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 TI3: Stability, Scenarios, and MHD |
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Chair: Fatima Ebrahimi, Princeton Plasma Physics Laboratory Room: 103ABC |
Thursday, October 26, 2017 9:30AM - 10:00AM |
TI3.00001: Predict-first experimental analysis using automated and integrated MHD modeling Invited Speaker: Brendan C. Lyons The success of ITER and future fusion reactors would benefit significantly from theoretical predictions of stability and performance that have been validated against contemporary tokamaks. We present results of a predict-first analysis of transient-stability experiments using OMFIT [Meneghini, Nucl. Fusion 2015] integrated-modeling workflows. In particular, we look at the effect of shape variation on plasma response to 3D magnetic perturbations in order to predict access to ELM suppression. Beginning from equilibrium reconstructions of past experiments, we use EFIT [Lao, Nucl. Fusion 1985] to modify the shape of equilibria while other plasma parameters (e.g., beta, pedestal density) are held constant. Additional complexity in the workflows are considered, including using EPED [Snyder, Nucl. Fusion 2011] and NEO [Belli, PPCF 2012] to create pedestals with self-consistent pressure and bootstrap current profiles. We then use the autoC1 script, developed to automate linear M3D-C1 [Jardin, Comput. Sci. Discovery 2012] extended-MHD simulations, to assess the plasma response of each predicted equilibrium as though it is a reconstruction of a future experiment. Results from these workflows are used to guide subsequent experiments on DIII-D and KSTAR, and are then validated against 3D magnetics and ELM-suppression observations from the completed experiments. The validation is then used to develop workflows that would have more accurately predicted the experiment. In doing so, the physics models and ELM-suppression metrics considered are improved with every iteration of prediction and experiment. Lessons learned for best practices in predict-first studies and plans for future areas of application (e.g., disruption avoidance) are discussed. [Preview Abstract] |
Thursday, October 26, 2017 10:00AM - 10:30AM |
TI3.00002: Understanding the stability of the low torque ITER Baseline Scenario in DIII-D Invited Speaker: Francesca Turco Analysis of the evolving current density (J), pedestal and rotation profiles in a database of 200 ITER Baseline Scenario discharges in the DIII-D tokamak sheds light on the cause of the disruptive instability limiting both high and low torque operation of these plasmas. The m$=$2/n$=$1 tearing modes, occurring after several pressure-relaxation times, are related to the shape of the current profile in the outer region of the plasma. The q$=$2 surface is located just inside the current pedestal, near a minimum in J. This well in J deepens at constant betaN and at lower rotation, causing the equilibrium to evolve towards a classically unstable state. Lack of core-edge differential rotation likely biases the marginal point towards instability during the secular trend in J. New results from the 2017 experimental campaign establish the first reproducible, stable operation at T$=$0 Nm for this scenario. A new ramp-up recipe with delayed heating keeps the discharges stable without the need for ECCD stabilization. The J profile shape in the new shots is consistent with an expansion of the previous "shallow well" stable operational space. Realtime Active MHD Spectroscopy (AMS) has been applied to IBS plasmas for the first time, and the plasma response measurements show that the AMS can help sense the approach to instability during the discharges. The AMS data shows the trend towards instability at low rotation, and MARS-K modelling partially reproduces the experimental trend if collisionality and resistivity are included. The modelling results are sensitive to the edge resistivity, and this can indicate that the AMS is measuring the changes in ideal (kink) stability, to which the tearing stability index delta' is correlated. Together these results constitute a crucial step to acquire physical understanding and sensing capability for the MHD stability in the Q$=$10 ITER scenario. Work supported by US DOE under DE-FC02-04ER54698 and DE-FG02-04ER54761 [Preview Abstract] |
Thursday, October 26, 2017 10:30AM - 11:00AM |
