Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session NP8: Poster Session V: MHD and Waves; Inertial Confinement Fusion II and HEDP Diagnostics; C-Mod Tokamak; ITER and Magnetic Fusion Development |
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Room: Rosen Centre Hotel Grand Ballroom, 9:30am - 12:30pm |
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NP8.00001: MHD AND WAVES |
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NP8.00002: 2D Structure of RWM Plasma David Hannum, G. Fiksel, C.B. Forest, R.D. Kendrick The rotating wall machine is a linear screw-pinch built to study the role of different wall boundary conditions on the resistive wall mode (RWM). Its plasma is created by a hexagonal array of electrostatic guns. The central seven guns can be biased to discharge up to 1 kA of current. A probe inserted from the opposite end of the chamber combines magnetic pickup loops with singletip Langmuir electrodes. The loop signals are electronically integrated for a current measurement, while the singletip analysis employs a multivariable fitting routine on the I-V curve to derive the traditional Langmuir measurements. The probe can move over the length of the 1.2-meter long plasma column for 2D profiles of $q, T_e, n_e$ and $V_p$ in $r$ and $z$. Individual gun streams are seen to coalesce into a single plasma column, with density peaking in the center even as the plasma edge spreads out to the wall. [Preview Abstract] |
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NP8.00003: Identification of the resistive wall mode in the rotating wall machine W.F. Bergerson, C.B. Forest, G. Fiksel, D. Hannum, R. Kendrick, S. Olivia, J.S. Sarff The MHD stability properties of a line-tied plasma have been studied in a linear screw pinch device. Both an internal and external kink instability are observed to grow when the safety factor $q = \frac{4\pi^2 r^2 B_z}{\mu_0 I_p(r) L}$ approaches 1 inside the plasma. The growth rate of the internal kink is independent of the wall time, while the external kink growth scales with the wall time, as predicted by theory. After a brief growth phase, the modes saturate and create a helical equilibrium. Evidence of the internal modes suppressing the external kink will be presented. The main diagnostics for characterizing the MHD activity are a 2D array of 80 radial magnetic field pickup coils surrounding the plasma column, a segmented anode, which serves to measure current distribution inside the plasma, and an array of 40 poloidal and axial magnetic field coils just inside the conducting shell. These sensors identify a dominant poloidal mode m=1 and axial mode n=1 kink structure. This work was supported by the DoE. [Preview Abstract] |
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NP8.00004: Engineering design and construction of a spinning conducting shell to stabilize the RWM in a cylindrical, line-tied screw pinch Roch Kendrick, Cary Forest The primary goal of the rotating wall machine is to demonstrate the stabilization of the resistive wall mode using rotating metal walls. This poster describes the design and construction of a spinning shell for the rotating wall machine. The plasma is a 1 meter long, 10 cm radius cylindrical plasma column that has recently shown the existence of a resistive wall mode for sufficiently large currents. The plasma is surrounded by two shells: one is a 0.5 mm thick, stationary shell at the plasma boundary (with a shell time of 7 ms); and the second is a 0.5 mm thick shell at 12 cm radius, mounted inside a carbon fiber spinning tube. The carbon fiber tube is in turn supported by foil bearings and driven by an air turbine, technologies which should easily allow the shell to spin at rotation frequencies up to 100 Hz. These frequencies should be adequate for demonstrating the stabilization of the MHD. [Preview Abstract] |
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NP8.00005: The effect of finite parallel thermal conductivity on pressure-induced magnetic islands in three-dimensional equilibria M. Schlutt, C.C. Hegna A boundary layer analysis is used to investigate the formation of pressure-induced magnetic islands in three dimensional stellarator equilibria using a resistive MHD model. Previous analytic calculations have assumed effectively infinite heat conduction along the magnetic field lines. However, if the islands are sufficiently small in width, there is a competition between parallel and perpendicular transport processes due to the very long distance path along the magnetic field in the island region. The present calculation revisits the analytic island work using expressions for the pressure profile that account for the effect of finite parallel heat conductivity. The resulting analysis aims to provide a unifying theory for pressure-induced islands in three-dimensional equilibria encompassing both extreme limits of the parallel transport processes in the island region. [Preview Abstract] |
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NP8.00006: Experimental Study of Driven Magnetic Relaxation in a Laboratory Plasma S. Hsu, T. Madziwa-Nussinov, D. Sirajuddin, M. Light The physics goal of the Driven Relaxation Experiment (DRX) is to form and sustain simply-connected driven-relaxed plasmas above the first Jensen-Chu linear resonance of the force-free equation (where the first resonance is the ``flipped'' Rosenbluth- Bussac spheromak). As shown recently by Tang \& Boozer (PRL, 2005), the linear resonances are regularized in partially relaxed systems, thus removing the energy barriers which have been thought to constrain relaxed states to exist only below the first resonance. DRX will apply an ``over-driven'' boundary condition at the coaxial gun source, with $\lambda_{\rm gun} \approx 30$~m$^{-1} \approx 3\lambda_1$. The DRX power system (10~kV, 125~kJ) will form and sustain the $\sim 10$~eV plasma for about 0.5~ms, which is about 10 Sweet-Parker reconnection times and sufficient for the plasma to reach a driven-relaxed steady-state. We will measure the 2D structure of the equilibrium magnetic field and compare it with the first several linear eigenmodes of the force-free equation, and subsequently study the $k$-spectrum of the magnetic energy as well as the dynamics of relaxation. Other topics we will study include magnetic flux amplification and the role of boundary elongation on equilibrium/stability. Better understanding of these issues could lead to new ideas for confinement configurations. This poster will provide an overview of DRX and first experimental data. Supported by the LANL LDRD Program. [Preview Abstract] |
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NP8.00007: Physical Interpretations of Magnetohydrodynamic Invariants Martin Michalak, Bhimsen Shivamoggi Invariants arising from gauge transformations of magnetohydrodynamic equations are developed mathematically. Insights into physical interpretations of these invariants are considered. [Preview Abstract] |
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NP8.00008: Beyond the Intelligent-Shell concept: the Clean-Mode-Control Paolo Zanca, Lionello Marrelli, Gabriele Manduchi, Giuseppe Marchiori Due to their discrete nature a grid of active coils for the feedback control produces an infinite sequence of sideband harmonics in the magnetic field. If the sensors have the same periodicity as the coils, as in the Intelligent Shell scheme, the aliasing of the sidebands determines a systematic error on the Fourier analysis of the measurements. This is a drawback for the control of those perturbations that cannot be suppressed by the feedback, but only reduced in their saturation level, as the non-linear tearing modes of reversed field pinches. We have derived analytical formulas for the subtraction of the sidebands, implemented in a real-time correction algorithm of the Fourier analysis. The Fourier harmonics so obtained are used as feedback variable of a new control scheme named Clean-Mode-Control (CMC). The first tests of CMC in RFX-mod have given interesting results in the tearing modes control: besides a reduction of the edge radial field and the plasma surface distortion, systematic rotations with frequencies up to 100Hz are seen. These phenomena will be interpreted using a MHD model of the CMC. [Preview Abstract] |
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NP8.00009: Advances in Thermodynamic Mixed Materials Modeling for Resistive MHD John Luginsland, Michael Frese, Sherry Frese There is continued interest in the modeling of mixed materials for a variety of high current plasma applications ranging from advanced current interrupters via high voltage fuses to high energy density applications, such as high-dose x-ray production via intense electron emission and inertial fusion energy via z-pinch technology.~ The challenge with mixed materials is the constituent components may be very small physically, requiring high spatial resolution to capture the individual materials.~ NumerEx has developed a means to build mixed material models from the constituent equations of state, allowing drastically reduced resolution while still retaining the sufficient fidelity for the thermodynamic behavior.~ We show the protocol for starting with ``microscopic'' simulations where the individual materials are resolved, and using these calculations to build a ``macroscopic'' EOS and resistivity tables of a mixed, homogenized material. We demonstrate a new V{\&}V technique for comparing thermodynamic properties between experimental results and mixed materials simulations. [Preview Abstract] |
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NP8.00010: Pulsed Alfv\'en Wave Experiments in a Helicon Plasma Source Alex Hansen, Saeid Houshmandyar, Earl Scime Experiments to test a model for ion heating in the fast solar wind based on ion cyclotron damping of MHD turbulence driven by nonlinearly interacting, low frequency Alfv\'en waves [Matthaeus et. al., 1999], are being conducted in the West Virginia University HELIX (Hot hELIcon eXperiment) device in argon and helium plasmas. It is argued that counter-propagating waves arise from reflection of the waves off of a gradient in the Alfv\'en speed. The HELIX device has a similar speed gradient profile to that found in the solar corona: a short region of high Alfv\'en speed followed by an expansion region of lower Alfv\'en speed. Here we present measurements of pulsed Alfv\'en waves that have been launched via amplitude modulation of the steady-state RF drive of the plasma-creating (m = 1) helicon antenna. Measurements of wave magnetic field structure, wave phase speed, and the radial profile of the wave amplitude will be presented as a function of plasma density and magnetic field strength in the helicon source. [Preview Abstract] |
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NP8.00011: Alfv\'{e}n wave Measurements in HelCat at UNM Ralph Kelly, Christopher Watts, Yue Zhang, Mark Gilmore Neutral damping of Alfv\'{e}n waves has been theorized to be partially responsible for the heating of the sun's corona. As such, more research is needed to better understand the relationship between Alfv\'{e}n waves and ion and neutral density. This paper describes one method to launch and observe Alfv\'{e}n waves in a high-density Argon plasma. The Alfv\'{e}n waves are launched with a commercial inductor (emitter) and detected with a hand wound B-dot coil. Construction of the emitter, detector and amplifier circuit is described. The Argon plasma is created using the helicon source on HelCat, a linear plasma device at the University of New Mexico. HelCat is a 4 meter long, 50 cm diameter machine with a helicon source on one end and hot cathode source on the other. Initial data collected indicating the presence of Alfv\'{e}n waves in Argon plasma is presented. Neutral density will be adjusted by pre-ionizing the gas with the cathode and/or an external UV lamp. [Preview Abstract] |
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NP8.00012: Linear Theory of n=0 Geodesic Acoustic Mode H.V. Wong, H. Berk, T. Zhou The n=0 geodesic acoustic mode (GAM) has been observed in JET and D- III D experiments, and it is frequently accompanied with pronounced fast frequency chirping. A numerical investigation with the CASTOR code reveals that a global GAM mode arises if the continuum geodesic frequency has a local maximum as a function of radius. The global GAM properties are characterized by: a very small upward frequency shift from the continuum, a radially localized electrostatic component with poloidal numbers m=0,1 and with magnetic coupling to a nonlocalized m=2 component. Here we develop an analytic MHD theory of this n=0 global GAM in a toroidal plasma with r/R taken as order the square root of beta, with beta small. In the analysis we choose to start from the MHD quadratic form (with inertial terms) , we take the mode to primarily have a density perturbation and we find that the magnetic perturbation in the localization region of the mode to be one order of beta smaller than the density perturbation, though this component extends throughout the plasma. Indeed we verify that the existence of a linear global mode requires that the continuum GAM profile have a maximum as a function of radius. The theory shows that the frequency of the global GAM eigenmode is shifted from the maximum continuum GAM frequency, $\Omega_{Gm}$ by $\delta \omega =a (\beta r_{Gm})^2 \partial^2 \Omega_{Gm}$/ $\partial r^2$, where we developed an asymptotic matching scheme to determine the constant a. We found precise agreement of the asymptotic method with the numerical results for a specific q-profile in the high aspect ratio, low beta plasma. [Preview Abstract] |
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NP8.00013: Radial structures and nonlinear excitation of Geodesic Acoustic Modes Liu Chen, Fulvio Zonca In this paper, we show that GAMs constitute a continuous spectrum due to radial inhomogeneities. The existence of singular layer, thus, suggests linear mode conversion to short-wavelength kinetic GAM (KGAM) via finite ion Larmor radii. This result is demonstrated by derivations of the GAM mode structure and dispersion relation in the singular layer. At the lowest order in $k_r \rho_i$, with $k_r$ the radial wave vector and $\rho_i$ the ion Larmor radius, the well known kinetic dispersion relation of GAM is recovered. At the next relevant order, ${\rm O}(k_r^2\rho_i^2)$, we show that KGAM propagates in the low-temperature and/or high safety-factor domain; i.e., typically, radially outward, and a corresponding damping rate is derived. In this work, we also show that, while KGAM is linearly stable due to ion Landau damping, it can be nonlinearly excited by finite-amplitude DW turbulence via 3-wave parametric interactions. The resultant 3-wave system exhibits the typical prey-predator self-regulatory dynamics. [Preview Abstract] |
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NP8.00014: Spectral Gap of Shear Alfv\'{e}n Waves in a Periodic Array of Magnetic Mirrors Roger McWilliams, Yang Zhang, William Heidbrink, Heinz Boehmer, Guangye Chen, Boris Breizman, Stephen Vincena, Troy Carter, David Leneman, Walter Gekelman, Brian Brugman A multiple magnetic mirror array is formed at the LArge Plasma Device (LAPD), to study axial periodicity-influenced Alfv\'{e}n spectra. Shear Alfv\'{e}n Waves (SAW) are launched by antennas inserted in the LAPD plasma. From radial wave field scans with B-dot probes at many axial locations, SAW standing-wave formation and wave refraction in mirror cell(s) are observed. Alfv\'{e}n wave spectral gaps and continua are formed similar to wave propagation in other periodic media due to the Bragg effect. The width of the propagation gap scales with the modulation amplitude according to the solutions of Mathieu's equation. A 2-D finite-difference code modeling SAW in a mirror array configuration shows similar spectral features. Machine end-reflection conditions and damping mechanisms including electron-ion Coulomb collision and electron Landau damping are important for simulation. [Preview Abstract] |
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NP8.00015: Tearing mode transitions induced by electrostatic turbulence Fulvio Militello, Francois Waelbroeck, Richard Fitzpatrick, Wendell Horton The effect of electrostatic turbulence on the stability and propagation of the magnetic islands is investigated numerically. The physical model used is a 2-D version of the Hasegawa-Wakatani equations, which is the simplest model of electrostatic turbulence that takes into account the effect of magnetic shear and finite resistivity. Our equations are extended to include a curvature term, that makes the model linearly unstable to interchange instability. The problem is solved numerically in a slab box by using a finite difference, fully implicit code that uses PETSc libraries. Turbulence is found to cause transitions between the different roots for the propagation velocity of the mode. The transitions take the mode towards roots with slower propagation that are characterized by locally flattened profiles. At constant velocity, the effect of the turbulence is to increase the drive for the tearing mode but this effect may be compensated by the generally stabilizing effect of reducing the propagation velocity. [Preview Abstract] |
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NP8.00016: A feasibility study for the existence of precursor phenomena in magnetohydrodynamic waves Hogun Jhang The existence of precursor or forerunner waves, which propagate faster than the main signal, is a feature of linear wave propagation in a dispersive medium. Precursors were first predicted by Sommerfeld and Brillouin, and have been observed in the electromagnetic and the fluid surface wave propagation, in which non-monotonic dispersion relations prevail. In the present work, a feasibility study is carried out whether such a precursor wave exists in magnetized plasmas in the excitation and propagation of Alfven waves. The condition for the existence of the precursors is deduced from the dispersion relations of Alfven waves, and the characteristics of the precursors are investigated. Possible application of the present study to astrophysical and laboratory fusion plasmas are also discussed. Finally, an experimental setup is proposed for the observation of the Alfven precursor waves. [Preview Abstract] |
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NP8.00017: Nonlinear coupling between drift waves and streamers in LMD-U T. Yamada, S.-I. Itoh, T. Maruta, Y. Nagashima, S. Shinohara, K. Terasaka, M. Yagi, S. Inagaki, Y. Kawai, N. Kasuya, A. Fujisawa, K. Itoh Recent theories and simulations on drift wave turbulence revealed that the coupling between drift waves should generate streamers. A streamer is radially elongated and poloidally localized structure, therefore, it is expected to enhance the cross-field transport and have large effects on the plasma confinement. However, few streamers have been identified in toroidal and linear plasmas. We applied a 64-channel poloidal probe array to the LMD-U linear plasma in order to investigate poloidal structure of drift wave turbulence, and identify the occurrence of streamer events successfully. Furthermore, two-dimensional (poloidal wave number and frequency) spectra showed broadband fluctuations superposed with a couple of parent modes which satisfy the linear dispersion relation, and many quasi-modes which do not satisfy the dispersion relation. It should be highlighted that the first application of two-dimensional bi-spectral analysis revealed the cascading process from parent drift waves to quasi-modes and broadband components, and the process of a streamer generation. [Preview Abstract] |
