Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session TO3: PMI, Divertor, and Boundary Physics |
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Chair: Angela Capece, Princeton Plasma Physics Laboratory Room: Salon D |
Thursday, October 30, 2014 9:30AM - 9:42AM |
TO3.00001: Microinstabilities in the pedestal region David Dickinson, Benjamin Dudson, Howard Wilson, Colin Roach The regulation of transport at the pedestal top is important for the inter-ELM pedestal dynamics. Linear gyrokinetic analysis of the pedestal region during an ELM cycle on MAST has shown kinetic ballooning modes to be unstable at the knee of the pressure profile and in the steep pedestal region whilst microtearing modes (MTMs) dominate in the shallow gradient region inboard of the pedestal top. The transition between these instabilities at the pedestal knee has been observed in low and high collisionality MAST pedestals, and is likely to play an important role in the broadening of the pedestal. Nonlinear simulations are needed in this region to understand the microturbulence, the corresponding transport fluxes, and to gain further insight into the processes underlying the pedestal evolution. Such gyrokinetic simulations are numerically challenging and recent upgrades to the GS2 gyrokinetic code help improve their feasibility. We are also exploring reduced models that capture the relevant physics using the plasma simulation framework BOUT$++$. An electromagnetic gyrofluid model has recently been implemented with BOUT$++$ that has significantly reduced computational cost compared to the gyrokinetic simulations against which it will be benchmarked. [Preview Abstract] |
Thursday, October 30, 2014 9:42AM - 9:54AM |
TO3.00002: ABSTRACT WITHDRAWN |
Thursday, October 30, 2014 9:54AM - 10:06AM |
TO3.00003: Modelling of turbulence in X-point configurations with BOUT++ Brendan Shanahan, Ben Dudson, Peter Hill Simulations of instabilities and turbulence in X-point configurations are challenging due to the limitations of field-aligned coordinate systems: X-point dynamics are often extrapolated based on nearby flux surfaces, which could exclude relevant physics. The Flux Coordinate Independent (FCI) approach developed by Hariri et al. overcomes these limitations, and has been implemented in the BOUT++ framework. Here it is used to study turbulence in linear devices with X-point magnetic configurations. We compare the performance and accuracy of the FCI approach with other schemes, and present results of 3D drift-wave turbulence and flows in X-point configurations using two cold ion fluid models. Simulations have been performed to explore the feasibility of experimentally studying X-point configurations in linear plasma devices which indicate the effect of an externally applied X-point field. Preliminary studies of blobs and turbulence in toroidal geometries with X-points will be presented and the implications for tokamaks will be discussed. [Preview Abstract] |
Thursday, October 30, 2014 10:06AM - 10:18AM |
TO3.00004: Dynamic plasma-wall modeling of ELMy H-mode plasmas A. Pigarov, S. Krasheninnikov, E. Hollmann, T. Rognlien, E. Unterberg, C. Lasnier We discuss UEDGE-MB-W version of the 2-D transport code, which incorporates the Macro-Blob (MB) approach to simulate ELM plasma dynamics and various dynamic models for hydrogen inventory in the first wall (W). Results of time-dependent coupled simulations in various sequences of type-I ELMs with UEDGE-MB-W are presented. The temporal evolution of deuterium inventories of the pedestal plasma and wall and the calculated rates of particle deposition into wall during ELMs and of wall outgassing between ELMs are in agreement with experimental data on graphite-wall tokamaks. The fraction of pedestal particle losses deposited into the wall during ELMs is studied found to vary from 20 to 80{\%} depending on the sizes and frequencies of ELMs. Modeling results for discharge exhibiting the transition from small to giant type-I ELMs due to NBI decrease are analyzed, demonstrating the dominant role of wall outgassing in pedestal density built-up. Dynamic deposition/release equilibrium attained in the saturated wall in a sequence of ELMs and the roles of different PSI processes in generating gas release are analyzed. The role of transient events in formation of detached divertor plasma is studied. The dynamics of such plasma is modeled and ionization/recombination/radiation rates are compared to experimental data. [Preview Abstract] |
