Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session NI3: Flow and MHD in Stellarators and RFPs |
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Chair: John Sarff, University of Wisconsin Room: Ballroom BC |
Wednesday, October 31, 2012 9:30AM - 10:00AM |
NI3.00001: Direct Evidence of Magnetic Fluctuation-Driven Intrinsic Flow in a Toroidal Plasma Invited Speaker: Weixing Ding Exploiting fluctuation-driven torque is highly desirable for ITER where the efficiency of externally applied torque will likely be limited. Plasma pressure fluctuations, electric field fluctuations and magnetic field fluctuations have been long observed in various magnetic confinement configurations and can all play a role in intrinsic plasma flow. Electric and magnetic field fluctuations can act on plasma flow via the Reynolds stress or Maxwell stress, respectively. In addition, the interaction between plasma parallel pressure and magnetic field fluctuations can also generate a net torque (kinetic stress) that drives plasma flow parallel to magnetic field. This stress stems from the divergence of the magnetic-fluctuation-driven momentum flux. Direct experimental evidence of the kinetic stress and its relation to intrinsic flow has been observed in the MST reversed field pinch. Using advanced polarimetry and differential interferometry techniques, measurements of the torque amplitude, direction and spatial distribution are made in the plasma core. The magnitude of the force is approximately 0.5 N/m$^{3}$ at r/a$\sim $0.2 comparable to the observed flow acceleration. The stress is co-current directed in the core, and changes sign near mid-radius and the parallel flow profile has a similar shape. The correlated fluctuations generating the torque are related to tearing instabilities. The saturation mechanism of flow is qualitatively consistent with stochastic magnetic field induced damping. When magnetic fluctuations are suppressed by current profile control, the flow is also observed to decrease. These results strongly indicate the magnetic fluctuation-driven kinetic stress plays important role in plasma flow. [Preview Abstract] |
Wednesday, October 31, 2012 10:00AM - 10:30AM |
NI3.00002: RFX-MOD: A Multi-Configuration Fusion Facility for 3D Physics Studies Invited Speaker: Paolo Piovesan RFX-mod exploits its 192 active coils in both RFP and tokamak configurations with varying degree of 3D shaping, providing also a test bed for validating stellarator codes. This makes RFX a unique and flexible facility for comparative studies on 3D shaping and control, key tools for advanced fusion scenarios. The talk discusses how 3D fields allow access to RFP and tokamak advanced regimes. 3D fields are used to feedback control Single Helicity (SH) RFP equilibria with 1/7 helicity up to $\sim $2MA. This ensures record persistence of SH magnetic equilibrium, nearly 100{\%} of flat-top, avoiding back-transitions typical of spontaneous SH. It also allows accessing SH regimes with higher density (Greenwald fraction up to 0.5) presently inaccessible in spontaneous SH. 3D shaping opens for RFPs the path to helical divertor concepts similar to stellarator, through the control of the region of maximum PWI. Preliminary thoughts will be presented. Feedback on the 2/1 RWM in RFX tokamak allows for safe operation at q95$<$2, a almost unexplored promising regime. Forcing the 2/1 mode at finite small level, we produce a helical tokamak equilibrium with significant n=1 modulation and we find a new way to control sawteeth: their amplitude and period are inversely proportional to the 2/1 amplitude. This is reproduced with non-linear MHD code PIXIE3D. This new sawtooth control tool might be applied in larger tokamaks. The effects of different levels of 3D shaping on momentum transport are discussed. The strong 3D shaping in the SH RFP makes the flow follow a helical pattern. This flow and its shear increase with the level of 3D shaping, providing a way to control rotation. In tokamak the applied 2/1 non-resonant field -- though much smaller than in RFP - strongly affects toroidal flow: when is increased the plasma first decelerates and then accelerates in the opposite direction. This is compared with NTV theory. [Preview Abstract] |
Wednesday, October 31, 2012 10:30AM - 11:00AM |
NI3.00003: Self-consistent simulations of nonlinear MHD and profile evolution in stellarator configurations Invited Speaker: Mark Schlutt Self-consistent MHD equilibrium and nonlinear stability of 3D magnetic configurations are investigated using the extended MHD code NIMROD. In these calculations, initial conditions are given by 3D vacuum solutions with robust magnetic surfaces. We examine two classes of problems: those with current-driven instabilities and those with pressure-driven instabilities. Ohmic discharges in the Compact Toroidal Hybrid (CTH) are simulated [1]. The vacuum magnetic field of CTH is initialized and current is driven by specifying a toroidal electric field at the vessel boundary. The driven current penetrates toward the core and raises the rotational transform profile. Island formation is observed that is linked to the n=5 periodicity of the device. A prominent feature of these simulations is the coalescence of n/m=5/10 islands to n/m=1/2 islands when the rotational transform exceeds 0.5. At high levels of current drive, complete flux surface destruction is observed. Comparison with CTH data will be presented. Finite beta discharges in a straight stellarator are simulated. Vacuum magnetic fields are applied to produce stellarator-like rotational transform profiles with iota(0)$\le $0.5 and iota(0)$\ge $0.5. The vacuum magnetic fields are either helically symmetric or spoiled by the presence of magnetic harmonics of incommensurate helicity. As heat is added to the system, pressure-driven instabilities are excited when a critical $\beta $ is exceeded. These instabilities may cause disruption, or they may saturate nonlinearly as the equilibrium evolves. In all of these studies, anisotropic heat conduction is allowed with k$_{par}$/k$_{perp}$ = 10$^{5}$--10$^{7}$. Due to the finite parallel heat conduction, in some cases an equilibrium state persists that has a stochastic edge region which supports a pressure gradient. \\[4pt] [1] M.G. Schlutt, et al., submitted to Nuclear Fusion (2012). [Preview Abstract] |
