Center For Plasma Material Interactions Research Articles

Overview of Liquid-Metal PFC R&D at the University of Illinois Urbana-Champaign

The design and implementation of future flowing liquid-lithium plasma-facing components (LLPFCs) will be dependent on several factors. Of course, one of the most important is the need to be able to deal with high heat fluxes incident on the surface of the LLPFCs, but there are also several other important liquid-metal behaviors that have been identified for their critical impact on the feasibility of a LLPFC. One of these is the ability to constantly wet 100% of the plasma-facing component area and the best way to achieve that. Another key point is knowing and understanding the erosion and corrosion of the surfaces subject to a flowing liquid-lithium system and the ability for hydrogen and helium uptake by the system. The Center for Plasma Material Interactions (CPMI) has been tasked with looking at these various issues. The Mock-up Entry module for EAST device was used to investigate wetting and erosion effects and to design a suitable distribution and collection system with a liquid-lithium loop. The vapor shielding effects of lithium on the surface were also modeled and studied. A model coupling CRANE, an open-source global reaction network solver, and Zapdos, a plasma transport solver, is being developed to better understand the dynamics of the vapor cloud. Experiments on the Magnum-PSI at the Dutch Institute for Fundamental Energy Research have been carried out to study the vapor shielding effect and obtain experimental benchmarks to verify the model. Also, initial experiments using the Hybrid Illinois Device for Research and Applications have been performed to understand the pumping effects of lithium on helium. Experiments with a drop of liquid lithium (~100 mg) into a helium plasma have shown the ability of lithium to take out the cold recycling helium gas as well as hydrogen and oxygen impurity gases. The improvement in plasma performance was significant, and further understanding of this effect will have impacts on how future LLPFCs will be designed. Further investigation into the exact mechanism for helium pumping by lithium needs to be performed in the future. This paper presents a summary of the results obtained at the CPMI.

A distillation column for hydrogen isotope removal from liquid lithium

Recovery of tritium from plasma-facing components in fusion devices will be vital to future full-scale operation. Liquid, low-Z materials have demonstrated many inherent advantages over solid first wall materials. To this end, a thermal treatment method in the form of a distillation column for extraction of hydrogen isotopes from liquid lithium has been designed, developed, and constructed at the Center for Plasma-Material Interactions at the University of Illinois at Urbana-Champaign. Use of induction heating and lithium condensation stages are the two qualities that set this design apart from other thermal treatment systems. Induction heating capabilities were modeled using the COMSOL Multiphysics software, which were validated when commissioning the physical heater module. Proof-of-concept tests were performed in the prototype column, which were undertaken as batch processes to investigate the efficacy with which the column could remove hydrogen gas from lithium-rich and lithium hydride-rich samples. All of the tests reported used lithium hydride as a surrogate for lithium deuteride and lithium tritide. The design process and results from the initial tests will be discussed, along with the envisioned placement of this treatment scheme in a fully-functional lithium loop.

Latest Results From the Hybrid Illinois Device for Research and Applications (HIDRA)

The Hybrid Illinois Device for Research and Applications (HIDRA) is a toroidal fusion device at the University of Illinois at Urbana–Champaign, Urbana, IL, USA. HIDRA is the former WEGA stellarator that was operated at the Max Planck Institut fur Plasmaphysik, Greifswald, Germany. The machine is a five-period, $l=2, m=5$ stellarator, with major radius $R_{0}=0.72$ m, and minor radius $a=0.19$ m. Initial heating is achieved with 2.45-GHz electron cyclotron resonance heating and an on-axis magnetic field of $B_{0}=0.087$ T that can go as high as $B_{0}=0.5$ T. HIDRA has the ability to operate as both a stellarator and a tokamak, initially operating in the stellarator mode. The focus of research on HIDRA will be doing dedicated studies on plasma-material interactions (PMIs) using the wealth of knowledge and experience at the Center for Plasma Material Interactions, Urbana, IL, USA. In early 2016, the first experiments were performed on HIDRA. This paper presents some of the first results obtained from the machine such as initial magnetic fields’ measurements and plasma discharges. It also shows the development of the control system being currently implemented and introduces HIDRA-materials analysis tool, the in situ PMI facility that will be mounted on HIDRA in the near future to further enhance the diagnostics and material testing experiments meant to be conducted on the machine.

