovich, Daniel Gordon, Antonio Ting Recent theoretical work [1] on electro-optic shock produced from the interaction of intense laser radiation with high density plasma suggests that second harmonic radiation will be emitted at the Cherenkov angle. This angle is related to the ratio of the second harmonic phase velocity to the fundamental phase velocity. Experimental observation of this electro-optic shock radiation is currently underway at the U.S. Naval Research Laboratory. Initial results show the existence of a second harmonic ring. Spectrographic diagnostics are currently being developed and will soon be online. Preliminary results and analysis will be presented. \\[0pt] [1] D. F. Gordon, et al, PRL, accepted for publication (2008). [Preview Abstract] |
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PP6.00069: A compact X-ray to Gamma-ray FEL Frederico Fiuza, Luis Silva We study the possibility of using the magnetic mode driven by an intense radiation source impinging on a relativistic ionization front as a plasma undulator with periods that can be orders of magnitude smaller than those of conventional undulators, allowing for the generation of ultrashort-wavelength radiation with modest electron-beam energies, and therefore strongly decreasing the size of FEL systems. The effects of the plasma background on the resonance condition and on the growth rate of the generated radiation are investigated, as well as the role of the associated beam-plasma instabilities in the FEL mechanism. In order to check the validity of our theoretical predictions, we have performed simulations both with GENESIS 1.3, for an equivalent magnetic field structure, and with OSIRIS 2.0, in order to also include the plasma effects. The analysis reveals a good agreement between theory and simulation results, illustrating the possibility of using short-period plasma undulators to produce a compact X-ray to Gamma-ray FEL. [Preview Abstract] |
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PP6.00070: Terahertz radiation source from an electrostatic field using a laser-produced ionization front Hirofumi Nishimai, Takeshi Higashiguchi, Noboru Yugami, Patric Muggli THz sources are developing for various applications such as a test of semiconductor materials. The photoconductive (PC) antenna emits the THz radiation with a low power of the order of nW level. A plasma-based DC to AC radiation converter (DARC), which was proposed by W. B. Mori {\it et al}. (UCLA), can directly convert from an electrostatic field to an electromagnetic wave. When a laser-produced ionization front propagates through an electrostatic field, it generates a burst of current and a half- cycle pulse of radiation with high power. The radiation frequency of the DARC source depends on the electron density behind the ionization front and the wavenumber of the electrostatic field. In the present case, the temporal waveform of the THz radiation was observed. Two cycles wave with a period of 0.8 ps was observed. The Fourier spectrum of the temporal waveform has a peak of 1.2 THz with a bandwidth of 0.7 THz (FWHM). We study the gas-filled DARC source to produce the radiation with high output power. We characterize the frequency and the power of the radiation using a sampling method and a Si-bolometer, respectively. [Preview Abstract] |
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PP6.00071: Dynamic Manipulation of Electromagnetic Waves in Magnetized Plasmas: Compression, Frequency Shifting, and Release Yoav Avitzour, Gennady Shveys A new approach to manipulating the duration and frequency of microwave pulses using magnetized plasmas is demonstrated. The plasma accomplishes two functions: (a) slowing down and spatially compressing the incident wave, and (b) modifying the propagation properties (group velocity and frequency) of the wave in the plasma during a uniform in space adiabatic in time (USAT) variation of the magnitude and/or direction of the magnetic field. The increase of the group velocity results in the shortening of the temporal pulse duration. Although it has long been recognized that electromagnetic pulses can be slowed down and compressed in plasma, it was also understood that the outgoing pulses are going to return to their original length upon exiting the plasma. We show here that it is possible to dynamically change plasma conditions during the time period when the pulse is traveling in the plasma in such a way that the pulse emerges from the plasma compressed and/or frequency shifted. It is demonstrated that the simplest plasma geometries (electromagnetic wave propagating either at a small angle to the magnetic field in an infinite plasma, or inside a plasma-filled metallic waveguide) enable such dynamic manipulation of a microwave pulse using USAT variation of the external magnetic field B0.\\[0pt] [1] Y. Avitzour and G. Shvets, Phys. Rev. Lett. 100, 065006 (2008). [Preview Abstract] |
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PP6.00072: Remote Detection of Chemicals Using Femto-Second Laser Induced Breakdown Spectroscopy Tariq Ahmido, Antonio Ting, Prabhakar Misra Laser-Induced Breakdown Spectroscopy (LIBS) is being used for the analysis of surface contaminant pollutants and chemical compounds by focusing a high power femtosecond laser beam onto a contaminated surface. A femtosecond laser has the broad bandwidth that allows pulse compression by group velocity dispersion in air to achieve high power and high intensity at controlled remote distances [1]. A short laser pulse ($\sim $ 50 fs) produced by a Ti: Sapphire laser at 800 nm wavelength is propagated and focused in the laboratory to initiate LIBS. The research focuses on the detection of atoms, ions and chemical radicals present in the plasma generated by the high intensity laser. Emission radiation from the breakdown of contaminant is spectrally analyzed for signatures of the constituent chemical compounds. Currently, proof-of-concept studies are in progress, using representative chemicals such as sodium nitrate. Preliminary results will be presented. [1] I. Alexeev, et. al, APL 84, 4080(2004) [Preview Abstract] |
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PP6.00073: C-MOD |
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PP6.00074: Progress Using LHCD and ICRF on Advanced Tokamak Discharges in Alcator C-Mod C.E. Kessel, A.E. Hubbard, P.T. Bonoli, R. Parker, Y. Lin, A.E. Schmidt, S. Scott, S. Shiraiwa, A.C.C. Sips, G. Wallace, R. Wilson, S. Wolfe, S. Wukitch Advanced tokamak discharges on Alcator C-Mod are targeting high non-inductive current fraction with bootstrap and lower hybrid (LH) current drive, and high confinement from ICRF heated H-modes. Plasma currents in the range of 450-600 kA are coupled with up to 3.5 MW of ICRF and 0.6-0.9 MW of LH. Longer plasma current rampup times are used to allow the ICRF and LH to modify the current profile. Delay of the sawtooth onset is significant using the LH by itself and in combination with the ICRF. ICRF H-modes in the rampup show they are not as effective at delaying the sawtooth as LH. Time dependent simulations of the discharges with the TSC and LSC code (ray-tracing with 1D FP) have helped to determine the magnitude of the LHCD and the resulting effects on the safety factor profile. Analysis with GENRAY-CQL3D (ray-tracing with 2D FP) will be given. [Preview Abstract] |
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PP6.00075: Development of an operational scenario for a current hole plasma using TSC on Alcator C-MOD S. Shiraiwa, P. Bonoli, A. Hubbard, J. Ko, O. Meneghini, R. Parker, G. Wallace, S. Wolfe, S. Wukitch, C. Kessel, S. Scott, R. Wilson, Y. Takase A current hole develops when the q-profile is extremely deeply reversed and the toroidal current density becomes zero around the center of a tokamak plasma. In order to investigate the possibility of creating a current hole on Alcator C-mod, TSC and LSC (ray-tracing with 1D FP) simulations have been carried out. Using experience from the OH-less experiments on JT-60U[1] it is found that the plasma evolves towards a current hole configuration with a moderate LHCD power ($\sim$ 1MW) at a plasma current of 300kA, when a compound spectrum is employed. The plasma produced is nearly $100\%$ driven by LHCD in steady-state with a very small bootstrap current, providing a good test for our understanding of the current hole. These simulations are being used to formulate an experiment on Alcator C-mod. A possibility of increasing the bootstrap current by additional ICRF heating will also be discussed. [1] S. Shiraiwa, et al., Phys. Rev. Lett. 92, 035001 (2004) [Preview Abstract] |
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PP6.00076: Progress in Lower Hybrid Current Drive Experiments on Alcator C-Mod. Ronald Parker, Paul Bonoli, Arturo Dominguez, Amanda Hubbard, Jerry Hughes, Alex Ince-Cushman, Jinseok Ko, Orso Meneghini, Miklos Porkolab, John Rice, Andrea Schmidt, Shunichi Shiraiwa, Greg Wallace, John Wright, Randy Wilson, Steve Scott Lower hybrid current drive experiments on Alcator C-Mod have continued during the 2008 campaign with power up to 1.2 MW. LH driven current density profiles have been determined based on the MSE measured poloidal field. The profiles are broader than their inductive counterparts, with central q values above unity (consistent with the cessation of sawteeth) and lower internal inductance. Simulations of driven current profiles using GENRAY and CQL3D are in reasonable agreement with experiment providing a phenomenological spatially diffusive term is included. The value of the diffusion coefficient needed for agreement is consistent with results of time-dependent non-perturbative measurements of the energetic bremsstrahlung profiles. Additional results concerning observation of strong counter rotation in LHCD discharges, coupling of LH waves in the presence of ICRH and detection of LH waves by reflectometry will be described. [Preview Abstract] |
