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Ultrasfast Dynamics in Dense Hydrogen Explored at Flash

Description: The short pulse duration and high intensity of the FLASH (Free-electron LASer in Hamburg) allows us to generate and probe homogeneous warm dense non-equilibrium hydrogen within a single extreme ultraviolet (EUV) light pulse. By analyzing the spectrum of the 13.5 nm Thomson scattered light we determine the plasma temperature and density. We find that classical models of this interaction are in good agreement with our dense plasma conditions. In a FEL-pump FEL-probe experiment droplets of liquid hydrogen and their scattering behavior for different pump-probe setups were observed under 20{sup o} and 90{sup o}. We find that the scattering behavior of the scattered intensity depends on the scattering angle.
Date: August 1, 2011
Creator: Hilbert, V; Zastrau, U; Neumayer, P; Hochhaus, D; Toleikis, S; Harmand, M et al.
Partner: UNT Libraries Government Documents Department

Ignition Failure Mode Radiochemical Diagnostics Initial Assessment

Description: Radiochemical diagnostic signatures are well known to be effective indicators of nuclear ignition and burn reaction conditions. Nuclear activation is already a reliable technique to measure yield. More comprehensively, though, important quantities such as fuel areal density and ion temperature might be separately and more precisely monitored by a judicious choice of select nuclear reactions. This report details an initial assessment of this approach to diagnosing ignition failures on point-design cryogenic National Ignition Campaign targets. Using newly generated nuclear reaction cross section data for Scandium and Iridium, modest uniform doping of the innermost ablator region provides clearly observable reaction product differences between robust burn and failure for either element. Both equatorial and polar tracer loading yield observable, but indistinguishable, signatures for either choice of element for the preliminary cases studied.
Date: April 20, 2007
Creator: Fortner, R; Bernstein, L; Cerjan, C; Haan, S W; Harding, R; Hatchett, S et al.
Partner: UNT Libraries Government Documents Department

Magnetic stochasticity in gyrokinetic simulations of plasma microturbulence

Description: Analysis of the magnetic field structure from electromagnetic simulations of tokamak ion temperature gradient turbulence demonstrates that the magnetic field can be stochastic even at very low plasma pressure. The degree of magnetic stochasticity is quantified by evaluating the magnetic diffusion coefficient. We find that the magnetic stochasticity fails to produce a dramatic increase in the electron heat conductivity because the magnetic diffusion coefficient remains small.
Date: February 12, 2010
Creator: Nevins, W M; Wang, E & Candy, J
Partner: UNT Libraries Government Documents Department

Imaging with Spherically Bent Crystals or Reflectors

Description: This paper consists of two parts: Part I describes the working principle of a recently developed x-ray imaging crystal spectrometer, where the astigmatism of spherically bent crystals is being used with advantage to record spatially resolved spectra of highly charged ions for Doppler measurements of the ion-temperature and toroidal plasmarotation- velocity profiles in tokamak plasmas. This type of spectrometer was thoroughly tested on NSTX and Alcator C-Mod, and its concept was recently adopted for the design of the ITER crystal spectrometers. Part II describes imaging schemes, where the astigmatism has been eliminated by the use of matched pairs of spherically bent crystals or reflectors. These imaging schemes are applicable over a wide range of the electromagnetic radiation, which includes microwaves, visible light, EUV radiation, and x-rays. Potential applications with EUV radiation and x-rays are the diagnosis of laserproduced plasmas, imaging of biological samples with synchrotron radiation, and lithography.
Date: June 1, 2010
Creator: Bitter, M.; Hill, K. W.; Scott, S.; Ince-Cushman, A.; Reinke, M.; Podpaly, Y. et al.
Partner: UNT Libraries Government Documents Department

INTEGRATED NEUTRAL BEAM MEASUREMENTS IN A TOKAMAK ENVIRONMENT

Description: Tokamaks are, in many respects, the most promising avenue for the development of fusion power. The continual improvement in the performance of these devices and their understanding of them is due in greater measure to the development of accurate plasma diagnostics. Many of the most crucial measurements required to assess their progress on these experiments are based in one way or another upon collisional interactions of injected neutral beams with the plasma. These measurements include such fundamental parameters as the ion temperature, rotation, and density profiles, electric and magnetic field structure, and local studies of the plasma turbulent transport. Maximizing the obtained information for a given geometry of plasma, beams, and possible viewchords represents an interesting challenge to the experimentalist. Advances in detector and analysis techniques allow them to take full advantage of the beam/plasma emission for these measurements.
Date: September 1, 2000
Creator: THOMAS, D.M.
Partner: UNT Libraries Government Documents Department

