Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session UM5: Mini-conference on the First Microns of the First Wall: Graphite Wall Sputtering and Evolution |
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Chair: Sergei Krasheninnikov, University of California at San Diego Room: Rosen Centre Hotel Salon 11/12 |
Thursday, November 15, 2007 2:00PM - 2:20PM |
UM5.00001: In-situ investigations of material migration in recent JET campaign A. Kreter To improve understanding of material erosion and migration in view of the planned ITER-like wall in JET, a set of in-situ material migration diagnostics has recently been installed, including quartz microbalance (QMB) systems. These techniques are focused on the identification and the quantification of processes determining the carbon migration by time resolved measurements. Studies across the recent JET 2006-07 campaigns show that the major characteristics of carbon transport in the divertor can be summarised as follows: (i) The transport is mainly line-of-sight, with particles predominately sputtered at the strike point (SP) positions and travelling over distances of up to several centimetres across the magnetic field; (ii) The amount of eroded carbon depends on the surface type. The highest rates are obtained after a shift of the strike-point to the ``fresh'' layers deposited in previous discharges (``history'' effect); (iii) there is a clear non-linear increase of the erosion rates in the inner divertor with increasing ELM energies. This ELM induced erosion is attributed to the thermal decomposition of carbon layers. [Preview Abstract] |
Thursday, November 15, 2007 2:20PM - 2:40PM |
UM5.00002: Chemical Sputtering of graphite surfaces by slow H and D Atomic and Molecular projectiles F.W. Meyer, H. Zhang, H.F. Krause Because of its high thermal conductivity, excellent shock resistance, absence of melting, low activation, and low atomic number, there is significant technological interest in using graphite as a plasma-facing component on present and future fusion devices, despite its poor chemical erosion and sputtering properties. As divertor designs evolve, the interest in the erosion characteristics of the carbon surfaces is shifting to progressively lower impact energies. Results are presented of chemical sputtering yields for ATJ graphite and HOPG impacted by H$^{+}$(D$^{+})$, H$_{2}^{+}$(D$_{2}^{+})$and H$_{3}^{+}$(D$_{3}^{+})$ in the energy range 5-250 eV/amu. The measurements serve as benchmarks for in house MD simulations [Physica Scripta \textbf{T128}, 50 (2007)] of the chemical sputtering process that seek to incorporate more realistic many-body potentials and to expand the reaction pathway to include vibrational and/or electronic excited states. Comparison between same velocity atomic and molecular ion impact at energies as low as 5 eV/amu will be described [J. Nucl. Mater. \textbf{357}, 9(2006)]. In addition, the isotope effect in methane production by H and D incident ions will be discussed. [Preview Abstract] |
Thursday, November 15, 2007 2:40PM - 3:00PM |
UM5.00003: MD Simulations of Plasma-Surface Interactions of Deuterated Carbon P.S. Krstic, C.O. Reinhold, S.J. Stuart We study plasma-surface interactions (PSI) in the first few nanometers of carbon walls, at fluences of $\sim $10$^{20}$~D~m$^{-2}$ and impact energies in range of tens of eV. Using an atomistic classical molecular dynamics (MD) approach we are able to take into account the full configuration space of impact plasma particles (atoms, molecules), including their rovibrational states, as well as to describe with the same level of detail the PSI outcome, i.e. of particles ejected by chemical sputtering and reflection. Understanding of the relevant PSI processes, reflecting the deuterated surface microstructure is obtained [\textit{New J. Phys.} \textbf{9}, 209 (2007), \textit{Europhys. Lett.} \textbf{77}, 33002 (2007)], as well as a good agreement with recent in-house beam-surface experiments. This increases confidence in our MD calculations for impact of plasma particles distributed at a given temperature. Our results show that the products of the PSI of deuterated carbon are significantly different from those of ``pure'' carbon, highlighting the importance of accounting for the hydrogen content in the walls. [Preview Abstract] |
Thursday, November 15, 2007 3:00PM - 3:20PM |
