D. L. Sadowski

THE ARIES-CS COMPACT STELLARATOR FUSION POWER PLANT

Abstract An integrated study of compact stellarator power plants, ARIES-CS, has been conducted to explore attractive compact stellarator configurations and to define key research and development (R&D) areas. The large size and mass predicted by earlier stellarator power plant studies had led to cost projections much higher than those of the advanced tokamak power plant. As such, the first major goal of the ARIES-CS research was to investigate if stellarator power plants can be made to be comparable in size to advanced tokamak variants while maintaining desirable stellarator properties. As stellarator fusion core components would have complex shapes and geometry, the second major goal of the ARIES-CS study was to understand and quantify, as much as possible, the impact of the complex shape and geometry of fusion core components. This paper focuses on the directions we pursued to optimize the compact stellarator as a fusion power plant, summarizes the major findings from the study, highlights the key design aspects and constraints associated with a compact stellarator, and identifies the major issues to help guide future R&D.

The Science and Technologies for Fusion Energy With Lasers and Direct-Drive Targets

We are carrying out a multidisciplinary multi-institutional program to develop the scientific and technical basis for inertial fusion energy (IFE) based on laser drivers and direct-drive targets. The key components are developed as an integrated system, linking the science, technology, and final application of a 1000-MWe pure-fusion power plant. The science and technologies developed here are flexible enough to be applied to other size systems. The scientific justification for this work is a family of target designs (simulations) that show that direct drive has the potential to provide the high gains needed for a pure-fusion power plant. Two competing lasers are under development: the diode-pumped solid-state laser (DPPSL) and the electron-beam-pumped krypton fluoride (KrF) gas laser. This paper will present the current state of the art in the target designs and lasers, as well as the other IFE technologies required for energy, including final optics (grazing incidence and dielectrics), chambers, and target fabrication, injection, and tracking technologies. All of these are applicable to both laser systems and to other laser IFE-based concepts. However, in some of the higher performance target designs, the DPPSL will require more energy to reach the same yield as with the KrF laser.

Hydrodynamic characteristics of counter-current two-phase flow in vertical and inclined channels: effects of liquid properties

Abstract Flow patterns, counter-current flow limitation (flooding), and gas hold-up (void fraction) in counter-current flow in vertical and inclined channels were experimentally investigated. Tests were performed in a 2 m-long channel with 1.9 cm inner diameter, using air, and demineralized water, mineral and paraffinic oils, covering a surface tension range of 0.0128–0.072 N/m, and a liquid viscosity range of 1 × 10 −3 −1.85 × 10 −1 Ns/m 2 . The liquid and gas superficial velocity ranges for the tests with demineralized water were 0 ⩽ U GS ⩽ 54 cm/s and 1 ⩽ U GS ⩽ 299 cm/s, for tests with mineral oil were 0 ⩽ U LS ⩽ 23 cm/s and 1 ⩽ U GS ⩽ 248 cm/s, and for paraffinic oil were 0.15 ⩽ U LS ⩽ 11.6 cm/s and 1 ⩽ U GS ⩽ 224 cm/s, respectively. The examined channel angles of inclination with respect to the vertical line were 0, 30 and 68°. Flooding data were significantly different from pure water results only at very high liquid viscosities. The effect of liquid viscosity on gas hold-up and flow patterns was significant, furthermore, and several existing models and correlations were unable to correctly predict the data trends. With increasing the liquid viscosity the parameter range of the slug flow pattern expanded for all angles of inclination, and froth flow replaced the churn flow pattern in the vertical configuration and it replaced the churn/stratified and semi-stratified patterns in inclined configurations. The churn-stratified flow pattern is predominantly wavy stratified and is interrupted by upward-moving flooding-type waves. Semi-stratified is a periodic pattern where in each period the flow regime is initially wavy stratified while liquid accumulates in the bottom portion of the test section and forms a large liquid slug which subsequently moves upwards in the channel.

