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A Sensitive Radiation Monitor for a Large Laboratory Area

Description: An instrument has been built to survey releases of radioactivity within the 22-acre enclosure of the Savannah River Laboratory. The device consists of a 2-in.diameter x 2-in. sodium iodide scintillation crystal with associated detector counting and recording circuits which provide a permanent record of radioactivity movements within the Laboratory area. The monitor detects signals of 6.9 mu r/hr from a 0.48-curie cobalt-60 source located 265 ft away. (auth)
Date: November 1, 1963
Creator: Cathey, L. & Moyer, R. A.
Partner: UNT Libraries Government Documents Department

Toward more accurate loss tangent measurements in reentrant cavities

Description: Karpova has described an absolute method for measurement of dielectric properties of a solid in a coaxial reentrant cavity. His cavity resonance equation yields very accurate results for dielectric constants. However, he presented only approximate expressions for the loss tangent. This report presents more exact expressions for that quantity and summarizes some experimental results.
Date: May 1, 1980
Creator: Moyer, R. D.
Partner: UNT Libraries Government Documents Department

Determination of the uncertainties of reflection coefficient measurements of a microwave network analyzer

Description: A method that calculates the residual uncertainties of a microwave network analyzer for the frequency range of 300 kHz to 50 GHz is described. The method utilizes measurements on NIST-certified standards (such as an airline or load) plus additional measurements to estimate the combined standard uncertainties for measurements using the network analyzer. The uncertainties of the standards are incorporated by means of a Monte Carlo technique. The uncertainties assigned to a network analyzer then provide the basis for estimating the uncertainties assigned to devices measured using a network analyzer. The results of this method for characterizing network analyzer uncertainties are presented for several connector types.
Date: April 1, 1998
Creator: Duda, L.E. & Moyer, R.D.
Partner: UNT Libraries Government Documents Department

MODELING OF COUPLED EDGE STOCHASTIC AND CORE RESONANT MAGNETIC FIELD EFFECTS IN DIVERTED TOKAMAKS

Description: Attaining the highest performance in poloidally diverted tokamaks requires resonant magnetic perturbation coils to avoid core instabilities (locked, resistive wall and neoclassical tearing modes). These coils also perturb the pedestal and edge region, causing varying degrees of stochasticity with remnant islands. The effects of the DIII-D locked mode control coil on the edge and core of Ohmic plasmas are modeled with the field line integration code TRIP3D and compared with experimental measurements. Without detailed profile analysis and field line integration, it is difficult to establish whether a given response is due to a ''core mode'' or an ''edge stochastic boundary.'' In diverted Ohmic plasmas, the boundary stochastic layer displays many characteristics associated with such layers in non-diverted tokamaks. Comparison with stochastic boundary results from non-diverted tokamaks indicates that a significant difference in diverted tokamaks is a ''focusing'' of the magnetic field line loss into the vicinity of the divertor.
Date: June 1, 2002
Creator: EVANS, T.E. & MOYER, R.A.
Partner: UNT Libraries Government Documents Department

Experimental and Numerical Studies of Separatrix Splitting and Magnetic Footprints in DIII-D

Description: A numerical field line integration code is used to study the structure of divertor footprints produced by small non-axisymmetric magnetic perturbation in the DIII-D tokamak. The numerical modeling results are compared to experimental infrared camera data which show a splitting of the divertor target plate heat flux into several distinct peaks when an n=3 magnetic perturbation from the DIII-D I-coil is applied. The heat flux splitting consistently appears when the n=3 perturbation is applied and disappears when the perturbation is removed. The magnitude of the splitting implied by the numerical modeling is a factor of 3 smaller than the splitting seen in the experimental data. These results suggest that the plasma response to the edge resonant applied n=3 magnetic perturbation produces an amplification of the vacuum magnetic footprint structure on the divertor target plates. These results may have significant implications for the ITER divertor design.
Date: May 15, 2006
Creator: Evans, T; Joseph, I; Moyer, R; Fenstermacher, M; Lasnier, C & Yan, L
Partner: UNT Libraries Government Documents Department

DIII-D Edge Plasma, Disruptions, and Radiative Processes. Final Report

Description: The scientific goal of the UCSD-DIII-D Collaboration during this period was to understand the coupling of the core plasma to the plasma-facing components through the plasma boundary (edge and scrape-off layer). To achieve this goal, UCSD scientists studied the transport of particles, momentum, energy, and radiation from the plasma core to the plasma-facing components under normal (e.g., L-mode, H-mode, and ELMs), and off-normal (e.g., disruptions) operating conditions.
Date: January 1, 2001
Creator: Boedo, J. A.; Luckhardt, S.C. & Moyer, R. A.
Partner: UNT Libraries Government Documents Department

