Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session GI2: Pedestal and Low-Temperature Physics |
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Chair: Venkattraman Ayyaswamy, University of California, Merced Room: 102ABC |
Tuesday, October 24, 2017 9:30AM - 10:00AM |
GI2.00001: Prediction and realization of ITER-like pedestal pressure in the high-$B$ tokamak Alcator C-Mod Invited Speaker: Jerry Hughes Fusion power in a burning plasma will scale as the square of the plasma pressure, which is increased in a straightforward way by increasing magnetic field: $P_{fus} \sim p^2 \sim B^4$. Experiments on Alcator C-Mod, a compact high-$B$ tokamak, have tested predictive capability for pedestal pressure, at toroidal field $B_T$ up to $8 \mbox{T}$, and poloidal field $B_P$ up to $1 \mbox{T}$. These reactor-like fields enable C-Mod to approach an ITER predicted value of $90 \mbox{kPa}$. This is expected if, as in the EPED model, the pedestal is constrained by onset of kinetic ballooning modes (KBMs) and peeling-ballooning modes (PMB), yielding a pressure pedestal approximately as $p_{ped} \sim B_T \times B_P$. One successful path to high confinement on C-Mod is the high-density ($\bar{n}_e > 3 \times 10^{20} \mbox{m}^{-3}$) approach, pursued using enhanced D-alpha (EDAs) H-mode. In EDA H-mode, transport regulates both the pedestal profiles and the core impurity content, holding the pedestal stationary, at just below the PBM stability boundary. We have extended this stationary ELM-suppressed regime to the highest magnetic fields achievable on C-Mod, and used it to approach the maximum pedestal predicted by EPED at high density: $p_{ped} \approx 60 \mbox{kPa}$. Another approach to high pressure utilizes a pedestal limited by PBMs at low collisionality, where pressure increases with density and EPED predicts access to a higher ``Super H'' solution for $p_{ped}$. Experiments at reduced density ($\bar{n}_e < 2 \times 10^{20} \mbox{m}^{-3}$) and strong plasma shaping ($\delta > 0.5$) accessed these regimes on C-Mod, producing pedestals with world record $p_{ped} \approx 80 \mbox{kPa}$, at $T_{ped} \approx 2 \mbox{keV}$. In both the high and low density approaches, the impact of the pedestal on core performance is substantial. Our exploration of high pedestal regimes yielded a volume-averaged pressure $\langle p \rangle > 2 \mbox{atm}$, a world record value for a magnetic fusion device. The results hold promise for the projection of pedestal pressure and overall performance of high field burning plasma devices. [Preview Abstract] |
Tuesday, October 24, 2017 10:00AM - 10:30AM |
GI2.00002: Toroidally asymmetric density profiles and turbulence induced by applied 3D fields in DIII-D Invited Speaker: Robert Wilcox Small non-axisymmetric magnetic field perturbations that are applied to tokamaks (dB/B\textasciitilde 10\textasciicircum -4) are shown to produce 3D plasma equilibrium changes that alter the stability of microturbulence modes in the pedestal region. Measurements from the DIII-D tokamak show that the density gradient scale length in the pedestal at the outboard midplane changes with the toroidal phase of the applied 3D fields, and this change is qualitatively reproduced using two-fluid M3D-C1 modeling. This modeling shows that the density may be non-constant within flux surfaces in the pedestal region when 3D fields are applied. The pressure gradient and resulting diamagnetic rotation are large in the pedestal, so that the ion and electron fluid velocities differ significantly, necessitating the use of a two-fluid model to resolve this effect. Calculated changes to surface topology using single fluid modeling are shown to be too small to affect turbulence stability directly, through modification of the curvature and local magnetic shear, without the additional changes to the 3D density profiles that arise from two-fluid modeling. In DIII-D experiments and simulations, field-aligned helical flux tubes in the pedestal region with an increase in normalized density gradients correspond to locations with increased broadband density fluctuation amplitudes, measured using beam emission spectroscopy and Doppler backscattering. These pedestal effects may help explain the longstanding mystery of density pumpout, by which 3D fields used for control of ELMs and other properties in tokamaks lead to a rapid decrease in plasma density, and improve understanding of the heat and particle fluxes into the scrape-off layer and to the divertor. [Preview Abstract] |
Tuesday, October 24, 2017 10:30AM - 11:00AM |
GI2.00003: Tamed stability and transport using controlled non-axisymmetric fields in KSTAR Invited Speaker: Yongkyoon In Meticulously orchestrated non-axisymmetric fields enabled KSTAR to explore various paths to tame plasma stability and transport in a very rigorous manner. Given an extremely low level of intrinsic non-axisymmetry, KSTAR has now established high-precision 3-D field control capability that can not only robustly suppress edge localized modes using resonant magnetic perturbation (RMP), but also exclusively alter plasma rotation without invoking particle and energy transport. In highly shaped plasmas (triangularity of $\delta $ \textasciitilde 0.6), we have secured low-n RMP ELM suppressions in a wide range of edge safety factors at q$_{\mathrm{95\thinspace =\thinspace }}$3.4 -- 6.4. One of the best n$=$1 RMP ELM suppressions has been sustained for more than 30 secs (comparable to wall saturation time), satisfying a low edge collisionality ($\nu $* \textasciitilde 0.2), close to ITER-target. Beyond a typical 3-row RMP configuration, we have newly succeeded in suppressing ELMs with n$=$1 off-midplane RMPs only, whose configuration is nearly orthogonal to conventional helical RMP structure. Also, a-priori calculation of ideal plasma response led us to identify an optimal window of ELM suppression distinctively separated from mode-lockings. With RMP configuration fixed, a gradual torque control between perpendicular and parallel components helped us access the onset of ELM suppression, strongly supporting a theoretical hypothesis of $\omega_{\mathrm{\bot Ñ,e}}$ \textasciitilde 0 as necessary condition for ELM-suppression, consistent with direct measurement by ECEI. Among non-resonant fields that are quite sensitive to a variation of q$_{\mathrm{95\thinspace }}$in NTV physics, a `most-quiescent' configuration has been identified to fully decouple plasma rotation from particle and energy transport. In support of ITER, KSTAR has conducted a series of experiments regarding RMP misalignment impact and its compatibility with detached plasmas. Preliminary experimental analysis suggests that divertor heat flux might not be sufficiently diffused by misalignment alone, once RMP-driven ELM suppression takes place. [Preview Abstract] |
