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Three-dimensional Reconstruction of Dust Particle Trajectories in the NSTX

Description: Highly mobile incandescent dust particles are routinely observed on NSTX using two fast cameras operating in the visible region. An analysis method to reconstruct dust particle trajectories in space using two fast cameras is presented in this paper. Position accuracies of a few millimeters depending on the particle's location have been achieved and particle velocities between 10 and 200 m/s have been observed. 2008 American Institute of Physics. __________________________________________________
Date: March 6, 2009
Creator: Boeglin, W. U.; Roquemore, A. L. & Maqueda, R.
Partner: UNT Libraries Government Documents Department

Intermittency in the Scrape-off Layer of the National Spherical Torus Experiment During H-mode Confinement

Description: A gas puff imaging diagnostic is used in the National Spherical Tokamak Experiment [M. Ono, et al., Nucl. Fusion 40, 557 (2000)] to study the edge turbulence and intermittency present during H-mode discharges. In the case of low power Ohmic H-modes the suppression of turbulence/blobs is maintained through the duration of the (short lived) H-modes. Similar quiescent edges are seen during the early stages of H-modes created with the use of neutral beam injection. Nevertheless, as time progresses following the L-H transition, turbulence and blobs reappear although at a lower level than that typically seen during L-mode confinement. It is also seen that the time-averaged SOL emission profile broadens, as the power loss across the separatrix increases. These broad profiles are characterized by a large level of fluctuations and intermittent events.
Date: November 22, 2010
Creator: Maqueda, R. J.; Stotler, D. P. & Zweben, S. J.
Partner: UNT Libraries Government Documents Department

Intermittent Divertor Filaments in the National Spherical Torus Experiment and Their Relation to Midplane Blobs

Description: While intermittent filamentary structures, also known as blobs, are routinely seen in the low-field-side scrape-off layer of the National Spherical Torus Experiment (NSTX) (Ono et al 2000 Nucl. Fusion 40 557), fine structured filaments are also seen on the lower divertor target plates of NSTX. These filaments, not associated with edge localized modes, correspond to the interaction of the turbulent blobs seen near the midplane with the divertor plasma facing components. The fluctuation level of the neutral lithium light observed at the divertor, and the skewness and kurtosis of its probability distribution function, is similar to that of midplane blobs seen in Dα; e.g. increasing with increasing radii outside the outer strike point (OSP) (separatrix). In addition, their toroidal and radial movement agrees with the typical movement of midplane blobs. Furthermore, with the appropriate magnetic topology, i.e. mapping between the portion of the target plates being observed into the field of view of the midplane gas puff imaging diagnostic, very good correlation is observed between the blobs and the divertor filaments. The correlation between divertor plate filaments and midplane blobs is lost close to the OSP. This latter observation is consistent with the existence of ‘magnetic shear disconnection’ due to the lower X-point, as proposed by Cohen and Ryutov (1997 Nucl. Fusion 37 621).
Date: May 19, 2010
Creator: Maqueda, R. J. & Stotler, D. P.
Partner: UNT Libraries Government Documents Department

Visible imaging of edge turbulence in NSTX

Description: Edge plasma turbulence in tokamaks and stellarators is believed to cause the radical heat and particle flux across the separatrix and into the scrape-off-layers of these devices. This paper describes initial measurements of 2-D space-time structure of the edge density turbulence made using a visible imaging diagnostic in the National Spherical Torus Experiment (NSTX). The structure of the edge turbulence is most clearly visible using a method of gas puff imaging to locally illuminate the edge density turbulence.
Date: June 13, 2000
Creator: Zweben, S.; Maqueda, R.; Hill, K.; Johnson, D. & al, et
Partner: UNT Libraries Government Documents Department

Design, Installation and Performance of the New insulator for NSTX CHI Experiments