Target Plate Conditions During Stochastic Boundary Operation on DIII-D

Description: A major concern for large tokamaks like ITER is the presence of edge localized modes (ELMs) that repeatedly send large bursts of particles and heat into the divertor plates. Operation with resonant magnetic perturbations (RMP) at the boundary of DIII=D has suppressed ELMs for values of q95 {approx} 3.7. At the target plate, the conditions during ELM suppressed operation for both high and low collisionality are observed by a set of radially distributed Langmuir probes. At high collisionality (n*{approx}1), the target plate particle flux and temperature drops by > 30% during ELM suppression. At low collisionality (n*{approx}0.1), the core density, target plate density, and target plate particle flux drop but the plate electron temperature increases after the ELMs are suppressed. The ELM-suppressed target plate heat flux is nearly the same as the heat flux between ELMs but the (5X higher) transient heat flux peaks due to ELMs are eliminated.
Date: May 15, 2006
Creator: Watkins, J; Evans, T; Moyer, R; Lasnier, C & Rudakov, D
Partner: UNT Libraries Government Documents Department

Study of the phase transition dynamics of the L to H transition

Description: A highly radiating zone (MARFE) just above the divertor X-point has been used to access the marginal transition regime P{sub sep} {approx} P{sub thres} to study the existence of a critical point for the L to H transition. Phase transition models predict that at the critical point, the transition duration increases and the plasma parameters vary continuously between L-mode and H-mode. In these experiments, the L to H transition duration increased 50--100 times over fast transitions. However, the evolution of E{sub r} shear, edge density gradient, H-mode pedestal, and fluctuations is essentially unchanged from that in fast transitions. The only difference is in the speed with which and the degree to which the fluctuation amplitudes are transiently reduced. This difference is understandable in terms of the time scales for fluctuation amplitude reduction ({le} 100 {micro}s) and edge pressure gradient increase (several ms), provided the edge fluctuations are pressure-gradient driven.
Date: September 1, 1997
Creator: Moyer, R. A.; Rhodes, T. L. & Rettig, C. L.
Partner: UNT Libraries Government Documents Department

A radial transmission line material measurement apparatus

Description: A radial transmission line material measurement sample apparatus (sample holder, offset short standards, measurement software, and instrumentation) is described which has been proposed, analyzed, designed, constructed, and tested. The purpose of the apparatus is to obtain accurate surface impedance measurements of lossy, possibly anisotropic, samples at low and intermediate frequencies (vhf and low uhf). The samples typically take the form of sections of the material coatings on conducting objects. Such measurements thus provide the key input data for predictive numerical scattering codes. Prediction of the sample surface impedance from the coaxial input impedance measurement is carried out by two techniques. The first is an analytical model for the coaxial-to-radial transmission line junction. The second is an empirical determination of the bilinear transformation model of the junction by the measurement of three full standards. The standards take the form of three offset shorts (and an additional lossy Salisbury load), which have also been constructed. The accuracy achievable with the device appears to be near one percent.
Date: May 1, 1993
Creator: Warne, L. K.; Moyer, R. D.; Koontz, T. E. & Morris, M. E.
Partner: UNT Libraries Government Documents Department

Enhanced scrape-off layer plasma in DIII-D double-null discharges

Description: In this paper, the authors examine a denser and broader scrape-off layer (SOL) plasma, first seen in VH mode, in the DIII-D tokamak. The enhanced SOL appears in many types of double-null (DN) discharges and is not a property of VH-mode only. The DN enhanced SOL density and temperature profiles exhibit a 5--6 cm broad profile outside the separatrix. For DN and single-null (SN) boundary geometry with similar core plasma conditions, the enhanced SOL is only observed in high triangularity discharges. The origin of the enhanced SOL is, however, not yet understood.
Date: July 1, 1994
Creator: Watkins, J. G.; Jong, R. A. & Moyer, R. A.
Partner: UNT Libraries Government Documents Department

Calculation of the Thermal Footprint of Resonant Magnetic Perturbations in DIII-D

