190 Matching Results

Search Results

Advanced search parameters have been applied.

Preliminary Failure Modes and Effects Analysis of the US Massive Gas Injection Disruption Mitigation System Design

Description: This report presents the results of a preliminary failure modes and effects analysis (FMEA) of a candidate design for the ITER Disruption Mitigation System. This candidate is the Massive Gas Injection System that provides machine protection in a plasma disruption event. The FMEA was quantified with “generic” component failure rate data as well as some data calculated from operating facilities, and the failure events were ranked for their criticality to system operation.
Date: October 1, 2013
Creator: Cadwallader, Lee C.
Partner: UNT Libraries Government Documents Department

Thermal ablation of plasma-facing surfaces in tokamak disruptions: Sensitivity to particle kinetic energy

Description: Ablation damage to solid targets with high heat flux impulses is generally greater high-energy electron beam heat sources compared to low-energy plasma guns. This sensitivity to incoming particle kinetic energy is explored with computer modelling; a fast-running routine (DESIRE) is developed for initial scoping analysis and is found to be in reasonable agreement with several experiments on graphite and tungsten targets. If tokamak disruptions are characterized by particle energies less than {approximately}1 keV, then we expect plasma guns are a better analogue than electron beams for simulating disruption behavior and testing candidate plasma-facing materials.
Date: February 1, 1996
Creator: Ehst, D.A. & Hassanein, A.
Partner: UNT Libraries Government Documents Department


Description: OAK B202 TOKAMAK EQUILIBRIA WITH CENTRAL CURRENT HOLES AND NEGATIVE CURRENT DRIVE. Several tokamak experiments have reported the development of a central region with vanishing currents (the current hole). Straightforward application of results from the work of Greene, Johnson and Weimer [Phys. Fluids, 3, 67 (1971)] on tokamak equilibrium to these plasmas leads to apparent singularities in several physical quantities including the Shafranov shift and casts doubts on the existence of this type of equilibria. In this paper, the above quoted equilibrium theory is re-examined and extended to include equilibria with a current hole. It is shown that singularities can be circumvented and that equilibria with a central current hole do satisfy the magnetohydrodynamic equilibrium condition with regular behavior for all the physical quantities and do not lead to infinitely large Shafranov shifts. Isolated equilibria with negative current in the central region could exist. But equilibria with negative currents in general do not have neighboring equilibria and thus cannot have experimental realization, i.e. no negative currents can be driven in the central region.
Date: June 1, 2002
Creator: CHU, M.S. & PARKS, P.B.
Partner: UNT Libraries Government Documents Department

Disruptions, loads, and dynamic response of ITER

Description: Plasma disruptions and the resulting electromagnetic loads are critical to the design of the vacuum vessel and in-vessel components of the International Thermonuclear Experimental Reactor (ITER). This paper describes the status of plasma disruption simulations and related analysis, including the dynamic response of the vacuum vessel and in-vessel components, stresses and deflections in the vacuum vessel, and reaction loads in the support structures.
Date: December 31, 1995
Creator: Nelson, B.; Riemer, B.; Sayer, R.; Strickler, D.; Barabaschi, P.; Ioki, K. et al.
Partner: UNT Libraries Government Documents Department

Scientific basis and engineering design to accommodate disruption and halo current loads for the DIII-D tokamak

Description: Plasma disruptions and halo current events apply sudden impulsive forces to the interior structures and vacuum vessel walls of tokamaks. These forces arise when induced toroidal currents and attached poloidal halo currents in plasma facing components interact with the poloidal and toroidal magnetic fields respectively. Increasing understanding of plasma disruptions and halo current events has been developed from experiments on DIII-D and other machines. Although the understanding has improved, these events must be planned for in system design because there is no assurance that these events can be eliminated in the operation of tokamaks. Increased understanding has allowed an improved focus of engineering designs.
Date: October 1996
Creator: Anderson, P. M.; Bozek, A. S.; Hollerbach, M. A.; Humphreys, D. A.; Luxon, J. L.; Reis, E. E. et al.
Partner: UNT Libraries Government Documents Department