Improvements in the CHERS system for DT experiments on TFTR

Description: Improvements in the charge exchange recombination spectroscopy (CHERS) system have resulted in accurate measurements of T{sub i} and V{sub {phi}} profiles during DT experiments. These include moving the spectrometer detector array and electronics farther away from the tokamak to a low neutron flux location. This relocation has also improved access to all components of the system. Also, a nonplasma-viewing calibration fiber system was added to monitor the change in fiber transmission due to the high flux DT neutrons. Narrowband filtered light transmitted through the calibration fiber is now used as a reference for the VO measurement. At the highest neutron flux of {approximately} 2.5 {times} 10{sup 18} neutrons/see (fusion power {approximately} 6.2 MW) a modest 5% decrease in fiber transmission was observed. Corrections for transmission loss are made and T{sub i} (r,t) and absolute V{sub phi} (r,t) profiles are automatically calculated within four minutes of every shot.
Date: March 1, 1995
Creator: Bush, C.E.; Bell, R. & Synakowski, E.J.
Partner: UNT Libraries Government Documents Department

Simulation of ion-temperature-gradient turbulence in tokamaks

Description: Results are presented from nonlinear gyrokinetic simulations of toroidal ion temperature gradient (ITG) turbulence and transport. The gyrokinetic simulations are found to yield values of the thermal diffusivity significantly lower than gyrofluid or IFS-PPPL-model predictions. A new phenomenon of nonlinear effective critical gradients larger than the linear instability threshold gradients is observed, and is associated with undamped flux-surface-averaged shear flows. The nonlinear gyrokineic codes have passed extensive validity tests which include comparison against independent linear calculations, a series of nonlinear convergence tests, and a comparison between two independent nonlinear gyrokinetic codes. Our most realistic simulations to date have actual reconstructed equilibria from experiments and a model for dilution by impurity and beam ions. These simulations highlight the need for still more physics to be included in the simulations
Date: October 14, 1998
Creator: Cohen, B I; Dimits, A M; Kim, C; Mattor, N; Nevins, W M; Parker, S E et al.
Partner: UNT Libraries Government Documents Department

Shear flow effects on ion thermal transport in tokamaks

Description: From various laboratory and numerical experiments, there is clear evidence that under certain conditions the presence of sheared flows in a tokamak plasma can significantly reduce the ion thermal transport. In the presence of plasma fluctuations driven by the ion temperature gradient, the flows of energy and momentum parallel and perpendicular to the magnetic field are coupled with each other. This coupling manifests itself as significant off-diagonal coupling coefficients that give rise to new terms for anomalous transport. The authors derive from the gyrokinetic equation a set of velocity moment equations that describe the interaction among plasma turbulent fluctuations, the temperature gradient, the toroidal velocity shear, and the poloidal flow in a tokamak plasma. Four coupled equations for the amplitudes of the state variables radially extended over the transport region by toroidicity induced coupling are derived. The equations show bifurcations from the low confinement mode without sheared flows to high confinement mode with substantially reduced transport due to strong shear flows. Also discussed is the reduced version with three state variables. In the presence of sheared flows, the radially extended coupled toroidal modes driven by the ion temperature gradient disintegrate into smaller, less elongated vortices. Such a transition to smaller spatial correlation lengths changes the transport from Bohm-like to gyrobohm-like. The properties of these equations are analyzed. The conditions for the improved confined regime are obtained as a function of the momentum-energy deposition rates and profiles. The appearance of a transport barrier is a consequence of the present theory.
Date: March 1, 1995
Creator: Tajima, T.; Horton, W.; Dong, J.Q. & Kishimoto, Y.
Partner: UNT Libraries Government Documents Department