Tuesday, October 24, 2017 11:00AM - 11:30AM |
GI2.00004: Adaptive plasma for cancer therapy: physics, mechanism and applications Invited Speaker: Michael Keidar One of the most promising applications of cold atmospheric plasma (CAP) is the cancer therapy. The uniqueness of plasma is in its ability to change composition in situ. Plasma self-organization could lead to formation of coherent plasma structures. These coherent structures tend to modulate plasma chemistry and composition, including reactive species, the electric field and charged particles. Formation of coherent plasma structures allows the plasma to adapt to external boundary conditions, such as different cells types and their contextual tissues. In this talk we will explore possibilities and opportunities that the adaptive plasma therapeutic system might offer. We shall define such an adaptive system as a plasma device that is able to adjust the plasma composition to obtain optimal desirable outcomes through its interaction with cells and tissues. The efficacy of cold plasma in a pre-clinical model of various cancer types such as lung, bladder, breast, head, neck, brain and skin has been demonstrated. Both \textit{in-vitro} and \textit{in-vivo} studies revealed that cold plasmas selectively kill cancer cells. Recently mechanism of plasma selectivity based on aquaporin hypothesis has been proposed. Aquaporins (AQPs) are the confirmed membrane channels of H$_{\mathrm{2}}$O$_{\mathrm{2}}$ and other large molecules. We have demonstrated that the anti-cancer capacity of plasma could be inhibited by silencing the expression of AQPs. Additional possible cell feedback mechanism was recently discovered. It is associated with production of reactive species during direct CAP treatment by cancer cells. Selective production of hydrogen peroxide by different cells can lead to adaptation of chemistry at the plasma-cell interface based on the cellular input. In particular we have found that the discharge voltage is an important factor affecting the ratio of reactive oxygen species to reactive nitrogen species in the gas phase and this correlates well with effect of hydrogen peroxide production by cells. [Preview Abstract] |
Tuesday, October 24, 2017 11:30AM - 12:00PM |
GI2.00005: Novel diagnostics for direct measurements of radical densities in atmospheric pressure plasma jets Invited Speaker: Erik Wagenaars Atmospheric-pressure plasma jets (APPJs) are widely studied for potential applications in industry and healthcare, e.g. surface modification of plastics, plasma medicine and photoresist removal. These plasmas can operate in open air, remain at room temperature and still have a non-equilibrium chemistry. Even though the exact mechanisms through which APPJs affect target surfaces remain largely unknown, it is clear that reactive species play a pivotal role in the success of APPJs. Therefore, reactive species diagnostics of APPJs play an important role in further developing our understanding of the plasma chemistry and will enable increases in treatment efficacy. Two-photon Absorption Laser Induced Fluorescence (TALIF) is a well-known technique for the measurement of absolute densities of atomic radicals such as O, N and H. Unfortunately, application of this technique on APPJs that are operating under realistic conditions for applications, i.e. in open air and with complex admixtures, is not straightforward. The highly collisional environment of APPJs means that collisional quenching of the laser-excited state becomes significant and needs to be taken into account. For well-controlled atmospheres and simple admixtures the effect can be estimated using quenching coefficients, however under realistic operating conditions the identity and density of the quenching partners is unknown due to the complexity of the plasma chemistry. I will present a picosecond TALIF diagnostic which uses a sub-nanosecond laser and iCCD camera that allows the measurement of the quenching-affected fluorescence decay rate directly, enabling absolute measurements of O and N density maps in the open-air effluent of an APPJ. [Preview Abstract] |
Tuesday, October 24, 2017 12:00PM - 12:30PM |
GI2.00006: Equation of state for two-dimensional dusty plasma liquids and its applications. Invited Speaker: Yan Feng Laboratory dusty plasma consists of free electrons, free ions, and micro-sized dust particles with thousands of negative elementary charges. Due to their extremely low charge-to-mass ratio, these dust particles are strongly coupled, arranging themselves like atoms in liquids or solids. Due to the shielding effects of electrons and ions, dust particles interact with each other through the Yukawa potential, so that simulations of Yukawa liquids or solids are used to study properties of dusty plasmas. In the past two decades, the properties of liquid 2D dusty plasmas have been widely studied from experiments to theories and simulations. However, from our literature search, we have not found a quantitative and comprehensive study of properties of 2D liquid dusty plasmas over a wide range of plasma conditions. Here, from molecular-dynamics simulations of Yukawa liquids, we have obtained a concise equation of state (EOS) for the 2D liquid dusty plasmas from empirical fitting, which contains three quantities of the internal pressure, the coupling parameter, and the screening parameter. From this EOS, different thermodynamical processes can be directly derived, such as isotherms, isobars and isochores. Also, various physical properties of 2D liquid dusty plasmas, like the bulk modulus of elasticity, can be analytically derived, so that the sound speeds can be obtained. Finally, an analytical expression of the specific heat for 2D liquid dusty plasmas has been achieved. [Preview Abstract] |
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