A high power impulse magnetron sputtering model to explain high deposition rate magnetic field configurations

High Power Impulse Magnetron Sputtering (HiPIMS) is one of the recent developments in the field of magnetron sputtering technology that is capable of producing high performance, high quality thin films. Commercial implementation of HiPIMS technology has been a huge challenge due to its lower deposition rates compared to direct current Magnetron Sputtering. The cylindrically symmetric “TriPack” magnet pack for a 10 cm sputter magnetron that was developed at the Center for Plasma Material Interactions was able to produce higher deposition rates in HiPIMS compared to conventional pack HiPIMS for the same average power. The “TriPack” magnet pack in HiPIMS produces superior substrate uniformity without the need of substrate rotation in addition to producing higher metal ion fraction to the substrate when compared to the conventional pack HiPIMS [Raman et al., Surf. Coat. Technol. 293, 10 (2016)]. The films that are deposited using the “TriPack” magnet pack have much smaller grains compared to conventional pack DC and HiPIMS films. In this paper, the reasons behind the observed increase in HiPIMS deposition rates from the TriPack magnet pack along with a modified particle flux model is discussed.

High Deposition Rate Symmetric Magnet Pack for High Power Pulsed Magnetron Sputtering

Abstract High power pulsed magnetron sputtering is a promising physical vapor deposition technique with two minor challenges that obstruct its broader implementation in industry and its use by researchers. The first challenge is the availability of low cost HPPMS power supplies with output power under 2 kW. Such power supplies are suited for circular planar magnetrons with target diameters between 50 mm to 150 mm. The second challenge is the overall lower deposition rates of HPPMS when compared with direct current magnetron discharges. The “e” magnet pack designed for a 100 mm sputter magnetron which was developed by the Center for Plasma Material Interactions at the University of Illinois at Urbana Champaign in collaboration with Kurt J. Lesker Company was capable of producing twice higher deposition rates in HPPMS compared to a conventional magnet pack. The cylindrically symmetric “TriPack” magnet pack presented here was developed based on magnetic field design solutions from the “e” magnet pack in order to keep the high deposition rates, but improve deposition uniformity, without the need for substrate rotation. The new cylindrically symmetric magnet pack for 100 mm diameter targets, along with a specially designed cooling well provides stable operation at 2 kW average power, even with low-temperature melting-point target materials. The deposition rates from the TriPack magnet pack is compared with a commercial conventional magnet pack for DC and HPPMS power supplies.

Debris transport analysis at the intermediate focus of an extreme ultraviolet light source

As extreme ultraviolet light lithography matures, critical deficits in the technology are being resolved. Research has largely focused on solving the debris issue caused by using warm (Te ∼ 30 eV) and dense (ne ∼ 1020 cm−3) plasma to create 13.5-nm light. This research has been largely focused on the mitigation of the debris between the plasma and the collector optics. The next step of debris mitigation is investigated, namely the effect of debris mitigation on the transport of undesired contaminants to the intermediate focus (IF). In order to investigate emissions from the IF, the Center for Plasma-Material Interactions at the University of Illinois at Urbana-Champaign has developed the Sn intermediate focus flux emission detector. The effects of a secondary RF-plasma, buffer gas flow rate, chamber pressure, and charged plate deflection are investigated. By increasing the chamber pressure to 10 mTorr, flowing 1000 sccm Ar buffer gas, and utilizing charged particle deflection, it is possible to reduce the measured number of post-IF species by greater than 99%. Furthermore, it is shown that typical debris mitigation techniques lead to the development of a plasma near the IF that can be detrimental to post-IF optics.

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High power pulsed magnetron sputtering is a promising physical vapor deposition technique with two minor challenges that obstruct its broader implementation in industry and its use by researchers. The first challenge is the availability of low cost HPPMS power supplies with output power under 2 kW. Such power supplies are suited for circular planar magnetrons with target diameters between 50 mm to 150 mm. The second challenge is the overall lower deposition rates of HPPMS when compared with direct current magnetron discharges. The “ε” magnet pack designed for a 100 mm sputter magnetron which was developed by the Center for Plasma Material Interactions at the University of Illinois at Urbana Champaign in collaboration with Kurt J. Lesker Company was capable of producing twice higher deposition rates in HPPMS compared to a conventional magnet pack. The cylindrically symmetric “TriPack” magnet pack presented here was developed based on magnetic field design solutions from the “ε” magnet pack in order to keep the high deposition rates, but improve deposition uniformity, without the need for substrate rotation. The new cylindrically symmetric magnet pack for 100 mm diameter targets, along with a specially designed cooling well provides stable operation at 2 kW average power, even with low-temperature melting-point target materials. The deposition rates from the TriPack magnet pack is compared with a commercial conventional magnet pack for DC and HPPMS power supplies.

