In KSTAR experiments, various disruptive events are identified by a local 2D electron temperature (Te) fluctuation diagnostics known as the electron cyclotron emission imaging diagnostics. We will introduce distinct 2D Te patterns of different disruptive events to elucidate the importance of 2D measurements for early detection of the events. Observations include off-normal sawtooth crashes,...
Near-future burning plasma tokamaks will need to run disruption-free or with very few (<1%) unmitigated disruptions, therefore predicting disruptions on new tokamaks when they begin operating and disruption data is sparse, will be crucial to their success. This letter introduces a Hybrid Deep Learning (HDL) architecture for disruption prediction that achieves high predictive accuracy on the...
While mitigation will play a critical role in reducing the impact of disruptions in ITER, it should be used only as a "rarely-used last resort" [Strait et al. 2019], i.e. reliable disruption avoidance strategies are necessary, the most important of which is the stabilisation of magnetic islands. Indeed, in the JET tokamak equipped with an ITER-like wall, 95\% of disruptions are preceded by...
This contribution presents a novel application of a semi-supervised learning algorithm to detect disruption precursors in a large dataset, given a relatively small number of manually labelled examples. Preliminary analysis applying the label propagation [1] and label spreading [2] algorithms for detection of H-L back transitions demonstrates a reasonably high detection accuracy (~70% of...
ITER could benefit from a new generation of disruption mitigation systems with fast response time, high-velocity radiative payload injection for core deposition, and very high overall system reliability. Systems under current investigation such as the Electromagnetic Particle Injector (EPI), the two-stage gas gun, and the Shell Pellet concept may offer some or all of these capabilities....
Relativistic runaway electrons emit synchrotron radiation, which can be used to experimentally diagnose features of the runaway electron distribution function. In this contribution, we present the analysis of visible-light camera images of synchrotron emission from runaways in the ASDEX Upgrade discharge #35628. We perform both forward (solution of a fluid-kinetic equation system) and backward...
ASDEX Upgrade (AUG) is installing a Shattered Pellet Injector (SPI), expecting start of operation in the 2021 campaign. The primary goal of the project is to study the impact of different SPI shard size distributions [1] -- realized by different shatter angles -- on the disruption mitigation characteristics of SPI. The project will also aid the understanding of pellet shard penetration and...
Disruption mitigation is one of the major challenges for ITER and future tokamaks. As an alternative to massive gas and shattered pellet injection, an electromagnetic pellet injection (EPI) mechanism has been proposed that would offer a fast response time and high enough speed to deposit the payloads in the plasma core [1]. This technique is expected to be tested during the next NSTX-U...
Global and local forces on the wall of COMPASS-U [1,2] tokamak during plasma disruptions are calculated with CarMa0NL code [3] for scenarios with positive and negative triangularities. The COMPASS-U is a high-field tokamak presently in the final design phase. It will operate at the toroidal magnetic field, plasma current and elongation up to B = 5 T, I = 2 MA and κ = 1.8, respectively. Large...
The complex interplay of physics phenomena, which can cause plasma disruptions, hinders the development of predicting models. Recently, satisfactory Machine Learning predictors have been deployed on different devices. These models extract information from the high-dimensional data spaces of fusion experiments and help to detect and classify disruptions. Nevertheless, Machine Learning...
DIII-D and EAST are developing and qualifying a comprehensive disruption control toolset to help ensure ITER success. The 2019-2020 DIII-D program began a focused effort, called the “Disruption Free Protocol” (DFP), to qualify disruption control techniques including emergency shutdown and continuous regulation of proximity to stability boundaries.
A new control architecture has been...
In a commercially viable tokamak-based fusion power plant, the burning plasma must be steered such that major disruptions do not occur at all. In present experimental devices, disruptions usually appear as sudden major events leading to plasma termination and damage to the plasma facing components. Via post-discharge analysis often one is able to identify a precursor or a chain of events that...
Two critical issues in ITER disruptions are the
thermal load during the thermal quench (TQ) and the asymmetric wall
force produced during the current quench (CQ).
Simulations of asymmetric wall force during disruptions [1] with M3D [2]
were shown consistent with JET data.
These results have been extended with M3D-C1 [3] simulations and
compared with additional JET data.
The results...
Direct measurements of current flows during vertical displacement events (VDEs) have been carried out on COMPASS to better understand current pattern distribution within vessel structure and divertor. Especially, asymmetric VDEs resulting in sideways forces on the vacuum vessel, are potentially challenging for ITER. The theoretical understanding of these events is not yet conclusive. The...
