Passive Runaway Electron Coil for Tokamak Disruption Mitigation
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Solution Overview
Problem
Active coils are not practical for high field and high current devices in tokamaks, as they are not effective in producing field energies of megajoules in a few milliseconds, and existing solutions fail to efficiently mitigate runaway electrons before they accelerate to high energy.
Innovation Solution
A passive runaway electron mitigation coil (REMC) with a non-axisymmetric shape is energized by disruption-induced voltage, creating stochastic magnetic fields that cause electrons to be lost rapidly, preventing them from accelerating and distributing energy to minimize damage by overlapping tearing mode regions and amplifying fields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If actively driven coils are used for runaway electron mitigation, then electron mitigation is achieved, but the device cannot produce field energies of megajoules in a few milliseconds and is not practical for high field and high current devices
Solution Approach 1:
The passive coil system utilizes the disruption-induced voltage and mutual inductance with the plasma to self-energize during disruptions, eliminating the need for external power supplies and complex active drive systems while achieving the required megajoule-level field energies
Solution Approach 2:
Instead of using actively driven coils that require external power, the invention inverts the approach by using a passive coil that is energized by the plasma's own disruption-induced voltage and mutual inductance, turning the plasma from a controlled object into the energy source
2Reliability
If the REMC produces a larger perturbing field to overcome avalanche and Compton scattering growth terms, then runaway electron mitigation effectiveness is improved, but the forces on the coil increase
Solution Approach 1:
The coil's L/R time constant is specifically tuned to be long relative to the current quench duration to maximize coupled currents and mitigation effectiveness, while the characteristic decay time is optimized to balance field strength against mechanical forces on the coil structure
3Power
If the REMC is mounted on the outboard side to be spaced from the vacuum vessel wall, then higher driven currents are achieved, but the coil requires additional mounting structure
Solution Approach 1:
The vertical stability coils serve dual functions: providing vertical stability control during normal operation and acting as structural mounting supports for the passive runaway electron mitigation coil during disruptions, eliminating the need for separate mounting structures
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The REMC effectively expels runaway electrons from the tokamak plasma before they reach high energy, reducing damage to the wall by distributing energy diffusively and maximizing the perturbing field within engineering constraints, without requiring an external power supply, and minimizing forces on the coil.
Implementation Method 1
the REMC is driven by leveraging mutual inductance with the plasma during a current quench
Implementation Method 2
the non-axisymmetric shape of the coil produces resonant fields that open magnetic islands across the plasma minor radius
Implementation Method 3
opening tearing mode regions (or 'islands') in the plasma that upon overlapping, generate a stochastic magnetic field that in turn results in stochastic movement of electrons out of the plasma
Implementation Method 4
Runaway electrons follow (i.e. travel a path defined by) perturbed field lines out of the plasma and impact a random location of a wall or other surface
Data Source
AI summary
Described are concepts, structures and techniques for passive mitigation of relativistic electrons produced in a tokamak plasma. These electrons may be referred to herein as “runaway” electrons (REs). In embodiments, a passive runaway electron mitigation coil (REMC) comprises an electrical conductor having a non-axisymmetric loop shape comprising a plurality of portions arranged along paths defined by the surface of a torus along a toroidal direction. In operation, the REMC is energized by a disruption-induced voltage, and the resulting magnetic field stochasticity causes electrons to be lost more rapidly than a RE beam can form. The REMC is configured for outboard side mounting within a vacuum vessel.