One of the high priority research needs for the ITER project is the development of a solid physics basis of plasma disruptions and their mitigation. The thermal and electromagnetic loads taking place during these events pose important constraints on the lifetime of tokamak components [1, 2]. The extrapolation of these loads from experimental data to new machines entails large uncertainties,...
During vertical displacement events (VDEs) plasma column hits the wall and scrape-off layer (``halo'') currents can reach significant amplitudes [1]. Therefore, the related electromagnetic loads on plasma facing components (PFCs) should be thoroughly evaluated to guarantee their structural integrity. Modelling of halo currents for next generation tokamaks is a challenging task. For example,...
Thermal quench (TQ) marks the point of no return in a tokamak
disruption. It not only brings a thermal load management issue at the
divertor plates and first wall, but also determines the runaway
seeding for the subsequent current quench (CQ). There are two ways to
trigger a TQ, one is the globally stochastic magnetic field lines that
connect the hot core plasma to the cold boundary,...
Analysis of Variability in Pre-Disruption Plasma Parameters and their Effect on Runaway Electron Generation using the JET data-base on RE
V.V. Plyusnin(1) , C. Reux(2), V.G. Kiptily(3), S. Gerasimov(3), S. Jachmich(4),
M. Lehnen(4), O. Ficker(5), E. Joffrin(2) and JET contributors*
EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK
(1) Instituto de Plasmas e...
Pellet injection is used in tokamaks and stellarators for fuelling, ELM pacing, diagnostics, and disruption mitigation. Injection of shattered pellets is a critical part of the envisaged ITER disruption mitigation system.
A highly localized plasmoid initially expands predominantly along the magnetic field lines. These assimilation dynamics play a critical role in determining the post-pellet...
The vertical motion of the plasma current during the current quench phase of the uncontrolled major disruption is described analytically. The presented filament-based model interprets the vertical displacement event in the ideal wall limit as an adiabatically slow evolution of the plasma equilibrium. The initial pre-disruption equilibrium becomes unstable in a pitchfork bifurcation. The...
Timely detection and prevention of plasma disruptions is essential for next-step tokamaks. Plasma disruption is a multi-step process in which the loss of plasma vertical position control and the quenching of the plasma current are typically among the last events that precede complete plasma deconfinement. Given the major role of abnormal behavior of the plasma current and vertical position in...
Because of the need to obtain a real-time predictive model of the complex disruption behaviour, a great effort has been devoted in the last decades to apply data-driven models to disruption mitigation and avoidance, starting from the first black-box neural network approaches to the more physics-based Machine Learning (ML) models up to the latest models based on Deep Learning techniques. In the...
At present, the massive gas injection (MGI) and shattered pellet injection (SPI) techniques are regarded as the primary injection methods for disruption and RE mitigation. Both of them have their own character and can be used in different applications.
In order to combine the advantages of gas injection and pellet injection for avoidance and mitigation of disruptions, a new hybrid...
Kinetic instabilities in the MHz range have been observed during current quench in DIII-D disruption experiments (A. Lvovskiy et al., PPCF 60, 124003 (2018)). In this talk we show the new updates on kinetic-MHD simulation of current quench modes using M3D-C1. It is found that this mode is mainly compressional Alfvén eigenmode (CAE) and has large parallel perturbed magnetic field component. The...
Mitigation as last line of defense against disruptions needs to simultaneously mitigate electromagnetic forces, heat loads, and runaway electrons (REs) for a safe operation of ITER like tokamaks. Simulations help to prepare robust mitigation scenarios and need to capture various non-linearly interacting physics processes simultaneously in a self-consistent way.
JOREK is an extended...
A helical coil designed to passively generate non-axisymmetric fields during a plasma disruption has been shown, via electromagnetic analysis, linear MHD modeling, and relativistic drift orbit tracing, to be effective at deconfining runaway electrons (REs) on a time scale significantly faster than the plasma current quench. Magnetic equilibria from DIII-D RE experiments are used to calculate...
Magnetic flux surfaces are abruptly destroyed and the plasma is no longer confined during thermal quench [1]. For a representative unmitigated disruption occurring in ITER with full deuterium-tritium (DT) performance, about 350 MJ of thermal energy and up to 1 GJ of magnetic energy may be released to the divertor and first-wall (FW) surfaces during several milli-seconds, leading to serious...
Plasma fueling and disruption mitigation studies are two key aspects of the future fusion reactors. To meet these challenges, development of pellet injectors and associated technologies has been initiated in India to cater the domestic fusion program. As a first step towards it, a single pellet injector system (SPINS-IND) is successfully developed. The injector, depending on the barrel and...
Any disruption mitigation system requires a trigger to trigger the corresponding remedial actions. Such trigger is the final step of a chain of events. This chain starts with an alarm that recognises an incoming disruption followed by interlocks protecting particular systems (for example, plasma heating systems). This contribution is a review of a specific disruption predictor that is...