Description: The effect of resonant magnetic perturbations on heat transport in DIII-D H-mode plasmas has been calculated by combining the TRIP3D field-line tracing code with the E3D two-fluid transport code. Simulations show that the divertor heat flux distribution becomes non-axisymmetric because heat flux is efficiently guided to the divertor along the three-dimensional invariant manifolds of the magnetic field. Calculations demonstrate that heat flux is spread over a wider area of the divertor target, thereby reducing the peak heat flux delivered during steady-state operation. Filtered optical cameras have observed non-axisymmetric particle fluxes at the strike-point and Langmuir probes have observed non-axisymmetric floating potentials. On the other hand, the predicted magnitude of stochastic thermal transport is too large to match the pedestal plasma profiles measured by Thomson scattering and charge exchange recombination spectroscopy. The Braginskii thermal conductivity overestimates the expected heat transport in the pedestal because the mean free path is longer than estimates of the parallel thermal correlation length, and collisionless transport models are probably required for accurate description. However, even the collisionless estimates for electron thermal transport are too large by one to two orders of magnitude. Thus, it is likely that another mechanism such as rotational screening of resonant perturbations limits the stochastic region and reduces transport inside of the pedestal.
Date: September 14, 2007
Creator: Joseph, I; Evans, T; Moyer, R; Fenstermacher, M; Groth, M; Kasilov, S et al.
Partner: UNT Libraries Government Documents Department

Stochastic Transport Modeling of Resonant Magnetic Perturbations in DIII-D

Description: Three-dimensional two-fluid simulations of heat transport due to resonant magnetic perturbations of tokamaks have been computed by coupling the TRIP3D field line tracing code to the E3D edge transport code. The predicted electron temperature contours follow the new separatrix represented by the perturbed invariant manifold structure of the X-point in qualitative agreement with X-point TV observations. However, preliminary modeling predicts that the resulting stochastic heat transport is greater than that measured in low-collisionality ELM suppression experiments in DIII-D H-mode plasmas. While improved determination of transport coefficients is definitely required, possible explanations include plasma screening of resonant perturbations, invalid treatment of the edge as a fluid, or insufficient understanding of stochastic heat transport.
Date: June 1, 2006
Creator: Joseph, I; Moyer, R A; Evans, T E; Schaffer, M J; Runov, A M; Schneider, R et al.
Partner: UNT Libraries Government Documents Department

Comparison of Edge Plasma Perturbation During ELM Control Using One vs Two Toroidal Rows of RMP Coils in ITER Similar Shaped Plasmas on DIII-D

Description: Large Type-I edge localized modes (ELMs) were suppressed by n = 3 resonant magnetic perturbations (RMPs) from a set of internal coils (I-coil) in plasmas with an ITER similar shape at the ITER pedestal collisionality, {nu}*{sub e} {approx} 0.1 and low edge safety factor (q{sub 95} {approx} 3.6), with either a single toroidal row of the internal RMP coils or two poloidally separated rows of coils. ELM suppression with a single row of internal coils was achieved at approximately the same q{sub 95} surface-averaged perturbation field as with two rows of coils, but required higher current per coil. Maintaining complete suppression of ELMs using n = 3 RMPs from a single toroidal row of internal coils was less robust to variations in input neutral beam injection torque than previous ELM suppression cases using both rows of internal coils. With either configuration of RMP coils, maximum ELM size is correlated with the width of the edge region having good overlap of the magnetic islands from vacuum field calculations.
Date: May 21, 2008
Creator: Fenstermacher, M E; Evans, T E; Osborne, T H; Schaffer, M J; deGrassie, J S; Gohil, P et al.
Partner: UNT Libraries Government Documents Department

Reciprocating and fixed probe measurements of n{sub e} and T{sub e} in the DIII-D divertor

Description: This paper describes divertor density and temperature measurements using both a new reciprocating Langmuir probe (XPT-RCP) which plunges vertically above the divertor floor up to the X-point height and swept, single, Langmuir probes fixed horizontally across the divertor floor. These types of measurements are important for testing models of the SOL and divertor which then are used to design plasma facing components in reactor size tokamaks. This paper presents an overview of the new divertor probe measurements and how they compare with the new divertor Thomson scattering system. The fast time response of the probe measurements allows detailed study of ELMs.
Date: November 1, 1996
Creator: Watkins, J.G.; Moyer, R.A.; Cuthbertson, J.W.; Buchenauer, D.A.; Carlstrom, T.N.; Hill, D.N. et al.
Partner: UNT Libraries Government Documents Department

