Public Abstract

DE-SC0010719: DECIPHERING THE ROLE OF MIXED-MATERIAL DEPOSITION AND TEMPERATURE ON LITHIUM-COATED PFCS IN NSTX-U HIGH-PERFORMANCE PLASMAS: COLLABORATIVE UIUC & UTK PROPOSAL

Award Status: Inactive

Institution: Board of Trustees of the University of Illinois, Champaign, IL
UEI: Y8CWNJRCNN91
DUNS: 041544081

Most Recent Award Date: 08/22/2017
Number of Support Periods: 5
PM: King, Joshua

Current Budget Period: 09/01/2017 - 08/31/2018
Current Project Period: 09/01/2014 - 08/31/2018
PI: Allain, Jean Paul

Supplement Budget Period: N/A

Public Abstract

Although oxygen is required to sustain life, oxygen sucks the life out of fusion by radiating away too much power from the high-temperature plasma. Accordingly, great efforts are expended to reduce the oxygen found in fusion facilities. Researchers at NSTX have long used lithium wall conditioning as a method for improving plasma performance. These improvements include elimination of otherwise virulent edge plasma instabilities, and an improvement in the energy confinement of the plasma, both of which are correlated with a reduction of neutrals that ‘recycled’ at the plasma facing components. Until recently, researchers assumed that the lithium was primarily responsible for these benefits, although the precise mechanism remained unknown. Contributing to the mystery is the fact that walls of NSTX are made of carbon in the form of graphite tiles. Lithium tends to seep into graphite, so it was unclear why any lithium would be left to capture anything that landed on the surface of the tiles. Instead, it appears that the lithium interacts with both the carbon in the tiles, and the oxygen that is naturally embedded in them as well as oxygen carried by the evaporated plume of lithium, to create a new plasma-facing wall that contains all three elements. Moreover, the mystery grew deeper noting that the ambient vacuum in NSTX would improve with more lithium since it would effectively “getter” these impurities. How could then lithium pump hydrogen having gettered impurities such as oxygen? The work by Prof. J.P. Allain’s team at the University of Illinois at Urbana-Champaign and formerly at Purdue University have shed light on how effectively this special wall surface can improve plasma performance. These studies have demonstrated the strong reaction that takes place when deuterium, the hydrogen isotope used in NSTX plasmas, comes into contact with lithium and oxygen at the plasma-facing wall of the fusion facility. Researchers first used a highly sensitive measurement technique called “X-ray photoelectron spectroscopy,” or XPS, to detect the chemistry of the top few nanometers of the lithium-covered graphite tiles in NSTX experiments. Researchers then used computer simulations led by collaborator Dr. Predrag Krstic (now at SUNY Stony Brook and formerly at Univ. Tennessee and Oak Ridge National Lab) to replicate the contact between deuterium and graphite tiles impregnated with lithium and oxygen. Results showed that the lithiated graphite captured much of the deuterium, mirroring what occurred in NSTX. (See “Deuterium Uptake in Magnetic-Fusion Devices with Lithium-Conditioned Carbon Walls” by P. S. Krstic et al. in: Physical Review Letters 110, 105001 (2013).) The next phase of this work will now extend to another important aspect of lithium’s performance and that is its dependence on temperature. All former experiments including computational simulations ignored any temperature effects, primarily due to the difficulty in simulating the same. Most thermodynamic processes have time scales of the order of microseconds to seconds or even minutes. Computational atomistic simulations have largely focused on “prompt” mechanisms of the chemistry near picoseconds to nanoseconds. However, the plasma interactions with many of the lithiated graphite surfaces in NSTX can be raised to temperatures beyond the melting point of lithium and this could have serious implications to the operation of NSTX-Upgrade plasmas that intend to raise overall heat fluxes by factors of two or more. One essential question is whether these lithium-conditioned surfaces would be able to sustain the beneficial low-recycling regimes so far achieved at higher power fluxes to these wall surfaces. In addition, complex surfaces involving boronization conditioning and re-deposition from eroded graphite will also need to be evaluated in the context of hydrogen retention by lithium coatings. Moreoever, lithium coatings are intended for future use with molybdenum or tungsten in future NSTX-U operation scenarios. Therefore, another important question is whether the chemical interactions found between lithium, graphite and oxygen remain with these new high-Z metals, which are part of a plan in NSTX-U to cover the lower divertor floor in the future. For this task, PI Prof. J.P. Allain has teamed up with Prof. Brian Wirth of the University of Tennessee at Knoxville to address this problem using both state-of-the art in-situ experiments at the University of Illinois and multi-scale computational simulation codes used by Wirth’s team. Prof. Wirth brings a wealth of experience in the simulation of multi-scale spatio-temporal behavior of irradiated tungsten and will extend his efforts to study lithium coatings on these substrates and graphite. Long-pulse PFC studies will be coordinated with an established collaboration at the Dutch Institute for Fundamental Energy Research (DIFFER) under the direction of Dr. Greg DeTemmerman.

