Research, operational limits in fusion
THE REVERSED-FIELD PINCH (RFP)
Effect of Thermal Conduction on Pressure-driven Modes in the Reversed-field Pinch
Nuclear Fusion, vol. 52, no. 12, s. 123012, 2012.
Summary: In this study, it is shown that linear stability of pressure driven resistive modes in the reversed-field pinch (RFP) require kinetic effects, such as finite Larmor radius stabilization. Classical linearized resistive magnetohydrodynamic (MHD) stability theory predicts unstable pressure-driven modes even at low plasma pressure for the RFP. Including also allegedly stabilizing thermal conduction effects we here show that pressure-driven resistive g-modes are stabilized only for very low values of plasma pressure.
Confinement Scaling Laws for the Conventional Reversed-field Pinch
Physical Review Letters, vol. 85, no. 2, s. 322-325, 2000.
Summary: We have here, for the first time, determined theoretical limits for RFP confinement behaviour. The results indicate significant degradation from the standard picture of RFP confinement, but are in good agreement with experimental results. They emphasize the importance of experimentally demonstrating control of the RFP current profile in order to improve energy confinement. A series of high resolution, 3D, resistive MHD numerical simulations of the reversed-field pinch are performed to obtain scaling laws for poloidal beta and energy confinement at Lundquist numbers approaching 10**6. Optimum plasma conditions are attained by taking the transport coefficients to be classical, and by ignoring radiation losses and resistive wall effects. We find that poloidal beta scales as I**(-0.40) and that the energy confinement time scales as I**0.34 for fixed I/N, with aspect ratio 1.25. I is the plasma current.
Numerical Studies of Confinement Scalings for the Dynamo Free Reversed-field Pinch
Nuclear Fusion, vol. 47, no. 1, s. 9-16, 2007.
Summary: In the RFP, tearing modes associated with the dynamo fluctuations are responsible for reduced energy- and particle confinement. In this numerical study, it is observed that by implementing current profile control (CPC) in the RFP, a dynamo-free state can be achieved. Scaling laws are determined for radial magnetic field, energy confinement time, poloidal beta and temperature. Confinement is improved substantially as compared with the conventional RFP - the temperature reaches reactor relevant levels by ohmic heating alone. The focus of this study is on obtaining principal theoretical optimization of confinement in the RFP by implementing CPC, thus investigating the reactor viability of the concept.
THE Z-PINCH
Large Larmor Radius Stability of the Z-pinch
Physical Review Letters, vol. 72, s. 2399, 1994.
Summary: This is the first theoretical evidence that kinetic finite Larmor radius effects cannot in themselves produce a stabilized zpinch. The linear m = 0 stability of the z pinch in the collisionless, large ion Larmor radius regime is examined using the Vlasov fluid model. The results reveal a strong equilibrium dependence. The uniform current density equilibrium shows a reduction in growth rate when the average ion Larmor radius is about one-fifth of the pinch radius. Complete stabilization of m = 0 modes is only achieved in unphysical cases where the pressure is relatively high at the plasma boundary.
Linear Stability of the High Temperature, Dense Z-pinch
Physical Review Letters, vol. 74, s. 2698, 1995.
Summary: Results are presented on the linear stability of the collisionless m = 1 mode in a dense zpinch. It is shown that a reduction in growth rate by a factor of about 10 (when compared to the zero Larmor radius result) is possible by initializing the zpinch with a sufficiently low line density. With the completion of this work we conclude that linear, large Larmor radius effects cannot stabilize the high temperature, dense zpinch. Such pinches will always exhibit linear m = 0 or m = 1 instabilities with growth times comparable to the radial Alfvén transit time.
