Pellet Injection Research Articles

Recipes for pellet generation and launching in the ASDEX Upgrade SPI

In support of the ITER disruption mitigation system (DMS), a highly flexible, triple-barrel shattered pellet injection (SPI) system was installed at ASDEX Upgrade and tested in 240 dedicated discharges in the 2022 experimental campaign. Prior to the tokamak experiments, the system was commissioned and characterised in a laboratory environment. In this paper we discuss the experience gained from 2000 pellet launches on the freezing and launching process for 4 mm and 8 mm diameter pellets made from deuterium, neon and their mixtures. Different amounts of neon inside the pellet as well as the pellet heating recipes show a significant impact on pellet quality and successful launching. Pellet recipes were developed pushing the lower boundary of successful pellet launches. Depending on pellet size and composition, velocities between 60 m/s and 750 m/s were achieved.

Implementation of low temperature spectrometers for the JET high resolution Thomson scattering diagnostic for disruption plasma measurements.

This work presents a system upgrade of the High Resolution Thomson Scattering (HRTS) diagnostic [Pasqualotto etal., Rev. Sci. Instrum. 75, 3891-3893 (2004)] on JET that allows it to measure low temperature (1-500eV) plasma pre- and post-Thermal quench (TQ), which would help us further understand the Shattered Pellet Injection (SPI) physics. The upgrade was done by connecting optic fibers from the original HRTS system to four spectrometers specialized in measuring low temperature plasmas. The upgraded system allows for the measurement of low temperature plasma at up to 12 spatial points, which can be flexibly distributed throughout the JET outer midplane profile during the dedicated SPI experiments. In other JET experiments, four previously unused groups of fibers were used to measure the central plasma to provide disruption data without changing the standard HRTS system. The low temperature Thomson scattering system was installed, commissioned, and cross-calibrated against the standard HRTS diagnostic in a calibration pulse. The system worked reliably during the JET experiments and provided electron density and temperature profiles for pre- and post-TQ low temperature plasmas in the SPI campaign.

Radiation asymmetry in JET disruption mitigation experiments with shattered pellet injection

In ITER, to mitigate the deleterious effects of plasma disruptions, massive quantities of radiating impurities will be injected into the disrupting plasma by shattered pellet injectors (SPI) to pre-emptively radiate away the stored thermal and magnetic energy (Lehnen et al Proc. 27th IAEA Fusion Energy Conf. (FEC 2018) (Gandhinagar, India) EX/P7-12). However, asymmetries in the radiation pattern could result in intense photon flashes during the thermal quench that could locally damage or erode the stainless steel plasma-facing surface of the diagnostic port plugs (Pitts et al 2015 J. Nucl. Mater. 463 748–75). Experiments have been undertaken at JET to assess the potential dependence of the radiated power asymmetry on plasma energy during SPI mitigated disruptions. Calculations of the toroidal asymmetry in the radiated power indicate that the toroidal peaking factor is largest near the SPI position and decreases with the plasma stored energy, which is a promising result in view of radiation heat loads during mitigated disruptions in ITER.

Interaction of SPI pellets with plasma on JET and associated disruptions

The presented data refer to the Shattered Pellet Injector (SPI) experiments carried out at JET in 2019–2020. This paper is a full journal version of the data originally presented as posters at TMPDM_2020 and EPS_2021. This paper presents various aspects of the interaction of pellets with plasma and associated disruptions. The experiment was performed with I p = (1.1–3.1) MA plasmas and mainly with Ne + D2 pellet composition, but also with Ar pellets. The Current Quench (CQ) time, τ 80−20, is the key characteristic of mitigation effectiveness. A pellet with a high content of Ne or Ar can reduce the CQ duration below the upper required JET threshold. Plasmas with high (thermal + internal poloidal magnetic) pre-disruptive plasma energy require a high content of Ne pellets to obtain a short CQ duration. Pellets with a small amount of Ne (and accordingly large amount of D), instead of causing a mitigated CQ, create the conditions for a ‘cold’ Vertical Displacement Events (VDE). The SPI was applied to plasma with different status: mainly to normal (‘healthy’) plasma, i.e. not prone to disruption, post-disruptive and VDE plasma. This study shows that SPI effectiveness in terms of CQ duration and, accordingly, EM loads does not depend on the state of the plasma, whether it is ‘healthy’ or post-disruptive plasma. SPI has been shown to reduce the axisymmetric vertical vessel reaction forces by about (30–40) % compared to unmitigated disruptions. On JET, the VDE, whether ‘hot’ or ‘cold’, always creates the conditions for a toroidal asymmetry in the plasma, so the VDE on the JET is referred to as Asymmetric VDE (AVDE). The interrupting of VDE and prevention of AVDE with SPI has been demonstrated. Thus, the effectiveness of disruption mitigation using SPI has been confirmed.