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PP6.00077: Interaction of Lower Hybrid Waves with the Scrape Off Layer G.M. Wallace, R.R. Parker, P.T. Bonoli, A.E. Schmidt, D.G. Whyte, S.J. Wukitch, J.R. Wilson The Lower Hybrid Current Drive (LHCD) system on Alcator C-Mod was designed to drive current in the core plasma, however several interesting Scrape Off Layer (SOL) phenomena have been observed during recent operation. Visible imaging during plasma discharges shows bright stripes of plasma emission, tilted along the magnetic field lines and modulated with LH power, across the LH launcher face. Post-campaign inspection of the launcher has shown significant melting of the waveguide walls in the same location as the bright stripes seen on the camera, indicating a localized interaction between the high power LH waves and the SOL plasma. Additionally, parallel current densities of $\sim $500 kA/m$^{2}$ are observed on divertor Langmuir probes just outside the separatrix during high power LH operation above n$_{e0}$=10$^{20}$m$^{-3}$. The plasma density in the outer SOL is also observed to increase when the LH is turned on. These observations indicate LH power is absorbed in the SOL, especially at higher density. [Preview Abstract] |
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PP6.00078: TOPLHA and ALOHA: comparison between Lower Hybrid wave coupling codes Orso Meneghini, J. Hillairet, M. Goniche, R. Bilato, D. Voyer, R. Parker TOPLHA and ALOHA are wave coupling simulation tools for LH antennas. Both codes are able to account for realistic 3D antenna geometries and use a 1D plasma model. In the framework of a collaboration between MIT and CEA laboratories, the two codes have been extensively compared. In TOPLHA the EM problem is self consistently formulated by means of a set of multiple coupled integral equations having as domain the triangles of the meshed antenna surface. TOPLHA currently uses the FELHS code for modeling the plasma response. ALOHA instead uses a mode matching approach and its own plasma model. Comparisons have been done for several plasma scenarios on different antenna designs: an array of independent waveguides, a multi-junction antenna and a passive/active multi-junction antenna. When simulating the same geometry and plasma conditions the two codes compare remarkably well both for the reflection coefficients and for the launched spectra. The different approach of the two codes to solve the same problem strengthens the confidence in the final results. [Preview Abstract] |
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PP6.00079: Proposal for the Study of LH Wave Propagation Using Reflectometry on Alcator C-Mod A. Dominguez, P. Bonoli, E. Marmar, R. Parker, S. Shiraiwa, G. Wallace The Alcator C-Mod tokamak currently uses lower hybrid range of frequency (LHRF) waves as an auxiliary current drive and heating source. Measuring the radial penetration of these waves and comparing the results to theory and simulation is important for the understanding of LHRF wave physics in tokamaks. The use of O-mode reflectometry is proposed as an experimental tool for this purpose. Initial proof of principle experiments investigating the viability of this proposal were carried out during the 2008 campaign in which high frequency low power waves (54.8GHz at $\sim$5mW) were injected into the low field side midplane of the plasma in O-mode propagation during LH operation. The reflected signal is found to have frequency components consistent with modes caused by the modulation of the reflectometry cutoff density layer by the LHRF wave. Parametric decay instability (PDI) components can also be seen in the reflected waves. In this poster, the results of initial proof of principle experiments carried out on the Alcator C-Mod are presented as well as a proposal to fully implement the technique for more extensive measurements. [Preview Abstract] |
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PP6.00080: Transport Studies in Alcator C-Mod ITB Plasmas C.L. Fiore, P.T. Bonoli, D. Ernst, M.J. Greenwald, A. Ince-Cushman, L. Lin, E.S. Marmar, M. Porkolab, J.E. Rice, S. Wukitch, W. Rowan, I. Bespamyatnov, P. Phillips Internal transport barriers occur in C-Mod plasmas that have off-axis ICRF heating and also in Ohmic H-mode plasmas. These ITBs are marked by highly peaked density and pressure profiles, as they rely on a reduction of particle and thermal flux in the barrier region which allows the neoclassical pinch to peak the central density without reducing the central temperature. Enhancement of several core diagnostics has resulted in increased understanding of C-Mod ITBs. Ion temperature profile measurements have been obtained using an innovative design for x-ray crystal spectrometry and clearly show a barrier forming in the ion temperature profile. The phase contrast imaging (PCI) provides limited localization of the ITB related fluctuations that increase in strength as the central density increases. Simulation of triggering conditions, integrated simulations with fluctuation measurements, parametric studies, and transport implications of fully ionized boron impurity profiles in the plasma are under study. A summary of these results will be presented. [Preview Abstract] |
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PP6.00081: Studies of Turbulence and Transport in Alcator C-Mod H-Mode Plasmas with Phase Contrast Imaging and Comparisons with GYRO M. Porkolab, L. Lin, E.M. Edlund, J.C. Rost, C.L. Fiore, M. Greenwald, D. Mikkelsen We present recent experimental measurements of turbulence and transport in C-Mod H-Mode plasmas with and without internal transport barriers (ITB)\footnote{C.L. Fiore et al., Fusion Sci. Technol., \textbf{51}, 303 (2007). } using the phase contrast imaging (PCI) diagnostic\footnote{M. Porkolab et al., IEEE Trans. Plasma Sci. \textbf{34}, 229 (2006). } and compare the results with GYRO predictions.\footnote{J. Candy et al., Phys. Rev. Lett., \textbf{91}, 045001 (2003).} In plasmas without ITB, the fluctuation above 300 kHz observed by PCI agrees with ITG in GYRO simulation, including the direction of propagation, wavenumber spectrum, and absolute intensity within experimental uncertainly (+/-75{\%}). After transition to ITBs, the observed overall fluctuation intensity increases. GYRO simulation in the core shows that ITG dominates in ITBs but its intensity is lower than the overall experimental measurements which may also include contributions from the plasma edge. These results, as well as the impact of varying $\nabla $T$_{i}$, $\nabla $n, and ExB shear on turbulence will be discussed. [Preview Abstract] |
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PP6.00082: Comparison of the Characteristics of Striations Formed During Li Pellet Injection and Turbulent ExB Flows from the Gyro-Kinetic Simulation GYRO Brock Bose, Earl Marmar, David Mikkelsen, Martin Greenwald Using an ultra high speed CCD camera, (frame rate up to 500 kHz) and a stereoscopic imaging system the detailed three dimensional evolution of striations formed in the ablation clouds of injected lithium pellets has been recorded on the Alcator C-Mod tokamak. The striations move primarily in the poloidal direction during the first 10 $\mu $s after their formation and show distinctly different behaviors in ohmic L-mode plasmas and ICRH heated H-mode plasmas. During ohmic L-mode plasmas the direction in which the striations are emitted oscillates from the positive to negative ion diamagnetic direction over a radial scale of 10-20 ion gyro radii, and they move with speeds of up to 5 km/s. On the other hand, during ICRH heated H-mode plasmas the striations are predominantly emitted in the negative ion diamagnetic direction, again with speeds of up to 5 km/s. During the 2007 and 2008 campaigns the stereo-imaging system was employed to make a survey of the characteristics of striation trajectories in a variety of background plasmas. The characteristics, such as velocity distributions and the spatial scales of the striation trajectories are compared with simulated turbulent ExB flows from the non-linear gyro-kinetic solver GYRO. [Preview Abstract] |
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PP6.00083: Pedestal and confinement properties under shape and magnetic topology variation on Alcator C-Mod J.W. Hughes, B. Lipschultz, D. Whyte, E.S. Marmar, M. Greenwald, A.E. Hubbard, R.M. McDermott Recent work on Alcator C-Mod has examined the influence of magnetic topology and equilibrium shape on edge pedestal structure and plasma confinement. H-mode pedestal parameters show a striking sensitivity to the ion $\nabla B$ drift direction, relative to the active x-point position, with considerable variability observed when the distance between separatrices is on the order of the pedestal width ($\sim$5mm) or less, {\it i.e.} very near double null (DN). Near DN H-modes can have improved confinement factors ($H_{98}>1$) as a result of elevated pedestal temperature ($T_{\mbox{ped}}$), with the edge regulated by benign small edge-localized modes (ELMs) or continuous modes. Such operational regimes with no large ELMs are desirable for ITER and other future devices. Discharges with L-mode-like particle confinement, yet with $H_{98}\approx1$ and $T_{\mbox{ped}}\approx 1\mbox{keV}$, were maintained steady-state by operating with high current, strong shaping and unfavorable $\nabla B$ drift direction, while holding input power below the L-H threshold to prevent particle barrier formation. The pedestal and confinement properties of these improved ELM-free regimes will be compared to those of typical H-modes. [Preview Abstract] |
