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 GI1: Runaways, Divertors, and Edge Physics |
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Chair: Glen Wurden, Los Alamos National Laboratory Room: Acadia |
Tuesday, October 28, 2014 9:30AM - 10:00AM |
GI1.00001: Relativistic Runaway Electrons Invited Speaker: Boris Breizman This talk covers recent developments in the theory of runaway electrons in a tokamak with an emphasis on highly relativistic electrons produced via the avalanche mechanism. The rapidly growing population of runaway electrons can quickly replace a large part of the initial current carried by the bulk plasma electrons. The magnetic energy associated with this current is typically much greater than the particle kinetic energy. The current of a highly relativistic runaway beam is insensitive to the particle energy, which separates the description of the runaway current evolution from the description of the runaway energy spectrum. A strongly anisotropic distribution of fast electrons is generally prone to high-frequency kinetic instabilities that may cause beneficial enhancement of runaway energy losses. The relevant instabilities are in the frequency range of whistler waves and electron plasma waves. The instability thresholds reported in earlier work have been revised considerably to reflect strong dependence of collisional damping on the wave frequency and the role of plasma non-uniformity, including radial trapping of the excited waves in the plasma. The talk also includes a discussion of enhanced scattering of the runaways as well as the combined effect of enhanced scattering and synchrotron radiation. A noteworthy feature of the avalanche-produced runaway current is a self-sustained regime of marginal criticality: the inductive electric field has to be close to its critical value (representing avalanche threshold) at every location where the runaway current density is finite, and the current density should vanish at any point where the electric field drops below its critical value. This nonlinear Ohm's law enables complete description of the evolving current profile. [Preview Abstract] |
Tuesday, October 28, 2014 10:00AM - 10:30AM |
GI1.00002: Achievement of Runaway Electron Energy Dissipation by High-Z Gas Injection in DIII-D Invited Speaker: E.M. Hollmann Disruption runaway electron (RE) formation followed by RE beam-wall strikes is a concern for future tokamaks, motivating the study of mitigation techniques to reduce the RE beam energy in a controlled manner. A promising approach for doing this is the injection of high-Z gas into the RE beam. Massive (100 torr-l) injection of high-Z gas into RE beams in DIII-D is shown to significantly dissipate both RE magnetic and kinetic energy. For example, injection of argon into a typical 300 kA current RE beam is observed to cause a drop in kinetic energy from 50 kJ to 10 kJ in 10 ms, thus rapidly reducing the damage-causing capability of the RE beam. Both the RE kinetic energy and pitch angle are important for determining the resulting wall damage, with high energy, high pitch angle electrons typically considered most dangerous. The RE energy distribution is found to be more skewed toward low energies than predicted by avalanche theory. The pitch angle is not found to be constant, as is frequently assumed, but is shown to drop from sin($\theta{)} \sim 1$ for energies less than 1 MeV to sin($\theta{)} \sim 0.2$ for energies greater than 10 MeV. Injection of high-Z impurities does not appear to change the overall shape of the energy or pitch angle distributions dramatically. The enhanced RE energy dissipation appears to be caused primarily via collisions with the cold plasma leading to line radiation. Synchrotron power loss only becomes significant in the absence of high-Z impurities, while radial transport loss of REs is seen to become dominant if the RE beam moves sufficiently close to the vessel walls. The experiments demonstrate that avalanche theory somewhat underestimates collisional dissipation of REs in the presence of high-Z atoms, even in the absence of radial transport losses, meaning that reducing RE wall damage in large tokamaks should be easier than previously expected. [Preview Abstract] |