Description: Coaxial Helicity Injection (CHI), a non-inductive method to initiate plasma and generate toroidal plasma current, is being investigated in the National Spherical Torus Experiment (NSTX). The center stack and outer vacuum vessel are separated by insulating gaps at the top and bottom of the slim central column so that a high voltage (up to 2 kV) can be applied between them from a pulsed power supply or a capacitor bank to initiate an arc discharge. In the presence of a suitable poloidal magnetic field, the discharge is initiated at the lower gap (the injector gap) and because of the strong toroidal field develops a helical structure resulting in substantial toroidal plasma current being driven. In NSTX, up to 390 kA of toroidal current has been generated for an injected current of 25 kA. The early investigations of CHI however frequently developed arcs across the insulator at the top of the machine (the absorber gap), which terminated the desired discharge. This arcing greatly restricted the operational space available for CHI studies. During 2002, the absorber region was modified to suppress these arcs. The new design includes a new ceramic insulator on the high field side of the absorber region with a much longer tracking distance between conducting elements at the different potentials. Furthermore, two new coils were installed near the absorber to provide the ability to minimize the poloidal field connecting the center stack and outer vacuum vessel. During the subsequent experimental campaign, CHI operation was less prone to arcing in the absorber and those arcs that did occur did not terminate the main discharge.)
Date: March 5, 2008
Creator: Mueller, D; Chrzanowski, J; Gates, D; Menard, J; Raman, R; Jarboe, T R et al.
Partner: UNT Libraries Government Documents Department

Three-Dimensional Neutral Transport Simulations of Gas Puff Imaging Experiments

Description: Gas Puff Imaging (GPI) experiments are designed to isolate the structure of plasma turbulence in the plane perpendicular to the magnetic field. Three-dimensional aspects of this diagnostic technique as used on the National Spherical Torus eXperiment (NSTX) are examined via Monte Carlo neutral transport simulations. The radial width of the simulated GPI images are in rough agreement with observations. However, the simulated emission clouds are angled approximately 15 degrees with respect to the experimental images. The simulations indicate that the finite extent of the gas puff along the viewing direction does not significantly degrade the radial resolution of the diagnostic. These simulations also yield effective neutral density data that can be used in an approximate attempt to infer two-dimensional electron density and temperature profiles from the experimental images.
Date: September 19, 2003
Creator: Stotler, D.P.; DIppolito, D.A.; LeBlanc, B.; Maqueda, R.J.; Myra, J.R.; Sabbagh, S.A. et al.
Partner: UNT Libraries Government Documents Department

Images of Edge Turbulence in NSTX

Description: The 2-D structure of edge plasma turbulence has been measured in the National Spherical Torus Experiment (NSTX) by viewing the emission of the Da spectral line of deuterium. Images have been made at framing rates of up to 250,000 frames/sec using an ultra-high speed CCD camera developed by Princeton Scientific Instruments. A sequence of images showing the transition between L-mode and H-mode states is shown.
Date: July 16, 2004
Creator: Zweben, S.J.; Bush, C.E.; Maqueda, R.; Munsat, T.; Stotler, D.; Lowrance, J. et al.
Partner: UNT Libraries Government Documents Department

Large Area Divertor Temperature Measurements Using A High-speed Camera With Near-infrared FiIters in NSTX

Description: Fast cameras already installed on the National Spherical Torus Experiment (NSTX) have be equipped with near-infrared (NIR) filters in order to measure the surface temperature in the lower divertor region. Such a system provides a unique combination of high speed (> 50 kHz) and wide fi eld-of-view (> 50% of the divertor). Benchtop calibrations demonstrated the system's ability to measure thermal emission down to 330 oC. There is also, however, signi cant plasma light background in NSTX. Without improvements in background reduction, the current system is incapable of measuring signals below the background equivalent temperature (600 - 700 oC). Thermal signatures have been detected in cases of extreme divertor heating. It is observed that the divertor can reach temperatures around 800 oC when high harmonic fast wave (HHFW) heating is used. These temperature profiles were fi t using a simple heat diffusion code, providing a measurement of the heat flux to the divertor. Comparisons to other infrared thermography systems on NSTX are made.
Date: April 5, 2011
Creator: Lyons, B. C.; Zweben, S. J.; Gray, T. K.; Hosea, J.; Kaita, R.; Kugel, H. W. et al.
Partner: UNT Libraries Government Documents Department

Solenoid-free Plasma Start-up in NSTX using Transient CHI

Description: Experiments in NSTX have now unambiguously demonstrated the coupling of toroidal plasmas produced by the technique of CHI to inductive sustainment and ramp-up of the toroidal plasma current. This is an important step because an alternate method for plasma startup is essential for developing a fusion reactor based on the spherical torus concept. Elimination of the central solenoid would also allow greater flexibility in the choice of the aspect ratio in tokamak designs now being considered. The transient CHI method for spherical torus startup was originally developed on the HIT-II experiment at the University of Washington.
Date: November 3, 2008
Creator: Raman, R.; Nelson, B. A.; Mueller, D.; Jarboe, T. R.; Bell, M. G.; LeBlanc, B. et al.
Partner: UNT Libraries Government Documents Department

