Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session UI2: Innovative Confinement Concepts and High Beta |
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Chair: Jonathan Menard, Princeton Plasma Physics Laboratory Room: Adam's Mark Hotel Plaza Ballroom EF |
Friday, October 28, 2005 9:30AM - 10:00AM |
UI2.00001: Production and Study of High-Beta Plasma Confined by a Superconducting Dipole Magnet Invited Speaker: The \urllink{Levitated Dipole Experiment (LDX)}{http://psfcwww2.psfc.mit.edu/ldx/} is a new research facility that is exploring the confinement and stability of plasma created within the dipole field produced by a strong superconducting magnet. Unlike other configurations in which stability depends on curvature and magnetic shear, MHD stability of a dipole derives from plasma compressibility. Theoretically, the dipole magnetic geometry can stabilize a centrally-peaked plasma pressure that exceeds the local magnetic pressure ($\beta > 1$), and the absence of magnetic shear allows particle and energy confinement to decouple. In this presentation, the first experiments using the LDX facility are reported. Long-pulse, quasi-steady state microwave discharges lasting up to 12 seconds have been produced that are consistent with equilibria having peak beta values of 10\%. Detailed measurements have been made of discharge evolution, plasma dynamics and instability, and the roles of gas fueling, microwave power deposition profiles, and plasma boundary shape. In these initial experiments, the high-field superconducting floating coil was supported by three thin supports and later the coil will be magnetically levitated. The plasma was created by multi- frequency electron cyclotron resonance heating at 2.45 and 6.4 GHz, and a population of energetic electrons, with mean energies above 50 keV, dominated the plasma pressure. Creation of high-pressure, high-beta plasma is only possible when intense hot electron interchange instabilities are stabilized sufficiently by a high background plasma density. A dramatic transition from a low-density, low-beta regime to a more quiescent, high-beta regime is observed when the plasma-fueling rate and confinement times are sufficiently long. External shaping coils are seen to modify the outer plasma boundary and affect the transition. [Preview Abstract] |
Friday, October 28, 2005 10:00AM - 10:30AM |
UI2.00002: Transport and fluctuations in high temperature spheromak plasmas Invited Speaker: A systematic analysis is presented of thermal transport in a driven spheromak that extends well into the collisionless regime and spans a wide range of magnetic fluctuation levels. The relationship between internal fluctuations and energy/helicity transport is of fundamental interest to many self-organized configurations in laboratory and space plasmas and the subject of ongoing multi-institutional collaborations. With the recent achievement on the SSPX spheromak[1] of electron temperature $T_e \sim 350eV$ in the core, and good confinement (core electron thermal diffusivity $\chi _e <10m^2/\sec $ for $T_e >200eV)$, we are now comparing heat transport in the experiment with a variety of models including classical, Bohm, and stochastic[2]/diffusive[3]/open[4] field lines. Using Thomson scattering to measure $T_e $, $n_e $ profiles and the CORSICA equilibrium code to calculate internal current profiles from magnetic probe fits, we find that $\chi _e $ decreases as $T_e $ increases, a scaling behavior more classical-like than Bohm or open field line models would indicate. Lower $T_e $ and higher $\chi _e $ is observed in the transition region between the core and the separatrix where NIMROD 3d resistive MHD calculations[5] show the possible existence of chaotic field lines. We will also discuss plans including multi-pulse Thomson scattering and neutral beam heating. [1] E.B. Hooper, et al., Nucl. Fusion 39, 863 (1999). [2] A.B. Rechester and M.N. Rosenbluth, Phys. Rev. Lett. 40, 38 (1978). [3] J.D. Callen, Phys. Rev. Lett. 94, 055002 (2005). [4] R.W. Moses, et al., Phys. Plasmas 8, 4839 (2001). [5] B.I. Cohen, et al., Phys. Plasmas 12, 056106 (2005). This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. [Preview Abstract] |
Friday, October 28, 2005 10:30AM - 11:00AM |
UI2.00003: Access to Sustained High-Beta With Internal Transport Barrier and Negative Central Shear in DIII-D Invited Speaker: High values of normalized pressure ($\beta_N\sim\,$4) and safety factor ($q_{min}\sim\,$2) have been sustained simultaneously for $\sim$2~s in \hbox{DIII-D}, suggesting a possible path to high fusion performance, steady-state tokamak scenarios with a large fraction of bootstrap current. The combination of internal transport barrier and negative central magnetic shear results in high confinement ($H_{89p}>\,$2.5) and good bootstrap current alignment ($ƒ_{BS}\sim\,$60\%). Previously, stability limits in plasmas with core transport barriers have been observed at moderate values of $\beta_N$ ($<$3) [R.C.\ Wolf, Plasma Phys.\ Control.\ Fusion ${\bf 45}$, R1 (2003)] because of the pressure peaking which normally develops from improved core confinement. In recent \hbox{DIII-D} experiments the internal transport barrier is clearly observed in the ion temperature and rotation profiles at $\rho\sim\,$0.5 but not in the electron temperature profile, which is very broad. The misalignment of $T_i$ and $T_e$ gradients may help avoid a local pressure peaking. Furthermore, at low internal inductance $\sim$0.6, the current density gradients are close to the vessel and the ideal kink modes are strongly wall-coupled. Simultaneous feedback control of both external and internal sets of n=1 magnetic coils was used to maintain optimal error field correction and resistive wall mode stabilization, allowing operation above the free-boundary beta-limit. Large particle orbits at high safety factor in the core help to broaden both the pressure and the beam-driven current profiles, favorable for steady state operation. At plasma current flattop and $\beta \sim\,$5\%, a noninductive current fraction of $\sim$90\% has been observed. Stability modeling shows the possibility for operation up to the ideal-wall limit at $\beta \sim$6\%. [Preview Abstract] |