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PP6.00084: Magnetized ion collection by oblique surfaces including self-consistent drifts: Mach-probes of arbitrary shape I.H. Hutchinson A complete analytic theory for magnetized Mach-probes, when cross-field diffusion is neglected, is presented. It is shown that the full self-consistent quasi-neutral fluid drift equations around an ion-collecting probe of arbitrary 3-D shape, in a magnetized isothermal plasma with background parallel and perpendicular flow, can be solved exactly. The resulting flux to the probe (per unit area perpendicular to $B$) is $n_\infty c_s \exp(-1 -M_\parallel + M_\perp\cot\theta)$, where $\theta$ is the angle between the surface and $B$ in the plane of background-drift. This exponential dependence is in good agreement with prior numerical fits of the diffusive case. The (background) perpendicular Mach number, $M_\perp$, is that arising from the sum of ExB and, counter-intuitively, {\it electron} (not ion) diamagnetic drifts. Fluid displacements in the magnetic presheath are important, and included in this expression, but give rise to small additional terms at some orientations. Temperature-gradient diamagnetic drifts can be added, but only approximately: both electron and ion drifts contribute. Corrections of order Larmor radius divided by electrode-dimensions are also evaluated. They can bias the results for small probes. [Preview Abstract] |
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PP6.00085: Parallel and Perpendicular Plasma Flows in Alcator C-Mod Noah Smick, Brian LaBombard, Bruce Lipschultz, John Rice, Kenneth Marr, Rachel McDermott, Alex Ince-Cushman, Alex Graff Three new scanning Gundestrup probes have been operating on Alcator C-Mod for the 2007 and 2008 run campaigns. Two of the probes are located on the on the LFS and HFS midplanes, and one is near the outboard divertor leg. Each probe is equipped with a four-electrode Gundestrup geometry and is capable (in principle) of measuring the parallel and perpendicular component of SOL plasma flow. Parallel flow results show a strong correlation to magnetic topology, particularly on the high field side where near-sonic flows exist toward the active x-point. This observation supports the idea of a ballooning-like transport asymmetry. The possibility that these flows couple to core toroidal rotation is investigated. Contrary to expectation, the perpendicular flow shows a strong feature in the electron diamagnetic direction in the vicinity of the separatrix. This may be connected to recent theoretical work by Hutchinson$^{1}$ which suggests the inclusion of an electron diamagnetic term in the Gundestrup formulation. [1] I. H. Hutchinson, Poster this session. [Preview Abstract] |
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PP6.00086: Comparison of neoclassical flow theory with CXRS measurements for the H-mode pedestal on Alcator C-Mod Kenneth Marr, Bruce Lipschultz, Rachael McDermott, Peter Catto, Andrei Simakov Neoclassical theory, applied in the Pfirsch-Schluter regime to the plasma edge, describes poloidal (and toroidal) flow on a flux surface. Using measured pressure gradients we calculate low-field side poloidal pedestal flow profiles and compare them with flow profiles derived from the CXRS diagnostics on Alcator C-Mod. Given low- and high-field edge velocity measurements we can also calculate the ratio of impurity densities in those regions. The calculated radial profile of poloidal velocity exhibits the same strong peaking (shape and magnitude) in the region of steep gradients as seen in the measured profiles. Terms based on the electric field in the toroidal flow calculation are oppositely directed from the gradient terms, resulting in flatter flow profiles. Density calculations show an in-out asymmetry of up to a factor of 4 between the high- and low-field sides of the plasma with the effect strongest in the steep gradient region. [Preview Abstract] |
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PP6.00087: Main ion toroidal rotation in the edge of the Alcator C-Mod tokamak A. Graf, D. Whyte, K. Marr, M. May, P. Beiersdorfer The main ion toroidal rotation and temperature is measured spectroscopically at the inner wall midplane plasma edge $\pm$1 cm around the last closed flux surface. A four channel high resolution transmission grating spectrometer monitors the Doppler shift and width of the deuterium Balmer beta emission line shape at 4860 {\AA}, every 8 ms for the duration of the discharge. The measurement is localized using a radially oriented neutral deuterium gas puff which enhances the emission via charge exchange with the fuel ions. Comparisons with impurity rotation and temperature from B V deduced in a similar fashion will be given under various plasma conditions. Also, a proof of principle measurement of helium rotation and temperature in an L-mode helium majority plasma has been accomplished by monitoring the diagnostic neutral beam induced charge exchange enhancement of the Paschen $\alpha$ transition in He II at 4686 {\AA}. This work was performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. [Preview Abstract] |
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PP6.00088: Study of Ion-Temperature and Intrinsic Rotation-Velocity Profiles on the Alcator C-Mod Tokamak K.W. Hill, M. Bitter, S.D. Scott, A. Ince-Cushman, M. Reinke, J.E. Rice, P. Beiersdorfer, M.F. Gu, S.G. Lee Measurements of radial profiles (r/a=0-0.75) of ion temperature (Ti) and toroidal rotation velocity (v$\phi )$ are made on C-Mod with a spatially resolving x-ray crystal spectrometer ($\sim $1 cm resolution) at 20 ms intervals via Doppler broadening and shifting of He-like and H-like Ar lines. Reversals of the rotation direction during transitions to H-mode, during formation of internal transport barriers, and during LHCD injection provide information on intrinsic rotation mechanisms. The data analysis techniques, Ti and v$\phi $ profile results, analysis of background resulting from fusion neutrons, and predictions of performance on the international tokamak ITER and other tokamaks will be presented. [Preview Abstract] |
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PP6.00089: Direct comparison of GEMR edge turbulence simulations with Alcator C-Mod SOL turbulence measurements S. Scott, B. Scott, S. Zwebens, J. Terry, B. Labombard, J. Hughes, D. Stotler We report a direct comparison of measured SOL turbulence in Alcator C-Mod plasmas to a computational model. The turbulence measurements were made with gas puff imaging (GPI) and Langmuir probes, and the simulations were performed by the GEMR gyrofluid electromagnetic code [Phys. Plasmas 12 (2005) 102307]. Plasma conditions were chosen the match the capabilities of the computational model: near-circular, Ohmic, inner wall limited, with B=2.9-5.4 T and Ip=0.4-0.8 MA at fixed q(a). The measured radial and poloidal correlation lengths in the C-Mod SOL were approximately a factor 1.5-2 larger than those predicted by GEMR, and both measured and computed correlation lengths showed a slow decrease with B, with relatively little change with SOL density. The measured and simulated autocorrelation times and frequency spectra were very similar at low B, but the measured autocorrelation time increased with B more strongly than in the simulations. Additional comparisons and limitations of these comparisons will be described. [Preview Abstract] |
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PP6.00090: Light Impurity Transport Studies in Alcator C-Mod Igor Bespamyatnov, William Rowan, Kenneth Gentle, Perry Phillips, Catherine Fiore, Robert Granetz Time-dependent profiles of fully stripped light impurity are measured on Alcator C-Mod using CXRS and used for impurity transport studies. Experiments are proposed to further understanding. Boron profiles were measured for L-, H-mode and ITB plasmas. Profile analysis reveals outward convection for some H-mode plasmas and strong inward convection for ITB plasmas. TRANSP will be used to compare particle and impurity transport. Measurements of radial electric field are used to study turbulent transport. Shear suppression estimates coupled with ITG growth rate prediction in linear GS2 runs provide clues as to the important turbulent modes. Quasilinear estimates for impurity transport fluxes were recently developed, and quantitative comparisons with experiment are in fair agreement. Since the impurity source for existing measurements is time-independent, there are limitations on the transport measurements. Removal of these limitations with modulated or pulsed sources will be discussed. [Preview Abstract] |
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PP6.00091: Improved absolute calibration of Thomson scattering diagnostics on the Alcator C-Mod tokamak Y. Ma, J.W. Hughes, A.E. Hubbard The Thomson scattering (TS) diagnostics on Alcator C-Mod have been upgraded to provide measurements with improved radial spatial resolution of 1cm in the range of $r/a \le 0.6$. To accurately obtain electron densities ($n_e$), we absolutely calibrate TS using two independent methods. First, we backfill the vessel with deuterium or hydrogen gas at room temperature and measure the anti-Stokes Raman scattering from the TS Nd:YAG lasers. Second, we take advantage of the fact that measurements of second-order harmonic electron cyclotron emission (ECE) from plasma are cut off at certain $n_e$ and radial locations in a known magnetic geometry. These cutoffs allow us to cross-calibrate simultaneous TS $n_e$ measurements with ECE diagnostics, in specially designed lower-field ($B_T \approx 4\mbox{T}$), high-density ($\bar{n}_e > 2\times 10^{20} \mbox{m}^{-3}$) discharges. Results from both techniques are analyzed and used to produce reasonable $n_e$ profiles for plasma discharges. The reliability of each technique is assessed, and discrepancies between the two techniques are discussed. The ultimate goal is to refine the gas calibration technique such that dedicated discharges are not necessary for TS calibration. [Preview Abstract] |
