Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session NP6: Poster Session V: Dusty and Low Temperature Plasmas; Basic Plasma Physics and Simulation; MST, RFX-MOD, and Reversed Field Pinch; NSTX and Spherical Torus; General Spherical Torus |
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Room: Marsalis A/B, 9:45am - 12:45pm |
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NP6.00001: DUSTY AND LOW TEMPERATURE PLASMA |
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NP6.00002: Conducting Particles in a Complex Plasma Jorge Carmona, Ke Qiao, Victor Zhang, Jay Murphee, Lorin Matthews, Truell Hyde Complex plasma containing conducting grains is common across a number of research environments. In space such plasmas can be formed from dust left over after the formation of stellar and protoplanetary systems where iron rich meteors, containing remnants of primordial solar system material, offer a source for micron-sized metallic dust particles. Conducting dust particles are also common contaminants within both semi-conductor processing systems and fusion devices where they can create a host of problems. In this research, comparison and contrast studies were conducted employing both non-conducting and conducting particles immersed in a capacitively coupled RF plasma generated within a GEC reference cell. The data collected was analyzed employing both Voronoi diagrams and pair correlation functions with the results then compared to theoretical prediction. [Preview Abstract] |
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NP6.00003: Dipole-dipole interactions of charged magnetic grains Erik Remkus, Jonathan Perry, Lorin Matthews, Truell Hyde The interaction of charged dust grains is an important process in fields as diverse as planet formation and plasma processing of silicon wafers for computer chips. The interaction of the dust grains depends in part on the material properties of the initial dust population (conducting, non-conducting, ferrous or non-ferrous). The effects that electrostatic and magnetic forces have on the coagulation of dust in a protoplanetary disk are examined by modeling the interactions between extended dust aggregates -- specifically looking at how the arrangement of charge over the aggregate surface or the inclusion of magnetic material produces dipole-dipole interactions which affect the orientation of grains as they collide and stick. Numerical models are used to simulate the charging and coagulation of dust aggregates formed from ferrous and non-ferrous materials. Analysis of the resulting dust populations demonstrate the effect of grain material on the structure of the aggregate, characterized by the fractal dimension. The aggregate structure not only governs the growth rate of aggregates, but also determines how well the grains couple to the gas in the protoplanetary disk. An improved understanding of the dynamics of aggregating dust grains and how these dynamics depend on grain materials will lead to greater knowledge of the early processes leading to planet formation. [Preview Abstract] |
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NP6.00004: Diffraction of Dust Acoustic Waves Su-Hyun Kim, Jonathon R. Heinrich, Robert L. Merlino We present observations of the diffraction of dust acoustic (DA) waves around various objects. The DA waves are excited spontaneously in a dusty plasma produced in a DC glow discharge. The waves are imaged by laser light scattering and digitally recorded using a CCD camera. We have studied the diffraction of DA waves by a dielectric rod and by a rectangular slit of variable width. The wavelengths of the DA waves are comparable to the effective size of the object which is determined by the dust void around it. The void is formed due to the expulsion of the negatively charged dust particles by the object which also acquires a negative charge. The results of the diffraction by the rod are compared with the diffraction pattern produced by sound waves incident on a cylinder. [Preview Abstract] |
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NP6.00005: Dust Acoustic Wave Excitation in a Plasma with Warm Dust M. Rosenberg, E. Thomas, Jr., L. Marcus, R. Fisher, J.D. Williams, R.L. Merlino Measurements of the dust acoustic wave dispersion relation in dusty plasmas formed in glow discharges at the University of Iowa [1] and Auburn University [2] have shown the importance of finite dust temperature effects. The effect of dust grains with large thermal speeds was taken into account using kinetic theory of the ion-dust streaming instability [3]. The results of analytic and numerical calculations of the dispersion relation based on the kinetic theory will be presented and compared with the experimental results. [1] E. Thomas, Jr., R. Fisher, and R. L. Merlino, Phys. Plasmas 14, 123701 (2007). [2] J. D. Williams, E. Thomas Jr., and L. Marcus, Phys. Plasmas 15, 043704 (2008). [3] M. Rosenberg, E. Thomas Jr., and R. L. Merlino, Phys. Plasmas 15, 073701 (2008). [Preview Abstract] |
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NP6.00006: Using ion flows parallel and perpendicular to gravity to modify dust acoustic waves E. Thomas, R. Fisher Recent studies of dust acoustic waves have shown that the dust kinetic temperature can play an important role in determining the resulting dispersion relation [M. Rosenberg, et al., Phys. Plasmas, 15, 073701 (2008)]. In these studies, it is believed that ion flows play a dominant role in determining both the kinetic temperature of the charged microparticles as well as providing the source of energy for triggering the waves. In this presentation, results will be presented on the effects of ion flow on spatial structure and velocity distribution of dust acoustic waves. Here, the waves will be formed in dusty plasmas consisting of 3 $\pm $ 1 micron diameter silica microspheres. Two separate electrodes will be used to modify the ion flow in the plasma -- one parallel to the direction of gravity and one perpendicular to the direction of gravity. Particle image velocimetry (PIV) techniques will be used to observe the particles and to measure their velocity distributions. [Preview Abstract] |
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NP6.00007: Probability of metastable states in Yukawa clusters Patrick Ludwig, Hanno Kaehlert, Henning Baumgartner, Michael Bonitz Finite strongly coupled systems of charged particles in external traps are of high interest in many fields. Here we analyze the occurrence probabilities of ground- and metastable states of spherical, three-dimensional Yukawa clusters by means of molecular dynamics and Monte Carlo simulations and an analytical method. We find that metastable states can occur with a higher probability than the ground state, thus confirming recent dusty plasma experiments with so-called Yukawa balls [1]. The analytical method [2], based on the harmonic approximation of the potential energy, allows for a very intuitive explanation of the probabilities when combined with the simulation results [3].\\ $[1]$ D. Block, S. K\"{a}ding, A. Melzer, A. Piel, H. Baumgartner, and M. Bonitz, Physics of Plasmas \textbf{15}, 040701 (2008)\\ $[2]$ F. Baletto and R. Ferrando, Reviews of Modern Physics \textbf{77}, 371 (2005)\\ $[3]$ H. K\"{a}hlert, P. Ludwig, H. Baumgartner, M. Bonitz, D. Block, S. K\"{a}ding, A. Melzer, and A. Piel, submitted for publication (2008)\\ [Preview Abstract] |
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NP6.00008: Dust Particle Contamination Detection Mechanisms James Creel, Jorge Carmona Reyes, Lorin Matthews, Truell Hyde The existence of dust within magnetic confinement fusion (MCF) reactors has always been an issue; recently it has garnered new attention as a mechanism for degraded confinement performance and increased safety risks. These consequences arise in part due to the fact that such dust can erode and adhere to chamber walls, become chemically toxic or radioactive, and promote arcing between the plasma and the chamber. Unfortunately, due to the operating regimes of the bulk plasma (luminosity, temperature, etc.), methods for detecting such dust are currently limited. This paper will discuss a dust tracking system currently employed within a light gas gun (LGG), exploring the combination as one possible mechanism for developing new dust detection diagnostics. [Preview Abstract] |
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NP6.00009: Magnetic field structure generation in collisional dusty plasmas Padma Kant Shukla, Fernando Haas A perpendicular ion drift is proposed as a possible mechanism for the generation of magnetic field structures in a highly collisional dusty plasma. The basic dissipation mechanism is assumed to be the dust-neutral momentum exchange, so that plasmas with a small ionization fraction are natural candidates for experiments. The model reduces to a nonlinear partial differential equation for the vector potential. The conditions for linear instability are presented. Possible stationary states are periodic arrangements for the magnetic field, described by a Lienard equation. The fully depleted (ion-dust) case is also considered in detail. Applications of the present work to magnetic field structures in planetary rings, comets and low-temperature dusty plasma experiments are discussed. A necessary condition for the validity of the model is a sufficiently slow time-scale of the generated magnetic fields in dusty plasmas. [Preview Abstract] |
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NP6.00010: Laboratory plasma with nanoparticles Scott Robertson, G. Bano, W. Handley, X. Wang An experiment has been constructed that creates dusty plasma with nanometer-sized particles that is similar to the ionosphere in which there are smoke particles from the ablation of meteoroids. A Zn vapor source is used to create a smoky gas of particles that are up to 100 nanometers in diameter. These are seen both by laser scattering and by electron microscopy. The Zn is evaporated into 5 Torr of argon and the DC discharge is run at 500 mTorr. The plasma density is 10$^{8}$ -- 10$^{9}$ cm$^{-3}$ and the electron temperature is 1 eV. Plasma oscillations near 1 kHz, possibly dust acoustic waves, are detected by a probe when the oven is at 460 C when nanoparticles are present, but these oscillations are absent at 380 C when particles are absent. Analysis is complicated by the presence of variable amounts of Zn vapor. An afterglow plasma is created by interrupting the discharge periodically. Particles diffusing toward the wall are collected by a plate behind a screen mesh. The arrival time of these particles is 10 -- 30 ms after the disappearance of the electron signal. The particles arrive more quickly if the bias on the collector is increased, which supports the interpretation that this signal is from the nanoparticles. [Preview Abstract] |
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NP6.00011: One-to-two dimensional transition in small Debye clusters T.E. Sheridan, K.D. Wells We study the transition from one-dimensional to two-dimensional configurations of small clusters of monodisperse dust particles levitated in plasma. Particles are confined by a highly-anisotropic two-dimensional potential well in the Dusty ONU experimenT (DONUT). The well anisotropy, as determined by measuring the center-of-mass oscillation frequencies in the $x$ and $y$ directions using Brownian motion, is found to be $\omega_{0y}^2/\omega_{0x}^2=30.7$. For $n\le9$ particles, the cluster is in a linear, one-dimensional configuration. The addition of one more particle ($n=10$) causes a zig-zag transition to a two-dimensional ``barred-elliptical'' configuration with an ``elliptical'' nucleus and linear tails. As more particles are added the nucleus grows and the tails decline until the cluster becomes an oval. These results are found to be in good agreement with Monte Carlo calculations of particle configurations. The Monte Carlo calculations show that in a linear configuration normal modes are either longitudinal or transverse, so that for low energies the cluster dynamics are one-dimensional. [Preview Abstract] |
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NP6.00012: Observation of Dust Particle Gyromotion in a Magnetized Dusty Plasma C.S. Compton, W.E. Amatucci, G. Gatling, E. Tejero In dusty plasma research, gyromotion of the dust has been difficult to observe experimentally. Previous experiments by Amatucci et al. have shown gyromotion of a single dust particle [1]. This early work was performed with alumina dust that had a size distribution and non-uniformly shaped particles. In the current experiment, evidence of spherical, monodispersed, dust particles exhibiting gyromotion has been observed. Silica particles 0.97 micrometers in diameter are suspended in a DC glow discharge argon plasma.~The experiment is performed in the Naval Research Laboratory's DUsty PLasma EXperiment (DUPLEX Jr.). DUPLEX is a 61-cm tall by 46-cm diameter acrylic chamber allowing full 360 degree optical access for diagnostics. The neutral pressure for the experiment is 230 mTorr with a 275 V bias between the circular electrodes. The electrodes have a separation of 4 cm. A strong magnetic field is created by 2 pairs of neodymium iron boride magnets placed above and below the anode and cathode respectively. The resulting field is 1.4 kG. The dust particles are illuminated with a 25 mW, 672 nm laser. Images are captured using an~intensified~CCD camera and a consumer digital video cassette recorder. Recent evidence of gyromotion of spherical, monodispersed, dust particles will be presented. [1] Amatucci, W.E., et al., Phys. Plasmas, 11, 2097 (2004) [Preview Abstract] |
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NP6.00013: Frequency threshold for ion beam formation in expanding RF plasma Saikat Chakraborty Thakur, Zane Harvey, Ioana Biloiu, Alex Hansen, Robert Hardin, William Przybysz, Earl Scime We observe a threshold frequency for ion beam formation in expanding, low pressure, argon helicon plasma. Mutually consistent measurements of ion beam energy and density relative to the background ion density obtained with a retarding field energy analyzer and laser induced fluorescence indicate that a stable ion beam of 15 eV appears for source frequencies above 11.5 MHz. Reducing the frequency increases the upstream beam amplitude. Downstream of the expansion region, a clear ion beam is seen only for the higher frequencies. At lower frequencies, large electrostatic instabilities appear and an ion beam is not observed. The upstream plasma density increases sharply at the same threshold frequency that leads to the appearance of a stable double layer. The observations are consistent with the theoretical prediction that downstream electrons accelerated into the source by the double layer lead to increased ionization, thus balancing the higher loss rates upstream [1]. 1. M. A. Lieberman, C. Charles and R. W. Boswell, J. Phys. D: Appl. Phys. \textbf{39} (2006) 3294-3304 [Preview Abstract] |
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NP6.00014: Evaporation studies of lithiated ATJ graphite Kenzo Ibano, Vijay Surla, David Ruzic Lithium evaporation conditioning of ATJ graphite tiles at the diverter region of the National Spherical Torus Experiment (NSTX) occasionally showed remarkable improvements to the plasma performance. [1] To obtain these effects consistently, a deeper understanding of the lithium behavior in the ATJ graphite is required. In particular, the evaporation as a function of temperature of the intercalated lithium from the graphite surface is an important concern. Therefore ATJ graphite samples were examined in the Ion-Surface Interaction Experiment (IIAX) at the University of Illinois. After a certain amount of lithium was evaporated to the ATJ graphite by an in-situ lithium heater, the amount of evaporation from the sample surface as a function of temperature was observed using a dual quartz crystal microbalance system. The experiments are repeated with similar samples that have been saturated by D. These results are modeled and compared to lithiated graphite sputtering studies. Chemical sputtering effects are also considered. [1] H. W. Kugel et al., 18th PSI conference, submitted to J. Nucl. Mater. (2008) [Preview Abstract] |
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NP6.00015: Ambipolar Potential of a Collisionless Scrape-off-Layer Erik Granstedt, Leonid Zakharov Low-recycling plasma-facing-components may permit fusion devices to attain higher edge temperatures, reducing the global energy transport. Reduced collisionality at the edge would raise questions about the plasma-wall interaction, including the magnitude of the ambipolar potential. We describe an analytic model to give qualitative insight into the value of the ambipolar potential in the SOL, and in particular its dependence on the temperature of the SOL particle source. The SOL is treated as a magnetic mirror; curvature is neglected; a square-well (spatially uniform) potential and magnetic field are assumed; and only classical end-losses are considered. The key difference from previous studies of magnetic mirrors is that a substantial fraction of the incident particle flux is injected directly into the loss region, hence the passing particle density cannot be neglected. The model predicts the ambipolar potential to decrease with temperature, and eventually flatten in the collisionless limit where the passing particle density is small. Larger mirror ratios increase ion trapping, resulting in a higher potential to maintain quasineutrality by confining more electrons. An estimate of the SOL current in the case of unequal end-wall potentials is also given. [Preview Abstract] |
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NP6.00016: The role of presheaths in establishing anisotropy at the sheath edge Noah Hershkowitz, Dongsoo Lee, Greg Severn Presheaths provide ion acceleration to the Bohm velocity. We consider modifications to the parallel and perpendicular IVDFs associated with the presheath. Laser-induced fluorescence (LIF) data obtained with a diode laser near a negatively-biased plate give the transverse and parallel metastable ion velocity distribution function profiles (measured with respect to the normal to the plate) in a low temperature, low pressure, DC multidipole argon discharge plasma. For a neutral pressure of 0.3~mTorr, the transverse temperature increases along the presheath from 0.026~eV in the bulk plasma to 0.058~eV at the presheath sheath boundary. This result is compared with PIC code simulations\footnote{A. Meige et al., {\em Phys. Plasmas}, 14, 032104 (2007)} and experimental results\footnote{N. Claire et al., {\em Phys. Plasmas}, 13, 062103 (2006)} found in the literature. The general dependence on presheath characteristics on neutral pressure is discussed. Apparently contradictory experiments and PIC code results are resolved. [Preview Abstract] |
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NP6.00017: Characteristics of High-Density Helicon Plasma with Large Diameter Excited by a Flat Spiral Antenna Shunjiro Shinohara, Kenji Tanaka, Taisei Motomura, Katsuhiko Murakami Helicon sources have been extensively investigated because of their efficient plasma production, and large diameter (45-74 cm), high-density ($\sim $ 10$^{13}$ cm$^{-3})$ plasmas have been produced, using flat spiral antennas [1,2]. However, an effect of the magnetic field configuration on plasma performance has not been fully investigated. In addition, a short axial length, i.e., low aspect ratio, in some cases is desirable. These subjects are investigated using a vacuum chamber, whose diameter and axial length are 40 and 120 cm, respectively, with a movable end plate to change the axial plasma length, and also eight magnetic field coils. With the increase in the degree of the convergent field, threshold rf power for density jumps decreases. Furthermore, the lower threshold power to have density jumps is found, increasing the argon fill pressure. Detailed results in addition to the shorter axial length case will also be presented. [1] S. Shinohara \textit{et al,}, Jpn. J. Appl. Phys. \textbf{35} (1996) 4503, [2] S. Shinohara and T. Tanikawa, Rev. Sci. Instrum. \textbf{75} (2004) 1941. [Preview Abstract] |
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NP6.00018: Characteristics of Helicon-Plasma Produced Using a Segmented Multi-Loop Antenna II Takao Tanikawa, Shunjiro Shinohara It has been demonstrated that a large-diameter (up to 74 cm), large-volume (up to 2.1 m$^{3})$ helicon plasma can be produced very efficiently (production of $\sim $10$^{15}$ ion-electron pairs per one-watt of rf power) by using a flat spiral antenna, which is installed just outside an insulator window at the end of a cylindrical vacuum vessel and is immersed in a non-uniform magnetic field [1,2]. The same principle has been applied to a smaller device (20 cm in diameter and 1 m in axial length), but using a somewhat different type of flat antenna, namely a segmented multi-loop antenna [3]. By changing the electrical connections among the antenna segments, it is possible to excite the azimuthal modes of higher than $m$ = 0 as well as the $m$ = 0 mode itself, where $m$ is the azimuthal mode number. Under certain conditions, it is possible to produce a non-uniform plasma along the uniform magnetic field due probably to the neutral depletion in the main plasma production region. The discharge characteristics of the device will be presented. [1] S. Shinohara and T. Tanikawa, Rev. Sci. Instrum. \textbf{75}, 1941 (2004); Phys. Plasmas \textbf{12}, 044502 (2005). [2] T. Tanikawa and S. Shinohara, Thin Solid Films \textbf{506-507}, 559 (2006). [3] T. Tanikawa \textit{et al.}, Bull. Ameri. Phys. Soc. \textbf{51} (7), 164 (2006). [Preview Abstract] |