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NP8.00018: Magnetic field effects on velocity shear-driven ion cyclotron instabilities E. Tejero, A. Eadon, E. Thomas, W.E. Amatucci Flows generated in plasmas due to the presence of mutually perpendicular electric and magnetic fields are of relevance to both the fusion and space plasma communities. In particular, inhomogeneity in the flow can have both stabilizing and destabilizing effects on the plasma depending on the magnitude and scale length of the inhomogeneity. Studies in the ALEXIS device, a 170-cm long, 10.2-cm diameter magnetized plasma column, seek to determine the stability regimes for driven inhomogeneous flows. This poster focuses on the effect of the axial magnetic field strength on the behavior of ion cyclotron waves observed in ALEXIS plasmas containing radially localized dc electric fields. This presentation discusses the clear correlation observed between sheared azimuthal E x B plasma flows and the spatial localization of ion cyclotron instabilities observed in ALEXIS. Further, it is shown that the effect on the oscillations due to varying the magnetic field is consistent with a Doppler-shifted frequency resonating with a harmonic of the ion cyclotron frequency. [Preview Abstract] |
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NP8.00019: Study of the low-frequency ion-cyclotron turbulence in a cylindrical plasma magnetically confined Steve Jaeger, Abdelilah Ajendouz, Cedric Brault, Amine Erradi, Thiery Pierre, Alexandre Escarguel, Cyril Rebont, Nicolas Claire, Eric Faudot, Stephane Heuraux, Kamal Quotb A cylindrical column of magnetized plasma is produced by means of the hot cathode laboratory device MISTRAL using various gases (Neon, Argon, Krypton). In usual experimental conditions, strongly nonlinear low frequency instabilities in the ion cyclotron range are recorded. A transition towards ion cyclotron turbulence appears to be triggered by a threshold value of the radial electric field. In order to investigate the characteristics of the instabilities and the phenomena responsible for the destabilization of the plasma column, detailed measurements are performed using various diagnostics (probe arrays, ultra-fast and intensified cameras, Laser Induced Fluorescence, spectroscopy, microwave resonance). The results are compared with both a theoretical model of the ion cyclotron instability and Particle in Cell simulation of the instability. [Preview Abstract] |
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NP8.00020: Focusing light with subwavelength aperture due to surface plasma K.R. Chen Diffraction limits any optical system; in fact, it affects all wave fields. As a reminiscent of Heisenberg uncertainly principle for quantum mechanics, diffraction limit restricts the minimum value of the product of divergent angle and width of light beam and sets the achievable smallest spot size of a focused light at the order of wavelength. We study an innovative approach and the physical mechanism using surface plasma to focus propagating light to a width below the scale of wavelength and are investigating the possibility to break the diffraction limit and its implications in fundamental physics. This process is simulated with finite-difference time domain (FDTD) method; the dispersive material such as silver film is modeled by auxiliary difference equation (ADE) method with Drude poles and thus the phasor polarization current is calculated. Such a focused light may be useful to imaging, sensing, lithograph, optical storage, photonic circuit, and many other applications. [Preview Abstract] |
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NP8.00021: Energy transfer via Weibel and two-stream instabilities in two-temperature electron-ion plasmas Jaehong Park, Eric Blackman, Xianglong Kong, Chuang Ren Whether an efficient collisonless temperature equilibration mechanism exists for a two-temperature ion-electron plasma, with $T_i > T_e$, is important for understanding astrophysical phenomena such as radiatively inefficient accretion flows and relativistic collisionless shocks in Gamma-ray bursts. Here we study whether the two-stream and Weibel instabilities driven by proton counter-streaming and/or temperature anisotropy can be such a mechanism. Analysis and PIC simulations show that the Weibel instability induces only a weak electro-ion coupling in either non-magnetized [Ren, Blackman, and Fong, Phys. Plasmas, 14:012901 (2007)] or magnetized plasmas. The two-stream instability is found to be more effective than the Weibel instability for the electron-ion coupling. This work is supported by the U.S. Department of Energy under Grant Nos. DE-FC02-04ER54789 and DE-FG02-06ER54879. [Preview Abstract] |
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NP8.00022: Dynamic manipulation of electromagnetic waves in magnetized plasmas: deceleration, compression and acceleration Yoav Avitzour, Gennady Shvets We study the properties of electromagnetic fields propagating in magnetized plasmas with emphasis on propagation modes with very low group velocities. The dramatic slowdown of a wave entering such a plasma results in compression of the wave energy. In plasma, unlike in other media that demonstrate low group velocity, the compressed energy is stored in the longitudinal component of the EM field and is not transferred to the medium. The compressed wave can then be used for electron acceleration in the plasma. We also demonstrate temporal compression of the transmitted wave by rotating the magnetic field or increasing its amplitude while the wave is in the plasma. Depending on the plasma frequency, the transmitted signal can be upshifted or unchanged in frequency. We present analytical and numerical results for the problem in one, two and three dimensions. [Preview Abstract] |
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NP8.00023: Properties of Whistler Spheromaks Reiner Stenzel, J. Manuel Urrutia, Kyle Strohmaier A loop antenna inserted into a large magnetized laboratory plasma is used to induce a field-reversed configuration (FRC) in a large laboratory plasma. Upon reversal of the oscillating antenna current the FRC splits into two emerging spheromaks. The magnetic structures develop helicity of opposite signs, propagates in the whistler mode along the ambient field away from the antenna. The propagation speed of these ``whistler spheromaks'' or 3D vortices decreases with increasing amplitude. When two counter-propagating spheromaks collide they merge into a stationary FRC whose axis frequently precesses in the direction of the toroidal electron drift. Whistler spheromaks and FRCs are subject to a non-collisional (inertial or stochastic) damping mechanism: Electrons are accelerated along a neutral line by a parallel electric field. Magnetic energy is converted into electron kinetic energy within one cycle. The electron distribution is non-Maxwellian and likely anisotropic since the energized electrons give rise to an instability of whistler modes at different frequencies. [Preview Abstract] |
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NP8.00024: Whistler instabilities in EMHD spheromaks and FRCs J. Manuel Urrutia, Kyle Strohmaier, Reiner Stenzel In a large laboratory plasma anisotropic electron distributions are produced by accelerating electrons in magnetic null regions. These null regions are predominantly the toroidal null lines of field-reversed configuration (FRC) and spheromaks in the parameter regime of electron MHD (EMHD). The electrons gain energy from an inductive electric field along the separator thereby converting magnetic energy into electron kinetic energy. The non-adiabatic electron motion near null points produces non-equilibrium distributions which give rise to kinetic instabilities. The emission of whistlers from toroidal electron current rings is observed. Frequency spectra, amplitudes and wave magnetic field distributions are measured. Of particular interest is the source region which is not a toroidal rf current but two opposing poloidal current layers. Since the source region is of order of the whistler wavelength the instability appears to be absolute rather than convective. Possible instability mechanisms will be discussed. [Preview Abstract] |
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NP8.00025: Experiments on triggered whistler emissions Kyle Strohmaier, J. Manuel Urrutia, Reiner Stenzel Emission of whistler modes from a laboratory plasma with locally anisotropic electrons has been observed. In order to measure the spatial growth rate of the possibly convective whistler instability test whistler waves have been injected into the source region. The frequency is chosen near the most unstable mode ($< 7$ MHz $\simeq 0.3\omega_{ce}$). It is observed that the test wave is not amplified but the whistler emission is greatly enhanced. The emission can be distinguished from the test wave by its different field topology and frequency which chirps downward in time. Thus, the test wave triggers an enhanced emission of an absolute whistler instability. These findings will be compared with observations of triggered emissions in the magnetosphere. [Preview Abstract] |
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NP8.00026: Large-amplitude electron oscillations with spatially inhomogeneous ion densities? Barbara Abraham-Shrauner No analytic solutions for large-amplitude electron oscillations in a cold plasma with immobile ions have been found for spatially inhomogeneous ion densities. The one-dimensional (spatial) electrons obey two fluid equations and two Maxwell equations for the electric field. The Eulerian variables are transformed to Lagrangian variables. The problem is then reduced to the equation of motion of an equivalent particle in one dimension and the Lagrangian time (orbital) given as an integral over the position. The expressions for the electron oscillations with immobile and spatially homogeneous ion densities are recovered. Several possible spatially inhomogeneous ion densities are explicitly solved but the electron density vanishes, an unphysical result. A particular generic solution for the electron position as a function of the Lagrangian position and time is shown by Lie symmetry methods to require spatially homogeneous ion densities. More general forms of spatially varying ion densities give a simple condition for a nonzero electron density. However, for the Jacobian elliptic functions secular terms that destroy the oscillation occur if either the modulus of the elliptic function or a coefficient of the Lagranian time depends on the Lagrangian position. The key restriction is Gauss' law. These results restrict perturbation solutions. [Preview Abstract] |
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NP8.00027: Modified Budden problem associated with an energetic-particle population A.N. Kaufman, A.J. Brizard, E.R. Tracy Our main motivation is to investigate what new effects are introduced in standard heating and/or current-drive scenarios when a non-Maxwellian population of energetic particles (e.g., fusion alphas) is taken into account. In particular, we investigate how energy from a wave supported by a population of energetic particles (e.g., Bernstein wave) can be transferred to a bulk-ion wave through the intermediary of a magnetosonic wave. For this purpose, a three-wave Budden model with two resonance layers is constructed that allows recirculation of energy fluxes around a rectangle in ray phase space. The transmission, reflection, and conversion coefficients for this extended Budden problem are calculated by ray phase- space methods and the modular-eikonal approach [1,2]. The analytical and numerical results show that all of the connection coefficients exhibit interference effects that depend on an interference phase that can be calculated from the coupling constants at each conversion point and the area enclosed by the rectangle. When one of the three waves is a negative-energy wave supported by an inverted energetic-particle population, the magnitude of the conversion coefficients can exceed 100\%. Such amplification effects may provide a new form of alpha- channeling. \newline $\left[1\right]$ Y.~M.~Liang, {\it et al.}, Phys.~Lett.~A {\bf 193}, 82 (1994). \newline $\left[2\right]$ A.~J.~Brizard, {\it et al.}, Phys.~Plasmas {\bf 5}, 45 (1998). [Preview Abstract] |
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NP8.00028: A new normal form for multidimensional mode conversion E. Tracy, A. Richardson, N. Zobin, A. Kaufman Linear conversion occurs when two wave types are locally resonant in a nonuniform plasma [1]. In recent work, we have shown how to incorporate a ray-based approach to mode conversion in numerical algorithms [2,3] for the most common type of conversion. Here, we present a new formulation that can deal with more general cases [4]. We exploit a new normal form for the 2X2 dispersion matrix defined such that the diagonals Poisson- commute with the off-diagonals (at leading order). Therefore, if we use the diagonals as ray Hamiltonians, the off- diagonals will be constant. Thus, the 2X2 dispersion matrix in normal form has a very natural physical interpretation: the diagonals are the uncoupled ray Hamiltonians and the off-diagonals are the coupling. We further discuss how to incorporate the normal form into ray tracing algorithms. 1] E. Tracy, A. Kaufman and A. Brizard, Phys. Plasmas {\bf 10} (2003) 2147. 2] A. Jaun, E. Tracy and A. Kaufman, Plasma Phys. Control. Fusion {\bf 49} (2006) 43. 3] E. Tracy, A. Kaufman and A. Jaun, to appear in Phys. Plasmas. 4] A. Kaufman, E. Tracy and A. Brizard, Phys. Plasmas {\bf 12} (2005) 022101. 5] E. Tracy and A. Kaufman, PRL {\bf 91} (2003) 130402. [Preview Abstract] |
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NP8.00029: Quadratic Effects in Conversion Andrew Richardson, Eugene Tracy, Allan Kaufman Phase space ray-tracing techniques can be used to solve wave problems exhibiting mode conversion [1,2]. The (x,k)-dependence of the dispersion matrix, $\mathbf{D}$, is linearized near the conversion, and the matrix is then converted back to an operator. The resulting coupled equations can be solved for the local fields. Matching these local solutions onto uncoupled WKB far-field solutions gives scattering coefficients which can be used to treat the mode conversion as a ray-splitting process. In this work, we study the effects of quadratic terms in $\mathbf{D}$ near a mode conversion. We show that for one spatial dimension, $\mathbf{D}$ can be put into normal form, where the diagonals contain quadratic corrections, and the off-diagonals are the constant coupling. The quadratic terms introduce phase corrections to the far-field coupled WKB solutions, while the local solutions have both amplitude and phase corrections. These corrections allow for better matching at the conversion, which we illustrate by comparing the asymptotic solution with a numerical solution for the 1-D conversion. 1] A. Jaun, E. Tracy and A. Kaufman, Plasma Phys. Control. Fusion 49, 43-67 (2007). 2] E. Tracy, A. Kaufman and A. Jaun, to appear, Phys. Plasmas (2007). [Preview Abstract] |
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NP8.00030: Path Integrals and Mode Conversion N. Zobin, A.S. Richardson, E.R. Tracy The phase space path integral arises naturally in the ray-tracing approach to the solution of wave equations, including those which exhibit mode conversion. The wave evolution operator is related to the exponential of the dispersion matrix, and the path integral can be used to find this exponential using the method of operator symbols. Thus, the path integral can be used even in nonstandard conversions which are not of the ``avoided crossing'' type, and we hope to exploit this to develop new types of ray-based treatments for these conversions. In order to do this, a deeper understanding of the nature of this path integral will be useful. Here we show that the phase space path integral can be interpreted as a Fourier transform on the space of measures using a new formalism for infinite dimensional Fourier transforms proposed by Zobin. Approximate expressions for this type of Fourier transform are developed using entropy arguments and, in the classical limit where smooth ray trajectories exist, we recover the familiar sum over all histories. We also give an example for the case of a discrete phase space, in which smooth paths are not defined, but where the Fourier transform over measures can still be evaluated using entropic techniques. [Preview Abstract] |
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NP8.00031: Direct comparison of full-wave and ray-tracing methods for a simple model of multi-dimensional mode conversion Y. Xiao, A. Richardson, E. Tracy Mode conversion can occur in a nonuniform plasma when two waves of different character are locally resonant. Jaun et al. have recently developed a numerical ray-tracing algorithm for realistic tokamak models that accounts for the ray splitting that occurs at conversions [1,2]. Here we present a comparison of ray-based and full-wave methods by considering a simple model consisting of a pair of coupled wave equations in two spatial dimensions. The two spatially-dependent wave speeds, $c_1(x,y)$ and $c_2(x,y)$ are distinct for almost all $(x,y)$, and are equal only along a line where conversion occurs. We launch a WKB-type wave packet in channel $1$. There is initially no excitation in channel $2$. Absorbing boundary conditions are used to avoid reflections which would complicate the results. From the full-wave output, we compute the initial energy density as a function of position and consider its evolution along a family of rays which undergo conversion. These full-wave results are then compared to the ray-based predictions. [1] A.Jaun, E.Tracy and A.Kaufman, Plasma Phys. Control. Fusion {\bf 49}, 43-67 (2007). [2] E.Tracy, A.Kaufman and A.Jaun, to appear in Phys. Plasmas. [Preview Abstract] |
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NP8.00032: Laboratory investigation of whistler and lower hybrid wave characteristics William Amatucci, David Blackwell, Gurudas Ganguli, George Gatling, David Walker, Chris Compton An experimental investigation of the generation and propagation of whistler and lower hybrid waves is underway in the NRL Space Physics Simulation Chamber. Wave propagation is being investigated in conditions simulating the Earth's radiation belt environment. These studies are carried out in both homogeneous plasma and plasma containing density structures. In homogeneous plasma, resonance cone propagation of the waves is observed, consistent with theoretical predictions. In plasma containing a density depletion layer, wave ducting within the layer has been observed. For these experiments, we have fabricated and tested transmitting and receiving magnetic loop antennas and electric field dipole receiving antennas. Preliminary comparisons of the two antenna styles indicate that loop antennas couple significantly more wave power into the plasma. Efforts are currently underway to further quantify these observations. Experimental results related to the propagation characteristics of whistler/lower hybrid waves under these conditions will be presented. [Preview Abstract] |
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NP8.00033: Superposition of Parallel and Perpendicular Flow Velocity Shears in Magnetized Hybrid-Ion Plasmas Toshiro Kaneko, Shuichi Tamura, Ryuta Ichiki, Rikizo Hatakeyama Low-frequency instabilities modified by ion flow velocity shears are investigated using concentrically three-segmented ion and electron emitters in a modified double-ended Q-machine. When each of the emitters is individually biased, the perpendicular and parallel ion flow shears can be generated and superimposed on each other. The fluctuation amplitude of the drift wave which has an azimuthal mode number m=3 increases with increasing the parallel shear strength. When the perpendicular shear is superimposed on the parallel shear, the drift wave of m=3 changes into that of m=2 through a broadband turbulence state. Furthermore, the parallel shear strength required for the excitation of the drift wave becomes large with a decrease in the azimuthal mode number. On the other hand, the effects of a negative ion as one of the hybrid ions on the drift wave in the presence of the positive-ion flow shear are also investigated. The negative ion stabilizes the shear-modified drift wave, which is the opposite result to a number of earlier studies on the negative ion plasmas. [Preview Abstract] |