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PP6.00092: Design and Construction of a Multi-Pulse Laser Blow-Off System for Impurity Transport Studies on Alcator C-Mod Nathan Howard, M. Greenwald, J. Terry, J. Rice A new laser blow-off system for impurity injection on Alcator C- Mod is currently under construction. Design goals include: multiple impurity injections during a single plasma pulse, remote manipulation of the ablated spot size, and a laser pulse capable of ablating a wide range in target Z. This is achieved with the use of a 650 mJ, ND:YAG laser operating at up to 10 Hz, fast beam steering via a 2-D piezoelectric mirror mount able to move spot locations in 100 ms, and a remote controllable optical train allowing spot sizes to vary from approximately 1 to 8 mm. Alcator C-Mod's extensive diagnostic capabilities (soft X-ray, Vacuum Ultraviolet (VUV), charge exchange spectroscopy, etc.) together with the improvements over standard laser blow-off systems allow for detailed studies of the impurity transport dependencies and mechanisms. The impurity injector is scheduled to be installed on Alcator C-Mod for the 2009 run campaign. [Preview Abstract] |
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PP6.00093: Development of the Alcator C-Mod FIR Polarimeter P. Xu, J.H. Irby, J. Bosco, A. Kanojia, R. Leccacorvi, E.S. Marmar, P. Michael, R. Murray, R. Vieira, S. Wolfe, D.L. Brower, W.X. Ding, D.K. Mansfield A multi-chord FIR polarimetry diagnostic is being developed for the Alcator C-Mod Tokamak to be used to determine the q-profile and to study density and magnetic field fluctuations. This poloidally viewing system using retro-reflectors on the inner wall will have geometry and fields similar to those planned for ITER. The optical layout will be discussed, as well as simulations of the expected Faraday and Cotton-Mouton signal levels, and the plans to integrate these data into EFIT. Details of the hardware being developed and procured including the FIR laser system, the laser power and frequency control system, optical components, detectors, beam position feedback system, and inner wall retro-reflectors and shutter will be presented. [Preview Abstract] |
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PP6.00094: Study and optimization of boronization in Alcator C-Mod Roman Ochoukov, Dennis Whyte, Bruce Lipschultz The new edge diagnostic, S$^{3}$ (Surface Science Station) was used to study the effectiveness of boronization utilizing electron cyclotron (EC) discharge plasmas (90-80{\%} He, 10-20{\%} B$_{2}$D$_{6})$ in Alcator C-Mod. The boron (B) deposition was measured with a pair of quartz microbalances (QMBs) the faces of which were oriented parallel and perpendicular to the applied magnetic field. The plasma density profile was measured with a Langmuir probe. It was found that the B deposition rate peaks (at $\sim $3.5 nm/min) away from the EC resonance near the upper hybrid (UH) resonance. The radial B deposition profile follows the ion saturation current profile, implying that the B deposition is primarily ionic. The application of the vertical magnetic field (B$_{vert})$ was found to narrow the plasma density and B deposition profiles near the UH resonance, thus localizing the deposition. A Monte Carlo simulation of B deposition on realistic surfaces requires a B ion temperature of $\sim $1 eV, whose finite gyroradius explains the B deposition pattern on the QMB surfaces perpendicular and parallel to the magnetic field. [Preview Abstract] |
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PP6.00095: Current Saturation Avoidance with Real-Time Control using DPCS M. Ferrara, I. Hutchinson, S. Wolfe, J. Stillerman, T. Fredian Tokamak ohmic-transformer and equilibrium-field coils need to be able to operate near their maximum current capabilities. However if they reach their upper limit during high-performance discharges or in the presence of a strong off-normal event, shape control is compromised, and instability, even plasma disruptions can result. On Alcator C-Mod we designed and tested an anti-saturation routine which detects the impending saturation of OH and EF currents and interpolates to a neighboring safe equilibrium in real-time. The routine was implemented with a multi-processor, multi-time-scale control scheme, which is based on a master process and multiple asynchronous slave processes. The scheme is general and can be used for any computationally-intensive algorithm. USDoE award DE- FC02-99ER545512. [Preview Abstract] |
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PP6.00096: Observation of TAEs destabilized by energetic particles in Alcator C-Mod J. Sears, A. Bader, W. Burke, G. Kramer, R.R. Parker, J.A. Snipes Toroidicity-induced Alfv\'{e}n Eigenmodes (TAEs) are weakly damped MHD modes in toroidal plasmas. The modes occur at discrete frequencies near $\omega_{TAE}=v_A/2qR$, $\left( v_A=B/\sqrt{\mu_0\rho} \right)$ in a gap of the continuous spectrum of Alfv\'{e}n waves. In Alcator C-Mod L-mode plasmas with ICRF heating up to $4.5$ MW, damping rates of stable TAEs have been measured to decrease from $\sim5\%$ at $2.5$ MW of ICRF to $\sim0.5\%$ at $4$ MW of ICRF. Unstable modes are also observed during ICRF heating at $3.5$ MW and higher. Measurements of charge exchanged neutral particles indicate that the damping decreases as the population of energetic particles near the mode resonance increases. Measured TAE structure, frequency and damping rate are compared to computational results from NOVA-K. [Preview Abstract] |
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PP6.00097: SOL reflectometer for Alcator C-mod Cornwall Lau, Yijun Lin, Greg Wallace, Steve Wukitch, Greg Hanson, John Wilgen The study of antenna-plasma interactions during RF heating and current drive is greatly influenced by the SOL density profile. A swept-frequency X-mode reflectometer is being built for Alcator C-mod to measure the SOL density profiles in front of the new Lower Hybrid Launcher and the new ICRF antenna. Six pairs of launchers will give measurements at the top, middle, and bottom of both the ICRF and LHRF antennas. The system is planned to operate between 100 and 145 GHz at sweep rates from 10 $\mu $s to 1 ms and will cover a density range of approximately 10$^{16}$ to 10$^{20}$ cm$^{-3}$ at 5-5.4 T. Due to the strong density fluctuations in the SOL, the system will use both differential phase and full phase techniques to get the best possible measurement. Design and preliminary test data from the electronics and waveguide runs will be presented. A 3D ray tracing code to study the expected reflectometry measurement of the density profile will also be shown. [Preview Abstract] |
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PP6.00098: Overview of ICRF Experiments in Alcator C-Mod S.J. Wukitch, Y. Lin, P.T. Bonoli, A. Hubbard, B. LaBombard, B. Lipschultz, M. Porkolab, J.E. Rice, D. Whyte We seek to develop ICRF as a reliable actuator to optimize overall plasma performance with minimum negative impact on the plasma. In addition to heating and current drive, an RF flow drive actuator has been long sought because it potentially offers an external tool for manipulating transport via flow shear and stabilizing macro- and micro-instabilities. Here we report the first demonstration of efficient RF toroidal flow drive by mode converted waves. We also emphasize validating the full-wave physics models through comparison with experiments including direct fast wave absorption regimes. For the first time in C-Mod, we have observed evidence of direct fast wave electron heating despite low single pass absorption. An additional challenge for ICRF utilization is mitigation and control of impurities associated with ICRF operation attributed to RF sheaths. We have measured the sheath potential in the presence of ICRF and identified their dependence on linkage to antenna, confinement mode, boronization, and insulating limiters. Experimental results from a number of experiments will be presented. [Preview Abstract] |
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PP6.00099: Measurements of Mode Converted ICRF Waves with Phase Contrast Imaging in Alcator C-Mod N. Tsujii, M. Porkolab, E.M. Edlund, L. Lin, Y. Lin, J.C. Wright, S.J. Wukitch The Phase Contrast Imaging (PCI) system in Alcator C-Mod has been used to measure MHD modes, turbulence and rf waves[1]. Rf waves are measured by adding the optical heterodyne method to the usual homodyne PCI method. In D-3He(-H) plasmas, the mode converted ion cyclotron wave (MC ICW) was observed in correlation to strong plasma flow drive[2]. The MC ICW was in the vicinity of the D-3He hybrid layer, and in good agreement with predictions from TORIC, a 2-D full-wave solver in an axisymmetric plasma[3]. Line-integrated density fluctuations have been synthesized from TORIC simulations, and directly compared with PCI measurements. [1]M. Porkolab et al., IEEE Trans. Plasma Sci., 34, 229 (2006) [2]Y. Lin (Invited talk at this meeting) [3]J.C. Wright, P.T. Bonoli, M. Brambilla et al., Phys. Plasmas, 11, 2473 (2004) [Preview Abstract] |
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PP6.00100: Measurements of Fast Ion Distribution in ICRF Heated Plasmas A. Bader, P. Bonoli, R. Granetz, R. Parker, J. Sears, S. Wukitch Alcator C-Mod uses ICRF for the bulk auxiliary heating and relies primarily on hydrogen minority heating scenarios. Measuring the resulting hydrogen ion distribution provides an opportunity to validate ICRF upgraded simulation capability that includes non-Maxwellian ions. The Compact Neutral Particle Analyzer (CNPA) is a diagnostic employed on Alcator C-Mod to measure this fast ion distribution function. The diagnostic consists of silicon diode detectors that have a direct view of the plasma from the top of the vessel. Hydrogen particles neutralize, exit the plasma and impinge on the detector producing pulses proportional to their energy. Analysis of the pulse height distribution can provide the energy distribution of the fast ion tail, serving as a benchmark for simulation results and allowing for an assessment of the simulation algorithm and physics kernel. In this poster, we will present results from the detector in the most recent campaigns. We will also describe the calibration of the detector and lastly we will provide plans for expanding the detector in the upcoming campaign. [Preview Abstract] |