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NP6.00019: Wave Characteristics of Large-Diameter, High-Density Helicon Plasma with Short Axial Length Taisei Motomura, Kenji Tanaka, Katsuhiko Murakami, Shunjiro Shinohara, Takao Tanikawa, Shamrai Konstantin We have developed a large-diameter (73.8 cm) helicon device with an axial length of 486 cm that utilizes a flat spiral antenna at ISAS/JAXA [1]. It has been realized that a shorter axial length is desirable in certain applications. Therefore, the axial length has been shortened in a range of 12 to 123 cm by installing a movable termination plate. Even with very short axial length, the plasma density can exceed 10$^{12}$ cm$^{-3}$ ($p_{Ar}$ = 0.75 mTorr) with the input rf power of less than 4 kW, showing a rather high plasma production efficiency. When the axial length is relatively longer, the excitation of the higher order radial eigenmodes is found to be correlated with the magnetic field configuration and the radial density profile. When the axial length is further shortened, the axial wave structure tends to become standing wave like, where its wavelength depends on the plasma density and axial length as is expected. The details of the experimental results will be discussed in the presentation. [1] S. Shinohara and T. Tanikawa, Rev. Sci. Instrum. \textbf{75}, 1941 (2004) {\&} Phys. Plasmas \textbf{12}, 044502 (2005). [Preview Abstract] |
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NP6.00020: Dust Devil Dynamics W. Horton, H. Miura A dust devil is a rotating updraft, with coherent structures ranging from small (H/D $\sim$ 5m/1m) to large (H/D $\sim$ 1000 m/10 m). Common in west Texas and Arizona, dust devils are formed unstable stratification of the air by solar heating over a sandy floor. Unstable gravity waves grow exponentially in the low density, hot air, rising into the upper layer of stably stratified atmosphere creating the large, 3D vortex. Dust devils are common on Mars. On Earth radio noise and electrical fields greater than 100kV/m are inferred [Kok J. F., N. O. Renno (2006), Geophys. Res. Lett., 33, L19S10]. Dust devils pick up small dirt and dust particles. The whirling charged dust particles (30 -50 microns) create a magnetic field that fluctuates between 3 and 30 times each second. The electric fields created assist the vortices in lifting materials off the ground and into the atmosphere. We use the theory and simulation tools of fusion plasma physics to describe dust devils. The Grad-Shafranov equation governs the vorticity dynamics and gives a solution for steady axisymmetric flows. The high core velocity is limited by the vortex model with viscous dissipation. The Reynolds number is not large, so these structures are well represented with super computers, in contrast to collisionless plasmas. \vspace{1mm} Research supported by NIFS, Japan and the NSF through ATM-0638480 at UT Austin. [Preview Abstract] |
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NP6.00021: BASIC PLASMA PHYSICS AND SIMULATION |
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NP6.00022: Active Magnetic Experiment: a magnetic bubble in the ionospheric stream Alessandro Biancalani, Francesco Ceccherini, Francesco Pegoraro A space plasma experiment is discussed which consists of a magnetized plasma bubble interacting with the ionospheric plasma. The magnetized plasma inside the magnetized bubble is tied to the dipole magnetic field generated inside the satellite. The parameters of the bubble are discussed in relation to the parameters of the ambient plasma and the plasma phenomena that can be investigated are indicated. The requirements on the plasma and the (earth orbiting) satellite parameters for the interaction between the satellite and the ionospheric-magnetospheric plasma to be ``collective'' are examined. The miniaturization of the obstacle from a planet magnetosphere to a satellite magnetosphere leads to plasma regimes that are characterized by very different dimensionless numbers. Although a physically significant scaling of the magnetosphere solar- wind interaction may not be possible important information about the nonlinear dynamics of collisionless plasmas can be obtained by a relatively simple, satellite-based experiment involving a magnetized plasma bubble tied by a dipole magnetic field generated inside the satellite. [Preview Abstract] |
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NP6.00023: Magnetic Detachment and Plume Control in Escaping Magnetized Plasma Paul Schmit, Nathaniel Fisch The model of two-fluid, axisymmetric, ambipolar magnetized plasma detachment from thruster guide fields is extended to include plasmas with non-zero injection angular velocity profiles. Certain plasma injection angular velocity profiles are shown to narrow the plasma plume, thereby increasing exhaust efficiency. As an example, we consider a magnetic guide field arising from a simple current ring and demonstrate plasma injection schemes that more than double the fraction of useful exhaust aperture area, more than halve the exhaust plume angle, and enhance magnetized plasma detachment. [Preview Abstract] |
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NP6.00024: Measuring 3D Plasma Velocity in the TS-4 Compact Toroid Setthivoine You, Alexander Balandin, Yasushi Ono Experiments at the University of Tokyo shoot two compact toroids at each other to form a single compact toroid with strong plasma flows. The plasma flows have been observed with conventional ion Doppler spectroscopy, and up to now, have been constrained to the toroidal component of the velocity. We present the progress in implementing an additional Doppler spectroscopy diagnostic, set up to determine the 3D plasma fluid velocity profile by vector tomographic reconstruction. The line-integrated measurements are collected from 70 locations around the plasma into a 35 channel fiber-bundle array and recorded on a CCD array after passing through the 1m monochromator. The reconstruction of the solenoidal component of the velocity vectors uses the spherical harmonics expansion method [1]. Realistic simulations of the experimental setup determines the required location and minimum number of line-of-sights (projections) for an acceptable reconstruction. Experimental results will be discussed. The diagnostic will help determine the ion self-helicity of a compact toroid in the context of two-fluid MHD relaxation theory [2]. [1] AL Balandin, Y Ono, J. Comp. Phys., 202 (2005) 52-64 [2] L Steinhauer, A Ishida, Phys. Rev. Lett., 79, 18 (1997) [Preview Abstract] |
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NP6.00025: Transport of parallel momentum by drift-Alfven turbulence Chris McDevitt, Pat Diamond An electromagnetic gyrokinetic formulation is utilized to calculate the turbulent radial flux of parallel momentum for a strongly magnetized plasma in the large aspect ratio limit. For low beta plasmas, away from transport barriers, the level of momentum transport induced by microturbulence is found to be well described within the electrostatic approximation. However, near regions of steep equilibrium profile gradients, strong electromagnetic contributions to the momentum flux are uncovered. In particular, the magnitude of transport induced by the off-diagonal residual stress component of the momentum flux induced by drift wave turbulence can be quenched for sufficiently steep pressure gradients. This quenching mechanism is distinct from ExB shear decorrelation, since it allows for the level of off-diagonal turbulent transport to be strongly mitigated, without extinguishing the underlying microturbulence. In contrast, the level of transport induced by a given Alfvenic branch of the drift-Alfven dispersion relationship typically increases as the pressure gradient steepens, allowing for an alternate avenue of momentum transport. A homogeneous calculation of momentum transport induced by Alfvenic turbulence suggests that an imbalance in the Elsasser populations is required in order to introduce a finite level of off-diagonal momentum transport for the simplified geometry utilized. [Preview Abstract] |
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NP6.00026: Interactions Between Magnetic Flux Ropes in a Laboratory Plasma Eric Lawrence, Walter Gekelman, Stephen Vincena The behavior and interaction of magnetic flux ropes have long been topics of interest to solar and space plasma physicists, but few laboratory experiments have been performed as it is necessary to have a relatively collisionless plasma and currents with significant self-generated fields. Movable lanthanum hexaboride (LaB$_6$) cathodes have been developed to study the 3D dynamics of flux ropes in the Large Plasma Device (LaPD). Each 2.5~cm LaB$_6$ cathode can produce current densities of 5-20 A/cm$^2$ and $\Delta B/B \sim 10\%$. The background plasma ($n \sim 2 \times 10^{12}$ cm${}^{-3}$, $d \sim 60$ cm, $L \sim 18$ m, and $\tau_{\rm rep}= 1$ s) is produced with a DC discharge using a pulsed barium oxide-coated cathode. The two current channels are created by biasing the LaB$_6$ cathodes with respect to a grid anode at the opposite end of the chamber. They are emitted parallel to each other and the background $B$ field. {\boldmath $J$} $\times$ {\boldmath $B$} forces cause the currents to move across the field and interact. The role of reconnection in these interactions will be investigated through detailed volume measurements of the magnetic field and current density. Data from Langmuir probes and microwave horn antennas will also be presented. [Preview Abstract] |
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NP6.00027: LES-MHD Models for Turbulence George Vahala, Tao Wang, Min Soe, Linda Vahala Large Eddy Simulation (LES) models for MHD turbulence are required when the Lundquist and Reynolds numbers become too high for adequate spatial resolution. All previous LES-MHD models are usually based on the filtered equations for the velocity and magnetic field. To achieve closure one must approximate the nonlinear fluctuation terms. For the mean velocity field it is typically assumed that the effect of the nonlinear velocity-magnetic fluctuations results only in an eddy viscosity, while for the mean magnetic field closure is typically achieved by the introduction of an eddy resistivity. Here we present a new LES-MHD model based on the Elsasser representation of MHD. Using the same logic as in previous LES-MHD models, we now find on transforming back from the Elsasser variables that the mean velocity and magnetic field evolutions are each dependent on both an eddy viscosity and eddy resistivity. A lattice Botlzmann representation is then determined for this new LES-MHD model. [Preview Abstract] |
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NP6.00028: Linear stability of resistive interchange modes in a cylindrical Reversed-field pinch plasma Mio Suzuki, Fatima Ebrahimi, Dalton Schnack Experimental techniques of current profile control and pellet injection have yielded RFP discharges that are free of current driven instabilities and achieve beta of 26{\%} [1]. Resistive interchange modes are expected to be the dominant instability in these plasmas. Using the NIMROD code, we calculate the linear stability of these modes in a tearing mode stable cylindrical RFP equilibrium for a range of beta. A growth rate spectrum showing the fastest growing modes is obtained. According to linear theory [2], the growth rate of interchange mode is increases with azimuthal mode number, $m$. The result also suggests that all the modes tend to localize around a particular range of rational surfaces with increasing localization for larger $m$. We also present a linear simulation including gyroviscosity to investigate the stabilizing finite Larmor Radius (FLR) effect on interchange modes. In accordance with earlier theoretical work [3] , the suppression of growth rate is observed where \textit{k$\rho $}$_{i }\sim $1. [1] M. D. Wyman et al., Phys. Plasmas 15, 010701 (2008) [2] B. Coppi, J. M. Greene, and J. L. Johnson, Nucl. Fusion 6, 101 (1966) [3] T. E. Stringer, Nucl. Fusion 15, 125 (1975) [Preview Abstract] |
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NP6.00029: Ideal Kink Modes In a Line-Tied Screw Pinch With Finite Plasma Pressure* V.V. Mirnov, V.A. Svidzinski, H. Li A new method for computing ideal magnetohydrodynamic linear eigenmodes in a cylindrical screw pinch with line-tying boundary conditions is presented. In this method, plasma volume is reflected over one of the end plates, and equations and field components are continued on the extended volume with the continuation rules prescribed by the line-tying boundary conditions. Field components in the combined volume are expanded in Fourier series in the axial coordinate. The resulting set of coupled differential equations is solved numerically in the radial coordinate yielding growth rates and eigenmodes for the system. Example of an m=1 (m is poloidal mode number) internal kink instability in force-free plasma equilibrium with uniform pressure is considered. In contrast to a periodic screw pinch, marginally stable perturbations are essentially compressible in line-tied geometry. Finite compressibility makes the mode more stable in addition to usual line-tying stabilization in zero pressure plasma. The critical length corresponding to marginal stability increases with the increase of plasma beta. A universal axial dependence for marginally stable density perturbations $\rho $(r, z) = $\rho $(r) exp(- i z / q(r)) is predicted analytically and confirmed numerically (q = r B$_{z}$/B$_{\theta }$ ). $^{\ast }$The work was supported by the U.S. D.O.E. and N.S.F. [Preview Abstract] |
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NP6.00030: Why the resistive magnetohydrodynamic equations do not form a closed system Robert Johnson The usual resistive MHD equations, defined as a system of 14 scalar equations in 14 scalar variables, conventionally are determined to be complete and soluble. These equations are a combination of Navier-Stokes and a subset of Maxwell's. However, one of the vector equations, Faraday's law, is actually an identity when viewed from the potential formulation of electrodynamics, hence does not determine any degrees of freedom. The error arises from attributing six degrees of freedom to the electromagnetic fields, when the Maxwell field tensor has only four physical degrees of freedom which are coupled to sources given by the four-current. Only by reinstating Gauss' law does the system of equations become closed, bringing the number of scalar equations and degrees of freedom into agreement with the number twelve. The homogeneous Maxwell equations are recognized as the Bianchi identity for the field tensor coupled to the sources through the inhomogeneous field equations. [Preview Abstract] |
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NP6.00031: Dynamics of conical wire array implosions on a central plasma David Martinez, Radu Presura, Sandra Wright, Chris Plechaty, Stephan Neff Sheared plasma flows are seen in Earth's magnetosphere and have been theorized to stabilize conducting plasma columns. A conical wire array has been shown to produce an axially flowing pinching plasma. Adding a wire on the axis of the array, we can create an axial plasma flow with a radial velocity gradient. These experiments were conducted on Zebra, a 2 TW pulsed power device located at the Nevada Terawatt Facility. From experimental observation we know that the center conductor pulls a significant amount of the array current and ablates. This plasma then acts as a target for the imploding array, creating the desired flow profile. This presentation will discuss the change in dynamics of the conical wire array by adding a center wire that is either straight or helically perturbed. Cylindrical wire arrays with a center wire will also be presented for comparison. [Preview Abstract] |
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NP6.00032: Grid generation and adaptation via Monge-Kantorovich optimization in 2D and 3D Gian Luca Delzanno, Luis Chacon, John M. Finn In a recent paper [1], Monge-Kantorovich (MK) optimization was proposed as a method of grid generation/adaptation in two dimensions (2D). The method is based on the minimization of the L$_{2}$ norm of grid point displacement, constrained to producing a given positive-definite cell volume distribution (equidistribution constraint). The procedure gives rise to the Monge-Amp\'{e}re (MA) equation: a single, non-linear scalar equation with no free-parameters. The MA equation was solved in Ref. [1] with the Jacobian Free Newton-Krylov technique and several challenging test cases were presented in squared domains in 2D. Here, we extend the work of Ref. [1]. We first formulate the MK approach in physical domains with curved boundary elements and in 3D. We then show the results of applying it to these more general cases. We show that MK optimization produces optimal grids in which the constraint is satisfied numerically to truncation error. [1] G.L. Delzanno, L. Chac\'{o}n, J.M. Finn, Y. Chung, G. Lapenta, \textit{A new, robust equidistribution method for two-dimensional grid generation}, submitted to Journal of Computational Physics (2008). [Preview Abstract] |
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NP6.00033: Fully implicit adaptive mesh refinement solver for 2D MHD B. Philip, L. Chacon, M. Pernice Application of implicit adaptive mesh refinement (AMR) to simulate resistive magnetohydrodynamics is described.\footnote{B. Philip, L. Chac\'on, M. Pernice, {\em J. Comput. Phys.}, in press (2008)} Solving this challenging multi-scale, multi-physics problem can improve understanding of reconnection in magnetically-confined plasmas. AMR is employed to resolve extremely thin current sheets, essential for an accurate macroscopic description. Implicit time stepping allows us to accurately follow the dynamical time scale of the developing magnetic field, without being restricted by fast Alfven time scales. At each time step, the large-scale system of nonlinear equations is solved by a Jacobian-free Newton-Krylov method together with a physics-based preconditioner. Each block within the preconditioner is solved optimally using the Fast Adaptive Composite grid method, which can be considered as a multiplicative Schwarz method on AMR grids. We will demonstrate the excellent accuracy and efficiency properties of the method with several challenging reduced MHD applications, including tearing, island coalescence, and tilt instabilities. [Preview Abstract] |
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NP6.00034: Adaptive Mesh Refinement in Calculations of the Rayleigh-Taylor Instability David Fyfe, Gopal Patnail, Doug Schwer, C. Richard DeVore, Kevin Olson This paper describes our initial efforts to use of the package PARAMESH to create an Adaptive Mesh Refinement (AMR) version of NRL's FASTRAD3D code. AMR allows dynamic mesh refinement near unstable interfaces without necessitating refinement globally. In spherical coordinates, AMR alleviates the numerical stability requirements associated with the origin. PARAMESH was designed to create an MPI-based AMR code from a block structured serial code such as FASTRAD3D. FASTRAD3D is a compressible hydrodynamics code containing the physical effects relevant for the simulation of high-temperature plasmas including inertial confinement fusion (ICF) Rayleigh-Taylor unstable direct drive laser targets. Here we present some initial calculations of the Rayleigh-Taylor instability in Cartesian coordinates. Our first calculations will be gravitationally induced. Our second set of calculations will involve laser driven planar targets. For the moment we ignore radiation effects. We will include classical flux-limited Spitzer thermal conduction and inverse bremmstrahlung laser energy absorption in the latter calculations. [Preview Abstract] |
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NP6.00035: Kinetic Plasma Simulation Using a Quadrature-based Moment Method David J. Larson The recently developed quadrature-based moment method [Desjardins, Fox, and Villedieu, \textit{J. Comp. Phys.} \textbf{227} (2008)] is an interesting alternative to standard Lagrangian particle simulations. The two-node quadrature formulation allows multiple flow velocities within a cell, thus correctly representing crossing particle trajectories and lower-order velocity moments without resorting to Lagrangian methods. Instead of following many particles per cell, the Eulerian transport equations are solved for selected moments of the kinetic equation. The moments are then inverted to obtain a discrete representation of the velocity distribution function. Potential advantages include reduced computational cost, elimination of statistical noise, and a simpler treatment of collisional effects. We present results obtained using the quadrature-based moment method applied to the Vlasov equation in simple one-dimensional electrostatic plasma simulations. In addition we explore the use of the moment inversion process in modeling collisional processes within the Complex Particle Kinetics framework. [Preview Abstract] |
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NP6.00036: Fluid-kinetic Simulation via Pseudo-Particle Method : 1D Nonlinear Simulation of the Weibel instability Shuhei Numazawa, Zensho Yoshida We developed a new numerical scheme, Pseudo-particle method, which can capture both fluid and kinetic aspects of plasmas in the single scheme. It enables us to simulate phenomena fluidically or kinetically by changing the property of pseudo-particles, due to the scale length of phenomena and the structure of phase space. Within the numerical scheme, we extend the Smoothed Particle Hydrodynamics (SPH) method and a numerical method for fluid plasma via pseudo-particle method is developed. This extension of SPH to two-fluid theory is firstly accomplished. Using the scheme, we simulated a nonlinear development of the Weibel instability in 1D configuration. We clarify the validity of fluid simulation via the extended SPH and the difference between fluid simulation and kinetic simulation using Particle in Cell method. Then we report how fluid simulation and kinetic simulation can be transited in the single scheme. [Preview Abstract] |