Seebeck coefficient measurements of lithium isotopes

Lithium, owing to its many advantages, is of immense interest to the fusion community for its use as plasma facing component (PFC) material. Various experiments are under progress in the Center for Plasma Material Interactions (CPMI) at the University of Illinois at Urbana Champaign (UIUC) aimed at understanding the plasma–lithium interactions. In one such experiment called Solid/Liquid Lithium Divertor Experiment (SLiDE), it was recently observed that the flow of liquid lithium in the presence of magnetic fields is dominated by thermoelectric Magnetohydrodynamic (TEMHD) effects. To describe these results accurately, a knowledge of the thermoelectric properties of lithium is essential. For this purpose, an apparatus to measure the Seebeck coefficient of lithium was developed. Using this apparatus, the Seebeck coefficient of lithium as a function of temperature has been obtained. The Seebeck coefficient of lithium-7 is found to gradually increase from 11 μV/K to 25 μV/K, as the temperature is raised from 25 °C to 240 °C. These measurements are in good agreement with Kendall’s thermoelectric measurements on natural Li. Furthermore, using the same apparatus, the thermoelectric curve of lithium-6 is obtained and for the first time are reported in this paper.

Lithium research as a plasma facing component material at the University of Illinois

Plasma facing component (PFC) materials are critical to the development of future fusion reactor. While several different PFC materials have been considered in the past, there is an increased interest in the scientific community on the use of lithium as a future PFC material, after its initial success demonstrated by the TFTR “supershots” and more recent work performed on the CDX-U and NSTX spherical tori. The worldwide usage of lithium in tokamaks has grown rapidly over the past few years because of these results. The Center for Plasma-Material Interactions (CPMI) at the University of Illinois at Urbana-Champaign (UIUC) has been actively involved in lithium research since its foundation and currently, there are three experimental facilities dedicated to studying lithium interactions relevant to fusion conditions namely: 1. Ion surface InterAction eXperiment (IIAX); 2. Divertor Erosion and Vapor Shielding eXperiment (DEVeX); and 3. Solid/Liquid Lithium Divertor Experiment (SLiDE). One of the significant advantages of these experimental facilities is the ability to study the basic physics of the phenomenon taking place. In this review paper, a brief description of the experimental facilities and their capabilities is provided and the interested reader is advised to look in future publications for the recent results.

Ionic debris measurement of three extreme ultraviolet sources

Generation of debris in extreme ultraviolet (EUV) light sources is an inherent and real threat to the lifetime of collection optics. Debris measurement of these sources is useful to enable source suppliers to estimate collector lifetime. At the Center for Plasma Material Interactions (CPMI) at the University of Illinois, an Illinois calibrated spherical sector electrostatic energy analyzer (ICE) was built to measure the ion debris flux in absolute units. In addition to ion flux, the detector is also capable of identifying different ion species present in the plasma utilizing energy-to-charge ratio discrimination. The lifetime of the collector optics is calculated using the measured ion flux. In the current investigation we compare the measurement of ion debris production in three different EUV sources: the Energetiq EQ-10M, the AIXUV-100, and the XTREME XTS 13-35. In the EQ-10M source, three angular measurements are coupled with three variations in operating pressure to measure consequent effects on debris production. These measurements reveal four predominant ion species in the energetic debris analysis: C+, Si+, Xe+, and Xe2+. The amount of debris is reduced as pressure is increased. Various debris mitigation methods are implemented in the AIXUV-100 source and results reveal that four ion species are observed (Ar+, Xe+, Xe2+, and W+), though there does not seem to be a dominant species. The first mitigation technique, backstreaming argon toward the source, reduces the amount of Ar+, Xe2+, and W+, yet increases the amount of Xe+ The increase in Xe+ flux is explained based on charge exchange phenomena. The ICE machine was then attached 1.92 m away from the pinch of XTS 13-35 source, and placed at 25° away from the normal line. The comparison of results reveals that the XTS 13-35 and the EQ-10M sources produced comparable amounts of energetic ion flux per watt of EUV light produced. The AIXUV-100 source generated more ion debris flux per watt of EUV light than the other two sources, though it should be noted that the AIXUV-100 source was capable of producing more than ten times the amount of EUV light power compared to any of the other sources.