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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.

The magnetohydrodynamic (MHD) ballooning stability of a compact, quasiaxially symmetric stellarator (QAS), expected to achieve good stability and particle confinement is examined with a method that can lead to estimates of global stability. Making use of fully 3D, ideal-MHD stability codes, the QAS beta is predicted to be limited above 4% by ballooning and high-n kink modes. Here MHD stability is analyzed through the calculation and examination of the ballooning mode eigenvalue isosurfaces in the 3-space [s, alpha, theta(subscript ''k'')]; s is the edge normalized toroidal flux, alpha is the field line variable, and theta(subscript ''k'') is the perpendicular wave vector or ballooning parameter. Broken symmetry, i.e., deviations from axisymmetry, in the stellarator magnetic field geometry causes localization of the ballooning mode eigenfunction, with new types of nonsymmetric, eigenvalue isosurfaces in both the stable and unstable spectrum. The isosurfaces around the most unstable points i n parameter space (well above marginal) are topologically spherical. In such cases, attempts to use ray tracing to construct global ballooning modes lead to a k-space runaway. Introduction of a reflecting cutoff in k(perpendicular) to model numerical truncation or finite Larmor radius (FLR) yields chaotic ray paths ergodically filling the allowed phase space, indicating thatmore » the global spectrum must be described using the language of quantum chaos theory. However, the isosurface for marginal stability in the cases studied are found to have a more complex topology, making estimation of FLR stabilization more difficult.« less

The TRANSP code is used to construct comprehensive, self-consistent models for ITER discharges. Plasma parameters are studied for two discharges from the ITER ``Interim Design`` database producing 1.5 GW fusion power with a plasma current of 21 MA and 20 toroidal field coils generating 5.7 T Steady state profiles for T{sub ion}, T{sub e}, n{sub e}, Z{sub eff}, and P{sub rad} from the database are specified. TRANSP models the full up/down asymmetric plasma boundary within the separatrix. Effects of high-energy neutral beam injection, sawteeth mixing, toroidal field ripple, and helium ash transport are included. Results are given for the fusion rate profiles, and parameters describing effects such as alpha particle slowing down, the heating of electrons and thermal ions, and the thermalization rates. The deposition of 1 MeV neutral beam ions is predicted to peak near the plasma center, and the average beam ion energy is predicted to be half the injected energy. Sawtooth mixing is predicted to broaden the fast alpha profile. The toroidal ripple losses rate of alpha energy is estimated to be 3% before sawtooth crashes and to increase by a factor of three to four immediately following sawtooth crashes. Assumptions for the thermal He transport and the He recycling coefficient at the boundary are discussed. If the ratio of helium and energy confinement times, {tau}*{sub He}/{tau}{sub E} is less than 15, the steady state fusion power is predicted to 1.5 GW or greater. The values of the transport coefficients required for this fusion power depend on the He recycling coefficient at the separatrix. If R{sub rec} is near 1, the required He diffusivity must be much larger than that measured in tokamaks. If R{sub rec} {le} 0.50, and if the inward pinch is small, values comparable to those measured are compatible with 1.5 GW.

Insufficient helium exhaust can seriously degrade the performance of any fusion reactor and has recently been identified as a potentially significant issue for ITER. The effects of variations of the ratios D{sup He}/{chi}i, D{sup He}/D{sup D}, v{sup He}/D{sup He} and of the sawtooth period on helium exhaust in ITER are studied with BALDUR code transport simulations. Recent measurements of D{sup He}/{chi}i, at TFTR, of D{sup He}/D{sup D} at TEXTOR of v{sup He}/D{sup He} at TFTR, TEXTOR, and JT-60 are found compatible with sustained ignition and helium exhaust requirements for ITER. Ignition is found to be critically sensitive to the ratio v{sup He}/D{sup He}, particularly at the plasma edge, and to reduced sawtooth period. These critical dependencies are moderated for reduced helium recycling at the separatrix and for hollow particle diffusivity profiles. 17 refs., 9 figs., 1 tab.

