Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session JO5: MHD Equilibrium and Stability, Reconnection |
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Chair: Nikolas Logan, Princeton Plasma Physics Lab Room: OCC B113-114 |
Tuesday, November 6, 2018 2:00PM - 2:12PM |
JO5.00001: Magnetic Island detection using dual tangential soft X-ray imaging system on magnetically confined devices Satoshi Ohdachi, Xiaodi D Du, Morgan Shafer, Yasuhiro Suzuki, Todd Evans Dual tangential soft X-ray imaging system was developed for the detection of the asymmetric deformation of the plasma, caused by the RMP application or the MHD instabilities. From the difference of the two images measured at different toroidal locations, deformation of the plasma can be visualized. This system successfully detected the islands-chain of the born-locked modes observed in DIII-D Tokamak [1] using the forward modelling. If the local deformation is determined only by the experimental data, this system is applicable to wider range of issues. Though this kind inverse problem can be not solved in general, it becomes possible if we assume that the perturbation is localized on the rational surfaces and aligned to the magnetic field line. The inverse problem of dual tangential SX system with appropriate regularization will be discussed. [1] Xiaodi Du, et. al., “Direct Measurements of Internal Structures of Born-locked Modes and the Key Role in Triggering Tokamak Disruption”, this conference |
Tuesday, November 6, 2018 2:12PM - 2:24PM |
JO5.00002: Nonlinear plasma response and flow relaxation induced by resonant magnetic perturbation in Rutherford regime Ping Zhu, Xingting Yan, Zihan Li, Wenlong Huang Externally applied non-axisymmetric magnetic fields such as error field and resonant magnetic perturbation (RMP) are known to influence the plasma momentum transport and dynamics through plasma response in a tokamak, whereas the evolution of plasma response itself strongly depends on the plasma flow as well. Such a nonlinear interaction between the two may be captured in a quasilinear model for the coupled system of torque balance and magnetic island evolution equations in the Rutherford regime. For a more complete and self-consistent account, we solve for the nonlinear plasma response and the associated flow relaxation induced by a single-helicity RMP for a circular-shaped limiter tokamak equilibrium with an initial uniform toroidal flow, using the initial-value, full MHD simulation code NIMROD. Time evolution of the parallel flow or "slip frequency'' profile and its asymptotic relaxation to steady state obtained from the NIMROD simulations qualitatively agree with the theory calculations. The difference suggests the nontrivial contribution of non-resonant or kink response to the flow dynamics. |
Tuesday, November 6, 2018 2:24PM - 2:36PM |
JO5.00003: Multi-branch resistive wall instabilities in a resistive plasma Guangzhou Hao, Sanxiang Yang, Yueqiang Liu, Zhengxiong Wang Most of the theory and the modelling efforts on the resistive wall mode (RWM) instability are based on the ideal plasma assumption (i.e., without plasma resistivity). Previous works shown that toroidal favorable average curvature (i.e. GGJ) effect associated with the resistive layer has a stabilization effect on the RWM . In this work, we apply the full toroidal stability code MARS-F to investigate the GGJ effect on the RWM stability in a toroidal resistive plasma. An important conclusion is that there are two instability branches of the RWM, when the GGJ effect is taken into account. The behavior of these two branches (both mode growth rate and mode real frequency) rather different while varying both the Lundquist number and the toroidal plasma rotation frequency. However, only one branch can be found when GGJ effect is excluded. Qualitatively similar results can be obtained by numerically solving the RWM dispersion relation, which includes the resistive layer physics associated with the GGJ effect. |
Tuesday, November 6, 2018 2:36PM - 2:48PM |
JO5.00004: Fast and pervasive heat transport induced by multiple locked modes in DIII-D Qiming Hu, Xiaodi D Du, Qingquan Yu, Nikolas C Logan, Egemen Kolemen, Raffi Nazikian The nonlinear MHD code TM1 [1] is used to understand the transition process from multiple locked modes (LMs) to thermal quench (TQ) in DIII-D. It is found that the co-existence of 2/1, 3/1 and 4/1 locked islands flattens the temperature at the corresponding rational surfaces (RS) and produces a large (~50%) reduction in the central temperature T_{e}. This modeling reproduces the DIII-D experimental results well. The modeled T_{e} profile from 2/1 to 4/1 RS is nearly flattened even in cases with no island overlap. The observed reduction in the edge T_{e}, however, requires island overlap within the TM1 model. The modeled T_{e} profile is reduced further when applying larger EFs that drive larger island widths, wider edge stochastic regions and secondary island structures. These results indicate that the co-existence of multiple LMs deteriorate plasma thermal confinement more than the sum of their isolated impacts would and that this may be responsible for the fast TQ observed prior to major disruptions. [1] Yu Q., Phys. Plasmas 10 (2003) 797 |