Tuesday, October 28, 2014 10:30AM - 11:00AM |
GI1.00003: Broadening of the divertor heat flux footprint with increasing number of ELM filaments in NSTX Invited Speaker: Joon-Wook Ahn We report on the broadening (narrowing) of the ELM heat flux footprint with increasing (decreasing) number of filamentary striations from in-depth thermography measurements in NSTX. Edge localized modes (ELMs) represent a challenge to future fusion devices, due to the high heat fluxes on plasma facing surfaces. One ameliorating factor has been that the divertor heat flux characteristic profile width ($\lambda_{\mathrm{q}})$ has been observed to broaden with the size of ELM, as compared with the inter-ELM $\lambda_{\mathrm{q}}$, which keeps the peak heat flux (q$_{\mathrm{peak}})$ from increasing.\footnote{T. Eich \textit{et al}., J. Nucl. Mater. \textbf{415}, S856 (2011)}$^,$\footnote{S. Devaux \textit{et al}., J. Nucl. Mater. \textbf{415}, S865 (2011)} In contrast, $\lambda_{\mathrm{q}}$ has been observed to narrow during ELMs under certain conditions in NSTX, for both naturally occurring\footnote{J-W. Ahn \textit{et al}., J. Nucl. Mater. \textbf{438}, S317 (2013)} and 3-D fields triggered\footnote{J-W. Ahn \textit{et al}., Plasma Phys. Control. Fusion \textbf{56}, 015005 (2014)} ELMs. Fast thermographic measurements and detailed analysis demonstrate that the ELM $\lambda_{\mathrm{q}}$ increases with the number of observed filamentary striations, $i.e.,$ profile narrowing (broadening) occurs when the number of striations is smaller (larger) than 3-4.\footnote{J-W. Ahn \textit{et al}., submitted to Phys. Rev. Lett. (2014)} With profile narrowing,\footnote{Ahn, J. Nucl. Mater (2013)} q$_{\mathrm{peak}}$ at ELM peak times is inversely related (proportional) to $\lambda_{\mathrm{q}}$ (the ELM size), exacerbating the heat flux problem. Edge stability analysis shows\footnote{D.P. Boyle \textit{et al}., Plasma Phys. Control. Fusion \textbf{53}, 105011 (2011)} that NSTX ELMs almost always lie on the current-driven kink/peeling mode side with low toroidal mode number (n$=$1-5), consistent with the typical numbers of striations in NSTX (0-8); in comparison 10-15 striations are normally observed in intermediate-n peeling-ballooning ELMs, e.g., from JET.\footnote{Devaux, J. Nucl. Mater (2011)} The NSTX characteristics may translate directly to ITER, which is also projected to lie on the low-n kink/peeling stability boundary.\footnote{P.B. Snyder \textit{et al}., Nucl. Fusion \textbf{51}, 103016 (2011)} [Preview Abstract] |
Tuesday, October 28, 2014 11:00AM - 11:30AM |
GI1.00004: A convective divertor utilizing a 2nd-order magnetic field null Invited Speaker: Thomas Rognlien New results motivate a detailed study of a magnetic divertor concept characterized by strong plasma convection near a poloidal magnetic field (B$_{\mathrm{p}}$) null region. The configuration is that of a near-2nd-order B$_{\mathrm{p}}$ null (B$_{\mathrm{p}}\propto \Delta $r$^{\mathrm{2}})$, as in a snowflake divertor [1,2]. The concept has 2 key features: (A) Convection spreads the heat flux between multiple divertor legs and further broadens the heat-flux profile within each leg, thereby greatly reducing target-plate heat loads [2]. (B) The heat flux is further reduced by line radiation in each leg in detachment-like ionization zones. Theory indicates that convective turbulence arises when the poloidal plasma beta, $\beta_{\mathrm{p}}=$2$\mu _{\mathrm{0}}$nT/B$_{\mathrm{p}}^{2}$ \textgreater \textgreater 1. Measurements in TCV [4] now more fully quantify earlier NSTX and TCV observations of plasma mixing [5.6], and related modeling of TCV indicates that strongly enhanced null-region transport is present [7]. Convective mixing provides a stabilizing mechanism to prevent the ionization fronts (hydrogenic and impurity) from collapsing to a highly radiating core MARFE. Also, the radiating zone maps to a very small region at the midplane owing to the very weak B$_{\mathrm{p}}$ in the convective region, thus minimizing its impact on the core plasma. Detailed calculations are reported that combine features A and B noted above. The plasma mixing mechanisms are described together with the corresponding transport model implemented in the 2D UEDGE edge transport code [2]. UEDGE calculations are presented that quantify the roles of mixing, impurity radiation, and detachment stability for a realistic snowflake configuration. Work in collaboration with D.D. Ryutov, S.I. Krasheninnikov, and M.V. Umansky.