The buildup of helium ash in the proposed ITER experiment has been studied in a series of simulations with the BALDUR transport code. Using radially dependent thermal diffusivities which were scaled from JET values, we studied the role of particle transport coefficients and edge recycling on helium poisoning of ignition. A sustained ignition was obtained when the exhaust of helium from the edge plasma was allowed to exceed 10% of the helium flux into the edge plasma from the core plasma, and the ratio of particle (He ion) to thermal diffusivities, D/{chi}, was larger than 1/4. The simulations included the effects of sawtooth oscillations, radiative as well as conductive energy loss channels, and density profile variations. 29 refs., 11 figs.

The possibility of obtaining ignition in TFTR by means of very centrally peaked density profiles is examined. It is shown that local central alpha heating can be made to exceed local central energy losses (''central ignition'') under global conditions for which Q greater than or equal to 1. Time dependent 1-D transport simulations show that the normal global ignition requirements are substantially relaxed for plasmas with peaked density profiles. 18 refs., 18 figs.

In order to achieve the stated objectives of key future experiments such as CIT, it will be necessary to maintain plasmas with confinement properties significantly improved over those observed in auxiliary-heated L-mode-type tokamak discharges. This underscores the importance of understanding the physics responsible for the favorable trends exhibited, for example, by ''supershot'' discharges in TFTR and by H-mode-type divertor plasmas. In addressing this general issue, the present paper reports results of theoretical investigations dealing with: the onset conditions of the microinstabilities most likely to influence enhanced confinement behavior; gyrokinetic particle simulations and analytic studies of the saturation and transport properties of the most persistent forms of these instabilities; and detailed comparisons of experimental results with predictions from transport codes using appropriate microinstability-based diffusivities and with local diffusivities from various theoretical models. 19 refs., 2 tabs.

This paper reports on results of theoretical studies dealing with: (1) the use of microinstability-based thermal transport models to interpret the anomalous confinement properties observed in key tokamak experiments such as TFTR and (2) the likely consequences of the presence of such instabilities for future ignition devices. Transport code simulations using profile-consistent forms of anomalous thermal diffusivities due to drift-type instabilities have yielded good agreement with the confinement times and temperatures observed in TFTR under a large variety of operating conditions including pellet-fuelling in both ohmic- and neutral-beam-heated discharges. With regard to achieving an optimal ignition margin, the adverse temperature scaling of anomalous losses caused by drift modes leads to the conclusion that it is best to operate at the maximum allowable density while holding the temperature close to the minimum value required for ignition.

Experiments with plasmas having nearly equal concentrations of deuterium and tritium have been carried out on TFTR. To date, the maximum fusion power has been 10.7 MW, using 39.5 MW of neutral-beam heating, in a supershot discharge and 6.7 MW in a high-{beta}{sub p} discharge following a current ramp-down. The fusion power density in the core of the plasma has reached 2.8 MWm{sup {minus}3}, exceeding that expected in the International Thermonuclear Experimental Reactor (ITTER). The energy confinement time, {tau}{sub E}, is observed to increase in D-T, relative to D plasmas, by 20% and the n{sub i}(O){center_dot}{tau}{sub E} product by 55%. The improvement in thermal confinement is caused primarily by a decrease in ion heat conductivity in both supershot and limiter-H-mode discharges. Extensive lithium pellet injection increased the confinement time to 0.27 s and enabled higher current operation in both supershot and high-{beta}{sub p} discharges. First measurements of the confined alpha particles have been performed and found to be in good agreement with TRANSP simulations assuming classical confinement. Measurements of the alpha ash profile have been compared with simulations using particle transport coefficients from helium gas puffing experiments. The loss of energetic alpha particles to a detector at the bottom ofmore » the vessel is well described by the first-orbit loss mechanism. No loss due to alpha-particle-driven instabilities has yet been observed. ICRF heating of a D-T plasma, using the second harmonic of tritium, has been demonstrated. D-T experiments on TFTR will continue both to explore the physics underlying the ITER design and to examine some of the physics issues associated with an advanced tokamak reactor.« less