Tuesday, November 6, 2018 2:48PM - 3:00PM |
JO5.00005: Modification of the Alfvén spectrum by 3D density inhomogeneities James Oliver, Sergei Sharapov, Boris Breizman, David Terranova, Donald Spong Alfvén eigenmodes driven unstable by energetic particles are routinely observed in tokamak plasmas. The most frequently observed Alfvén eigenmodes are gap modes, which consist of poloidal harmonics coupled by inhomogeneity in the magnetic field. Further coupling can be introduced by 3D inhomogeneities in the plasma density, for example, transiently during the assimilation of injected pellets. The material deposited by the pellet temporarily breaks the toroidal and poloidal symmetry of the density profile, causing additional coupling of poloidal and toroidal harmonics. This additional coupling modifies the Alfvén continuum and discrete eigenmode spectrum. In JET, we observe significant changes to Alfvén eigenmodes during pellet injection [1]. From observed changes in the frequency of the Alfvén eigenmodes, information about the changes in the plasma density due to pellets can be inferred. To use Alfvén eigenmodes for MHD spectroscopy of pellet injected plasmas, we generalised the 3D MHD codes Stellgap [2] and AE3D [3] to incorporate 3D density profiles. Hence, we obtain the Alfvén continuum and discrete eigenmode spectrum for tokamak plasmas with pellet injection. We compare this numerical work to analytical results of mode coupling due to density inhomogeneities. |
Tuesday, November 6, 2018 3:00PM - 3:12PM |
JO5.00006: New results for the finite ion orbit width effect on NTM threshold Koki Imada, Howard R Wilson, Jack W Connor, Alexandra Dudkovskaia, Peter Hill For successful operation of future tokamaks, such as ITER, it is essential to control neoclassical tearing modes (NTMs): plasma instabilities characterized by the evolution of magnetic islands. Experiments show that sufficiently small magnetic islands heal themselves and shrink away. Possible origins of this threshold have been suggested, e.g. the effects of finite radial transport, finite Larmor radius and finite banana width. However, an NTM control system requires a deeper understanding of the threshold physics, for predicting the threshold island width w_{c}. This paper investigates the effect of finite particle orbit width on the NTM threshold in toroidal geometry, considering a small island width w comparable to the ion poloidal Larmor radius ρ_{θi}. We find that the perturbed ion distribution function profile shifts radially by O(ρ_{θi}), relative to the magnetic island location. When w ~ ρ_{θi}, this shift substantially restores the density gradient that is flattened across the island, weakening the bootstrap current drive for the island growth. The result is the threshold behavior, with the critical island width scaling as: w_{c} ~ 2.7ρ_{θi}, in the absence of other effects. These results will provide valuable information toward the development of effective NTM control system for ITER. |
Tuesday, November 6, 2018 3:12PM - 3:24PM |
JO5.00007: Effects of thin wall on resistive wall modes in KTX Rui Han, Ping Zhu, Wei Bai, Tao Lan, Wandong Liu Keda Torus eXperiment (KTX) is a newly built reversed field pinch (RFP) device with a copper thin shell. The stability and control of resistive wall modes (RWMs) are essential for the sustained operation of KTX in high parameter regimes. In this work, we investigate the effects of thin wall on the RWM in KTX, based on a cylindrical α-θ model of the RFP equilibrium. We first evaluate the analytical dispersion relation for RWM formulated from the MHD energy principle, and then calculate the dominant linear growth rate of RWM using the full MHD code NIMROD. The dependences of RWM growth rate on the position and the conductivity of vacuum wall for KTX are in qualitative agreement between the two types of calculations. The radial location for the transition from RWM to external kink mode is identified to be 0.53m, same for various wall resistivity conditions, which is well beyond the KTX copper wall location (r=0.4m). |
Tuesday, November 6, 2018 3:24PM - 3:36PM |
JO5.00008: Magnetic Reconnection Sustained by the Thermonuclear Heating of the Electron Population Renato Gatto, Bruno Coppi Endogenous modes associated with a finite electron temperature gradient can be sustained by the electron heating rate due to the charged reaction products in fusion burning plasmas [1]. These modes, that are of the quasi-flute type, are localized around selected rational magnetic surfaces [1], involve the effects of finite and anisotropic electron thermal conductivities and depend directly on the amplitude of the perturbed electron temperature and of the reconnected fields. In this case, the effective longitudinal thermal conductivity on the relevant rational surfaces is shown to be decreased, relative to its collisional value, by the reconnected magnetic field component. If the relevant decrease is significant, rational magnetic surfaces can acquire a favorite role in the process of plasma heating by fusion reaction. [1] B. Coppi, et al., Nucl. Fusion, 55, 053011 (2015). |
