Finite drift orbit effects in a tokamak pedestal

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, September 2009."September 2009." Cataloged from PDF version of thesis.Includes bibliographical references (p. 100-105).This thesis aims at better understanding of the tokamak pedestal, which is a defining feature of the so-called "High Confinement Mode" or "H Mode" of tokamak operation. This region is characterized by a drastic plasma density drop over a relatively short radial distance, typically of order of the poloidal ion gyroradius (p,,). Experiments demonstrate that H Mode plasmas have superior transport properties compared to other known regimes, making them important for practical fusion energy generation. However, the nature of this improvement is still poorly understood and this thesis provides key new insights. According to experiments and simulations, plasmas in a tokamak are turbulent and therefore their physics can only be addressed with a formalism that retains short perpendicular wavelengths such as gyrokinetics. To be applicable in the pedestal, the formalism must also be capable of treating background scales as short as p, and conveniently accounting for the effects of finite ion drift orbits whose size scales with p,, as well. To this end, we develop a special version of gyrokinetics that employs canonical angular momentum in place of the standard radial gyrokinetic variable. Using this formalism to find the leading order ion distribution function we conclude that the background ion temperature profile in the H Mode regime cannot have a steep p,, wide pedestal similar to the one observed for the plasma density. Having obtained this result, we next deduce that a strong electric field is inherently present in a subsonic pedestal to sustain ion pressure balance, making the ExB drift enter the leading order streaming operator in the kinetic equation. We proceed by analyzing novel features that the existence of the pedestal introduces in collisionless zonal flow, the dominant mechanism controlling the anomalous transport. In particular, we find that due to the electric field modifying ion orbits, the zonal flow residual in the pedestal is enhanced over its core value. This allows us to suggest a new scenario for the pedestal formation. Since the turbulence level is lowered, we are led to consider neoclassical mechanisms of plasma transport by retaining collisions in our gyrokinetic equation. Then, we observe that the ExB drift entering the gyrokinetic equation makes the neoclassical ion heat conductivity sensitive to the pedestal electric field. Next, with the help of the same technique we evaluate the neoclassical poloidal ion flow. Importantly, we predict that once the equilibrium electric field goes beyond a certain value this flow changes its direction. This result elucidates the discrepancy between the conventional banana regime predictions and recent experimental measurements of the poloidal impurity flow performed at Alcator C-Mod.by Grigory Kagan.Ph.D

Ion Thermal Decoupling and Species Separation in Shock-Driven Implosions

Anomalous reduction of the fusion yields by 50% and anomalous scaling of the burn-averaged ion temperatures with the ion-species fraction has been observed for the first time in D[superscript 3]He-filled shock-driven inertial confinement fusion implosions. Two ion kinetic mechanisms are used to explain the anomalous observations: thermal decoupling of the D and [superscript 3]He populations and diffusive species separation. The observed insensitivity of ion temperature to a varying deuterium fraction is shown to be a signature of ion thermal decoupling in shock-heated plasmas. The burn-averaged deuterium fraction calculated from the experimental data demonstrates a reduction in the average core deuterium density, as predicted by simulations that use a diffusion model. Accounting for each of these effects in simulations reproduces the observed yield trends.United States. National Nuclear Security Administration (Grant DE-NA0001857)University of Rochester. Fusion Science Center (Grant 415023-G)National Laser User’s Facility (Grant DE-NA0002035)University of Rochester. Laboratory for Laser Energetics (Grant 415935-G)Lawrence Livermore National Laboratory (Grant B600100

Exploration of the Transition from the Hydrodynamiclike to the Strongly Kinetic Regime in Shock-Driven Implosions

Clear evidence of the transition from hydrodynamiclike to strongly kinetic shock-driven implosions is, for the first time, revealed and quantitatively assessed. Implosions with a range of initial equimolar D[superscript 3]He gas densities show that as the density is decreased, hydrodynamic simulations strongly diverge from and increasingly overpredict the observed nuclear yields, from a factor of ∼2 at 3.1  mg/cm[superscript 3] to a factor of 100 at 0.14  mg/cm[superscript 3]. (The corresponding Knudsen number, the ratio of ion mean-free path to minimum shell radius, varied from 0.3 to 9; similarly, the ratio of fusion burn duration to ion diffusion time, another figure of merit of kinetic effects, varied from 0.3 to 14.) This result is shown to be unrelated to the effects of hydrodynamic mix. As a first step to garner insight into this transition, a reduced ion kinetic (RIK) model that includes gradient-diffusion and loss-term approximations to several transport processes was implemented within the framework of a one-dimensional radiation-transport code. After empirical calibration, the RIK simulations reproduce the observed yield trends, largely as a result of ion diffusion and the depletion of the reacting tail ions.United States. Dept. of Energy (Grant DE-NA0001857)United States. Dept. of Energy (Grant DE-FC52-08NA28752)University of Rochester. Fusion Science Center (5-24431)National Laser User’s Facility (DE-NA0002035)University of Rochester. Laboratory for Laser Energetics (415935-G)Lawrence Livermore National Laboratory (B597367

Enhancement of the Bootstrap Current in a Tokamak Pedestal

The strong radial electric field in a subsonic tokamak pedestal modifies the neoclassical ion parallel flow velocity, as well as the radial ion heat flux. Existing experimental evidence of the resulting alteration in the poloidal flow of a trace impurity is discussed. We then demonstrate that the modified parallel ion flow can noticeably enhance the pedestal bootstrap current when the background ions are in the banana regime. Only the coefficient of the ion temperature gradient drive term is affected. The revised expression for the pedestal bootstrap current is presented. The prescription for inserting the modification into any existing banana regime bootstrap current expression is given.United States. Dept. of Energy (Grant No. DE-FG02- 91ER54109