Plasma pressure and flows during divertor detachment

Description: MHD theory applied to tokamak plasma scrape-off layer (SOL) equilibria requires Pfirsch-Schlueter current, which, because the magnetic lines are open, normally closes through electrically conducting divertor or limiter components. During detached divertor operation the Pfirsch-Schlueter current path to the divertor target is sometimes blocked, in which case theory predicts that the plasma develops a poloidal pressure gradient around the upstream SOL and a corresponding parallel flow, in order to satisfy all the conditions of MHD equilibrium. This paper reports the only known examples of detached diverted plasma in the DIII-D tokamak with blocked Pfirsch-Schlueter current, and they show no clear SOL poloidal pressure differences. However, the predicted pressure differences are small, near the limit of detectability with the available diagnostics. In the more usual DIII-D partially detached divertor operation mode, the Pfirsch-Schlueter current appears to never be blocked, and no unusual poloidal pressure differences are observed, as expected. Finally, a local overpressure is observed just inside the magnetic separatrix near the X-point in both attached and detached Ohmically heated plasmas.
Date: August 1, 1998
Creator: Schaffer, M.J.; Brooks, N.H.; Boedo, J.A.; Isler, R.C. & Moyer, R.A.
Partner: UNT Libraries Government Documents Department

Reactor operation environmental information document

Description: This volume is a reactor operation environmental information document for the Savannah River Plant. Topics include meteorology, surface hydrology, transport, environmental impacts, and radiation effects. 48 figs., 56 tabs. (KD)
Date: December 1, 1989
Creator: Bauer, L.R.; Hayes, D.W.; Hunter, C.H.; Marter, W.L. & Moyer, R.A.
Partner: UNT Libraries Government Documents Department

Studies of Impurity Assimilation During Massive Argon Gas Injection in DIII-D

Description: Fast shutdown of discharges using massive gas injection (MGI) is a promising technique for reducing tokamak wall damage during disruptions [1]. An outstanding concern, however, is the generation of runaway electrons (RE) during the shutdown. Although RE formation observed during MGI in present-day experiments is quite small (typically <1% of the main plasma current I{sub p} in DIII-D), it is thought that even this small RE current could be amplified to significant levels in reactor-scale tokamaks such as ITER [2]. It is expected that complete collisional suppression of any potential RE amplification during the CQ can be achieved for suppression parameters {gamma}{sub crit} {triple_bond} E{sub crit}/E{sub {psi}} > 1, where E{sub crit} = [2{pi}e{sup 3}ln{Lambda}(2n{sub e} + n{sub B})]/mc{sup 2} is the critical electric field [2] and E{sub {psi}} {approx} -[({mu}{sub 0}l{sub i})/4{pi}][-({partial_derivative}I{sub p}/{partial_derivative}t)+ {alpha}{sub L}(I{sub W}/{tau}{sub W})] is the toroidal electric field resulting from the decay of the plasma current I{sub p}. n{sub e} is the free electron density, n{sub B} is the bound electron density, {alpha}{sub L} {approx} 2[ln(8R/r{sub w})-2]/l{sub i} is the ratio of external (outside conducting wall) to internal (inside conducting wall) self-inductance, I{sub w} is the wall current, and {tau}{sub w} is the wall time. The densities required to achieve {gamma}{sub crit} > 1 are typically quite large, e.g. n{sub tot} {triple_bond} n{sub e} + n{sub B}/2 {approx} 10{sup 16} cm{sup -3} for DIII-D. To have a possibility of achieving the required density in the DIII-D plasma (with volume V{sub p} {approx} 20 m{sup 3}), an MGI system using argon must be able to deliver of order 10{sup 22} argon atoms to the plasma within the shutdown timescale of about 10 ms.
Date: June 27, 2007
Creator: Hollmann, E; Jernigan, T; Parks, P; Baylor, L; Boedo, J; Combs, S et al.
Partner: UNT Libraries Government Documents Department