UM5.00004: Structural Studies of Carbon Dust Samples Exposed to NSTX Plasma Yevgeny Raitses, Charles H. Skinner, Fuming Jiang, Thomas S. Duffy, Angus Pacala Raman spectroscopy offers a sensitive measure of the microstructure of carbon dust. We present Raman spectra of dust samples exposed to the NSTX plasma, unexposed dust, carbon deposits produced in an arc discharge, and heat-treated (1500-2000 \r{ }C) carbon samples all made from graphite material. The main difference in the measured Raman spectrum is that for the exposed dust samples, the high energy G-mode peak (Raman shift $\sim $1580 cm$^{-1})$ is much weaker than the defect-induced D-mode peak (Raman shift $\sim $ 1350 cm$^{-1})$, while for the unexposed samples, similarly to commercial graphite, the ratio of G-mode to D-mode peaks is always larger than 1. These results indicate that the production of carbon dust particles in NSTX involves modifications of the physical and chemical structure of the original graphite and these modifications are similar to those induced by the arc plasma to carbon particles. [Preview Abstract] |
Thursday, November 15, 2007 3:20PM - 3:40PM |
UM5.00005: Hydrogen dynamics under strong plasma-wall coupling A.Yu. Pigarov, S.I. Krasheninnikov, A. Pletzer The newly developed time-dependent one-dimensional code WALLPSI for wall temperature and erosion rates as well as for trapped, absorbed, and mobile hydrogen inside the wall is discussed. The code is a part of integrated model FACETS for core/edge/wall transport. To study the basic physic process, the code is coupled to the 1D edge plasma transport code. The results of self-consistent plasma/neutral/wall modelling which show strong plasma-wall coupling are presented. Variation of hydrogen inventory in the wall in response to changing plasma impact is discussed. Thermal instability of plasma in contact with hydrogen saturated wall is analyzed. [Preview Abstract] |
Thursday, November 15, 2007 3:40PM - 4:00PM |
UM5.00006: Investigation of Plasma Surface Interactions with the PISCES ELM Laser System K.R. Umstadter, M. Baldwin, J. Hanna, R. Doerner, T. Lynch, T. Palmer, G.R. Tynan When an ELM occurs in tokamaks, up to 30{\%} of the pedestal energy can be deposited on the wall of the tokamak causing heating {\&} material loss due to sublimation, evaporation and melt splashing of plasma facing components (PFCs) and expansion of the ejected material into the plasma. We have explored heat pulses using an electrical power circuit to draw electrons from the plasma to heat samples ohmically. This system is limited in power to $\sim $250kJ/m$^{2}$ at the minimum pulse width of 10ms and depletes the plasma column, complicating spectroscopy. We have completed calculations that indicate that a pulsed laser system can be used to simulate the heat pulse of ELMs. We are integrating laser systems into the existing PFC research program in PISCES, a laboratory facility capable of reproducing plasma-materials interactions expected during normal operation of large tokamaks. Two Nd:YAG lasers capable of delivering up to 50J of energy over various pulsewidths are used for the experiments. Laser heat pulse only, H+/D+ plasma only, and laser+plasma experiments were conducted and initial results indicate that metals behave very differently while exposed to plasma and simultaneous heat pulses. We will also discuss initial results for carbon PFCs and material transport into the plasma. Supported by US DoE grant DE-FG02-07ER-54912. [Preview Abstract] |
Thursday, November 15, 2007 4:00PM - 4:20PM |
UM5.00007: Atomistic Simulations of Energetic Particle Interactions with the First Wall. Roger Stoller Atomistic simulations of the interactions between energetic particles and a fusion reactor first wall have been carried out using molecular dynamics to investigate both primary damage formation in the structural first wall material and the sputtering of surface atoms which can lead to contamination of the plasma. In the case of damage formation in the structural material, the results provide a quantitative measure of the effect of a nearby free surface on the evolution of atomic displacement cascades, and the nature of the residual damage produced. This damage is characterized by the total number of point defects (vacancies and interstitials), as well as the number and size distribution of point defect clusters produced. A sufficient number of simulations have been completed to statistically evaluate variations between surface-influenced and bulk cascades. Surface sputtering from the molecular dynamics simulations is compared to the results obtained with a more simple, and widely-used Monte Carlo model (SRIM). [Preview Abstract] |
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