Wednesday, October 31, 2012 11:00AM - 11:30AM |
NI3.00004: 3D effects on viscosity and generation of toroidal and poloidal flows in LHD Invited Speaker: Kenichi Nagaoka Transport is strongly affected by the breaking of toroidal symmetry as seen in the resonant magnetic perturbation experiment in tokamaks, topology bifurcation in reversed field pinches and intrinsically in helical plasmas. In these experiments, the transport parallel to the magnetic field contributes to radial diffusion process, which is recognized as 3D effect on transport. This effect is most significant in the momentum transport, because the plasma flow is sensitive to the change in topology of magnetic field. In this paper, recent experimental results on damping and driving mechanism of plasma flow in 3D topology are discussed. The parallel viscosity is relatively large in the edge region of helical plasmas, but, perpendicular viscosity due to turbulence is dominated in the core region in LHD similar to tokamaks. Significant intrinsic toroidal flow in the co-direction is observed at the mid-radius of the plasma where a large ion temperature gradient (ion ITB) exists, which is driven by turbulence in the nested magnetic surface. The flattening of electron and ion temperature profiles and toroidal flow profile has been observed associated with the topology change from nesting magnetic flux surface to stochastic magnetic field structure. The significant reduction of toroidal flow indicates the increase of viscosity. A large poloidal flow driven by difference of ion and electron parallel transports along the field line was observed in the edge region where the field lines are stochastic and open. The topological change of magnetic field structure contributes to the formation of strong radial electric field shear, which should have strong impact on heat and particle transport at LCFS. [Preview Abstract] |
Wednesday, October 31, 2012 11:30AM - 12:00PM |
NI3.00005: Simulation Study on Neoclassical Poloidal Viscosity in Helical Plasmas Invited Speaker: Shinsuke Satake In helical plasma confinement devices such as LHD, CHS and TU-Heliac, biasing experiments have been carried out to study the relationships among the ExB rotation, neoclassical poloidal viscosity (NPV), JxB torque of biasing current, and plasma confinement properties. In earlier studies using simple analytic formulae, it has been suggested that the transition phenomena of plasma transport found in the biasing experiments is attributed to nonlinear dependence of NPV on poloidal Mach number of the ExB rotation speed, or Mp. To study the NPV dependence on Mp in LHD biasing plasmas more in detail, we have applied FORTEC-3D drift-kinetic Monte-Carlo simulation code which can evaluate NPV precisely in realistic 3-D magnetic configurations. This is the first application of the massive neoclassical transport simulation to study the dependence of NPV on the magnetic configuration and rotation speed. In LHD plasmas, neoclassical transport properties such as radial particle transport and viscosity can be controlled by shifting the magnetic axis position. Our simulation study revealed that the NPV is drastically reduced if magnetic axis moves from 3.75m to 3.53m. As the biasing voltage, or Mp increases, it is found that the local maximum of NPV appears when $\vert $Mp$\vert \sim $1, at which the transition of plasma transport properties is expected to happen. The transition Mp value is much smaller than that is predicted from simple analytic estimations. Comparing with the data from LHD biasing experiments, we confirmed that Mp near the electrode is about unity when a transition occurs, and also found that the peak NPV value at $\vert $Mp$\vert \sim $1 agrees with the magnitude of JxB torque at the transition point. This suggests that our simulation successfully explains the nonlinear dependence of NPV and can give a quantitative evaluation of NPV in realistic LHD biasing experiment. [Preview Abstract] |
Wednesday, October 31, 2012 12:00PM - 12:30PM |
NI3.00006: Measurement and Modeling of Large Helical Flows in the HSX Stellarator Invited Speaker: Alexis Briesemeister Symmetry in a device's magnetic field strength allows large flows to develop, which may reduce turbulent transport. Although symmetry is an inherent feature of tokamaks and other axisymmetric devices, stellarators typically do not have a direction of symmetry. The quasihelically symmetric HSX stellarator is the only device with a helical direction of approximately constant magnetic field strength. We present here first results that verify the capability for the class of quasisymmetric stellarators to have large intrinsic flows. Flow velocities of up to 20 km/s along the helical direction, with no external momentum injection, have been measured using charge exchange recombination spectroscopy in HSX. Measurements are made using the 529 nm C+5 line at 10 radial locations from two viewing directions allowing the flow direction and magnitude to be determined. These measured flows are compared to the neoclassical values calculated by the PENTA code [1]. A non-momentum conserving collision operator is used when solving the drift kinetic equation for stellarators, which typically have large flow damping in all directions. HSX's parallel flow is under-predicted by an order of magnitude by the non-momentum conserving calculations, but good agreement is seen with parallel flows calculated by PENTA when a momentum conservation correction technique [2] is applied. In addition to verifying a key attribute of quasisymmetric stellarators, these results validate a neoclassical code that can calculate plasma flows in a wide range of toroidal devices from perfectly axisymmetric systems to fully 3D configurations. This allows the effects of symmetry breaking magnetic field components, which can increase flow drive as well as damping, to be studied.\\[4pt] [1] D. Spong, Phys. Plas. 12 (2005) 056114.\\[0pt] [2] H. Sugama, S. Nishimura, Phys. Plas. 9 (2002) 4637. [Preview Abstract] |
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