Solenoid-free Plasma Start-up in NSTX using Transient CHI

Description: Experiments in NSTX have now unambiguously demonstrated the coupling of toroidal plasmas produced by the technique of CHI to inductive sustainment and ramp-up of the toroidal plasma current. This is an important step because an alternate method for plasma startup is essential for developing a fusion reactor based on the spherical torus concept. Elimination of the central solenoid would also allow greater flexibility in the choice of the aspect ratio in tokamak designs now being considered. The transient CHI method for spherical torus startup was originally developed on the HIT-II experiment at the University of Washington.
Date: November 3, 2008
Creator: Raman, R.; Nelson, B. A.; Mueller, D.; Jarboe, T. R.; Bell, M. G.; LeBlanc, B. et al.
Partner: UNT Libraries Government Documents Department

Recent progress on the National Spherical Torus Experiment (NSTX)

Description: Recent upgrades to the NSTX facility have led to improved plasma performance. Using 5MW of neutral beam injection, plasmas with toroidal {beta}{sub T} (= 2{mu}{sub 0}<p>/B{sub T}{sup 2} where B{sub T} is the vacuum toroidal field at the plasma geometric center) > 30% have been achieved with normalized {beta}{sub N} (={beta}{sub T}aB{sub I}/I{sub p}) {approx} 6% {center_dot} m {center_dot} T/MA. The highest {beta} discharge exceeded the calculated no-wall {beta} limit for several wall times. The stored energy has reached 390kJ at higher toroidal field (0.55T) corresponding to {beta}{sub T} {approx} 20% and {beta}{sub N} = 5.4. Long pulse ({approx}1s) high {beta}{sub p} ({approx}1.5) discharges have also been obtained at higher B{sub {phi}} (0.5T) with up to 6MW NBI power. The highest energy confinement times, up to 120ms, were observed during H-mode operation which is now routine. Confinement times of {approx} 1.5 times ITER98pby2 for several {tau}{sub E} are observed during both H-Mode and non-H-Mode discharges. Calculations indicate that many NSTX discharges have very good ion confinement, approaching neoclassical levels. High Harmonic Fast Wave current drive has been demonstrated by comparing discharges with waves launched parallel and anti-parallel to the plasma current.
Date: January 1, 2002
Creator: Maqueda, R. J. (Ricardo J.); Wurden, G. A. (Glen A.); Gates, D. A.; Bell, M. G.; Bialek, J.; Bigelow, T. et al.
Partner: UNT Libraries Government Documents Department

Local scrape-off layer control using biased electrodes in NSTX

Description: An experiment was designed to test the theory that biased electrodes can affect the local scrape-off layer (SOL) width by creating a strong radial ExB drift [Cohen, R.H. and Ryutov, D.D, Nucl. Fusion 37, 621 (1997)]. These electrodes were located near the outer midplane in the SOL of the National Spherical Torus Experiment (NSTX). The electrodes were biased at up to �100 Volts, and the radial profile of the plasma between them was measured by an array of Langmuir probes. The biasing caused large changes in the local SOL profiles at least qualitatively consistent with this theory.
Date: April 24, 2009
Creator: Zweben, S. J.; Maqueda, R. J.; Roquemore, A. L.; Bush, C. E.; Kaita, R.; Marsala, R. J. et al.
Partner: UNT Libraries Government Documents Department

Non-inductive Solenoid-less Plasma Current Start-up in NSTX Using Transient CHI

Description: Coaxial Helicity Injection (CHI) has been successfully used in the National Spherical Torus Experiment (NSTX) for a demonstration of closed flux current generation without the use of the central solenoid. The favorable properties of the Spherical Torus (ST) arise from its very small aspect ratio. However, small aspect ratio devices have very restricted space for a substantial central solenoid. Thus methods for initiating the plasma current without relying on induction from a central solenoid are essential for the viability of the ST concept. CHI is a promising candidate for solenoid-free plasma startup in a ST. The method has now produced closed flux current up to 160 kA verifying the high current capability of this method in a large ST built with conventional tokamak components.
Date: May 23, 2007
Creator: Raman, R; Jarboe, T R; Nelson, B A; Bell, M G; Ono, M; Bigelow, T et al.
Partner: UNT Libraries Government Documents Department

