Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session NI3: Technology and Fundamental Plasma Physics |
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Chair: David Graves, University of California, Berkeley Room: Plaza F |
Wednesday, November 13, 2013 9:30AM - 10:00AM |
NI3.00001: Hydrogen Ionic Plasma and Particle Dynamics in Negative Ion Source for NBI Invited Speaker: Katsuyoshi Tsumori Three negative-ion-based neutral beam injectors (NBIs) have been developed for plasma heating in the Large Helical Device. The NBIs achieve successfully the nominal injection power and beam energy [1-3], and understanding of the production and transport mechanisms of H$^-$ ion is required to obtain more stable high power beam. In the ion source development, we have found hydrogen ionic plasmas with extremely low electron density are produced in the beam extraction region [4]. The plasma is measured with a combination of an electrostatic probe, millimeter-wave interferometer and cavity ring down (CRD) [4-6]. It has been observed for the first time that the charge neutrality of the ionic plasma is broken with H$^-$ extraction and electrons compensate the extracted H$^-$ charge [4]. The influence of the extraction field widely affects to the ionic plasma in the extraction region [4, 6]. Two-dimensional particle-in-cell simulation (2D-PIC) has been applied to investigate the particle transport and reproduces the production of the ionic plasma and electron compensation due to H$^-$ extraction [4, 7]. In particle model, produced H$^-$ ions leave from the Cs covered PG surface in opposite direction to beam extraction. The direction can be changed with the electric field and collective effect due to the presence of plasma. A new technique using CCD camera with H$\alpha $ filter applied to measure the two-dimensional distribution of H$^-$ density [8]. In the ionic plasma, H$\alpha $ light is emitted via electron-impact excitation and mutual neutralization processes with H$^-$ ion and proton. Comparing the results obtained with optical emission spectroscopy, electrostatic probe and CRD, it is shown the H$\alpha $ emission is dominated with the mutual neutralization. By subtracting the CCD images with and without beam extraction, it becomes clear that H$^-$ ions are extracted not directly from the PG surface but from the bulk of the ionic plasma [8]. The result suggests the initial energy of H$^-$ ion is dumped rapidly in the ionic plasma.\\[4pt] [1] Y. Takeiri et al. {\it Rev. Sci. Instrum.} {\bf 71} (2000) 1225.\\[0pt] [2] O. Kaneko et al. {\it Nucl. Fusion} {\bf 43} (2003) 692.\\[0pt] [3] Tsumori et al. {\it Fusion Sci. and Technol.} {\bf 58} (2010) 489.\\[0pt] [4] Tsumori et al. {\it Rev. Sci. Instrum.} {\bf 83} (2012) 02B116.\\[0pt] [5] Nagaoka et al. {\it AIP Conference Proceedings} {\bf 1390} (2010) 374.\\[0pt] [6] Nakano et al. {\it AIP Conference Proceedings} {\bf 1515} (2012) 237.\\[0pt] [7] Fukuyama et al. {\it AIP Conference Proceedings} {\bf 1515} (2012) 74.\\[0pt] [8] Ikeda et al. {\it Plasma Fusion Res.} {\bf 8} (2013) 1301036. [Preview Abstract] |
Wednesday, November 13, 2013 10:00AM - 10:30AM |
NI3.00002: Differentiating the role of lithium and oxygen in retaining deuterium on lithiated plasma-facing components Invited Speaker: Chase Taylor Lithium wall conditioning has been implemented in nearly a dozen fusion devices, resulting in significantly improved plasma performance. Improvements are manifest as a reduction and eventual elimination of edge localized modes, reduced edge neutral density, reduced deuterium recycling, and some reduction in impurities. Initially, researchers assumed that lithium, via a direct lithium-deuterium bond, was directly responsible for these improvements. Our experiments and atomistic simulations have revealed that lithium coatings play a much more indirect role in improving plasma performance [1]. The presence of oxygen in tokamaks is ubiquitously viewed as unfavorable. However, recent results show that lithium reduces oxygen impurities and surprisingly uses the oxygen to retain deuterium. Experiments using X-ray photoelectron spectroscopy identify that oxygen immediately begins to accumulate on lithium conditioned surfaces [2]. Tight-binding density functional theory simulations tested various carbon matrices with and without lithium, oxygen, and hydrogen, and identified that oxygen plays the key role in retaining deuterium. In fact, a simulated PFC with 20{\%} oxygen in carbon retains more deuterium than does 20{\%} lithium in carbon. Recent experiments implanted oxygen in graphite to match simulations; however, we were unable to achieve the simulated results because all implanted oxygen was released upon deuterium bombardment. We therefore conclude that while oxygen retains deuterium, lithium plays an indispensible role in this process. Lithium attracts and retains oxygen, and then oxygen binds and retains deuterium. \\[4pt] [1] P. S. Krstic, J. P. Allain, C. N. Taylor, et al., Phys. Rev. Lett. 110, 105001 (2013).\\[0pt] [2] C. N. Taylor, B. Heim, and J. P. Allain, J. Appl. Phys. 109, 053306 (2011). [Preview Abstract] |
Wednesday, November 13, 2013 10:30AM - 11:00AM |
NI3.00003: The in-situ diagnosis of plasma-wall interactions on magnetic fusion devices with accelerators Invited Speaker: Zachary Hartwig We present the first in-situ, time-resolved measurements of low-Z isotope composition and deuterium retention over a large plasma-facing component (PFC) surface area in a magnetic fusion device. These critical measurements were made using a novel diagnostic technique based on the analysis of induced nuclear reactions from PFC surfaces on the Alcator C-Mod tokamak. Achieving an integrated understanding of plasma physics and materials science in magnetic fusion devices is severely hindered by a dearth of in-situ PFC surface diagnosis. Plasma-wall interactions, such as the erosion/redeposition of PFC material, the evolution of PFC surface isotope composition, and fusion fuel retention present significant plasma physics and materials science challenges for long pulse or steady-state devices. Our diagnostic uses a compact ($\sim$1 meter), high-current ($\sim$1 milliamp) radio-frequency quadrupole accelerator to inject $\sim$1 MeV deuterons into the vacuum vessel. We control the tokamak's magnetic fields -- in between plasma shots -- to steer the deuterons to PFC surfaces, where they induce high-Q nuclear reactions with low-Z isotopes in the first $\sim$10 microns of material. Analysis of the induced gamma and neutron energy spectra provides quantitative reconstruction of PFC surface conditions. This nondestructive, in-situ technique achieves PFC surface composition measurements with plasma shot-to-shot time resolution and 1 centimeter spatial resolution over large PFC areas. [Preview Abstract] |
