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].
Particle pinch in gyrokinetic simulations of closed field-line systems.
Gyrokinetic simulations of small-scale turbulent transport in a closed magnetic field-line plasma geometry are presented. The simulations are potentially applicable to dipolar systems such as the levitated dipole experiment (LDX) [J. Kesner, Plasma Phys. Rep. 23, 742 (1997).] and planetary magnetospheres, as well as simpler systems such as the Z pinch. We report here for the first time the existence of a robust particle (and weaker temperature) pinch regime, in which the particles are transported up the density gradient. The particle pinch is driven by non-MHD entropy-mode turbulence at k(⊥) ρ(i) ∼ 1 and particle pinch appears at larger η ≡ L(n)/L(T) ≳ 0.7, consistent with quasilinear theory. Our results suggest that entropy-mode transport will drive the LDX plasma profiles toward a state with η ∼ 0.7 and pressure gradients that are near marginal ideal MHD interchange-mode stability.
Long Ceramic Beam Tubes for Accelerator Magnets
The ceramic beam tubes for the fast and abort kickers and the bucker and pinger magnets used at Fermilab must meet a number of exacting requirements. The tubes must be long and sufficiently straight so as not to limit magnet design. They must have wall strength capable of withstanding the atmospheric and mechanical forces encountered during construction and operation. Vacuum tight ceramic to stainless steel transitions must be of reasonable cost; they must also withstand high temperature processing and remain vacuum tight to the 10/sup -9/ Torr range. To remove the possibility of static charge buildup each tube must have a surface coating of indium oxide applied to the inner wall. This thin coating is difficult to achieve and requires the most careful attention to detail. The methods used at Fermilab to achieve these goals are presented in this paper.