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NP8.00034: Microwave heating of over-dense plasmas in the TJ-K stellarator Alf K{\"o}hn, Gregor Birkenmeier, Hendrik H{\"o}hnle, Eberhard Holzhauer, Walter Kasparek, Mirko Ramisch, Ulrich Stroth In the stellarator TJ-K, over-dense plasmas are generated by means of microwaves at 2.45 GHz and 8.25 GHz. The plasma is characterized by densities $\le 10^{18}$ m$^{-3}$ and electron temperatures $\le$ 20 eV. At the given plasma parameters, absorption of the O or X-waves at the fundamental cyclotron resonance is only possible through the Bernstein wave, which can be generated due to an O-X-B or X-B mode conversion process. Furthermore, absorption is possible at the upper-hybrid resonance or, if a R-wave is generated after multiple reflections between cut-off layer and wall, at the cyclotron resonance. In order to sort out the importance of the different heating mechanisms, experimental studies have been carried out in a wide parameter range. Power-modulation experiments with Langmuir-probe arrays and wave-field measurements are used to detect the local power-deposition profile. Dominant absorption is found at the upper-hybrid resonance. In order to optimise the O-X-B mode-conversion efficiency, a novel array antenna has been developed. Thus the angle between the microwave beam and the flux surfaces can be modified by tuning the microwave frequency. First experiments with this antenna will be presented. The experimental results are compared with simulations from a full-wave code. [Preview Abstract] |
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NP8.00035: INERTIAL CONFINEMENT FUSION II AND HEDP DIAGNOSTICS |
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NP8.00036: A Simple Model for ICF Double Shell Target Performance Mordecai Rosen Hohlraum-driven double-shell capsules are being considered as ignition / moderate gain systems for the National Ignition Facility. We present a simple model for the performance of these double shell capsules by focusing on the dynamics of the inner Au shell and the DT gas within. We calculate the post shock conditions of the DT, followed by its adiabatic compression. At stagnation time, the ultra-high density Au shell is treated simply as a Fermi-degenerate system. We equate the peak kinetic energy of that shell to the internal energies of the DT and the Au upon stagnation. We close the system of equations via a pressure balance between the Au and the DT. This ``hydro phase'' of the analytic calculation results in predictions for peak DT density and temperature. We then calculate ignition criteria for this system and add a simple model for fusion burn-up and gain. Results from all of this are compared with published results of the simulations of Amendt et. al. (PoP 14, 056312 [2007]). We use our analytic formulae to study parameter variations such as DT initial density and radius as they vary with ICF driver scale. [Preview Abstract] |
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NP8.00037: 2D LMJ target design for early ignition experiments Stephane Laffite, Stephane Liberatore, Pascal Loiseau Achieving ignition is one of the objective of the ``Laser MegaJoule'' (LMJ), which will deliver up to 1.8 MJ and 600 TW. Several indirect drive target were designed to reach ignition on this facility. The nominal point design target, called A1040, is composed of a doped plastic capsule in a gold cylindrical hohlraum. Other experiments are planned before the laser being totally built. They include the attempt of ignition and burn of an ICF capsule with 40 laser quads (of 4 beams each), delivering up to 1.2 MJ and 390 TW. We present here the 2D design of two target requiring 1 MJ, 300 TW and 1.2 MJ, 360 TW of laser energy and power. For both targets, the hohlraum is cylindrical, made up with gold. Yield is about 10 for the smaller one, more than 15 for the other one. The beam passage through the laser entrance holes, the radiation symmetry and the susceptibility to laser-plasma interactions are analyzed. [Preview Abstract] |
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NP8.00038: 3D simulations of thermally induced expansion of beryllium microstructure in gas--filled NIF ignition targets M.M. Marinak, N.R. Barton, R. Becker, S.W. Haan, J.D. Salmonson X-ray preheat in the baseline gas-filled NIF ignition target is calculated to heat solid portions of the beryllium ablator hundreds of degrees Kelvin. Anisotropy in the resulting thermal expansion, due to crystal properties, causes the interfaces between the beryllium and cryogenic fuel, as well as the internal beryllium interfaces, to distort before the grains are melted by passage of the first shock. We quantify these effects for the full duration of the implosion. A 3D polycrystalline model is employed in ALE3D, a multiphysics arbitrary Lagrange Eulerian code, to calculate this expansion and the response to the first shock. It models the anisotropic elastic and plastic response, resolving individual grains. Perturbations in the fields and interfaces are then linked to a 3D HYDRA simulation of the remainder of the implosion. High-resolution simulations resolve modes up to $\ell \approx 2000$. These perturbations add to those originating from the native roughness on the embedded interfaces. We compare the magnitudes and spectral content of the different perturbation seeds. [Preview Abstract] |
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NP8.00039: Strength and surface roughness of Be under laser irradiation S.N. Luo, E.N. Loomis, S.R. Greenfield, D.L. Paisley, D. Swift, T. Tierney, R. Johnson, N. Hoffman, H. Lorenzana To examine the microstructure effect on dynamic strength and surface roughness of Be under laser irradiation, we have conducted dynamic loading experiments at the Trident laser facility on rolled foil, single crystal disk, equal channel angular extruded (ECAE) foil with 0.9\% atom Cu doping, and sputtered Be with 0.9\% atom Cu doping. Direct and confined laser ablation have been applied to Be samples at 532 nm and 1064 nm for 1D strain loading. The drive pulse, ranging from 2 ns to 2000 ns in duration, is shaped temporally as square, Gaussian, half Gaussian (ramping and Taylor-release), etc. The time-resolved diagnostics include point- and line-imaging velocity interferometers (VISARs), and transient imaging displacement interferometer (TIDI) for two-dimensional out-of-plane displacement measurement. Data with simultaneous VISAR and TIDI measurements have been acquired on ECAE Be-Cu, rolled Be foils and (0001) single crystal Be. For example, an ECAE Be-Cu sample with an initial roughness of $<$30 nm (the surface opposite to the drive side) was loaded into the plastic regime with a peak stress of $\sim$4.5 GPa during Taylor-release-type confined ablation. The flow stress was found to be about 1.9 GPa from VISAR measurements, and the roughness was about 300 nm shortly after breakout. [Preview Abstract] |
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NP8.00040: Stability study of planar targets using standard and adiabat shaping pulses Marina Olazabal-Loume, Ludovic Hallo It has been recently proposed to reduce the ablative Rayleigh-Taylor instability growth by using the adiabat shaping technics. In this work, the relaxation adiabat shaping scheme [B. Betti et al., Phys. Plasmas 12, 042703 2005] is considered. A prepulse is followed by a relaxation period, when the laser is turned off. We report here a parametric study of picket's parameters carried out with a code dedicated to the linear stability analysis, on the basis of spherical realistic simulations including full physics. It is shown that the set picket/relaxation time mainly determines the target stability and that the adiabat shaping scheme modifies the perturbed state before and at the beginning of the main acceleration. Consequences of small laser pulse shape variations on the target hydrodynamics and on perturbations behaviour are studied. [Preview Abstract] |
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NP8.00041: Progress Toward Kelvin-Helmholtz instabilities in a High-Energy-Density Plasma on the Nike Laser E.C. Harding, R.P. Drake, Y. Aglitskiy, V.V. Dwarkadas, R.S. Gillespie, M.J. Grosskopf, C.M. Huntington, N. Gjeci, D.A. Campbell, D.C. Marion In the realm of high-energy-density (HED) plasmas, there exist three primary hydrodynamic instabilities: Rayleigh-Taylor (RT), Richtmyer-Meshkov (RM), and Kelvin-Helmholtz (KH). Although the RT and the RM instabilities have been observed in the laboratory, no experiment to our knowledge has cleanly diagnosed the KH instability. While the RT instability results from the acceleration of a more dense fluid into a less dense fluid and the RM instability is due to shock deposited vorticity onto an interface, the KH instability is driven by a lifting force generated by velocity shear at a perturbed fluid interface. Understanding the KH instability mechanism in HED plasmas will provide essential insight into detailed RT-spike development, mass stripping, many astrophysical processes, as well as laying the groundwork for future transition to turbulence experiments. We present 2D simulations and data from our initial attempts to create a pure KH system using the Nike laser at the Naval Research Laboratory. [Preview Abstract] |
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NP8.00042: ABSTRACT WITHDRAWN |
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NP8.00043: Pre-imposed ripple evolution on planar CH targets with a high-{Z} overcoat Max Karasik, Y. Aglitskiy, V. Serlin, J.L. Weaver, J.W. Bates It has been found previously that a thin (400-800$\AA$) high-Z overcoat on the laser side of the target can be effective in suppressing laser imprint [S. P. Obenschain et al. Phys. Plasmas 9, 2234 (2002)]. In those and subsequent experiments, it was found that the overcoat also results in an apparent delay in growth of a front-surface pre-imposed sinusoidal ripple without affecting the RT growth rate. The cause of this apparent delay is hypothesized to be a change in the ablative Richtmyer-Meshkov (RM) phase of the ripple evolution, which forms the seed for the subsequent RT growth, due to the initial x-ray ablation with the high-Z overcoat. In order to investigate this effect, experiments are performed with a pre-imposed ripple amplitude large enough to be observable from the beginning of the laser pulse. Evolution of the ripple due to ablative RM oscillation and transition to RT growth is then observed by face-on x-ray radiography using Bragg reflection from a curved crystal coupled to an x-ray streak camera. X-ray flux from the high-Z overcoat is monitored using absolutely calibrated time-resolved x-ray spectrometers. Simultaneous side-on radiography using a curved crystal allows target trajectory measurement for comparison with simulations. This work is supported by US DOE/NNSA. [Preview Abstract] |
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NP8.00044: Simulation of Thin-Shell Capsule Compression in a Hohlraum with Diagnostic Ports Mark Schmitt, Robert Goldman, Robert Kirkwood Observation of the compression of thin spherical shells (``thin-shells'') is currently being pursued as a method to determine the early-time radiation drive symmetry inside laser-driven hohlraums for the National Ignition Campaign. Observation of thin shell symmetry during implosion requires x-ray backlighting of the capsule, typical performed through diagnostic holes placed near the waist of the high-atomic-number (e.g. gold) hohlraum. Radiation losses through these holes reduce the temperature inside the hohlraum and introduce additional asymmetry (beyond that from the laser drive) in the radiation environment. Simulations to assess the effects of laser drive and diagnostic holes on capsule implosion symmetry have been performed. We compare the results of these simulations to experiments performed on the Omega laser. [Preview Abstract] |
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NP8.00045: Techniques to determine electron and ion temperatures in D2/3He filled capsules M.A. Gunderson, D.C. Wilson, J.F. Benage, G.A. Kyrala, L.A. Welser-Sherrill, H. Makaruk, C.K. Li, J. Frenje, R. Petrasso, B. Yaakobi, W. Garbett In order to match yields in ICF implosion experiments, the ability to determine temperatures in the imploded capsules is crucial. ``Mix'' is commonly used to adjust the simulation to match the experimental yield, but to be more confident that the mix model is being applied correctly, it is very important to know the temperature of the imploded fuel region. To acquire information on ion temperatures in the imploded fuel, we have neutron time-of-flight data, proton spectroscopy, and the ratio of D+3He proton to D+D neutron yield. To determine electron temperatures, we have spectral emission data from spectroscopic tracers (Krypton, Argon) and the slope and shape of the continuum spectrum. In fact, the spectral emission data can also be used to determine the density in the imploded fuel. A summary of the temperature measurements using these techniques will be presented. Work supported by US DOE/NNSA, performed at LANL, operated by LANS LLC under Contract DE-AC52-06NA25396. [Preview Abstract] |
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NP8.00046: ABSTRACT WITHDRAWN |
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NP8.00047: Low velocity ion stopping in multicomponent plasmas for WDM production Bekbolat Tashev, Claude Deutsch In order to comply with several converging international endeavors (Berkeley, Darmstadt, Princeton...) aiming at heating thin solid foils into warm dense matter(solid density and eV temperature range)through the impact of intense and low velocity ion beams we focus our attention on multispecies target.Collisional and dielectric approaches are contrasted in the Bragg peak region of current operational interest.We evidence a critical ion projectile velocity Vp such as target electron stopping turns comparable to target ion stopping.Various simplifying approximations qualifying target ion stopping are thus thoroughly compared. The given critical velocity being somewhat reminiscent of an equivalent one in MFE associated with tokamak anomalous heating,we extend our investigation to the heating of magnetized targets as well. [Preview Abstract] |
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NP8.00048: Simulations of ion beam heated targets for warm dense matter (WDM) physics and inertial fusion energy J. Barnard, A. Friedman, M. Marinak, L.J. Perkins, J. Armijo, F. Bieniosek, E. Henestroza, M. Leitner, B.G. Logan, R. More, P. Ni, G. Penn, P. Roy, P. Seidl, J. Wurtele, A. Zeballos, A. Zylstra, R. Davidson, L. Grisham, I. Kaganovich, C. Debonnel, P. Stoltz, S. Veitzer We present simulations and analysis of ion beam heating of foil targets in the WDM regime for prospective experiments on the Neutralized Drift Compression Experiment (NDCX-1) and its proposed upgrade (NDCX-II). The simulations were carried out using the multi-physics rad/hydro code HYDRA$^{2}$, as well as the 1D codes DPC$^{3 }$and DISH$^{4}$. Calculations of droplet radius evolution and ion energy deposition refinements were carried out. Initial simulations of direct drive capsules using temporally tailored ion beams will also be presented. $^{2}$M. M. Marinak, et al, Phys. Plasmas\textbf{ 8}, 2275 (2001); $^{3}$R. More, et al, JQSRT \textbf{99}, 409 (2006); $^{4}$DISH is a Deeply Simplified Hydrodynamics code authored by R. More, June 2007. [Preview Abstract] |
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NP8.00049: Heavy-ion direct-drive T-lean targets for self-T breeding and plasma MHD direct conversion B. Grant Logan, L. John Perkins, Kai N. LaFortune, John J. Barnard Transverse and longitudinal beam compression in neutralizing plasma enable heavy ion beam direct drive in the ablative rocket regime at high rocket efficiency with ion ranges a fraction of the initial ablator thickness for low adiabat implosions. Ions can couple energy into thick fuel capsule ablators at the peak in rocket efficiency as efficiently as x-rays do in hohlraums, but without conversion loss of beam energy into x-rays. High ablation velocities with heavy ion direct drive mitigate hydrodynamic instabilities like x-ray drive. An analytic implosion model with a heavy-ion dE/dx deposition model, together with hydrodynamic implosion calculations (LASNEX and HYDRA) explore beam requirements for heavy ion direct drive for small 1 MJ drive DT targets and larger Tritium-lean ($>$ 90{\%} DD) targets. Both model and implosion codes indicate ion beams can couple $>$15{\%} of their incident energy into compressed fuel assemblies. Increasing ion energy during the drive pulse can reduce the parasitic beam losses on ablated plasma. [Preview Abstract] |
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NP8.00050: Process Model of the Gas Recovery System in an IFE reactor Charles Gentile, Maria Aristova It is necessary to develop a detailed representative model for the fuel recovery system (FRS) in the prospective direct drive inertial fusion energy (IFE) reactor. In order to observe the interaction of all components, a chemical process model is developed as part of the conceptual design phase of the project. Initially, the reactants, system structure, and processes are defined using the known contents of the vacuum vessel exhaust. The output, which will include physical properties and chemical content of the products, is analyzed to determine the most efficient and productive system parameters. The results of the modeling will be presented in this paper. This modeling exercise will be instrumental in optimizing and closing the fusion fuel cycle in the IFE power reactor. [Preview Abstract] |
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NP8.00051: Possible Radiochemical Signatures for Imploding Capsule Diagnostics on the National Ignition Facility Charles Cerjan, Mark Stoyer, Rob Hoffman, Peter Amendt, Jeff Colvin One of the most important and challenging issues confronting capsule implosion experiments will be a quantitative evaluation of the implosion dynamics. Given the extreme conditions under which these experiments will occur, developing robust, sensitive diagnostics will be difficult. Radiochemical signatures might provide important insight into material mixing and laser drive asymmetry that would complement planned x-ray and nuclear diagnostics since the relevant nuclear reactions sample the core conditions directly. Simulations of both single-shell and double-shell target configurations with selected radiochemical tracers indicate that several experimentally accessible isotopic ratios would be suitable diagnostics for the expected implosion conditions. [Preview Abstract] |
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NP8.00052: Simulating Mono-energetic Proton Radiographs of Inertial Confinement Fusion Experiments using the Geant4 Monte Carlo Particle Transport Toolkit M.J.-E. Manuel, F.H. Seguin, C.K. Li, J.R. Rygg, J.A. Frenje, R.D. Petrasso, R. Betti, O. Gotchev, J. Knauer, F. Marshall, D.D. Meyerhofer, V.A. Smalyuk Proton radiography has been used to image Inertial Confinement Fusion (ICF) capsules during their implosions as well as to quantitatively measure magnetic fields generated by laser-plasma interactions at the OMEGA laser facility. An imploded, D$^{3}$He-filled capsule provides mono-energetic, $\sim $15-MeV protons for radiographing another capsule. We are developing simulated models of these experiments using the Geant4 Monte Carlo Particle Transport Toolkit (G4). Of particular interest are the limitations on spatial resolution caused by scattering effects. Experimental and simulated results will be presented for different experiments and models. This work was performed in part at the LLE National Laser User's Facility (NLUF), and was supported in part by US DOE, LLNL, LLE and FSC at Univ. Rochester. [Preview Abstract] |