Runaway electron dynamics in ITER disruptions with shattered pellet injections

This study systematically explores the parameter space of disruption mitigation through shattered pellet injection in ITER with a focus on runaway electron (RE) dynamics, using the disruption modeling tool Dream. The physics fidelity is considerably increased compared to previous studies, by e.g. using realistic magnetic geometry, resistive wall configuration, thermal quench onset criteria, as well as including additional effects, such as ion transport and enhanced RE transport during the thermal quench. The work aims to provide a fairly comprehensive coverage of experimentally feasible scenarios, considering plasmas representative of both non-activated and high-performance DT operation, different thermal quench onset criteria and transport levels, a wide range of hydrogen and neon quantities injected in one or two stages, and pellets with various characteristic shard sizes. Using a staggered injection scheme, with a pure hydrogen injection preceding a mixed hydrogen-neon injection, we find injection parameters leading to acceptable RE currents in all investigated discharges without activated runaway sources. Dividing the injection into two stages is found to significantly enhance the assimilation and minimize RE generation due to the hot-tail mechanism. However, while a staggered injection outperforms a single stage injection also in cases with radioactive RE sources, no cases with acceptable RE currents are found for a DT-plasma with a 15MA plasma current.

The integration and tests of the shattered pellet injection system on EAST

A SPI system specifically designed for EAST has been integrated and commissioned to carry out plasma rapid shutdown. This system is capable of injecting pellets with quantities ranging from 7 to 14 Pa m3 and at velocities between 150 and 400 m/s, as determined after conducting a series of bench tests. A two-stage differential pumping system designed to remove the propellant gas has had its efficiency assessed through the application of pumping equations and the fourth-order Runge-Kutta routine. For the purpose of facilitating remote control of the SPI system, a control system based on EPICS has been developed. This system enables an automated workflow for pellet formation and launch, which includes stages such as pellet formation, strengthening pellet, awaiting trigger, launching pellet, baking gun barrel, and cooling cold head, with the entire process taking 600 s. Furthermore, rapid shutdown experiments using Ne shattered pellet injection have been conducted, demonstrating the system's high reliability and reproducibility on EAST.

Divertor-safe nonlinear burn control based on a SOLPS parameterized core-edge model for ITER

For ITER operations, the range of desirable burning-plasma regimes with high fusion power output will be restricted by various operational constraints. These constraints include the saturation of ITER’s various heating and fueling actuators such as the neutral beam injectors, the ion and electron cyclotron heating systems, the gas puffing system, and the deuterium–tritium pellet injectors. In addition to these actuator constraints, the H-mode power threshold, divertor detachment, and the heat load on the divertor targets may apply limitations to ITER’s operational space. In this work, Plasma Operation Contour (POPCON) plots that map the aforementioned constraints to the temperature-density space are used to investigate which constraints are most limiting towards accessing regimes with high fusion power output. The presented POPCON plots are based on a control-oriented core-edge model that couples the nonlinear density and energy response models for the core-plasma region with SOLPS4.3 parameterizations for conditions in the edge-plasma regions (scrape-off-layer and divertor). Using this control-oriented core-edge model, a nonlinear burn controller, which aims to regulate the plasma temperature and density in the core-plasma region, is constructed in this work. This controller is augmented with an online optimization scheme that governs the control references such that the plasma can be guided towards regimes with high fusion powers while protecting the divertor targets from dangerously high heat loads. A closed-loop simulation study illustrates the capability of this burn control scheme.

Plasmoid drift and first wall heat deposition during ITER H-mode dual-SPIs in JOREK simulations

The heat flux mitigation during the thermal quench (TQ) by the shattered pellet injection (SPI) is one of the major elements of disruption mitigation strategy for ITER. It’s efficiency greatly depends on the SPI and the target plasma parameters, and is ultimately characterised by the heat deposition on to the plasma facing components. To investigate such heat deposition, JOREK simulations of neon-mixed dual-SPIs into ITER baseline H-mode and a ‘degraded H-mode’ with and without good injector synchronization are performed with focus on the first wall heat flux and its energy impact. It is found that low neon fraction SPIs into the baseline H-mode plasmas exhibit strong major radial plasmoid drift as the fragments arrive at the pedestal, accompanied by edge stochasticity. Significant density expulsion and outgoing heat flux occurs as a result, reducing the mitigation efficiency. Such drift motion could be mitigated by injecting higher neon fraction pellets, or by considering the pre-disruption confinement degradation, thus improving the radiation fraction. The radiation heat flux is found to peak in the vicinity of the fragment injection location in the early injection phase, while it relaxes later on due to parallel impurity transport. The overall radiation asymmetry could be significantly mitigated by good synchronization. Time integration of the local heat flux is carried out to provide its energy impact for wall heat damage assessment. For the baseline H-mode case with full pellet injection, melting of the stainless steel armour of the diagnostic port could occur near the injection port, which is acceptable, without any melting of the first wall tungsten tiles. For the degraded H-mode cases with quarter-pellet SPIs, which have 1/4 total volume of a full pellet, the maximum energy impact approaches the tolerable limit of the stainless steel with un-synchronized SPIs, and stays well below such limit for the perfectly synchronized ones.

