Magnetic Confinement Research Research Articles

Fast sawtooth reconnection at realistic Lundquist numbers

Magnetic reconnection, a ubiquitous phenomenon in astrophysics, space science and magnetic confinement research, frequently proceeds much faster than predicted by simple resistive MHD theory. Acceleration can result from the break-up of the thin Sweet–Parker current sheet into plasmoids, or from two-fluid effects decoupling mass and magnetic flux transport over the ion inertial length or the drift scale depending on the absence or presence of a strong magnetic guide field. We describe new results on the modelling of sawtooth reconnection in a simple tokamak geometry (circular cylindrical equilibrium) pushed to realistic Lundquist numbers for present day tokamaks. For the resistive MHD case, the onset criteria and the influence of plasmoids on the reconnection process agree well with earlier results found in the case of vanishing magnetic guide fields. While plasmoids are also observed in two-fluid calculations, they do not dominate the reconnection process for the range of plasma parameters considered in this study. In the two-fluid case they form as a transient phenomenon only. The reconnection times become weakly dependent on the S-value and for the most complete model—including two-fluid effects and equilibrium temperature and density gradients—agree well with those experimentally found on ASDEX Upgrade

Calculation of 3-D Magnetic Fields Produced by MHD Active Control Systems in Fusion Devices

An accurate control of the magnetic boundary of a thermonuclear plasma is an important issue in magnetic confinement research. The development of methods for the active control of magnetohydrodynamic instabilities and correction of error fields is mandatory in view of fusion reactors or experimental test reactors under design. Recently, a very effective control scheme, named clean mode control (CMC), has been proposed in a reversed field pinch experiment (RFX-mod). The CMC is based on the real-time correction (cleaning), under simplifying hypothesis, of the sideband harmonics in the magnetic field produced by the discrete local active coils. In this paper, we focus on the problem of carefully computing the penetration of sidebands through a realistic load assembly, but still with a simplified cylindrical geometry, to allow for a detailed comparison with the crude CMC algorithm without the additional complication of the coupling between the poloidal harmonics due to the toroidal geometry.

Overview of Innovative PMI Research on NSTX-U and Associated PMI Facilities at PPPL

Developing a reactor compatible divertor and managing the associated plasma material interaction (PMI) has been identified as a high priority research area for magnetic confinement fusion. Accordingly on NSTXU, the PMI research has received a strong emphasis. With ~15 MW of auxiliary heating power, NSTX-U will be able to test the PMI physics with the peak projected divertor plasma facing component (PFC) heat loads of up to 40-60 MW/m2. To support the PMI research, a comprehensive set of PMI diagnostic tools are being implemented. The snow-flake configuration can produce exceptionally high divertor flux expansion of up to ~50. Combined with the radiative divertor concept, the snowflake configuration has reduced the divertor heat flux by up to an order of magnitude in NSTX. Another area of active PMI investigation is the effect of divertor lithium coating (both in solid and liquid phases). The overall NSTX lithium PFC coating results suggest exciting opportunities for future magnetic confinement research including significant electron energy confinement improvements, H-mode power threshold reduction, the control of Edge Localized Modes (ELMs), and high heat flux handling. To support the NSTX-U/PPPL PMI research, there are also a number of associated PMI facilities implemented at PPPL/Princeton University including the Liquid Lithium R&D facility, Lithium Tokamak Experiment, and Laboratories for Materials Characterization and Surface Chemistry.

Chapter 4: Laser-Aided Plasma Diagnostics

Laser-aided diagnostics are widely applied in the field of high-temperature plasma diagnostics for a large variety of measurements. Incoherent Thomson scattering is used for highly localized measurements of the electron temperature and density in the plasma. Coherent Thomson scattering yields information on the fast ion population in the plasma and/or depending on the geometry and wavelength chosen electron density fluctuations. Interferometry and polarimetry are often combined in a single diagnostics setup to measure the electron density and the component of the magnetic field parallel to the laser chord. Density fluctuations can be measured by means of phase contrast imaging, scattering, and various other laser-aided techniques. This paper is primarily focused on laser diagnostics utilized in the mainstream magnetic confinement research (tokamaks and stellarators with some examples from other devices if applicable). In the paper a brief tutorial introduction in each of the techniques is given, followed by a description of some typical implementations on magnetic confinement devices and some examples of recent experimental results. For each of the techniques the potential application to the ITER tokamak is also discussed. The paper is not meant as a comprehensive and exhaustive review giving a proper tribute to all the work that has been done in this field over the years.