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NP6.00037: A Symplectic Energy and Momentum Conserving Spectral Particle Method David Fillmore, Peter Messmer abstract-Particle plasma models, including the particle-in-cell (PIC) model class, generally follow the trajectories of a large number of macro-particles under the influence of both external and internal electromagnetic fields. The charge and current density terms in the Maxwell equations are obtained by prescribing a shape function that is centered on each particle. The PIC methods are among the most efficient for plasmas out of local thermodynamic equilibrium, but, through the use of grid interpolations, suffer from numerical grid heating (or cooling). In this poster we describe a spectral particle method that conserves both energy and momentum, and is thus devoid of grid heating, but at the computational cost of scaling as the product of the number of particles and the number of spectral grid points, or wave vectors. The algorithm is derived from a discrete action principle which ensures that it is symplectic, or that it conserves phase space volume. We present some simple numerical examples for a collision free plasma, including the classic counter stream instability, and compare the symplectic algorithm to results of standard PIC solutions. [Preview Abstract] |
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NP6.00038: Statistical treatment of relativistic binary collisions in particle codes Fabio Peano, Michael Marti, Luis Silva, Gianni Coppa In particle-based algorithms, such as the PIC method, the effects of binary collisions are commonly described statistically, using Monte Carlo (MC) techniques. In the relativistic regime, stringent constraints on the sampling techniques are critical to ensure physically meaningful results and to avoid the systematic apperance of qualitatively wrong equilibrum distributions [1]. The general procedure for a physically consistent statistical treatment of relativistic collisions is provided within a general kinetic framework. Results from 3D ultrarelativistic MC simulations with $\sim $10$^{8}$ computational particles are presented, illustrating the technique and reproducing the correct equilibrium distribution, described by the J\"{u}ttner function, over several orders of magnitudes in energy and particle number. The systematic origin of conflicting results appeared in the recent literature (cf. [2]) is identified, and a simple interpretation of the numerical results recently presented in [2] is given. [1] F. Peano \textit{et al.}, submitted for publication (2008). [2] D. Cubero \textit{et al.}, Phys. Rev. Lett. \textbf{99}, 170601 (2007). [Preview Abstract] |
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NP6.00039: Calculation of the Nanbu-Trubnikov Kernel: Implications for Numerical Modeling of Coulomb Collisions* Chiaming Wang, Andris Dimits, Russel Caflisch, Bruce Cohen, Yanghong Huang We investigate the accuracy of and assumptions underlying the numerical binary Monte-Carlo collision operator due to Nanbu [K. Nanbu, Phys. Rev. \textbf{E 55} (1997)]. The numerical experiments that resulted in Nanbu's parameterized collision kernel are approximate realizations of the Coulomb-Lorentz pitch-angle scattering process, for which an analytical solution is available. It is demonstrated empirically that Nanbu's collision operator quite accurately recovers the effects of Coulomb-Lorentz pitch-angle collisions, or processes that approximate these even for very large values of the collisional time step. An investigation of the analytical solution shows that Nanbu's parameterized kernel is highly accurate for small values of the normalized collision time step, but loses some of its accuracy for larger values of the time step. Finally, a practical collision algorithm is proposed that for small-mass-ratio Coulomb collisions improves on the accuracy of Nanbu's algorithm. *Work performed for US DOE by UCLA under grant DE-FG02-05ER25710 and by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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NP6.00040: Accelerated Monte Carlo Methods for Coulomb Collisions Russel Caflisch, Richard Wang, Yanghong Huang, Bruce Cohen, Andris Dimits We present accelerated computational method for Coulomb collisions in a plasma, through significant improvements in our earlier hybrid method that combines a Monte Carlo particle simulation and a fluid dynamic solver in a single uniform method throughout phase space. We derive an improved formulation of the detailed balance constraint on the thermalization and dethermalization probabilities. We define a parameterized set of thermalization and dethermalization probabilities and optimize the choice of parameters to achieve the fastest computation time for a specified accuracy level. We mathematically analyze the validity of the thermalization and dethermalization step in the context of a simple drift-diffusion model that includes long range interactions as in Coulomb collisions. Finally, we formulate a higher order stochastic method for solving the drift diffusion model using a Milstein correction. [Preview Abstract] |
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NP6.00041: Efficient hybrid algorithms for collisional plasma simulation Y. Huang, A.M. Dimits, R.E. Caflisch, B.I. Cohen, C.M. Wang We report on the development of efficient hybrid simulation algorithms for plasma systems that span a wide range of collisionality. Investigations of their performance, using ion-sheath- and electron-transport-based test problems, are presented. In these schemes the distribution function for one or more charged species is decomposed of into kinetic (particle) and fluid components. The fluid component is treated via standard Eulerian fluid simulation. One class of algorithms [R. E. Caflisch, et. al., Multiscale Modeling and Simulation, SIAM, 2008, in press.] is based on a combination of standard fixed-weigh particle-in-cell (PIC) simulation and binary Monte-Carlo collision operators. Here, particles are created by sampling from the fluid component, and paired for collisions with the kinetic-component particles. The other class of algorithms is based on evolving-weight delta-f PIC schemes and collision-field algorithms. The performance these algorithms depends strongly on the particular sets of criteria for (a) exchange between the particle and fluid components and (b) creation, destruction, and retention of the simulation particles. [Preview Abstract] |
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NP6.00042: Test problems for dense kinetic plasmas John Luginsland, Andrew Christlieb Simulations of dense kinetic plasmas are an area of active research. A variety of devices from high power microwave devices to Hall thrusters involve situations with dense fluid-like plasmas coexisting with important kinetic non-Maxwellian particle distributions. These plasmas exhibit a range of length and time scales, making accurate simulation a challenging and computationally intensive task. The development of algorithms to handle these multi-scale circumstances is facilitated by high-fidelity test problems suitable for verification and validation studies. Specifically, we report on the development of ordinary differential equations appropriate for studying kinetic plasmas with short Debye lengths relative to the system size. Via analytic and numerical solutions, we study the impact of collective shielding on Langmuir waves. Furthermore, we extend these equations to account for typical particle weighting found in particle-in- cell (PIC) software. We compare these solutions to electrostatic PIC simulations with specific attention to computational artifacts, such as numerical heating. [Preview Abstract] |
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NP6.00043: Gyrokinetic Vlasov-Poisson simulation in slab geometry using the conservative IDO scheme Kenji Imadera, Yasuaki Kishimoto, Jiquan Li, Daisuke Saito, Takayuki Utsumi We have introduced the IDO-CF (Conservative Form of Interpolated Differential Operator) scheme [1], which is one of the multi-moment schemes and has been applied to various CFD problems, in solving a Vlasov-Poisson system. The IDO scheme is found to be efficient in capturing a sharp domain interface like shock propagation, and in introducing dissipations like particle collision and also external source/sink terms. Furthermore, the IDO-CF scheme has exact mass conservation properties, so that we can apply it to the problems that need long time scale simulations. We first apply the scheme in studying the nonlinear Landau damping and two-stream instability. We have investigated the conservation property of the total mass, energy and entropy, and found that the IDO-CF scheme allows stable simulation over many bounce periods keeping higher accuracy than other multi-moment schemes. We have also developed a gyrokinetic full-f Vlasov code with the IDO-CF scheme in studying the slab ITG driven turbulence. [1] Y.Imai et al., J. Comput. Phys. 227, 2263(2008). [Preview Abstract] |
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NP6.00044: MST, RFX-MOD AND REVERSED FIELD PINCH |
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NP6.00045: Overview of MST Results and Plans J.S. Sarff Improved confinement with high beta has been established in MST over its full range of plasma current capability using transient inductive current profile control. Both thermal electron and ion confinement are increased, and energetic electrons are observed to 100 keV. The global energy confinement time is 12 ms at high current and high temperature ($T_e$=2 keV, $T_i$ =1 keV), with $\beta_{tot}$=10\% (only Ohmic heating). Maximum $\beta_{tot}$=26\% is attained at lower current and temperature with D$_2$ pellet injection, without evidence of hard-beta-limit phenomena. Momentum transport associated with MHD tearing shows the fascinating behavior that the Maxwell and Reynolds turbulent stresses are both large but oppositely directed in sawtooth magnetic relaxation events. Momentum is transported rapidly in these events, presumably through the imbalance in the stresses. Electron temperature fluctuations associated with MHD tearing are measured using a multi-point, multi-pulse Thomson scattering diagnostic. A 5-250 kHz pulse-burst laser is under construction to extend the Thomson capability to high frequency. Lower hybrid and electron Bernstein wave injection are under development to provide more sustained current profile control and heating. X-ray emission from the plasma is observed for both waves at 175 kW injected power. Substantial new experimental capability will be provided by a recently installed programmable power supply for the toroidal field, a new 1 MW, 20 ms neutral beam injection system, and upgraded OFCD system. Supported by U.S. DoE and NSF. [Preview Abstract] |
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NP6.00046: High confinement high temperature plasmas in MST B.E. Chapman, A.F. Almagri, J.K. Anderson, K.J. Caspary, D.J. Clayton, D.J. Den Hartog, D.A. Ennis, G. Fiksel, S. Gangadhara, J.A. Goetz, R. O'Connell, R.M. Magee, S.C. Prager, J.A. Reusch, J.S. Sarff, H.D. Stephens, F. Bonomo, P. Franz, D.L. Brower, B. Deng, W.X. Ding, T. Yates, D. Craig With inductive modification of the current profile and reduction of magnetic fluctuations, the energy confinement time in MST had previously been increased ten-fold, to about 10 ms. However, this result was achieved at relatively low plasma current, 0.2 MA, and relatively low temperature, Te = 0.6 keV and Ti = 0.2 keV. We have now extended improved confinement to the upper range of MST's plasma-current capability, around 0.5 MA. Here, the ohmically heated electrons reach 2 keV, and ions are heated to well above 1 keV by magnetic reconnection occurring prior to improved confinement. The global energy confinement time in these plasmas is about 12 ms, a modest improvement over the confinement at low current. This corresponds to a global thermal diffusivity of about 5 m$^2$/s. Total beta (volume-averaged pressure/total field pressure at the plasma boundary) is about 10 percent. Supported by USDOE and NSF. [Preview Abstract] |
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NP6.00047: Pellet-fueled, high-$\beta $, improved confinement RFP plasmas K.J. Caspary, M.D. Wyman, B.E. Chapman, A.F. Almagri, J.K. Anderson, D.J. Den Hartog, F. Ebrahimi, D.A. Ennis, G. Fiksel, S. Gangadhara, J.A. Goetz, S.T. Limbach, R. O'Connell, S.P. Oliva, S.C. Prager, J.A. Reusch, J.S. Sarff, H.D. Stephens, F. Bonomo, P. Franz, D.L. Brower, B.H. Deng, W.X. Ding, T. Yates, S.K. Combs, C.R. Foust, D. Craig Pellet fueling of improved confinement MST plasmas has resulted in a four-fold increase in the density, reaching 4x10$^{19}$ m$^{-3}$, and a total beta of up to 26{\%}.~ At high beta, the Mercier limit for interchange stability is exceeded, but there is as yet no experimental evidence of interchange modes.~ The linear stability threshold for pressure-driven tearing modes is also exceeded, and one does observe increased tearing mode amplitudes.~ At high density, the energy confinement time is still improved, but the degree of improvement is reduced.~ This may be due to the tearing modes, suggesting that pressure-driven tearing could ultimately limit the achievable beta in the RFP.~ MST's pellet injector has now been upgraded to accommodate pellets with roughly twice the particle inventory used to achieve the above results.~ Tests with these larger pellets are underway.~ Work supported by U.S.D.O.E. [Preview Abstract] |
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NP6.00048: Measurement of $Z_{eff}$ and Radial Diffusion via X-ray Spectroscopy in MST D.J. Clayton, A.F. Almagri, J.K. Anderson, D.R. Burke, B.E. Chapman, R. O'Connell, R.W. Harvey Measured x-ray spectra and Fokker-Planck modeling are used to constrain the effective ionic charge $Z_{eff}$ and the radial particle diffusion coefficient $D_{r}$ in MST. A new single-photon counting Si detector measures 2-10 keV x rays while a multichord array of CdZnTe detectors measures the 10-150 keV range. Absolute calibration of the measured x-ray flux is required to find $Z_{eff}$ and $D_{r}$. The Fokker-Planck code CQL3D models the electron distribution function and predicts the x-ray spectrum resulting from bremsstrahlung. The code is run iteratively to find the $Z_{eff}$ and $D_{r}$ for which the predicted x-ray flux best matches the measurement. $Z_{eff}$ is then used to calculate quantities such as resistivity, ohmic power, and the energy confinement time. Results from standard RFP plasmas, with $D_{r}$ dependent on electron velocity, and improved confinement, pulsed parallel current drive plasmas, with $D_{r}$ independent of velocity, will be presented. Work supported by the USDOE. [Preview Abstract] |
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NP6.00049: Measuring power loss due to radiation and charge exchange in MST Jeff Waksman, Brett Chapman, Gennady Fiksel, Paul Nonn An array of photodiode-pyrobolometer pairs will be placed on MST to measure the spatial structure of the radiated power and charge exchange. Photodiodes (XUV detectors) measure photonic radiated power from about 10eV to 10keV, while pyrobolometers (thermal detectors) measure both photonic radiated power and power carried by charge-exchange neutrals. Compared to other thermal detectors, pyrobolometers have very good time resolution. To accurately calibrate the individual detectors, an electron gun producing a modulated square wave output has been set up to carefully calibrate each new pyrobolometer to be placed on MST. When viewing the MST plasma, subtraction of the data from the photodiode-pyrobolometer pairs allows one to determine the net power loss due to charge-exchange neutrals. These measurements are important in the calculation of ion energy balance, and it is potentially important in understanding the difference in the temperatures reached by majority and impurity ions during magnetic-reconnection ion-heating events. Since toroidal and poloidal asymmetries in charge exchange are possible, a distributed array of detector pairs will facilitate a better estimate of global power loss. Work supported by the U.S.D.O.E. . [Preview Abstract] |
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NP6.00050: Multicolor SXR Tomography on MST M.B. McGarry, J.A. Goetz, D.J. Den Hartog, B.E. Chapman, P. Franz, F. Bonomo, L. Marrelli We present soft-x-ray (SXR) tomographic images of the MST RFP plasma using both the multicolor and two-color configurations during pulsed parallel current drive (PPCD). With the application of PPCD, magnetic flux surfaces are partially restored, and discrete islands can form. The tomographic diagnostic is comprised of four 20-channel cameras at the same toroidal angle but different poloidal angles. The multicolor configuration uses different thickness Be filters on each camera, whereas the two-color configuration uses two pairs of Be filters, measuring four and two distinct energy ranges, respectively. SXR data at these different energies can then be analyzed and compared to magnetic measurements to investigate MHD core dynamics in MST plasmas. We also present the model we use to simulate SXR emissivity images based on user-specified plasma parameters. The model allows localized structures to be introduced, both in the temperature (\textit{Te}) and density profiles, to simulate the SXR structures that emerge during PPCD. The SXR emissivity model is used to show that \textit{Te} profiles can be obtained by the standard two-foil technique. We apply this technique to recent data and compare the \textit{Te }measurement to that of the Thomson scattering diagnostic. Work supported by U.S.D.O.E. [Preview Abstract] |
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NP6.00051: A toroidal CHERS diagnostic for MST R.M. Magee, D.J. Den Hartog, G. Fiksel, D. Craig, D.A. Ennis, S. Gangadhara Charge exchange recombination spectroscopy (CHERS) has been used on MST for several years to make localized measurements of the poloidal component of the impurity ion temperature and flow. These data have been used to constrain theoretical models of the RFP anomalous ion heating problem and measure the MHD dynamo in the core. The complementary toroidal measurements will allow reconstruction of the parallel and perpendicular temperature profiles and measurements of additional MHD dynamo components. This poster will describe the design of the diagnostic in light of two principal difficulties. The low signal-to-background ratio has been addressed by upgrading the 49 keV H$^{0}$ neutral beam to increase the charge exchange signal. The requirements of high temporal and spatial resolution have driven the optical design; the assembly maximally fills a high throughput spectrometer optimized for fast measurements (100 kHz) while maintaining a spatial resolution of 2-3 cm. Initial results will be presented. [Preview Abstract] |
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NP6.00052: Probe Measurements of Particle and Momentum Transport in the edge of MST M.C. Miller, A.F. Almagri, D. Craig, D.A. Ennis, G. Fiksel, S. Gangadhara, A. Kuritsyn, S.C. Prager, D. Stone, T.D. Tharp Direct measurements of fluctuation-induced particle transport, Maxwell stress, and Reynolds stress have been made in the edge plasma of the MST RFP. Particle transport, $\langle\widetilde{n} \widetilde{v_{r}}\rangle$, Reynolds stress, \mbox{$-\rho (\tilde{v} \cdot \nabla) \tilde{v}$}, and Maxwell stress, \mbox{$ \tilde{j} \times \tilde{B}$}, all depend on the correlation between fluctuating quantities. These fluctuations of have been measured in the edge of MST with probes. Whereas previous velocity measurements relied on measuring the drift velocity, {\boldmath $E \times B$, $v$} is now measured directly. A spectroscopic probe measured $v_{r}$, and a Mach probe measured $v_{\theta}$ and $v_{\phi}$. Density and magnetic field were measured with a triple tip Langmuir probe and pickup coil triplets respectively. In the edge of MST, during reconnection events, the Reynolds stress is measured to be an order of magnitude larger than the inertial term in the momentum balance equation, but it is balanced by the Maxwell stress. Particle transport increases dramatically during a reconnection event, going up several times its pre-crash value. This poster presents probe design, the details of the correlation techniques, and experimental results. Work is supported by the US DOE and the NSF. [Preview Abstract] |
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NP6.00053: Core Measurement of Magnetic Fluctuation-Induced Particle Flux in the MST Reversed Field Pinch W.X. Ding, D.L. Brower, G. Fiksel, D.J. Den Hartog, S.C. Prager, J.S. Sarff, T. Yates The cross product of fluctuating parallel particle flux and radial magnetic field fluctuations, commonly referred to as radial particle transport driven by stochastic magnetic field, has been measured in the high-temperature core of Reversed Field Pinch plasmas by using a fast laser-based Faraday rotation and interferometer system. Measurements show that convective electron particle flux can account for the equilibrium density change during a sawtooth crash at the magnetic axis. In addition, the stochastic field driven ion and electron fluxes are nearly the same with their difference being $\sim $1{\%}. A quasi-linear estimate of ion particle flux indicates that it scales as the ion sound speed and the square of radial magnetic field fluctuations. [Preview Abstract] |
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NP6.00054: Density Profile and Fluctuations in Quasi-Single-Helicity MST Plasmas W.F. Bergerson, D.L. Brower, B. Chapman, W.X. Ding, J.S. Sarff, S.C. Prager Numerous resonant and overlapping tearing modes of multiple helicity (MH) generate the stochastic magnetic field in reversed field pinch (RFP) plasmas. However, RFPs with one resistive tearing mode impart a single helicity (SH) to the magnetic field and are predicted to preserve good magnetic surfaces and reduce transport. Between the extremes of MH and SH is Quasi-Single-Helicity (QSH), a plasma with a dominant tearing mode and multiple smaller modes. Equilibrium density profiles and density fluctuations have been studied in the MST RFP by employing standard and differential interferometry techniques in 300 kA plasmas at modest density (0.5 * 10$^{13}$ m$^{-3}$). Initial comparison of the MH and QSH states indicates the latter has a hollow density profile, while the former is centrally peaked. This finding is maintained when looking at ensembled data. Modification to stochastic magnetic field driven particle transport during QSH provides a potential explanation for the equilibrium profile changes. Equilibrium magnetic and current density profiles measured by polarimetry will also be investigated. This work is supported by the DoE. [Preview Abstract] |
