Axisymmetric, High-β, Steady-State Plasma Torus: A "Wind-Tunnel" to Develop Whole Device Models
The development of experimentally-validated whole device models is a grand challenge of fusion energy science and a major goal of integrated simulation research. As explained in the 2009 Joint FES/ASCR Workshop Report [1], extreme-scale computing power will allow “improved versions” of large-scale simulations “to produce an experimentally validated integrated simulation capability for scenario modeling of the whole device.” Whole device models will integrate multi-scale physics (e.g. turbulence and transport, local and nonlocal coupling), magnetic geometry (e.g. anisotropy between parallel and perpendicular dynamics, interfaces between closed and open field lines), nonlinear and electromagnetic dynamics (e.g. instability-induced energy transfer, particle kinetics and resonances), and carefully designed validation experiments. This scientific challenge is unquestionably “grand.” Workshop participants called integrated predictive modeling both “urgent” and “one of the most difficult and least understood problems in fusion energy research” [1]. This white paper presents a cost-effective and near-term research step that makes progress towards this grand challenge by considering the simpler task of experimentally validated integrated simulation of a steady-state, high-β plasma torus confined by a simple axisymmetric current ring. Using our nation’s only toroidal confinement device with superconducting magnets, we can now make unparalleled measurements of fusion-relevant plasmas confined for several minutes and use these measurements for critical validation studies. We call this approach a “plasma wind tunnel”, because it is analogous to using low-cost instrumented objects in wind tunnels to validate CFD and turbulent hydrodynamics for various aerodynamical applications. In a manner of speaking, we want to follow in the footsteps of the Wright brothers. Just as the Wright brothers validated the means to predict lift and drag in their wind tunnel, we should build confidence in our whole-plasma capabilities by first demonstrating success with a fusion-relevant plasma torus that has the least complicated magnetic geometry and the least complicated particle kinetics. We believe the axisymmetric current ring is both the simplest magnetized plasma torus that can be used for whole-plasma modeling, and, also, the simplest plasma torus that incorporates features of any fusion device: multi-scale and boundary layer physics, nonlinear stability and turbulence phenomena, and important kinetic effects and particle resonances. Steady-state high-β plasmas have proven to be relatively easy to produce in the laboratory [2-7], and gyro-kinetic and gyro-fluid models have been successfully applied to this geometry, yielding important insights into the fundamental nonlinear turbulent transport processes [8-12]. Like a tokamak, the plasma torus confined by a current ring is sustained by sources of heat, particles, and toroidal momentum and involves turbulent transport across boundary layers, upgradient turbulent pinches [13,14], nonlinear fast particle dynamics [15], filamentary “blobs” [16], and turbulent cascades [17]. Unlike a tokamak, turbulent transport does not always lead to plasma loss. Instead turbulence causes self-organization, centrally-peaked profiles, and either an inward particle or an inward thermal pinch [11, 12] as the plasma approaches a state of minimum entropy production [18].
Transport induced by large scale convective structures in a dipole-confined plasma.
Convective structures characterized by E×B motion are observed in a dipole-confined plasma. Particle transport rates are calculated from density dynamics obtained from multipoint measurements and the reconstructed electrostatic potential. The calculated transport rates determined from the large-scale dynamics and local probe measurements agree in magnitude, show intermittency, and indicate that the particle transport is dominated by large-scale convective structures.
Thermal incompatibility analysis of metal-ceramic systems based on flexural displacement data.
The feasibility of simple tests or analytical methods for prediction of residual stress states in metal-ceramic (MC) prostheses has not been demonstrated. Biomaterial metal-ceramic strips have been proposed to provide sensitive measures of transient and residual stress states through the measurement of midpoint deflection after cooling from the ceramic sintering temperature. The objective of this study was to apply the elastic-viscoelastic analogy to calculate transient and residual midpoint deflections in MC biomaterial strips and to compare these values with deflections measured with a beam-bending viscometer (BBV). Calculations and measurements were made for five MC systems that were found from a clinical study to be "thermally compatible" systems. Metal strips, 64 mm in length, 3 mm wide, and either 0.5 mm, 1.0 mm, or 2.0 mm in thickness, were veneered with four 0.25-mm thick layers of opaque porcelain. Midpoint deflection of the MC strips (ceramic oriented in the posterior position) was measured during cooling from an initial temperature of 700 degrees C. In general, the directions of the measured residual deflections did not agree with the "textbook" convention that negative deflections are associated with positive thermal contraction mismatch (alpha(M) - alpha(c) > 0) regardless of metal thickness. For a metal thickness of 0.5 mm, the residual midpoint deflection for all thermal contraction mismatch cases, except one, was positive (upward deflection) whereas the residual midpoint deflections were all negative when the metal thickness was increased to 1 or 2 mm, independent of the thermal contraction mismatch. The best agreement between calculated and measured values of residual midpoint deflection (+16 microns vs. +14 +/- 2.3 microns, respectively was obtained for MC biomaterial strips with a Ni-Cr alloy (0.5 mm thick) while the largest difference (+346 microns vs. +61 +/- 43.8 microns) was obtained for MC bimaterial strips with a Au-Pd allow (0.5 mm thick). In all but one case, changes in deflection direction as a function of metal thickness were correctly predicted by the viscoelastic analysis. The results of this study indicate that a viscoelastic model is useful for estimating thermal compatibility conditions of MC systems.