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PP6.00101: INERTIAL CONFINEMENT FUSION AND LASER-PLASMA INTERACTIONS |
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PP6.00102: Long-Duration Backlighting Experiments on the Omega Laser A.B. Reighard, P.E. Young, M. Schneider, S.G. Glendinning, M. Foord, K. Lu, W.W. Hsing, R. Wallace, C. Sorce, S.A. MacLaren, T. Dittrich We have successfully demonstrated a 7.5 ns-duration pinhole-apertured backlighter at the Omega laser facility. Long-duration point-projection backlighting allows continuous imaging of evolving features in experiments planned for the National Ignition Facility. The backlighter consisted of a 20 $\mu$m diameter pinhole in a 75 $\mu$m thick Ta substrate separated from either a Zn emitter (8.9 keV) or a Ni emitter (7.9 keV) by a low-$Z$ continuous substrate. The continuous substrate prevented the shock from the laser-driven surface from reaching the substrate before 8 ns and helped minimize x-ray ablation of the pinhole substrate. We have successfully imaged a resolution target on a streak camera for 8 ns, measured both integrated and streaked spectra, and tested models for pinhole closure. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
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PP6.00103: Multi-Variable Sensitivity Studies of the Beryllium NIF Ignition Target Jay Salmonson, Stephen Haan, Daniel Clark, Debra Callahan We report the results of our continuing effort to optimize and control sensitivities for the Beryllium NIF ignition capsule. We study the latest (Revision 3) capsule by performing a sensitivity analysis which integrates 35 1D capsule design parameters with 2D surface roughness specifications for each of the seven distinct capsule layer interfaces as well as laser drive asymmetry specifications. First, we perform 10,000 1D simulations of a National Ignition Facility capsule, each varying 35 design parameters. Next, we select a random subset of these 10K 1D capsules which are then simulated as 2D full capsules with nominal specified surface roughnesses applied to each layer interface for Legendre modes 1 through 30. We also apply time dependent laser drive asymmetry as taken from an integrated hohlraum/capsule simulation for modes 2,4,6,8. As such we perform a set of NIF target calculations taking into account the most complete and realistic list of 1D and 2D design specifications to date to explore and diagnose the overall expected performance and robustness of the capsule. [Preview Abstract] |
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PP6.00104: NIF ignition target requirements, margins, and uncertainties: Rev3.1 S.W. Haan, J.D. Salmonson, D.S. Clark, D.A. Callahan, B.A. Hammel, L.J. Suter, M.J. Edwards, B.J. MacGowan, M.M. Marinak, D.H. Munro, B.K. Spears, J.D. Lindl We describe simulations of NIF ignition targets, concentrating on a Be-ablator design that uses 1.3 MJ to heat a hohlraum to 285eV. Requirements have been developed to define all aspects of the target, fabrication, laser pulse, and features of the pre-ignition experiments used to finalize the design. We describe a model, normalized to simulations, that characterizes the margin of the target as a function of input parameters and uncertainties. The model is used to quantify the impact of each requirement, and to project the probability of ignition, both shot-to-shot variations and given systematic errors. Backup targets are being kept active: other drive temperatures between 270 and 300eV; and CH and C ablators. This presentation emphasizes changes in the requirements in the last year, and the relative performance of the CH and Be designs. [Preview Abstract] |
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PP6.00105: Timing Sensitivity of Ignition Capsules to Hot Electrons Donald Meeker, Laurance Suter, Steve Haan, Harry Robey Under certain conditions approaching those expected in NIF hohlraums, the laser drive in ICF targets may produce hot electrons that can preheat the DT ice inside the ignition capsule. In addition to causing an increase in fuel entropy that directly reduces margin, the preheat can also change the optimal shock timing. The effect of the hot electrons is dependent on the time of creation of these electrons relative to the main laser drive. The capsules are most sensitive to electrons occurring very early in the pulse and are least sensitive to hot electrons created at peak laser power. The behavior of the capsule affected by the electron preheat will be discussed as well as methods to minimize their effect. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
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PP6.00106: Uncertainty Quantification in NIF Ignition Parameter Space Nelson Hoffman, John Edwards, Debra Callahan When we begin to ``tune'' the NIF target and laser pulse, we will need guidance about how observed capsule performance (e.g., shock timing, imploded shape, etc.) depends on the input parameters at our disposal (e.g., laser pulse shape, hohlraum length, etc.) So it will be necessary to have already predicted this dependence via numerical simulations. Besides being guides for tuning, such simulations also show the sensitivity of NIF capsules to uncertainty in the inputs. We present results of a study of NIF THD capsules, leading to creation of a ``surrogate model'' (i.e., an analytic fit) for the dependence of the observed capsule performance on the inputs. Treating the inputs as random variables with known distributions, the surrogate is sampled to predict distributions for the outputs, allowing quantitative estimates of uncertainty in the outputs. We show how uncertainty in the laser power during various parts of the pulse (with fixed cone ratio) leads to uncertainty in predictions of the imploded capsule's shape. [Preview Abstract] |
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PP6.00107: Simulations of high-mode Rayleigh-Taylor growth in NIF ignition capsules B.A. Hammel, S.W. Haan, M.J. Edwards, D. Clark, M.M. Marinak, M. Patel, J. Salmonson Hydrodynamic growth at mode numbers up $\sim $1000 is important for several unstable surfaces in ICF capsules. The ``buried'' ablator:fuel interface is unstable during acceleration, and supports short wavelength ($\sim $ 2 $\mu $m) Rayleigh-Taylor (R-T) growth. Roughness on the inner ablator surface grows by a factor of $\sim $1000, leading to mixing of the pusher into the dense fuel. The roughness of the inner DT fuel surface, which can include fine cracks, can also seed short wavelength growth at this interface. On the outer ablator surface, very high mode growth is stabilized by ablation and the density gradient, however, for some target designs modes up to $\sim $200 are important. Finally, features of the capsule ``fill-tube'' are $\sim $ 5 $\mu $m scale length, and can seed short wavelength growth at the ablation front and the ablator:fuel interface. To optimize capsule designs, we are performing 2D and 3D HYDRA simulations that resolve up to mode $\sim $1000. The results of this work will be presented. [Preview Abstract] |
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PP6.00108: Laboratory experiments to study supersonic astrophysical flows interacting with clumpy environments P.A. Rosen, J.M. Foster, B.H. Wilde, R. Coker, B.E. Blue, R.J.R. Williams, F. Hansen, C. Sorce, P. Hartigan, R. Carver, J. Palmer A wide variety of objects in the universe drive supersonic outflows through the interstellar medium which is often highly clumpy. These inhomogeneities affect the morphology of the shocks that are generated. The hydrodynamics is difficult to model as the problem is inherently 3D and the clumps are subject to a variety of fluid instabilities as they are accelerated and destroyed by the shock. We have been carrying out experiments on the University of Rochester's Omega laser to address the interaction of a shock wave with a single spherical particle. We use a 1.6-mm diameter, 1.2-mm length hohlraum to drive a composite plastic ablator (which includes bromine to prevent M-band radiation from preheating the experiment). The ablator acts as a ``piston'' driving a shock into 0.3 g/cc foam containing a 0.5-mm diameter sapphire or 1.0-mm diameter plastic sphere. We radiograph along two orthogonal lines of sight, using pinhole-apertured x-ray backlighters, to study the subsequent hydrodynamics. We present initial experimental results and multi-dimensional simulations of the experiment. [Preview Abstract] |
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PP6.00109: On the Numerical Modeling of Fluid Instabilities in the Small-Amplitude Limit Steven Zalesak, A.J. Schmitt, A.L. Velikovich The problem we wish to address is that of accurately modeling the evolution of small-amplitude perturbations to a time- dependent flow, where the unperturbed flow itself exhibits large-amplitude temporal and spatial variations. In particular, we wish to accurately model the evolution of small-amplitude perturbations to an imploding ICF pellet, which is subject to both Richtmyer-Meshkov and Rayleigh-Taylor instabilities. This modeling is difficult despite the expected linear evolution of the perturbations themselves, because these perturbations are embedded in a highly nonlinear, strongly- shocked, and highly complex flow field which in and of itself stresses numerical computation capabilities, and whose simulation often employs numerical techniques which were not designed with the proper treatment of small-amplitude perturbations in mind. We will review some of the techniques that we have found to be of use toward this end, including the imposition of a ``differentiability condition'' on the component numerical algorithms of the codes which implement such modeling, the appropriate representation of interfaces in an Eulerian hydrodynamics context, and the role of exact energy conservation. [Preview Abstract] |