TI3.00003: Advanced Tokamak Investigations in Full-Tungsten ASDEX Upgrade Invited Speaker: Alexander Bock The tailoring of the $q$-profile is the foundation of Advanced Tokamak (AT) scenarios. It depends on low collisionality $\nu^*$ which permits efficient external current drive and high amounts of intrinsic bootstrap current. At constant pressure, lowering $n_\mathrm{e}$ leads to a strong decrease of $\nu^* \sim {T_\mathrm{e}}^{-3}$.\\ After the conversion of ASDEX Upgrade to fully W-coated plasma facing components, radiative collapses of H-modes with little gas puffing due to central W accumulation could only be avoided partially with central ECRH. Also, operation at high $\beta$ with low $n_\mathrm{e}$ presented a challenge for the divertor. Together, these issues prevented meaningful AT investigations.\\ To overcome this, several major feats have been accomplished: Access to lower $n_\mathrm{e}$ was achieved through a better understanding of the changes to recycling and pumping, and optionally the density pump-out phenomenon due to RMPs. ECRH capacities were substantially expanded for both heating and current drive, and a solid W divertor capable of withstanding the power loads was installed. A major overhaul improved the reliability of the current profile diagnostics.\\ This contribution will detail the efforts needed to re-access AT scenarios and report on the development of candidate steady state scenarios for ITER/DEMO. Starting from the `hybrid scenario,' a non-inductive scenario ($q_{95}=5.3$, ${\beta_\mathrm{N}}=2.7$, ${f_\mathrm{bs}}>40\%$) was developed. It can be sustained for many $\tau_\mathrm{E}$, limited only by technical boundaries, and is also independent of the ramp-up scenario. The $\beta$-limit is set by ideal modes that convert into NTMs. The $T_\mathrm{i}$-profiles are steeper than predicted by TGLF, but nonlinear electromagnetic gyro-kinetic analyses with GENE including fast particle effects matched the experimental heat fluxes. We will also report on scenarios at higher $q_{95}$, similar to the EAST/DIII-D steady state scenario. The extrapolation of these scenarios to ITER/DEMO will be discussed. [Preview Abstract] |
Thursday, October 26, 2017 11:00AM - 11:30AM |
TI3.00004: Non-Inductively Driven Tokamak Plasmas at Near-Unity Toroidal Beta in the Pegasus Toroidal Experiment Invited Speaker: Joshua Reusch A major goal of the spherical tokamak research program is accessing a state of low internal inductance $l_{i}$, high elongation $\kappa $, high toroidal and normalized beta ($\beta_{t}$ and $\beta_{N})$, and low collisionality without solenoidal current drive. A new local helicity injection (LHI) system in the lower divertor region of the ultra-low aspect ratio Pegasus ST provides non-solenoidally driven plasmas that exhibit most of these characteristics. LHI utilizes compact, edge-localized current sources ($A_{inj}\sim $ 4 cm$^{\mathrm{2}}$, $I_{inj}\sim $ 8 kA, $V_{inj}\sim $ 1.5 kV) for plasma startup and sustainment, and can sustain more than 200 kA of plasma current. Plasma growth via LHI is enhanced by a transition from a regime of high kink-like MHD activity to one of reduced MHD activity at higher frequencies and presumably shorter wavelengths. The strong edge current drive provided by LHI results in a hollow current density profile with low $l_{i}$. The low aspect ratio ($R_{0}/a\sim $ 1.2) of Pegasus allows ready access to high $\kappa $ and MHD stable operation at very high normalized plasma currents ($I_{N}= I_{p}$\textit{/aB}$_{T}$ \textgreater $_{\mathrm{\thinspace }}$15). Thomson scattering measurements indicate $T_{e}\sim $ 100 eV and $n_{e}_{\mathrm{\thinspace }}\sim $ 1$\times $19 m$^{\mathrm{-3}}$. The impurity $T_{i}$ evolution is correlated in time with high frequency magnetic fluctuations, implying substantial reconnection ion heating is driven by the applied helicity injection. Doppler spectroscopy indicates $T_{i}\ge T_{e}$ and that the anomalous ion heating scales consistently with two fluid reconnection theory. Taken together, these features provide access to very high $\beta_{t}$ plasmas. Equilibrium analyses indicate $\beta_{t}$ up to $\sim $100{\%} and $\beta _{N}\sim $ 6.5 is achieved. At increasingly low $B_{T}$, the discharge disrupts at the no-wall ideal stability limit. In these high $\beta_{t}$ discharges, a minimum \textit{\textbar B\textbar } well forms over $\sim $50{\%} of the plasma volume. This unique magnetic configuration may be of interest for testing predictions of stabilizing drift wave turbulence and/or improving energetic particle confinement. [Preview Abstract] |
Thursday, October 26, 2017 11:30AM - 12:00PM |