Visible imaging of edge turbulence in NSTX

Description: Edge plasma turbulence in tokamaks and stellarators is believed to cause the radial heat and particle flux across the separatrix and into the scrape-off-layers of these devices. This paper describes initial measurements of 2-D space-time structure of the edge density turbulence made using a visible imaging diagnostic in the National Spherical Torus Experiment (NSTX). The structure of the edge turbulence is most clearly visible using a method of ''gas puff imaging'' to locally illuminate the edge density turbulence.
Date: June 21, 2000
Creator: Zweben, S.; Maqueda, R.; Hill, K.; Johnson, D.; Kaye, S.; Kugel, H. et al.
Partner: UNT Libraries Government Documents Department

Particle and Energy Transport in the SOL of DIII-D and NSTX

Description: The far scrape-off layer (SOL) radial transport and plasma-wall contact is mediated by intermittent and ELM-driven transport. Experiments to characterize the intermittent transport and ELMs have been performed in both DIII-D and NSTX under similar conditions. Both intermittent transport and ELMs are comprised of filaments of hot, dense plasma (n{sub e} {approx} 1 x 10{sup 13} cm{sup -3}, T{sub e} {approx} 400 eV) originating at the edge, transport both particles and heat into the SOL by convection, increasing wall interaction and causing sputtering and impurity release. Both intermittent filaments and ELMs leave the pedestal region at speeds of {approx}0.5-3 km/s, losing heat and particles by parallel transport as they travel through the SOL. The intermittency shows many similarities in NSTX and DIII-D, featuring similar size (2-5 cm), large convective radial velocity, ''holes'' inside and peaks outside the LCFS which quickly decay and slow down with radius. Whereas in DIII-D the intermittency decays in both intensity and frequency in H-mode, it chiefly decays in frequency in NSTX. In the low collisionality (v* = {pi}R{sub q{sub 95}}/{lambda}C) (v* {approx} 0.1, N{sub G} {approx} 0.3) case, the ELMs impact the walls quite directly and account for {approx}90% of the wall particle flux, decreasing to {approx}30% at (v* {approx} 1.0, N{sub G} > 0.6).
Date: October 9, 2006
Creator: Boedo, J; Maqueda, R; Rudakov, D; McKee, G; Kugel, H; Maingi, R et al.
Partner: UNT Libraries Government Documents Department

Progress on the FRX-L FRC plasma injector at LANL for magnetized target fusion

Description: The FRX-L Field Reversed Configuration plasma is now operational at Los Alamos National Laboratory. The goal of the project is to demonstrate the production of suitable FRC target plasmas for later MTF (Magnetized Target Fusion) implosion experiments which will first be carried out at the Air Force Research Laboratory in Albuquerque, New Mexico, in a few years' time. Expected plasma parameters in the 4 cm diameter, 30 cm long FRC are ne{approx}1017 cm-3, T{approx}100-300 eV, at 4-5 Tesla fields, with a lifetime of {approx}20 microseconds. The system includes a 0.5 T bias field, 70 kV 250 kHz ringing pre-ionization, and a 1.5 MA, 200 kJ main-theta coil bank. Maxwell rail gap plasma switches are used to start the PI bank, the main theta coil bank, and to crowbar the main bank. Initial results using the first diagnostic set of excluded flux loops, B-dot probes, visible light diodes, a fiber-optically coupled gated intensified visible spectrometer, and a 3.3 micron quadrature interferometer are presented. Future diagnostics include end-on bolometry, Thomson scattering, and a multi-chord fanned HeNe side-on interferometer. Multi-turn cusp and guide coils will be added later this year, to enable translation experiments into a cylindrical metal liner.
Date: January 1, 2001
Creator: Assmus, P. N. (Phillip N.); Feinup, W. J.; Intrator, Thomas; Langner, M. C. (Matthew C.); Maqueda, R. J. (Ricardo J.); Scott, K. J. et al.
Partner: UNT Libraries Government Documents Department