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PP6.00110: A Numerical Investigation of Shock Propagation through a Clumpy Medium M.R. Douglas, B.H. Wilde, B.E. Blue, A. Frank, J. M. Foster, P.A. Rosen, R. Williams, P. Hartigan High velocity flows into inhomogeneous environments are common occurrences in many astrophysical phenomena, and include the passage of strong shocks through ``clumps'' or ``clouds'' in the interstellar medium. To obtain an improved understanding of the complex hydrodynamics involved in shock-clump physics, a small number of laboratory experiments have been performed over the past few years which have focused on shock interactions with single clumps. Most recently, a series of Omega experiments have investigated the dynamics of planar shocks propagating through small-scale clumpy media with one to several clumps. As a possible extension to these latter experiments, 2-D and 3-D hydrodynamic calculations have been carried out to examine the effects of much higher number clumps on shock front morphology and shock speed reduction. These calculations utilize the existing experimental platform, which launches a near planar shock (M$\sim $5) into an RF (C$_{15}$H$_{12}$O$_{4})$ cylinder. The clumps are approximated by 100 $\mu $m diameter sapphire balls distributed randomly within a confinement sphere of 1-2 mm diameter. For the 3-D calculations, up to 500 balls in a 2 mm confinement sphere has been simulated. A description of the experimental platform and details of the simulations will be presented. [Preview Abstract] |
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PP6.00111: LMJ Target design with the A1040 CH-ignition capsule in a cocktail holraum G. Malinie, C. Boniface The A1040 indirect-drive ignition capsule was originally designed for the ``Full LMJ'' 240-beam configuration. An ``Ignition milestone'' has been scheduled, when the LMJ will be only partly completed, with a 160-beam, 2-cone configuration. A first approach to meet this milestone is to scale down the capsule and hohlraum of the full LMJ design. Here we use a different approach and show the A1040 ``as is'' can still meet the milestone, provided that a suitable cocktail-walled rugby hohlraum is used to drive the capsule. This is because this kind of hohlraum has a better energetic efficiency than the gold-walled cylinder originally used. From 1D and 2D integrated simulations, we investigate the influence of various parameters of the design, such as the shape of the four steps of the laser pulse, the density of the H/He gas filling of the hohlraum, and the effect of a thin gold coating on the outer surface of the polyimid window used to contain the gas. [Preview Abstract] |
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PP6.00112: Measurements of x-ray radiation generated during hohlraum window burnthrough David Bradley, Shon Prisbrey, David Braun, Gilbert Collins, Ralph Page, John Edwards, Otto Landen, Russell Wallace Current designs for the NIF ignition hohlraum include a low density fill-gas contained by polyimide windows positioned over the laser entrance holes. The windows, which are typically 0.5 $\mu $m thickness are designed to burn through during the first few hundred ps of the foot of the ignition pulse. The laser intensity on the window can approach 1 x 10$^{15}$ W/cm$^{2}$ and it is possible that the x-rays produced during burn-through could preheat the capsule and act as a seed for Rayleigh Taylor growth, or induce a low-mode asymmetry. We have made measurements of the absolute x-ray spectrum produced during polyimide window burn-through for different window thicknesses, laser intensities and beam overlap conditions spanning those expected during window burn-through during a NIF implosion. The Dante spectral channel signals compare favorably with similar channel spectra produced by Lasnex simulations and imply that, for current NIF designs, window burn-through should not be a problem. [Preview Abstract] |
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PP6.00113: X-ray energy flow and radiography measurements of evolving density perturbations A.S. Moore, P. Graham, M.J. Taylor, J.M. Foster, C. Sorce, A. Reighard, S. Maclaren, P. Young, G. Glendinning, B.E. Blue, C.A. Back, J. Hund X-radiation transport through plasma density gradients, such as N-waves, form a closely coupled system that is challenging to simulate. Such situations are a key component of the physics of laser-heated hohlraums occurring in the laser-heated cavity and also in the laser-entry and any diagnostic holes. In addition the similarity to some astrophysical conditions may mean that such experiments can be used as a laboratory-scale analogue for their investigation. To better understand these phenomena, we present results from a series of experiments performed at the Omega laser facility. Using a laser-heated hohlraum drive, a tantalum aerogel with an initial seed perturbation is heated, and x-rays initially free-stream through the perturbation before they fill with plasma and radiation transport becomes diffuse. We present energy flux measurements diagnosed using two different methods, and complementary radiography results that achieve sufficient contrast, despite the high background of the hohlraum, to enable the complex shock interactions and stagnation to be directly observed. These results are compared with simulations performed using a 2-D Eulerian hydrocode, which are able to reproduce the overall energetics, and much of the details of the deceleration shock and axial stagnation region at the centre of the slit. [Preview Abstract] |
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PP6.00114: Characterization of Ar and Kr K$\alpha $ self-emission in clustering gas jets Nathan Kugland, Tilo Doeppner, Paul Neumayer, Hyun-Kyung Chung, Carmen Constantin, Andreas Kemp, Siegfried Glenzer, Christoph Niemann K$\alpha $ self-emission from Ar and Kr has been characterized by x-ray spectroscopy and x-ray imaging. The K$\alpha $ x-rays were produced by irradiating clustering Ar and Kr gas jets with ultra-high intensity (10$^{20}$ W/cm$^{2})$, ultra-short pulse (150-700 fs) lasers. These K$\alpha $ sources are very high contrast (defined as the ratio of K$\alpha $ to continuum radiation) and the K$\alpha $ emission area is millimeter-scale. Such qualities make these sources useful for high-contrast diffraction and for backlighting large objects in extended-source geometries. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory, through the Institute for Laser Science and Applications, under contract DE-AC52-07NA27344. The authors also acknowledge support from Laboratory Directed Research and Development Grant No. 08-LW-004. [Preview Abstract] |
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PP6.00115: Design of Asymmetrically driven hohlraum experiments on OMEGA Stuart McAlpin, Mark Stevenson, Kelly Vaughan, John Foster, Mark Taylor A campaign of experiments is planned on the OMEGA laser to asymmetrically drive an imploding capsule within a hohlraum. This will act as a stringent test of the modeling of both the conditions inside the hohlraum and the evolution of complex hydrodynamic systems. These experiments will be modelled using a two step approach. A pure Lagrangian code linked to an Eulerian code is used to capture the late-time hydrodynamics and a single-step ALE (Arbitrary Lagrangian Eulerian) code is used as a fully integrated test. A number of techniques have been identified which potentially offer significant control of both the spatial and temporal asymmetry of the drive on the capsule. These will be tested systematically in two stages. The drive as a function of position and time will be measured by following the ablation front in aerogel spheres. The effect of the drive on the late time hydrodynamics will be tested using the implosion and potential jet formation in GDP coated glass capsules. In both cases the evolution of the configuration will be determined using titanium area backlighting at 4.7 keV combined with a gated x-ray imaging system. [Preview Abstract] |
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PP6.00116: Hot electron conversion physics in extremely high intensity laser matter interactions Brian Chrisman, Yasuhiko Sentoku, Andreas Kemp The Fast Ignition experiment relies on core heating due to high energy conversion from an ignition laser to hot electrons, particularly hot electrons in the few MeV energy range. In a previous investigation [Chrisman et. al., PoP2008], high populations of MeV electrons in super intense regimes ($>10^{20}$W/cm$^2$) were observed and conversion efficiency was found to scale with intensity. Further 2D PIC simulations investigate the observed absorption scaling with intensity. In particular, large static magnetic fields (100MG or more) are found in the interaction region. Under these large magnetic fields, the electron's cyclotron frequency could be higher than the laser frequency, causing a novel regime of high absorption due to an effective de-phasing of locally oscillating electrons in the laser fields. [Preview Abstract] |
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PP6.00117: Critical requirements for ignition and burn of impact fast ignition Masakatsu Murakami, Hideo Nagatomo, Tomoyuki Johzaki, Hiroshi Azechi Impact fast ignition (IFI) is a new scheme of inertial confinement fusion research as an alternative to the standard type of fast ignition using petawatt (PW) laser. Although IFI does not need PW laser, it is required to achieve a super-high velocity of the order of 1000 km/s to convert the kinetic energy into the thermal energy corresponding to 5-10 keV, that are enough to occur ignition of the fuel. To clarify the critical requirements of IFI, two-dimensional simulations have been conducted. As a result it has turned out that ignition can be attained at a velocity of 1500 km/s under some accompanied conditions. It is also important to achieve a high in-flight density of 2-3 g/cc, which is expected to undergo further compression due to the geometrical convergence and shock compression. We present a theoretical model to describe such a high compression under high Mach acceleration of the impactor shell. [Preview Abstract] |