TI3.00005: Exploring nuclear reactions relevant to Stellar and Big-Bang Nucleosynthesis using High-Energy-Density plasmas at OMEGA and the NIF Invited Speaker: M. Gatu Johnson Thermonuclear reaction rates and nuclear processes have been explored traditionally by means of accelerator experiments, which are difficult to execute at conditions relevant to Stellar Nucleosynthesis (SN) and Big Bang Nucleosynthesis (BBN). High-Energy-Density (HED) plasmas closely mimic astrophysical environments and are an excellent complement to accelerator experiments in exploring SN and BBN-relevant nuclear reactions. To date, our work using HED plasmas at OMEGA and NIF has focused on the complementary $^{\mathrm{3}}$He$+^{\mathrm{3}}$He, T$+^{\mathrm{3}}$He and T$+$T reactions. First studies of the T$+$T reaction indicated the significance of the $^{\mathrm{5}}$He ground-state resonance in the T$+$T neutron spectrum. Subsequent T$+$T experiments showed that the strength of this resonance varies with center-of-mass (c-m) energy in the range of 16-50 keV, a variation that is not fundamentally understood. Studies of the $^{\mathrm{3}}$He$+^{\mathrm{3}}$He and T$+^{\mathrm{3}}$He reactions have also been conducted at OMEGA at c-m energies of 165 keV and 80 keV, respectively, and the results revealed three things. First, a large cross section for the T$+^{\mathrm{3}}$He-$\gamma $ branch can be ruled out as an explanation for the anomalously high abundance of $^{\mathrm{6}}$Li in primordial material. Second, the results contrasted to theoretical modeling indicate that the mirror-symmetry assumption is not enough to capture the differences between T$+$T and $^{\mathrm{3}}$He$+^{\mathrm{3}}$He reactions. Third, the elliptical spectrum assumed in the analysis of $^{\mathrm{3}}$He$+^{\mathrm{3}}$He data obtained in accelerator experiments is incorrect. Preliminary data from recent experiments at the NIF exploring the $^{\mathrm{3}}$He$+^{\mathrm{3}}$He reaction at c-m energies of \textasciitilde 60~keV and \textasciitilde 100 keV also indicate that the underlying physics changes with c-m energy. In this talk, we describe these findings and future directions for exploring light-ion reactions at OMEGA and the NIF. The work was supported in part by the US DOE, LLE, and LLNL. [Preview Abstract] |
Thursday, October 26, 2017 12:00PM - 12:30PM |
TI3.00006: Dynamics of Plasma Jets and Bubbles Launched into a Transverse Background Magnetic Field Invited Speaker: Yue Zhang A coaxial magnetized plasma gun has been utilized to launch both plasma jets (open B-field) and plasma bubbles (closed B-field) into a transverse background magnetic field in the HelCat (Helicon-Cathode) linear device at the University of New Mexico [1]. These situations may have bearing on fusion plasmas (e.g. plasma injection for tokamak fueling, ELM pacing, or disruption mitigation) and astrophysical settings (e.g. astrophysical jet stability, coronal mass ejections, etc.). The magnetic Reynolds number of the gun plasma is $\sim100$, so that magnetic advection dominates over magnetic diffusion. The gun plasma ram pressure, $\rho _{jet}V_{jet}^{2}>B_{0}^{2}/2\mu _{0}$ , the background magnetic pressure, so that the jet or bubble can easily penetrate the background B-field, $B_{0}$. When the gun axial B-field is weak compared to the gun azimuthal field, a current-driven jet is formed with a global helical magnetic configuration. Applying the transverse background magnetic field, it is observed that the $n =1$ kink mode is stabilized, while magnetic probe measurements show contrarily that the safety factor q(a) drops below unity. At the same time, a sheared axial jet velocity is measured. We conclude that the tension force arising from increasing curvature of the background magnetic field induces the measured sheared flow gradient above the theoretical kink-stabilization threshold [2], resulting in the emergent kink stabilization of the injected plasma jet. In the case of injected bubbles, spheromak-like plasma formation is verified. However, when the spheromak plasma propagates into the transverse background magnetic field, the typical self-closed global symmetry magnetic configuration does not hold any more. In the region where the bubble toroidal field opposed the background B-field, the magneto-Rayleigh-Taylor (MRT) instability has been observed. Details of the experiment setup, diagnostics, experimental results and theoretical analysis will be presented. \par $\ast$~~This work performed in collaboration with D. Fisher, A. G. Lynn, M Gilmore, and S. C. Hsu. \par $\ast\ast$~The author's present address: Univ. of Washington \par [1] M. Gilmore et al., J. Plasma Phys. 81, 345810104 (2015). \par [2] U Shumlak et al., Phys. Rev. Lett. 75(18):3285 (1995). [Preview Abstract] |
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