Reduced Physics Model of the Tokamak Scrape-off-Layer for Pulse Design

The dynamic interplay between the core and the edge plasma has important consequences in the confinement and heating of fusion plasma. The transport of the Scrape-Off-Layer (SOL) plasma imposes boundary conditions on the core plasma, and neutral transport through the SOL influences the core plasma sourcing. In order to better study these effects in a self-consistent, time-dependent fashion with reasonable turn-around time, a reduced model is needed. In this paper we introduce the SOL Box Model, a reduced SOL model that calculates the plasma temperature and density in the SOL given the core-to-edge particle and power fluxes and recycling coefficients. The analytic nature of the Box Model allows one to readily incorporate SOL physics in time-dependent transport solvers for pulse design applications in the control room. Here we demonstrate such a coupling with the core transport solver TRANSP and compare the results with density and temperature measurements, obtained through Thomson scattering and Langmuir probes, of an NSTX discharge. Implications for future interpretive and predictive simulations are discussed.Comment: 8 pages, 10 figure

Modeling and Physics Design of a Lithium Vapor Box Divertor

Diverted operation, where magnetic field lines intersect with plasma facing components (PFCs) far from the main plasma in a region known as the divertor, has long been seen as necessary for any future tokamak fusion device. However, a consequence of this is that the target, the set of PFCs which intersect the field lines, faces extremely high heat flux. In fact, any solid material would be destroyed if faced with unmitigated reactor-level heat fluxes. Divertor detachment, whereby the power of the plasma is dissipated before striking a material surface, is a proposed solution to this problem. This power dissipation is typically achieved by impurity injection. Divertor detachment with medium-Z impurities can result in radiating regions within the last closed flux surface, which has the tendency to reduce plasma performance via degradation of confinement at the plasma edge. The lithium vapor box divertor seeks to detach via near-target lithium evaporation with condensation of the lithium vapor at walls further upstream. A vapor gradient would thus form, with high density lithium vapor at the target, and low density lithium beyond the condensation region. This vapor gradient results in a stable detachment front, and the low-Z lithium cannot form a radiating region within the main plasma. In this thesis, we show Stochastic PArallel Rarefied-gas Time-accurate Analyzer (SPARTA) and Scrape-Off Layer Plasma Simulator (SOLPS) predictions for the effect of a lithium vapor box divertor in a tokamak. Using SOLPS, fuel puffing is found to have a significant effect on the plasma via increases to the bulk ion friction force acting on the lithium plasma fluid. PFC geometry choices are examined and compared with lithium evaporation in the open divertor geometry on the NSTX-U tokamak. Divertor closure is found to have significant benefits in reducing upstream lithium content. In high power cases, where the unmitigated heat flux to the target is found to be 65MW/m$^2$, different closure designs are considered. Single baffling of the divertor is found to have benefits when compared to a slot divertor geometry for both heat flux and upstream lithium content reduction, as well as isolation of the divertor cooling from the outer midplane. The baffled geometry is found to be resistant to flow reversal in the far SOL where main ion flow is weak, thus the baffles eliminate a path for lithium contamination of the main plasma. The baffled system is able to reach sub-5 MW/m$^2$ heat fluxes at the cost of lithium density around 5$\%$ of the electron density at the outer midplane. The results of this study are tested for their sensitivity to choices of transport coefficients, upstream pumping rate, and puffing location. Even when transport coefficients are reduced to provide less particle flow from the core and higher heat flux at the target, sub-10MW/m$^2$ solutions are available to the lithium vapor box from an unmitigated 92 MW/m$^2$. Private Flux Region (PFR) puffing is seen to be more effective at reducing upstream lithium content while Common Flux Region (CFR) puffing is seen to be more effective at heat flux reduction. The efficacy of both puffing locations is increased by increases to the divertor recycling coefficient. Reducing pumping at walls upstream of the baffles improves the effect of the puffs, leading to cases with lower upstream lithium content for less heat flux. Ultimately, predictions for the upstream lithium content depend heavily on several assumptions made in the input to the code. With more conservative transport parameters $n_{Li}/n_e$ around 0.07 could be expected in order to reduce the target heat flux to sub-10MW/m$^2$

Comparing lithium vapor box designs in a high heat flux scenario using SOLPS-ITER