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PP6.00118: Fokker-Planck modeling of fast electron transport including hydrodynamic plasma response R.J. Kingham, C.P. Ridgers, R.G. Evans, S.J. Rose, A.P.L. Robinson The fast electron beam required for the fast ignition approach to ICF has to couple $\sim $10kJ of energy from the critical surface to the core in $\sim $10ps. The intense pressure gradients and B-fields generated by the beam should be capable of significantly modifying the density of the background plasma. Over several picoseconds there is sufficient time for ions to be expelled, forming a channel, especially at densities up to and including solid density. Hybrid code simulations have shown that a static density channel can induce hollowing of a fast electron beam [1]. We are developing a new 2-D Vlasov-Fokker-Planck code for simulating fast electron transport in regimes relevant to fast ignition. It uses a spherical harmonic representation of the electron distribution (as used in KALOS and [1]) and self-consistently includes hydrodynamic response of the plasma for the first time. Its kinetic description of all electrons is essential when the beam density approaches the background density and hydro is essential for correctly capturing the dynamics of multi-ps duration beams. Progress on its development will be reported. [1] A. P. L. Robinson \textit{et al.}, Plasma Phys. Control. Fusion \textbf{50}, 065019 (2008) [Preview Abstract] |
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PP6.00119: Laser Channeling in mm-Scale Underdense Plasmas for Fast Ignition C. Ren, G. Li, R. Yan, J. Tonge, T.-L. Wang, W.B. Mori Recent 2D PIC simulations for laser channeling in mm-scale underdense plasmas show many new phenomena that were not present in previous 100 m-scale experiments and simulations, including plasma buildup to above critical density in front of the laser, laser hosing/refraction, channel bifurcation and self-correction, and electron heating to relativistic temperatures [G. Li et al., PRL\textbf{ 100, }125002 (2008)\textbf{.]} The channeling speed is much less than the linear group velocity of the laser. A scaling from the simulations shows that low-intensity channeling pulses are preferred to minimize the required energy. Significant improvement of the transmission of the ignition pulse in a preformed channel has been demonstrated. New 3D PIC simulations show that the channeling speed is larger in 3D than in 2D due to stronger laser self-focusing. This work was supported by the U.S. DoE under Grants No. DE-FG02-06ER54879, DE- FC02-04ER54789, DE-FG52-06NA26195, and DE-FG02-03ER54721. Simulations were carried out at NERSC through an INCITE grant and on the UCLA DAWSON Cluster under Grant No. NSF-Phy-0321345. [Preview Abstract] |
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PP6.00120: Vlasov Simulation of Ion Acceleration in the Field of an Intense Laser Incident on an Overdense Plasma Magdi Shoucri, Mathieu Charbonneau-Lefort, Bedros Afeyan We study the interaction of a high intensity laser with an overdense plasma. When the intensity of the laser is sufficiently high to make the electrons relativistic, unusual interactions between the EM wave and the surface of the plasma take place. We use an Eulerian Vlasov code for the numerical solution of the one-dimensional two-species relativistic Vlasov-Maxwell equations [1]. The results show that the incident laser steepens the density profile significantly. There is a large build-up of electron density at the plasma edge, and as a consequence a large charge separation that is induced under the action of the intense laser field. This results in an intense quasistatic longitudinal electric field generated at the surface of the plasma which accelerates ions in the forward direction. We will show the details of the formation of the longitudinal edge electric field and of electron and ion phase-space structures. [1] M. Charbonneau-Lefort, M. Shoucri, B. Afeyan , Proc. of the EPS Conference, Greece (2008). [Preview Abstract] |
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PP6.00121: Implosion Dynamics of an Advanced Cone-guided Target for Fast Ignition Hideo Nagatomo, Tomoyuki Johzaki, Atsushi Sunahara, Hitoshi Sakagami, Kunioki Mima We have been studied the formation of high-density and high-areal-density core plasma in cone-guided non-spherical implosion for Fast Ignition. Recently, we have proposed an advanced cone-guided target for fast ignition. In the target, the tip of the cone consists of low Z material in order to reduce the scattering hot electrons which are produced inside the cone. We have performed 2-D radiation hydrodynamics simulations, and the results suggest that inner surface of the cone is irradiated by the radiation and gold plasma expand into the cone, if the tip of cone consists of low Z material. This ablated gold plasma may interfere with the heating short pulse laser. To avoid the effect, we have proposed to coat the inner surface of the cone, as well as the outer surface coating [1]. Taking account of the modification, we have designed an advance cone-guided target for FIREX-I experiment which is carried on at ILE Osaka Univ. [1] H. Nagatomo, et. al, Phys. Plasmas, 14 056303 (2007). This work is supported by MEXT, Grant-in Aid for Creative Scientific Research (15GS0214). [Preview Abstract] |
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PP6.00122: PIC simulations of the two-plasmon-decay instabilities for direct-drive ICF R. Yan, A. Maximov, C. Ren, G. Li Preheating of fuel shells due to the hot electrons generated from the two-plasmon-decay (TPD) instability could be a potential source of yield reduction in direct-drive inertial confinement fusion. Using a particle-in-cell (PIC) code OSIRIS, we study the linear growth and saturation of TPD under conditions relevant to experiments on the Omega laser system. We compare the linear growth rates with both theories and a fluid code and study the convection of modes due to density inhomogeneity. The mode saturation was found to be closely correlated to the plasma ion density fluctuations near the quarter-critical surface. The ion density fluctuations are driven by the ponderomotive force of the plasma waves in TPD. We show this by numerically solving the governing ion-acoustic wave equation with a ponderomotive force term from the measured local envelope of the two plasma waves. For I=2$\times$10$^{15}$W/cm$^{2}$, L=100$\mu$m, Te=1kev, the generated hot electrons have 1{\%} of the laser energy and a temperature of $\sim $30kev. [Preview Abstract] |
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PP6.00123: Kinetic Simulations of Stimulated Raman Scattering W. Rozmus, P.-E. Masson-Laborde, Z. Peng, D. Pesme, M. Casanova, S. Hueller, M. Albrecht-Marc, V. Yu. Bychenkov, C. Capjack Results of PIC simulations of the stimulated Raman scattering in one and two spatial dimensions are discussed. 2D simulation results at large kL/kD values are consistent with results by Yin et al. Phys. Plasmas 15, 013109 (2007). However, our interpretation differs in stressing the SRS reflectivity randomness due to frequency shift and transverse modulations of Langmuir waves by (i) Weibel instability due to the current of trapped particles and (ii) trapped particle modulational instability. Randomness due to the frequency shift is responsible for the first saturation of SRS. In 1D PIC simulations in inhomogeneous plasmas we have studied the interplay between trapped particle nonlinear frequency shift and plasma density inhomogeneity. [Preview Abstract] |
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PP6.00124: Assessing the 2$\omega _{pe}$ instability in ignition-scale hohlraums William Kruer, Nathan Meezan, David Strozzi, Edward Williams, Larry Suter, Sean Regan In recent experiments\footnote{S. P. Regan et. al. (submitted to Phys. Rev. Letters)} Sean Regan, \textit{et. al.} for the first time observed the 2$\omega _{pe}$ instability from window plasma in hohlraum targets. In addition, this instability has been predicted to operate near the edge of the inner beams in the ablator plasma and near the edge of the outer beams in the liner plasma. Fortunately only a small fraction of the laser energy was estimated to be at risk. A more quantitative assessment of the energy at risk will here be given. We also explore how strong collisionality restricts this instability in the Au wall plasma. We show that the instability threshold can be significantly reduced for laser beams with an angle of incidence of about 60 degrees due to the swelling of the laser field near its turning point. A simple model is given. It is also shown that for frequently cited plasma conditions, the SRS-scattered light wave can itself drive the 2$\omega _{pe}$ instability. This effect is relevant for the nonlinear saturation of SRS and the resulting heated electron generation. Some estimates are given. [Preview Abstract] |
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PP6.00125: Diagnosing Phase Space coherent structures by the test particle method Mathieu Charbonneau-Lefort, Bedros Afeyan We examine a number of diagnostics which can be deployed to understand the physical basis of coherent structures in phase space such as KEEN Waves [1,2]. We find that the test particle following method (where the self-consistent field is used as well as approximations thereof to follow a large number of particles) can be quite informative. These so called Lagrangian techniques can be used to build the statistical information crucial to understanding the self organization characteristics of KEEN waves and electron plasma waves. Their properties will be examined including the partition of phase space and modal truncation. We will consider VP and VM code generated data with these phase space diagnostics tools. [1] B. Afeyan, et al., Kinetic Electrostatic Electron Nonlinear (KEEN) Waves and their interactions driven by the ponderomotive force of crossing laser beams, Proc. IFSA, (Inertial Fusion Sciences and Applications 2003, Monterey, CA), B. Hammel, D. Meyerhofer, J. Meyer-ter-Vehn and H. Azechi, editors, 213, American Nuclear Society, 2004. [2] B. Afeyan, et al., Dynamically Self-Organized Structures in Vlasov Phase Space: Ponderomotively Driven KEEN Waves, Submitted to PRL, 2008. [Preview Abstract] |