MHD stability studies in reversed shear plasmas in TFTR

Description: MHD phenomena in reversed shear plasmas in TFTR are described during each of the three phases of the evolution of these discharges: the current ramp, high power neutral beam heating and after the beam power has been reduced. Theoretical analysis of discharges which disrupted in the high-{beta} phase indicates that the {beta} - limit is set by the ideal n = 1 infernal/kink mode. The mode structure of the disruption precursor reconstructed from the electron temperature data compares favorably with the predicted displacement vector from the ideal MHD model. In contrast, disruptions during the early and late phases are due to resistive instabilities, double tearing modes coupled to high-m edge modes. The resistive interchange mode, predicted to be unstable in reversed shear plasmas, is not seen in the experiment. Neo-classical tearing mode theory is shown to describe the non-disruptive MHD phenomena. A nonlinear resistive MHD simulation reproduces off-axis sawtooth-like crashes during the post-beam phase. The dependence of the {beta}-limit on the pressure peakedness and q{sub min} is discussed, showing a path to stable higher-{beta} regimes.
Date: December 31, 1996
Creator: Manickam, J.; Fredrickson, E. & Chang, Z.
Partner: UNT Libraries Government Documents Department

Modelling multi-ion plasma gun simulations of Tokamak disruptions

Description: The effect of impurity ions in plasma gun ablation tests of various targets is considered. Inclusion of reasonable amounts of impurity ({approximately}10%) is adequate to explain observed energy transmission and erosion measurements. The gun tests and the computer code calculations are relevant to the parameter range expected for major disruptions on large tokamaks.
Date: August 1, 1995
Creator: Ehst, D.A.
Partner: UNT Libraries Government Documents Department

Small-action Resonance in Hamiltonian Systems and Redistribution of Energetic Ions in Tokamaks

Description: It has been found that an arbitrary small perturbation in an integrable Hamiltonian system typically leads to driven resonance in the regions of the phase space where at least one of the action variables is sufficiently small. In particular, such a small-action resonance is shown to play a dominant role in the sawtooth-crash-induced disappearance of a strongly localized gamma-ray and neutron emitting region in a tokamak plasma, which was observed experimentally.
Date: July 1, 1999
Creator: White, R.B.; Lutsenko, V.V.; Kolesnichenko, Ya. I. & Yakovenko, Yu. V.
Partner: UNT Libraries Government Documents Department

High-{beta} disruption in tokamaks

Description: Three dimensional MHD simulations of high-{beta} plasmas show that toroidally localized high-n ballooning modes can be driven unstable by the local pressure steepening which arises from the evolution of low-n modes. Nonlinearly, the high-n mode becomes even more localized and produces a strong local pressure bulge which destroys the flux surfaces resulting in a thermal quench. The flux surfaces then recover temporarily but now contain large magnetic islands. This scenario is supported by experimental data.
Date: July 1, 1995
Creator: Park, W.; Fredrickson, E.D. & Janos, A.
Partner: UNT Libraries Government Documents Department

TAE modes and MHD activity in TFTR DT plasmas

Description: The high power deuterium and tritium experiments on TFTR have produced fusion a parameters similar to those expected on ITER. The achieved {beta}{sub {alpha}}/{beta} and the R{triangledown}{beta}{sub {alpha}} in TFRR D-T shots are 1/2 to 1/3 those predicted in the ITER EDA. Studies of the initial TFTR D-T plasmas find no evidence that the presence of the fast fusion {alpha} population has affected the stability of MHD, with the possible exception of Toroidal Alfven Eigenmodes (TAE`s). The initial TFTR DT plasmas had MHD activity similar to that commonly seen in deuterium plasmas. Operation of TFTR at plasma currents of 2.0--2.5 MA has greatly reduced the deleterious effects of MHD commonly observed at lower currents. Even at these higher currents, the performance of TFTR is limited by {beta}-limit disruptions. The effects of MHD on D-T fusion {alpha}`s was similar to effects observed on other fusion products in D only plasmas.
Date: March 1, 1995
Creator: Fredrickson, E.; Batha, S. & Bell, M.
Partner: UNT Libraries Government Documents Department