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NP6.00055: Interferometric Measurement of Density and Magnetic Fluctuations in MST Travis Yates, Weixing Ding, Troy Carter, David Brower Fluctuations play an important role in anomalous particle, momentum and energy transport. Knowledge of local density and magnetic field fluctuations allow for the measurement of particl flux. Simultaneous interferometry-polarimetry measurements with a bandwidth of $\sim $500 kHz and 8 cm chord spacing have been utilized to determine the core density and magnetic field fluctuations in MST. Local density and radial magnetic field fluctuation profiles can be reconstructed from line-integrated measurements by employing newly developed analysis techniques. These techniques model the fluctuation spatial distribution and include toroidal effects. Both the density fluctuation amplitude and phase relative to radial magnetic fluctuations are obtained. This allows evaluation of the convective magnetic fluctuation-induced electron particle flux, $\Gamma _r =v_{\vert \vert } \frac{<\tilde {n}\tilde {b}_r >}{B_0 }$. Spatial structure of fluctuations for modes m=1, n=1-15 are described. Work supported by U.S. Department Of Energy. [Preview Abstract] |
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NP6.00056: Electron temperature fluctuations from tearing modes in MST Hillary Stephens, Daniel Den Hartog, Chris Hegna, Rob O'Connell, Joshua Reusch Electron temperature fluctuations (temporal and spatial) associated with the largest magnetic tearing mode (m=1, n=6) have been observed in MST. The rational surface for the m=1, n=6 modes is well defined by the temperature structure in standard discharges. Evidence that the temperature fluctuations may be slightly out of phase with the magnetics is observed. Results are presented for both 400kA standard and Quasi-Single Helicity (QSH) MST discharges. During standard plasmas the temperature structure around the rational surface is approximately 20 percent of the minor radius with temperature fluctuations approximately 4 percent of the mean. This spatial structure may be associated with a magnetic island in the plasma. Electron temperature is measured using a multi-point, multi-pulse Thomson scattering diagnostic. Magnetic data is collected with toroidal and poloidal magnetic coil arrays. The data are collected over an ensemble of similar plasma shots with temperature measured at 21 spatial locations for two time points during standard discharges and four time points during QSH discharges. A Bayesian analysis is used on the ensemble to map the temperature structure near the tearing mode. The research was performed under appointment to the Fusion Energy Sciences Fellowship Program and supported by US DOE and CMSO. [Preview Abstract] |
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NP6.00057: Direct Comparison of Simulated Sawtooth Evolution to the Madison Symmetric Torus RFP J.A. Reusch, J.K. Anderson, D.J. Den Hartog, F. Ebrahimi, C.B. Forest, R. O'Connell, D.D. Schnack, H.D. Stephens It has long been believed that the reversed field pinch has stochastic magnetic field lines and exhibits Rechester-Rosenbluth thermal transport under standard operating conditions. In this poster, results from a set of first principles simulations using the resistive MHD code DEBS will be presented. The simulations are designed to match experimental data as closely as possible. To that end, both a Spitzer and a neoclassical resistivity model were tried for the cases of a fixed, experimentally measured resistivity. A freely evolving resistivity based on an evolving pressure profile was also examined. The fixed resistivity calculations were done at a Lundquist number of $10^6$ and the evolving pressure calculations had an average Lundquist number of approximately $4\times10^6$. The expected Rechester-Rosenbluth electron thermal diffusion, $\chi_{_{RR}} = v_{_{T_e}} \pi L_{_{eff}} \tilde{b}^2/B^2$, is compared to the measured thermal diffusion obtained through power balance. Furthermore, in the evolving pressure case, the temperature profile information is extracted and compared directly to Thomson scattering measurements. [Preview Abstract] |
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NP6.00058: A Pulse-Burst Laser System for Thomson Scattering D.J. Den Hartog, M.T. Borchardt, Y.M. Yang, J.R. Ambuel, D.J. Holly, H.E. Mattison, P.E. Robl A ``pulse-burst'' laser system is being constructed for addition to the Thomson scattering diagnostic on the MST reversed-field pinch. This laser will produce a burst of up to 200 approximately 1 J Q-switched pulses at repetition rates 5--250 kHz. The laser will operate at 1064 nm and is a master oscillator, power amplifier (MOPA) system. Variable pulse-width drive (0.1--20 ms) of the flashlamps is accomplished by IGBT switching of large electrolytic capacitor banks. In the near term, these flashlamp power supplies will be adapted to drive the flashlamps in the two existing commercial Nd:YAG lasers used for Thomson scattering on the MST RFP. This will enable these lasers to produce a burst of up to 40 pulses at repetition frequencies $\le $ 1 kHz. The burst train of laser pulses will enable the study of $T_{e}$ and $n_{e}$ dynamics in a single MST shot. [Preview Abstract] |
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NP6.00059: Energetics of ion heating during magnetic reconnections in Reversed Field Pinch Gennady Fiksel, A.F. Almagri, S.C. Prager, J.S. Sarff, C.R. Sovinec It has been pointed out [S. Gangadhara et. al., PoP 15, 056121, 2008] that the magnetic energy released during reconnection events in reversed field pinch plasma is directly related to the magnitude of ion heating. In deuterium plasmas about $20\%$ of the released magnetic energy was deposited into the bulk ion thermal energy. We extend the measurements of ion heating to hydrogen and helium plasmas. The bulk ion temperature was measured with a Rutherford scattering diagnostic, and the deposited fraction of magnetic energy was found to vary as the square root of the ions mass. It depends weakly on the ion charge, plasma density, and the magnetic field. The mechanism by which the ions are heated is still not identified. We speculate that it may be stochastic in nature based on similar mass scaling in Fermi-like stochastic heating in capacitive discharges. Global ion heating arises only when both core and edge magnetic reconnection occur. During so-called ``core reconnection only'' events the core resonant magnetic modes reach a substantial amplitude but the edge resonant modes are absent. The released magnetic energy and, subsequently, the ion heating are very small. There are indications that during these events the core modes may have an ideal character, thus not resulting in the reconnection of the equilibrium magnetic field. [Preview Abstract] |
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NP6.00060: Experimental Study of High Frequency Magnetic Fluctuations in MST Y. Ren, A. Almagri, S.C. Prager, G. Fiksel, J.S. Sarff Reversed field pinch plasmas exhibit a broad spectrum of magnetic fluctuations, dominated by low frequency tearing modes ($\sim 10-30$ KHz) which are important for magnetic self- organization and transport. However, the origin of higher frequency fluctuations remains unclear. Here we propose that magnetic energy nonlinearly cascades from the tearing mode fluctuations to the shorter wavelength, high frequency fluctuations. This is suggested by observations that the high frequency power always increases or decreases in concert with the tearing mode amplitudes. The portion of the power spectrum adjacent to the low frequency tearing modes exhibits a power law similar to expectations from relevant nonlinear inertial cascade models. However, the power falls more rapidly at higher frequency, suggesting dissipation is important in a large portion of the spectrum. Interestingly, these spectral features resemble magnetic turbulence measurements in space plasmas. Radial profile measurements using a magnetic probe array show that the high frequency fluctuations are locally resonant modes which have $k_{||}\ll k_{\perp}$. The k-spectra also exhibit power law structure similar to the frequency spectra. An electrostatic probe is used to measure the electrostatic components of the high frequency fluctuations. The possible types of high frequency fluctuations will be discussed. This work is supported by NSF and DoE. [Preview Abstract] |
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NP6.00061: Measurements of Linear and Nonlinear Hall Reconnection T.D. Tharp, A.F. Almagri, A.V. Kuritsyn, V.V. Mirnov, S.C. Prager, J.S. Sarff, D. Craig, M.C. Miller Previous measurements in MST have established that two-fluid Hall effects produce a dynamo during sawtooth relaxation events, and therefore two-fluid dynamics are important when evaluating the macroscopic effects of reconnection. This was established by measuring the nonlinear Hall term $(J_1 \times B_1)$ in the axisymmetric (flux-surface averaged) Ohm's Law. Here, we report measurements of terms in the \em non\em -axisymmetric Ohm's Law, including the reconnection electric field and terms that may balance this field. In particular, the linear Hall term, $(J_1 \times B_0 + J_0 \times B_1)$, and nonlinear three wave interaction Hall terms are considered. These fluctuation measurements are a more direct indicator of the role of two-fluid effects on reconnection. Measurements are performed by probes in the vicinity of the reversal surface to measure reconnection associated with modes of poloidal mode number m=0. Results of these measurements lead to an estimation of the extent to which classical resistivity can balance Ohm's Law at the reconnection x-point. [Preview Abstract] |
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NP6.00062: Measurements with a Heavy Ion Beam Probe during Improved Confinement in MST D.R. Demers, X. Chen, P.M. Schoch, P. Fimognari The Heavy Ion Beam Probe (HIBP) in operation on the Madison Symmetric Torus (MST) is acquiring measurements in the core region of improved confinement plasmas. These discharges, also known as pulsed poloidal current drive (PPCD), are characterized by periods during which magnetic turbulence and transport are suppressed. Simultaneous measurements of \~{n} and $\phi $\~{ } will enable us to explore the contribution of electrostatic fluctuations to transport and measurements of E$_{r}$ will allow us to examine localized structures associated with non-ambipolar particle transport. The issues presented by the reversed field pinch plasma and the dynamic nature of PPCD - including computation of magnetic equilibria; HIBP sample volume localization, size, and orientation; and fluctuation sensitivity -- will be discussed, and measurements will be presented. [Preview Abstract] |
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NP6.00063: Localized SXR Emission During Electron Bernstein Wave Injection in MST Jay Anderson, Cary Forest, Andrew Seltzman The electron Bernstein wave has been suggested as a solution to the RFP confinement problem: sustained, off-axis current drive stabilizes the resistive tearing modes which govern thermal transport. A staged experiment to test the feasibility of EBW heating and current drive is underway on MST. Experiments ($\sim10^5$ W) aimed at a demonstration of EBW heating have produced a localized increase in SXR emission. This measured emission is consistent with modeling in its location, energy spectrum and dependence on radial diffusion within the plasma. Preliminary analysis indicates that the emission is strongest in the region where ray tracing predicts maximum deposition of the injected power. The multi-chord SXR camera used is sensitive to 4-7 keV photons which is consistent with Fokker-Plank modeling of EBW injection. The enhanced SXR emission vanishes quickly when radial diffusion in the plasma is high (as indicated by m=0 magnetic activity); this is also consistent with Fokker-Plank modeling. An increase of boron emission (and presumably boron within the plasma) is also observed during EBW injection. This presents an alternative explanation to the enhanced SXR emission; recent efforts have been made to isolate the two effects. [Preview Abstract] |
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NP6.00064: HXR Observations with Lower Hybrid Current Drive on MST M.C. Kaufman, J.A. Goetz, D.R. Burke, C.B. Forest, S.C. Prager Lower Hybrid current drive has been offered as a means to reduce tearing f\/luctuations and improve confinement in the reversed field pinch. Experiments at 160 kW input power show robust coupling to a variety of plasmas. Hard x-ray (HXR) bremsstrahlung emission from rf-generated fast electrons with energies up to and beyond 60 keV has been observed using CdZnTe detectors. Monte Carlo simulations and simple modeling of the antenna demonstrate that near-fields (within a cm of the antenna) exceeding 3 kV/cm can drive thermal electrons to energies above 30 keV, consistent with experimental observations. HXR emission far from the antenna is spatially localized around the antenna with a toroidal spread of about 60 degrees. Power launched in the co-current drive direction generates an order of magnitude more flux than in the counter-current drive direction. Moreover, the HXR spectra of the co-current drive plasma emission are qualitatively different from both the counter-current plasma emission as well as the near-field emission. The HXR spectra from the co-current drive exhibits the signature of a bi-Maxwellian distribution while the other cases do not, corroborating a different production mechanism. However, this two-temperature spectrum is distinct from a typical fast electron tail seen with rf drive in tokamaks.\newline Work supported by US DOE Contract DE-FC02-05ER54814. [Preview Abstract] |
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NP6.00065: X-ray diagnosis of Lower Hybrid experiments on the MST D.R. Burke, A.F. Almagri, D.J. Clayton, C.B. Forest, J.A. Goetz, M.C. Kaufman, K. Makwana, R. O'Connell, S.C. Prager Inductive auxiliary current drive has been shown to transiently reduce tearing fluctuations and improve confinement in the Reversed Field Pinch (RFP). Lower hybrid waves are predicted to be capable of supplying steady state stabilizing current on MST. The RFP offers a unique set of challenges for lower hybrid wave injection from both a diagnostic and a modeling perspective. Toroidal loop voltage cannot be used as an diagnostic, since driven current is expected to be mostly poloidal. Thus, the main diagnostic of the effects of lower hybrid waves on the plasma is measurement of the x-ray spectrum. MST plasmas are diagnosed by CdZnTe detectors for x-rays above 10 keV. Silicon photodiodes are being employed to add spectral discrimination from 2 to 10 keV. Ray tracing with the GENRAY code and Fokker-Planck modeling with the CQL3D code are used to predict flux. Hard x-rays with energies up to 60 keV have been measured and are toroidally localized to within 60$^{\circ}$ of the antenna. The unique challenges presented by this explored and modeling results are compared with experiment. Direct current measurements using a Rogowski coil are being attempted. [Preview Abstract] |
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NP6.00066: Oscillating Field Current Drive Experiments on MST K.J. McCollam, F. Ebrahimi, J.S. Sarff, D.R. Stone, D.J. Den Hartog, G. Fiksel, R. O'Connell, S.C. Prager, D.L. Brower, B.H. Deng, W.X. Ding, D. Craig Oscillating field current drive (OFCD) is a proposed method for efficient, steady-state toroidal plasma current sustainment using AC poloidal and toroidal loop voltages. OFCD is added to a standard RFP in the MST device, increasing the net plasma current by about 10{\%}. In recent experiments the OFCD input power and frequency and MST baseline current have been varied to test their effects on the OFCD-added current, but it has not yet been improved beyond previously improved levels. The amount of added current is likely to be strongly affected by magnetic fluctuation activity, which is modulated significantly by OFCD. The added current computed by a 3D resistive-MHD code using experimental voltage parameters closely matches the measured current. OFCD's effect on confinement is a key issue in evaluating its feasibility for full sustainment. In these partial-sustainment tests the cycle averages for both the energy confinement time and total beta are about the same as in the standard RFP without OFCD. Helicity balance studies are ongoing which include contributions from potential and magnetic fluctuations as measured in the plasma edge. This work is supported by the US DOE. [Preview Abstract] |
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NP6.00067: Fluctuation-Induced Helicity Transport in Oscillating Field Current Drive Tests on MST D.R. Stone, A. Almagri, G. Fiksel, K.J. McCollam, M.C. Miller, J.S. Sarff In oscillating field current drive (OFCD), poloidal and toroidal frequency-matched ac magnetic fields are inductively applied to the plasma in order to drive dc plasma current. The fields interact to inject net magnetic helicity into the plasma depending on their phase difference.~Measurements of helicity balance for OFCD tests in MST are done both to better understand the role of helicity in plasma dynamics and to aid in optimizing OFCD performance.~The equilibrium helicity injection rate is roughly balanced by equilibrium helicity dissipation, but helicity transport or dissipation due to fluctuating quantities may be needed for precise balance. Helicity transport due to fluctuations has been measured in MST's edge with and without OFCD using a radial-magnetic-field probe combined with a triple Langmuir probe to measure plasma potential. Helicity transport over the course of the OFCD cycle is measured for different OFCD frequencies and phases. Helicity transport is also compared for sawtooth crashes with and without OFCD. This work was supported by the US DOE. [Preview Abstract] |
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NP6.00068: Programmable Power Supply for MST's Toroidal Field D.J. Holly, J.R. Adney, A.E. Anders, M.T. Borchardt, B.E. Chapman, J.C. Morin, J.S. Sarff, A.A. Squitieri We have installed a new power supply on MST to provide programmable control of the toroidal magnetic field and poloidal loop voltage. The supply uses 128 high power IGBTs in a modular series-parallel H-bridge configuration to produce a bipolar output of about 25 MW with a bandwidth of several kHz. The Plasma Control System developed by GA will control the supply output waveform. The new supply's programmability will allow us to optimize and extend inductive current profile control using Pulsed Parallel Current Drive, a technique which reduces magnetic tearing fluctuations and the resultant energy transport. We will also use the supply to test alternate plasma startup techniques which can reduce V-s consumption in the poloidal field system; and for Oscillating Field Current Drive experiments, where the supply will give us the new ablility to generate non-sinusoidal waveforms. Initial results will be presented. [Preview Abstract] |
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NP6.00069: Linear Ion Temperature Gradient Driven Drift Modes in a Reversed Field Pinch Plasma Varun Tangri, Paul Terry, R.E. Waltz In Reversed Field Pinch (RFP) plasmas, tearing modes are usually considered responsible for global confinement losses. However, recently improved confinement has been observed when tearing modes are suppressed through current profile control. Although diffusivity has improved, it still exceeds classical and neoclassical estimates. The new mechanism is poorly understood but believed to be associated with smaller scale instabilities, and estimates of diffusivities are lower than observations. In such discharges, it is plausible that short scale ITG may play a significant role in particle and heat confinement. In this work, we examine the linear stability of the ion temperature gradient mode in the RFP geometry. We use a new, simple equilibrium (zero beta, concentric circle) model for RFP machines, which is similar to the well-known s-alpha model. Using the code GYRO in a collsionless limit, linear gyrokinetic simulations in real toriodal RFP geometry have been performed. We also make comparisons with simple calculations and potential relevance of the slab and toroidal branches is also analyzed. [Preview Abstract] |
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NP6.00070: Kinetic effects in RFP plasma Vladimir Svidzinski, Hui Li, Brian Albright Strong tearing mode activity is present at sawtooth crashes in the Madison Symmetric Torus reversed field pinch (RFP). It is believed that tearing modes are responsible for strong ion heating and change in plasma flow profile at the crash. Our results based on both linear and nonlinear resistive MHD models showed that the spatial scale of velocity, electric field and current profiles in the tearing mode near resonance surface is comparable to ion gyroradius. The ion gyroradius is relatively large in RFPs because of smaller equilibrium magnetic field. In these conditions both two fluid and kinetic effects can be significant. We study ion kinetic effects on tearing modes in RFP plasmas. We consider RFP-like equilibrium in plane geometry and solve for linear eigenmodes in resistive MHD, two fluid and fully kinetic models. In the first two models we solve an eigenvalue problem, in the last we use particle in cell code VPIC and follow linear time evolution of the fastest growing mode. We analyze how the scale of plasma flow and flow amplitude in the mode are effected by the finite ion gyroradius effect, to what plasma component (ions or electrons) the magnetic energy of initially unstable equilibrium is transfered. Results of this analysis will be presented. [Preview Abstract] |