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NP8.00053: The CR-39 Coincidence Counting Technique for Enhanced Signal-to-Background in a Large Range of Charged-Particle and Neutron Measurements at OMEGA and the NIF D.T. Casey, J.A. Frenje, S.C. McDuffee, C.K. Li, J.R. Rygg, F.H. Seguin, R.D. Petrasso, V.Yu. Glebov, D.D. Meyerhofer, S. Roberts, T.C. Sangster CR-39 detectors have been used extensively in several types of nuclear diagnostic applications for diagnosing Inertial Confinement Fusion plasmas produced at the OMEGA laser facility. A coincidence counting technique (CCT) is now being used to identify particle tracks in CR-39 when large background noise levels are present, such as when measuring products of secondary reactions. Two orders of magnitude improvement in the signal to background ratio can be achieved in some applications. In this paper, we describe the effect of the CCT when applied to a large range of charged-particle and neutron data obtained from measurements performed at OMEGA and for proposed work at the NIF. This work was supported in part by the U.S. Department of Energy (Grant No. DE-FG03-03SF22691), LLE (subcontract Grant No. 412160-001G), LLNL (subcontract Grant No. B504974). [Preview Abstract] |
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NP8.00054: NIF Conceptual Design Studies of Bang Time Diagnostics Using d-t Fusion Gamma Rays Joseph Mack, Carlton Young, Scott Evans, Hans Herrmann, Robert Malone, Michael Moran, Vladimir Glebov Bang time and reaction history measurements are essential components of diagnosing failure-modes for ICF implosions on the National Ignition Facility (NIF). Fusion gamma rays are the preferred observable, as they offer the most direct link to deuterium-tritium (d-t ) burn. NIF requirements dictate time resolution and timing accuracy of $<$10 ps and $<$50 ps, respectively. Current approaches use Gas Cherenkov Detectors (GCDs) that convert d-t fusion gamma rays to optical Cherenkov photons, which are collected and recorded by an appropriately fast system. GCD systems, based on ultra-fast photomultiplier tubes and streak cameras, have been developed and fielded successfully at the Omega laser facility. A comparative study of streak-camera-based designs, using optical transport and light pipes, are presented. Trade-off analyses are provided based on achievable throughput and bandwidth. Related studies are also described that attempt to optimize the most advantageous aspects of the case studies. [Preview Abstract] |
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NP8.00055: A Neutron Streak Camera Designed for ICF Fuel Ion Temperature Jiabin Chen, Hua Liao, Ming Chen A neutron streak camera was designed for inertial confinement fusion (ICF) fuel ion temperature diagnostic. It is made of a 1 cm thick $\times $8 cm diam piece of 3{\%} benzophenone quenched plastic scintillator with about a 190 ps FWHM and a streak tube (55ps time resolution) with large-area photocathode ($\phi $30 mm) showed no slit. The electron beam from the photocathode is focused into a little spot ($\phi $1mm). Then the spot is scanned directly and multiplied by an internal microchannel plate. This greatly improves the sensitivity of the tube. The neutron streak camera combines the advangtages of scintillation detector (with high neutron detection efficiency) and of streak camera (with fast time response). The whole detection system time resolution is 300ps and can record neutron time of flight signals from ICF implosion target with yields of 10$^{7}$ DT neutron per shot. [Preview Abstract] |
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NP8.00056: Innovative High-Pressure Helium Neutron Detector G.A. Chandler, M.S. Derzon Neutron detection systems are required to provide critical data for inertial confinement fusion experiments at Sandia National Laboratories. Critical measurements include Neutron spectroscopy, Neutron bang-time and Neutron Imaging. Present detector systems including scintillators coupled to photomultipliers, scintillating fiber arrays, diamond photoconductive detectors, and other systems, have been developed for these measurements. These detectors all have their limitations with regards to sensitivity, time response, energy resolution, spatial resolution and background rejection. An innovative high-pressure Helium detector is proposed that appears to have many beneficial performance characteristics with regards to making these neutron measurements in the high bremsstrahlung environments found in high energy density physics experiments on the fast pulsed power facilities at Sandia. Calculations of the performance characteristics of these detectors will be presented. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Dept. of Energy under contract No. DE-AC04-94AL85000. [Preview Abstract] |
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NP8.00057: Sandia National Laboratories' Z-Petawatt Laser Facility: A Progress Report G.R. Bennett, P.K. Rambo, B.W. Atherton, E. Brambrink, A.D. Edens, M. Geissel, J.L. Porter, J. Schwarz, I.C. Smith Sandia National Laboratories' Z accelerator, which is currently being upgraded and will become operational again 2007, includes the Z-Beamlet Laser (ZBL) system [P. K. Rambo \textit{et al}., Appl. Opt. \textbf{44,} 2421 (2005)] for x-ray imaging support. ZBL is a long-pulse, multi-kJ, TW-class device. For higher energy x-ray requirements on Z, and possible fast ignition studies, an additional laser, the short-pulse, multi-kJ, PW-class Z-Petawatt Laser (ZPW), is presently under construction. In the first phase, 50 J, 0.5 ps pulses have been generated, with pulse compression via commercially-available gratings. In the second phase, with the main cavity slab amplifiers operated at higher gain, followed by beam expansion onto larger (94 cm) Nova gold gratings, an energy enhancement to several hundred J will be achieved. In the final phase, full aperture 4-pass amplification through the main amps, and compression via large multilayer dielectric gratings, will lead to 2 kJ in 1-10 ps. [Preview Abstract] |
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NP8.00058: Characterization Studies of Microchannel Plates for Use in High-speed, Time-gated X-ray Cameras Ming Wu, Craig Kruschwitz, Ken Moy, Greg Rochau X-ray detectors based on straight-channel microchannel plates (MCPs) are a powerful diagnostic tool for two-dimensional, time-resolved imaging and time-resolved X-ray spectroscopy in the fields of laser-driven inertial confinement fusion and fast Z-pinch experiments. Understanding the behavior of MCPs as used in such detectors is critical to understanding the data obtained. This presentation reports on recent efforts to characterize MCPs under direct current and pulsed voltage bias and in both saturated and unsaturated regimes. Experiments have been performed using an intense, short-pulse ($<$1 ps full-width half-maximum) ultraviolet laser. These data are compared to the results of a Monte Carlo code we have developed to describe MCP electron cascade dynamics. We present results from measurements and simulations of MCP gain, saturation behavior, gate profiles, and spatial resolution. In general, good agreement between the experimental data and simulations was obtained. [Preview Abstract] |
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NP8.00059: Supersonic Heat Wave Propagation in Laser-Produced Underdense Plasmas K.B. Fournier, M. Tanabe, H. Nishimura, S. Fujioka, K. Nagai, A. Iwamae, N. Ohnishi, F. Girard, M. Primout, B. Villette, D. Babnneuau, S. Moon, S.B. Hansen, M. Tobin, K. Mima Intense, multi-keV X-ray sources are required for radiographic applications in laboratory astrophysics and ICF. Low-density targets are favorable for efficient laser-to-X-ray conversion because supersonic energy deposition leads to volumetric heating with low hydrodynamic losses. We report on recent X-ray generation experiments at the GEKKO XII laser. Ti-doped SiO$_2$ aerogel-filled (3.3 mg/cm$^3$, 3-6 atom\% Ti) Be or CH cylinders were irradiated with nine laser beams with a total of 1 kJ energy in a 2.5~ns square pulse, at 351~nm wavelength. Laser irradiance at the entrance of cylinder was 1.4${\times}$10$^{14}$ W/cm$^2$. The observed heat wave clearly shows two different phases in terms of propagation velocities. The measured heat-front propagation velocity was 1.4$\times$10$^8$ cm/s, which is a Mach number of 10 for the given conditions. Electron temperature in the heated target was derived from time-resolved X-ray spectra. By changing observation points, electron temperature profiles of the heat wave along the cylinder axis were obtained at different times. [Preview Abstract] |
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NP8.00060: X-ray Diagnostic Calibration with a Small Picosecond Laser Facility C. Reverdin, M. Paurisse, A. Duval, D. Husson, C. Rubbelynck The broadband x-rays emission of a target irradiated by a laser can be sometimes used to calibrate detectors. X-ray calibration is mainly done in the continuous mode with synchrotron radiation because high intensity monochromatic radiation can be obtained. Such calibration results can be sometimes irrelevant as x-ray plasma diagnostics are operated in pulsed mode (for instance with CVD diamond detectors). At CEA-DIF we have a small picosecond laser facility called EQUINOX with 0.3 J at 800 nm. The laser is focused inside a target chamber on a solid target and produces intense radiation in the 100 eV - 2000 eV range. The x-ray source is routinely monitored with a pinhole camera for the source dimension measurement and with x-ray diodes for the total level flux. In addition an x-ray transmission grating spectrometers, a crystal spectrometer and a single count CCD camera measure the x-ray spectrum between 200 eV and 15 keV. The absolute calibration of those sets of spectrometers allows us full characterised x-ray emission spectra. Typical duration is less than 100ps The spectrum can be adjusted by changing the target material and by x-ray filters. Some examples of calibration will be shown, such as CVD diamonds{\ldots} [Preview Abstract] |
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NP8.00061: High Dynamic Range Single Shot Third Order Autocorrelator John Morrison, Enam Chowdhury, Tony Link, Dustin Offermann, Vladimir Ovchinnikov, Rebecca Weber, Linn van Woerkom, Richard Freeman In experiments involving high power ultra-intense laser interactions with solid targets, characterizing the pre-pulse of the laser pulse is extremely important. The pre-pulse determines the initial conditions of the target onto which the main intense pulse impinges. This information is necessary for accurate experimental analysis and computer simulations for benchmarking efforts and is critical for understanding particle acceleration/transport relevant to Fast Ignition Inertial Confinement Fusion. Current techniques require a large number of shots without significant fluctuations, which are unavailable on low repetition rate lasers. The details of a few to single shot, 10 ps window, third order autocorrelator with high contrast developed on a high power laser will be presented. [Preview Abstract] |
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NP8.00062: Design of the optical backscatter diagnostic for laser plasma interaction measurements on NIF J.D. Moody, P. Datte, E. Ng, K. Maitland, W. Hsing, B.J. MacGowan, D.H. Froula, P. Neumayer, L. Sutter, N. Meezan, S.H. Glenzer, R.K. Kirkwood, L. Divol, S. Andrews, J. Jackson, A. Mackinnon, I. Jovanovic, R. Beeler, L. Bertolini, M. Landon, S. Alvarez, T. Lee, P. Watts We describe the design of the backscatter diagnostic for NIF laser-plasma interaction (LPI) studies. It will initially be used to validate the 280 eV point design hohlraum and select phase plates for the ignition experiments. Backscatter measurements are planned for two separate groups of 4 beams (a quad). One quad is 30$^{\circ}$ from the hohlraum axis and the other at 50$^{\circ}$. The backscatter measurement utilizes 2 instruments for each beam quad. The full aperture backscatter system (FABS) measures light backscattered into the final focus lens of each beam in the quad. The near backscatter imager (NBI) measures light backscattered outside of the beam quad. Both instruments must work in conjunction to provide spectrally and temporally resolved backscatter power. We describe the design of the diagnostic and its capabilities as well as plans for calibrating it and analyzing the resulting data. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. [Preview Abstract] |
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NP8.00063: Development of the High Sensitivity Multi Channel Bulk Absorption Laser Calorimeter Chunxiao Su, Cunbang Yang, Wenhong Li The ICF experiments performed at Shenguang II facility need to measure the total energy of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) as well as the weak backscattered SRS and SBS. Generally used laser calorimeter whose sensitivities are rather low, cannot meet the needs of such measurements. Because of long signal cable and AC power disturbance, simply boosting the amplifier gain will lead to the weak signal submerged by noise. Equipped with third order low pass active filters and software smooth filter, the multi channel bulk absorption laser calorimeter introduced in this paper, can significantly restrain the noise. Thus, the sensitivity of laser calorimeter can be greatly improved about two orders higher by means of boosting the amplifier gain than that of the normal type laser calorimeter. Using the new type laser calorimeter in the ICF experiments performed at Shenguang II facility, we obtained results different from that before. The experimental arrangement and the results are presented. [Preview Abstract] |
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NP8.00064: Electronic Measurement of Microchannel Plate Pulse Height Distribution C.M. Huntington, E.C. Harding, M.E. Lowenstern, R.P. Drake Microchannel plates are a central component to the x-ray framing cameras used in many plasma experiment diagnostic systems. Seeking to improve the photon-to-electron conversion efficiency of x-ray cameras we will characterize the pulse-height distribution of the electron output from a microchannel plate. Replacing the framing camera's phosphor-coated fiber optic screen with a charge-collection plate and coupling to a low-noise multichannel analyzer, we will quantify the total charge generated per photon event over a range of x-ray energies and incident fluxes. Hypothesizing that plate saturation is a function of incident photon flux, we will calculate the saturation regime for microchannel plates operated in a single-plate configuration. The electronically-measured pulse height distribution will be compared to the same data collected via a purely-optical method, as described previously (E. C. Harding and R. P. Drake, Rev. Sci. Instrum. 77, 10E312 (2006)). [Preview Abstract] |
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NP8.00065: C-MOD TOKAMAK |
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NP8.00066: Progress and Prospects of Advanced Integrated Scenarios on Alcator C-Mod A.E. Hubbard, P.T. Bonoli, C. Fiore, B. LaBombard, B. Lipschultz, Y. Lin, E. Marmar, R. Parker, M. Porkolab, A.E. Schmidt, G. Wallace, S.M. Wolfe, S. Wukitch, C. Kessel, J.R. Wilson ``Advanced'' scenarios, with a high non-inductive current and greater degree of control over current and pressure profiles, offer significant potential advantages over conventional tokamak operation. Key issues for application to burning plasmas such as ITER, and for DEMO, include obtaining improved confinement with low external torque and particle sources, and with coupled electrons and ions. External current drive in high confinement plasmas with an edge transport barrier, and compatibility with high divertor heat fluxes, are also important. The Alcator C- Mod integrated scenarios program focuses on addressing these challenges. Key new tools include a lower hybrid current drive system for current profile control and a cryopump for density control. Promising results from recent experiments using both of these will be reported. Modeling shows that scenarios with high non-inductive current fraction are achievable, with increased LHCD power. [Preview Abstract] |
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NP8.00067: Simulations and Experiments on Modifying the q-profile for Advanced Tokamak Discharges on Alcator C-Mod C. Kessel, S. Scott, R. Wilson, A. Hubbard, P. Bonoli, J-S. Ko, Y. Lin, R. Parker, A. Schmidt, D. Terry, G. Wallace, S. Wolfe, S. Wukitch As part of the advanced tokamak scenario development on Alcator C-Mod, time-dependent simulations using the Tokamak Simulation Code (TSC) and experiments are examining the impact of ion cyclotron radio frequency (ICRF) heating and Lower Hybrid (LH) heating and current drive on plasma evolution. Here the ICRF heating is obtained by using the hydrogen minority scheme at 80 MHz with BT of 5.4 T. The LH utilizes 4.6 GHz and a phasing of 90 degrees co-CD. Slower plasma current ramps, earlier diverting with heating, H-mode transition, and either ICRF or LH heating, as well as both together, are examined. The sawtooth onset, li, surface voltage, motional stark effect (MSE), and profile data are being used to categorize the impact and constrain the simulations where possible. Experiments have shown that LH powers less than 1/4 of the injected ICRF power can significantly delay the sawtooth onset when injected during rampup. While on the other hand, the ICRF power is found to be critical for accessing the H-mode. [Preview Abstract] |
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NP8.00068: Current Profile Modification By Lower Hybrid Waves in Alcator C-Mod R.R. Parker, P.T. Bonoli, A.E. Hubbard, J. Ko, M. Porkolab, A.E. Schmidt, D.E. Terry, G.M. Wallace, S.M. Wolfe, J.C. Wright, S.D. Scott, J.R. Wilson Driving current off-axis is a prerequisite for realizing steady-state, high-performance Advanced Tokamak (AT) regimes. Lower Hybrid Current Drive (LHCD) is well suited for this purpose since the driven current is typically deposited beyond r/a = 0.5. An important step toward the goal of forming AT regimes with LHCD in Alcator C-Mod is validation of ray-tracing and full-wave codes regarding the location of the LH driven current. In LH experiments on C-Mod, bremsstrahlung produced by fast electrons carrying the RF-induced current indicates that the current is driven off-axis at a location determined by the parallel index of refraction. This is in qualitative agreement with expectation and is supported by indirect evidence such as decreased li, sawtooth stabilization, and changes in q(r) as inferred from MSE. The results will be compared with the predictions of ray-tracing (GENRAY) and full-wave codes (TORIC) coupled with Fokker-Planck codes that follow the self-consistent evolution of f(v). [Preview Abstract] |
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NP8.00069: Lower Hybrid Coupling Experiments on Alcator C-Mod G.M. Wallace, P.T. Bonoli, A.E. Hubbard, Y. Lin, R.R. Parker, A.E. Schmidt, C.E. Kessel, J.R. Wilson The Alcator C-Mod Lower Hybrid launcher couples RF waves at 4.6 GHz via 4 rows of 22 phased waveguides. Directional couplers in the launcher structure measure forward and reflected power in each waveguide, while six Langmuir probes mounted to the front of the antenna grill monitor density at the plasma edge and act as RF probes for the observation of parametric decay. Parametric decay spectra grow exponentially with line averaged electron density in the regime $\omega $ = 3-6 $\omega _{lh}$. Measurements of the coupling of lower hybrid waves have been performed at power levels approaching 1 MW. Edge density, launched n$_{\vert \vert }$ spectrum, and plasma shape have been adjusted to optimize coupling in Ohmic and ICRF heated L- and H-mode plasmas. Preliminary results show that deleterious effects of ICRF on LH coupling are reduced following boronization, particularly in H-mode. Experimentally observed coupling results will be compared to simulations from a coupling code (M. Brambilla, Nuc. Fus., 16:47-54, 1976.). [Preview Abstract] |