Thursday, October 30, 2014 10:18AM - 10:30AM |
TO3.00005: The role of divertor collisionality on SOL transport: a study on the ITER stepladder D. Carralero, H.W. M\"uller, M. Groth, M. Komm, J. Adamek, G. Birkenmeier, M. Brix, F. Jankyc, P. Hacek, S. Marsen, F. Reymold, C. Silva, U. Stroth, M. Wischmeier, E. Wolfrum The L-mode SOL density transition is experimentally investigated in three tokamaks of the ITER stepladder (COMPASS, AUG and JET). The results confirm the relevance of divertor detachment in the SOL profile transition: Both AUG and JET show significant changes in the outer midplane density profiles and increase in the size and velocity of filaments at the same densities in which the LFS divertor surpasses its ion saturation roll-over. Meanwhile, the COMPASS divertor remains attached and no clear signs of transition are observed in the midplane. Furthermore, according to a filament model by Myra et al., only the changes of collisionality in the divertor are sufficient to trigger the transition in both cases. An extrapolation from these results indicates that this broadening is to be expected in ITER despite having a collisionless SOL in the main chamber, since the disconnection of filaments will take place in the partially detached divertor. [Preview Abstract] |
Thursday, October 30, 2014 10:30AM - 10:42AM |
TO3.00006: Tungsten and molybdenum transport studies in the TEXTOR tokamak Armin Weckmann, Per Petersson, Petter Stroem, Marek Rubel, Jan Coenen, Arkadi Kreter, Peter Wienhold Understanding the transport of high-Z metals in tokamaks is important for magnetic controlled fusion devices. Two separate experiments aiming at the determination of tungsten and molybdenum migration were performed in the TEXTOR tokamak by means of controlled injection of volatile compounds:WF$_{6}$ and MoF$_{6}$. Nitrogen-15 tracer was also seeded for plasma edge cooling. Spectroscopy measurements were done for edge and core plasma. The experiments done on the last day before the shut-downs were followed by comprehensive analyses of plasma facing components (PFC), test limiters and collector probes. Surface studies with ion and electron spectroscopy techniques enabled mapping of the metal content both in toroidal and poloidal position in the whole torus. The material balance and the global and local deposition patterns were determined. The results explain gradual migration of high-Z metals by prompt re-deposition and re-erosion processes. The differences between W and Mo will be addressed. Data on He and N-15 retention will also be presented. [Preview Abstract] |
Thursday, October 30, 2014 10:42AM - 10:54AM |
TO3.00007: Fluence Resolved Growth of Nanostructured Tungsten ``Fuzz'' Peter Fiflis, Davide Curreli, David Ruzic Growth of nanostructures on the surface of tungsten plasma facing components is anticipated in the next generation of experimental fusion reactors. Determining the mechanisms underlying tungsten fuzz growth is an important step towards mitigation of fuzz formation. Nanostructured tungsten was produced on ohmically heated tungsten wires in a helicon plasma source (maximum flux of 2.5e21 m$^{-2}$s$^{-1}$), asymmetry in the setup allows for investigation of temperature and flux effects in a single sample. An effort at elucidating the mechanism of formation was made by inspecting SEM micrographs of the nanostructured tungsten at successive fluence steps of helium ions up to 1e27 m$^{-2}$. To create these micrographs a single tungsten sample was exposed to the plasma, removed and inspected with an SEM, and replaced into the plasma. The tungsten surface was marked in several locations so that each micrograph is centered within 200 nm of each previous micrograph. Pitting of the surface (dia. 20 $\pm$ 10 nm, fl. 5$+$/-2e25 m$^{-2}$) followed by surface roughening (9$+$/-2e25 m$^{-2}$) and finally tendril formation (30$+$/-10 nm, 2$+$/-1e26 m$^{-2}$) is observed. [Preview Abstract] |