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NP6.00071: Numerical Studies of Nonlinear Two Fluid Tearing Modes in Cylindrical RFPs J.R. King, C.R. Sovinec, V.V. Mirnov The NIMROD code is used to run 3D two fluid computations in cylindrical geometry. Profiles similar to the Madison Symmetric Torus RFP are used with a reduced aspect ratio to limit the number of modes. The structure and dynamo effects of single helicity non-reversed states are examined and compared with a single fluid model. Consistent with analytical predictions [1], NIMROD linear results show rotation of the two fluid tearing instability due to the relatively strong field line curvature in the RFP configuration. Previous results in 2D slab geometry have shown the linear hall dynamo is present in a narrow region around the rational surface, but it is broadened and exhibits a fine structure along the separatrix in the nonlinearly saturated state. This fine structure is a challenge to model in 3D, and resolution requirements are investigated. The sensitivity of the nonlinear Hall dynamo to the relative size of the ion sound gyroradius and the resistive scale is investigated. Studies of reversed states allow for the examination of both the stable and unstable resonant modes and their nonlinear coupling. [1] V.V. Mirnov, et al., Proc. of 21st IAEA Fusion Energy Conf, TH/P3-18 (2006) [Preview Abstract] |
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NP6.00072: Overview of the recent results of RFX-mod M. Spolaore The RFX-mod Reversed Field Pinch (RFP) experiment (R=2m /a=.459 m) is exploring RFP physics at record high plasma currents and is equipped with a state-of-the-art MHD stability control system composed by 192 radial field coils, each independently driven, that completely cover the surface of the device. Thanks to a model-based algorithm and various feedback control schemes the system represents the most advanced tool for the magnetic boundary control providing a successful phase decoupling and amplitude reduction of tearing modes, as well as a full suppression of multiple Resistive Wall modes. Good quality and well reproducible discharges up to 1.6 MA plasma current lasting up to 0.5 s are now currently obtained. As the plasma current increases, a new spontaneous self-organization towards single axis helical states, with improved confinement properties and internal electron transport barriers, is observed. This is very promising for the future perspectives of the configuration. An overview of the results obtained during the 2008 campaign will be presented. [Preview Abstract] |
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NP6.00073: Current density filaments measured by electrostatic, magnetic and optical diagnostics in RFX-mod N. Vianello, M. Spolaore, M. Agostini, V. Antoni, R. Cavazzana, E. Martines, G. Serianni, P. Scarin, E. Spada, M. Zuin Edge turbulence is ubiquitous in fusion devices and characterized by the formation of coherent structures which are believed to play a relevant role in driving particle losses. These structures have been observed also in the edge region of the RFX-mod Reversed Field Pinch device. In order to gain insight into their origin and features an original probe system has been used measuring both magnetic and electrostatic fluctuations simultaneously and on the same location with a high time resolution. This insertable probe head allows the direct measurements of several plasma parameters including local vorticity patterns and current density fluctuations. An array of toroidally distributed sensors allows following the structures along the main flow direction. It is found that in the cross-field plane bursts correspond to pressure structures and are related to current density filaments mainly oriented along the magnetic field. These results are compared with those provided by the measured relationship between structures on HeI emitted radiation observed by the Gas Puffing Imaging system and magnetic fluctuations. The presence and features of the current density filaments are assessed at different plasma current regimes. [Preview Abstract] |
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NP6.00074: Helical states with ordered magnetic topology in the Reversed Field Pinch D. Bonfiglio, S. Cappello, M. Gobbin, G. Spizzo The reversed field pinch (RFP) configuration for magnetic confinement has shown to develop helical configurations characterized by good magnetic surfaces both in experiments and visco-resistive 3D MHD numerical computations [1]. In the RFX-mod experiment, quasi-single helicity (QSH) states with ordered magnetic topology have been found to develop both spontaneously during high current discharges [2] and in a stimulated way through the periodic oscillation of the toroidal flux (so-called OPCD technique) [3]. In both cases, the expulsion of the separatrix of the dominant mode has proved to be the key for significant chaos healing [4], as expected by theory [5]. In this work, we present results of visco-resistive 3D MHD numerical modeling aiming at clarifying the mechanism and the conditions for separatrix expulsion and chaos healing in spontaneous and stimulated cases. The effect is investigated by reconstruction of the magnetic topology through field line tracing algorithms and by study of test particle dynamics. [1] S. Cappello, Plasma Phys. Control. Fusion \textbf{46, }B313 (2004) {\&} references therein. [2] M. Valisa et al., invited oral, EPS Conf. on Plasma Physics (2008). [3] D. Terranova et al., Phys. Rev. Lett. \textbf{99, }095001 (2007). [4] R. Lorenzini et al., Phys. Rev. Lett. \textbf{101, }025005 (2008). [5] D. F. Escande, R. Paccagnella et al., Phys. Rev. Lett. \textbf{85, }3169 (2000). [Preview Abstract] |
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NP6.00075: Density limit in the Reversed Field Pinch M.E. Puiatti, M. Agostini, S. Cappello, P. Innocente, R. Lorenzini, R. Paccagnella, I. Predebon, P. Scarin, G. Spizzo, D. Terranova, M. Valisa A phenomenological density limit is observed in the Reversed-Field Pinch, arising when the density, normalized to the Greenwald density $n_G = I/\pi a^2$, exceeds values $\simeq 0.4 \div 0.5$. In this paper we connect the presence of toroidally localized patterns of increased radiation and density, observed experimentally in the RFX, to the loss of electrons driven by $m=0$, $n \ge 1$ islands in the plasma edge. This loss of electrons causes a toroidally localized inversion of the plasma flow in the edge, which is at the basis of the radiation and density accumulation. This phenomenon will be studied by means of the guiding centre code \textsc{Orbit}\footnote{R. B. White and M. S. Chance, Phys. Fluids \textbf{27}, 2455 (1984)}, using as input the experimentally inferred eigenfunctions for the MHD $m=0,1$,$n$ modes. [Preview Abstract] |
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NP6.00076: Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge Paolo Scarin, Matteo Agostini, Roberto Cavazzana, Fabio Sattin, Gianluigi Serianni, Monica Spolaore, Nicola Vianello The spatial scales of edge turbulence structures and the corresponding toroidal phase velocity v$\phi$ are routinely measured in RFX-mod with a Gas Puffing Imaging (GPI) diagnostic for the whole plasma pulse duration and in all plasma conditions without the frozen turbulence hypothesis. A new Thermal Helium Beam (THB) diagnostic has been coupled to the GPI to measure at the same location (about 45mm) the edge profiles of electron density and temperature via the line intensity ratio technique. In the edge region of fusion devices and in RFX-mod as well coherent structures (blobs) are commonly observed. In order to investigate their birth and the possible drive instability mechanism, the link between the e-folding characteristic length Lp of the edge electron pressure profile and the dimensions of the coherent structures (blobs) is studied, as well as the ion sound radius $\rho$s (ion temperature is measured at the edge region by the Doppler broadening of BII lines). [Preview Abstract] |
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NP6.00077: First experiments of impurity LBO injection in RFX-mod Alessandro Fassina, Alberto Alfier, Federica Bonomo, Lorella Carraro, Gianluca Spizzo, Maria Ester Puiatti, Roberto Pasqualotto, Marco Valisa A system for impurity injection via laser blow-off has been recently installed on the RFX-mod experiment to study impurity transport. The system layout will be described, in terms of the laser, its path and optics, the ablation chamber and the target manipulator. Measurements of the impurity time-of-flight have been performed before installing the system on RFX-mod. An overview of nickel LBO experiments in discharges at 1.5 MA in presence of QSH will be presented. Nickel line emissions in XUV and VUV spectral range and SXR continuum emission are described and compared with the reconstructions obtained with a 1-dimensional collisional-radiative impurity transport model [1,2]. Injection process has been traced with aid of TS diagnostic, bolometry, and SXR tomography, displaying the presence of radiation structures inside the plasma. An upgrade of the optical system is planned: this will provide a more uniform energy deposition on the target, improving the ablation efficiency. [1] M.Mattioli et al. J.Phys.B 34, 127 (2001) [2] M.Mattioli et al. J.Phys.B 37, 13(2004) [Preview Abstract] |
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NP6.00078: An MHD-integrated model for impurity and core transport studies in RFP plasmas I. Predebon, R. Paccagnella, M.E. Puiatti, M. Valisa We present a 1D numerical tool for the study of core transport in the Reversed Field Pinch, built with a self-consistent MHD model. The code solves the safety factor evolution assuming an Ohm's law with an $\alpha$-dynamo term and a Spitzer's like resistivity; the MHD equations are coupled with continuity and heat balance equations; part of the code RITM\footnote{M.~Z. Tokar, {\em Plasma Phys. Control. Fusion\/} {\bf 36}, 1819 (1994)} has been integrated so as to compute the neutral and impurity related terms in the main gas equations. The code has been conceived in order to allow the inclusion of various transport models. As a first case, to simulate the chaotic multiple-helicity configuration, typical of low current/high density plasmas, a stochastic model has been implemented with the inclusion of collisionality. Some examples are shown, concerning the effect of strong density variations on the discharge sustainment. The role of the dynamo and the incompatibility of the resistivity with a pure Spitzer's law are discussed. [Preview Abstract] |
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NP6.00079: Nonlocal L\'{e}vy flight transport in the RFX G. Spizzo, R.B. White, S. Cappello, L. Marrelli Particle transport in a toroidal plasma confinement device has been shown to be non-diffusive when magnetic chaos is present\footnote{G. Spizzo, R. B. White and S. Cappello, Phys. Plasmas \textbf{14}, 102310 (2007)}. The effect is illustrated by numerical modelling of the magnetic structure and associated particle transport in conditions relevant for the reversed-field pinch experiment at the Consorzio RFX, Padova, Italy. A nonlocal Montroll equation, modified to take account of causality, inhomogeneity, finite boundaries and the very different behavior of trapped and passing particles is used to model this transport. The necessary input to the Montroll equation consists of a generalized distribution (kernel) of L\'{e}vy flight times and distances $p(r',r-r';t-t')$. A guiding center code is used to numerically construct this function, and the Montroll equation is used to examine the transport. The Montroll equation is found to reproduce the observed subdiffusive transport as well as the associated phenomenological pinch effect. [Preview Abstract] |
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NP6.00080: NSTX AND SPHERICAL TORUS |
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NP6.00081: NSTX Facility/Research Status and Plans Masayuki Ono The NSTX team completed the FY 2008 run with a record number of plasma discharges with newly implemented facility and diagnostic capabilities. The dual lithium evaporator (LITER) system was introduced to provide complete toroidal coverage of the lower divertor with a solid lithium film which prouced improved plasma confinement and ELM-free discharges. LITER together with EF/RWM feedback control system led to record pulse length / high performance discharges on NSTX. The 75 ch PCHERS and Fast Ion D-alpha diagnostic systems were also successfully commissioned. For the 2009 run, a liquid-lithium divertor target to achieve lower collisionality, the HHFW antenna upgrade to double its power handling capability, and a BES diagnostic to extend the localized turbulence measurements and an MSE-LIF system for Er and B measurements will be introduced. For longer term upgrades, a new center stack to enable $\sim $ 1 T, 2 MA, and 5 s operation is planned along with a second NBI for providing current profile control. These upgrades will enable NSTX to study fully non-inductive operations over much expanded plasma parameter space with lower collisionality and higher plasma temperature needed for the next-step STs. [Preview Abstract] |
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NP6.00082: Plasma performance improvements from optimized error field correction in NSTX J.E. Menard, S.P. Gerhardt, D.A. Gates, S.A. Sabbagh The active suppression of n=1 resonant field amplification (RFA) of intrinsic error fields was previously shown to lead to pulse length extension at high beta in NSTX. The correction of intrinsic n=3 error fields was also found to maintain/increase plasma rotation near the plasma boundary resulting in further pulse length extension for operation above the no-wall limit. More recently, the optimal n=3 error field correction (EFC) was determined as a function of plasma current indicating that n=3 intrinsic EF is most likely related to the PF or TF coil system rather than the OH coil as is the case for the n=1 intrinsic EF. Importantly, n=2 error fields were also investigated and measured to be small, indicating odd-n (n=1 and 3) EFs are most prominent in NSTX. Finally, the time response of the n=1 RFA suppression has been optimized by optimizing the low-pass filtering and proportional gain to more robustly control n=1 RFA and unstable n=1 RWMs. Overall, the combined n=3 EFC and n=1 RFA and RWM control has been instrumental in reliably increasing the duration of operation above the no-wall limit. This improved control was used in achieving record pulse- lengths on NSTX and is being applied to a wide range of operating scenarios in NSTX. [Preview Abstract] |
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NP6.00083: Nonlinear Simulation of Plasma Response to the NSTX Error Field J.A. Breslau, J.K. Park, A.H. Boozer, W. Park In order to better understand the effects of the time-varying error field\footnote{J.E. Menard, \textit{et al}., \textit{Nucl. Fus}. \textbf{47}, S645 (2007).} in NSTX on rotation braking, which impedes RWM stabilization, we model the plasma response to an applied low-$n$ external field perturbation using the resistive MHD model in the M3D code.\footnote{W. Park, \textit{et al}., \textit{Phys. Plasmas} \textbf{6}, 1796 (1999).} As an initial benchmark, we apply an $m$=2, $n$=1 perturbation to the flux at the boundary of a non-rotating model equilibrium and compare the resulting steady-state island sizes with those predicted by the ideal linear code IPEC.\footnote{J.K. Park, et al., \textit{Phys. Plasmas} \textbf{14}, 052110 (2007).} For sufficiently small perturbations, the codes agree; for larger perturbations, the nonlinear correction yields an upper limit on the island width beyond which stochasticity sets in. We also present results of scaling studies showing the effects of finite resistivity on island size in NSTX, and of time-dependent studies of the interaction between these islands and plasma rotation. The M3D-C1 code\footnote{S.C. Jardin, \textit{et al}., \textit{J. Comp. Phys}. \textbf{226}, 2146 (2007).} is also being evaluated as a tool for this analysis; first results will be shown. [Preview Abstract] |
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NP6.00084: NSTX Experiments with Lithium Plasma-Facing Components -- Recent Results and Future Plans H.W. Kugel, D. Mansfield, M.G. Bell, R. Bell, D. Gates, S. Gerhardt, R. Kaita, J. Kallman, S. Kaye, B. LeBlanc, R. Majeski, D. Mueller, S. Paul, A.L. Roquemore, P.W. Ross, H. Schneider, C.H. Skinner, J. Timberlake, S. Sabbagh, V. Soukhanovskii, W.R. Wampler, R. Maingi, J. Wilgen, J.P. Allain, R. Raman Recent NSTX high power, H-mode divertor experiments have shown significant and recurring benefits from active lithium coatings on PFC's. Two lithium evaporators were used to routinely evaporate lithium onto the NSTX lower divertor region between discharges. The major improvements in plasma performance from these lithium depositions include: 1) plasma density reduction as a result of lithium deposition; 2) suppression of ELMs; 3) improvement of energy confinement in a low-triangularity equilibrium; 4) improvement in plasma performance for standard, high-triangularity discharges; 5) reduction of the required HeGDC time between discharges; 6) increased pedestal electron and ion temperature; 7) reduced SOL plasma density; and 8) reduced edge neutral density. The next step in this work is installation of a liquid lithium divertor (LLD). Physics design and progress in implementation will be discussed. Work supported by USDOE Contract DE-AC02-76-CH03073. [Preview Abstract] |
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NP6.00085: Improved NSTX Plasma Performance with Lithium Aerosol Injection Dennis Mansfield, Lane Roquemore, Henry Kugel, Rajesh Maingi Elemental lithium in the form of dry aerosol particles has been injected continuously into the scrape-off layer of NSTX discharges in an attempt to affect H-Mode performance by real-time wall conditioning. The average particle diameter was 44 microns and typical injection fluxes were in the range of 1 -- 30 mg/sec. In all cases, the discharges were remarkably tolerant of this method of introducing lithium and plasma parameters were generally improved. However, especially significant reductions in OH flux consumption, ELMS amplitude and D$\alpha $ emission were accompanied by an increase in plasma confinement when lithium injection began prior to the L-H transition. [Preview Abstract] |
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NP6.00086: On some Challenges of Thomson Scattering Measurement in NSTX Benoit LeBlanc The large field of view of the Multi-Point Thomson Scattering (MPTS) diagnostic, on the horizontal midplane of the NTSX device, makes the installation of a viewing dump unpractical. Hence one challenge that sometimes occurs during measurement is the presence of high background radiation, which can precludes normal operation of the diagnostic, particularly since the inception of lithium evaporation. A palliative measure has been the installation of polarizer into the viewing optics, which successfully increased the detection dynamic range in presence of high background radiation. Nevertheless cases still occur when detection saturation is present. Some background radiation sources have been identified to be plasma-wall interaction and metallic impurity accumulation. Thomson scattering analysis with a reduced number of spectral channels is being considered as a means to deal with high background-light plasmas. Another challenge recently appeared in the form of significant window coating, observed in 2007 and 2008. An in-situ illumination apparatus has been installed to monitor the transmission during the 2008 run. Details and discussions will be presented along with the effects on the $T_{e}$ and $n_{e}$ measurements. This work is supported by United States DOE contract DE-AC02- 76CH03073. [Preview Abstract] |
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NP6.00087: Effects of lithium on chemical sputtering, physical sputtering, and deuterium pumping on graphitic substrates Chase Taylor, Jean-Paul Allain, Bryan Heim, Henry Kugel, Robert Kaita, Charles Skinner, Richard Majeski Lithium research has been conducted in various tokamak devices such as TFTR, CDX-U, FTU, T-11M and NSTX, as a means of enhancing plasma performance. Lithium has been found to reduce hydrogen recycling and facilitate H-mode power threshold. No systematic data is available on depumping properties of lithiated graphite structures and lithium film structures deposited on alternate substrates. Since most of the plasma- wetted surface during liquid lithium divertor (LLD) runs will consist of lithiated graphite, Purdue University is implementing a phased approach to study the effects of lithiated graphite surfaces on the LLD in NSTX. This approach includes design of a witness sample probe to examine erosion/redeposition mechanisms at the NSTX scrape-off-layer during experimental campaigns using the LLD. This work includes studies of D, He bombardment and D pumping on lithiated graphite systems using in-situ surface characterization techniques in the Omicron and PRIHSM facilities at Purdue University. [Preview Abstract] |
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NP6.00088: Momentum Transport in Electron-Dominated Spherical Torus Plasmas Stanley Kaye, Wayne Solomon, Ron Bell, Benoit LeBlanc, Fred Levinton, Jon Menard, Greg Rewoldt, Steve Sabbagh, Weixing Wang, Howard Yuh NSTX plasmas operate in an electron-dominated transport regime due to suppression of ion-scale modes by large ExB shearing rates. In this regime, the ion thermal diffusivity is neoclassical, and can be up to a factor of 30 greater than the momentum diffusivity. The momentum diffusivity, however, is much larger than the neoclassical value. Analysis of perturbative experiments that used applied n=3 magnetic fields to brake the plasma rotation indicate inward pinch velocities up to 40 m/s and perturbative momentum diffusivities larger by a factor of several than those values inferred from steady-state analysis with a zero pinch velocity assumed. The inferred pinch velocity values are consistent with values based on theories in which low-k turbulence drives the inward momentum pinch. Thus, in NSTX, the momentum transport can be a better probe of low-k turbulence, since unlike the ion energy transport, the neoclassical driven momentum transport is near zero. Understanding the source of the momentum transport, and how it scales to larger devices operating at lower collisionality, is critical to the performance of future devices. [Preview Abstract] |
