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

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

Electro-diffusion in a plasma with two ion species

Electric field is a thermodynamic force that can drive collisional inter-ion-species transport in a multicomponent plasma. In an inertial confinement fusion (ICF) capsule, such transport causes fuel ion separation even with a target initially prepared to have equal number densities for the two fuel ion species. Unlike the baro-diffusion driven by ion pressure gradient and the thermo-diffusion driven by ion and electron temperature gradients, electro-diffusion has a critical dependence on the charge-to-mass ratio of the ion species. Specifically, it is shown here that electro-diffusion vanishes if the ion species have the same charge-to-mass ratio. An explicit expression for the electro-diffusion ratio is obtained and used to investigate the relative importance of electro- and baro-diffusion mechanisms. In particular, it is found that electro-diffusion reinforces baro-diffusion in the deuterium and tritium mix, but tends to cancel it in the deuterium and helium-3 mix.Comment: Submitted to Phys. Plasmas on 2012-03-06 (revised version 05/13/2012

Bounce-free spherical hydrodynamic implosion

In a bounce-free spherical hydrodynamic implosion, the post-stagnation hot core plasma does not expand against the imploding flow. Such an implosion scheme has the advantage of improving the dwell time of the burning fuel, resulting in a higher fusion burn-up fraction. The existence of bounce-free spherical implosions is demonstrated by explicitly constructing a family of self-similar solutions to the spherically symmetric ideal hydrodynamic equations. When applied to a specific example of plasma liner driven magneto-inertial fusion, the bounce-free solution is found to produce at least a factor of four improvement in dwell time and fusion energy gain.Comment: accepted by Phys. Plasmas (Nov. 7, 2011); for Ref. 11, please see ftp://ftp.lanl.gov/public/kagan/liner_evolution.gi