H. Mynick

Radial diffusion coefficient for counter-passing MeV ions in the TFTR Tokamak

The radial diffusion coefficient for confined counter-passing MeV ions is evaluated in a new way, using measurements of escaping D-D fusion products. A class of passing ions near the plasma centre is measured whose orbits could eventually diffuse across their passing/trapped boundary, resulting in unconfined trapped orbits which can be detected at the plasma wall. The analysis indicates that the passing MeV ion diffusion coefficient D is smaller than ≈0.1 m2/s, which is small compared to the thermal diffusivities of ≈1 m2/s for these plasmas. The same experiment and analysis should be applicable to alpha particles in D-T plasmas.

Physics issues in the design of high-beta, low-aspect-ratio stellarator experiments

High-beta, low-aspect-ratio ~‘‘compact’’ ! stellarators are promising solutions to the problem of developing a magnetic plasma configuration for magnetic fusion power plants that can be sustained in steady state without disrupting. These concepts combine features of stellarators and advanced tokamaks and have aspect ratios similar to those of tokamaks ~2‐4!. They are based on computed plasma configurations that are shaped in three dimensions to provide desired stability and transport properties. Experiments are planned as part of a program to develop this concept. A b54% quasi-axisymmetric plasma configuration has been evaluated for the National Compact Stellarator Experiment ~NCSX!. It has a substantial bootstrap current and is shaped to stabilize ballooning, external kink, vertical, and neoclassical tearing modes without feedback or close-fitting conductors. Quasi-omnigeneous plasma configurations stable to ballooning modes at b54% have been evaluated for the Quasi-Omnigeneous Stellarator ~QOS! experiment. These equilibria have relatively low bootstrap currents and are insensitive to changes in beta. Coil configurations have been calculated that reconstruct these plasma configurations, preserving their important physics properties. Theory- and experiment-based confinement analyses are used to evaluate the technical capabilities needed to reach target plasma conditions. The physics basis for these complementary experiments is described.

Radially global gyrokinetic simulation studies of transport barriers

Improvements in tokamak transport have recently been obtained in a variety of operational modes through the formation of transport barriers, or good confinement radial zones. Here global nonlinear three‐dimensional toroidal gyrokinetic simulation is used to study three effects that are linearly stabilizing and may cause the formation of transport barriers, namely, sheared toroidal rotation, reversed magnetic shear, and peaked density profiles. The effect of toroidal shear flow on ion heat diffusivity is found to be relatively weak compared to mixing‐length expectations based on linear calculations. In contrast, it is found that weak or negative magnetic shear (s<1/2) in combination with a peaked density profile relative to the temperature profile greatly suppresses ion‐temperature‐gradient‐driven turbulence in the central region of global nonlinear simulations. Similar features are seen experimentally in reversed magnetic shear tokamak plasmas. There is some nonlocal penetration (∼20–30ρi) of the turbulen...

Physics Design of a High-beta Quasi-axisymmetric Stellarator

Note: 8th Toki 11th International Stellarator Conference, Toki-City, Japan, September/October 1997, Proc. published in J. Plasma and Fusion Res., SERIES, Vol. 1, 429 - 432 (1998) Reference CRPP-CONF-1998-055 Record created on 2008-05-13, modified on 2016-08-08

Quasilinear diffusion in stochastic magnetic fields: Reconciliation of drift‐orbit modification calculations

An apparent difference between two calculations of drift‐orbit modifications to the spatial diffusion rate of particles in stochastic magnetic fields is addressed [H. E. Mynick and J. A. Krommes, Phys. Rev. Lett. 43, 1506 (1979); J. R. Myra and P. J. Catto, Phys. Fluids B 4, 176 (1992)]. The calculations are reconciled by noting the relevance of an inequality which was not discussed in these studies. It is shown, both analytically and by Monte Carlo simulation, that the diffusion coefficient can be sensitive to the spectral width of the magnetic turbulence relative to a finite Larmor radius parameter.

Physics of compact stellarators

Recent progress in the theoretical understanding and design of compact stellarators is described. Hybrid devices, which depart from canonical stellarators by deriving benefits from the bootstrap current which flows at finite beta, comprise a class of low aspect ratio A<4 stellarators. They possess external kink stability (at moderate beta) in the absence of a conducting wall, possible immunity to disruptions through external control of the transform and magnetic shear, and they achieve volume-averaged ballooning beta limits (4%–6%) similar to those in tokamaks. In addition, bootstrap currents can reduce the effects of magnetic islands (self-healing effect) and lead to simpler stellarator coils by reducing the required external transform. Powerful physics and coil optimization codes have been developed and integrated to design experiments aimed at exploring compact stellarators. The physics basis for designing the national compact stellarator will be discussed.

Anomalous losses of deuterium–deuterium fusion products in the Tokamak Fusion Test Reactor*

In the Tokamak Fusion Test Reactor (TFTR) [International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Wurzburg, Paper No. A‐2‐2 (International Atomic Energy Agency, Vienna, 1993)] there have been at least three types of anomalous loss of alpha‐like deuterium–deuterium (D–D) fusion products: (1) a magnetohydrodynamic (MHD)‐induced loss of D–D fusion products correlated with Mirnov and fishbone‐type oscillations and sawtooth crashes, (2) a slow ‘‘delayed’’ loss of partially thermalized D–D fusion products occurring without large MHD activity, and (3) ion cyclotron resonance heating (ICRH)‐induced loss of D–D fusion products ions observed during direct electron heating experiments, and possibly also during 3He minority heating. In this paper each of these will be reviewed, concentrating on those due to MHD activity, which are the largest of these anomalous losses. The experimental results are compared with numerical models of various fusion product transport mechanisms.

Alpha-particle physics in the tokamak fusion test reactor DT experiment

A summary is presented of recent alpha-particle experiments on the tokamak fusion test reactor. Alpha particles are generally well confined in MHD-quiescent discharges, and alpha heating of electrons has been observed. The theoretically predicted toroidicity-induced Alfv?n eigenmode has been seen in discharges of of alpha power, but only in plasmas with weak magnetic shear.

MeV ion confinement in the TFTR tokamak

In this paper the confinement of the MeV ions that are created by D–D reactions in the TFTR tokamak [in Plasma Physics and Controlled Nuclear Fusion Research 1988 (IAEA, Vienna, 1989), Vol. 1, p. 27] is described. The ions that escape from the plasma are measured by a new type of detector located just outside the plasma edge. Most measurements made with this detector are consistent with the first‐orbit loss of these ions. Exceptions are correlated with strong magnetohydrodynamic activity, and a preliminary explanation is presented.

Turbulent optimization of toroidal configurations

Recent progress in ?turbulent optimization? of toroidal configurations is described, using a method recently developed for evolving such configurations to ones having reduced turbulent transport. The method uses the GENE gyrokinetic code to compute the radial heat flux Qgk, and the STELLOPT optimization code with a theory-based ?proxy? figure of merit Qpr to stand in for Qgk for computational speed. Improved expressions for Qpr have been developed, involving further geometric quantities beyond those in the original proxy, which can also be used as ?control knobs? to reduce Qgk. Use of a global search algorithm has led to the discovery of turbulent-optimized configurations not found by the standard, local algorithm usually employed, as has use of a mapping capability which STELLOPT has been extended to provide, of figures of merit over the search space.