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Plasma Physics (Physics/ECE/NE 922) Seminar
“Disruption physics gaps encountered in the SPARC design pertinent to the design of the ARC power plant”
Date: Monday, April 1st
Time: 12:00 pm - 1:15 pm
Place: 1227 Engineering Hall
Speaker: Ryan Sweeney, MIT PSFC/CFS
Abstract: R. Sweeney on behalf of the SPARC and ARC Teams
Addressing climate change with a tokamak-based fusion power plant requires that plasma disruptions pose a low risk to day-to-day operation or to early end of life for in-vessel components. Advancing disruption resilience requires a tokamak that can create a reactor-scale environment for testing proposed solutions but can also survive disruptions if the solution fails, enabling a fast-learning cycle. Commonwealth Fusion Systems (CFS) is developing disruption resilient tokamaks equipped with robust plasma control software to meet this need. SPARC is a compact (R=1.85 m), high field (12.2 T) and current (8.7 MA) tokamak under construction in Devens, MA, designed to start operations in 2026. Its first mission is to demonstrate Q > 1 and will then be used to answer questions critical to the design of the ARC fusion power plant. SPARC is conservatively designed to structurally survive a battery of worst case disruption events and to tolerate melt events with tungsten based first wall components and no active cooling, and is equipped with state-of-the-art mitigation systems and diagnostics to retire the disruption risks for ARC. This talk will start with a brief overview of the status of the SPARC project with a focus on disruption systems. The remainder of the talk will address the disruption physics that set the most challenging requirements and drove actuator decisions for SPARC, highlighting where the larger community can complement the physics basis for the ARC power plant now under design. Disruptions are a known challenge, and the SPARC and ARC Teams welcome support as we sprint toward the solution in an effort to deploy fusion on the timescale that climate change requires.

Supplementary text on open disruption physics questions, with suggestions for studies: The predicted plasma current quench durations, based on ITPA scalings, drive the structural design of the SPARC vacuum vessel and in-vessel components. Improved physics understanding of these timescales would better inform the ARC design. A massive gas injection (MGI) system with six toroidally and poloidally distributed valves is the first mitigation technology that will be deployed on SPARC. Empirical and simulation comparisons of MGI with shattered pellet injection (SPI) and other alternatives would better inform the actuator decision for ARC. A novel runaway electron mitigation coil (REMC) is designed for SPARC and predictions suggest it will fully prevent runaway formation during disruptions at high current. Exploration of other runaway prevention solutions could provide optionality for the ARC design. Parallel heat fluxes from unmitigated and even some mitigated thermal quenches pose a risk to melting plasma facing tiles and other in-vessel components in SPARC. CFS and IPP Garching are exploring this physics on AUG; exploration on other machines would complement this planned dataset. The magnetic energy outside of the vacuum vessel can significantly increase the severity of disruption heat fluxes. Further understanding of this conversion would help to design ARC for these loads. While ARC will be designed to operate a single plasma scenario, SPARC is tasked with identifying the optimal ARC scenario and thereby must explore parameter space. Demonstrating physics-based disruption observers and avoidance could support SPARC operation, as well as physics studies of disruption causes including fusion-relevant material failure modes.
Host: Prof. Carl Sovinec
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