Friday, October 28, 2005 11:00AM - 11:30AM |
UI2.00004: Long Pulse High Performance Plasma Scenario Development for the National Spherical Torus Experiment Invited Speaker: NSTX is targeting long pulse high performance, non-inductive sustained operations at low aspect ratio. The modeling of these plasmas provides a framework for experimental planning and identifies the tools to access these regimes. Here, two scenarios are examined, the first near-term with t$_{flattop}>\tau _{J}$, and the second, longer term goal obtaining $\beta \approx $40{\%} for t$_{flattop}>>\tau _{J}$. Simulations based on NBI-heated plasmas with the free-boundary Tokamak Simulation Code (TSC) and TRANSP are made to understand the impact of various modifications and identify the requirements for 1) high elongation and triangularity, 2) density control to optimize the current drive, 3) rotation/wall and/or feedback stabilization to operate above the no-wall limit, 4) Electron Bernstein Waves for off-axis heating/current drive (H/CD), and 5) High Harmonic Fast Wave for H/CD. Comparison of the profile evolution with experiment, including time-resolved kinetic and current profile measurements, provides the required benchmarking. An integrated scenario is constructed using TSC/TRANSP to provide the transport evolution and (H/CD) source modeling, supported by RF and stability analyses using CURRAY, GENRAY/CQL3D, JSOLVER/BALMSC/PEST2, DCON, and VALEN. Important factors include the energy confinement, Z$_{eff}$, early heating/H-mode, broadening of the NBI-driven current profile from fast ion MHD, and maintaining q(0) and q$_{min}>$1.0. Simulations show that the near-term goal can be reached at I$_{P}$=800 kA, B$_{T}$=0.5 T, $\kappa \approx $2.5, $\beta _{N}\le $5, $\beta \le $15{\%}, f$_{NI}$=92{\%}, and q(0)$>$1.5 with NBI H/CD, density control, and similar global energy confinement to experiments. The non-inductive sustained high $\beta $ plasmas can be reached at I$_{P}$=1.0 MA, B$_{T}$=0.35 T, $\kappa \approx $2.5, $\beta _{N}\le $9, $\beta \le $43{\%}, f$_{NI}$=100{\%}, and q(0)$>$1.5 with NBI H/CD and 3.0 MW of EBW H/CD, density control, and 25{\%} higher global energy confinement than experiments. [Preview Abstract] |
Friday, October 28, 2005 11:30AM - 12:00PM |
UI2.00005: Progress Toward Fully Noninductive, High Beta Conditions in DIII-D Invited Speaker: The \hbox{DIII-D} Advanced Tokamak program is aimed at developing a scientific basis for steady state (SS), high performance operation in future devices. This requires 100\% noninductive (NI) operation with high bootstrap current fraction ($f_{BS}$) and toroidal beta. Recent progress in this area includes demonstration of fully NI conditions with $\beta_T=\,$3.6\%, $\beta_N =\,$3.5, and $H_{89} =\,$2.4 using off-axis electron cyclotron current drive (ECCD). The equilibrium measurements indicate that the NI current profile is well aligned, with little inductively driven current remaining anywhere in the plasma. The duration of this state is limited by pressure profile evolution, leading to MHD instabilities after about 1~s. Stationary conditions are maintained in similar discharges ($\sim$90\% NI) for one current relaxation time, limited only by the 2~s duration of the present ECCD systems. These experiments achieve the necessary fusion performance and $f_{BS}$ to extrapolate to the ITER Q=5 SS scenario. Developing an understanding of the complex interactions between bootstrap current, external current drive, transport and MHD stability requires sophisticated integrated modeling tools. A major effort is ongoing to apply such tools, using both empirical and theory-based (GLF23) transport models, to both plan and interpret these experiments. These comparisons validate the models to allow their use in planning future DIII-D experiments with higher power and longer pulse ECCD and fast wave in a pumped double-null configuration. The models predict our ability to control the current and pressure profiles to reach full noninductivity with increased, $f_{BS}$, and duration. The same modeling tools are applied to ITER, predicting favorable prospects for the success of the ITER SS scenario. [Preview Abstract] |
Friday, October 28, 2005 12:00PM - 12:30PM |
UI2.00006: Reduced Neoclassical Flow Damping with Quasi-Symmetry: Measurements and Modeling from HSX Invited Speaker: The quasisymmetric stellarator concept may represent a solution to a conundrum in toroidal confinement. We wish to have the good neoclassical transport properties associated with a symmetric system, including the minimal flow damping in the direction of symmetry, as in a tokamak. At the same time, we desire an inherently steady state reactor, as in a stellarator. The Helically Symmetric eXperiment (HSX) was developed to test this concept; the quasi-helically symmetric base configuration can be modified using ``trim-coils'' to break the quasisymmetry, leading to configurations with the transport qualities of a traditional stellarator. We have constructed system of biased electrodes and Mach probes for flow damping studies, as well as arrays of H-alpha detectors to estimate the flow damping caused by ion-neutral friction. We have demonstrated that the flow damping in the quasisymmetric configuration is reduced compared to the symmetry-broken configuration. Neoclassical modeling has demonstrated that the reduction is in the amount predicted by theory, but that there is additional damping beyond the contribution of neoclassical parallel viscosity. This is similar to the results of large tokamaks, where toroidal flow damping is anomalously large. This research was funded by the U.S. DOE. [Preview Abstract] |
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