Mode Structure of Disruption Precursors in TFTR Enhanced Reversed Shear Discharges

Description: The mode structure of the disruption precursors in the TFTR enhanced reversed shear parameters has been studied by using T(subscript e) fluctuation profiles and q profiles obtained from Electron Cyclotron Emission (ECE) and Motional Stark Effect (MSE) measurements. The observed profiles of the radial displacement associated with the MHD modes were consistent with the displacement profiles expected from the ideal MHD external kink mode. The observed mode frequencies differ from the plasma toroidal rotation frequency measured with CVI charge-exchange recombination light. The independence of the mode frequency from the plasma rotation frequency supports the ideal MHD hypothesis. Possible causes of the frequency difference are discussed.
Date: January 1, 1998
Creator: Fredrickson, E.; Taylor, G.; Manickam, J.; Okabayashi, M.; Batha, S. & al, et
Partner: UNT Libraries Government Documents Department

Magnetic and thermal energy flow during disruptions in DIII-D

Description: The authors present results from disruption experiments where they measure magnetic energy flow across a closed surface surrounding the plasma using a Poynting flux analysis to measure the electromagnetic power, bolometers to measure radiation power and IR scanners to measure radiation and particle heat conduction to the divertor. The initial and final stored energies within the volume are found using the full equilibrium reconstruction code EFIT. From this analysis they calculate an energy balance and find that they can account for all energy deposited on the first wall and the divertor to within about 10%.
Date: July 1, 1996
Creator: Hyatt, A.W.; Lee, R.L.; Humphreys, D.A.; Kellman, A.G.; Taylor, P.L.; Cuthbertson, J.W. et al.
Partner: UNT Libraries Government Documents Department

Initial operation of the divertor Thompson scattering diagnostic on DIII-D

Description: The first Thomson scattering measurements of n{sub e} and T{sub e} in the divertor region of a tokamak are reported. These data are used as input to boundary physics codes such as UEDGE and DEGAS and to benchmark the predictive capabilities of these codes. These measurements have also contributed to the characterization of tokamak disruptions. A Nd:YAG laser (20 Hz, 1 J, 15 ns, 1064 nm) is directed vertically through the lower divertor region of the DIII-D tokamak. A custom, aspherical collection lens (f /6.8) images the laser beam from 1-21 cm above the target plates into eight spatial channels with 1.5 cm vertical and 0.3 cm radial resolution. 2D mapping of the divertor region is achieved by sweeping the divertor X-point location radially through the fixed laser beam location. Fiber optics carry the light to polychromators whose interference filters have been optimized for low T{sub e} measurements. Silicon avalanche photo diodes measure both the scattered and plasma background light. Temperatures and densities are typically in the range of 5-200 eV and 1 - 10 x 10{sup 19} m{sup -3} respectively. Low temperatures, T{sub e} < 1 eV, and high densities, n{sub e} > 8x10{sup 20} m{sup -3} have been observed in detached plasmas. Background light levels have not been a significant problem. Reduction of the laser stray light permits Rayleigh calibration. Because of access difficulties, no in-vessel vacuum alignment target could be used. Instead, an in situ laser alignment monitor provides alignment information for each laser pulse. Results are compared with Langmuir probe measurements where good agreement is found except for regions of high n{sub e} and low T{sub e} as measured by Thomson scattering.
Date: May 1996
Creator: Carlstrom, T. N.; Hsieh, C.L. & Stockdale, R. E.
Partner: UNT Libraries Government Documents Department