Uncontrolled termination of post-disruption relativistic runaway electron (RE) current can cause deep localized melting of the plasma facing components and poses a serious challenge to the successful operation of fusion grade tokamaks, including ITER. While RE deconfinement depends on the timescale of flux-surface reformation, the magnetohydrodynamic (MHD) plasma stability itself is affected...
ITER adopts massive particle injection using shattered pellet injection (SPI) as a basic mitigation method to mitigate three major risk factors that can occur in the process of plasma disruption: heat load, electro-magnetic load, and runaway electrons. The injected particles composed of a combination of hydrogen and neon increase the density of plasma through the assimilation process to...
Advanced tokamak reactors require a low disruptivity ceiling to reach commercial viability. The damaging impact of plasma disruptions on machine components can greatly reduced the lifetime of a device. A precursor to disruptions is the locking dynamic of rotating MHD events that are often neoclassical tearing modes (NTM). The drag of electromagnetic and fluid viscosity torques can cause the...
The cause of the thermal quench (TQ) in tokamak disruptions has not been well understood.
Recent work identified the TQ in JET locked mode disruptions with
a resistive wall tearing mode (RWTM) [1].
New research finds a similar instability in DIII-D locked mode shot 154576 [2]. The instability is studied with simulations, theory, and comparison to experimental data. Linear theory and...
This work extends the recent modeling of runaway electron (RE) mitigation in Ref. {1} by including an avalanche RE source {2} in the Kinetic Orbit Runaway electrons Code (KORC). Our main finding is that REs produced by the avalanche source are the primary contributor to transient high heat loads observed at plasma-facing component (PFC) surfaces as shown in Fig. [1]. The magnitude of the...
We report comprehensive investigation of Alfvénic instabilities driven by runaway electrons (REs) during the current quench in the DIII-D tokamak. These instabilities are observed as toroidal magnetic field fluctuations in the frequency range of 0.1–3 MHz and correlate with increased RE loss from the plasma which candidates them to be responsible for non-sustained RE beams and motivates a...
Machine learning techniques have been applied successfully in EAST plasma equilibrium reconstruction and disruption prediction. Regression neural network models are trained to identify the plasma center position and calculate equilibrium plasma parameters including li,β_p,κ,q_0 and a_minor with magnetic diagnostic signals as input features [1][2]. The results on test dataset show good...
Present contribution aims at comparing the different kinds of disruptions that occurred in the last JET with ITER-like wall (ILW) campaigns with Tritium and Deuterium-Tritium fuels.
Last campaigns performed in JET-ILW with a D fuel showed that the majority (around 80%) of disruptions follow two main paths [1]. The first path (temperature hollowing, TH) is strictly related with the influx of...
Control is necessary to keep fusion plasmas stable. This requests a set of real-time diagnostics. These sensors and/or data acquisition systems are prone to failure, especially under the demanding environments of a fusion reactor that has cryogenic and extreme hot conditions, high neutron production and high magnetic fields. Current real-time control algorithm assume the sensors as correct...
Disruption prediction requires an understanding of the routes that a plasma may take from being in a healthy state to a disruption, such as the analysis carried out on JET [1]. Of particular concern are those routes that give very little warning of an imminent disruption because they give little potential to either take avoiding or mitigating action. We will use the MAST high speed visible...
The capability to terminate plasma pulses safely is an important goal towards the optimization of operational scenarios in tokamaks, so it is of great importance to study the physical phenomena involved in plasma disruptions and to develop precursors for avoidance and/or mitigation actions. The development of tearing modes (TMs) inside the plasma is a major cause of disruptions. It has been...
Disruption prediction and avoidance is critical for ITER and reactor-scale tokamaks to maintain steady plasma operation and to avoid damage to device components. Physics-based disruption event characterization and forecasting (DECAF) research determines the relation of events leading to disruption, and forecasts event onset. The analysis has access to data from multiple tokamaks to best...
Disruption is a major obstacle for tokamaks to be commercially viable reactors. Accurately predicting an incoming disruption and deploying disruption mitigation system is one of the keys to solve this problem. Today’s machine learning based disruption predictors do have great performance if given good enough data to train. But future tokamak will not provide good enough data before damaging...
Achieving acceptably low disruptivity on ITER and future reactors will require active monitoring and control of the proximity of operating points to unsafe regions. Although mitigation strategies can protect the infrastructure from disruptions and engineering limits, maximizing scientific or economic output of a device demands avoiding triggering mitigation systems while optimizing...
ITER will require exceptionally low disruptivity while pushing the limits of plasma performance. Ensuring robust stability will require a comprehensive strategy, and must include the continuous regulation of the proximity to stability and controllability limits, also called “Proximity Control.” DIII-D has been developing a Proximity Control architecture [1] which modifies control targets and...