SOLPS calculations of lithium vapor box divertor designs on NSTX-U are presented. Predictive high power simulations (Pheat=10 MW, qpeakunmitigated∼65 MW/m2) are used to compare and contrast two divertor designs. Specifically a baffled “box” divertor, where a region of neutral density is allowed to build up, is compared to a more typical slot divertor geometry. It is found that significant differences in lithium containment lead to profoundly different viability of the two designs. These differences are seen to be due to far SOL flow patterns that change based on the presence of baffling as well differences in efficiency of the lithium evaporator. Outer-midplane (OMP) separatrix lithium content is found to be strongly detrimental to upstream temperature when nLi/ne>0.1 is reached. This regime of high upstream contamination is avoided via baffling. The reduction in upstream lithium allows access to low heat flux solutions below 5 MW/m2 with very little reduction to upstream temperature from the unmitigated, 65 MW/m2 solution. The slot is able to reach sub-10 MW/m2 heat fluxes though raising the evaporation rate much further reduces the upstream temperature, such that the range of stable evaporation rates with low heat flux to the target is small. Higher performance solutions (low heat flux and low upstream lithium content) are accessible by controlling recycling coefficients of deuterium on the walls above the box

Reduced physics model of the tokamak Scrape-Off-Layer for pulse design

The dynamic interplay between the core and the edge plasma has important consequences in the confinement and heating of fusion plasma. The transport of the Scrape-Off-Layer (SOL) plasma imposes boundary conditions on the core plasma, and neutral transport through the SOL influences the core plasma sourcing. In order to better study these effects in a self-consistent, time-dependent fashion with reasonable turn-around time, a reduced model is needed. In this paper we introduce the SOL Box Model, a reduced SOL model that calculates the plasma temperature and density in the SOL given the core-to-edge particle and power fluxes and recycling coefficients. The analytic nature of the Box Model allows one to readily incorporate SOL physics in time-dependent transport solvers for pulse design applications in the control room. Here we demonstrate such a coupling with the core transport solver TRANSP and compare the results with density and temperature measurements, obtained through Thomson scattering and Langmuir probes, of an NSTX discharge. Implications for future interpretive and predictive simulations are discussed

The effect of inlet conditions on the performance and flowfield structure of a non-premixed swirl-stabilized distributed reaction

A model reactor is used to investigate the extent to which the overall performance and detailed flowfield structure of a non-premixed swirl-stabilized distributed reaction are sensitive to modest changes in inlet conditions (e.g., fuel injection angle, inlet geometry, and swirl vane solidity). Measurements of combustor performance are based on exhaust plane species concentration profiles (HC, CO2, CO, O2), combustion efficiency, and visual observation of combustor stability. The detailed flowfield structure is established by spatially mapping the axial and azimuthal velocity fields using two-component laser anemometry, and the temperature field using a thermocouple probe. The results show that relatively modest changes in inlet conditions can dramatically affect the flowfield structure. As an example, for the model reactor evaluated the addition of a small step at the outer boundary of the swirler yields significantly lower centerline axial velocities and a more uniform thermal structure in the recirculation zone. Furthermore, a modest reduction in swirl vane solidity transforms the aerodynamic structure of the recirculation zone from an off-axis to a central, on-axis structure. These results explain, in part, the contradictory conclusions drawn from data acquired in non-premixed swirl-stabilized distributed reactions, and establish that comparisons and generalizations of such flows require, at a minimum, (1) careful measurements and specification of the inlet conditions, (2) detailed measurements of the flow structure, and (3) an assessment of the effects of modest changes in key operating and configurational variables. © 1988 Combustion Institute

Liquid Engine Test Facilities Assessment

The John C. Stennis Space Center (SSC) requested The Aerospace Corporation to examine the current testing capability of all existing large liquid engine test facilities located in the United States. That information along with projected liquid rocket engine development was used to examine future liquid rocket engine testing facilities needs in the coming decade. Current domestic liquid engine test facilities capabilities, when examined against engine concepts for the coming decade, indicate there are ample facilities offering altitude simulation during test. In addition, it was observed that many contractor facilities have limited ambient test capability of larger thrust engines under current consideration. Finally, it was concluded that diminished contractor participation engine development testing will drive this activity to the government sector. Only three facilities are seen as key contributors to engine testing in the coming decade, namely John C. Stennis Space Center (SSC), Marshall Space Flight Center (MSFC), and Air Force Research Laboratory (AFRL). Past rocket engine test experience was evaluated as a possible resource for projecting future engine test needs. A database comprised of various engine models and the level of testing performed to flight qualify those systems for their first flight was constructed. For comparison purposes in this study, development and qualification efforts were totaled and treated as one test program. Based on experience with past Air Force programs, the time on the test stand accounts for typically 50% or more of the total program time. Historical data show that the time to design and develop new engines has increased over the last 40 years, most likely due to scarcer resources in today's funding environment