3D modelling of thermal loads during unmitigated vertical displacement events in ITER and JET

This paper presents a physics-based 3D modeling workflow that couples MHD simulations with field line tracing and thermal response to predict and validate disruption-induced heat loads in JET and ITER, ultimately demonstrating the resilience of ITER's tungsten first wall against vertical displacement events.

The Gist

The "Giant Sparkler" Problem: Keeping the World’s Biggest Fusion Reactor from Melting

Imagine you are trying to build the world’s most powerful, high-tech campfire inside a massive, expensive metal lantern. This campfire (the fusion plasma) is incredibly hot—millions of degrees—and it’s held in place by invisible magnetic "hands."

Now, imagine that suddenly, those magnetic hands slip. Instead of a steady, controlled glow, the fire suddenly wobbles, slams against the side of the lantern, and surges wildly. This is what scientists call a Disruption (specifically a "Vertical Displacement Event"). If this happens in a machine like ITER (the massive fusion reactor being built in France), the heat could be so intense and localized that it actually melts the metal walls of the machine.

This paper is about a new, high-tech "digital simulator" designed to predict exactly where those "burn marks" will appear so we can build better protection.

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1. The Problem: The "Flashlight" vs. The "Floodlight"

In the past, scientists used math models that assumed the heat would spread out evenly, like a floodlight illuminating a whole wall. They thought, "If we know the total energy, we can just spread it across the whole surface."

But this paper proves that's a mistake. Because of the way the magnetic fields wobble during a disruption, the heat doesn't act like a floodlight; it acts like a high-powered laser or a focused flashlight. Instead of warming the whole wall slightly, it hits one tiny spot with incredible intensity. If you only prepare for a floodlight, your "laser" will punch right through your defenses.

2. The Solution: The "Digital Twin" Workflow

The researchers created a three-step "digital stress test" to see what would happen:

1. The Chaos Simulator (JOREK): They use a supercomputer to simulate the "wobble." It’s like a high-speed video of a car crash, showing exactly how the magnetic fields twist and turn.
2. The Map Maker (Field Line Tracing): They take that chaotic movement and trace the "heat paths." Imagine following a single spark from a fire to see exactly which part of the floor it lands on.
3. The Heat Tester (Wall Response): They take those "sparks" and apply them to a 3D model of the reactor's walls to see if the metal actually melts.

3. The "Reality Check" (The JET Test)

Before they trusted this model for the giant ITER reactor, they tested it on a smaller, existing machine called JET.

• They ran the simulation against real-life "crashes" that had already happened at JET.
• The Result: The simulation was spookily accurate. When the real JET machine melted a specific piece of beryllium (a metal used for armor), the computer predicted that exact spot would melt. This gave the scientists the "green light" to trust the model for the much bigger ITER.

4. The Big News: Tungsten is a Superhero

The paper compares two different types of "armor" for the reactor:

• Beryllium (The Old Armor): Used in older machines. It’s like using a chocolate bar to protect a house from a blowtorch—it melts easily.
• Tungsten (The New Armor): This is what ITER will use. Tungsten has one of the highest melting points of any metal. It’s like using a thick steel shield.

The researchers found that while the "laser-like" heat hits the ITER walls very hard, the Tungsten armor is incredibly resilient. It might get very hot, and it might melt slightly at the very edges of the panels, but it is much more likely to survive the "crash" than the old materials were.

Summary: Why does this matter?

Building a fusion reactor is like trying to bottle a star. If the star "sneezes," it could damage the bottle. This paper provides the high-definition weather map for those "plasma sneezes." By knowing exactly where the heat will strike, engineers can design the "armor" of the future to ensure that even when things go wrong, the machine stays safe and ready to try again.

Technical Summary

Technical Summary: 3D Modelling of Thermal Loads during Unmitigated Vertical Displacement Events in ITER and JET

1. Problem Statement

Predicting three-dimensional (3D) thermal loads during tokamak disruptions is a critical challenge for the success of ITER. During disruptions, specifically Unmitigated Vertical Displacement Events (UVDEs), the rapid loss of thermal and magnetic energy can cause severe melting of Plasma-Facing Components (PFCs).

