Search

A comparison of three sets of Lithium Tokamak Experiment-β (LTX-β) discharges is presented, each with progressively more lithium evaporatively deposited on the stainless steel plasma facing components (PFCs). Multiple observations independently indicate a reduction in recycling with increasing lithium deposition - plasma current, discharge duration, density pumpout, and edge electron temperatures increase while plasma density and neutral influx decrease. These measurements make use of several new operational and diagnostics upgrades that have been installed on LTX-β to enable recycling analysis and quantification. Installation of an upgraded Supersonic Gas Injector (SGI) provides access to rapid density pumpout, enabling estimation of effective particle confinement times. Edge density and temperature are measured using a new, movable, low field side, off-midplane, swept single langmuir probe in addition to the LTX-β core Thomson scattering system. The neutral particle influx from the high field side limiter is measured using a hydrogen Lyman-α array. Prospects for future modeling and analysis integrating all of these measurements is discussed.

Strong poloidal refueling asymmetry in the DIII-D tokamak is inferred from line radiation measurements. Synthetic diagnostics in neutral transport modeling coupled to gyrokinetic simulations illuminate implications for the plasma flow profile in the scrape-off layer of single-null beam-driven discharges. Recycling occurs primarily either on the inner or outer divertor legs, depending on the toroidal magnetic field direction. By reversing the toroidal magnetic field, the observed line radiation asymmetry is nearly eliminated or reversed. It is determined that, while relatively simple physics can describe the observed ionization asymmetry, predicting the overall brightness of the hydrogenic Lyman- α signal requires detailed simulation of the plasma and resulting turbulence. To this end, kinetic plasma simulations fully coupled to comprehensive neutral transport calculations—a novel capability—provide first-principles reproduction of Lyman- α observations on DIII-D.

The observation that fast ions stabilize ion-temperature-gradient-driven microturbulence has profound implications for future fusion reactors. It is also important in optimizing the performance of present-day devices. In this work, we examine in detail the phenomenology of fast ion stabilization and present a reduced model which describes this effect. This model is derived from the high-energy limit of the gyrokinetic equation and extends the existing ‘dilution’ model to account for nontrivial fast ion kinetics. Our model provides a physically-transparent explanation for the observed stabilization and makes several key qualitative predictions. Firstly, that different classes of fast ions, depending on their radial density or temperature variation, have different stabilizing properties. Secondly, that zonal flows are an important ingredient in this effect precisely because the fast ion zonal response is negligible. Finally, that in the limit of highly-energetic fast ions, their response approaches that of the ‘dilution’ model; in particular, alpha particles are expected to have little, if any, stabilizing effect on plasma turbulence. We support these conclusions through detailed linear and nonlinear gyrokinetic simulations. [ABSTRACT FROM AUTHOR]