ITER vacuum vessel dynamic stress analysis of a disruption

Description: Dynamic stress analysis of the International Thermonuclear Experimental Reactor vacuum vessel loaded by disruption forces was performed. The deformation and stress results showed strong inertial effects when compared to static analyses. Maximum stress predicted dynamically was 300 MPa, but stress shown by static analysis from loads at the same point in time reached only 80 MPa. The analysis also provided a reaction load history in the vessel`s supports which is essential in evaluating support design. The disruption forces were estimated by assuming a 25-MA plasma current decaying at 1 MA/ms while moving vertically. In addition to forces developed within the vessel, vertical loadings from the first wall/strong back assemblies and the divertor were applied to the vessel at their attachment points. The first 50 natural modes were also determined. The first mode`s frequency was 6.0 Hz, and its shape is characterized by vertical displacement of the vessel inner leg. The predicted deformation of the vessel appeared similar to its first mode shape combined with radial contraction. Kinetic energy history from the analysis also correlated with the first mode frequency.
Date: December 1994
Creator: Riemer, B. W.; Conner, D. L.; Strickler, D. J. & Williamson, D. E.
Partner: UNT Libraries Government Documents Department

Disruption mitigation studies in DIII-D

Description: Data on the discharge behavior, thermal loads, halo currents, and runaway electrons have been obtained in disruptions on the DIII-D tokamak. These experiments have also evaluated techniques to mitigate the disruptions while minimizing runaway electron production. Experiments injecting cryogenic impurity killer pellets of neon and argon and massive amounts of helium gas have successfully reduced these disruption effects. The halo current generation, scaling, and mitigation are understood and are in good agreement with predictions of a semianalytic model. Results from killer pellet injection have been used to benchmark theoretical models of the pellet ablation and energy loss. Runaway electrons are often generated by the pellets and new runaway generation mechanisms, modifications of the standard Dreicer process, have been found to explain the runaways. Experiments with the massive helium gas puff have also effectively mitigated disruptions without the formation of runaway electrons that can occur with killer pellets.
Date: January 1, 1999
Creator: Taylor, P.L.; Kellman, A.G. & Evans, T.E.
Partner: UNT Libraries Government Documents Department

Investigation of the effect of resistive MHD modes on spherical torus performance in CDX-U

Description: Resistive MHD modes and associated effects on spherical torus performance are investigated in the CDX-U device for Ip {le} 100 kA. Presently, the growth of resistive MHD modes (n=1/m=3 or n=1/m=2) as the edge q[q(a)] is lowered toward 3.5 appears to limit the maximum current achievable in CDX-U. For low q(a) discharges, a prominent rotating hot spot can be seen with the soft x-ray array, indicative of a magnetic island associated with a n=1/m=1 mode. The edge mode, which is n=1/m=3 or n=1/m=2, can be seen by the soft x-ray and edge magnetic pick up coil array. The growth of those modes in space and amplitude eventually leads to an Internal Reconnection Event (IRE). Prior to the IRE, strong mode-mixing takes place suggesting magnetic island overlap. The IRE causes a rapid heat loss from the core causing a strong plasma elongation and current spike due to the plasma inductance drop. With an appropriate discharge control, a MHD quiescent high confinement regime with over twice the central electron temperature relative to the MHD active regime has been found. To assess the halo-induced effects during the MHD events, a pair of segmented Rogowski coils were installed on the center stack. The observed halo-induced current fraction is generally small (less than 5% of the total plasma current) even for the case of forced disruption.
Date: January 1, 1997
Creator: Ono, M.; Stutman, D. & Hwang, Y.S.
Partner: UNT Libraries Government Documents Department

Ballooning instability precursors to high {beta} disruptions

Description: Strongly ballooning modes have been found as precursors to high {beta} disruptions on TFTR. The modes are typically localized to a region spanning about 60{degree} in the toroidal direction. The toroidal localization is associated with lower frequency, global Magneto-Hydro-Dynamic (MHD) activity, typically an ideal n = 1 kink mode. They have moderate to high frequency (f = 10--20 f{sub rot}), implying toroidal mode numbers in the range n = 10--20. The growth rates for the modes are large, of order 10{sup 4}/sec.
Date: December 1, 1995
Creator: Fredrickson, E.D.; McGuire, K.M. & Chang, Z.Y.
Partner: UNT Libraries Government Documents Department