\\[4pt] [1] D.D. Ryutov et al., PPCF \textbf{54} (2012) 124050.\\[0pt] [2] T.D. Rognlien et al., J. Nucl. Mat. \textbf{438} (2013) S418.\\[0pt] [3] D.D. Ryutov et al., accepted, Physica Scripta (2014).\\[0pt][4] W. Vijvers et al., Nucl. Fusion \textbf{54} (2014) 023009.\\[0pt] [5] V.A. Soukhanovskii et al., Nucl. Fusion \textbf{51} (2011) 012001 and Phys. Plasmas \textbf{19} (2012) 082504.\\[0pt] [6] H. Reimerdes et al., PPCF \textbf{55} (2013) 124027.\\[0pt] [7] T. Lunt et al., PPCF \textbf{56} (2014) 035009. [Preview Abstract] |
Tuesday, October 28, 2014 11:30AM - 12:00PM |
GI1.00005: X-point-position-dependent intrinsic rotation in the edge of TCV Invited Speaker: Timothy Stoltzfus-Dueck A simple transport-based theoretical model predicts that intrinsic toroidal rotation in the tokamak edge should depend strongly on $R_X$, the major-radial position of the X-point, including a sign change to counter-current rotation for adequately outboard X-point. To test the prediction, an $R_X$ scan was conducted in Ohmic L-mode shots on TCV, in both USN and LSN configurations. The strong linear dependence on $R_X$ was experimentally observed, with quantitative magnitude corresponding to a realistic value for the theory's corresponding input parameter. Although peaked rotation profiles complicate the comparison of absolute rotation values, the data is consistent with the predicted sign change. The core rotation profile shifted fairly rigidly with the edge rotation value, maintaining a relatively constant core rotation gradient. Core rotation reversals, triggered accidentally in a few shots, had little effect on the edge rotation velocity. Edge rotation was modestly more counter-current in USN than LSN discharges. [Preview Abstract] |
Tuesday, October 28, 2014 12:00PM - 12:30PM |
GI1.00006: The influence of Filaments in the Private Flux Region on Divertor Power and Particle Deposition Invited Speaker: James Harrison Recent advances in imaging of the MAST divertor have revealed, for the first time, evidence for filaments in the private flux region (PFR). Detailed analysis of the image data shows 3 distinct types of fluctuations occurring within the divertor volume: highly sheared filaments in the SOL originating from the outer midplane, high frequency (\textgreater 50kHz) filaments near the separatrix of the outer divertor leg and filaments in the private flux region originating from inner divertor leg. With the need to extrapolate divertor performance from existing machines to future devices, these observations can contribute to our quantitative understanding of transport in the PFR. In particular, they suggest that transport in the PFR is, at least in part, driven by turbulence, which may not be well captured by the Eich/Wagner description of the divertor footprint [1], expressed in terms of exponential decay in space above the X-point and Gaussian spreading below the X-point. The PFR filaments are observed to move largely parallel with the flux surfaces in a way equivalent to a toroidal angular velocity of order 2x10$^{4}$ rad/s in H-mode, and slower by a factor of order 2 in L-mode. During their transit parallel to the flux surfaces across the PFR, the filaments eject plasma in bursts, away from the separatrix, deeper into the private flux region. Correlation analysis suggests that they are generated by processes local to the inner divertor leg, as there is a weak correlation between fluctuations in the SOL and PFR above what is expected from line integration effects. Scaling of filament properties with machine operating parameters, such as plasma current, density and auxiliary heating power will be presented, together with a comparison with data from divertor Langmuir probes and IR thermography to estimate the role PFR filaments play in determining the width of the divertor footprint. \\[4pt] [1] T. Eich et al., Phys. Rev. Lett. 107, 215001. [Preview Abstract] |
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