The generation of runaway electrons during major disruptions in International Thermonuclear Experimental Reactor is unacceptable. Disruption Mitigation System (DMS) designed in ITER should be a reliable tool for suppression of RE and mitigate other detrimental consequences of disruptions, such as heat and mechanical loads. Elaboration of the RE database and its comprehensive analysis should...
The ITER Plasma Control System (PCS) is responsible for real-time regulation of plasma state and stability in order to satisfy the ITER physics mission and respond to fault conditions while minimizing the frequency of plasma-terminating disruptions [1]. Each of the control functions of the PCS work together to continuously prevent, asynchronously avoid, or mitigate (unavoidable) disruptions....
A novel path to runaway electron mitigation in tokamaks found by combining an impurity-free (deuterium) background plasma with current-driven kink excitation at low safety factor (qa) is being explored for its application to ITER and beyond. Realization of this scheme requires primary or secondary injection of deuterium and promotion of kink instability via limited actuation by the poloidal...
At this moment in time we cannot guarantee the development of disruption free tokamak plasma. The first non-disruptive tokamak pulse was obtained on the TM-2 tokamak in 1962. Thus, the TM-2 experiments manifested Shafranov’s predictions for MHD stable plasmas. Disruptions were undesirable but tolerable on small and medium scale machines. However, during the 1971-73 runaway electrons (RE) study...
ITER adopts a strategy that distributes radiated power evenly during the disruption mitigation and reduces the time to prepare pellets, using simultaneous multiple shattered pellet injections (SPIs) [1]. However, since there were no existing devices with perfectly symmetric SPIs, as planned in ITER [2], sufficient studies have not been conducted on the effects of simultaneous multi-injections....
The disruption mitigation technology remains the key issue of safe and reliable device operation in future large tokamaks including ITER. In this report, we analyze a novel approach aiming at an essential reduction of seeds causing the avalanche runaway electron generation after the thermal quench (TQ) but does not use injection into the device vacuum vessel a large mass of gas, liquid or...
Since the first observations of disruptions on TM-2 tokamak in 1962 with their specific characteristic “negative” (i.e., opposite in direction to plasma current) spike in loop voltage, the disruptions became and remain the most troublesome effect in tokamak projections to the power sources. Tolerable on middle size machines, disruptions in TFTR powerful supershots sometimes led to two months...
Large amounts of runaway electrons can be generated during ITER disruptions which could lead to severe damage and limit the lifetime of the plasma facing components (PFCs). Indeed, the control and mitigation of the runaway electrons constitute one of the priorities of the disruption mitigation system (DMS) in ITER [1], the injection of high-Z impurities by Shattered Pellet Injection (SPI)...
The JET baseline scenario [1] is being developed to achieve high fusion performance and sustained fusion power. However, with higher plasma current and higher input power, an increase in pulse disruptivity is being observed. Although there is a wide range of possible disruption causes [2], the present disruptions seem to be closely related to radiative phenomena such as impurity accumulation,...
Data-driven models for disruption prevention are being developed across many different experimental devices currently in operation and with the aim of designing viable solutions to prevent disruptions on next-generation devices. Many current machine learning approaches lean towards interpretable predictive algorithms to guarantee a seamless integration with the plasma control system (PCS) and...
In the last decades, lacking solid and detailed theoretical understanding, machine learning tools have been deployed in various Tokamaks to predict the occurrence of disruptions. Their results clearly outperform empirical descriptions of the plasma stability limits. On the other hand, all the machine learning techniques applied in practice show very poor “physics fidelity” (their mathematical...
The 2018 ITER Research Plan states that “Operation of ITER will have to strongly focus on avoiding disruptions with a high success rate and on mitigating those in which avoidance techniques fail” (1). We address the situation where an off-normal event leads to the appearance of a large island that threatens to cause a disruption. On JET, 95% of the disruptions are preceded by the appearance...
NIMROD 3D MHD modeling of dispersive shell pellet (DSP) injection into DIII-D supports anticipated strengths of the concept for disruption mitigation, e.g. high radiated energy fraction, and finds unanticipated benefits for runaway electron (RE) loss during a two-stage current redistribution [1]. DSP, a concept demonstrated on DIII-D [2], comprises a thin shell of low-Z material (diamond in...
Mitigation of disruption electro-magnetic load with SPI on JET-ILW
The disruption mitigation system at ITER consists of shattered pellet injectors (SPI) that can inject up to 24 pellets from three different equatorial port plugs, which will be dedicated to the mitigation of electro-magnetic loads (EML), thermal loads and the avoidance and suppression of runaway electrons. Recently the JETILW...