Although oxygen is required to sustain life, oxygen sucks the life out of fusion by radiating away too much power from the high-temperature plasma. Accordingly, great efforts are expended to reduce the oxygen found in fusion facilities. Researchers at NSTX have long used lithium wall conditioning as a method for improving plasma performance. These improvements include elimination of otherwise virulent edge plasma instabilities, and an improvement in the energy confinement of the plasma, both of which are correlated with a reduction of neutrals that ‘recycled’ at the plasma facing components. Until recently, researchers assumed that the lithium was primarily responsible for these benefits, although the precise mechanism remained unknown.

Contributing to the mystery is the fact that walls of NSTX are made of carbon in the form of graphite tiles. Lithium tends to seep into graphite, so it was unclear why any lithium would be left to capture anything that landed on the surface of the tiles. Instead, it appears that the lithium interacts with both the carbon in the tiles, and the oxygen that is naturally embedded in them as well as oxygen carried by the evaporated plume of lithium, to create a new plasma-facing wall that contains all three elements. Moreover, the mystery grew deeper noting that the ambient vacuum in NSTX would improve with more lithium since it would effectively “getter” these impurities. How could then lithium pump hydrogen having gettered impurities such as oxygen?

The work by Prof. J.P. Allain’s team at the University of Illinois at Urbana-Champaign and formerly at Purdue University have shed light on how effectively this special wall surface can improve plasma performance. These studies have demonstrated the strong reaction that takes place when deuterium, the hydrogen isotope used in NSTX plasmas, comes into contact with lithium and oxygen at the plasma-facing wall of the fusion facility. Researchers first used a highly sensitive measurement technique called “X-ray photoelectron spectroscopy,” or XPS, to detect the chemistry of the top few nanometers of the lithium-covered graphite tiles in NSTX experiments. Researchers then used computer simulations led by collaborator Dr. Predrag Krstic (now at SUNY Stony Brook and formerly at Univ. Tennessee and Oak Ridge National Lab) to replicate the contact between deuterium and graphite tiles impregnated with lithium and oxygen. Results showed that the lithiated graphite captured much of the deuterium, mirroring what occurred in NSTX. (See “Deuterium Uptake in Magnetic-Fusion Devices with Lithium-Conditioned Carbon Walls” by P. S. Krstic et al. in: Physical Review Letters 110, 105001 (2013).)

The next phase of this work will now extend to another important aspect of lithium’s performance and that is its dependence on temperature. All former experiments including computational simulations ignored any temperature effects, primarily due to the difficulty in simulating the same. Most thermodynamic processes have time scales of the order of microseconds to seconds or even minutes. Computational atomistic simulations have largely focused on “prompt” mechanisms of the chemistry near picoseconds to nanoseconds. However, the plasma interactions with many of the lithiated graphite surfaces in NSTX can be raised to temperatures beyond the melting point of lithium and this could have serious implications to the operation of NSTX-Upgrade plasmas that intend to raise overall heat fluxes by factors of two or more. One essential question is whether these lithium-conditioned surfaces would be able to sustain the beneficial low-recycling regimes so far achieved at higher power fluxes to these wall surfaces. In addition, complex surfaces involving boronization conditioning and re-deposition from eroded graphite will also need to be evaluated in the context of hydrogen retention by lithium coatings. Moreoever, lithium coatings are intended for future use with molybdenum or tungsten in future NSTX-U operation scenarios. Therefore, another important question is whether the chemical interactions found between lithium, graphite and oxygen remain with these new high-Z metals, which are part of a plan in NSTX-U to cover the lower divertor floor in the future.

For this task, PI Prof. J.P. Allain has teamed up with Prof. Brian Wirth of the University of Tennessee at Knoxville to address this problem using both state-of-the art in-situ experiments at the University of Illinois and multi-scale computational simulation codes used by Wirth’s team. Prof. Wirth brings a wealth of experience in the simulation of multi-scale spatio-temporal behavior of irradiated tungsten and will extend his efforts to study lithium coatings on these substrates and graphite. Long-pulse PFC studies will be coordinated with an established collaboration at the Dutch Institute for Fundamental Energy Research (DIFFER) under the direction of Dr. Greg DeTemmerman.