Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session GI3: Islands, Kinks and Sawteeth |
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Chair: Jill Foley, Nova Photonics, Inc Room: Centennial II |
Tuesday, November 3, 2009 9:30AM - 10:00AM |
GI3.00001: A New View of Internal Kink Modes and Their Relation to the Sawtooth Instability Invited Speaker: Understanding the physics of sawteeth is crucial for avoiding large sawtooth crashes in tokamaks. Analyses of equilibria reconstructed through consecutive cycles of the ramp and crash phases of DIII-D sawtooth experiments have shed new light on the interplay of the underlying ideal MHD instabilities, the cross section, and the profiles, and a consistent picture is emerging. The discharges comprised two low beta oval and bean shaped discharges and a single null ICRF heated discharge, chosen since they have very different sawtooth characteristics. The crash in the bean is fast and violent but the oval is slow with negligible energy loss. The ICRF discharge exhibited giant sawteeth. The aim is to understand the different characteristics through analysis of both the crash and ramp phases since they are inextricably linked through the triggering mode --- the onset from the linear threshold and the crash by the nonlinear consequences. The results are surprising in view of the conventional sawtooth picture. The underlying ideal mode can have characteristics more like the quasi-interchange mode and this leads to the qualitatively different crash features observed experimentally. Also, the ideal stability does not necessarily degrade during the ramp as $q_0$ decreases; the sawtooth is generally triggered by weakening of the kinetic stabilization. MHD relaxation events observed during the ramp are found to be associated with a nonmonotonic $q$ profile near unity and an underlying quasi-interchange mode, with apparent reconnection at the inner $q=1$ surface. The crash onset itself is described qualitatively by a Porcelli-like model and the return to $q\sim 1$ by full reconnection. However, if the model is to be a quantitative predictor of the crash onset, it will require plausible but specific adjustments to some key terms and coefficients. [Preview Abstract] |
Tuesday, November 3, 2009 10:00AM - 10:30AM |
GI3.00002: Theoretical and JET experimental evidence of a new sawtooth control mechanism by off-axis toroidally propagating ICRF waves Invited Speaker: A new explanation has been given [J. P. Graves, et al, Phys. Rev. Lett. 102, 065005 (2009)] for the highly effective nature of sawtooth control using off-axis toroidally propagating ion cyclotron resonance waves in tokamaks. Energetic passing ions influence the internal kink mode when the distribution of ions is asymmetric in the parallel velocity, a natural feature of co or counter propagating ICRH waves. Dedicated JET experiments have been devised in order to eliminate an alternative explanation of modified sawteeth involving magnetic shear modification. Low concentration He-3 minority was employed because the large drag current of the background plasma yields very poor current drive efficiency. Nevertheless, these ITER relevant ICRH scenarios in JET demonstrate that sawteeth can be controlled effectively. Modelling of these experiments quantifies the role played by the fast ions. Importantly, the extreme sensitivity of the fast ion mechanism to RF deposition location is consistent with the variation of the observed sawtooth period over a change in the magnetic field strength of only a few percent. The success of these JET experiments in controlling sawteeth, and validating the theory, greatly improves the prospect of using the planned ICRH system in ITER to shorten or lengthen sawteeth. [Preview Abstract] |
Tuesday, November 3, 2009 10:30AM - 11:00AM |
GI3.00003: Islands in the Stream: The Effect of Plasma Flow on Tearing Stability Invited Speaker: Reducing plasma flow clearly decreases the stability of tearing modes in multiple regimes (sawtooth, hybrid) in both high- and low-aspect-ratio tokamaks (DIII-D, JET, NSTX, each with distinct means of lessening rotation). Further, reducing flow makes preexisting ``saturated" islands larger at the same beta. Thus lower plasma flow impairs high-beta operation owing both to the destabilization and to the impact of tearing-mode islands. The physics is that {\em flow shear} (not flow) at the tearing rational surface is classically stabilizing, making the tearing stability index $\Delta^\prime$ more negative (more stable). In this picture, with profiles and all else the same, the critical beta at which neoclassical tearing modes (NTMs) destabilize is proportional to $-\Delta^\prime$ and hence lower flow and flow shear lead to destabilization at lower beta. Similarly, if destabilized, the saturated NTM island width is proportional to $-\beta/\Delta^\prime$ and thus increases as flow and flow shear are reduced. A working model for a significant level of stabilizing shear in the plasma toroidal angular flow $-d\Omega_\phi/dr$ at a given rational surface is of order of the inverse of the product of the local values of the parallel magnetic shear length $L_s$ and the Alfv\`en time $\tau_A$. Experimental data are fitted for the effect of this normalization of flow shear in a simple empirical model for both onset and saturation of tearing modes. Most theoretical literature is on the consequence of flow shear on classical tearing stability at zero beta; tokamaks at high beta have large magnetic Prandtl number (an issue for the sign of the flow effect) and very large Lundquist number. It is in this regime that theory will be compared to experimentally based empirical models. The consequence for future tokamaks with low rotation may be lower tearing stability than now expected. [Preview Abstract] |