The ARIES Advanced and Conservative Tokamak Power Plant Study

Abstract Tokamak power plants are studied with advanced and conservative design philosophies to identify the impacts on the resulting designs and to provide guidance to critical research needs. Incorporating updated physics understanding and using more sophisticated engineering and physics analysis, the tokamak configurations have developed a more credible basis compared with older studies. The advanced configuration assumes a self-cooled lead lithium blanket concept with SiC composite structural material with 58% thermal conversion efficiency. This plasma has a major radius of 6.25 m, a toroidal field of 6.0 T, a q95 of 4.5,a βtotal N of 5.75, an H98 of 1.65, an n/nGr of 1.0, and a peak divertor heat flux of 13.7 MW/m2. The conservative configuration assumes a dual-coolant lead lithium blanket concept with reduced-activation ferritic martensitic steel structural material and helium coolant, achieving a thermal conversion efficiency of 45%. The plasma has a major radius of 9.75 m, a toroidal field of 8.75 T, a q95 of 8.0, a βtotal N of 2.5, an H98 of 1.25, an n/nGr of 1.3, and a peak divertor heat flux of 10 MW/m2. The divertor heat flux treatment with a narrow power scrape-off width has driven the plasmas to larger major radius. Edge and divertor plasma simulations are targeting a basis for high radiated power fraction in the divertor, which is necessary for solutions to keep the peak heat flux in the range 10 to 15 MW/m2. Combinations of the advanced and conservative approaches show intermediate sizes. A new systems code using a database approach has been used and shows that the operating point is really an operating zone with some range of plasma and engineering parameters and very similar costs of electricity. Other papers in this issue provide more detailed discussion of the work summarized here.

Experimental and numerical investigation of the thermal performance of the gas-cooled divertor plate concept

The helium-cooled plate-type divertor concept proposed by Malang was designed to accommodate a surface heat load of ˜10 MW/m2. This design can potentially reduce the number of modules needed for the divertor by over two orders of magnitude compared with other concepts, thereby significantly reducing coolant delivery system complexity and manufacturing costs. While previous analyses have predicted that the plate design can accommodate heat fluxes of 10 MW/m2, no experimental data have been published to date to validate such analyses. Experiments have therefore been conducted using air as the coolant at Reynolds numbers similar to those proposed for the actual helium-coolant operating conditions on an instrumented test module with cross-sectional geometry identical to the prototypical plate-type divertor. A second test module where the planar jet exiting the inlet manifold is replaced by a two-dimensional hexagonal array of circular jets over the entire top surface of the inlet manifold has also been tested. The thermal performance of both test modules with and without a porous metallic foam layer in the gap between the outer surface of the inlet manifold and the cooled surfaces was directly compared to test the numerical simulations of Sharafat which predict that the metallic foam significantly enhances heat transfer.

Experimental Evaluation of the Thermal Hydraulics of Helium-Cooled Divertors

Abstract Current predictions suggest that the target plate of a divertor, as one of the few solid surfaces directly exposed to the plasma of a magnetic fusion energy reactor, will be subject to steady-state heat fluxes as great as 10 MW/m2. Developing appropriate methods for cooling these divertors with helium is therefore a major technological challenge for plasma-facing components. This paper reviews dynamically similar experimental studies and numerical simulations of the thermal-hydraulic performance of two helium-cooled divertor concepts, the helium-cooled divertor with multiple-jet cooling (HEMJ) and the helium-cooled flat plate divertor, as well as a variant of the HEMJ, the so-called finger-type divertor, performed as part of the ARIES study. The results from these studies are extrapolated to prototypical conditions and used to predict the maximum average heat flux and coolant pumping power requirements for these divertor concepts. These extrapolations can be used to estimate how changes in the operating conditions, such as the helium inlet temperature and the maximum temperature of the divertor pressure boundary, affect thermal performance. Finally, the correlations from these extrapolations are used in the system code developed by the ARIES study.

Experimental Studies of the Thermal Performance of Gas-Cooled Plate-Type Divertors

Abstract The helium-cooled plate-type divertor can reduce the number of divertor modules while accommodating heat fluxes q” up to 10 MW/m2 incident on tungsten-alloy armor. Dynamically similar experimental studies were performed to evaluate the thermal performance of variants of this divertor design at conditions that spanned the prototypical operating Reynolds number Re of 3.3 × 104. In the studies, a jet of air issuing from 0.5 mm and 2 mm wide slots impinged on and cooled a heated planar surface 2 mm away from the slot, then flowed through either a 2 mm wide channel or an array of cylindrical pin fins. The studies indicate that the fins, which increase the cooled surface area by a factor of 3.76, increase the effective heat transfer coefficient (HTC) by as much as 160% at a relatively modest increase in pressure drop of less than 40%. These experimental results were used to determine the thermal performance of the actual plate design with helium cooling under prototypical conditions. Although the benefit of the fins is reduced because the fin efficiency decreases as the HTC increases, the predictions suggest that the fins could increase the maximum q” that can be accommodated by this design to ˜18 MW/m2. Alternatively, for a given heat flux (e.g. 10 MW/m2), adding fins could allow operation of the divertor at lower coolant flow rates, and hence pumping powers.