DIII-D Studies of Massive Gas Injection Fast Shutdowns for Disruption Mitigation

Description: Injection of massive quantities of gas is a promising technique for fast shutdown of ITER for the purpose of avoiding divertor and first wall damage from disruptions. Previous experiments using massive gas injection (MGI) to terminate discharges in the DIII-D tokamak have demonstrated rapid shutdown with reduced wall heating and halo currents (relative to natural disruptions) and with very small runaway electron (RE) generation [1]. Figure 1 shows time traces which give an overview of shutdown time scales. Typically, of order 5 x 10{sup 22} Ar neutrals are fired over a pulse of 25 ms duration into stationary (non-disrupting) discharges. The observed results are consistent with the following scenario: within several ms of the jet trigger, sufficient Ar neutrals are delivered to the plasma to cause the edge temperature to collapse, initiating the inward propagation of a cold front. The exit flow of the jet [Fig. 1(a)] has a {approx} 9 ms rise time; so the quantity of neutrals which initiates the edge collapse is small (<10{sup 20}). When the cold front reaches q {approx} 2 surface, global magnetohydrodynamic (MHD) modes are destabilized [2], mixing hot core plasma with edge impurities. Here, q is the safety factor. Most (>90%) of the plasma thermal energy is lost via impurity radiation during this thermal quench (TQ) phase. Conducted heat loads to the wall are low because of the cold edge temperature. After the TQ, the plasma is very cold (of order several eV), so conducted wall (halo) currents are low, even if the current channel contacts the wall. The plasma current profile broadens and begins decaying resistively. The decaying current generates a toroidal electric field which can accelerate REs; however, RE beam formation appears to be limited in MGI shutdowns. Presently, it is thought that the conducted heat flux and halo current ...
Date: June 19, 2006
Creator: Hollmann, E; Jernigan, T; Antar, G; Bakhtiari, M; Boedo, J; Combs, S et al.
Partner: UNT Libraries Government Documents Department

DIII-D Studies of Massive Gas Injection Fast Shutdowns for Disruption Mitigation

Description: Injection of massive quantities of gas is a promising technique for fast shutdown of ITER for the purpose of avoiding divertor and first wall damage from disruptions. Previous experiments using massive gas injection (MGI) to terminate discharges in the DIII-D tokamak have demonstrated rapid shutdown with reduced wall heating and halo currents (relative to natural disruptions) and with very small runaway electron (RE) generation [1]. Figure 1 shows time traces which give an overview of shutdown time scales. Typically, of order 5 x 10{sup 22} Ar neutrals are fired over a pulse of 25 ms duration into stationary (non-disrupting) discharges. The observed results are consistent with the following scenario: within several ms of the jet trigger, sufficient Ar neutrals are delivered to the plasma to cause the edge temperature to collapse, initiating the inward propagation of a cold front. The exit flow of the jet [Fig. 1(a)] has a {approx} 9 ms rise time; so the quantity of neutrals which initiates the edge collapse is small (<10{sup 20}). When the cold front reaches q {approx} 2 surface, global magnetohydrodynamic (MHD) modes are destabilized [2], mixing hot core plasma with edge impurities. Here, q is the safety factor. Most (>90%) of the plasma thermal energy is lost via impurity radiation during this thermal quench (TQ) phase. Conducted heat loads to the wall are low because of the cold edge temperature. After the TQ, the plasma is very cold (of order several eV), so conducted wall (halo) currents are low, even if the current channel contacts the wall. The plasma current profile broadens and begins decaying resistively. The decaying current generates a toroidal electric field which can accelerate REs; however, RE beam formation appears to be limited in MGI shutdowns. Presently, it is thought that the conducted heat flux and halo current ...
Date: September 29, 2006
Creator: Hollmann, E; Jernigan, T; Antar, G; Bakhtiari, M; Boedo, J; Combs, S et al.
Partner: UNT Libraries Government Documents Department

Edge Stability and Transport Control with Resonant Magnetic Perturbations in Collisionless Tokamak Plasmas

Description: A critical issue for fusion plasma research is the erosion of the first wall of the experimental device due to impulsive heating from repetitive edge magneto-hydrodynamic (MHD) instabilities known as 'edge-localized modes' (ELMs). Here, we show that the addition of small resonant magnetic field perturbations completely eliminates ELMs while maintaining a steady-state high-confinement (H-mode) plasma. These perturbations induce a chaotic behavior in the magnetic field lines, which reduces the edge pressure gradient below the ELM instability threshold. The pressure gradient reduction results from a reduction in particle content of the plasma, rather than an increase in the electron thermal transport. This is inconsistent with the predictions of stochastic electron heat transport theory. These results provide a first experimental test of stochastic transport theory in a highly rotating, hot, collisionless plasma and demonstrate a promising solution to the critical issue of controlling edge instabilities in fusion plasma devices.
Date: June 13, 2006
Creator: Evans, T E; Moyer, R A; Burrell, K H; Fenstermacher, M E; Joseph, I; Leonard, A W et al.
Partner: UNT Libraries Government Documents Department