Thursday, October 30, 2014 10:54AM - 11:06AM |
TO3.00008: Flux threshold of He-ion-beam induced nano-fuzz growth on hot tungsten below and above the displacement damage threshold energy Hussein Hijazi, Mark E. Bannister, Chad M. Parish, Harry M. Meyer III, Fred W. Meyer Measurements of nano-fuzz growth on linear plasma devices have shown that below the displacement damage energy threshold, a minimum He-ion flux is required for nano-fuzz formation. We report comparative measurements of nano-fuzz flux thresholds below and above the displacement damage energy threshold using well characterized He ion beams at the ORNL MIRF. He-ion-beam flux distributions were optimized and measured at 218 and 2000 eV prior to ion beam impact on W coupons heated to about 1000 deg. C. After exposure times ranging from 4200 to 7200 seconds, the beam spots were examined by SEM over a 0.5 mm x 0.5 mm grid, which was spatially correlated to the measured flux distributions. In this manner, we were able to obtain, in a single ion beam exposure, the flux dependence of the observed surface morphology changes at each of the two energies. At 218 eV, for fluxes below 1.5x10$^{16}$/cm$^{2}$s, ordered surface structures are observed, with great grain-to-grain variability, together with blisters and pinholes, while above this flux value, individual grain characteristics disappear, and nano-fuzz growth is observed. At 2 keV, very similar surface morphologies are observed, but the flux threshold for nano-fuzz formation has almost doubled, to 2.5 - 3x10$^{16}$/cm$^{2}$s. Possible reasons for this increase will be discussed. [Preview Abstract] |
Thursday, October 30, 2014 11:06AM - 11:18AM |
TO3.00009: Creation of deuterium protective layer below the tungsten surface Predrag Krstic, Igor Kaganovich, Edward Startsev By cumulative irradiation of both pre-damaged and virgin surfaces of monocrystal tungsten by deuterium atoms of impact energy of few tens of eV, we simulate by classical molecular dynamics the creation of a deuterium protective layer. The depth and width of the layer depend on the deuterium impact energy and the diffusion rate of deuterium in tungsten, the latter being influenced by the tungsten temperature and damage. Found simulation results are in concert with the experimental results, found recently in DIFFER [1]. \\[4pt] [1] M.H.J. 't Hoen~et al~2013 , Phys. Rev. Lett.~111, 225001 (2013) [Preview Abstract] |
Thursday, October 30, 2014 11:18AM - 11:30AM |
TO3.00010: Simulation of tungsten dust impact on ITER-like plasma edge R.D. Smirnov, S.I. Krasheninnikov, A.Yu. Pigarov, T.D. Rognlien The large stationary or intermittent particle and heat fluxes in future fusion devices, such as ITER, can damage plasma-facing components leading to production of metallic dust and droplets. Transport and ablation of such dust present an important mechanism of impurity contamination of fusion plasmas. We investigate impact of tungsten dust on ITER-like edge plasmas using the DUSTT/UEDGE code. Different scenarios of tungsten dust injection in ITER divertor and upstream plasmas are modeled. It is demonstrated that injection of the dust with rates as low as a few mg/s can already cause unacceptably high relative concentrations of tungsten impurities in the core-edge, which can limit ITER operational regimes to the point of ignition inaccessibility. Larger rates of tungsten dust injection, of order of a few 10mg/s, are shown to lead to divertor plasma thermal instability and discharge termination. It is also found that accumulation of high-Z impurities in plasma edge produces large scale plasma oscillations causing undesirable periodic variations of divertor heat load. Implications of the obtained results on ITER dust production limits are discussed. [Preview Abstract] |
Thursday, October 30, 2014 11:30AM - 11:42AM |