Recent results from the DIII-D Tokamak

Description: The goal of the DIII-D program is to provide the integrated basis for commercially attractive steady state fusion power plants. Significant progress toward this goal has been achieved, enabled by system improvements including an error field correction coil, an expanded diagnostic set, a digital plasma control system, and high power rf systems. Simultaneous improvements in both the confinement and stability have been achieved during both VH-mode and negative central shear discharges. Fully non-inductive discharges with high bootstrap current fraction have been obtained. The divertor program has demonstrated simultaneous reduction of divertor heat flux and effective particle control using gas puffing and an in-vessel cryopump. Control of the wall particle inventory, He exhaust, and characterization of the scrapeoff layer and divertor plasma have been achieved. Progress has also been made in addressing additional specific needs for ITER: investigation of basic transport scaling, disruption characterization and avoidance, material erosion, and steady state beta limits.
Date: October 1995
Creator: Kellman, A.G.
Partner: UNT Libraries Government Documents Department

Erosion damage of nearby plasma-facing components during a disruption on the divertor plate

Description: Intense energy flow from the disrupting plasma during, a thermal quench will cause a sudden vapor cloud to form above the exposed divertor area. The vapor-cloud layer has been proved to significantly reduce the subsequent energy flux of plasma particles to the original disruption location. However, most of the incoming plasma energy is quickly converted to intense photon radiation emitted by heating of the vapor cloud. This radiation energy can cause serious erosion damage of nearby components not directly exposed to the disrupting, plasma. The extent of this ``secondary damage`` will depend on the divertor design, disrupting plasma parameters, and design of nearby components. The secondary erosion damage of these components due to intense radiation can exceed that of the original disruption location.
Date: September 1, 1996
Creator: Hassanein, A. & Konkashbaev, I.
Partner: UNT Libraries Government Documents Department

DIII-D research operations. Annual report to the US Department of Energy, October 1, 1994--September 30, 1995

Description: The DIII-D research program funded by the U.S. Department of Energy (DOE) is aimed at developing the knowledge base for an economically and environmentally attractive energy source for the nation and the world. The DIII-D program mission is to advance fusion energy science understanding and predictive capability and improve the tokamak concept. The DIII-D scientific objectives are: (1) Advance understanding of fusion plasma physics and contribute to the physics base of ITER through extensive experiment and theory iteration in the following areas of fusion science - Magnetohydrodynamic (MHD) stability - Plasma turbulence and transport - Wave-particle interactions - Boundary physics plasma neutral interaction (2) Utilize scientific understanding in an integrated manner to show the tokamak potential to be - More compact by increasing plasma stability and confinement to increase the fusion power density ({Beta}{tau}) - Steady-state through disruption control, handling of divertor heat and particle loads and current drive (3) Acquire understanding and experience with environmentally attractive low activation material in an operating tokamak. This report contains the research conducted over the past year in search of these scientific objectives.
Date: September 1, 1996
Partner: UNT Libraries Government Documents Department

Optimization of negative central shear discharges in shaped cross sections

Description: Magnetohydrodynamic (MHD) stability analyses of Negative Central Shear (NCS) equilibria have revealed a new understanding of the limiting MHD instabilities in NCS experiments. Ideal stability calculations show a synergistic effect between cross section shape and pressure profile optimization; strong shaping and broader pressure independently lead to moderately higher {Beta} limits, but broadening of the pressure profile in a strongly dee-shaped cross- section leads to a dramatic increase in the ideal {Beta} limit. Localized resistive interchange (RI) modes can be unstable in the negative shear region and are most restrictive for peaked pressure profiles. Resistive global modes can also be destabilized significantly below the ideal P limit. Experiments largely confirm the general trends, and diagnostic measurements and numerical stability calculations are found to be in good qualitative agreement. Observed disruptions in NCS discharges with L-mode edge and strongly peaked pressure, appear to be initiated by interactions between the RI, and the global ideal and resistive modes.
Date: October 1, 1996
Creator: Turnbull, A.D., Chu, M.S., Taylor, T.S., Casper, T.A., Rice, B.W.; Greene, J.M., Greenfield, C.M., La Haye, R.J., Lao, L.L., Lee, B.J.; Miller, R.L., Ren, C., Strait, E.J., Tritz, K.; Rettig, C.L., Rhodes, T.L. & Sauter, O.
Partner: UNT Libraries Government Documents Department

