Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session UO4: Scrape-Off Layer & Plasma-Material Interaction |
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Chair: Brian Nelson, University of Washington Room: 230 A |
Thursday, November 3, 2016 2:00PM - 2:12PM |
UO4.00001: The effects of the Snowflake Divertor on upstream SOL profiles. C.K. Tsui, J.A. Boedo, S. Coda, B. Labit, R. Maurizio, F. Nespoli, H. Reimerdes, C. Theiler, M. Spolaore, N. Vianello, T. Lunt, W.A.J. Vijvers, N. Walkden The Snowflake Divertor creates separated volumes within the SOL and divertor that feature strikingly different ne, Te profiles, and decay lengths, as measured with a scanning probe. Profiles were taken at the outer midplane of TCV plasmas with snowflake divertors as well as just above the X-points within the region of enhanced $\beta $pol. Density shoulders in the far SOL in single null plasmas are relaxed by secondary X-points, while effects are more complex in the near SOL. These changes were observed whether the secondary X-point was placed in the low field side SOL, or in the high field side SOL. Additionally, target profiles measured with IR camera and Langmiur probes that were taken in the divertor leg opposite the secondary X-point also show features on the flux surface corresponding to the secondary X-point. Fluctuation statistics from the reciprocating probe as well as comparisons made between upstream and downstream measurements are considered for their implications on SOL transport. [Preview Abstract] |
Thursday, November 3, 2016 2:12PM - 2:24PM |
UO4.00002: Level crossings and excess times of intermittent fluctuations in scrape-off layer plasmas Audun Theodorsen, Odd Erik Garcia Transient transport events associated with the radial motion of filamentary structures in the scrape-off layer (SOL) may cause significant heat load on the plasma facing components. It is therefore of interest to describe the statistical properties of intermittent plasma fluctuations in the SOL. Investigations of SOL plasmas in the Alcator C-Mod and TCV tokamaks show that the plasma fluctuations are well described by a stochastic model consisting of uncorrelated pulses arriving according to a Poisson process. The pulse have a fixed, exponential shape and exponentially distributed amplitudes. Here, we extend the stochastic model by deriving the joint probability density function of the signal and its time derivative. This result is used to find the rate of positive level crossings over and the mean time above a prescribed signal threshold. Previously, discrepancies between predictions of excess time statistics based on the assumption of normally distributed signals and measurement data have been interpreted as a sign of intermittency. The model presented here quantifies the level of intermittency and only requires parameters obtained from the lowest moments and the correlation function of the signal itself. [Preview Abstract] |
Thursday, November 3, 2016 2:24PM - 2:36PM |
UO4.00003: SOL density profile formation and intermittent ion fluxes to the first wall in JET Nicholas Walkden, F Militello, G Matthews, J Harrison, D Moulton, A Wynn, B Lipschultz, C Guillemaut The ion flux in the scrape-off layer (SOL) of a tokamak is highly non-diffusive due to the radial propagation of intermittent burst events known as filaments. As a result the formation of mean profiles in the SOL and the flux incident on the outer wall are strongly impacted by transient events. This has been investigated over a series of pulses in an Ohmic L-mode horizontal target configuration in JET. Broadening of the SOL density profile is reduced as plasma current is increased or the density is decreased. The mean and variance of the ion flux at the outer wall change concurrently with this broadening. Upon renormalization the PDFs of the ion flux at the outer-wall collapse indicating universality in the dynamics of their constituent fluctuations. This universality is shown to result from a balance between the duration and frequency of burst events which keeps the intermittency parameter constant. These measurements will be compared to synthetically produced measurements created using a stochastic framework based on filamentary dynamics. Through this comparison possible models of filamentary dynamics will be assessed and compared quantitatively to gain an understanding of the processes underlying density profile formation and fluxes to the outer wall of JET. [Preview Abstract] |
Thursday, November 3, 2016 2:36PM - 2:48PM |
UO4.00004: ABSTRACT WITHDRAWN |
Thursday, November 3, 2016 2:48PM - 3:00PM |