Experimental Study of Ion Heating and Acceleration During Magnetic Reconnection

Description: Ion heating and acceleration has been studied in the well-characterized reconnection layer of the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)]. Ion temperature in the layer rises substantially during null-helicity reconnection in which reconnecting field lines are anti-parallel. The plasma out flow is sub-Alfvonic due to a downstream back pressure. An ion energy balance calculation based on the data and including classical viscous heating indicates that the ions are heated largely due to non-classical mechanisms. The Ti rise is much smaller during co-helicity reconnection in which field lines reconnect obliquely. This is consistent with a slower reconnection rate and a smaller resistivity enhancement over the Spitzer value. These observations indicate strongly that non-classical dissipation mechanisms can play an important role both in heating the ions and in facilitating the reconnection process.
Date: October 24, 2000
Creator: Hsu, S.C.; Carter, T.A.; Fiksel, G.; Ji, H.; Kulsrud, R.M. & Yamada, M.
Partner: UNT Libraries Government Documents Department

Thomson scattering from laser plasmas

Description: Thomson scattering has recently been introduced as a fundamental diagnostic of plasma conditions and basic physical processes in dense, inertial confinement fusion plasmas. Experiments at the Nova laser facility [E. M. Campbell et al., Laser Part. Beams 9, 209 (1991)] have demonstrated accurate temporally and spatially resolved characterization of densities, electron temperatures, and average ionization levels by simultaneously observing Thomson scattered light from ion acoustic and electron plasma (Langmuir) fluctuations. In addition, observations of fast and slow ion acous- tic waves in two-ion species plasmas have also allowed an independent measurement of the ion temperature. These results have motivated the application of Thomson scattering in closed-geometry inertial confinement fusion hohlraums to benchmark integrated radiation-hydrodynamic modeling of fusion plasmas. For this purpose a high energy 4{omega} probe laser was implemented recently allowing ultraviolet Thomson scattering at various locations in high-density gas-filled hohlraum plasmas. In partic- ular, the observation of steep electron temperature gradients indicates that electron thermal transport is inhibited in these gas-filled hohlraums. Hydrodynamic calcula- tions which include an exact treatment of large-scale magnetic fields are in agreement with these findings. Moreover, the Thomson scattering data clearly indicate axial stagnation in these hohlraums by showing a fast rise of the ion temperature. Its timing is in good agreement with calculations indicating that the stagnating plasma will not deteriorate the implosion of the fusion capsules in ignition experiments.
Date: January 12, 1999
Creator: Moody, J D; Alley, W E; De Groot, J S; Estabrook, K G; Glenzer, S H; Hammer, J H et al.
Partner: UNT Libraries Government Documents Department

Study of micro-instabilities in toroidal plasmas with negative magnetic shear

Description: The micro-instabilities driven by a parallel velocity shear, and a temperature gradient of ions are studied in toroidal plasmas with negative magnetic shear. Both the fluid and the gyro-kinetic formulations are investigated. It is found that for a broad range of parameters, the linear growth rates of the modes are lower, and the threshold temperature gradient {eta}{sub icr} is higher for plasmas with negative magnetic shear compared to plasmas with positive magnetic shear of equal magnitude. The reduction in the growth rate (with negative shear), although not insignificant, does not seem to be enough to account for the dramatic improvement in the confinement observed experimentally. Other possible physical mechanisms for the improved confinement are discussed.
Date: March 1, 1996
Creator: Dong, J.Q.; Zhang, Y.Z.; Mahajan, S.M. & Guzdar, P.N.
Partner: UNT Libraries Government Documents Department

Thermal effects in intense laser-plasma interactions

Description: We present an overview of a new warm fluid model that incorporates leading-order kinetic corrections to the cold fluid model without making any near-equilibrium assumptions. In the quasi-static limit we obtain analytical expressions for the momentum spread and show excellent agreement with solutions of the full time-dependant equations. It is shown that over a large range of initial plasma temperatures, the fields are relatively insensitive to the pressure force. We discuss implications of this work for model validation.
Date: October 22, 2004
Creator: Shadwick, B.A.; Tarkenton, G.M. & Esarey, E.H.
Partner: UNT Libraries Government Documents Department

