UPGRADE OF THE MATERIALS ANALYSIS PARTICLE PROBE (MAPP-U) TO DECIPHER THE IMPACT OF LITHIUM-BASED SURFACES ON NSTX-U PLASMA BEHAVIOR

Understanding the plasma wall interaction (PWI) remains a critical issue for the feasibility of thermonuclear

magnetic fusion energy solutions. Key issues with PWI mechanisms in fusion tokamak reactors include:

evolution of surface chemistry and its role on hydrogen retention. In particular how low-Z coatings such

as lithium can impact the behavior of plasma at the edge and in the core. PMI (plasma-material interactions)

are particularly important for strategies that involve low-recycling regimes and the use of lithium PFS

(plasma-facing surfaces) to attain them, as in the case of NSTX-U. Recent reports have indicated the

importance of access to the evolving plasma-facing surface during and in-between plasma discharges.

Changes in surface chemistry and morphology due to ion bombardment and the difficulty of diagnosing

plasma-facing surfaces, especially reactive surfaces, complicate the development of a predictive

understanding of the wall and its interaction with the plasma. Consequently, this impairs the ability to

design advanced PFC materials for future plasma-burning fusion reactors and appropriate PMI code

validation.

The Materials Analysis Particle Probe (MAPP) is an established and on-going PMI probe diagnostic system

compatible with the highly chemically reactive system of lithium and boron coatings adopted by the NSTXU

research program [1-5]. MAPP is the first PMI diagnostic to capture the surface physics and chemistry

in-vacuo in a fusion tokamak system and correlate this data to controlled plasma shots. Currently MAPP

captures this information at a fixed radial location at the NSTX-U outboard divertor region. The MAPP

diagnostic has enabled understanding of the near-surface and surface chemistry of complex evolving

lithiated and borated carbon-based PFC surfaces retention and transport of hydrogen. Coupled to atomistic

simulations in collaboration with P. Krstic of Stony Brook U. MAPP has been very successful in achieving

high-impact scientific research in its current grant period evidenced by two invited review articles and over

20 peer-reviewed manuscripts and over 40 contributed and invited presentations at both national and

international conferences..

Need for Renewal: Given the extended shut-down of NSTX-U and its current recovery schedule, a need

to complete PhD student theses and ensure the appropriate diagnostic component upgrade and testing for

MAPP-U is submitted for consideration.

Key Aims of Renewal Proposal: This extension proposal focuses on three primary aims: 1) upgrading

MAPP with advanced diagnostics to measure hydrogen retention directly and in-situ erosion, 2) conduct

post-mortem characterization of NSTX-U PFC samples and testing of MAPP-U on Proto-MPEX at ORNL,

and 3) re-establish high-fidelity OES (optical emission spectroscopy) along the MAPP probe surface and

final integration of the new MAPP-U and commissioning in time for NSTX-U startup in 2022. The last

goal is one that existed before the shutdown and was in progress in collaboration with V. Soukhanovskii at

LLNL.

MAPP-U will allow time-resolved PMI data by tuning plasma shots that vary from 1 second and stepped

up to longer pulses (up to 5-10 seconds for NSTX upgrade), the surface chemistry can be assessed under

the context of 'long-time' 'high-power' operation with ramifications to materials design for future steadystate

devices, such as PMI studies of refractory alloy metals. Furthermore MAPP will enable the study of

other advanced materials including liquid-metals and hybrid (solid/liquid) systems and their controlled

exposure to designed plasma shots to guide materials and component options. Testing with MAPP will

include but not limited to: testing of new nanostructured lowZ/highZ hybrid materials considered by PPPL

collaborators, tungsten-based nanocomposites and alloys, and no-flow and slow-flow liquid-metals of

interest to PPPL collaborators. Below are the three primary aims in more detail with an overall research

and timeline plan.