In preparation for testing a lithium-helium heat exchanger at Sandia, unexpected rapid failure of the mild steel lithium preheater due to liquid metal embrittlement occurred when lithium at ~400 °C flowed into the preheater then at ~200 °C. This happened before the helium system was pressurized or heating with electron beams began. The paper presents an analysis of the preheater plus a discussion of some implications for fusion.
The 2016 DIII-D DiMES tungsten (W) leading edge experiment in support of ITER studied heat loads in helium (He) plasmas with ELMs and compound ELMs (C-ELMs). The regime was close to the threshold for L-mode to H-mode transitions. In shots with ECH, C-ELMs produced in H-L back-transitions dominated the transient particle exhaust. Their duration (10–20 ms) was much longer than regular ELMs, and their heat loads higher by 2–3 times. The C-ELMS reduced the plasma density significantly, and some triggered (automated) gas puffing. This regime, with high particle and heat exhaust, may have relevance for He plasmas in ITER's start-up phase. Regular ELMs occurred during neutral beam (NB) powered shots. This paper describes new analyses on heat loads during and between C-ELMs in ECH-powered shot 166843. The results are generally consistent with a geometric solution for the parallel heat load at the plasma edge striking the leading edge. The power in the C-ELMs is sufficient to cause a temperature rise of ∼
Additive Manufacturing (AM) can create novel and complex engineered material structures. Features such as controlled porosity, micro-fibers and/or nano-particles, transitions in materials and integral robust coatings can be important in developing solutions for fusion subcomponents. A realistic understanding of this capability would be particularly valuable in identifying development paths. Major concerns for using AM processes with lasers or electron beams that melt powder to make refractory parts are the power required and residual stresses arising in fabrication. A related issue is the required combination of lasers or e-beams to continue heating of deposited material (to reduce stresses) and to deposit new material at a reasonable built rate while providing adequate surface finish and resolution for meso-scale features. Some Direct Write processes that can make suitable preforms and be cured to an acceptable density may offer another approach for PFCs.
Nygren, Richard E.; Matthews, Guy M.; Morgan, Thomas M.; Silburn, Scot S.; Rosenfeld, John R.; North, Mark N.; Talligro, Alberto T.; Stavila, Vitalie S.
This review of the potential of robust plasma facing components (PFCs) with liquid surfaces for applications in future D/T fusion device summarizes the critical issues for liquid surfaces and research being done worldwide in confinement facilities, and supporting R&D in plasma surface interactions. In the paper are a set of questions and related criteria by which we will judge the progress and readiness of liquid surface PFCs. Part-II (separate paper) will cover R&D on the technology-oriented aspects of liquid surfaces including the liquid surfaces as integrated first walls in tritium breeding blankets, tritium retention and recovery, and safety.
This paper advances a vision for plasma facing components (PFCs) that includes the following points. The solution for plasma facing materials likely consists of engineered structures in which the layer of plasma facing material (PFM) is integrated with an engineered structure that cools the PFM and may also transition with graded composition. The key to achieving this PFC architecture will likely lie in advanced manufacturing methods, e.g., additive manufacturing, that can produce layers with controlled porosity and features such as micro-fibers and/or nano-particles that can collect He and transmutation products, limit tritium retention, and do all this in a way that maintains adequate robustness for a satisfactory lifetime. This vision has significant implications for how we structure a development program.
Past numerical investigations of the performance of porous media to enhance heat transfer in helium-cooled devices neglected the susceptibility of multi-channel heat sinks to parallel flow instabilities even though experimental evidence suggests it may be a problem for narrow channel devices. In previous work, our simulations have shown that helium micro-jets do not experience changes in flow distribution due to non-uniform heating. However, jets are difficult to fabricate for large area refractory metal components. The same is not true for narrow channel devices filled with porous media. Although these refractory devices are easier to fabricate, the effects of downstream hot gas expansion can influence the incoming flow distribution in multi-channel configurations.
Until 2012, Sandia participated regularly in non-fusion R&D that was supported primarily through our collaborations with companies in the DOE program for Small Business Innovative Research but also in some work-for-others contracts. In this work, funds were recovered from collaborating institutions for the staff time and materials used, but FES had supported the facility itself and in doing so enabled the contributions to the non-fusion R&D below.