UO4.00005: Extinguishing ELMs in detached radiative divertor plasmas Alexander Pigarov, Sergei Krasheninnikov, Thomas Rognlien In order to avoid deleterious effects of ELMs on PFCs in next-step fusion devices it has been suggested to operate with small-sized ELMs naturally extinguishing in the divertor. Our modeling effort is focusing at extinguishing type-I ELMs: conditions for expelled plasma dissipation; efficiency of ELM power handling by detached radiative divertors; and the ELM impact on detachment state. Here time-dependent modeling of a sequence of many ELMs was performed with 2-D edge plasma transport code UEDGE-MB-W which incorporates the Macro-Blob (MB) approach to simulate non-diffusive filamentary transport and various ``Wall'' (W) models for time-dependent hydrogen wall inventory and recycling. Three cases were modeled, in which extinguishing ELMs are achieved due to: (i)intrinsic impurities via graphite sputtering, (ii)extrinsic impurity gas puff (Ne), and (iii)$=$(i)$+$(ii). For each case, we performed a series of UEDGE-MB-W runs scanning the deuterium and impurity inventories, pedestal losses and ELM frequency. Temporal variations of the degree of detachment, ionization front shape, recombination sink strength, radiated fraction, peak power loads, OSP, impurity charge states, and in/out asymmetries were analyzed. We discuss the onset of extinguishing ELMs, conditions for not burning through and enhanced plasma recombination as functions of scanned parameters. Efficiencies of intrinsic and extrinsic impurities in ELM extinguishing are compared. [Preview Abstract] |
Thursday, November 3, 2016 3:00PM - 3:12PM |
UO4.00006: Developing the Science and Technology for the Material Plasma Exposure eXperiment (MPEX) Juergen Rapp, Theodore Biewer, Timothy Bigelow, John Caughman, Richard Goulding, Arnold Lumsdaine The Material Plasma Exposure eXperiment (MPEX) is a device planned to address scientific and technological gaps for the development of viable plasma facing components for fusion reactor conditions (FNSF, DEMO). MPEX is designed to deliver those plasma conditions with a novel Radio Frequency plasma source able to produce high density plasmas and heat electron and ions separately with Electron Bernstein Wave (EBW) heating and Ion Cyclotron Resonance Heating (ICRH) with a total installed power of 800 kW. The science and technology for this source system is currently being tested on Proto-MPEX. This is a linear device utilizing 12 water-cooled copper coils able to achieve peak magnetic fields of 1.6T. The currently total installed heating power (for helicon, EBW and ICRH) is 330kW. An overview of the status of this development program is given with an outlook to the next steps. [Preview Abstract] |
Thursday, November 3, 2016 3:12PM - 3:24PM |
UO4.00007: Repetitive tabletop plasma focus to produce a tunable damage factor on materials for fusion reactors Leopoldo Soto, Cristian Pavez, Maria Jose Inestrosa-Izurieta, Jose Moreno, Sergio Davis, Biswajit Bora, Gonzalo Avaria, Jalaj Jain, Luis Altamirano, Miguel Panizo, Raquel Gonzalez, Antonio Rivera Future thermonuclear reactors, both magnetic and inertial confinement approaches, need materials capable of withstanding the extreme radiation and heat loads expected from high repetition rate plasma. A damage factor (F$=$q$\tau^{\mathrm{\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} }})$ in the order of 10$^{\mathrm{4}}$ (W/cm$^{\mathrm{2}})$s$^{\mathrm{1/2\thinspace }}$ is expected. The axial plasma dynamics after the pinch in a tabletop plasma focus of hundred joules, PF-400J, was characterized by means of pulsed optical refractive diagnostics. The energy, interaction time and power flux of the plasma burst interacting with targets was obtained. Results show a high dependence of the damage factor with the distance from the anode top where the sample is located. A tunable damage factor in the range 10- 10$^{\mathrm{5}}$(W/cm$^{\mathrm{2}})$s$^{\mathrm{1/2}}$ can be obtained. At present the PF-400J operating at 0.077 Hz is being used to study the effects of fusion-relevant pulses on material target, including nanostructured materials. A new tabletop device to be operated up to 1Hz including tunable damage factor has been designed and is being constructed, thus thousand cumulative shots on materials could be obtained in few minutes. The scaling of the damage factor for plasma foci operating at different energies is discussed. [Preview Abstract] |
Thursday, November 3, 2016 3:24PM - 3:36PM |
UO4.00008: He bombardment of WEST tungsten grades: surface morphology changes and flux dependence H. Hijazi, C. Martin, F. W. Meyer, M. E. Bannister, M. Cabie, A. Campos, J-L. Gardarein, Y. Corre, M. Richou, Y. Addab, P. Roubin We report measurements of the surface morphology changes induced by He ion bombardment of WEST grades polycrystalline tungsten at conditions relevant for the WEST He campaign (T$=$400-1000 \textdegree C and flux range 0.3-5.10$^{\mathrm{20}}$ m$^{\mathrm{-2}}$s$^{\mathrm{-1}})$. 218 eV He impact energy bombardments were carried out at the ORNL MIRF, using a high-flux deceleration module and beam flux monitor. Surface analyses were performed at the PIIM laboratory using electron microscopy techniques (FIB-SEM and EBSD). At fluxes below 2.10$^{\mathrm{20}}$ m$^{\mathrm{-2}}$s$^{\mathrm{-1}}$, nano-wavy structures and pinholes are observed on individual grains, together with sub-surface bubbles. Interestingly, the wavy structures and pinholes were found preferentially on grains with surface orientations near 101 and 001, respectively. At fluxes above 2.10$^{\mathrm{20}}$ m$^{\mathrm{-2}}$s$^{\mathrm{-1}}$, the individual grain-to-grain variability disappears and the entire surface is covered by nano-fuzz structures. These results suggest that, at around 2.10$^{\mathrm{20}}$ m$^{\mathrm{-2}}$s$^{\mathrm{-1}}$, ion beam bombardment produces significant sub-surface damages with a high bubble density due to He saturation leading to a possible scenario that bubbles burst to form pinholes and then nanofuzz. Detailed analyses of the correlation between the grain orientation and the wavy structure as well as of the surface erosion, roughness and emissivity are underway. [Preview Abstract] |