Wednesday, November 13, 2013 11:00AM - 11:30AM |
NI3.00004: Direct measurement of turbulent resistivity Invited Speaker: M.D. Nornberg We have directly measured the vector turbulent emf in a two-vortex flow of liquid sodium in the Madison Dynamo Experiment. Using a novel probe design, we simultaneously measure magnetic and flow fluctuations to determine their correlated effect on mean-field induction. Through our electromagnetic model for the flow-induced mean magnetic field, constrained by measurements throughout the flow, we construct the vector mean current density at the probe location. With this information we are able to construct the mean-field model for the $\alpha$ and $\beta$-effect terms of the turbulent emf and compare them with the direct measurement of the time averaged correlated fluctuations. The measured turbulent emf is anti-parallel with the mean current and is almost entirely described by an enhanced resistivity. The residual turbulent resistivity presents a difficulty for establishing the onset of the kinematic dynamo in a laboratory turbulent flow in that the effective magnetic Reynolds number is reduced making it more difficult to exceed the critical $Rm$. We have demonstrated that this enhanced resistivity can be mitigated by eliminating the largest-scale eddies. By tailoring the large-scale flow, we have achieved flows operating near threshold for dynamo self-excitation. [Preview Abstract] |
Wednesday, November 13, 2013 11:30AM - 12:00PM |
NI3.00005: Effective Potential Theory for Transport Coefficients across Coupling Regimes Invited Speaker: Scott D. Baalrud Plasmas in several modern experiments, including dense, ultracold and dusty plasmas, can reach strong coupling where the Coulomb potential energy of interacting particles exceeds their average kinetic energy. Understanding how the many-body physics of correlations affects plasma transport properties in this regime is interesting both from a basic physics standpoint and as a practical matter. Transport coefficients are essential input required for accurate hydrodynamic modeling of these systems, which can include weakly coupled and strongly coupled components simultaneously. We discuss a physically motivated extension of plasma transport theory that is computationally efficient and versatile enough that it can be applied to essentially any transport property [1]. Like conventional plasma theories, ours is based on a binary collision picture, but where particles interact via an effective potential that accounts for average affects of the intervening medium. This includes both correlations and screening. Hypernetted chain (HNC) theory, which is a well-established approximation for the pair correlation function, is used to derive the effective potential. The theory is shown to compare well with ion velocity relaxation in an ultracold plasma experiment [2], as well as classical molecular dynamics simulations of temperature relaxation in electron-ion plasmas [3], and diffusion in both one-component plasmas and ionic mixtures [4].\\[4pt] [1] Baalrud and Daligault, PRL 110, 235001 (2013).\newline [2] Bannasch, et al, PRL 109, 185008 (2012).\newline [3] Dimonte and Daligault, PRL 101, 135001 (2008).\newline [4] Daligault, PRL 108, 225004 (2012). [Preview Abstract] |
Wednesday, November 13, 2013 12:00PM - 12:30PM |
NI3.00006: Experimental discrimination between ion temperature and hydromotion in turbulent plasmas. Invited Speaker: Yitzhak Maron Distinguishing between energy placed in hydrodynamic motion of plasma from thermalization of the ions is of fundamental significance for laboratory plasma physics, astrophysics, and hydrodynamics, including high energy density (HED) plasmas, where energy placed in hydrodynamic motion contributes neither to radiation nor to fusion reactivity, whereas ion temperature does. Yet distinguishing ion temperature from hydromotion in HED plasmas has been regarded to be very difficult, since Doppler-broadened line shapes of emission lines can be due to either effect. However, two novel spectroscopic methods have been developed and implemented. The first method is based on determining the rate of heat transfer from ions to electrons by measuring the total ion kinetic energy, its dissipation rate, the total radiation from the plasma, and the electron density and temperature [1]. The second method is based on the effect of the ion-ion coupling on the shape of Stark-broadened lines [2]. This method requires an independent determination of the electron density, and the Doppler broadening of the emission line should be small. The experiments were performed using neon z-pinch plasmas. Required were observations with high-resolution in spectrum, space, and time, augmented by line-shape and time-dependent CR and radiation-transport modeling. The ion temperature was found to be significantly lower than the total ion kinetic energy. The dissipation time of the hydromotion was determined. The data also allowed for assessing reliably the pressure and energy balance in the stagnation phase of the imploding plasma [3]. Implications on various HED plasmas in large systems will be discussed.\\[4pt] [1] E. Kroupp et at., PRL 98 (2007), PRL 107 (2011).\\[0pt] [2] D. Alumot et al., ICOPS 2012, ICOPS 2013.\\[0pt] [3] Y. Maron et al., PRL 2013. [Preview Abstract] |
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