Studies of turbulent liquid sheets for protecting IFE reactor chamber first walls

Abstract The HYLIFE-II conceptual inertial fusion energy (IFE) reactor design uses stationary and oscillating slab jets, or liquid sheets, to create a protective pocket that allows target injection and driver beam propagation while protecting the chamber first walls from neutrons, X-rays and target debris. Thick liquid wall protection can, therefore, reduce reactor chamber size and increase chamber lifetime in commercial IFE reactors. Minimizing driver beam interference and irradiation of the final focus magnets places stringent requirements upon the surface smoothness of the stationary liquid sheets that shield the heavy-ion driver array. Experiments were carried out to determine how nozzle and flow conditioner design affect surface smoothness in turbulent liquid sheets at Reynolds numbers up to 130 000. The free-surface was directly imaged using planar laser-induced fluorescence (PLIF) at downstream distances up to 25 times the sheet thickness at the nozzle exit. The data are processed to determine the probability of finding fluid at any given spatial location. The surface smoothness of sheets of water issuing from nozzles with various contractions and rectangular or nearly elliptical exits into atmospheric pressure air were quantified and compared. The free-surface characteristics of liquid sheets issuing from unblocked and partially blocked (2.5% of total area) but otherwise identical flow conditioners were compared to investigate the robustness of flow conditioning elements over long operation times. These results on nearly prototypical turbulent stationary liquid sheets address a number of technical feasibility issues for thick liquid protection in IFE reactors.

Dynamically Similar Studies of the Thermal Performance of Helium-Cooled Finger-Type Divertors With and Without Fins

Abstract Experimental studies based upon dynamic similarity have been used to evaluate the thermal performance of several modular helium-cooled tungsten divertor designs, including a configuration similar to the helium-cooled modular divertor with multiple jets (HEMJ). Until recently, all of these experiments used air, instead of helium, as the coolant. The average Nusselt number and loss coefficient were determined from cooled surface temperature and pressure drop data. Correlations were developed for the Nusselt number and loss coefficient as a function of the Reynolds number then used to predict the thermal performance of the divertor under prototypical conditions when cooled with high-temperature, high-pressure helium. Recently, experiments were performed using helium and argon to confirm the dynamic similarity assumption. The results indicated that the previous experiments with air, which were performed at the prototypical nondimensional coolant mass flow rate, or Reynolds number, did not account for the differences in the fraction of the incident power conducted through the walls of the divertor versus that convected, i.e., removed, by the coolant. Dimensional analysis and numerical simulations suggest that for a given divertor geometry this fraction can be characterized by the ratio of the thermal conductivities of the divertor material and the coolant. Nusselt number correlations were developed to include the effect of the thermal conductivity ratio. Based on these correlations, the predicted maximum heat flux values that can be accommodated by the HEMJ-like configuration are reduced by [approximately]20% from previous estimates. The results also suggest that the maximum heat flux that can be accommodated by this design can be increased by as much as 19% by adding an array of cylindrical pin fins on the cooled pressure boundary. However, as expected, adding the fins increases the pumping power for the coolant by [approximately]16%. As a fraction of maximum total incident thermal power, however, the pumping power decreases by 2% when the fins are added due to the significant increase in the maximum heat flux.

Verification of Thermal Performance Predictions of Prototypical Multi-Jet Impingement Helium-Cooled Divertor Module

Abstract An experimental investigation of the thermal performance of the Helium-Cooled Multi-Jet (HEMJ) modular divertor design developed by the Karlsruhe Research Center (FZK) was previously performed at Georgia Tech using air at Reynolds numbers (Re) spanning those at which the actual He-cooled divertor is to be operated. More recently, another experimental investigation was performed by the Georgia Tech group for a similar finger-type divertor module using both air and He as coolants. The results of these experiments suggest that, in addition to matching Re, dynamic similarity between the air and He experiments requires that a correction be made to account for the differences in the relative contributions of convection and conduction (through the divertor walls) to the overall heat removal rate by the module. This correction factor depends on the thermal conductivity ratio of the solid to the coolant. Experiments similar to those previously conducted have therefore been performed using air, argon, or He as coolant for test sections constructed of brass or steel thus covering a wide range of thermal conductivity ratio. The resultant correlation between Re, the heat removal rate, and the thermal conductivity ratio from these experiments can be used to predict the thermal performance of HEMJlike divertors at prototypical operating conditions.