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NP8.00070: Recent ICRF results on Alcator C-Mod Yijun Lin, S. Wukitch, A. Binus, A. Parisot, M. Reinke Alcator C-Mod utilizes 8 MW ICRF source power in the ion cyclotron range of frequencies (ICRF) to heat plasma in the D(H) or D(He3) minority regimes. The power is injected via two 2-strap and one 4-strap antennas. Recently, we have installed a fast ferrite tuning (FFT) system on one of the 2-strap antennas. The FFT system was shown to maintain the transmitter matchings to antenna loading in a variety of plasmas, L-mode, H-mode, ELMs and pellet injection, with power reflection less than 3{\%}. It can handle forward RF power up to 0.9 MW in typical H-mode plasmas. To understand the role ICRF sheaths play in impurity penetration, discharges with different magnetic topologies will be compared and presented. We will also report on experiments where sawtooth control in the presence of energetic ions is performed utilizing mode conversion current drive. For central seed current in advanced tokamak discharges, fast wave heating and current drive is a candidate scenario and modeling and initial experimental investigations will presented. Finally, new modeling and preliminary experimental result on the synergy between the mode converted IBW and lower hybrid current drive will also be presented. [Preview Abstract] |
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NP8.00071: Collisionality dependence of density peaking in H-mode plasmas in Alcator C-Mod D.R. Mikkelsen, M. Greenwald, J. Candy, R. Waltz Recent results from Alcator C-Mod confirmed earlier AUG and JET findings that spontaneous peaking of the density profile in H-mode plasmas depends on collisionality. Quasi-linear transport estimates based on gyrokinetic stability calculations (C. Angioni, et al., Phys. Plasmas 12, (2005) 112310) found a pinch that was restricted to a range of collisionality completely outside the range of experimental data. We will present nonlinear, `full-radius', gyrokinetic turbulence simulations with experimentally relevant levels of impurities to determine the degree to which these added levels of completeness can bring theory into harmony with experiment. Particular attention will be paid to the significance of including multiple impurity species and two hydrogenic species. We will also explore the sensitivity of the results to the detailed form of the collision operator, including one that is momentum conserving. [Preview Abstract] |
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NP8.00072: Ohmic ITBs in Alcator C-Mod William L. Rowan, Igor O. Bespamyatnov, C.L. Fiore, A. Dominguez, A.E. Hubbard, A. Ince-Cushman, M.J. Greenwald, L. Lin, E.S. Marmar, M. Reinke, J.E. Rice, K. Zhurovich Internal transport barrier (ITB) plasmas can arise spontaneously in Ohmic Alcator C-Mod plasmas. The operational prescription for the ITB include formation of an EDA H-mode in a toroidal magnetic field that is ramping down and a subsequent increase in the toroidal magnetic field. Like ITBs generated with off-axis ICRF heating, these have peaked pressure profiles which can be suppressed by on-axis ICRF heating. Recent work on onset conditions for the ICRF generated ITB (K. Zhurovich, et al., To be published in Nuclear Fusion) demonstrates that the broadening of the ion temperature profile due to off-axis ICRF reduces the ion temperature gradient and suppreses the ITG instability driven particle flux as the primary mechanism for ITB formation. The object of this study is to examine the characteristics of Ohmic ITBs to find whether this model for onset is supported. [Preview Abstract] |
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NP8.00073: The role of turbulent suppression in the triggering ITBs on C-Mod K. Zhurovich, C.L. Fiore, D.R. Ernst, P.T. Bonoli, M.J. Greenwald, A.E. Hubbard, J.W. Hughes, E.S. Marmar, D.R. Mikkelsen, P. Phillips, J.E. Rice Internal transport barriers can be routinely produced in C-Mod steady EDA H-mode plasmas by applying ICRF at $\vert $r/a$\vert \quad \ge $ 0.5. Access to the off-axis ICRF heated ITBs may be understood within the paradigm of marginal stability. Analysis of the T$_{e}$ profiles shows a decrease of R/L$_{Te}$ in the ITB region as the RF resonance is moved off axis. Ti profiles broaden as the ICRF power deposition changes from on-axis to off-axis. TRANSP calculations of the T$_{i}$ profiles support this trend. Linear GS2 calculations do not reveal any difference in ETG growth rate profiles for ITB vs. non-ITB discharges. However, they do show that the region of stability to ITG modes widens as the ICRF resonance is moved outward. Non-linear simulations show that the outward turbulent particle flux exceeds the Ware pinch by factor of 2 in the outer plasma region. Reducing the temperature gradient significantly decreases the diffusive flux and allows the Ware pinch to peak the density profile. Details of these experiments and simulations will be presented. [Preview Abstract] |
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NP8.00074: Low Density ITB Studies Using the Upgraded C-Mod Reflectometry System A. Dominguez, E. Edlund, C.L. Fiore, L. Lin, E.S. Marmar, J.A. Snipes, M. Porkolab, G.J. Kramer, W.L. Rowan The Alcator C-Mod reflectometry system was recently upgraded in two ways: The low frequency channels were changed from amplitude modulation - in which two microwave signals, slightly separated in frequency, are injected into the plasma - to baseband, where a single frequency is used, in order to improve density fluctuation measurements. The second change, a variable frequency channel operating over the range from 122GHz to 140GHz (with corresponding density cutoffs of 1.84-2.43x10$^{20}$m$^{-3})$ has been installed in collaboration with PPPL. Initial results from the upgraded system are presented, including the study of low density Internal Transport Barriers. Using O-mode waves, the reflectometry system is able to radially localize density fluctuations on the low field side along the tokamak midplane. It can, therefore, be used to probe the foot of low density ITBs. The corresponding reflectometry data will be compared to those of other fluctuation diagnostics, including Phase Contrast Imaging and magnetic pick-up coils. [Preview Abstract] |
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NP8.00075: Localized Temperature Scale Length Measurements in Alcator C-Mod With Central RF Heating Perry Phillips, Kenneth Gentle, William Rowan The electron temperature scale length $L_{T_e } =\raise0.7ex\hbox{${T_e }$} \!\mathord{\left/ {\vphantom {{T_e } {\nabla T_e }}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{${\nabla T_e }$}$can be measured locally using ECE and small $\raise0.7ex\hbox{${\Delta B}$} \!\mathord{\left/ {\vphantom {{\Delta B} B}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$B$}\approx 1\% $ changes in toroidal field. These measurements are extremely useful because they have high accuracy, are independent of the ECE calibration and they can have high spatial resolution. Alcator C-Mod has the ability to rapidly change the toroidal field to allow these measurements. The heterodyne ECE system on Alcator C-Mod has 32 closely spaced channels $\left( {\Delta R\approx 7mm} \right)$allowing very high spatially resolved temperature and $L_{T_e } $profiles. The experiments were conducted with central RF heating that allowed variation of the heat flux in the confinement region. This heat flux may then be compared to the changes in the $L_{T_e } $ as the RF power is varied. $L_{T_e } $is a key parameter in investigation of heat transport and the role of a turbulence threshold above a critical gradient that results in profile stiffness. Results will be presented in this poster. [Preview Abstract] |
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NP8.00076: Drift Wave Turbulence Studies with the Phase Contrast Imaging Diagnostic in Alcator C-Mod L. Lin, M. Porkolab, E.M. Edlund, D.R. Ernst, M. Greenwald, N. Tsujii The Phase Contrast Imaging diagnostic (PCI) in Alcator C-Mod has measured density fluctuations with frequencies up to 500 kHz and wavenumbers up to 20 cm$^{-1}$, which corresponds to $k_{R}$\textit{$\rho $}$_{s}$\textit{$\sim $}1.6. Furthermore, as the density increases in ohmic plasmas, the observed relative density fluctuation level decreases in the ``linear'' ohmic regime (low density, Alcator scaling, \textit{$\tau $}$_{kin}\propto n_{e})$, whereas it increases in the high density ``saturated'' ohmic regime. Recent upgrades have enabled the PCI system to localize the short wavelength turbulence in the ETG range and resolve the direction of propagation (i.e., electron vs. ion diamagnetic direction) of even the longer wavelength turbulence in the ITG/TEM range. Initial analysis of the observed turbulence in purely ohmic plasmas indicates that $\vert $\textit{\~{n}}$_{+}$\textit{/\~{n}}$_{-}\vert $ in the frequency range of 100-400 kHz and wavenumber range of 2-8 cm$^{-1}$ decreases monotonically but remains above 1.0 as the density is reduced. Here \textit{\~{n}}$_{+}$\textit{/\~{n}}$_{-}$ is the ratio of density fluctuations propagating in the ion diamagnetic direction to those propagating in the electron direction. We will compare these measurements with gyro-kinetic code predictions (e.g., GS2, GYRO). [Preview Abstract] |
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NP8.00077: Impurity poloidal rotation and other CXRS measurements for 0.1 $<\rho <$ 1.0 in Alcator C{\-}Mod plasmas. Igor Bespamyatnov, William Rowan, Kenneth Gentle, Robert Granetz, Dexter Beals The new wide-view poloidal CXRS array was installed and successfully tested during the C{\-}Mod's 2007 experimental campaign. The 19 new poloidal chords traverse the outer half of the C{\-}Mod plasma allowing measurement of B$^{+5}$ rotation, temperature, and density profiles for 0.1 $<\rho <$ 1.0. Along with the toroidal flow velocity measured by the toroidal CXRS array, these measurements are sufficient to investigate the validity of neoclassical theory. This work focuses on the top of the plasma pedestal 0.5 $<\rho <$ 0.90, where neoclassical theory is expected to be applicable and where the maximum CXRS enhancement is observed. Our previous study showed that the region of the plasma edge 0.90 $<\rho <$ 1.0, contained high density and temperature gradients that vary during the L to H mode transition so that comparison with neoclassical theory is very difficult. Radial electric field profiles will also be presented. The changes of the impurity rotation profiles and E$_{r}$ evaluation play a role in understanding of the physics of the L to H transition. [Preview Abstract] |
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NP8.00078: Impurity Temperature and Velocity Profiles in L-mode and H-mode from the New Edge Charge Exchange Diagnostic on Alcator C-Mod R.M. McDermott, B. Lipschultz, K. Marr, D. Whyte, J.W. Hughes The edge Charge Exchange Spectroscopy (CXS) system on Alcator C-Mod has been upgraded to include a beam viewing toroidal periscope. This periscope is designed to work in conjunction with the pre-existing edge poloidal CX periscope enabling concurrent measurements of the poloidal and toroidal velocity as well as the temperature and density of the B$^{+5 }$ions in the edge pedestal region (0.8 $<$ r/a $<$ 1.08). Temporally resolved radial profiles of B$^{+5 }$brightness, temperature, and velocity have been obtained in both L and H mode plasmas. Although the time resolution of the edge CXS diagnostic (6.2ms) is not sufficient to capture the evolution of edge profiles during standard L-H transitions, the temperature profiles obtained before and after the transition show excellent agreement with electron temperature measurements from the edge Thomson Scattering diagnostic. The obtained velocity profiles indicate that the poloidal velocity is the dominant term in the calculation of E$_{r}$ and H-mode poloidal velocity profiles indicate strong E$_{r}$ shear just inside the last closed flux surface. [Preview Abstract] |
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NP8.00079: Velocity measurements from the pedestal-viewing CXRS system on Alcator C-Mod Kenneth Marr, Bruce Lipschultz, Rachael McDermott The evolution of velocity profiles can be used to study the toroidal momentum transport and shear that purportedly play an important role in the formation and intensity of the H-mode. For the 2007 run campaign the charge-exchange spectroscopy diagnostic has been upgraded for better spatial and temporal resolution of the pedestal region of the plasma. The `pedestal' is the area near the separatrix where the density forms a steep gradient during H-mode. The diagnostic utilizes injected neutrals at both the high- and low-field edges of the plasma to spatially localize the measurement of toroidal and poloidal velocities at the intersection of the line of sight with the beam of injected neutrals. The upgrade added more views of the plasma, including background (away from the injection) views, and new spectrometers. Measured velocities trend as expected; increasing in magnitude into H-mode and into the core. These results are compared to similar diagnostics on Alcator C-Mod and their relation to momentum transport is discussed. Specific focus will be on v and Ti profile evolution during the transition from L- to H-mode. [Preview Abstract] |
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NP8.00080: Inference of Ion-Temperature and Rotation-Velocity Profiles from a Spatially Resolving X-Ray Crystal Spectrometer on Alcator C-Mod K.W. Hill, M. Bitter, P. Beiersdorfer, A. Ince-Cushman, Ming-Feng Gu, S.G. Lee, M. Reinke, J.E. Rice, S.D. Scott A new x-ray crystal spectrometer capable of providing spatially ($\sim $1.5 cm) and temporally ($\sim $10 ms) resolved, high resolution spectra of He-like Ar K$\alpha$ lines has been installed on Alcator C-Mod. The imaging spectrometer consists of a spherically bent crystal and three Pilatus II 2d pixel detectors. Spectra are simultaneously measured from 12 -- 45 chords covering the region r/a = 0 - 0.8 with count rates of 0.1 -- 5.0 MHz per chord, enabling measurement with good statistics. Preliminary ion-temperature (T$_{i})$ and rotation-velocity (v$_{\phi })$ profiles are inferred from the Doppler widths and shifts of the chordally integrated spectral lines. The data analysis techniques, T$_{i}$ and v$_{\phi }$ profile results, analysis of background resulting from fusion neutrons, and predictions of performance on ITER and other tokamaks will be presented. [Preview Abstract] |
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NP8.00081: Application of new fitting routines on edge Thomson scattering profiles from Alcator C-Mod N. Gierse, J.W. Hughes, B. LaBombard, B. Lipschultz The edge plasma region plays a key role in the performance of tokamak fusion devices, as transport in this region directly affects the energy confinement properties of the discharge and determines particle and heat loads on the first wall and the divertor. Recent experiments in Alcator C-Mod show strong evidence that transport in edge is governed by critical gradients physics in both L-mode and H-mode. To examine profiles from experimental millimeter resolution Thomson scattering edge data a modified tanh fit is routinely performed which reports the global pedestal parameters. The goals of this work are to apply new fitting techniques (e.g. B-splines), and then compare results to find the most suitable fitting routine which does not impose artificial constraints on the result. The results of such fits for a wide variety of L- and H-mode profiles are then stored in a recently constructed database with complete profile information. This allows us to conduct profile studies over a wide range in C-Mod operation space and over a range of radial locations in the edge region. [Preview Abstract] |
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NP8.00082: Results From Stereoscopic Imaging of the Ablation of Injected Li Pellets in the Alcator C-Mod Tokamak. B. Bose, E. Marmar, D. Mikkelsen, M. Greenwald, S. Zweben 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 lithium plasma during the ablation 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 $s after their formation and show distinctly different behaviors in ohmic L-mode plasmas and ICRH heated H-mode plasmas. During ohmic L-mode plasmas the direction in which the striations are emitted oscillates from the positive to negative ion diamagnetic direction on a length scale of 10-20 ion gyro radii, and they move with speeds of up to 5 km/s. On the other hand, during ICRH heated H-mode plasmas the striations show a distinct propensity to be emitted in the negative ion diamagnetic direction, and again move with speeds of up to 5 km/s. The measured characteristics of the striations will be compared with the theoretical predictions of Parks' striation theory [1]. In addition we are exploring possible correlations between striation characteristics and properties of self-generated zonal flows within the plasma. [1] Parks PB 1996 Plasma Phys, Contrl. Fusion \textbf{38 }571 [Preview Abstract] |
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NP8.00083: Design and operation of a novel divertor cryopumping system in Alcator C-Mod B. LaBombard, B. Beck, J. Bosco, R. Childs, D. Gwinn, J. Irby, R. Leccacorvi, S. Marazita, N. Mucic, S. Pierson, Y. Rokhman, P. Titus, R. Vieira, J. Zaks, A. Zhukovsky C-Mod's recently installed upper-divertor cryopump is unique among the world's tokamaks, employing an array of gas-pumping slots that penetrate the upper divertor target. This geometry enables the use of a single toroidal loop of liquid helium, operating in an efficient heat transfer regime with low or no helium flow. A system pumping speed of 9,600 l/sec for D$_{2}$ gas has been achieved, matching that of a full-scale prototype system. Neutral pressures in the pumping slots during upper-null plasmas (USN) are found to meet or exceed pressures in the lower divertor's private flux region during lower-null (LSN) -- evidence that the pumping-slot geometry is performing as intended. Very high steady-state pumping throughputs (exceeding $\sim $140 torr-l/s) have been demonstrated in USN. Reliable and efficient operation of the pump has been established, synchronized with the C-Mod shot cycle and consuming 60 to 90 liters of liquid helium during a full day of operation. [Preview Abstract] |
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NP8.00084: The Instrumental Function of the new X-ray Imaging Crystal Spectrometer on Alcator C-Mod M. Bitter, K.W. Hill, D. Mikkelson, S. Scott, A. Ince-Cushman, M. Reinke, J.E. Rice, P. Beiersdorfer, M.F. Gu A new high-resolution X-ray imaging crystal spectrometer was installed on Alcator C-Mod to determine the radial profiles of the ion temperature and toroidal plasma rotation velocity from the Doppler widths and Doppler shifts of spectral lines from He- and H-like argon. The instrument consists of two spherically bent crystals and high count rate, semi-conductor diode arrays, so-called PILATUS II detector modules, which are arranged in the Johann configuration. The poster will present analytical and numerical calculations of the instrumental function and the observed spectral line profiles. The results obtained from these calculations will be compared with the experimental data. [Preview Abstract] |
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NP8.00085: Spectroscopic Diagnostics using a Visible Transmission Grating Spectrometer at the Alcator C-Mod Tokamak A. Graf, M. May, P. Beiersdorfer, D. Whyte, B. Labombard, N. Smick, K. Marr A high throughput, f/\#$\sim$3.5, transmission grating spectrometer for visible light (350-670 nm), is being used to make localized measurements of bulk flow velocities and temperatures of impurity ions and deuterium atoms, in the edge and scrape off layer of C-Mod. The flows and temperatures of the main atoms are investigated by detecting D$_{\beta}$ emission from charge exchange recombined deuterons which is enhanced by providing a local atom source from a gas puff. The CCD detector used with the spectrometer allows a simultaneous measurement from 4 different spatial points with a time resolution $\geq$10 ms/frame. Comparison between our results and the edge flows measured by a Mach probe and those from other spectroscopic diagnostics will be given. A higher time resolution ($\sim$100 $\mu$s/frame) is possible by using a special readout mode of the CCD. This allows time resolved electron density measurements from Stark broadened D$_{\beta}$ during disruptions. [Preview Abstract] |