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NP6.00089: NSTX transport study via density fluctuation measurement of FIReTIP system M. Johnson, K.C. Lee, C.W. Domier, N.C. Luhmann, Jr., H. Park, S.A. Sabbagh The density fluctuation level measured by the Far Infra Red Tangential Interferometry/Polarimetry (FIReTIP) system on National Spherical Torus Experiment (NSTX) is compared with the energy confinement for the study of turbulence transport. In the 2008 campaign, FIReTIP system was operational with 6 channels including a new channel (CH5) at the tangency radius of 132 cm. Since CH5 measures line integrated density over the turbulence dominated region from the pedestal to Scrape-Off-Layer, the density fluctuation level measured by CH5 provides correlated information with the turbulence induced transport of NSTX boundary. The density fluctuation levels are compared with the stored energy from EFIT equilibria for L-mode and H-mode cases. The possible relation to the turbulent diffusion based on the gyrocenter shift will be discussed. [Preview Abstract] |
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NP6.00090: Changes in edge turbulence characteristics across the L-H transition in NSTX S. Kubota, W.A. Peebles, N.A. Crocker, R. Maingi, C.E. Bush, R.J. Maqueda, S.J. Zweben, R.E. Bell, B.P. LeBlanc, T.S. Hahm, G.J. Kramer The fast evolution of turbulence and profiles are characterized across the L-H transition in NSTX. New millimeter-wave diagnostics are used to track the electron density profile and turbulence properties near the plasma edge. In NSTX, the rapid build-up of impurities in the edge following the L-H transition allows only a $\leq$10 ms window for reflectometry measurements across the transition barrier. The ultra-fast FMCW reflectometers (13-53 GHz) now have a $<$8.5 $\mu$s sweep capability, which allows the system to be swept across the entire outboard plasma, and therefore over the turbulence correlation length, faster than the turbulence decorrelation time. Substitution of the temporal correlation of the reflectometer signal for spatial correlation allows radial correlation length estimates in $<$0.5 ms. Fluctuation levels and mean/fluctuating flow velocities (zonal flows) are estimated with the poloidal correlation reflectometer. The connection between the local turbulence properties and the $E\times B$ shear will be discussed. [Preview Abstract] |
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NP6.00091: Spontaneous Intrinsic Rotation and Changes in Turbulence in Ohmic H-modes on NSTX C.E. Bush, R.E. Bell, S. Kubota, J.-W. Ahn, R.J. Maqueda, S.J. Zweben, B.P. LeBlanc, K.C. Lee, E. Mazzucato, J.B. Wilgen, E.D. Fredrickson, R. Raman, L. Delgado-Aparicio, D. Stutman, K. Tritz, S.M. Kaye Spontaneous spin-up in intrinsic toroidal and poloidal rotation of impurity ions (C and He) was observed at the edge of ohmic plasmas during L-H mode transitions in NSTX. The rotation was reduced again after the H-L back transition. The changes in rotation were accompanied by changes in plasma fluctuations and turbulence observed over the plasma minor radius from the far SOL to deep in the core using diagnostics such as fast reflectometry and Langmuir probes near the chamber walls, gas puff imaging (GPI), and microwave reflectometry and high-k scattering in the plasma core. Intrinsic rotation increases of 10s of km/s were observed using an edge rotation diagnostic and reflectometry showed the long wavelength correlation length in the plasma core to drop sharply at the L-H transition. GPI and a fast radial (midplane) scanning probe showed a strong decrease in plasma turbulence (blob activity and \~{n}/n) at the plasma edge. The observed intrinsic velocity scalings are consistent with a variety of toroidal devices. Discussions of these results relative to H-mode physics will be presented. [Preview Abstract] |
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NP6.00092: Ion Power Balance in NSTX P.W. Ross, R.E. Bell, D.A. Gates, S. Gerhardt, S.M. Kaye, B.P. LeBlanc, S.S. Medley, R.B. White, M. Podesta Experiments and simulations have been performed to investigate power balance of thermal ions in NSTX discharges. The neutral beam sources were modulated to affect the input power into the plasma. The modulations occurred on two time scales. For some discharges, the modulations lasted 30 ms which is approximately than the thermalization time of beam ions. In other discharges, the modulations lasted for 60 ms, or slightly longer than the energy confinement time. The faster time scale was used to determine the deposition profile of the fast ions, and the slower time scale was used to study the effects of the beam ions on the thermal ions. Grad-Shafranov reconstructions of the plasma were performed using magnetic, temperature, and MSE measurements as constrains. The input power to the ions was calculated using the TRANSP code, which performs a time dependent transport analysis. For the longer modulation time, the inferred confinement in some cases appears to be better than neoclassical, while the shorter modulation time fit better with theoretical predictions. NPA measurements show almost no fast ions for some discharges but a very strong signal for other discharges. Comparison of the fast particle drive MHD spectrum is made between discharges with the longer and shorter modulation times. This work was supported by DoE contract No. DE-AC02-76CH03073. [Preview Abstract] |
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NP6.00093: Simulations of ETG-driven turbulent transport in NSTX D.R. Mikkelsen, S.M. Kaye, J. Candy, R.E. Waltz The gyrokinetic code GYRO [1] is used to simulate turbulent transport driven by ETG modes in two sets of conditions based on NSTX experiments. Many simulations are located at r/a=0.6 in a deuterium L-mode plasma with Te=1.1Ti, Zeff=1.5, q=1.3, and shear=1. We find that the experimentally determined ExB shearing rate controls the level of transport by limiting the size of the turbulent eddies. The ExB shearing rate removes the need to include modes with $k_{\theta} \rho_i < 1$ in the simulation, thereby reducing the cost of using a gyrokinetic treatment for both electrons and ions and allowing simulations with $m_D/m_e$ up to 1600 that demonstrate convergence in this parameter. The ExB shear may also be responsible for the absence of runaway turbulence (previously reported by others for shear$>$0.4) in our simulations with adiabatic-ions. Preliminary work at r/a=0.34 in a helium L-mode plasma with Te=1.5Ti, Zeff=2.5, q=2.4, finds that the magnetic shear, $\hat{s}$=-0.3, has a strong stabilizing influence. \newline [1] J. Candy and R.E. Waltz, J. Comput. Phys. 186 (2003) 545 [Preview Abstract] |
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NP6.00094: Gyrokinetic Turbulence Simulation Challenges in the NSTX Spherical Torus J.L. Peterson, G.W. Hammett, D. Mikkelsen, S. Kaye, J. Candy, R.E. Waltz Recent progress in the numerical simulation of plasma turbulence has led to a greater understanding of the mechanisms behind anomalous heat and particle losses in tokamaks. However, the source of turbulent transport in machines with smaller aspect ratios, such as the National Spherical Torus Experiment (NSTX), remains elusive. Leading contenders for explaining transport in spherical tori include turbulence driven by the Electron Temperature Gradient (ETG) mode and microtearing modes. The coupling of ITG and ETG turbulence complicates simulations in regular tokamaks. However, the flow-driven suppression of long wavelength modes in NSTX may reduce the resolution requirements for ETG simulations. Reduced ion models, which still include the enhancement of zonal flows on the ion gyroradius scale, may also speed up ETG simulations. Here we present some linear studies of microtearing and ETG modes for NSTX parameters using the GYRO nonlinear gyrokinetic code\footnote{J. Candy, R. E. Waltz et al., J. Phys. Conf. Ser. {\bf 78}, 012008 (2007).} and some initial nonlinear studies of the resulting turbulence. [Preview Abstract] |
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NP6.00095: Ion loss during TAE avalanches in NSTX E.D. Fredrickson, D. Darrow, G. Kramer, N.N. Gorelenkov, S.S. Medley, B. LeBlanc, R.E. Bell, W.W. Heidbrink, D. Liu, M. Podesta, N.A. Crocker, S. Kubota, F.M. Levinton, H. Yuh Non-linear interactions of multiple Toroidal Alfv\'{e}n Eigenmodes (TAE) can result in explosive mode growth and enhanced losses of fast ions in a repetitive cycle of TAE bursts called avalanches. The mode structure and mode amplitudes are measured with arrays of reflectometers and Mirnov coils. The amplitude of individual modes, and rms amplitude of all TAEs are found to increase by an order of magnitude, coupled with strong downward frequency chirps in the final burst. Fast ion redistribution is seen for the energies $>$ 30 keV with a Neutral Particle Analyzer diagnostic. The plasma equilibrium is reconstructed using Thomson scattering profile and multi-channel Motional Stark Effect data. The NOVA-k code has been used to simulate the eigenmode structures, matched to the measured radial profiles and mode frequencies and are used to simulate the effect of the TAE on fast ion transport with ORBIT. [Preview Abstract] |
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NP6.00096: Alfv\'{e}n Cascade modes in NSTX:~fast-ion transport and MHD spectroscopy N.A. Crocker, E.D. Fredrickson, N.N. Gorelenkov, W.W. Heidbrink, G.J. Kramer, R.E. Bell, D. Darrow, J. Hosea, S. Kubota, B.P. LeBlanc, K.C. Lee, F.M. Levinton, J.E. Menard, M. Podesta, G. Taylor, H. Yuh Alfv\'{e}n Cascades (AC) occur in reverse shear plasmas and can transport fast-ions (e.g. fusion $\alpha $'s, beam ions or RF-accelerated ions), impacting plasma performance. This transport is studied with neutral particle analyzers, a scintillator fast lost ion probe and a fast-ion D alpha (FIDA) spectrometer. AC structure measurements via reflectometery and interferometery, in conjunction with structure calculations by the NOVA-K eigenmode code and orbit calculations by the ORBIT code, are employed to study how ACs transport fast-ions. ACs also enable MHD spectroscopy, the determination of $q_{min}$ from AC frequency. In spherical tori, this requires understanding the influence of acoustic coupling and pressure gradients on AC frequency, which is studied by changing species (He and D) and density (0.5 -- 1.5 x 10$^{19}$ m$^{-3})$, and modifying $T_{e}$ with high harmonic fast wave heating. Comparisons are made with NOVA-K. [Preview Abstract] |
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NP6.00097: Neutral Beam Ion Loss Accompanying Bursting MHD in NSTX Plasmas Douglass Darrow, Eric Fredrickson, Neal Crocker, Nikolai Gorelenkov, Lane Roquemore, Kouji Shinohara NSTX plasmas exhibit a variety of fast particle driven MHD instabilities, including bursts of Energetic Particle Modes (EPMs), Reverse Shear Alfv\'{e}n Eigenmodes (RSAEs), and Beta-induced Alfv\'{e}n Acoustic Eigenmodes (BAAEs). These are all driven by the 80-90 kV D beam ions. Loss of fast ions is observed during EPM bursts and during RSAEs. The EPMs rapidly develop into multiple toroidal mode numbers present concurrently, and loss of beam ions over a wide range of pitch angles is observed. This loss is interpreted as a stochastization of the particle phase space by the modes. During the RSAE upward frequency sweep, beam ion loss is also seen, but over only a limited pitch angle range, in contrast to the EPM bursts. Fast ion loss is also seen during TAE avalanches or concurrent multiple-n TAEs, but this loss also is seen over a limited range in pitch angle, again suggesting the loss mechanism differs from that in the EPM bursts. [Preview Abstract] |
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NP6.00098: Correlations of Neutral Particle Analyzer Diagnostics with Instability Bursts D. Liu, W.W. Heidbrink, M. Podest\`a, E.D. Fredrickson, S.S. Medley, R.B. White The extensive set of fast-ion diagnostics [neutron detectors, E$\vert \vert $B-type neutral particle analyzer (NPA), four-chord solid state neutral particle analyzer array (SSNPA) and Fast-ion D-alpha diagnostic (FIDA)] on National Spherical Torus Experiment (NSTX) provides a good test-bed for the study of fast ion confinement. A cross-correlation analysis has been performed on the NSTX 2007 and 2008 run campaign data in order to find the correlation between NPA/SSNPA signals and instability bursts. It is shown that sawteeth and large fishbones often cause bursts at outer chords of SSNPA and NPA and drops in the neutron rate, which indicate fast ion loss. It is also observed that Toroidicity-induced Alfven Eigenmode (TAE) avalanches always cause bursts at some chords of SSNPA. High energy channels respond earlier than low energy channels. Examples of experimental data and comparison with ORBIT simulation will be presented along with physical explanations. [Preview Abstract] |
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NP6.00099: ABSTRACT WITHDRAWN |
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NP6.00100: Overview of Neoclassical Tearing Mode Studies in NSTX Stefan Gerhardt, R.J. La Haye, E.J. Strait, R.J. Buttery, D. Brennan, E. Fredrickson, D. Gates, M. Maraschek, J.E. Menard, S.A. Sabbagh Both the m/n=2/1 and 3/2 neoclassical tearing modes have been observed in high-performance plasma regimes in NSTX. These instabilities are observed as fluctuations in the soft X-rays and Mirnov arrays, as sources of strong rotation damping, and as flat regions in the electron temperature profile. The neoclassical nature of the islands is confirmed by the observed linear dependence of the saturated island width on beta-poloidal, and restabilization of the islands as beta-poloidal is reduced. The 2/1 mode is often, but not always, observed to grow without any clear seeding event. The required drive for the 2/1 mode when it strikes, i.e. beta-poloidal or the bootstrap current at q=2, increases with increasing toroidal flow shear. A combination of lithium surface coatings and dynamic error field correction often eliminate the 2/1 mode for the duration of the discharge. The role of error fields in modifying the 2/1 onset threshold will also be described. Modeling efforts and future experimental research plans will be discussed. [Preview Abstract] |
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NP6.00101: Investigation of the Complex Relationship Between Plasma Rotation and Resistive Wall Mode Stabilization in NSTX J.W. Berkery, S.A. Sabbagh, H. Reimerdes, R. Betti, B. Hu, J. Manickam Recent resistive wall mode (RWM) experiments in NSTX show that the role of plasma rotation in mode stabilization is more complex than past tokamak research indicates. These experiments show that the RWM can be destabilized in high rotation plasmas while low rotation plasmas can be stable, which calls into question the concept of a threshold or ``critical'' plasma rotation for stability. Theoretical stabilization mechanisms are tested against experimental discharges with various plasma rotation profiles created by applying either n=2 or 3 non-resonant magnetic braking. Kinetic modification of the ideal stability criterion is calculated using experimental equilibrium reconstructions. Analysis of multiple NSTX discharges predicts near-marginal mode growth rates just before RWM instability is experimentally observed, with trapped ions providing the dominant kinetic resonances. Increasing or decreasing the rotation in the calculation drives the prediction farther from the marginal point, showing that unlike simpler ``critical'' rotation theories, kinetic theory allows for a more complex relationship between plasma rotation and RWM stability. Supported by U.S. DOE Contracts DE-FG02-99ER54524 and DE-AC02-76CH03073. [Preview Abstract] |
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NP6.00102: Multi-energy SXR characterization of actively stabilized resistive wall modes in NSTX Luis F. Delgado-Aparicio, Kevin Trittz, Dan Stutman, Michael Finkenthal, Jeffrey Levesque, Steve Sabbagh, Ronald Bell, Benoit LeBlanc, Stephen Paul A fast and compact multi-energy soft X-ray (ME-SXR) array is used for the determination of time and space-resolved SXR emissivity in different energy ranges during active stabilization of resistive wall modes (RWM) in NSTX. The insensitivity of ME-SXR to stray fields helps to discriminate between the RWM and parasitic perturbations. Fast electron temperature measurements are obtained from ratios of these emissivity profiles by modeling the slope of the continuum radiation. The amplitude of the core and edge electron temperature ($T_{e}$) modulations associated with actively-stabilized resistive RWMs is of the order of 50-100 eV ($\sim10\%$). Their time history is in good agreement with the slow evolution of the $n=1$ magnetic perturbation measured by the poloidal and radial RWM coils. Together with the magnetics, the ME-SXR data suggests that in NSTX the mode is not entirely `rigid' and that acting with the stabilizing coils on its external structure may transfer some of the perturbation to the interior of the plasma. [Preview Abstract] |
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NP6.00103: Results and Advances in VALEN Resistive Wall Mode Modeling James Bialek, Long Poe Ku, Allen Boozer, Steven Sabbagh The resistive wall mode (RWM) code VALEN now has two versions: a single-mode and a new multi-mode version. The single-mode version of VALEN was used to analyze the RWM performance of the ITER `VAC02' internal coil design. It was found that ITER scenario 4 may be stabilized up to beta normal of 3.8, well above the no wall beta normal limit of 2.5, with modest power and current requirements. The single-mode version of VALEN was also used to analyze upgrades to the RWM control system in the NSTX device. The new version of VALEN allows the inclusion of multiple plasma modes, both unstable and stable for multiple values of toroidal mode number. Both the multi-mode formulation and multi-mode calculations for HBT-EP and NSTX will be presented. [Preview Abstract] |
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NP6.00104: Progress on High Harmonic Fast Wave Heating and Current Drive on NSTX P.M. Ryan, L.A. Berry, E.F. Jaeger, J.B. Wilgen, R.E. Bell, J.C. Hosea, B.P. LeBlanc, C.K. Phillips, G. Taylor, E.J. Valeo, J.R. Wilson, H. Yuh The recent improvement of the 30 MHz HHFW heating efficiency at lower toroidal wavenumbers in helium plasmas [1] has been extended to deuterium operation at B$_{T}$(0) = 0.55 T on NSTX. The key to effective power penetration of the edge plasma is the reduction of the plasma density near the Faraday screen/first wall [2]. For deuterium plasmas, it was necessary to use lithium wall conditioning to control the density rise that often accompanies high power RF operation, particularly at the lower toroidal wavenumbers achievable with the 12-element phased-array launcher. The HHFW power deposition at k$_{\vert \vert }$ = -8 m$^{-1}$ is comparable to that of k$_{\vert \vert }$ = -14 m$^{-1}$, and core heating at k$_{\vert \vert }$ = -3 m$^{-1}$ has now been observed, albeit at lower efficiency. Central electron temperatures of 5 keV have been achieved in both deuterium and helium plasmas with 3.1 MW at k$_{\vert \vert }$ = -14 m$^{-1}$ (-150$^{o}$ relative phase shift). Central heating of NBI-driven H-mode plasmas has been observed for both k$_{\vert \vert }$ = 14 and 8 m$^{-1}$. [1] Hosea, J. et al, Physics of Plasmas 15, 056104 (2008) [2] Hosea, J. et al, poster at this conference [Preview Abstract] |
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NP6.00105: Edge Plasma Properties for HHFW Heating on NSTX J. Hosea, R.E. Bell, B.P. LeBlanc, C.K. Phillips, G. Taylor, J.R. Wilson, E.F. Jaeger, P.M. Ryan, J. Wilgen HHFW core plasma heating efficiency in helium discharges is found to improve markedly on NSTX when the density at the antenna is below that for the onset of perpendicular wave propagation (n$_{onset} \quad \propto $ B*k$_{\vert \vert }$/$\omega )$. Lithium wall conditioning has resulted in lower edge density in deuterium discharges giving substantial improvement in core heating for 90$^{o}$ antenna phase and the first significant core heating for 30$^{o}$. Since core wave damping is so high in NSTX, the observed RF power losses in the plasma edge are driven in the vicinity of the antenna. PDI surface losses account for $\sim $ 16{\%} - 25{\%} of the lost power assuming the power is lost via ion collisions with the electrons. Spectroscopic measurements suggest that energetic edge ions could also be lost on direct loss orbits, especially when propagating field amplitudes are peaked up near the antenna (lower k$_{\vert \vert })$. The implication of this loss process for explaining the resilience of edge rotation to NBI during HHFW heating and as an important loss mechanism to include in advanced RF codes will be discussed. [Preview Abstract] |
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NP6.00106: Numerical Modeling of HHFW Heating and Current Drive on NSTX C.K. Phillips, R.E. Bell, J.C. Hosea, B.P. LeBlanc, G. Taylor, E.J. Valeo, J.R. Wilson, L.A. Berry, E.F. Jaeger, P.M. Ryan, J.B. Wilgen, P.T. Bonoli, J.C. Wright, R.W. Harvey, H.Y. Yuh High harmonic fast wave (HHFW) heating and current drive, at frequencies up to 15 times the fundamental deuterium cyclotron frequency, are being studied on NSTX. Recent experiments indicate that the core heating efficiency depends strongly on the antenna phasing and plasma conditions [1], and improves significantly at higher toroidal magnetic fields. Wave propagation, absorption and current drive characteristics for L-mode and H-mode NSTX discharges have been analyzed using both ray tracing and full wave models. Simulations obtained with the AORSA and TORIC full codes agree reasonably well with Motional Stark Effect measurements of the driven current, and indicate the importance of trapping effects on the driven current profile. Collisional damping effects on the wave absorption, particularly in edge regions, will be considered. [1] J. C. Hosea, \textit{et al}, Phys. Plasmas \textbf{15}, 056104 (2008). [Preview Abstract] |