Sandia has developed and tested mockups armored with W rods over the last decade and pioneered the initial development of W rod armor for International Thermonuclear Experimental Reactor (ITER) in the 1990's. We have also developed 2D and 3D thermal and stress models of W rod-armored plasma facing components (PFCs) and test mockups and are applying the models to both short pulses, i.e. edge localized modes (ELMs), and thermal performance in steady state for applications in C-MOD, DiMES testing and ITER. This paper briefly describes the 2D and 3D models and their applications with emphasis on modeling for an ongoing test program that simulates repeated heat loads from ITER ELMs.
An experiment at Sandia National Laboratories confirmed that a ternary salt (Flinabe, a ternary mixture of LiF, BeF{sub 2} and NaF) had a sufficiently low melting temperature ({approx}305 C) to be useful for first wall and blanket applications using flowing molten salts that were investigated in the Advanced Power Extraction (APEX) Program.[1] In the experiment, the salt pool was contained in a stainless steel crucible under vacuum. One thermocouple was placed in the salt and two others were embedded in the crucible. The results and observations from the experiment are reported in the companion paper.[2] The paper presented here will cover a 3-D finite element thermal analysis of the salt pool and crucible. The analysis was done to evaluate the thermal gradients in the salt pool and crucible and to compare the temperatures of the three thermocouples. One salt mixture appeared to melt and to solidify as a eutectic with a visible plateau in the cooling curve (i. e, time versus temperature for the thermocouple in the salt pool). This behavior was reproduced with the thermal model. Cases were run with several values of the thermal conductivity and latent heat of fusion to see the parametric effects of these changes on the respective cooling curves. The crucible was heated by an electrical heater in an inverted well at the base of the crucible. It lost heat primarily by radiation from the outer surfaces of the crucible and the top surface of the salt. The primary independent factors in the model were the emissivity of the crucible (and of the salt) and the fraction of the heater power coupled into the crucible. The model was 'calibrated' using (thermocouple) data and heating power from runs in which the crucible contained no salt.
In the authors initial high heat flux tests on small mockups armored with W rods, done in the small electron beam facility (EBTS) at Sandia National Laboratories, the mockups exhibited excellent thermal performance. However, to reach high heat fluxes, they reduced the heated area to only a portion ({approximately}25%) of the sample. They have now begun tests in their larger electron beam facility, EB 1200, where the available power (1.2 MW) is more than enough to heat the entire surface area of the small mockups. The initial results indicate that, at a given power, the surface temperatures of rods in the EB 1200 tests is somewhat higher than was observed in the EBTS tests. Also, it appears that one mockup (PW-10) has higher surface temperatures than other mockups with similar height (10mm) W rods, and that the previously reported values of absorbed heat flux on this mockup were too high. In the tests in EB 1200 of a second mockup, PW-4, absorbed heat fluxes of {approximately}22MW/m{sup 2} were reached but the corresponding surface temperatures were somewhat higher than in EBTS. A further conclusion is that the simple 1-D model initially used in evaluating some of the results from the EBTS testing was not adequate, and 3-D thermal modeling will be needed to interpret the results.
The 1999 US-Japan Workshop on High Heat Flux Components and Plasma Surface Interactions in Next Step Fusion Devices was held at the St. Francis Hotel in Santa Fe, New Mexico, on November 1-4, 1999. There were 42 presentations as well as discussion on technical issues and planning for future collaborations. The participants included 22 researchers from Japan and the United States as well as seven researchers from Europe and Russia. There have been important changes in the programs in both the US and Japan in the areas of plasma surface interactions and plasma facing components. The US has moved away from a strong focus on the ITER Project and has introduced new programs on use of liquid surfaces for plasma facing components, and operation of NSTX has begun. In Japan, the Large Helical Device began operation. This is the first large world-class confinement device operating in a magnetic configuration different than a tokamak. In selecting the presentations for this workshop, the organizers sought a balance between research in laboratory facilities or confinement devices related to plasma surface interactions and experimental research in the development of plasma facing components. In discussions about the workshop itself, the participants affirmed their preference for a setting where ''work-in-progress'' could be informally presented and discussed.