TO3.00011: Lithium-Metal Infused Trenches: Progress toward a Divertor Solution D.N. Ruzic, P. Fiflis, M. Christenson, M. Szott, W. Xu, S. Jung, T.W. Morgan, K. Kalathiparambil The application of liquid metal, especially liquid lithium, as a plasma facing component (PFC) has the capacity to offer a strong alternative to solid PFCs by reducing damage concerns and enhancing plasma performance. The Liquid-Metal Infused Trenches (LiMIT) concept is a liquid metal divertor alternative which employs thermoelectric current from either plasma or external heating in tandem with the toroidal field to self-propel liquid lithium through a series of trenches. LiMIT has been tested in several devices, namely HT-7, the UIUC SLiDE and TELS facilities and Magnum PSI at heat fluxes of up to 3 MW/m$^{-2}$. Results of these experiments, including velocity and temperature measurements, power handling considerations, and preliminary vapor shielding results will be discussed, focusing on the 117 shots performed at Magnum scanning magnetic fields and heat fluxes up to $\sim$ 0.3 T and 3 MW/m$^{-2}$. Concerns over tritium retention and MHD droplet ejection will additionally be addressed. LiMIT has also been proposed to function as a limiter on the EAST moveable limiter arm and tests have been performed with a prototype module inclined at various angles. [Preview Abstract] |
Thursday, October 30, 2014 11:42AM - 11:54AM |
TO3.00012: Effect of Energetic Plasma Flux on Flowing Liquid Lithium Surfaces Kishor Kalathiparambil, Soonwook Jung, Michael Christenson, Peter Fiflis, Wenyu Xu, Mathew Szott, David Ruzic An operational liquid lithium system with steady state flow driven by thermo-electric magneto-hydrodynamic force and capable of constantly refreshing the plasma exposed surface have been demonstrated at U of I [1]. To evaluate the system performance in reactor relevant conditions, specifically to understand the effect of disruptive plasma events on the performance of the liquid metal PFCs, the setup was integrated to a pulsed plasma generator. A coaxial plasma generator drives the plasma towards a theta pinch which preferentially heats the ions, simulating ELM like flux, and the plasma is further guided towards the target chamber which houses the flowing lithium system. The effect of the incident flux is examined using diagnostic tools including triple Langmuir probe, calorimeter, rogowski coils, Ion energy analyzers, and fast frame spectral image acquisition with specific optical filters. The plasma have been well characterized and a density of $\sim$ 10$^{21}$ m$^{-3}$, with electron temperature $\sim$ 10 -- 20 eV is measured, and final plasma velocities of 34 -- 74 kms$^{-1}$ have been observed. Calorimetric measurements using planar molybdenum targets indicate a maximum plasma energy (with 6 kV plasma gun and 20 kV theta pinch) of 0.08 MJm$^{-2}$ with plasma divergence effects resulting in marginal reduction of 40 $\pm$ 23 J in plasma energy. Further results from the other diagnostic tools, using the flowing lithium targets and the planar targets coated with lithium will be presented. [1] D. N. Ruzic et. al, Nuc. Fusion \textbf{52} 102002 (2011) [Preview Abstract] |
Thursday, October 30, 2014 11:54AM - 12:06PM |
TO3.00013: HIDRA: A new device for PFC and PMI development Daniel Andruczyk, David N. Ruzic, Jean Paul Allain, Davide Curreli A toroidal plasma device is being constructed at the University of Illinois dedicated in part as a toroidal liquid-metal PFC technology test bench. The Hybrid Illinois stellarator/tokamak Device for Research and Applications (HIDRA) is a medium sized classical stellarator (previously WEGA, IPP Greifswald) with, R $=$ 0.72 m, a $=$ 0.19 m, B \textless 0.5 T and will be able to reach T$_{\mathrm{e}} = $ 10-50 eV, n$_{\mathrm{e}} = $ 10$^{17}$-10$^{18}$ m$^{-3}$ with plasmas running up to several minutes. A critical knowledge gap for liquid-metal PFCs is their integration and performance in asymmetric confinement fusion environments. HIDRA will be used to evaluate technologies such as TEMHD driven flows for the first wall, help address key questions including whether a full toroidal liquid-metal loop can operate in a toroidal machine, test low recycling regimes and whether D can be removed and recycled easily. Also, UIUC's experience with in-situ diagnostics will open up new opportunities for innovative Material Application Testing (HIDRA-MAT). [Preview Abstract] |
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