Thursday, November 3, 2016 3:36PM - 3:48PM |
UO4.00009: The temperature and ion energy dependence of deuterium retention in lithium films Luxherta Buzi, Bruce E. Koel, Charles H. Skinner Lithium conditioning of plasma facing components in magnetic fusion devices has improved plasma performance and lowered hydrogen recycling. For applications of lithium in future high heat flux and long pulse duration machines it is important to understand and parameterize deuterium retention in lithium. This work presents surface science studies of deuterium retention in lithium films as a function of surface temperature, incident deuterium ion energy and flux. Initial experiments are performed on thin (3-30 ML) lithium films deposited on a single crystal molybdenum substrate to avoid effects due to grain boundaries, intrinsic defects and impurities. A monoenergetic and mass-filtered deuterium ion beam was generated in a differentially pumped Colutron ion gun. Auger electron spectroscopy and X-ray photoelectron spectroscopy were used to identify the elemental composition and temperature programmed desorption was used to measure the deuterium retention under the different conditions. [Preview Abstract] |
Thursday, November 3, 2016 3:48PM - 4:00PM |
UO4.00010: Studies of lithiumization and boronization of ATJ graphite PFCs for NSTX-U Javier Dominguez, Felipe Bedoya, Predrag Krstic, Jean Paul Allain, Anton Neff, kara Luitjohan We examine and compare the effects of boron and lithium conditioning on ATJ graphite surfaces bombarded by low-energy deuterium atoms on deuterium retention and chemical sputtering. We use atomistic simulations and compare them with experimental in-situ ex-tempore studies with X-ray photoelectron spectroscopy (XPS), to understand the effects of deuterium exposure on the chemistry in lithiated, boronized and oxidized amorphous carbon surfaces. Our results are validated qualitatively by comparison with experiments and with classical-quantum molecular dynamic simulations. We explain the important role of oxygen in D retention for lithiated surfaces and the suppression of the oxygen role by boron in boronized surfaces. The calculated increase of the oxygen role in deuterium uptake after D accumulation in a B-C-O surface configuration is discussed. The sputtering yield per low-energy D impact is significantly smaller in boronized surfaces than in lithiated surfaces. [Preview Abstract] |
Thursday, November 3, 2016 4:00PM - 4:12PM |
UO4.00011: A compact self-flowing lithium system for use in an industrial neutron source. Kishor Kumar Kalathiparambil, Matthew Szott, Brian Jurczyk, Chisung Ahn, David Ruzic A compact trench module to flow liquid lithium in closed loops for handling high heat and particle flux have been fabricated and tested at UIUC. The module was designed to demonstrate the proof of concept in utilizing liquid metals for two principal objectives: i) as self-healing low Z plasma facing components, which is expected to solve the issues facing the current high Z components and ii) using flowing lithium as an MeV-level neutron source. A continuously flowing lithium loop ensures a fresh lithium interface and also accommodate a higher concentration of D, enabling advanced D-Li reactions without using any radioactive tritium. Such a system is expected to have a base yield of \textasciitilde 10e7 n/s. For both the applications, the key success factor of the module is attaining the necessary high flow velocity of the lithium especially over the impact area, which will be the disruptive plasma events in fusion reactors and the incident ion beam for the neutron beam source. This was achieved by the efficient shaping of the trenches to exploit the nozzle effect in liquid flow. The compactness of the module, which can also be scaled as desired, was fulfilled by the use of high Tc permanent magnets and air cooled channels attained the necessary temperature gradient for driving the lithium. The design considerations and parameters, experimental arrangements involving lithium filling and attaining flow, data and results obtained will be elaborated. [Preview Abstract] |
Thursday, November 3, 2016 4:12PM - 4:24PM |