Cornelis Marius Braams

Cornelis Marius “Kees” Braams, a fusion pioneer and emeritus professor in plasma physics at Utrecht University, died in De Heeg, the Netherlands, on 29 July 2003, following a heart attack. For three decades, Braams was the personification of fusion research in the Netherlands: He was a founder and the first director of the Foundation for Fundamental Research on Matter (FOM) Institute for Plasma Physics “Rijnhuizen.”Braams was born in ‘s Hertogenbosch, the Netherlands, on 5 July 1925. Following World War II, he studied physics and mathematics at Utrecht, where he graduated in 1952 with a master’s degree in physics. In that same year, Nature published his explanation of the phenomenon of the so-called Tippe Top, a toy that spontaneously turns itself upside down when spinning. He conducted part of his thesis studies at MIT from 1952 to 1954. In 1956, he received his PhD from Utrecht for his thesis, entitled “Energy Levels of Calcium Isotopes,” under the guidance of Pieter Endt.In the following years, Braams was actively involved in the foundation of Rijnhuizen, for which he was the director from 1959 until his retirement in 1987. His scientific interests in fusion research were concentrated mainly on the transition regime from gas discharges to plasmas. He tried to answer questions such as, Can a rotating gas blanket be used to stabilize a toroidal arc? And can the combustion process in a reactor be maintained by removing the ashes by means of diffusion to and from a coldgas blanket?In 1962, Braams became a professor of plasma physics at Utrecht and mentored a large number of doctoral students, whom he taught to be critical and independent scientists. During the year 1963–64, he worked at the Boeing Scientific Research Laboratories in Seattle, Washington, in the Plasma Physics Laboratory.Braams was very active in building the joint European fusion research network. He was a member of numerous national and international committees and councils, very often in the role of chairman. He was especially instrumental in the realization of the Joint European Torus, or JET, project and, in 1979, became the first chairman of its scientific council. In the Netherlands, he reanimated the Dutch Physical Society during his chairmanship in the 1970s. During all those public activities, he managed to find time for his other passions. With his sons, he designed and handcrafted a seaworthy yacht. His beautiful collection of exotic trees on the park grounds surrounding Rijnhuizen is a living testimony to his strong love for nature.At his retirement, Braams was awarded a royal decoration by Queen Beatrix of the Netherlands. He later saw it as his duty to record the scientific history of fusion research of magnetically confined plasmas. He did this in his own style, by checking all the facts, preferably by contacting the main characters directly. The result is a book he wrote with Peter Stott, entitled Nuclear Fusion, Half a Century of Magnetic Confinement Research (Institute of Physics, 2002). Braams was a passionate physicist, ever ready to fight pseudoscience, especially if fusion was involved. Whether it was ball lightning or cold fusion, he built his criticisms on a solid and thorough scientific analysis.Until his death, Braams worked a few days a week in “his” institute, where he still had an office. He remained especially active in engaging public interest in fusion research, for which he had always been an untiring and well-spoken advocate.Cornelis Marius BraamsFOM INSTITUTE FOR PLASMA PHYSICSPPT|High resolution© 2004 American Institute of Physics.

Advanced computations in plasma physics

Scientific simulation in tandem with theory and experiment is an essential tool for understanding complex plasma behavior. In this paper we review recent progress and future directions for advanced simulations in magnetically confined plasmas with illustrative examples chosen from magnetic confinement research areas such as microturbulence, magnetohydrodynamics, magnetic reconnection, and others. Significant recent progress has been made in both particle and fluid simulations of fine-scale turbulence and large-scale dynamics, giving increasingly good agreement between experimental observations and computational modeling. This was made possible by innovative advances in analytic and computational methods for developing reduced descriptions of physics phenomena spanning widely disparate temporal and spatial scales together with access to powerful new computational resources. In particular, the fusion energy science community has made excellent progress in developing advanced codes for which computer run-time and problem size scale well with the number of processors on massively parallel machines (MPP’s). A good example is the effective usage of the full power of multi-teraflop (multi-trillion floating point computations per second) MPP’s to produce three-dimensional, general geometry, nonlinear particle simulations which have accelerated progress in understanding the nature of turbulence self-regulation by zonal flows. It should be emphasized that these calculations, which typically utilized billions of particles for thousands of time-steps, would not have been possible without access to powerful present generation MPP computers and the associated diagnostic and visualization capabilities. In general, results from advanced simulations provide great encouragement for being able to include increasingly realistic dynamics to enable deeper physics insights into plasmas in both natural and laboratory environments. The associated scientific excitement should serve to stimulate improved cross-cutting collaborations with other fields and also to help attract bright young talent to plasma science.

FUSION POWER

This article surveys extensively fusion development under the following topics: US research directions; inertial confinement fusion; foreign fusion efforts; fusion issues; fusion applications; and arguments for fusion development. Dr. Dingee points out that, despite persuasive arguments for development, fusion has as yet attracted no substantial constituency; and that winning greater support for fusion may thus require a considerable technical breakthrough (namely, proof of scientific feasibility or achievement of energy breakeven) - or a new focus on an energy source such as hybrids, which offer a nearer-term payoff than pure fusion. Dr. Dingee says the next major facility for magnetic confinement research (to be built in late 1980s) has not yet been selected, but will probably be an engineering test facility; there are similar plans for inertial confinement. Whichever type is chosen, the first experimental power reactor is scheduled for the first few years of the 2000's, this to be followed by commercial demonstration of fusion power in the 2010 to 2020 time frame. He points out, finally, that the complex technical and institutional issues are being considered in a climate in which the benefits of nuclear energy itself are being questioned; and that there is little doubt that future developmentmore » is tied to overall decisions the nation will make regarding the value of nuclear energy.« less