Glenn Bateman

A Monte-Carlo model of neutral-particle transport in diverted plasmas

The transport of neutral atoms and molecules in the edge and divertor regions of fusion experiments has been calculated using Monte-Carlo techniques. The deuterium, tritium, and helium atoms are produced by recombination at the walls. The relevant collision processes of charge exchange, ionization, and dissociation between the neutrals and the flowing plasma electrons and ions are included, along with wall-reflection models. General two-dimensional wall and plasma geometries are treated in a flexible manner so that varied configurations can be easily studied. The algorithm uses a pseudocollision method. Splitting with Russian roulette, suppression of absorption, and efficient scoring techniques are used to reduce the variance. The resulting code is sufficiently fast and compact to be incorporated into iterative treatments of plasma dynamics requiring numerous neutral profiles. The calculation yields the neutral gas densities, pressures, fluxes, ionization rates, momentum-transfer rates, energy-transfer rates, and wall-sputtering rates. Applications have included modeling of proposed INTOR/FED poloidal divertor designs and other experimental devices.

Theory-based transport simulations of TFTR L-mode temperature profiles

The temperature profiles from a selection of Tokamak Fusion Test Reactor (TFTR) L‐mode discharges [17th European Conference on Controlled Fusion and Plasma Heating, Amsterdam, 1990 (EPS, Petit‐Lancy, Switzerland, 1990, p. 114)] are simulated with the 1 (1)/(2) ‐D baldur transport code [Comput. Phys. Commun. 49, 275 (1988)] using a combination of theoretically derived transport models, called the Multi‐Mode Model [Comments Plasma Phys. Controlled Fusion 11, 165 (1988)]. The present version of the Multi‐Mode Model consists of effective thermal diffusivities resulting from trapped electron modes and ion temperature gradient (ηi) modes, which dominate in the core of the plasma, together with resistive ballooning modes, which dominate in the periphery. Within the context of this transport model and the TFTR simulations reported here, the scaling of confinement with heating power comes from the temperature dependence of the ηi and trapped electron modes, while the scaling with current comes mostly from resistiv...

THEORY-BASED TRANSPORT MODELING OF TFTR

Transport simulations of TFTR temperature and density profiles have been carried out with the Nordman-Weiland transport model for ηi and trapped electron modes (including impurities and finite Larmor radius effects) combined together with resistive ballooning mode and neoclassical transport models. The Nordman-Weiland model is combined together with models for transport driven by pressure gradients to compute the coefficients for the transport of ion heat, hydrogen ions, and electron heat. The predictions of this model are compared with the experimentally observed profiles for TFTR L-mode and supershot plasmas.

Multiple mode model of tokamak transport

Theoretical models for radial transport of energy and particles in tokamaks due to drift waves, rippling modes and resistive ballooning modes have been combined in a predictive transport code. The resulting unified model has been used to simulate low confinement mode (L-mode) energy confinement scalings. The dependence of global energy confinement on electron density for the resulting model is also described.

Physics aspects of the Compact Ignition Tokamak

The Compact Ignition Tokamak (CIT) is a proposed modest-size ignition experiment designed to study the physics of alpha particle heating. The basic concept is to achieve ignition in a modest-size minimum cost experiment by using a high plasma density to achieve nT{sub E} {approx} 2 x 10{sup 20}s/m{sup 3} required for ignition. The high density requires a high toroidal field (10 T). The high toroidal field allows a large plasma current (10 MA) which provides a high level of ohmic heating, improves the energy confinement, and allows a relatively high beta ({approx} 6%). The present CIT design also has a high degree of elongation (k {approx} 1.8) to aid in producing the large plasma current. A double null poloidal divertor and pellet injection are part of the design to provide impurity and particle control, improve the confinement, and provide flexibility for improving the plasma profiles. Auxiliary heating is expected to be necessary to achieve ignition, and 10-20 MW of ICRF is to be provided.

Saturated tearing modes in toroidal geometry

A quasilinear method is developed for determining saturated tearing mode magnetic island widths in axisymmetric toroidal plasmas with arbitrary cross‐sectional shape, aspect ratio, or plasma pressure (beta). The method is applied to compute magnetic island widths in force‐free toroidal plasmas with aspect ratio as low as 2.0 and elongation between 1.0 and 2.0. It is found that current suppression within the magnetic island strongly increases the saturated width while current peaking reduces width. The effects of current profile, geometry, and harmonic mixing are also studied.

Time-dependent simulations of a Compact Ignition Tokamak

Detailed simulations of the Compact Ignition Tokamak are carried out using a 1-1/2-D transport code. The calculations include time-varying densities, fields, and plasma shape. It is shown that ignition can be achieved in this device if somewhat better than L-mode energy confinement time scaling is possible. We also conclude that the performance of such a compact, short-pulse device can depend greatly on how the plasma is evolved to its flat-top parameters. Furthermore, in cases such as the ones discussed here, where there is not a great deal of ignition margin and the electron density is held constant, ignition ends if the helium ash is not removed. In general, control of the deuterium--tritium density is equivalent to burn control. 48 refs., 15 figs.

Systematic comparison of a theory-based transport model with a multi-tokamak profile database

A theoretical model of flux-surface-averaged radial transport in tokamaks has been tested and calibrated against a well-documented set of temperature and density profiles from a pre-defined set of discharges from seven tokamaks. The transport theory includes neoclassical, drift/ηi, circulating electron mode, kinetic ballooning, neoclassical MHD, and resistive ballooning effects. Allowing for no explicitly adjustable free parameters, the nominal theory results are compared with experimental density and temperature profiles from the reference set of discharges from Alcator-C, ASDEX, ISX-B, TFTR, DIII-D, and JET. Profile results are also given for an additional set of six discharges including Alcator-C, ASDEX, DIII-D, TFTR, JET, and JFT-2M in addition to five TFTR discharges as part of a ρ* and β scan. Employed is a statistical model of calibration/modeling and measurement variances allowing detailed analysis of results and further calibration of the model along with a well defined procedure for setting the time dependent boundary conditions at an appropriate location just inside the inner-most closed magnetic flux surface.

Systematic comparison of a theory‐based transport model with a multi‐tokamak profile database

A theoretical model of flux‐surface‐averaged radial transport in tokamaks has been tested and re‐calibrated against a well‐documented set of temperature and density profiles from a pre‐defined set of discharges from six tokamaks. The transport theory includes neoclassical, drift/ηi, circulating electron mode, kinetic ballooning, neoclassical magnetohydrodynamic (MHD), and resistive ballooning effects. Allowing for no explicitly adjustable free parameters and no a posteriori exclusion of data subsets, the nominal theory reproduced observed temperatures and electron densities with relative error about two orders of magnitude smaller than the range over which machine parameters and resulting plasma parameters varied in the reference discharges examined. An important feature of this study is a well‐defined procedure for setting boundary conditions at an appropriate location just inside the inner‐most ‘‘closed’’ magnetic flux surface.

Transport simulations of TFTR experiments to test theoretical models for χe and χi

Predictions with the 1½-D BALDUR transport code, using recent theory based models for thermal and particle transport, are compared with measured profiles of electron plasma density and electron and ion temperatures for Ohmic, L-mode and supershot discharges on TFTR. The profile consistent drift wave model is found to overestimate the ion temperatures at high heating powers so that a third mode or loss process is needed in addition to drift wave transport (trapped electron mode and ηi mode) and an edge loss model. None of several versions of the local multiple mode models, using the 1989 Carreras-Diamond resistive ballooning mode, gives Te, Ti within 20% for all three TFTR regimes studied.