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PP6.00126: Parametric Instabilities in Structured Crossing Laser Beams Bedros Afeyan, Mathieu Charbonneau-Lefort, Andrew Schmitt, Robert Lehmberg We examine the theory of parametric instabilities in structured laser beams. Spatial intensity variations of lasers (speckle patterns or filamented beams) can imprint their fluctuations onto the plasma which then can influence the propagation of other beams which have crossed it. This leads to a theory where one is able to calculate the effective growth rates of parametric instabilities in the presence of multiple crossing laser beams. The effect of plasma inhomogeneity and temporal variations of the laser field patterns due to induced spatial incoherence (ISI) or smoothing by spectral discretion (SSD) are also included. Examples will be drawn from both Direct and Indirect Drive laser fusion target physics. [Preview Abstract] |
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PP6.00127: PIC simulations of stimulated Brillouin scattering driven by white light Bruno Brandao, Luis Silva, Robert Bingham A generalized Wigner-Moyal statistical theory of radiation [1,2] is used to obtain a general dispersion relation for Stimulated Brillouin Scattering (SBS) driven by a spatially stationary radiation field with arbitrary statistics. Our results show that the growth rate of SBS can be reduced by 1/3 for a bandwidth of 0.3 nm, for typical NIF parameters. We also discuss and describe an implementation of the pump wave's bandwidth in the PIC code OSIRIS 2.0 and perform simulations of broadband effects in parametric instabilities, focusing on the dependence of the growth rate of SBS on the intensity and wave number of a broadband pump field. The evolution of the strength of the instability as a function of the bandwidth ($\sigma )$ is also studied, retrieving the theoretically expected 1/$\sigma $~dependence. [1] J. E. Santos and L. O. Silva, J. Math. Phys. \textbf{46}, 102901 (2005) [2] J. E. Santos, L. O. Silva, and R. Bingham, Phys. Rev. Lett. \textbf{98}, 235001 (2007) [Preview Abstract] |
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PP6.00128: pF3d simulations of nonlinear backward stimulated Raman scatter in a multi-speckle environment E.S. Dodd, B. Bezzerides, D.F. DuBois, H.X. Vu Kinetic simulations of backward stimulated Raman scattering (BSRS) have shown that, in regimes of strong Landau damping of the BSRS Langmuir wave (LW), the reflectivity can exceed that predicted by linear analysis [1]. This is a result of electron trapping in the LW, which decreases Landau damping, and creates a frequency shift. Above a threshold, determined by the competition of trapping and collisional diffusion, the frequency shift becomes the dominant saturation mechanism for BSRS. This includes the transverse modulational instability [2]. However, one must use a code that models the nonlinear microscopic behavior along with the macroscopic evolution of the laser beam and background plasma. Here, we discuss work on implementing an empirical model for this effect in the pF3d code [3]. The model has been tested by comparing pF3d single-hot-spot simulations against theoretical calculations of the inflation threshold. We will discuss our current effort, using pF3d, to understand how the onset of nonlinear LW behavior is affected by inter-speckle interactions. [1] H. X. Vu, et al., \textit{Phys. Plasmas} \underline {14} 012702 (2007). [2] H. A. Rose, and L. Yin, \textit{Phys. Plasmas} \underline {15} 042311 (2008). [3] R. L. Berger, et al., \textit{Phys Plasmas} \underline {5} 4337 (1998). [Preview Abstract] |
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PP6.00129: SuperGaussian distribution functions in inhomogenous plasmas Jean-Pierre Matte In plasmas heated by a narrow laser beam, the shape of the distribution function is influenced by both the absorption, which tends to give a superGaussian (DLM) distribution function [1], and the effects of heat flow, which tends to make the distribution more Maxwellian, when the hot region is considerably wider than the laser beam [2]. Thus, it is only at early times that the deformation is as strong as predicted by our uniform intensity formula [1]. A large number of electron kinetic simulations of a finite width laser beam heating a uniform density plasma were performed with the electron kinetic code FPI [1] to study the competition between these two mechanisms. In some cases, the deformation is approximately given by this formula if we average the laser intensity over the entire plasma. This may explain why distributions were more Maxwellian than expected in some experiments [3]. \\[0pt] [1] J.-P. Matte et al., Plasma Phys. Contr. Fusion 30, 1665 (1988) [2] S. Brunner and E. Valeo, Phys. Plasmas 9, 923 (2002) [3] S.H. Glenzer et al., Phys. Rev. Lett. 82, 97 (1999). [Preview Abstract] |
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PP6.00130: Plasma Wave Packets in Density Gradients in 1D and 2D PIC Simulations Jay Fahlen, B. Winjum, T. Grismayer, V. Decyk, F.S. Tsung, J. Tonge, W. Mori Fully self-consistent simulations of Stimulated Raman Scattering in one and two dimensions indicate that the finite extent of the plasma wave, both longitudinally and transversely, can strongly affect the saturation, convection, and recurrence of the instability. Here we present 1D and 2D electrostatic, externally driven particle-in-cell simulation results that help to understand the effect of spatial localization and density gradients on plasmons. As plasmon packets convect, the rear edge of the packet damps as new particles trap and phase mix, leading to `etching' of the packet rear. In SRS, the recurrence rate depends on this velocity and will increase for increasing etching rates. A simple model accurately predicts the etching rate observed in the simulations. In two dimensions, trapped and detrapped particle effects and transverse amplitude variation lead to localization and the eventual destruction of the wave. A suite of new diagnostics, including particle tracking, and energy fluxes (ES Poynting flux), brings greater understanding to these effects. [Preview Abstract] |
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PP6.00131: Simulations of Stimulated Raman Backscatter B.J. Winjum, J.E. Fahlen, F.S. Tsung, W.B. Mori In fully self-consistent particle-in-cell (PIC) simulations, the onset and saturation of Stimulated Raman Scattering (SRS) is quite complicated. Fully kinetic simulations (both PIC and Vlasov) have shown that inflation, nonlinear frequency shifts, trapped particle sideband instabilities, beam modes, pump depletion, plasma wave convection, rescatter, plasma length, and ion motion all play a role. We have performed an extensive array of simulations using the PIC code OSIRIS over the past several years, looking at these effects in detail. This poster will summarize the results. We will show how SRS onset is dependent on the length available for gain, how saturation depends on both the frequency shifts and pump depletion (the relative amount depends on parameters), how sidebands and beam modes affect the electron phasespace and grow significantly only after saturation, how saturated plasma waves convect as a packet and lead to Raman scatter at a shifted frequency, and how recurrence is affected by convection, shifted frequencies, and pump depletion. [Preview Abstract] |
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PP6.00132: Two- and Three-Dimensional Kinetic Simulations of Stimulated Raman Scatter in NIF Ignition Conditions B. Langdon, B. Still, D. Hinkel, B. Lasinski, D. Strozzi, E. Williams Multidimensional simulations of stimulated Raman scatter (SRS) are performed in plasma conditions derived from the region identified in pF3d simulations as the principal site of SRS activity. These may be the first large speckle-scale particle-in-cell simulations of SRS in NIF ignition conditions, in this case for a 285 eV point design: $n_e = 10^{21}$/cm$^3$, T$_e$ = 2 keV, $\lambda_0$ = 0.351 $\mu$m, speckle intensity $I\sim10^{15}$ W/cm$^2$. A principal finding is that the Langmuir transverse modulational instability (1) plays a major role in limiting SRS in ignition targets by wavefront bowing and breakup, disrupting the spatial coherence of the Langmuir waves that reflects light. This effect was previously demonstrated in simulations (2) under quite different conditions, with lower T$_e$ and higher $I\lambda_0^2$. We analyze its energetic significance and the heated electron distributions in the context of NIF ignition. (1) H. Rose, Phys. Plasmas 12, 012318 (2005). (2) L. Yin, B. Albright, K. Bowers, W. Daughton, and H. Rose, Phys. Rev. Lett. 99, 265004 (2007); Phys. Plasmas 15, 013109 (2008). [Preview Abstract] |
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PP6.00133: Particle-in-Cell Simulations of the 2$\omega_p$/HFHI Instability Frank Tsung, B.B. Afeyan, W.B. Mori A particle-in-cell code OSIRIS is used to investigate the two plasmon instability in nonuniform plasmas using various density profiles relevant to inertial confinement fusion (ICF). Our simulations show that for systems not too far above threshold, the agreements between theory and simulations are excellent. Furthermore, for ICF relevant systems, the high-frequency hybrid instability (HFHI), where one of the daughter waves can have mixed polarization, is important. Building on these experiences, we have begun to investigate nonlinear effects on a longer timescale, such as the saturation mechanism, the spectrum of the fast electrons at saturation, the relaxation and recurrence of the instability, and ion effects. [Preview Abstract] |
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