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NP8.00086: Effect of secondary beam neutrals on MSE: Experiment Jinseok Ko, Steve Scott, Bill Rowan, Bob Granetz, Ian Hutchinson, Howard Yuh It has been conjectured that the anomalous results obtained previously during beam-into-gas calibrations of the Motional Stark Effect (MSE) diagnostic on Alcator C-Mod are caused by the `secondary' beam neutrals - the neutrals that ionize following collisions with the torus gas and then re-charge exchange at a random gyro angle. Based on this conjecture, the C- Mod diagnostic neutral beam (DNB) was pivoted toroidally to impart a non-zero parallel beam velocity. In this new configuration, spectroscopic measurements and beam-into-gas calibrations confirm that the beam-into-gas anomaly is caused by the secondary beam neutrals. In addition, we report initial results from the beam-into-plasma calibration using plasma edge sweeping and plasma current ramping. [Preview Abstract] |
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NP8.00087: Effect of secondary beam neutrals on MSE: Theory S. Scott, J. Ko, I. Hutchinson, H. Yuh A standard calibration technique for Motional Stark Effect ({\sc mse}) diagnostics is to compare the polarization direction of Doppler-shifted H$\alpha$ emission from a diagnostic neutral beam ({\sc dnb}) that is fired into a gas-filled torus to the pitch angle inferred from known toroidal and vertical fields. However, the polarization direction of H$\alpha$ emission from `secondary' beam neutrals that ionize, gyrate about field lines, and then charge exchange a second time differs from the polarization direction of the `primary' beam neutrals and thus confuses the calibration results. We compute the ratio of secondary-to-primary H$\alpha$ emission, $I_s/I_p$, as a function of torus pressure for 50 keV hydrogen atoms in Alcator C-Mod. For helium gas, $I_s/I_p$ is about unity at P=1 mTorr for the {\sc dnb} in its recently re-oriented configuration ($7^{\circ}$ from perpendicular). The effect on the {\sc mse} calibration of H$\alpha$ emission from these secondary beam neutrals is calculated by adding the Stokes vectors for all secondary-beam gyro angles whose Doppler shift lies within the {\sc mse} filter passband. The computed calibration error increases linearly with torus pressure and has distinct dependencies on MSE viewing geometry and pitch angle which are in qualitative agreement with recent measurements. [Preview Abstract] |
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NP8.00088: Design of a new X-mode edge reflectometer for Alcator C-mod Cornwall Lau, Greg Hanson, John Wilgen, Yijun Lin, Steve Wukitch The study of antenna-plasma interactions during RF heating and current drive often requires high temporal and spatial resolution density profiles of the SOL in front of the ICRF antenna. A new swept-frequency X-mode reflectometer is being built for Alcator C-mod to measure the edge density profiles in front of the future E port antenna. Due to the presence of strong ICRF heating and large density fluctuations, density profile measurements are difficult. This reflectometer is thus designed to use both differential-phase and full-phase reflectometry techniques to allow for the best results to be obtained.~~ 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. Design of this new reflectometer and initial results from modeling and testing will be presented. [Preview Abstract] |
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NP8.00089: Development of Absolute Calibration of the Phase Contrast Imaging Diagnostic and Experimental Tests in Alcator C-Mod N. Tsujii, M. Porkolab, E.M. Edlund, L. Lin The Phase Contrast Imaging (PCI) system in Alcator C-Mod is used to measure density perturbations from MHD modes, turbulence and RF waves. Recently, an absolutely calibrated system has been installed. This system consists of a set of transducers which cover frequency from 30 kHz to 200 kHz, and wavenumber from 5.5 cm$^{-1}$ to 36.6 cm$^{-1}$. The amplitude and phase of the transducer wavefronts are measured using a calibrated microphone. We will present the system design and modeling of this calibration system. Initial results, including a comparison with experimental measurements will also be discussed, if available. [Preview Abstract] |
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NP8.00090: Recent upgrades to Thomson scattering diagnostics on Alcator C-Mod for improved core spatial resolution Y. Ma, J.W. Hughes, K. Zhurovich, A. Hubbard The Thomson Scattering (TS) diagnostics on C-Mod employ two Nd:YAG lasers, each with a nominal 1.3J, 8ns pulse and 30Hz pulse rate, directed along the same vertical path to measure the electron temperature and density profiles. The core TS diagnostic set has recently been upgraded by adding 8 new compact polychromators in order to deliver core plasma measurements with improved spatial resolution. Up to 16 core detector channels are now available, with each $T_e,n_e$ measurement localized to 4mm in radial coordinates. Optimal positioning of collection fibers concentrates the measurement locations in the internal transport barrier region (typically centered at r/a~0.5), yielding an approximate spatial resolution of 15mm, allowing more accurate study of the plasma gradient scale lengths in this regime. The new polychromators demonstrate high signal-to-noise ratio (S/N) when operated during C-Mod discharges. Calibration results, analysis and plasma measurements will be presented. [Preview Abstract] |
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NP8.00091: Real-time Detection of Locked Modes S. Angelini, R.S. Granetz, S.M. Wolfe Disruptions are one of the largest problems facing tokamaks. In a large-scale experiment such as ITER, disruptions would cause crippling damage and severe setbacks in experimentation. One method for disruption mitigation involves the use of a gas jet which has been tested on both normally running plasmas and vertical displacement events (VDEs) on Alcator C-Mod. In both cases, the jet was successful in mitigating disruption effects. The gas jet has not yet been tested on other types of disruptions. Locked-mode major disruptions are easily created in C-Mod and could be used to test the effectiveness of the gas jet as a mitigation method if the jet could be fired early enough. It has been empirically observed that the electron cyclotron emissions (ECE) signal displays a flattening of the normally-present sawteeth before the current quench occurs in certain locked-mode major disruptions. A procedure is being written which will detect the ECE flattening by reading changes in the standard deviation of the signal. This procedure will be programmed into the digital plasma control system (DPCS) for real-time testing. [Preview Abstract] |
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NP8.00092: Optimization of gas jet disruption mitigation Robert Granetz, Dennis Whyte, Matthew Reinke, James Terry Recent experiments on Alcator C-Mod have focused on finding the minimum amount of optimal gas mixture that is required for good disruption mitigation. We desire to reduce the total amount of gas injected into the vessel in order to minimize impact on diagnostics and to shorten the pumpout time between discharges. Previous experiments on C-Mod have shown that a mixture of 15\% argon/85\% helium optimizes the response time of the gas jet system by increasing the transit speed of the argon, while still realizing good mitigation of halo currents and thermal loads on the divertor surfaces. But those experiments were all done with the plenum pressure set to 70 bar, which results in a undesirably large gas load into the tokamak. It is found that the plenum gas pressure can be reduced substantially, and thus the gas load in the torus, while still maintaining good mitigation characteristics. Results will be presented. [Preview Abstract] |
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NP8.00093: Kalman Filter for the Real Time Estimation of the Vertical Position of C-Mod Plasmas M. Ferrara, I. Hutchinson, S. Wolfe, J. Stillerman, T. Fredian A Kalman filter has been implemented for the real-time estimation of the vertical position of C-Mod plasmas. The simulator Alcasim, which reproduces the full evolution of a discharge, is used to synthesize the filter matrices at a time point. The linear model is then employed in the filter for the full duration of the flattop and for different target plasmas, in order to test for robustness. The filter has been extensively tested in linear and full Alcasim simulations. The performance in terms of noise rejection and signal distortion is evaluated in comparison with standard filtering such as lowpass and bandpass. Results show excellent signal tracking and noise rejection. For real-time implementation, computation speed is also a requirement, therefore issues such as off-line training of the filter and linear model reduction are investigated. The experimental test of the filter in the C-Mod fast vertical control loop is expected shortly. This could demonstrate a possible solution to the problem of control degradation and AC losses expected on ITER because of measurement noise. USDoE award DE-FC02-99ER54512. [Preview Abstract] |
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NP8.00094: Mode structure and stability analysis of reversed shear Alfv\'en eigenmodes with NOVA-K Eric Edlund, Miklos Porkolab, Liang Lin, Naoto Tsujii, Steve Wukitch, Gerrit Kramer Experimental studies of reversed shear Alfv\'en eigenmodes (RSAEs) with phase contrast imaging (PCI) on the Alcator C-Mod tokamak will be presented. The PCI system measures the line integral of electron density fluctuations along 32 vertical chords passing through the plasma core, and was configured to measure frequencies up to 5 MHz and wavenumbers up to 8 $cm^{-1} $ for these studies. With the PCI system the location of RSAEs can be determined very accurately. In combination with the strong dependency of the RSAE frequency on the minimum value of q this leads to severe constaints for the equilibrium reconstruction of reversed shear discharges. Interpretation of the PCI signals is aided by simulations from NOVA-K calculations of the electron density fluctuations, via a synthetic PCI. This technique has been used to interpret observations of RSAEs during sawteeth and the current ramp-up. Applied to RSAEs during sawteeth, it is found that q reaches a minimum of about 0.97 prior to the crash. NOVA-K calculations of RSAE stability and corresponding constraints on the fast ion population will be presented. *Work supported by US DoE awards DE-FC02-99-ER54512 and DE-FG02-94-ER54235 and DE-AC02-76CH03073. [Preview Abstract] |
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NP8.00095: Influence of ICRF heating on the stability of TAEs J. Sears, W. Burke, R.R. Parker, J.A. Snipes, S. Wolfe Unstable toroidicity-induced Alfv\'{e}n eigenmodes (TAEs) can appear spontaneously due to resonant interaction with fast particles such as fusion alphas, raising concern that TAEs may threaten ITER performance. This work investigates the progression of stable TAE damping rates toward instability during a scan of ICRF heating power up to $3.1$ MW. Stable eigenmodes are identified in Alcator C-Mod by the Active MHD diagnostic. Unstable TAEs are observed to appear spontaneously in C-Mod limited L-mode plasmas at sufficient tail energies generated by $>3$ MW of ICRF heating. However preliminary analysis of experiments with moderate ICRF heating power show that TAE stability may not simply degrade with overall fast particle content. There are hints that the stability of some TAEs may be enhanced in the presence of fast particle distribution tails. Furthermore, the radial profile of the energetic particle distribution relative to the safety factor profile affects the ICRF power influence on TAE stability. [Preview Abstract] |
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NP8.00096: Surface Science Station (S$^{3})$: a real-time diagnostic of boronization in Alcator C-Mod Roman Ochoukov, Bruce Lipschultz, Dennis Whyte, Niels Gierse, Soren Harrison The recently installed Surface Science Station (S$^{3})$ was used to study the effectiveness of boronization utilizing electron cyclotron discharge (ECD) plasmas (90{\%}He 10{\%}D$_{2}$B$_{6})$ in Alcator C-Mod. The boron (B) deposition was measured in real time with a pair of quartz microbalances (QMBs) the faces of which were oriented parallel and perpendicular to the applied magnetic field. A maximum B deposition rate of $\sim $2.5 nm/min was measured on both QMBs for a chamber pressure of 15 mTorr. The peak deposition took place $\sim $10 cm outboard from the EC resonance. Decreasing the D$_{2}$B$_{6}$ throughput (pressure) decreased the B deposition rate, again for both QMBs. Overall, the evidence points to the B deposition being primarily ionic and that B ion Larmor radius effects lead to B deposition on surfaces parallel to the magnetic field (implying B ion temperatures in the range of 10-15 eV). Subsequent application of He-only ECDs showed evidence of the B layers being eroded {\&} redeposited by the plasma with rates 10x lower on the QMB face parallel to the magnetic field compared to the QMB face perpendicular to the field. [Preview Abstract] |
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NP8.00097: Diagnostics to Study Flow of Dust Particles in Scrape-Off Layer of Alcator C-Mod Plasmas Aaron Bader, Robert Granetz, Brian LaBombard, James Terry Dust transport and migration is not well understood in tokamaks. Furthermore, current numerical codes (DUSTT) have not been benchmarked with experiments. Interest in dust has increased over recent years as it may be a significant issue in machines with high duty cycles (ITER, DEMO) due to safety concerns. Measuring dust particle trajectories in the plasma can also give added information on Scrape-Off Layer (SOL) flows, since an important force on dust flow is a plasma drag force. In order to study dust particle trajectories in the SOL for different plasma densities and topologies, we have designed and installed a dust injector which injects Boron dust particles into the divertor region. The particles are viewed with a video camera. This provides us with a 2-D projection of the particle trajectories. For full 3-D trajectories we would need to install a second viewing camera. Initial data and results will be presented along with the successes and shortcomings of the diagnostic and future improvements. [Preview Abstract] |
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NP8.00098: Stationary ELM-free H-mode on TCV L. Porte, S. Alberti, E. Asp, A. Bortolon, A. Karpushov, Y. Martin, O. Sauter By heating an ohmic ELMy H-mode target using vertically launched 3$^{rd}$ harmonic X-mode ECRH (X3), it was possible to obtain coupled power up to $\approx $1.3MW which was much greater than the $\approx $These discharges often transited to an ELM-free H-mode regime with constant electron density and stored energy in which the stored energy and toroidal beta both doubled. The maximum, achieved toroidal beta was 2.5{\%} while the ideal beta-limit for these discharges was 3.5{\%}. The recycling light level was high compared to the baseline ohmic H-mode level and the fluctuations in the recycling light level were correlated with core MHD. The energy confinement time was high, H$_{IPB98(y,2)}\approx $1.7, and was limited by core MHD. Measurements of ion temperature profiles and rotation velocity showed that the mid-radius ion temperature increased from 500eV to 1keV while the plasma rotation increased also from 5kms$^{-1}$ to 50kms$^{-1}$. An overview of these experiments will be presented. [Preview Abstract] |
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NP8.00099: Physics insight from plasma shaping experiments in the TCV tokamak A. Pochelon, A. Bortolon, Y. Camenen, B.P. Duval, A. Marinoni, H. Reimerdes, A. Scarabosio The characterization of the effects of shaping on tokamak plasmas is crucial for the design of future devices, as it influences both transport and MHD stability. This is studied in TCV over a broad range of elongations $k\le $2.8, positive and negative triangularities $d$, limited/diverted configurations, with 4.5MW EC heating. We discovered a strong dependence of electron energy diffusivity on $d.$ The confinement time doubles as $d$ goes from +0.4 to -0.4 in L-mode. These studies allowed the separation of the effects of shape and collisionality on transport. Trapped Electron Modes turbulence (TEM) is expected to dominate in these conditions and gyrokinetic simulations confirm the strong dependence of anomalous diffusivity on $d$. The change of turbulence nature (TEM/ITG) in density ramps may play a role in the abrupt intrinsic toroidal rotation inversion found in TCV as the density is ramped in high current discharges with $d>$0. Shape also modifies MHD and disruptions. High $k$ is destabilizing (freq. sawteeth), high $\pm d$ is stabilizing, in agreement with MHD stability. Plasma shaping is effective in stabilizing modes and preventing disruptions, e.g. in the current ramp-up, important for the ITER high-current scenario. [Preview Abstract] |
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NP8.00100: Effect of radio-frequency power injection on impurity profile in JET plasmas L. Carraro, C. Angioni, C. Giroud, M.E. Puiatti, M. Valisa, P. Buratti, R. Buttery, I. Coffey, L. Garzotti, D. Van Eester, L. Lauro Taroni, K. Lawson, E. Lerche, P. Mantica, M. Mattioli, V. Naulin To maximize the reactivity of a fusion plasma the impurity content should be as low as possible in the plasma centre. RF power has been seen to flatten the impurity density profiles in various experiments, especially when applied to electrons and deposited in the centre. To evaluate the potentiality of such phenomenon as a means towards an active control of the impurity profiles, JET H-Mode and Hybrid discharges at ITER relevant collisionality ($\nu $eff$<$0.2), with and without RF power applied to electrons, have been transiently doped with traces of Ne, Ar, Ni and Mo. With RF the pinch parameter (ratio of convection velocity to diffusion) of all the injected impurities reduces remarkably or even reverses its sign and the impurity profiles are flattened or become hollow. Such beneficial effect is to be weighted by the increase of the metal content in the plasma that can result from the RF application. Quasi linear gyro-kinetic simulations (GS2) do not explain the observed outward pinch and refer the analysis to a full non linear approach. [Preview Abstract] |
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NP8.00101: Radiative type-III ELMy H-mode at JET: expansion of operational domain Juergen Rapp The radiative type-III ELMy H-mode seems a possible solution for an integrated ITER scenario, combining the constraints of power exhaust with acceptable confinement for an inductive ITER scenario at 17 MA / 5.3 T (q$_{95}\sim $2.6) with a power amplification factor of Q=10. At JET this scenario has been demonstrated at 2.5 MA / 2.0T with almost all parameters within the ITER target values. However, the confinement meets marginally the ITER requirements and the fuel dilution could be improved. New experiments were performed at higher plasma current, up to 3.0 MA. At lower densities, N$^{GW}$=0.85, the confinement was slightly improved from H$_{98(y,2) }\sim $ 0.73 to 0.83. The Z$_{eff}$ was reduced to values of 1.5 mainly due to the increased absolute density. The collisionality was reduced by a factor of 3. The new data show that there is no apparent dependence of the confinement on the collisionality. A review of the old large database of impurity seeded type-III ELMy H-modes together with the new results has been performed and a new analysis of the global confinement scaling with dimensionless variables is made. [Preview Abstract] |