UO4.00012: Investigation of tin-lithium eutectic as a liquid plasma facing material. David Ruzic, Matthew Szott, Michael Christenson, Ivan Shchelkanov, Kishor Kumar Kalathiparambil Innovative materials and techniques need to be utilized to address the high heat and particle flux incident on plasma facing components in fusion reactors. A liquid metal diverter module developed at UIUC with self circulating lithium has been successfully demonstrated to be capable of handling the relevant heat flux in plasma gun based tests and on operational tokamaks. The proper geometry of the liquid lithium trenches to minimize droplet ejection during transient plasma events have also been identified. Although lithium has proven to be effective in improved plasma performance and contributes to other advantageous factors like reduction in the fuel recycling, impurity gettering and, owing to the low Z, a significantly reduced impact on plasma as compared to the solid wall materials, it still poses several drawbacks related to its high reactivity and high vapor pressure at the relevant tokamak wall temperatures. The evaporation properties of a new eutectic mixture of tin and lithium (20{\%} Sn) shows that lithium segregates to the surface at melting temperatures and hence is an effective replacement for pure lithium. Also, the vapor from the eutectic is dominated by lithium, minimizing the entry of high Z Sn into the plasma. At UIUC experiments for the synthesis and characterization of the eutectic -- measurement of the critical wetting parameters and Seebeck coefficients with respect to the trench materials have been performed to ensure lithium wetting and flow in the trenches. The results will be presented. [Preview Abstract] |
Thursday, November 3, 2016 4:24PM - 4:36PM |
UO4.00013: Electron Emission from Nano and Microstructured Materials for Fusion and Plasma Discharge Applications Marlene Patino, Yevgeny Raitses, Richard Wirz Secondary electron emission (SEE) from plasma-facing walls can lead to adverse effects (e.g. increased plasma heat flux to the wall) in plasma devices, including plasma processing, confinement fusion, and plasma thrusters. Experimental and computational efforts of engineered materials with nm to mm-sized structures (grooves, pores, fibers) have previously shown a decrease in SEE for primary electrons incident normal and oblique to the material. Here we present SEE measurements from one such engineered material, carbon velvet with $\mu $m fibers, and from a plasma-structured material, tungsten fuzz with nm fibers. Results show two trends: (a) significant reduction in SEE at normal incidence for carbon velvet (75{\%} reduction) and tungsten fuzz (40-50{\%} reduction) over smooth graphite and tungsten, respectively, and (b) SEE from tungsten fuzz is nearly independent of incident angle (i.e. not a cosine dependence on incident angle observed for smooth materials). Hence, the reduction in SEE from tungsten fuzz over smooth tungsten is more pronounced (up to 63{\%}) at grazing angles. This is important for many plasma devices since in a negative-going sheath the potential structure leads to relatively high incident angles. [Preview Abstract] |
Thursday, November 3, 2016 4:36PM - 4:48PM |
UO4.00014: 3D ion flow measurements and simulations near a boundary at oblique incidence to a magnetic field Derek S. Thompson, Shane Keniley, Rinat Khaziev, Davide Curreli, Timothy N. Good, Miguel Henriquez, Julianne McIlvain, M. Umair Siddiqui, Earl E. Scime Boundaries at oblique incidence to magnetic fields are abundant in magnetic confinement plasmas. The ion dynamics near these boundaries has implications for applications such as tokamak divertor wall loading and Hall thruster channel erosion. We present 3D, non-perturbative measurements of ion velocity distribution functions (IVDFs), providing ion temperatures and flows upstream of a grounded stainless steel limiter plate immersed in an argon plasma, oriented obliquely to the background axial magnetic field ($\psi$ = 74$^{\circ}$). The spatial resolution of the measurements is sufficient to probe the kinetic details of magnetic presheath structures, which span several ion Larmor radii ($\sim$1 cm). Furthermore, we report probe measurements of electron density and temperature, and of local electric potential. To complement these measurements, results from particle-in-cell and Boltzmann models of the same region are presented. These models allow for point-to-point comparison of simulated and measured electrostatic structures and IVDFs at high spatial resolution. [Preview Abstract] |
(Author Not Attending)
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UO4.00015: Shock formation induced by poloidal flow and its effects on the edge stability in tokamaks JaeChun Seol, Kerchung Shaing, Ahmet Aydemir In the high confinement mode of tokamaks, magnitude of the radial electric field increases at the edge. Thus, the poloidal flow inside the transport barrier can be sonic when the edge pressure gradient is not steep enough to make the poloidal flow subsonic. When the poloidal Mach number is close to unity, a shock appears in the low field side and causes a large density perturbation. In this study, we describe a shock induced by the sonic poloidal plasma flow. Then, an entropy production across the shock is calculated. Finally, we introduce a simple model for Type III edge localized modes using the poloidal density variation driven by the sonic poloidal flow. [Preview Abstract] |
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