Tuesday, November 3, 2009 11:00AM - 11:30AM |
GI3.00004: Electron thermal transport within magnetic islands in the RFP Invited Speaker: Magnetic islands formed by tearing reconnection have a significant impact on the thermal characteristics of magnetically confined plasmas such as the reversed-field pinch (RFP). New Thomson scattering diagnostic capability on the MST RFP has enabled measurement of the thermal transport characteristics of islands. Electron temperature (Te) profiles can now be acquired at 25 kHz, sufficient to measure the effect of an island on the profile as it rapidly rotates by the measurement point. In standard plasmas with a spectrum of tearing modes, islands tend to flatten the Te profile across resonant surfaces, as observed in tokamaks. This flattening is characteristic of parallel heat conduction inside the island and is not the result of a stochastic field as it was previously thought. In striking contrast, a temperature gradient within an island is observed in improved confinement plasmas, brought about by a reduction in tearing instability. This suggests local heating and relatively good confinement within the island; local power balance calculations are underway to quantify the island thermal transport. Measurement of transport in 3D magnetic structures is key to understanding the self-organized helical equilibrium in the RFP in which one large island reorganizes the central magnetic field topology. The high rep-rate Thomson scattering diagnostic has also been used to resolve electron thermal transport during the periodic growth and decay of the tearing modes in MST. The measured electron heat conductivity is less than that predicted for transport in a fully stochastic magnetic field (Rechester-Rosenbluth). This suggests that remnant island structures are present, consistent with the direct observations described above. The stochastic field was modeled with 3D nonlinear resistive MHD simulations (DEBS code) with Lundquist numbers matching those in MST during standard discharges. [Preview Abstract] |
Tuesday, November 3, 2009 11:30AM - 12:00PM |
GI3.00005: The Role of Kinetic Effects, Including Plasma Rotation and Energetic Particles, in Resistive Wall Mode Stability Invited Speaker: Continuous, disruption-free operation of tokamaks requires stabilization of the resistive wall mode (RWM). Theoretically, the RWM is thought to be stabilized by energy dissipation mechanisms that depend on plasma rotation and other parameters, with kinetic effects being emphasized.\footnote{B. Hu et al., \textit{Phys. Plasmas} \textbf{12} (2005) 057301.} Experiments in NSTX show that the RWM can be destabilized in high rotation plasmas while low rotation plasmas can be stable, which calls into question the concept of a simple critical plasma rotation threshold for stability. The present work tests theoretical stabilization mechanisms against experimental discharges with various plasma rotation profiles created by applying non-resonant n=3 braking, and with various fast particle fractions. Kinetic modification of ideal stability is calculated with the MISK code, using experimental equilibrium reconstructions. Analysis of NSTX discharges with unstable RWMs predicts near-marginal mode growth rates. Trapped ions provide the dominant kinetic resonances, while fast particles contribute an important stabilizing effect. Increasing or decreasing rotation in the calculation drives the prediction farther from the marginal point, showing that unlike simpler critical rotation theories, kinetic theory allows a more complex relationship between plasma rotation and RWM stability. Results from JT-60U show that energetic particle modes can trigger RWMs\footnote{G. Matsunaga et al., IAEA FEC 2008 Paper EX/5-2.}. Kinetic theory may explain how fast particle loss can trigger RWMs through the loss of an important stabilization mechanism. These results are applied to ITER advanced scenario equilibria to determine the impact on RWM stability. [Preview Abstract] |
Tuesday, November 3, 2009 12:00PM - 12:30PM |
GI3.00006: Kinetic Analysis of Resistive Wall Modes in ITER Advanced Tokamak Scenario Invited Speaker: A fully kinetic analysis of the stability of resistive wall modes in tokamaks is important. However, it is also challenging: first, because the conventional gyrokinetic equation cannot recover MHD; and, second, because the coupling to Alfv\'en continuum damping requires high-resolution computation of a mode near its singular surfaces, which is difficult to achieve with usual non-adaptive codes. Our current effort is aimed at resolving these two difficulties. First, we will describe our new derivation of an extended gyrokinetic theory that can fully recover MHD. Second, we will describe our numerical work to implement this extended gyrokinetic theory in the non-perturbative and non-hybrid kinetic code AEGIS-K as an adaptation of our existing, well-benchmarked AEGIS code. Our numerical results show that n=1 resistive wall modes in the ITER advanced tokamak scenario can be fully stabilized by modestly low rotation with a rotation frequency (normalized to the Alfv\'en frequency at the magnetic axis) of about 0.75\%. Wave-particle resonances, shear Alfv\'en continuum damping, trapped particle effects, and the parallel electric field effects are all taken into account. The rotation frequency for full stabilization is much larger than the diamagnetic drift frequency; therefore, finite Larmor radius effects are negligible. We also find that the window for rotation stabilization window opens first near the ideal wall limit. [Preview Abstract] |
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