Runaway electrons (RE) are a major threat for a reliable operation of future tokamaks including ITER. Avoiding or dissipating them is therefore essential. Shattered Pellet Injection (SPI) is the disruption mitigation and RE avoidance method currently planned for ITER [1]. However, if this first line of defence is not efficient enough to prevent the formation of REs, SPI must also be able to...
A summary of recent progress on modeling and simulation of runaway electrons (RE) at Oak Ridge National Laboratory is presented including new results on the following problems: (i) Role of magnetic confinement, spatiotemporal transport of impurities, and electric and magnetic fields dynamics in the efficiency of impurity-based dissipation of RE. (ii) 3D spatiotemporal effects on the production...
The large disruption in the JET shot 38070 in 1995 demonstrated the possibility of large sideways forces due to asymmetric vertical disruption. The effect was missed in the theory, and JET engineers in 1996 gave their own explanation of forces $F_{sideways}$
$𝐹_{sideways,MN}$ =$\frac{𝜋}{2}𝐵_{tor,T}\cdot ∆𝑀_{Z,MA\cdot m}$ (1)
based on consideration of force balance in a...
Understanding the MHD activity leading to runaway electron (RE) beam termination might al-
low a path to avoid localized first-wall damage in fusion-grade tokamaks such as ITER. Recent
experiments at JET demonstrated the possibility of benign termination of RE current [1], when
deuterium pellets were injected (via SPI) onto a plateau-phase RE beam with argon impurities in
the background...
Active disruption avoidance and reliable off-normal event handling schemes need to be integrated in modern Plasma Control Systems (PCS) to predict the proximity to operational boundaries and to react activating different tasks according to the decisions taken in real-time (RT) by a supervisory layer. The access to high performance regimes, which requires to control at the same time several...
Notwithstanding the efforts exerted over many years, disruptions remain a major impediment on the route to a magnetic confinement reactor of the Tokamak type. Machine learning predictors, relying on adaptive strategies, have recently proved to achieve unprecedented performance on JET (with misclassifications of the order of a few per thousand both in terms of missed and false alarms) [1]. Such...
During shattered pellet injection (SPI) shutdowns in ITER, a high fraction of the plasma thermal energy must be radiated with a moderate degree of uniformity to avoid damages to the divertor and the first wall such as melting. DIII-D, J-TEXT, JET, and KSTAR now operate SPI systems and studies have begun to assess these requirements. For studies of gross dependencies of the radiation...
Effective disruption mitigation by shattered pellet injection (SPI) requires the assimilation of a sufficient quantity of the injected material by the plasma. Progress in understanding this SPI particle assimilation, based on experimental measurements and modeling, is described. When the pellet contains radiating impurities such as neon, the resulting disruption evolution is well described...
The current concept of the disruption mitigation system in ITER relies on the cryogenic pellet injection. At the same time, as observed recently in DIII-D, the cryogenic pellets practically explode at the edge of the runaway electron (RE) beam, and the resulting RE dissipation rate is virtually the same for both the massive gas injection and the shattered pellet injection [1]. The similar...
Early reaction to ultimately all, approaching disruption types is one of the major requirements for ITER and DEMO. This early reaction must be targeted on a prevention of the disruption. In present experiments mainly mitigation is applied routinely to specific disruption types. In the future, stability boundaries have to be identified for all expectable disruption types. The proximity to this...
The reliable operation of high performance tokamaks such as ITER necessitate efficient and robust Disruption Mitigation System (DMS), which in turn relies on a clear understanding of the interplay between injected materials and the magneto-hydrodynamic (MHD) modes, thus providing incentive for nonlinear 3D MHD modelling of disruption mitigation. In this report, we will present an overview of...
Disruption prediction and avoidance is critical in ITER and reactor-scale tokamaks to maintain steady plasma operation and to avoid damage to device components. The present status and results from the disruption event characterization and forecasting (DECAF) research effort are shown. The DECAF paradigm is primarily physics-based and provides quantitative disruption forecasting for disruption...
Unmitigated disruptions can cause severe damage on high-current tokamak devices such as ITER. The currently envisaged mitigation method is based on massive material injection (MMI). Recent progress in modelling the dynamics of REs during disruptions mitigated by MMI indicate a substantial increase in the avalanche multiplication gain during an ITER current quench compared to previous...