Tuesday, November 6, 2018 3:36PM - 3:48PM |
JO5.00009: Effect of background polarized light in magnetic equilibrium reconstruction Jinseok Ko, Steven Douglas Scott, Robert T Mumgaard In addition to the multi-channel motional Stark effect (MSE) diagnostic system for the Korea Superconducting Tokamak Advanced Research (KSTAR), the polychrometer-based MSE system with the same number of sight lines has been adopted for the simultaneous measurement and subtraction of the background polarized light. Two independent MSE systems are compared to estimate the effect of the background polarized light with intermediate plasma densities on the magnetic equilibrium reconstruction. The systematic errors introduced from the background polarized light evaluated from this comparative studies can be extrapolated for high-density plasma discharges such as those in ITER. |
Tuesday, November 6, 2018 3:48PM - 4:00PM |
JO5.00010: Fast BIE method for computing force-free magnetic fields in toroidal geometries Dhairya Malhotra, Antoine J Cerfon, Mike O'Neil We develop a boundary integral equation solver for computing Taylor relaxed states in a given region bounded by flux surfaces. The region can be bounded by a single toroidal surface or it can be the region between two nested toroidal surfaces. Such solutions can be used to study ideal magneto-hydrodynamic (MHD) equilibria with stepped pressure profiles in magnetically confined plasmas in tokamaks and stellarators. |
Tuesday, November 6, 2018 4:00PM - 4:12PM |
JO5.00011: Vacuum Solution for Solov'ev's Equilibrium Configuration in Tokamaks Tao Xu, Linjin Zheng, Mike Kotschenreuther, Richard Fitzpatrick, Francois L. Waelbroeck In this work we have revised the Solov'ev analytical solution for tokamak equilibrium. We point out that this solution in the Solov'ev formulation is inapplicable in the vacuum, since it assumes a finite current density even outside the separatrix. The realistic vacuum should be current-free. To amend this vacuum solution problem, we use the Green function method to compute the vacuum fields caused by the plasma current, together with the fields corresponding to the homogeneous solution to the Grad-Shafranov equation. To construct the full solution, we match the vacuum field with the Solov'ev solution on the last closed flux surface. The total solution is then extended to the vacuum region to get the realistic vacuum solution. We find that the actual vacuum solution is vastly different from the Solov'ev solution, especially the X-point structure. The X-point solution is not of hyperbola type. Sometimes, even extra X points occur on the outboard midplane. The results are important for understanding the X point and separatrix structure. |
Tuesday, November 6, 2018 4:12PM - 4:24PM |
JO5.00012: Relationship between the H-mode in tokamaks and the finite-Larmor-radius (FLR) modified Beltrami/Taylor state Wei-li W Lee, Roscoe B White It has been shown that the radial electric field at the H-mode pedestal is the result of charge separation between the ions and electrons due to the FLR effects in the presence of a sharp pressure gradient [1]. The resulting radial electric field, which compares well with many of the experiments as shown in Ref. [1], then gives rise to the ${\bf E} \times {\bf B}$ drift for both the electrons and the ions. Again, because of the FLR effects, it results in a net current in the poloidal direction, which modifies the gyrokinetic vorticity equation [2]. In the Beltrami/Taylor state [3], this current vanishes and the resulting gyrokinetic MHD equations [4] resemble those of Strauss [5], which are MHD stable for $q > 1$. Details will be presented. |
Tuesday, November 6, 2018 4:24PM - 4:36PM |
JO5.00013: The generalized offset toroidal rotation for total neoclassical toroidal viscosity torque in a tokamak Xingting Yan, Ping Zhu For each ion species, its contribution to neoclassical toroidal viscosity (NTV) torque vanishes at the conventional offset rotation for that species. In general, the amplitude of NTV torque of each species can become comparable, thus the total NTV torque can be quite different from those of single species alone. Thus a generalized offset toroidal rotation for total NTV torque may exist for which the total NTV torque vanishes, defining a natural relaxation level of toroidal rotation above/below which the total NTV torque tends to brake/accelerate the toroidal flow. In this work, we evaluate the generalized offset toroidal rotation for total NTV torque and its dependence on plasma parameters using a coupling scheme between NIMROD and NTVTOK codes developed in our previous work. We find that the generalized offset toroidal rotation for total NTV torque can deviate significantly from those determined from single species contributions, especially in low collisionality and slow rotation regimes where ion and electron contributions are often comparable and opposite. |
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