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NP6.00107: Comparison of a New Self-Adjoint Calculation of NSTX High Harmonic Current Drive with CQL3D R.W. Harvey, A.P. Smirnov, C.K. Phillips, G. Taylor, J.C. Hosea, D. McCune, B. LeBlanc A new general linear calculation of RF current drive has been implemented in the GENRAY all-frequencies RF ray tracing code, based on the Karney {\it et al.} [AIP Conf. Proc. 190 (1989)] relativistic Green function calculator, ADJ, using the self- adjoint collision operator property, generalized to non- circular plasmas in toroidal geometry, and coupled with full, bounce-averaged momentum-space RF quasilinear fluxes [T. Stix, Waves in Plasmas(1992)]. This calculation includes effects of momentum conservation, polarization and trapped electrons, and is directly comparable to the CQL3D Fokker-Plank code when collisional effects exceed QL diffusion. Under these conditions, the model will be accurate for low-collisionality, low-phase velocity CD, e.g., HHFW. We show good comparisons between linear damping and current drive calculations in GENRAY and CQL3D for a range of NSTX plasma conditions. We also show circumstances where agreement between the described ADJ-QL calculation differs by up to a factor of 2 from previous Ehst-Karney parameterized results. [Preview Abstract] |
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NP6.00108: Profiles of Fast Ions Accelerated by High-Harmonic Fast-Wave Heating in NSTX E. Ruskov, D. Liu, W. Heidbrink, M. Podesta, R. Bell, E. Fredrickson, J. Hosea, S. Medley, R. Harvey Combined neutral beam injection and 30-MHz high-harmonic fast-wave (HHFW) heating accelerate deuterium fast ions in the National Spherical Torus Experiment (NSTX). With 1.1 MW of HHFW power, the neutron rate is about two times larger than in comparison discharges without this source of plasma heating. A fast-ion tail above the injection energy is observed on a conventional E$\vert \vert $B neutral particle analyzer (NPA), on a 4-chord solid state neutral particle analyzer (SSNPA) array, and on a 16-channel fast-ion D-alpha (FIDA) diagnostic. At some angles, neutral particle analyzers can detect neutrals at twice the injection beam energy. The spatial profile of the accelerated fast ions measured by the D-alpha diagnostic is much broader than in conventional tokamaks, presumably due to the multiple resonance layers and large orbits in a spherical tokamak. In addition, compressional Alfven eigenmode activity is much stronger during HHFW, probably due to the bump-on-tail fast-ion velocity distribution which enhances the instability drive. The fast-ion distribution function calculated by the CQL3D Fokker-Planck code is compared with experimental data. [Preview Abstract] |
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NP6.00109: Investigation of EBW Coupling and Propagation in H-Mode Plasmas in NSTX S.J. Diem, G. Taylor, P.C. Efthimion, H. Kugel, B.P. LeBlanc, C.K. Phillips, J.B. Caughman, J.B. Wilgen, J. Preinhaelter, J. Urban, S.A. Sabbagh EBW emission (EBE) diagnostics and EBE modeling have been employed on NSTX to study oblique EBW to O-mode (B-X-O) coupling and propagation. Initial EBE measurements in H-mode plasmas exhibited strong emission before the L-H transition, but the emission rapidly decayed after the transition. EBE simulations show that EBW collisional damping prior to mode conversion (MC) can significantly reduce the measured EBE when T$_{e}$ $<$ 30 eV, explaining the observations. Lithium evaporation was used to significantly reduce EBW collisional damping near the MC layer. The lithium evaporation rate was increased from 0 to 19 mg/min to reduce n$_{e}$ and increase T$_{e}$ outside the LCFS in an H-mode plasma. With edge conditioning, an increase in T$_{e}$ near the fundamental MC layer from 10 eV (no Li) to $>$ 20 eV (with Li) was observed. As a result the measured B-X-O transmission efficiency increased from $<$ 10\% (no Li) to 60\% (with Li), consistent with EBE simulations. [Preview Abstract] |
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NP6.00110: Ramp-up of CHI initiated plasmas on NSTX D. Mueller, R. Raman, M.G. Bell, T.R. Jarboe, A.L. Roquemore, B.A. Nelson The small bore of the spherical torus (ST) concept presents a serious challenge for plasma-current initiation and ramp-up for next generation of STs. A method of non-inductive startup, referred to as transient coaxial helicity injection (Transient CHI), was successfully developed on the Helicity Injected Torus (HIT-II) experiment and employed on the National Spherical Torus Experiment (NSTX) to produce up to 160 kA of toroidal plasma current on closed flux surfaces without use of the central solenoid. The CHI formed plasmas have been successfully coupled to inductively driven current ramp-up and neutral beam heating. These plasmas with plasma current over 700 kA enter the H-Mode and have performance comparable to plasmas initiated by inductance alone and demonstrates the feasibility of coupling CHI initiated plasmas to other means of current ramp-up and sustainment. [Preview Abstract] |
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NP6.00111: GENERAL SPHERICAL TORUS |
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NP6.00112: Edge and Scrape-off Layer Diagnostics for the NSTX Liquid Lithium Divertor J. Kallman, R. Kaita, H. Kugel, R. Ellis, S. Gerhardt, M.A. Jaworski, A.L. Roquemore, S. Zweben A Liquid Lithium Divertor (LLD) is being installed in NSTX to provide particle pumping for density control in the divertor region. The design includes four toroidal breaks where we plan to place several diagnostics to monitor the LLD's effects on the edge plasma. A large radial Langmuir probe array will be constructed at one of these locations to straddle the LLD and the carbon tiles beyond its innermost edge. This array will consist of approximately 40 sets of triple probes whose close spacing ($\sim$ 0.5 mm) will enable instantaneous electron temperature and density measurements with sufficient radial resolution to cover a full strike point heat flux profile width. Sensors in the tiles at the other LLD toroidal breaks will also be described. [Preview Abstract] |
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NP6.00113: Dependence of SOL widths on plasma parameters in NSTX Joon-Wook Ahn, Rajesh Maingi, Jose Boedo, Vlad Soukhanovskii, Ben LeBlanc, Robert Kaita The dependence of various upstream Scrape-Off Layer (SOL) widths on the line-averaged density ($\overline n _e )$, plasma current ($I_{p})$, and power into the SOL ($P_{SOL})$ for H-mode plasmas was investigated, using the mid-plane fast reciprocating probe and Thomson scattering diagnostics, in the National Spherical Torus Experiment (NSTX). The heat flux width (\textit{$\lambda $}$_{q})$ at the divertor plate, measured by the IR camera, was also measured and compared with the upstream SOL widths. The edge density profile remains fixed during the H-mode, such that the separatrix density is constant even though $\overline n _e $ is ramping. Thus \textit{$\lambda $}$_{q}$, \textit{$\lambda $}$_{Te,}$ and \textit{$\lambda $}$_{ne}$ are insensitive to $\overline n _e $. \textit{$\lambda $}$_{Te}$ and \textit{$\lambda $}$_{jsat}$ have strong negative dependence on $I_{p}$, whereas there was only a very weak change in \textit{$\lambda $}$_{ne}$ when $I_{p}$ was varied. These empirical results have been compared with scaling laws in the literature. The \textit{$\lambda $}$_{Te}$ dependence on $I_{p}$ is consistent with an H-mode \textit{$\lambda $}$_{Te}$ scaling law, while the insensitivity of \textit{$\lambda $}$_{ne}$ to $\overline n _e $ is not consistent with the \textit{$\lambda $}$_{ne}$ scaling law. Dependence of decay lengths on plasma parameters in a wide range of plasma conditions will be presented. This work was supported by the US Department of Energy, contract numbers DE-FG02-03ER54731, DE-AC02-76CH03073, DE-AC05-00OR22725, and DE-AC52-07NA27344. [Preview Abstract] |
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NP6.00114: Separatrix location in NSTX Frederick Kelly, Rajesh Maingi, Ricky Maqueda, Jon Menard, Ben LeBlanc, Ron Bell, Stephen Paul The separatrix location and corresponding plasma parameters in NSTX were estimated for H-mode discharge 117125 containing both MARFEs and ELMs and for Type V ELMy H-mode discharge 128337. Since equilibrium reconstructions with LRDFIT did not accurately locate the LFS separatrix, a method based on the strong electron parallel heat conductivity was used to map the LFS magnetic flux surfaces to the HFS since the innermost Thomson scattering measurement of Te(R) is the most accurate. During a MARFE or at MARFE onset in NSTX shot 117125, this method estimated the electron temperature at the LFS separatrix, T$_{e,sep}$, to vary between 31 and 41 eV. At times with no MARFE or ELM, T$_{e,sep}$ ranged between 41 and 93 eV. These T$_{e,sep}$ values compare well with T$_{e,sep}$ values (28-35 eV) in TEXTOR just before MARFE onset.$^{1}$ In NSTX shot 128337 late in the Type V ELMy phase, T$_{e,sep}$ was estimated to be $\sim $100 eV. These separatrix locations place the E$_{r}$ well outside the separatrix. [1] F.A. Kelly, W.M. Stacey, J. Rapp and M. Brix, Phys. Plasmas 8 (2001) 3382. [Preview Abstract] |
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NP6.00115: Reduced-model (SOLT) validation from comparisons with GPI data on NSTX D.A. Russell, D.A. D'Ippolito, J.R. Myra, R. Maqueda, T.L. Munsat, D.P. Stotler, S.J. Zweben We model the evolution of vorticity, density, temperature and zonal fluid momentum in the outboard midplane of a tokamak with our SOLT code. Our edge region supports the electron drift wave instability, while sheath losses are isolated in the SOL. Curvature- and grad-B-driven charge separation are included everywhere, enabling blob transport of fluctuations from the edge into the SOL. Several features of boundary turbulence seen in gas-puff imaging (GPI) diagnostics on NSTX are compared with the corresponding \textit{synthetic} GPI diagnostics of the SOLT simulations, including distributions of intensity fluctuations (mean and median intensity vs. distance from the separatrix) and of blob size. The accuracy of inferring radial transport velocities directly from the GPI data is determined by comparing blob radial velocities computed from the motion of GPI images to the underlying ExB convecting velocities from the simulations. [Preview Abstract] |
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NP6.00116: Velocity Field Analysis of Edge Turbulence Images from NSTX B.T. Brown, T. Munsat, R. Maqueda, S.J. Zweben The HOP-V (Hybrid OPtical-flow Velocimetry) code has been developed for extracting time-resolved 2-D velocity maps from turbulence imaging diagnostics, including the NSTX Gas Puff Imaging (GPI) instrument. The HOP-V code combines optical-flow and local pattern-matching techniques to derive ``dense'' velocity fields at the full temporal resolution and a fraction of the spatial resolution of the underlying image frames, which in this case is 64x64 pixels, 300 timepoints, taken at 4 $\mu$s per frame at a spatial resolution of $\sim$4 mm. The code has been validated for a variety of artificial test patterns of convective flow, including highly sheared cases. Recent work includes statistical analysis of a large number of NSTX shots in both L-mode and H-mode, with an investigation into a wide variety of flow properties, including flow shear, the relationship between flow evolution and H-mode behavior, the connection between flow, filament birth and evolution, and NSTX operating parameters/regimes, and the identification of zonal flows. Recent results and outstanding questions are presented. [Preview Abstract] |
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NP6.00117: Maximum Controllable Displacement: Experimental Results from NSTX, and Comparison with Theoretical Results and Other Tokamaks Egemen Kolemen, David Gates Optimization of the ITER PF design requires information on the limits to axisymmetric stability control performance, as well as the safe margins in operation experienced in present-day devices with configurations relevant to ITER. Maximum controllable displacement, $\Delta Z_{max}$, the largest displacement for which the plasma is still controllable, was recently proposed as a performance metric for the power supplies of the vertical stabilization loop on ITER. $\Delta Z_{max}$ is obtained. Experiments, which turn off the vertical position control and then turn it back on after the plasma has moved a given distance, are performed in order to measure $\Delta Z_{max}$. The experimental measurements of $\Delta Z_{max}$ for the NSTX under various conditions relevant to ITER are presented. Employing a linear magnetohydrodynamics model of the plasma, the theoretical $\Delta Z_{max}$ is obtained. The results are compared to similar measurements from other Tokamaks (DIII-D, C-Mod) to establish model accuracies and validate design predictions for ITER. This work was supported by DoE contract No. DE-AC02-76CH03073. [Preview Abstract] |
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NP6.00118: Model-based Robust Shape Control Design for the National Spherical Torus Experiment (NSTX) Majed Alsarheed, Eugenio Schuster, Jim Leuer, Michael Walker, David Humphreys, David Gates Plasma shape and position control is a challenging problem due to the difficulties associated with real-time shape identification, plasma parameters measurement, and control method selection. The recent implementation of the real-time equilibrium reconstruction code rtEFIT on NSTX allows plasma shaping by controlling the magnetic flux at the plasma boundary. A non-model-based shape controller that exploits this capability has been recently proposed [1]. We describe current efforts to develop a model-based multi-input-multi-output (MIMO) H$_{\infty }$ controller to provide real-time shaping and position control in the presence of disturbances and uncertainties in the plasma parameters. The control design is based on linear plasma response models derived from fundamental physics assumptions. Computer simulation results illustrate the performance of the model-based shape control method. [1] D.A. Gates, et al., Nucl. Fusion \textbf{46 }(2006) 17--23. [Preview Abstract] |
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NP6.00119: Measurement of poloidal velocity on NSTX R.E. Bell, R. Feder A spectroscopic diagnostic to measure impurity ion poloidal velocity profiles using charge exchange recombination spectroscopy has been installed on NSTX. Up-down symmetric viewing chords in the plane of the neutral beam are matched with similar symmetric views off the neutral beam to measure both active and background emission. Eight fast (f/1.8-f/2) camera lenses at four locations make up the collection optics. Six f/1.8 spectrometers measure 276 spectra of C VI ions every 10 ms to obtain 75 active spatial channels and 63 background channels to achieve a spatial resolution of 0.7-1.7 cm after inversion. Inversions are necessary to obtain local values from the chord-averaged measurements. The background emission is fitted and inverted for subtraction from active views. Non-vertical symmetric views and independent toroidal velocity measurements allow the separation of the vertical and horizontal components of velocity induced by the energy-dependent charge-exchange cross section and ion gyromotion, which should eliminate the need for knowledge of atomic physics cross sections, halo neutral densities, level mixing, and excited beam populations when extracting the poloidal velocity. [Preview Abstract] |
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NP6.00120: Investigation of microtearing instability in NSTX with X-ray diagnostics K.L. Wong, K. Tritz, D.R. Smith, K. Hill, S. Kaye, A.L. Roquemore Microtearing instabilities can quantitatively explain the experimentally determined electron thermal conductivity in some NSTX discharges,\footnote{K.L. Wong et al., Phys. Plasmas \textbf{15}, 056108 (2008)} and this motivates the search for experimental evidence of these modes. The 46-chord soft X-ray camera\footnote{D. Stutman et al., Rev. Sci. Instrum. \textbf{74}, 1982 (2003)} can, in principle, see the modes during their initial growth phase before neighboring island chains overlap. The singular value decomposition\footnote{T. Dudok de Wit et al., Phys. Plasmas \textbf{1}, 3288 (1994)} technique is employed to get information on their mode structures and evolution dynamics. When stochastic magnetic field caused by overlapping islands becomes the dominant electron loss mechanism, the loss rate is expected to increase with the electron parallel velocity, and this should distort the electron velocity distribution function to deviate from local Maxwellian distribution. Such evidence may appear in the X-ray energy spectrum. An X-ray spectrometer is built specifically for this purpose. Preliminary result from this investigation will be presented. [Preview Abstract] |
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NP6.00121: NSTX high-speed color camera as a low-resolution survey spectrometer A.L. Roquemore, R.E. Bell, R. Kaita, D. Mueller, W. Davis, R.J. Maqueda, C. Bush NSTX has recently installed a high-speed 10-bit color camera having a wide-angle global view of the plasma. The camera is typically operated from 1.5 - 5 kHz depending on the desired spatial resolution. The high-speed aspect of the camera yields information on the overall plasma behavior, while the colors gives an indication of the dominant elements involved. For instance, neutral deuterium and helium are readily identified as a red and yellow glow respectively, and especially during the plasma fueling, their spectra can be easily discerned. With the introduction of lithium into NSTX, the camera readily reveals the orange glow from Li I and the bright green associated with the Li II state, depending on the local temperature. Narrow green filaments are often observed to spiral around the center stack or propagating along the last closed flux surface. Bright flares of Li are observed when the plasma interacts with material surfaces or even dust particles. Several methods of displaying the 10-bit color can be used to emphasize details of the discharges. Many example movies will be available to demonstrate the camera's capabilities. [Preview Abstract] |
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NP6.00122: The Motional Stark Effect (MSE) Diagnostic on the National Spherical Torus Experiment (NSTX) Fred Levinton, Howard Yuh A motional Stark effect(MSE) diagnostic for measurement of the magnetic field pitch angle has been installed on the National Spherical Torus Experiment (NSTX). The diagnostic system has a tangential view of a neutral beam, used for heating the plasma, providing good spatial resolution. The viewing optics covers from inboard of the magnetic axis to the outboard plasma edge. Presently 16 sightlines are instrumented and operating, with additional channels planned for the future. Due to the low magnetic field ($\sim$0.35 Tesla), several changes from typical MSE diagnostics were incorporated into the system. The optical system is configured using an aperture to reduce the geometric Doppler broadening from the heating beam and increase the polarization fraction. Another innovation was development of a birefringent Lyot filter with high throughput and high resolution. The filter has a bandwidth of 0.058 nm and transmission of $\sim$30\%. The polarization fraction is measured to be $\sim$30-40\% which combined with the large etendue results in a time resolution of $\sim$5 ms. [Preview Abstract] |
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NP6.00123: Development of a 2-D Ion Velocity Diagnostic on NSTX Using the SWIFT Camera S. Paul, R. Bell, A. Lane Roquemore, N. Nishino Using a beam-splitter and interference filter diagnostic, the development of a 2-D ion flow measurement of toroidal ion flows is being developed on NSTX. A high frame-rate, 12-bit Photron-USA Ltd. monochrome digital camera is used to view He II line emission at 468.6 nm on the inboard side of the plasma in NSTX. The image is split and imaged onto two distinct 64 x 64 pixel sections of the camera's 1024 x 1024 pixel detector. A mirror and the beam-splitter are adjusted so that each of the two sections view the same 30 cm x 30 cm region in the plasma. Each section is made to view through separate interference filters located in the near field. The filters are custom designed and have opposite but linear sloping passbands. In this way the Doppler shift causes the ratio of the light incident on each section of the detector to vary with toroidal velocity. A calibration of the relative gain and linearity of the detector and a wavelength calibration the shape of the passband of the filter, the ion velocity is calculated from the ratio of the intensities from the two images. The light level in helium discharges in NSTX is found to be adequate to make measurements from 250 to 2000 fps. Because the camera was found to be sensitive to magnetic fields, the discharges were taken at a 3 kG field strength, quite low for NSTX. Nevertheless, these helium discharges lasted up to .5 sec and 4 MW of neutral beam heating was injected. Adequate light was obtained for time resolution as short as 0.5 ms and about 500 images were recorded per discharge. [Preview Abstract] |
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NP6.00124: LoWEUS, the Long Wavelength Extreme Ultraviolet Spectrometer, on the NSTX tokamak Jaan Lepson, Peter Beiersdorfer, Manfred Bitter, Layne Roquemore, Gretchen Zimmer LoWEUS is a high-resolution ($\lambda / \delta\lambda = \sim 200 $) grazing-incidence grating spectrometer from the LLNL electron beam ion trap program that was installed on the NSTX tokamak in the spring of 2008 to monitor emission lines in the long wavelength extreme ultraviolet band. As currently set up, LoWEUS covers the 55-225 {\AA} region, which includes important emission lines of oxygen and iron, among others. Most importantly, it observes the emission from hydrogenlike and heliumlike lithium (Li$^{2+}$ and Li$^+$, respectively). LoWEUS is thus complementary to the XEUS (also from LLNL) and SPRED spectrometers. The initial operation of LoWEUS, which was cut short by a valve issue, enabled us to perform a survey of emission lines of intrinsic impurities in NSTX. [Preview Abstract] |