Ohmic Flux Consumption During Initial Operation of the NSTX Spherical Torus

Description: The spherical tokamak (ST), because of its slender central column, has very limited volt-second capability relative to a standard aspect ratio tokamak of similar plasma cross-section. Recent experiments on the National Spherical Torus Experiment (NSTX) have begun to quantify and optimize the ohmic current drive efficiency in a MA-class ST device. Sustainable ramp-rates in excess of 5MA/sec during the current rise phase have been achieved on NSTX, while faster ramps generate significant MHD activity. Discharges with Ip exceeding 1MA have been achieved in NSTX with nominal parameters: aspect ratio A=1.3--1.4, elongation k=2--2.2, triangularity d=0.4, internal inductance li=0.6, and Ejima coefficient CE=0.35. Flux consumption efficiency results, performance improvements associated with first boronization, and comparisons to neoclassical resistivity are described.
Date: October 5, 2000
Creator: Menard, J.; LeBlanc, B.; Sabbagh, S.A.; Bell, M.; Bell, R.; Fredrickson, E. et al.
Partner: UNT Libraries Government Documents Department

Edge Turbulence Imaging in the Alcator C-Mod Tokamak

Description: The 2-D radial vs. poloidal structure of edge turbulence in the Alcator C-Mod tokamak [I.H. Hutchinson, R. Boivin, P.T. Bonoli et al., Nuclear Fusion 41(2001) 1391] was measured using fast cameras and compared with 3-D numerical simulations of edge plasma turbulence. The main diagnostic is Gas Puff Imaging (GPI), in which the visible D(subscript alpha) emission from a localized D(subscript 2) gas puff is viewed along a local magnetic field line. The observed D(subscript alpha) fluctuations have a typical radial and poloidal scale of approximately 1 cm, and often have strong local maxima (''blobs'') in the scrape-off layer. The motion of this 2-D structure motion has also been measured using an ultra-fast framing camera with 12 frames taken at 250,000 frames/sec. Numerical simulations produce turbulent structures with roughly similar spatial and temporal scales and transport levels as that observed in the experiment; however, some differences are also noted, perhaps requiring diagnostic improvement and/or additional physics in the numerical model.
Date: November 26, 2001
Creator: Zweben, S.J.; Stotler, D.P.; Terry, J.L.; LaBombard, B.; Greenwald, M.; Muterspaugh, M. et al.
Partner: UNT Libraries Government Documents Department

Effect of Gas Fueling Location on H-mode Access in NSTX

Description: The dependence of H-mode access on the poloidal location of the gas injection source has been investigated in the National Spherical Torus Experiment (NSTX). We find that gas fueling from the center stack midplane area produces the most reproducible H-mode access with generally the lowest L-H threshold power in lower single-null configuration. The edge toroidal rotation velocity is largest (in direction of the plasma current) just before the L-H transition with center stack midplane fueling, and then reverses direction after the L-H transition. Simulation of these results with a 2-D guiding-center Monte Carlo neoclassical transport code is qualitatively consistent with the trends in the measured velocities. Double-null discharges exhibit H-mode access with gas fueling from either the center stack midplane or center stack top locations, indicating a reduced sensitivity of H-mode access on fueling location in that shape.
Date: October 9, 2003
Creator: Maingi, R.; Bell, M.; Bell, R.; Biewer, T.; Bush, C.; Chang, C.S. et al.
Partner: UNT Libraries Government Documents Department

High Speed Imaging of Edge Turbulence in NSTX

Description: The two-dimensional radial versus poloidal structure and motion of edge turbulence in NSTX (National Spherical Torus Experiment) were measured by using high-speed imaging of the visible light emission from a localized neutral gas puff. Edge turbulence images are shown and analyzed for Ohmic, L-mode (low-confinement mode) and H-mode (high-confinement mode) plasma conditions. Typical edge turbulence poloidal correlation lengths as measured using this technique are = 4 {+-} 1 cm and autocorrelation times are 40 {+-} 20 {micro}sec in all three regimes. The relative fluctuation level is typically smaller in H-mode than in L-mode, and transitions from H- to L-mode and can occur remarkably quickly (=30 {micro}sec). The two-dimensional images often show localized regions of strong light emission which move both poloidally and radially through the observed region at a typical speed of =10{sup 5} cm/sec, and sometimes show spatially coherent modes.
Date: March 2003
Creator: Zweben, S. J.; Maqueda, R.; Stotler, D. P.; Keesee, A.; Boedo, J.; Bush, C. et al.
Partner: UNT Libraries Government Documents Department