TRANSPORT FROM OVERLAPPING ELECTRON AND ION DRIFTWAVE INSTABILITIES

Description: The electron temperature gradient (ETG) mode is a likely contributor to electron thermal transport in tokamaks. The ETG modes are dominantly unstable for poloidal wavelengths shorter than the ion gyroradius (high-k) where the ion response is adiabatic. Thus, they do not directly produce ion thermal or momentum transport or particle transport. Two potential mechanisms whereby ETG modes could produce transport in these channels are explored in this paper: a nonlinear coupling between high-k ETG modes and ions at low-k and a direct coupling when ETG modes and ion temperature gradient (ITG) modes are unstable in overlapping wavenumber ranges. It will be shown that the particle and momentum transport required to match experiment is small compared to the ETG driven electron thermal transport. Even quasilinearly ETG modes can produce ion transport if the ITG and ETG modes are both unstable at low-k. The implications of this for transport will be explored at the quasilinear level. A new gyro-Landau-fluid (GLF) closure model has been constructed in order to build a transport model which can include the coupling between electron and ion modes including trapped particles. The first growth rate spectra from this model will be shown to give an accurate approximation to the kinetic linear growth rates of drift-ballooning modes in tokamaks.
Date: July 2, 2004
Creator: STAEBLER,G.M; KINSEY,J.E & WALTZ,R.E
Partner: UNT Libraries Government Documents Department

Fluctuation Measurements in Tokamaks with Microwave Imaging Reflectometry

Description: To study the mechanism of anomalous transport in tokamaks requires the use of sophisticated diagnostic tools for the measurement of short-scale turbulent fluctuations. In this article, we describe an attempt at developing a technique capable of providing a comprehensive description of plasma fluctuations with k(subscript parallel rho i) < 1, such as such as those driven by the Ion Temperature Gradient mode in tokamaks. The proposed method is based on microwave reflectometry, and stems from a series of numerical calculations showing that the spatial structure of fluctuations near the cutoff could be obtained from the phase of reflected waves when these are collected with a wide aperture optical system forming an image of the cutoff onto an array of phase sensitive detectors. Preliminary measurements with a prototype apparatus on the Torus Experiment for Technology Oriented Research 94 (TEXTOR-94) [U. Samm, Proceedings of the 16th IEEE Symposium on Fusion Engineering, 1995 (IEEE, Piscata way, NJ, 1995), p. 470] confirm the validity of these conclusions. Technical issues in the application of the proposed technique to tokamaks are discussed in this article, and the conceptual design of an imaging reflectometer for the visualization of turbulent fluctuations in the National Spherical Torus Experiment (NSTX) [M. Ono, et al., Nucl. Fusion 40, 557 (2000)] is described.
Date: December 3, 2001
Creator: Mazzucato, E.; Munsat, T.; Park, H.; Deng, B.H.; Domier, C.W.; N.C. Luhmann, Jr. et al.
Partner: UNT Libraries Government Documents Department

Size Scaling of Turbulent Transport in Magnetically Confined Plasmas

Description: Transport scaling with respect to device size in magnetically confined plasmas is critically examined for electrostatic ion temperature gradient turbulence using global gyrokinetic particle simulations. It is found, by varying device size normalized by ion gyroradius while keeping other dimensionless plasma parameters fixed, that fluctuation scale length is microscopic in the presence of zonal flows. The local transport coefficient exhibits a gradual transition from a Bohm-like scaling for device sizes corresponding to present-day experiments to a gyro-Bohm scaling for future larger devices.
Date: April 5, 2002
Creator: Lin, Z.; Ethier, S.; Hahm, T.S. & Tang, W.M.
Partner: UNT Libraries Government Documents Department

EXB-Drift, Current, and Kinetic Effects on Divertor Plasma Profiles During ELMs

Description: The transient heat load on divertor surfaces from Edge-Localized Modes (ELMs) in tokamaks can be very large and thus of concern for a large device such as ITER. Models for kinetic modifications to fluid models are discussed that should allow them to reasonably describe the long mean-free path regime encountered owing to the high electron and ion temperatures in the SOL during large ELMs. A set of two-dimensional (2D) simulations of the dynamic response of the scrape-off layer (SOL) plasma to an ELM is presented. The role of plasma currents and E x B motion is emphasized, which cause large changes in the response compared to models neglecting them.
Date: May 23, 2002
Creator: Rognlien, T.D. & Shimada, M.
Partner: UNT Libraries Government Documents Department

Development Of a Spatially Resolving X-ray Crystal Spectrometer For Measurement Of Ion-temperature (Ti) And Rotation-velocity (v) Profiles in ITER