Existing predictive models often rely on axisymmetric (2D) assumptions, which are physically questionable during disruptions. When the safety factor ($q_{95}$) drops below $\sim2$, large-scale external kink modes break toroidal symmetry, leading to highly localized heat fluxes. Previous 2D models (like DINA or TOKES) tend to over-localize these loads, potentially leading to inaccurate assessments of melting risks and the "disruption budget" for future nuclear devices.

2. Methodology

The authors developed and validated a physics-based, three-stage workflow to capture the 3D nature of these events:

• Stage 1: Nonlinear Resistive-MHD Simulations: Using the JOREK code (coupled with the STARWALL code), the team performed 3D simulations of the VDE. This captures the time-resolved magnetic fields, plasma source terms (parallel heat flux), and halo current channels, accounting for toroidal asymmetries ($n \in [0, 3]$).
• Stage 2: 3D Field Line Tracing: A post-processor maps the plasma power and current loads onto a high-fidelity 3D CAD model of the first wall (FW). This identifies "wetted" elements—those directly connected to the plasma via field lines—ensuring the geometry of the wall panels is accurately represented.
• Stage 3: Transient Thermal Response: For every wall element, a 1D transient heat-conduction equation is solved in the wall-normal direction. This model uses temperature-dependent thermal conductivity and specific heat capacity to determine if the surface temperature exceeds the melting threshold of the material (Beryllium for JET; Tungsten for ITER).

3. Key Contributions & Results

A. Validation in JET:
The workflow was validated against two JET discharges (#95110 and #84832).

• Global Dynamics: JOREK successfully reproduced experimental plasma current ($I_p$), vertical displacement, and toroidal asymmetries in halo currents.
• Melting Prediction: The model correctly predicted the absence of melting in the low-energy pulse (#95110) and the occurrence of Beryllium melting in the high-energy pulse (#84832).
• Phase Coupling: A crucial finding was that melting in JET was not caused by the Thermal Quench (TQ) or Current Quench (CQ) in isolation, but by their combined action: the TQ pre-heats the surface, allowing the subsequent CQ to drive the temperature above the melting point.

B. Predictions for ITER (2024 W-armour Baseline):
The model was applied to a worst-case 15 MA upward-going UVDE in ITER.

• Resilience of Tungsten (W): The analysis demonstrates that the ITER Tungsten first wall is significantly more resilient than the previous Beryllium design. Melting is predicted to be marginal and highly localized.
• Localization at Edges: Melting is not expected on the main panel surfaces but rather at the panel edges (near upper ports) where the 3D geometry causes field lines to strike the wall at near-normal incidence.
• 3D vs. 2D Discrepancy: 3D modeling revealed that toroidal asymmetries amplify peak temperatures by a factor of $\sim2$ during the CQ and $\sim3.5$ during the TQ. Conversely, 3D MHD effects broaden the heat footprint compared to 2D models, which would otherwise over-predict the severity of localized melting.
• Effective Deposition Area: The study quantified the "footprint" of the disruption, finding effective deposition areas ($A_{eff}$) of $\sim12\text{ m}^2$ for the TQ and $\sim64\text{ m}^2$ for the CQ.

4. Significance

This research provides a robust, validated tool for assessing the structural integrity of PFCs in future fusion devices.

1. Engineering Metrics: It establishes that halo current patterns can serve as a reliable proxy for the localization of conducted energy, which is vital when direct heat flux measurements are unavailable.
2. Physics Insight: It proves that the width of the heat-load footprint is governed by 3D magnetic topology (the extent to which open field lines penetrate the plasma core) rather than just resistive or diffusive processes.
3. Operational Confidence: The results suggest that the ITER 2024 Tungsten baseline is well-positioned to handle unmitigated VDEs, potentially easing the stringent requirements for disruption mitigation systems.