In preparation for the upcoming deuterium-tritium (D-T) campaign on JET, efforts are being dedicated to developing control systems able to identify and safely terminate plasmas that are evolving towards a compromised state. This could mean reaching a condition at risk of disruption or otherwise missing the goal of high-performance conditions, resulting in a waste of strictly budgeted nuclear...
Development of machine learning algorithms to predict the plasma evolution and how they can be used in control of the plasma to achieve combined high performance and high stability, and its application to fusion reactors is presented. Due to the nature of tokamak plasmas, operation in higher fusion gain increases the probably of instabilities which may then lead to disruptions and damage the...
Wave-particle interaction (WPI) can
produce effective pitch-angle scattering for electrons under
runaway acceleration by the parallel inductive electric field.
Enhanced pitch-angle scattering can impact the runaway energy gain in
two ways. The first is entirely in momentum space, in which the
resonant pitch-angle scattering sets up an energy barrier for
electrons that follows the...
Tokamak disruptions have the potential to create runaway electrons (RE) and if unmitigated, could cause severe localized damage. The physics of RE are not sufficiently understood and TCV has developed an extensive set of controls and diagnostics to contribute to this field of research. Successful mitigation of RE beams on TCV has been achieved through three techniques: 1) controlled ramp-down...
One of the most important open questions in runaway-generation research concerns the formation and survival of a seed population of fast electrons following the thermal quench of a tokamak disruption. Because of the large plasma current in future tokamaks such as ITER, such a seed current can multiply via the knock-on mechanism which may result in a significant fraction of the total plasma...
While disruptions caused by MHD instabilities occur only in the plasma scenario foreseen for the ITER high gain mission (ITER Baseline Scenario, IBS, q95~3), and they are essentially non-existent in the high-betaN, high-q95~5-6 Steady-State scenarios, disruptions caused by hardware failures can occur in any plasma. This presentation focuses on the physics causes for the disruptions in the IBS...
Successfully mitigating a disruption (once it is deemed unavoidable) remains a major challenge for ITER [1]. Shattered pellet injection (SPI) [2] has been chosen as the baseline ITER disruption mitigation system (DMS) due to its superior impurity mass delivery capabilities in the ITER environment relative to the only other mature alternative at the present time, massive gas injection (MGI)....
Shattered pellet injector systems have been installed on DIII-D, JET, and KSTAR and used to experimentally determine the effectiveness of the shattered pellet injection (SPI) process in mitigating the deleterious effects of a tokamak plasma disruption. The SPI process starts by desublimating deuterium, neon, or argon gas into the barrel of a pipe gun cooled to cryogenic temperatures to form a...
This work presents the integrated strategy aimed at the protection of the first wall (FW) from plasma transients, developed for the EU-DEMO design. The proposed strategy foresees the use of discrete limiters to protect the breeding blanket (BB) FW from direct contact with the plasma. The present FW design include the use of Eurofer coolant channels, able to withstand steady state heat fluxes...
Disruptions are a major issue for operation of future tokamaks like ITER and may generate runaway electrons (REs) which can melt the plasma facing components. The present ITER disruption mitigation strategy is to avoid the RE beam formation using Shattered Pellet Injection (SPI). If a RE beam is still generated, the thermal plasma cools down to 1-20 eV forming a so called companion plasma...
Although the thermomechanical loads generated by plasma disruptions in ITER require operations to strongly focus on disruption avoidance, these cannot be ruled out, especially during the early phases of the ITER Research Plan (IRP) [1] when scenarios are established and the operational space is extended. Even at relatively low plasma performance, these disruption loads can be severe on ITER,...
The importance of considering kinetic effects for the generation of relativistic runaway electrons (RE) in the presence of non-fully ionized impurities [1,2] is demonstrated in first-time integrated simulations of massive material injection (MMI), background plasma response, and RE generation in artificially induced disruptions in ASDEX Upgrade (AUG). Understanding the processes governing RE...
Future tokamaks will require robust disruption-mitigation systems (DMS) to prevent damage from extreme heat loads, electromagnetic stresses, and runaway electrons. The leading-candidate DMS is shattered-pellet injection (SPI) of impurities, which is being tested experimentally on several tokamaks and will be used on ITER. Sophisticated, predictive models that have been well-validated against...
Disruptions are one of the major concerns in ITER and other future tokamaks [1]. A particularly troublesome type of disruption is a vertical displacement event (VDE) where control of the vertical position of the plasma column is lost. In addition to heat, particle flux, and energetic electrons impacting the first wall, significant electromagnetic loads will arise.. For realistic modelling...