Description: Imaging x-ray crystal spectrometer #2;XCS#3; arrays are being developed as a US-ITER activity for Doppler measurement of Ti and v profiles of impurities #2;(W, Kr, and Fe)#3; with ~#4;7 cm (a/30)#3; and 10-100 ms resolution in ITER. The imaging XCS, modeled after a prototype instrument on Alcator C-Mod, uses a spherically bent crystal and 2D x-ray detectors to achieve high spectral resolving power (E / dE >#2;6000)#3; horizontally and spatial imaging vertically. Two arrays will measure Ti and both poloidal and toroidal rotation velocity profiles. The measurement of many spatial chords permits tomographic inversion for the inference of local parameters. The instrument design, predictions of performance, and results from C-Mod are presented.
Date: December 15, 2010
Creator: Hill, K W; Delgado-Aparico, L; Johnson, David; Feder, R; Beiersdorfer, P; Dunn, James et al.
Partner: UNT Libraries Government Documents Department

OBSERVATION OF ANOMALOUSLY BROAD GAUSSIAN HeII 4686A SPECTRAL LINES IN TORMAC

Description: Gaussian Heii 4686{angstrom} spectral lines having full widths at half maximum of over 2{angstrom} have been observed in the Tormac plasmas. Interpretation of these widths as due to Doppler broadening would give ion temperatures of > 100eV, in contradiction with other diagnostics. Indications are that these widths are not simply explained by either Doppler broadening or Stark broadening due to interparticle fields. Some evidence exists of nonthermal turbulent conditions, which could broaden the lines by the Stark effect. if there exist large enough electric fields, or by the Doppler effect, if there is appreciable mass motion.
Date: March 1, 1980
Creator: Shaw, R.S.; Coonrod, J.; Greenwald, M.; Levine, M.A.; Myers, B.R. & Vella, M.C.
Partner: UNT Libraries Government Documents Department

Letter: Meyerhofer

Description: This letter confirms that the Laboratory for Laser Energetics (LLE) was an important part of the FY10 NIF Polar Drive Exploding Pusher experiments on the National Ignition Facility (NIF). These experiments were designed by LLE to produce requested neutron yields to calibrate and qualify nuclear diagnostics. LLE built a deuterium-tritium filling system for the glass shells and provided them to LLNL for mounting. In FY10, four exploding pusher implosions were performed with measured neutron yields within a factor of two of requested and ion temperatures within 20% of requested. These implosions are proving to be an ideal platform for commissioning the nuclear diagnostic suite on the NIF and are achieving all of the objectives planned for this campaign.
Date: January 6, 2011
Creator: Mackinnon, A J
Partner: UNT Libraries Government Documents Department

Studies on Neutral Beam Injection into the SSPX Spheromak Plasma

Description: In the Sustained Spheromak Physics Experiment, (SSPX) ['Improved operation of the SSPX spheromak', R.D. Wood, D.N. Hill, E.B. Hooper, S. Woodruff1, H.S. McLean and B.W. Stallard, Nucl. Fusion 45 1582-1588 (2005)], plasmas with core electron temperatures reaching up to 500 eV at densities of 10{sup 20}/m{sup 3} have been sustained for several milliseconds, making them suitable as targets for neutral beam injection. High performance and further progress in understanding Spheromak plasma physics are expected if neutral beams are injected into the plasma. This paper presents the results of numerical 1.5 D modeling of the plasma to calculate neutral beam current drive and ion and electron heating. The results are presented for varying initial conditions of density, temperatures and profiles and beam energy, injection angle and power. Current drive efficiency (Ampere/Watt of absorbed power) of up to 0.08 can be achieved with best performance SSPX shots as target. Analyses of neutral beam heating indicate that ion temperatures of up to 1.5 keV and electron temperatures of up to 750 eV can be obtained with injection of about 1 MW of neutral beam for 5-10 ms and with diffusivities typically observed in SSPX. Injection targeting near the magnetic axis appears to be the best for heating and current drive. Effect of the current drive and evolution of SSPX equilibrium are discussed.
Date: October 19, 2007
Creator: Jayakumar, R; Pearlstein, L D; Casper, T A; Fowler, T K; Hill, D N; Hudson, B et al.
Partner: UNT Libraries Government Documents Department