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NP6.00125: High Bandwidth FIReTIP Operation on NSTX W-C. Tsai, C.W. Domier, K.C. Lee, N.C. Luhmann, Jr., H.K. Park The Multichannel Far Infrared Tangential Interferometer/Polarimeter (FIReTIP) system has great potential in monitoring high frequency density fluctuations and magnetic field profiles on the National Spherical Torus Experiment (NSTX). The measurements are essential in understanding transport physics issues in NSTX as well as for future devices such as ITER in which fundamental understanding of microturbulence MHD issues is essential. The relatively narrow video bandwidths of the current FIReTIP system (250 kHz) have limited the speed and accuracy in which the above variations may be monitored. New electronics are under development to (1) extend the video bandwidth to $>$4 MHz; and (2) develop a parallel set of electronics to monitor 30 MHz density fluctuations induced by high harmonic fast wave heating. Experimental details and test results of the new electronics, scheduled for installation on NSTX in Fall 2008, will be presented. [Preview Abstract] |
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NP6.00126: Preliminary report of the U.S. Spherical Torus Coordinating Committee on proposed ST Strategic Plan Y.K. Martin Peng, William Dorland, Donald Hillis, Rob LaHaye, Fred Levinton, Richard Majeski, Jon Menard, Steve Sabbagh, Aaron Sontag The U.S. Spherical Torus Coordinating Committee (STCC) was recently formed by DOE. The STCC is chartered to develop, support and promote the evolving roles of ST in the U.S. fusion program; coordinate milestones, plans, and longer term goals; review and report progress relative to funded R{\&}D; and represent and advocate ST Program nationally, and internationally through the IEA ST Executive Committee. The STCC is currently developing a draft ST Strategic Plan in discussion with the fusion community. The overarching components of the strategic plan, including mission; 15-year goal; strengths weaknesses opportunities; and strategic objectives are being developed as input to the present FESAC Magnetic Alternates Panel. The remaining components of the plan will include strategies, short-term goals/priorities/initiatives; and action items/plans. A preliminary report of the ST Strategic Plan will be presented for discussion and feedback at the meeting. [Preview Abstract] |
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NP6.00127: Initial operation and performance projections for the Lithium Tokamak eXperiment (LTX) R. Majeski, L. Berzak, T. Gray, R. Kaita, T. Kozub, D.P. Lundberg, J. Manickam, K. Snieckus, D. Stotler, T. Strickler, J. Timberlake, J. Yoo, L. Zakharov, C.L. Ellison, G.V. Pereverzev, V. Soukhanovskii LTX is a modest-sized spherical tokamak with R=0.4 m, a=0.26 m, $\kappa$=1.5, $B_{toroidal} <$ 3.4 kG, $I_{p} <$ 400 kA, and $\tau_{flattop} >$ 50 msec. First plasma is expected in September 2008. The research objective is to investigate modifications to equilibria and transport when global recycling is reduced to very low values ($<$50\%), by means of a conformal wall coated with a liquid lithium film. Initial operation will utilize a reduced Ohmic power supply, and lithium will not be introduced into the device until trials of all systems are successfully completed. The device itself, early discharges, and performance projections using the ASTRA simulation code will be discussed. MHD stability with a very close, conducting, conformal wall is also briefly discussed (see poster by J. Manickam). Diagnostics are discussed in companion posters by L. Berzak, D. Lundberg, and T. Strickler. [Preview Abstract] |
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NP6.00128: Construction of the Lithium Tokamak Experiment (LTX) Thomas Kozub, Richard Majeski, Robert Kaita, Laura Berzak, Daniel Lundberg, Trevor Strickler, Robert Woolley, Leonid Zakharov The Lithium Tokamak eXperiment (LTX)* will investigate the low recycling operating regime for magnetically confined plasmas using liquid lithium plasma facing surfaces. The engineering design and machine fabrication process will be presented. The most significant new feature of the LTX machine is the installation of a heated copper toroidal shell that will be operated at 300\r{ }C to 500\r{ }C. Its stainless steel plasma-facing liner will be internally coated with an evaporated layer of liquid lithium. The shell is comprised of four quadrants that have been fabricated in-house from explosively bonded stainless steel on copper to conform closely to the outer plasma flux surface. All internal components of the LTX machine have been designed and built to meet the simultaneous requirements for liquid lithium compatibility, high temperature operation, and electrical isolation. These requirements have led to unique design features, such as the method of supporting the shell quadrants, and construction of the new internal poloidal field coils. *Supported by US DOE contract {\#}DE-AC02-76CH-03073 [Preview Abstract] |
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NP6.00129: Thomson Scattering Measurements of Electron Temperature and Density on the Lithium Tokamak Experiment (LTX) T. Strickler, R. Majeski, R. Kaita, B. LeBlanc The Lithium Tokamak eXperiment (LTX) is a spherical tokamak in which the plasma is almost entirely surrounded by a lithium-coated plasma-facing shell (5 m$^{2}$ area) conformal to the last closed magnetic flux surface. Based on previous experimental results and simulations, it is expected that the low-recycling liquid lithium surfaces will result in higher temperatures at the plasma edge, flatter overall temperature profiles, centrally-peaked density profiles, and an increased confinement time. To test these predictions, the electron temperature and density profiles in LTX are measured by a multi-point Thomson scattering system (TVTS). The TVTS system employs a ruby laser (15 J, 30 ns pulse width) with a spectrometer/ICCD detector. Here, the performance of the TVTS system is discussed. Initially, Thomson scattering measurements will be performed at several points between minor radius r=0 to r=24 cm (outboard). Later, an edge view Thomson scattering system will be implemented to look at the region a few centimeters within the last closed flux surface. [Preview Abstract] |
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NP6.00130: Initial Magnetics Data from the Lithium Tokamak eXperiment L. Berzak, R. Kaita, T. Kozub, D.P. Lundberg, R. Majeski, T. Strickler, K. Snieckus, L. Zakharov, C.L. Ellison The Lithium Tokamak eXperiment (LTX) is a modest-sized spherical tokamak designed to investigate the novel, low-recycling lithium wall operating regime for magnetically confined plasmas. An extensive array of magnetic diagnostics is available to characterize the experiment, including 84 Mirnov coils (single and double axis, internal and external to the shell), 33 flux loops, 2 Rogowskii coils, and a diamagnetic loop. Diagnostics are specifically located to account for the presence of a secondary conducting surface and engineered to withstand both high temperatures and incidental contact with liquid lithium. A new data acquisition system utilizing MDSplus has been built and implemented, allowing shot by shot organized storage of data and calibrations. This new data acquisition system and broad set of magnetic diagnostics provide the first magnetic information on LTX plasmas. [Preview Abstract] |
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NP6.00131: Electron Density Diagnostics on the Lithium Tokamak eXperiment (LTX) D.P. Lundberg, L. Berzak, R. Kaita, T. Kozub, R. Majeski, K. Snieckus, T. Strickler, C.L. Ellison LTX presents a unique opportunity to diagnose plasmas with low levels of particle recycling. Electron density measurements are necessary to evaluate the effects on global particle transport, and supplement the magnetic sensor data for equilibrium reconstruction. A pair of 2mm interferometers are employed on LTX. The first system samples a horizontal chord, aligned in the radial direction, and reflected off of the centerstack. It is mounted on a translation table whose vertical position is moved between shots via a stepper motor. Chords from near midplane to within 90{\%} of the plasma radius can be sampled. The second interferometer measures a single vertical chord, with a line of sight through the core of the plasma. This fixed core density measurement allows shot-to-shot variations to be accounted for when the horizontal viewing optics are moved. Radial density profiles will be reconstructed with Abel inversion. The first density measurements are presented, and future plans for LTX density diagnostics are discussed. [Preview Abstract] |
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NP6.00132: Three Potential LiWall regimes in the Lithium Tokamak eXperiment (LTX) Jongsoo Yoo, Leonid Zakharov The incoming Lithium Tokamak eXperiment (LTX) is designed for a low recycling operation with the plasma inside a close fitting shell coated with a pumping layer of lithium. This creates a special situation for confinement when the thermo-conduction is suppressed due to elevated edge temperature and the energy confinement is determined by the particle diffusion. The so- called Reference Transport Model (RTM) in which the diffusion coefficient as well as the ion/electron thermo-conduction coefficients are equal to the ion neoclassical thermo- conductivity, seems to be relevant for the low recycling regime. The RTM has been implemented in the ASTRA-ESC code system of transport analysis for simulation of the LTX stationary plasma regimes. Three zero recycling reference regimes have been identified earlier for LTX. In the Ohmic regime with the gas puff the central electron temperature is expected to be at the level of 1 keV and $T_i\simeq0.15$ keV. The combination of Ohmic-NBI (10 keV neutral beam injection) heating can elevate the ion temperature to $\simeq0.6$ keV. With an enhanced NBI current, the third, hot-ion regime can be achieved with plasma fueling by NBI and $T_i > T_e$. The focus of the present studies is on the effects of residual finite recycling. [Preview Abstract] |
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NP6.00133: MHD stability limits in LTX Janardhan Manickam, Richard Majeski, Robert Kaita, Jongsoo Yoo, Leonid Zakharov The Lithium Tokamak eXperiment, LTX, is expected to open access to new regimes in plasma-profile space. Of particular interest is the regime characterized by nearly flat $T_e$, a broad $T_i$, and a high edge current density. Another unique feature of LTX is the close fitting liquid Lithium wall, intended primarily for controlling the recycling; it would also affect the stability. This report addresses the ideal MHD stability of LTX. Target profiles are obtained from the ASTRA transport simulation code and the PEST stability code is used for stability analysis. Preliminary indications are that the n=1 mode, where n is the toroidal mode number, is stabilized by the close fitting shell, and a second stability regime for kink modes is accessed, as $\beta$ approaches $\beta_N \sim 10$. Results for n=1 and higher-n, will be presented. [Preview Abstract] |
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NP6.00134: Overview of the Pegasus toroidal experiment A.C. Sontag, D.J. Battaglia, M.W. Bongard, J.A. Cole, C.C. Hegna, E.T. Hinson, B.A. Kujak-Ford, B.T. Lewicki, A.J. Redd, A.P. Robinson, A.R. Wiersma, G.R. Winz Extremely low aspect ratio (A$<$1.3) allows Pegasus to achieve $I_N>$12 and $\beta_t>$20\% Ohmically. $I_p$ and $q$-profile manipulation using programmable magnet coil currents suppress internal tearing modes. Washer-stack point-current sources (plasma guns) are used to initiate non-solenoidal discharges with toroidal plasma current in excess of 60 kA via DC helicity injection. Present research is aimed at understanding the physics of this startup technique to form discharges with Ip~200 kA without central induction. Low TF and high edge current allows potential study of peeling stability. Estimates using DCON indicate that the Pegasus plasma edge is peeling unstable, consistent with the observation of spatially coherent edge filaments and accompanying magnetic fluctuations. Planned upgrades to the center-column will increase the available Ohmic flux by a factor of 5-10 and the toroidal field by a factor of 5. These upgrades will support tests of the scalability of the point-source helicity injection and enable feasibility tests of advanced divertor configurations. [Preview Abstract] |
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NP6.00135: Non-solenoidal startup in Pegasus discharges A.J. Redd, D.J. Battaglia, M.W. Bongard, E.T. Hinson, B.A. Kujak-Ford, B.T. Lewicki, A.C. Sontag, G.R. Winz Recent Pegasus experimental studies are directed at developing non-solenoidal startup techniques for ST and tokamak applications. High-field-side magnetic helicity injection with washer-stack current-sources (plasma guns) produces discharges with toroidal current $I_p$ up to 50 kA, using static coil currents and only 3 kA of injected current $I_{inj}$. Discharges driven by low-field-side injection typically require outer-PF ramps for radial force balance, also providing inductive current drive, and have achieved $I_p$=80 kA using $I_{inj} \le$ 2 kA. In either injection geometry, $I_p$ persists for a significant interval after gun shutoff, while the plasmas relax into typical tokamak equilibria with well-defined edges. According to a semi-empirical model, the maximum gun-driven $I_p$ is determined by the helicity injection rate, radial force balance, kink stability, and the Taylor relaxation criterion. Higher helicity injection rates will extend the Pegasus operating space, allowing higher $I_p$ and normalized current $I_N$, and enabling both flux amplification studies and predictive testing of the $I_p$ model. [Preview Abstract] |
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NP6.00136: Characterization of edge instabilities in the Pegasus toroidal experiment M.W. Bongard, D.J. Battaglia, J.A. Cole, C.C. Hegna, E.T. Hinson, A.J. Redd, A.C. Sontag, G.R. Winz Field-aligned, rotating edge filamentary structures are observed on a routine basis in Pegasus Ohmic discharges. Imaging studies using a fast-framing camera indicate the filaments are large-scale, short-lived, coherent structures with an average lifetime varying between 10 and $\approx$150 $\mu$s. Measurements using a pair of toroidally separated, radially scannable internal magnetic probes indicate that these structures are electromagnetic in nature. They are clearly distinguished from typical $2/1$ tearing activity and appear as low amplitude, broadband ($\leq$150 kHz) magnetic turbulence. These fluctuations are not observed on probes far from the plasma edge, suggesting high poloidal mode number. The high edge current density ($j_{a}\sim100\ mbox{kA/m}^{2}$) and low toroidal field ($|B_{\phi,a}|\sim$ 0.1 T) typical in Pegasus may make the edge unstable to peeling modes. Additional magnetic probe arrays will be implemented, allowing a more accurate determination of $n$ and $m$, as well as estimates of filament propagation velocity. [Preview Abstract] |
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NP6.00137: Edge density and temperature measurements on the Pegasus toroidal experiment using Langmuir probe arrays E.T. Hinson, D.J. Battaglia, M.W. Bongard, A.J. Redd, A.C. Sontag, G.W. Winz Langmuir probes are being deployed on the Pegasus Toroidal Experiment to measure the edge electron density and temperature in both Ohmic and plasma gun-driven plasmas. Typical edge temperatures in the region of interest are expected to be less than 50 eV, and edge densities are estimated to be on the order of $10^{19}~{\rm m}^{-3}$. Each probe has four 1 mm diameter tungsten tips, extending several millimeters into the plasma, and is protected by a graphite housing. This first setup can be employed as a swept probe or a triple probe. Future efforts will focus on deploying an array of such probes. The first probe is mounted on a sliding shaft 10 cm below the outboard midplane, enabling the probe tips to be positioned several centimeters within the scrape-off layer. The probe provides a high time resolution measurement of the edge plasma parameters, including fluctuations, up to the megahertz scale. In Ohmic discharges, the Langmuir probe measurements will support edge stability studies. In helicity-driven plasmas, the Langmuir probes will be used to characterize the plasma edge in unrelaxed plasma streams, gun-driven edge current sheets, and in relaxed tokamak-like equilibria. [Preview Abstract] |
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NP6.00138: Pegasus power system facility upgrades B.T. Lewicki, B.A. Kujak-Ford, G.R. Winz Two key Pegasus systems have been recently upgraded: the Ohmic-transformer IGCT bridge control system, and the plasma-gun injector power system. The Ohmic control system contains two new microprocessor controlled components to provide an interface between the PWM controller and the IGCT bridges. An interface board conditions the command signals from the PWM controller. A splitter/combiner board routes the conditioned PWM commands to an array of IGCT bridges and interprets IGCT bridge status. This system allows for any PWM controller to safely control IGCT bridges. Future developments will include a transition to a polyphasic bridge control. This will allow for 3 to 4 times the present pulse length and provide a much higher switching frequency. The plasma gun injector system now includes active current feedback control on gun bias current via PWM buck type power supplies. Near term goals include a doubling or tripling of the applied bias voltage. Future arc bias system power supplies may include a simpler boost type system which will allow access to even higher voltages using existing low voltage energy storage systems. [Preview Abstract] |
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NP6.00139: Computational study of a non-ohmic flux compression startup method for spherical tokamaks J.B. O'Bryan, C.R. Sovinec, D.J. Battaglia, T.M. Bird A computational study is conducted on a method of non-ohmic startup for spherical tokamaks, wherein a toroidal current channel generated by helicity injection undergoes flux compression to produce current amplification. The current channel is electrostatically driven along magnetic field lines from a washer-gun plasma source mounted at the midplane on the outboard side of the Pegasus Toroidal Experiment (Univ. of Wisconsin). The current channel is implemented in NIMROD (nimrodteam.org) using a localized, volumetric current source. Temporally and spatially varying boundary conditions produce the flux compression using data from the equilibrium reconstruction of the vacuum fields. Nonlinear, resistive MHD computations are used in this investigation, which includes anisotropic, temperature-dependent thermal conduction with corrections for demagnetization effects and temperature-dependent resistivity [Braginskii, Reviews of Plasma Physics, 1965]. We discuss the effectiveness of the flux compression, including current distribution and plasma confinement, as predicted by these calculations. [Preview Abstract] |
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NP6.00140: The Dynamics of Flux Amplification of Coaxial Helicity Injection on the HIT-II Spherical Torus R.J. Smith, T.R. Jarboe, B.A. Nelson Internal magnetic probing results are combined with external magnetics, bolometry and injector circuit measurements to provide a detailed description of the dynamics of a Coaxial Helicity Injected (CHI) ST demonstrating poloidal flux amplification. The relaxation process is characterized by an initial, sub-\textit{ms} phase of rapid flux and plasma current buildup, a slower, \textit{ms} time scale phase of steady current buildup and a stagnation phase in which helicity drive is balanced by losses which is strongly associated with a large amplitude edge mode activity. Flux amplification behavior over an extensive parameter range is presented in juxtaposition to non-relaxation CHI discharges. Possible explanations for the observed relaxation and flux amplification will be discussed. [Preview Abstract] |
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NP6.00141: Recent Progress in TS-4 and UTST ST Merging Experiments Yasushi Ono For the past ten years, we have been investigating the merging startup of spherical tokamak in TS-4 and UTST experiments. Cause and mechanism for magnetic reconnection heating were directly measured for the first time by use of new two dimensional ion and electron measurements. Two ST plasmas were merged together in the axial direction for high-power reconnection heating/ startup without center-solenoid (CS) coil. A new finding is that ions and electrons were heated in the down-stream area and in the current sheet, respectively. The 2-D line-integrated data of spectral lines were measured by polychromators with ICCD cameras, and were transformed into local ion temperature data using the tomography reconstruction. The electron temperature was measured by a scanning electrostatic probe array. While the electron temperature outside the sheet was uniformly 5-6eV, it clearly peaked around the current sheet. While electrons were quickly hearted inside current sheet by its ohmic heating power, ions were heated in the downstream area by shock or viscosity damping of the reconnection outflow. The ion heating power $\sim $4MW was an order of magnitude larger than the electron heating power $\sim $0.2MW. This heating mechanism is consistent with the squared B scaling (B: poloidal field) of reconnection heating energy. We started the multiple merging experiment for CS-less rampup and current drive of ST and for new double axis divertor for its high MHD activities. [Preview Abstract] |
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