Although the stabilization of locked islands using RF-driven currents has been demonstrated experimentally in a pioneering series of experiments [1], experimental and theoretical research on RF island stabilization has continued to focus almost exclusively on stabilization during the rotating phase, before locking occurs. An emphasis on avoiding island locking has emerged from a concern about...
While experimental dependence of the pellet penetration depth can be approximated using simple scaling laws, underlying physics is extremely complex and requires challenges for multiscale and multiphysics modelling. The electron density of an ablation cloud changes over eight orders of magnitude from solid density ($10^{28}$ m$^{-3}$) to background one ($10^{20}$ m$^{-3}$) as it expands along...
Precise values for radiated power during tokamak disruptions are required to predict the effectiveness of mitigation techniques at preventing damage in net energy tokamaks like ITER and SPARC. Conventional approaches to calculating $ P_{rad} $ on JET assume a toroidally symmetric radiation source to allow fitting from foil bolometer arrays at two toroidal locations, which is otherwise an...
Future tokamaks will require robust disruption mitigation to prevent machine damage from thermal loads, electromagnetic forces, and runaway-electron (RE) impact. The leading-candidate for this is shattered-pellet injection (SPI), which is being tested experimentally on several tokamaks and will be used on ITER. Verified, predictive models are needed to project the performance of these systems...
Simulation of Ne/H Shattered Pellet Injection (SPI) into an ITER L-mode plasma is carried out by JOREK non-equilibrium impurity model with separate electron and ion temperature. The focus of this report is on the MHD dynamics, the density transport, the temperature profile evolution and relaxation during the loss of the core confinement, as well as the impact from the non-equilibrium impurity,...
D2 injection into mature runaway electron (RE) beams is found to enable access to a benign termination scenario that can mitigate MA-level RE currents without measurable wall heating. This result is enabled by the excitation of large and sudden MHD events (dB/B ~ 5%) that are found to promptly disperse the entire RE population over a large wetted area, with MHD accelerated by a recombined...
Benign termination of mega-ampere level runaway current has been
convincingly demonstrated in JET [1] and DIII-D [2],
establishing it as a leading candidate for runaway mitigation on
ITER. This comes in the form of a runaway flush by parallel streaming
loss along stochastic magnetic field lines formed by global
magnetohydrodynamic (MHD) instabilities, which are found to correlate with...
A highly flexible shattered pellet injection (SPI) system was installed in December 2021 on ASDEX Upgrade. This system provides three independent guide tubes (GTs) and the pellet velocity can be varied between 100-900 m/s depending on pellet size and composition. At the end of each GT, different shatter heads had been installed for characterization of the disruption behaviour for different...
Shattered Pellet Injection (SPI) has been chosen as the baseline disruption mitigation system for ITER. However, many questions remain regarding its operation, particularly under the presently envisaged operating scenario where several simultaneous and staggered SPIs are needed to inject high-Z radiating impurities prior to the thermal quench. Experiments on DIII-D used two SPIs with pellets...
The Disruption Mitigation System (DMS) is an essential plant system of ITER to reduce the deleterious effects of a disruption on in-vessel components. The design of the ITER DMS is based on shattered pellet injection (SPI) technology. This technique works by forming large cryogenic pellets of up to 28mm in diameter and firing them at high speeds towards a shattering section, where they...
The ITER disruption mitigation system (DMS) is based on shattered pellet injection (SPI) technology. The principle of operation is to form and accelerate cylindrical cryogenic pellets to high speeds towards a shattering unit, where the pellets disintegrate into a shower of fragments of different sizes, which enter the plasma for the mitigation process. The effectiveness of this mitigation...
The ITER disruption mitigation support laboratory is part of the ITER Disruption Mitigation System (DMS) Task Force programme to establish the physics and technology basis for the ITER DMS. The laboratory is located at the Centre for Energy Research, Budapest Hungary.
The aims include production, launching and shattering of 19x39 mm and 28.5x57 mm (d x L) H, D, Ne and mixture pellets, and...
Micro-particle injectors used for quenching a burning plasma in case of a disruptive instability have predominantly relied on well-established pneumatic drives. Pneumatic drives are limited by the slow thermal velocity of the propellent gas molecules. Response time is also limited due to mechanical valves present in the gas-feed system. Such methods of acceleration may be therefore unsuitable...
Large hydrogen pellets will be the primary injected material for the ITER disruption mitigation (DM) system, based on the shattered pellet injection (SPI) technique. Shattered pellet injection utilizes cryogenic cooling to desublimate gas into the barrel of a pipe gun, forming a solid cylindrical pellet. SPI systems deployed worldwide usually operate with deuterium as the primary pellet...