Passive Spectroscopy Bolometers, Grating- And X-Ray Imaging Crystal Spectrometers

Description: This tutorial gives a brief introduction into passive spectroscopy and describes the working principles of bolometers, a high-resolution grating spectrometer, and a novel X-ray imaging crystal spectrometer, which is of particular interest for profile measurements of the ion temperature and plasma rotation velocity on ITER and future burning plasma experiments.
Date: November 7, 2007
Creator: Bitter, M; Hill, K W; Scott, S; Paul, S; Ince-Cushmann, A; Reinke, M et al.
Partner: UNT Libraries Government Documents Department

Scrape-Off Layer Transport and Deposition Studies in DIII-D

Description: Trace {sup 13}CH{sub 4} injection experiments into the main scrape-off layer of low density L-mode and high-density H-mode plasmas have been performed in the DIII-D tokamak [Luxon{_}NF02] to mimic the transport and deposition of carbon arising from a main chamber sputtering source. These experiments indicated entrainment of the injected carbon in plasma flow in the main SOL, and transport toward the inner divertor. Ex-situ surface analysis showed enhanced {sup 13}C surface concentration at the corner formed by the divertor floor and the angled target plate of the inner divertor in L-mode; in H-mode, both at the corner and along the surface bounding the private flux region inboard of the outer strike point. Interpretative modeling was made consistent with these experimental results by imposing a parallel carbon ion flow in the main SOL toward the inner target, and a radial pinch toward the separatrix. Predictive modeling carried out to better understand the underlying plasma transport processes suggests that the deuterium flow in the main SOL is related to the degree of detachment of the inner divertor leg. These simulations show that carbon ions are entrained with the deuteron flow in the main SOL via frictional coupling, but higher charge state carbon ions may be suspended upstream of the inner divertor X-point region due to balance of the friction force and the ion temperature gradient.
Date: October 27, 2006
Creator: Groth, M; Allen, S; Boedo, J; Brooks, N; Elder, J; Fenstermacher, M et al.
Partner: UNT Libraries Government Documents Department

Steady State Turbulent Transport in Magnetic Fusion Plasmas

Description: For more than a decade, the study of microturbulence, driven by ion temperature gradient (ITG) drift instabilities in tokamak devices, has been an active area of research in magnetic fusion science for both experimentalists and theorists alike. One of the important impetus for this avenue of research was the discovery of the radial streamers associated the ITG modes in the early nineties using a Particle-In-Cell (PIC) code. Since then, ITG simulations based on the codes with increasing realism have become possible with the dramatic increase in computing power. The notable examples were the demonstration of the importance of nonlinearly generated zonal flows in regulating ion thermal transport and the transition from Bohm to GyroBoham scaling with increased device size. In this paper, we will describe another interesting nonlinear physical process associated with the parallel acceleration of the ions, that is found to play an important role for the steady state turbulent transport. Its discovery is again through the use of the modern massively parallel supercomputers.
Date: December 20, 2007
Creator: W.W. Lee, S. Ethier, R. Kolesnikov, W.X. Wang, and W.M. Tang
Partner: UNT Libraries Government Documents Department

Development of a Spatially Resolving X-Ray Crystal Spectrometer (XCS) for Measurement of Ion-Temperature (Ti) and Rotation-Velocity (v) Profiles in ITER

Description: Imaging XCS arrays are being developed as a US-ITER activity for Doppler measurement of Ti and v profiles of impurities (W, Kr, Fe) with ~7 cm (a/30) and 10-100 ms resolution in ITER. The imaging XCS, modeled after a PPPL-MIT instrument on Alcator C-Mod, uses a spherically bent crystal and 2d x-ray detectors to achieve high spectral resolving power (E/dE>6000) horizontally and spatial imaging vertically. Two arrays will measure Ti and both poloidal and toroidal rotation velocity profiles. Measurement of many spatial chords permits tomographic inversion for inference of local parameters. The instrument design, predictions of performance, and results from C-Mod will be presented.
Date: May 21, 2010
Creator: Hill, K. W.; Delgado-Aprico, L.; Johnson, D.; Feder, R.; Beiersdorfer, P.; Dunn, J. et al.
Partner: UNT Libraries Government Documents Department