Coherent fluctuations in the initial TFTR D-T experiments

Description: The initial operation of TFTR with approximately equal power in the tritium and deuterium neutral beam injectors has resulted in the production of fusion power in excess of 9MW and central {beta}{sub {alpha}} > 0.25%. This {beta}{sub {alpha}} is within a factor of 2--3 of the {beta}{sub {alpha}} in projections of ITER performance. Effects of this {alpha} population on TAE modes, sawteeth and fishbone activity are being searched for. The D-T plasmas are also being studied for evidence of changes in MHD activity which might be attributed to the fast {alpha} population. This paper reports on the activity in the Alfven range of frequencies in the D-T plasmas and on detailed measurements of the MHD activity preceding major disruptions in D-D and D-T.
Date: March 1, 1995
Creator: Fredrickson, E.; Chang, Z.Y. & Budny, R.V.
Partner: UNT Libraries Government Documents Department

Singular surfaces in the open field line region of a diverted tokamak

Description: The structure of the open field lines of a slightly nonaxisymmetric, poloidally diverted tokamak is explored by numerical integration of the field line equations for a simple model field. In practice, the nonaxisymmetry could be produced self-consistently by the nonlinear evolution of a free-boundary MHD mode, or it could be produced by field errors, or it could be imposed externally by design. In the presence of a nonaxisymmetric perturbation, the tokamak is shown to develop open field line regions of differing topology separated by singular surfaces. It is argued that the singular surfaces can be expected to play a role analogous to that of rational toroidal flux surfaces, in terms of constraining ideal MHD perturbations and thus constraining the free-energy that can be tapped by ideal MHD instabilities. The possibility of active control of free-boundary instabilities by means of currents driven on the open singular surfaces, which are directly accessible from the divertor plates, is discussed. Also discussed is the possibility of early detection of imminent disruptions through localized measurement of the singular surface currents.
Date: May 1, 1995
Creator: Reiman, A.
Partner: UNT Libraries Government Documents Department

Energy balance, radiation and stability during rapid plasma termination via impurity pellet injections on DIII-D

Description: Injections of impurity {open_quotes}killer{close_quotes} pellets on DIII-D have demonstrated partial mitigation of undesirable disruption phenomena; namely reducing the convected heat loss to the wall, and the halo current`s magnitude and toroidal asymmetry. However, the appearance of a runaway electron population and large magnetic fluctuations (B/B{sub T} {approx} 1%) is coincident with the measured rapid loss of the plasma`s thermal energy ({approx}1 MJ in 1 ms) due to impurity radiation. A numerical code is developed to simulate the impurity radiation and predict the rapid plasma cooling observed. The simulation predicts two mechanisms for the generation of runaway electrons: the {open_quotes}slideaway{close_quotes} of hot tail electrons due to rapid cooling or the transport of hot electrons into the thermally collapsed plasma due to instabilities. Pressure gradients caused by the rapid non-adiabatic cooling of the impurity are identified as the probable source of these instabilities which also lead to convective heat losses. Results of a modeling effort to optimize pellet content, impurity species and cooling time for the avoidance of instabilities and runaway electrons are shown.
Date: June 1997
Creator: Whyte, D. G.; Jernigan, T. C. & Luckhardt, S. L.
Partner: UNT Libraries Government Documents Department