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NP8.00102: Damping Rate Measurements of Medium n Alfv\'{e}n Eigenmodes in JET Alexander Klein, Duccio Testa, Joseph Snipes, Ambrogio Fasoli, Herv\'e Carfantan Alfv\'{e}n Eigenmodes (AE's) with mode numbers 5 $<$ n $<$ 20 are expected to be unstable in burning tokamaks and may lead to loss of fast particle confinement. The active MHD spectroscopy program at JET has already provided a wealth of information about low n (n $\le $ 2) AE's in the past decade, but a recently installed array of four antennas is capable of driving higher mode numbered (n $<$ 100, 30 $<$ f $<$ 350 kHz) perturbations. In the latest JET campaign, the damping rates for several types of AE's were measured parasitically in a wide range of tokamak scenarios. We review the active MHD diagnostic and present the first measurements of medium-n AE stability on JET, then describe future plans for the active MHD spectroscopy project. The data analysis involves a novel method for resolving multiple AE's that exist at identical frequencies, which uses techniques based on the SparSpec code. [Preview Abstract] |
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NP8.00103: Radial correlation reflectometry measurements on the JET tokamak A. Fonseca, B. Alper, R. Budny, L. Cupido, J. Fessey, A. Figueiredo, S. Hacquin, M.E. Manso, E. Mazzucato, L. Meneses, A. Sirinelli, M. Walsh Fluctuations and turbulence are believed to play an important role in anomalous transport of heat and particles in magnetic fusion devices. It is of vital importance to characterize transport mechanism in order to understand and control it. In correlation reflectometry (CR), two microwave beams with different frequencies are launched into the plasma from which the turbulence correlation length, $L_{r}$, and the density fluctuation level, $\raise0.7ex\hbox{${\tilde {n}}$} \!\mathord{\left/ {\vphantom {{\tilde {n}} n}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$n$}$ can be deduced; quantities that are important for the study of plasma turbulence and transport. In this paper, some results of $L_{r}$ obtained with the four X-mode CR systems installed at Joint European Torus (JET) are reported. Experimentally, it was observed that $L_{r}$ increases from the plasma edge to the plasma core. Also, $L_{r}$ decreases inside the Internal Transport Barriers (ITB) [1]. The results obtained at JET agree quiet well with the $L_{r}$ dependence measured in other tokamak machines. [1] - A.C.A. Figueiredo, et.al., \textit{in the 34}$^{th}$\textit{ EPS Conference on Plasma Physics, Warsaw, Poland, 2-7 July, (2007)}. [Preview Abstract] |
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NP8.00104: Factors that Influence RF Breakdown in Antenna Systems J.B.O. Caughman, F.W. Baity, D.A. Rasmussen, M. Aghazarian, C.H. Castano Giraldo, David Ruzic One of the main power-limiting factors in antenna systems is the maximum voltage that the antenna or vacuum transmission line can sustain before breaking down. The factors that influence RF breakdown are being studied in a resonant 1/4-wavelength section of vacuum transmission line terminated with an open circuit electrode structure. Breakdown can be initiated via electron emission by high electric fields and by plasma formation in the structure, depending on the gas pressure. Recent experiments have shown that a 1 kG magnetic field can influence plasma formation at pressures as low as 8x10-5 Torr at moderate voltage levels ($<$5 kV). Ultraviolet light, with energies near the work function of the electrode material, can induce a multipactor discharge and limit power transmission. Details of these experimental results, including the effect of electrode materials (Ni and Cu), will be presented. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725. Work supported by USDOE with grant DE-FG02-04ER54765 [Preview Abstract] |
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NP8.00105: ITER AND MAGNETIC FUSION DEVELOPMENT |
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NP8.00106: ITER Low-Field-Side-Reflectometer Design Considerations W.A. Peebles, P-A. Gourdain, T.L. Rhodes, L. Zeng, S. Kubota, G. Wang, E.J. Doyle The ITER environment will be extremely harsh with heat loads and neutron flux levels reaching unprecedented levels. Ensuring the availability, reliability and accuracy of profile monitoring capabilities in ITER represents a major challenge. In contrast to optical-based diagnostics millimeter-wave systems are well-suited to this harsh environment. The first detailed U.S. assessment of the low-field-side reflectometer system is presented. Relativistic effects occur at high temperatures introducing a number of measurement challenges. Large changes in the reflectometry cutoff location occur due to relativistic mass correction, which leads to density profile measurement in ITER becoming dependent on knowledge of the local electron temperature. Furthermore, cutoff contours can become hollow and electron cyclotron absorption increases. These effects restrict core access and significantly modify wave propagation in the core plasma. The impact of these issues on reflectometry measurement capabilities is described and potential solutions discussed. [Preview Abstract] |
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NP8.00107: Study of Doppler Backscattering for Plasma Rotation Measurements on ITER T.L. Rhodes, S. Kubota, W.A. Peebles, L. Schmitz, L. Zeng, E.J. Doyle, G. Wang Millimeter-wave based diagnostic systems are well suited to the harsh environment expected in ITER and other future burning plasmas. One such technique, Doppler reflectometry, has been proposed for plasma rotation measurements on ITER. In this technique radiation is injected at an angle with respect to the plasma edge and the Doppler shift of the density fluctuations monitored. This shift depends upon both the background ExB velocity as well as the intrinsic propagation velocity of the fluctuations. The physics of Doppler backscattering and its specific application to ITER are studied using full wave 2D simulations for ITER scenarios. Near the cutoff layer a long wavelength electric field pattern is formed roughly parallel to the flux surface. It is this field pattern that interacts strongly with the density fluctuations propagating in the poloidal direction, i.e. within the flux surface. Data from the DIII-D tokamak as well as the limitations and potential of this method for ITER will be presented and discussed. [Preview Abstract] |
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NP8.00108: Initial design of visible/IR camera optics for upper ports of ITER C.J. Lasnier, L.G. Seppala, K. Morris, M.E. Fenstermacher, M. Groth We show an initial optical design for the visible/IR camera systems that are a US responsibility for 6 of the ITER upper ports. Optics are enclosed in a tube with a entrance aperture through the blanket shield module. An aspheric collection mirror sends light to a flat mirror that redirects the beam along the port tube. Dogleg mirrors provide a jog in the beam, allowing for neutron shielding. The beam is spatially split into visible and IR beams inside the port flange, for separate vacuum windows. Spatial resolution is diffraction-limited by the aperture, which in turn depends on the size of the optics allowed in the port plug. For a view of the entire outer divertor plate with no intermediate focusing optics in the port tube, the spatial resolution is poorer than the specified 3mm. We show the resolution advantages of reducing the field of view and of adding a lens in the port plug. [Preview Abstract] |
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NP8.00109: System Design of the MSE Diagnostic for ITER M.A. Makowski, S.L. Allen, M. Gu, C.T. Holcomb, S. Lerner, K. Morris, N. Wong, R. Chipman System designs for both the core and edge motional Stark effect (MSE) diagnostics for ITER have been developed and continue to be refined. An integrated system model has been developed that includes the emission physics and full optical properties of the four mirror relay system. Emphasis has been placed on minimizing the polarization aberrations of the optical relay system. A mechanical design has also been developed that can be coupled to a neutronics code for rapid evaluation of the shielding efficacy as alternative designs are examined. As part of the design program, candidate mirror materials are being characterized and then incorporated into the optics model to evaluate their impact on system performance. An overview of the system will be presented, together with various aspects of the emission model and optical and mechanical designs. [Preview Abstract] |
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NP8.00110: Evaluation of a DNB for ITER-Based on Common Long-Pulse Positive Ion Source Technology D.M. Thomas, R.W. Callis, R.M. Hong, H.K. Chiu, M. von Hellermann The use of a diagnostic neutral beam (DNB) is critical for the ITER diagnostic mission in order to provide radially resolved profiles of helium ash density as well as ion temperature, density, and rotation profiles. However, the estimated performance requirements for a suitable beam have yet to be demonstrated for either negative or positive ion source technology. In this paper we re-examine the suitability of existing common long pulse source (CLPS) technology, as exemplified in the DIII-D and TFTR programs, to provide a positive-ion-based DNB that will satisfy the ITER mission requirements with high reliability and ease of modulation. Straightforward modifications of the existing ion sources are expected to provide greater than 50 A of full energy hydrogen atoms at 100~keV, with a peak current density at the shield aperture approaching 200 A/m$^{2}$. Estimated S/N for BES and CXRS measurements for specific ITER operating scenarios will also be presented. [Preview Abstract] |
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NP8.00111: Notch Filter Possibilities for ITER Stray Gyrotron Radiation Paul Woskov ITER will have multi megawatt gyrotron systems at 170, 120, and potentially at 60 GHz for ECH, current drive, NTM control, start up, and CTS diagnostics. The ITER environment will therefore have significant background levels of stay radiation at these frequencies that can pose a problem to a number of diagnostic systems. Several, narrowband reject filter approaches have been examined. These include fundamental mode waveguide (wg) filters, quasi-optical interference filters, and molecular absorption gases. Fundamental wg stop band filters are available for 60 and 120 GHz, but have limited pass bands and may not be readily extended to 170 GHz. An overmoded corrugated wg interference filter with $>$ 30 db rejection over a 40 MHz band, a free spectral range of 6.7 GHz, and minimum insertion loss of 0.6 dB has been tested over the 100-145 GHz range and could be used to over 170 GHz. The OCS molecule as a series of 12 GHz spaced rotational lines with peaks at 60.8, 121.6, and 170.3 GHz with absorptions of 0.4, 3.2, 8.0 dB/m, respectively that could be used at low pressure with slight gyrotron adjustment. The H$_{2}$S molecule has absorption at 168.8 GHz that at atmospheric pressure would rejection by 8.6 dB/m at 170 GHz with insertion loss $<$ 0.01 dB/m below 156 GHz. The relative merits of these notch filter approaches will be presented. [Preview Abstract] |
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NP8.00112: ITER Ion Cyclotron Transmission Line and Tuning System Design Options and Choices D.W. Swain, R.H. Goulding, D.A. Rasmussen The ITER ion cyclotron system is required to deliver 20 MW of power to the plasma for ion heating and for current drive over the 40- to 55-MHz frequency range. An externally tuned antenna module consisting of an array of 24 (4 toroidal x 6 poloidal) current straps is mounted in an equatorial port. The straps are connected (internal to the antenna) in upper and lower poloidal triplets. Eight rf sources are connected to the transmission line and tuning/decoupling system and then to the feeds for the triplets. Two external tuning options that will permit ICH operation with ELMy H-modes are under consideration. In one option, conjugate-T, plasma load resilient, circuits are connected to triplet pairs. The conjugate-T matching components are external to but near the antenna. An alternate design uses a series of combiner-splitters (ELM dumps) to protect the rf sources from reflected power from transient plasma load variations. This talk will describe the design options, evolution of the different designs, and the pros and cons of the options. Future design work and R{\&}D will be needed and will be described. Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725. [Preview Abstract] |
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NP8.00113: Effects of local ECCD driven by the optimized Equatorial and Upper EC Launchers on ITER C. Zucca, O. Sauter, M.A. Henderson, D. Farina, G. Ramponi The present ITER base-line design has the EC launchers providing only co-ECCD. A variant of the EC system was recently proposed to enlarge the physics programs covered by the Upper (UL) and Equatorial (EL) Launchers. This study aims to provide the potential range of the $q$ profile control achievable by this optimization, including the possibility to drive cnt-ECCD and central ECH. Since the EL can only drive co-ECCD, if ECH power is needed to assist the L-H transition during ramp-up, it can have detrimental effects on the final profiles, such as removing the reverse shear. Cnt-ECCD offers greater control of the plasma current density and provides, when balanced with co-ECCD, pure ECH with no net driven current. The performance of the EL in tailoring the $q$ profile by adding co-/cnt-ECCD is analyzed. Effects of current drive and deposition width on sawtooth control by UL are also discussed. The modeling is carried out with both equilibrium and transport codes. [Preview Abstract] |
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NP8.00114: 170 GHz ITER ECH Transmission Line Estimated Losses and Testing M.A. Shapiro, S.T. Han, J.R. Sirigiri, D. Tax, R.J. Temkin, P.P. Woskov, T.S. Bigelow, D.A. Rasmussen The US will build 24 Transmission Lines (TL) connecting the 170 GHz gyrotrons to the ITER tokamak. Losses in the 63.5 mm diameter corrugated waveguide are estimated using an approximate analytical theory as 11{\%} for a TL length of about 100 m. Experimental demonstration of such low loss transmission is a challenge. The results of estimation can be verified through precise measurements and advanced simulations. A set of 170 GHz corrugated waveguide TL components built by General Atomics is under test at MIT. High precision measurement techniques include a vector network analyzer and a radiometer. The miter bends are tested as critical TL components, since they are responsible for the largest amount of mode conversion and loss. The testing results are compared with advanced simulations using the HFSS code (Ansoft Corp.). Another critical issue is additional loss due to thermal or mechanical distortion of the miter bend mirror and the waveguide sections during 1 MW CW power transmission. The critical issues have been revealed in high power CW test of the 170 GHz TL using the 1 MW 170 GHz gyrotron at JAEA, Japan. The US is planning a high power test stand for testing the ITER TL. Advanced thermo-mechanical simulations and HFSS electromagnetic simulations will be conducted to explain the experimental results. [Preview Abstract] |
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NP8.00115: Digital Holography Density Diagnostic for ITER Disruption Mitigation Test Stand C.E. Thomas Jr., L.R. Baylor, S.K. Combs Digital Holography holds the promise of providing very fast high-resolution density data at a relatively inexpensive price. A proof-of-principle system to demonstrate feasibility of the diagnostic is being designed and built on the ITER Disruption Mitigation Test Stand at ORNL. Although holographic interferometry has long been used as a density diagnostic, this will be the first application of digital holography to this task. While risky due to its unproven nature, digital holography offers the opportunity to provide an unprecedented fine-grain measurement of gas and/or plasma density. In a crossed-sightline configuration, 3-D density diagnosis is possible. This opens the door to developing new physics understanding both for disruption mitigation technology and on experimental fusion research devices. Understanding usually leads to both control and improvement. Details of the diagnostic system design and expected performance will be presented. [Preview Abstract] |
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NP8.00116: Beryllium deposition on ITER first mirrors: layer morphology and influence on reflectivity Gregory De Temmerman, Matt Baldwin, Russ Doerner, Daisuke Nishijima, Ray Seraydarian, Klaus Schmid, Christian Linsmeier, Laurent Marot Metallic mirrors will be essential components of the ITER optical diagnostics whose reliability may be affected by mirror reflectivity changes due to erosion and/or deposition of impurities (C, Be). The present study aims to assess the effect of Be deposition on the reflectivity of Cu and Mo mirrors and to collect data on the morphology of these layers. Mirrors were exposed in PISCES-B to collect eroded material from graphite and beryllium targets exposed to a Be-seeded D$_{2}$ plasma. After exposure, relative reflectivity of the mirrors was measured and the deposited layers were studied using different surface analysis techniques. Be layers formed in PISCES-B exhibit high levels of porosity and a reflectivity much lower than that of pure Be. It is found that if Be deposition occurs on ITER first mirrors, the reflectivity of the coated mirrors will strongly depend on the layer morphology, which in turn depends on the deposition conditions. [Preview Abstract] |
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NP8.00117: Codeposition of deuterium with the ITER mix of materials Russ Doerner, Matt Baldwin, Gregory De Temmerman, Daisuke Nishijima, Ch Linsmeier, Jochim Roth, Klaus Schmid Tritium accumulation within the ITER vacuum vessel is expected to be dominated by codeposition. Since ITER will employ a unique combination of plasma-facing materials (Be, C and W), the codeposition properties of each of the materials, and combinations of the materials, must be evaluated. The database on codeposition is reviewed and new measurements from PISCES-B are used to understand apparent discrepancies found in the literature. The variables determining the deuterium content in the layers appear to be the surface temperature during the codeposition, the morphology of the layer and the energy of particles incident on the codepositing layers. In addition to retention in codeposited layers, the ease of removal of deuterium from codeposits will determine their importance for ITER. The release of deuterium during vacuum annealing will be compared for each of the three proposed ITER materials, as well as from mixtures of the materials. [Preview Abstract] |
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NP8.00118: Carbon Atom and Cluster Sputtering Yields under Low Energy Noble Gas Bombardment Eider Oyarzabal, Russ Doerner, Masashi Shimada, George Tynan Carbon atom and cluster (C$_{2}$ and C$_{3})$ sputtering yields are measured during different noble gas (Xe, Kr, Ar, Ne and He) bombardment from a plasma, for low incident energies (75 -225 eV). A quadrupole mass spectrometer (QMS) is used to detect the fraction of sputtered neutrals that is ionized in the plasma and to obtain the angular distribution by changing the angle between the target and the QMS aperture. A one dimensional Monte Carlo code is used to simulate the plasma and the sputtered particles from the sample to the QMS, and to obtain the elastic scattering crossections of C, C$_{2}$ and C$_{3}$ with the different bombarding gas neutrals by changing the distance between the sample and the QMS and fitting the simulation results to the experimental results. The total sputtering yield (C+C$_{2}$+C$_{3})$ for each bombarding gas is obtained from weight loss measurements and the sputtering yield for C, C$_{2}$ and C$_{3}$ is then calculated from the integration of the measured angular distribution, taking into account the scattering and ionization of the sputtered particles from the sample to the QMS. We observe a clear decrease of the cluster (C$_{2}$ and C$_{3})$ to atom sputtering ratio as the incident ion mass decreases. [Preview Abstract] |
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NP8.00119: A Fitting Routine to Obtain Temperature and Relative Spectral Emissivity from Mixed Material (C, W, Be) Surfaces in PSI Experiments Ray Seraydarian, Jeremy Hanna, Russ Doerner, Matt Baldwin A nonlinear least-squares fitting code has been written to fit a black body profile multiplied by a polynomial spectral emissivity to the data from an IR spectroscopy diagnostic on the PISCES-B machine. The result is the absolute surface temperature and the coefficients of the polynomial emissivity. Emissivity measurements are important for thermal balance calculations of first wall materials in ITER, especially under conditions for which mixed materials can be created (e.g., combinations of C, W and Be) for which no other emissivity data are available. A straightforward data-to-theory-function fit is made possible by an intensity calibration using a commercially available illumination source. The raw spectrum has sufficient detail (512 pixels) to see and eliminate atomic line emission and absorption features from the spectrum before fitting. Fitting code details and preliminary experimental data analysis will be presented and discussed. [Preview Abstract] |
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NP8.00120: Compact Stellarator Path to DEMO J.F. Lyon Issues for a DEMO reactor are sustaining an ignited/high-Q plasma in steady state, avoiding disruptions and large variations in power flux to the wall, adequate confinement of thermal plasma and alpha-particles, control of a burning plasma, particle and power handling, etc. Compact stellarators have key advantages -- steady-state high-plasma-density operation without external current drive or disruptions, stability without a close conducting wall or active feedback systems, and low recirculating power -- in addition to moderate plasma aspect ratio, good confinement, and high-beta potential. The ARIES-CS study established that compact stellarators can be competitive with tokamaks as reactors. Many of the issues for a compact stellarator DEMO can be answered using results from large tokamaks, ITER D-T experiments and fusion materials, technology and component development programs, in addition to stellarators in operation, under construction or in development. However, a large next-generation stellarator will be needed to address some physics issues: size scaling and confinement at higher parameters, burning plasma issues, and operation with a strongly radiative divertor. Technology issues include simpler coils, structure, and divertor fabrication, and better cost information. [Preview Abstract] |
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NP8.00121: Establishing Physical and Engineering Science Base to Bridge from ITER to Demo Y.-K. Martin Peng, M. Abdou, D. Gates, C. Hegna, D. Hill, F. Najmabadi, G. Navratil, R. Parker A Nuclear Component Testing (NCT) Discussion Group emerged recently to clarify how ``\textbf{\textit{a lowered-risk, reduced-cost approach can provide a progressive fusion environment beyond the ITER level to explore, discover, and help establish the remaining, critically needed physical and engineering sciences knowledge base for Demo.}}'' The group, assuming success of ITER and other contemporary projects, identified critical ``gap-filling'' investigations: plasma startup, tritium self-sufficiency, plasma facing surface performance and maintainability, first wall/blanket/divertor materials defect control and lifetime management, and remote handling. Only standard or spherical tokamak plasma conditions below the advanced regime are assumed to lower the anticipated physics risk to continuous operation ($\sim $2 weeks). Modular designs and remote handling capabilities are included to mitigate the risk of component failure and ease replacement. Aspect ratio should be varied to lower the cost, accounting for the contending physics risks and the near-term R{\&}D. Cost and time-effective staging from H-H, D-D, to D-T will also be considered. *Work supported by USDOE. [Preview Abstract] |
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NP8.00122: Crossing the Next Frontier R. Goldston, J. Menard, J. Brooks, R. Doerner, D. Gates, G.-Y. Fu, N. Gorelenkov, R. Kaita, S. Kaye, G. Kramer, H. Kugel, R. Majeski, M. Ono, C. Skinner, J. Strachan, J. Harris, R. Maingi, M. Kotschenreuther, S. Mahajan, P. Valanju, R. Nygren, M. Ulrickson, D. Ruzic, S. Sabbagh, V. Soukhanovskii The plasma-material interface is the next frontier in fusion science. ITER's approaches to heat flux and tritium retention do not extrapolate to Demo. Defining questions at this frontier include: Can extremely high radiated-power fraction be consistent with high confinement and low Z$_{eff}$? Can magnetic flux expansion or edge ergodization reduce heat loads sufficiently? Can tungsten survive with acceptable core radiation and tritium retention? Can liquid metals more effectively handle high heat flux, off-normal loads and tritium exhaust? Answers must be integrated with high-performance, fully steady state plasma operation, avoiding ELMs and eliminating disruptions. The vehicle to cross this frontier is a high-power-density plasma with long pulses, excellent diagnostic access, flexible first wall, divertor, heating, current drive and plasma control systems, extensive deuterium and trace tritium operation, and the ability to test a range of plasma-facing materials at reactor-relevant temperature. [Preview Abstract] |
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NP8.00123: Fusion Development Facility -- Mission and Overview R.D. Stambaugh, V.S. Chan, C.P.C. Wong A Fusion Development Facility (FDF) is proposed to make possible a fusion demonstration power plant (DEMO) as the next step after ITER. To make possible a DEMO of the ARIES-AT type, the mission of the FDF should be to carry forward Advanced Tokamak physics and enable development of fusion energy applications. FDF should demonstrate advanced physics operation of a tokamak in steady-state with burn, producing 100-250 MW fusion power with modest energy gain (Q$<$5) in a modest sized device. Full noninductive, high bootstrap operation will enable continuous operation for periods up to two weeks. FDF must further develop all elements of AT physics for an advanced performance DEMO. With neutron flux at the outboard midplane of 1-2 MW/m$^{2}$ and a goal of a duty factor of 0.3, FDF can produce fluences of 3-6 MW-yr/m$^{2}$ in ten years of operation. FDF will have a goal of producing its own tritium and building a supply to start up DEMO. The development of blankets suitable for tritium, electricity, and hydrogen production will be done in port modules. FDF, ITER, IFMIF, and other AT devices will provide the basis for a fusion DEMO power plant of the ARIES-AT type. [Preview Abstract] |
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NP8.00124: Physics-Based Performance Projections for Fusion Development Facility V.S. Chan, R.D. Stambaugh, M.S. Chu, R.K. Fisher, C.M. Greenfield, D.A. Humphreys, L.L. Lao, J.A. Leuer, T.W. Petrie, R. Prater, G.M. Staebler, H.E. St John, P.B. Snyder, A.D. Turnbull, M.A. Van Zeeland The Fusion Development Facility (FDF) is a fusion application development facility based on advanced tokamak physics with copper magnets and tritium breeding capability. Theory based stability and transport studies are used to validate the performance projections from a system study based on simplified models. Ideal global and edge stability limits established by further optimization of high performance equilibria obtained in existing experiments indicate that the FDF power density and neutron flux requirements can be met with strong shaping and feedback control. Transport analysis using physics-based transport model with an edge condition consistent with the pedestal stability limit indicate the FDF confinement requirement can also be achieved. Interesting opportunities for study of alpha physics and challenges on first walls will be discussed. [Preview Abstract] |
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NP8.00125: Numerical Study of Equilibrium, Stability, and Advanced Resistive Wall Mode Feedback Algorithms on KSTAR Oksana Katsuro-Hopkins, S.A. Sabbagh, J.M. Bialek, H.K. Park, J.Y. Kim, K.-I. You, A.H. Glasser, L.L. Lao Stability to ideal MHD kink/ballooning modes and the resistive wall mode (RWM) is investigated for the KSTAR tokamak. Free-boundary equilibria that comply with magnetic field coil current constraints are computed for monotonic and reversed shear safety factor profiles and H-mode tokamak pressure profiles. Advanced tokamak operation at moderate to low plasma internal inductance shows that a factor of two improvement in the plasma beta limit over the no-wall beta limit is possible for toroidal mode number of unity. The KSTAR conducting structure, passive stabilizers, and in-vessel control coils are modeled by the VALEN-3D code and the active RWM stabilization performance of the device is evaluated using both standard and advanced feedback algorithms. Steady-state power and voltage requirements for the system are estimated based on the expected noise on the RWM sensor signals. Using NSTX experimental RWM sensors noise data as input, a reduced VALEN state-space LQG controller is designed to realistically assess KSTAR stabilization system performance. [Preview Abstract] |
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NP8.00126: KSTAR Equilibrium Reconstruction with EFIT Code Kwang-Il You, D.K. Lee, S.H. Hahn, L.L. Lao For application to the KSTAR (Korea Superconducting Tokamak Advanced Research) device, we have made some modification to the EFIT code and installed it on our computing system. The main function of EFIT is reconstruction of plasma equilibrium using discharge data. After every discharge, the code will be automatically run for a chosen time array and the results will be stored in the same way as experimental data will be. An MDSplus system will be used as the data storage for KSTAR; therefore, the EFIT reads experimental data from the MDSplus server and writes the results to it. We have added some subroutines to EFIT for direct link with the MDSplus server and also converted EFIT to Fortran 95 form. Test runs of the code will be made by using plasma simulator in the KSTAR plasma control system. This paper will also present some results of equilibrium data obtained with the equilibrium mode of EFIT. [Preview Abstract] |
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NP8.00127: Criteria for Neoclassical Tearing Modes Suppression in KSTAR Y.S. Park, Y.S. Hwang In KSTAR, neoclassical tearing modes(NTMs) will be suppressed by using 170GHz electron cyclotron current drive(ECCD) system with steering mirrors that align the current deposition to NTM locations. As an initial stage of NTM suppression study, 1 MW ECCD power will be used to suppress m/n = 3/2 and 2/1 NTMs. To confirm the feasibility of successful suppression of the modes under the proposed KSTAR environment, modified Rutherford equation(MRE) which encapsulates stability of NTMs is constructed for the target equilibrium of KSTAR. The geometric coefficients in MRE are obtained by comparing saturated sizes of NTMs from ISLAND code [1] with the amounts of local bootstrap currents from ONETWO. Parameters related to the operation of ECCD are analyzed by TORAY-GA linear ray-tracing code. Due to the small ECCD power available at the initial stage of KSTAR, condition of the optimum ECCD modulation is considered in the analysis to maximize suppression performance. From the analyses, criteria such as the minimum ECCD power required for complete suppression of the modes and the optimum conditions of EC wave launch angle and modulation duty factor are derived for the successful NTM suppression in KSTAR. [1] C.N. Nguyen, G. Bateman and A.H. Kritz, Phys. Plasmas 11 3460 (2004) [Preview Abstract] |
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NP8.00128: Two-Dimensional Simulation of Edge Localized Modes in the Edge-SOL Region of the KSTAR Tokamak Ki Min Kim, Hyun-Sun Han, Sang Hee Hong H-mode plasmas in the tokamak are frequently perturbed by oscillating instabilities known as ELMs (Edge Localized Modes) in the edge-SOL (Scrape-Off Layer) region. Large losses of plasma particle and energy by ELMs have a critical influence on the degradation of plasma confinement, and the released energies can cause serious damages to the plasma facing components in the edge region. In order to investigate the effects of ELMs on the tokamak operation, a two-dimensional predictive simulation has been carried out for the KSTAR (Korea Superconducting Tokamak Advanced Research) tokamak by the B2 transport code. Double-null edge-SOL geometry and reference ELMy H-mode scenarios of KSTAR are considered, and the enhanced transports during an ELM period are reproduced with the modified transport coefficients at the edge. The duration and frequency of ELMs are varied to simulate type-I and type-III ELMs. Simulation results show the dynamics of the plasma properties focused on the heat fluxes on the divertor target during ELMs. In addition, parametric characteristics of the divertor heat load distributions are discussed. [Preview Abstract] |
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NP8.00129: SOL-Divertor Plasma Simulation in the KSTAR Tokamak with a Neutral Transport Model Using the TEP method Hyun-Sun Han, Ki Min Kim, Sang Hee Hong A two-dimensional numerical simulation has been carried out to analyze the transport phenomena of plasma and neutrals in Scrape- off Layer (SOL) and divertor region of the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. In this numerical work, the transmission and escape probability (TEP) method is used for a neutral transport model by adapting the GTNEUT [1] code, which is coupled with a plasma transport model based on the Braginskii's fluid formulation. Prior to combining the GTNEUT code with the plasma transport one, preliminary tests are conducted by comparison with a Monte Carlo method to check the numerical accuracy and efficiency of the neutral model. A performance improvement of computing time is achieved during the coupling processes by pre-computing the various transmission coefficients and setting up an interpolation lookup table. As results of the simulation, plasma density and temperature distributions in the SOL-divertor region are calculated for the baseline operation of the KSTAR tokamak. This integrated modeling method could be extended to the simulation of more complicated edge transport for the advanced tokamak operation including impurity transport. \newline [1] J. Mandrekas, Comput. Phys. Comm., 161, 36 (2004) [Preview Abstract] |
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NP8.00130: ECEI/MIR Optical Designs for KSTAR Z. Shen, T. Liang, X. Kong, C.W. Domier, N.C. Luhmann, Jr., H.K. Park 2-D plasma imaging diagnostic system is being developed for the KSTAR tokomak to image electron temperature profiles and fluctuations via Electron Cyclotron Emission Imaging (ECEI) and electron density fluctuations via Microwave Imaging Reflectometry (MIR). Optical designs have been developed for both low field (1.5-2.0 T) and high field (3.0-3.5 T) operation scenarios. Each ECEI system consists of a pair of 24 element mixer arrays producing 768 (24$\times $32) channel temperature fluctuation images. Each MIR system consists of a single 16 element mixer array producing 128 (16$\times $8) channel density fluctuation images. Use of in-vessel reflective optics permits both systems to view the KSTAR plasma through a relatively small vacuum window. System details, including preliminary optical and electronics designs for low and high field configurations, will be presented. [Preview Abstract] |
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NP8.00131: Experimental investigation of the novel x-ray tube for the KSTAR x-ray image crystal spectrometer Jun-Gyo Bak, Sang-Gon Lee, Min-Gap Bog, Uk-Won Nam, Myung-Kook Moon, Jong-Kyu Cheon A novel x-ray tube with an anode and a line filament has been developed for the \textit{in-situ} calibration of a two dimensional segmented position-sensitive, multi-wire proportional counter (2D detector) in the KSTAR x-ray image crystal spectrometer (XICS). For the investigation of the performance of the x-ray tube as a line x-ray source for the calibration of the 2D detector, the images from the x-ray tube with Cu and Al anodes are measured by using a pinhole and the 2D detector. The characteristics of the x-ray images, such as the width and length of the image, are investigated and two images from the x-ray tube using Cu and Al anodes are compared. This work will be needed for the application of the in-situ calibration of the XICS. In this work, the experimental investigation of the x-ray tube will be presented. [Preview Abstract] |
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NP8.00132: Commissioning activities of the initial magnetic diagnostics for KSTAR tokamak Sang Gon Lee, Jun Gyo Bak, Eun Mie Ka The initial magnetic diagnostics for the KSTAR superconducting tokamak including three Rogowski coils, five flux/voltage loops, and sixty-four magnetic field probes have been successfully installed. The Rogowski coils, flux/voltage loops, and magnetic field probes measure the total plasma current, poloidal flux and loop voltage, and local poloidal magnetic field for the plasma position control and equilibrium studies, respectively. Accurate position measurements after installation for all of these initial magnetic diagnostics and \textit{in situ }calibration for the Rogowski coils were finished. Data acquisition systems for these initial magnetic diagnostics are currently under preparation. Detail commissioning activities before the first plasma from these initial magnetic diagnostics will be presented. [Preview Abstract] |
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NP8.00133: EAST First Diverted Plasma Operations -- Plasma Control and Vertical Stabilization A.W. Hyatt, J.A. Leuer, D.A. Humphreys, G.L. Jackson, R.D. Johnson, B.G. Penaflor, D.A. Piglowski, M.L. Walker, A.S. Welander, D. Mueller, B.J. Xiao, Q.P. Yuan, H.Z. Wang, P. Fu, X. Gong, J. Luo, Y. Wan, J. Li EAST, the first operational fully superconducting tokamak, has a poloidal field (PF) coil set that is similar to ITER. The EAST digital plasma control system (PCS), based on the DIII-D PCS, allowed EAST to rapidly progress from first plasma to diverted operations in a few months, and GA personnel to remotely support the initial rollout of the PCS. Effective combined I$_{p}$, R$_{p}$, and Z$_{p}$ control with a fully independent PF coil set has been demonstrated. Careful current programming of the PF coils demonstrated stable diverted plasma operation. AC heating and breakdown concerns limited the PF coils' power supplies' bandwidth, voltage, and ability to counter the growth of n=0 instabilities when the plasma elongation kappa, $\kappa \ge $1.15. Auxiliary internal PF coils driven by a fast power supply (3 kHz, 5 kA, 600 V), provided effective vertical stabilization at $\kappa \ge $1.8. Examples and simulations will be shown. [Preview Abstract] |
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NP8.00134: Major Improvements in Diagnostic Neutral Beam Performance on HT-7 and EAST and Implications for CXRS He Huang, William L. Rowan, Yuejiang Shi, Jun Li, Donghai Ding, Chundong Hu, Baonian Wan To improve the sensitivity of the charge exchange recombination spectroscopy (CXRS) diagnostic on HT-7 and the prospects for CXRS on EAST, a magnetic filter was added to the magnetic bucket plasma source on the diagnostic neutral beam (DNB). The density of the main energy component of the beam was increased by a factor of 2, radically improving the prospects for CXRS. The effect of the magnetic filter is to isolate the plasma production region from the beam extraction region. Further improvements in operation of the plasma source will be undertaken by re-engineering gas delivery and power supplies to increase the operational regime. New proposals for more efficient plasma excitation will be discussed. Simulations with the expected improvement both for HT-7 and for EAST are used to guide CXRS optical designs for the devices. [Preview Abstract] |
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