Heat and particle exhaust in high-performance plasmas in Wendelstein 7-X

The paper reports for the first time the heat and particle exhaust at the plasma boundary through various edge diagnostics for the high-performance plasma obtained after pellet injection on Wendelstein 7-X. The plasma density at the edge is found to be reduced by a factor of 2 in the high-performance phase, supporting the previously reported density peaking at the plasma centre. The plasma beta effect on the magnetic topology is reflected by the appearance of the second strike line, which is well understood with simulation. However, during the rapid decay phase of the enhanced confinement, a transient localized heat flow of up to 16 MW m−2 is observed at the leading edge of a poorly cooled divertor component, which has not been understood but raises concerns about machine safety.

Core density profile control by energetic ion anisotropy in LHD

Electron and impurity ion density profiles have been controlled by using tangential and perpendicular neutral beams for plasma heating in a stellarator/heliotron for the first time. Reduced anisotropy of stored energy for energetic ion En⊥/Enǁ has resulted in an inward electron and impurity transport, forming a core electron density peaking. Increased anisotropy leads to a flat or hollowed electron density profile with an impurity exhaust in a core region [Yoshinuma et al., Nucl. Fusion 49, 062002 (2009)]. A high confinement state of particles in LHD has yet to be achieved, except for a temporal state of an electron density peaking created by a pellet injection. As a pioneering and crucial research result, the operation of energetic ion anisotropy by neutral beams has newly demonstrated that the direction of the radial transport of bulk and impurity ions can be controlled. At the same time, the overall plasma performance rises in neutron flux and stored energy. On the other hand, the increase in the anisotropy flattens the density profile. This new finding holds promise for a control knob of nuclear fusion reactors to enhance fusion power output.

Optimization of pellet design parameters to achieve deep fueling depth in EAST plasma with PAM code

Deep pellet fueling depth is necessary to achieve a high-density high confinement operation and to conduct some pellet-related researches in current devices, such as trigger of internal transport barrier, and to achieve high fusion power and tritium burn-up fraction in future fusion devices. The newly developed PAM code which can make a fast evaluation on pellet ablation and deposition is applied to optimize injection parameters to achieve deep pellet fueling. Systematic scans on pellet injection parameters including pellet injection positions, injection angles, sizes and speeds are performed for optimization purpose, while at the same time demonstrating flexibility and time efficiency of the PAM code. Dependences of the pellet fueling depth on these injection parameters are revealed by simulation results and analyzed. Simulation results indicate that pellet penetration contributes more to the deep pellet deposition than the ∇B -induced plasmoid drifts in low temperature plasmas, while deep pellet fueling in reactor relevant high temperature plasmas has to rely on plasmoid drifts. Though a shallow penetration is expected in high temperature plasmas, the ∇B -induced plasmoid drift is expected to be larger than that in relatively low temperature plasmas.

Fuelling of deuterium–tritium plasma by peripheral pellets in JET experiments

A baseline scenario of deuterium–tritium (D–T) plasma with peripheral high-field-side fuelling pellets has been produced in JET in order to mimic the situation in ITER. The isotope mix ratio is controlled in order to target the value of 50%–50% by a combination of tritium gas puffing and deuterium pellet injection. Multiple factors controlling the fuelling efficiency of individual pellets are analysed, with the following findings: (1) prompt particle losses due to pellet-triggered edge-localised modes (ELMs) are detected, (2) the plasmoid drift velocity might be smaller than that predicted by simulation, (3) post-pellet particle loss is controlled by transient phases with ELMs.The overall pellet particle flux normalised to the heat flux is similar to that in previous pellet fuelling experiments in AUG and JET.

Experimental environment for testing the shattered pellet injection of KSTAR

The dual shattered pellet injection (SPI) systems installed at the KSTAR (Korean Superconducting Tokamak Advanced Research) have been operational since 2019 to conduct disruption mitigation studies in support of ITER DMS (Disruption Mitigation System) design activities. The system has the advantage of firing pellets simultaneously from the two identical pellet injectors installed at 180-degree opposite positions on KSTAR. Each injector is capable of fabricating and firing three pellets of different sizes (diameter, length). The characteristics of the initial pellet (geometry, velocity, etc.) can be verified using a microwave cavity (MWC) system and an imaging optical pellet diagnostics (OPD) system.Each SPI system must achieve specific conditions related to pellet fabrication and firing prior to plasma experiments. We realized that we didn't have the sufficient time and conditions to do off-line testing in 2022, so we have developed a suitable solution. First, we have to disconnect the SPI system from the KSTAR vacuum vessel and build a separate test chamber in the injection line. Inside the new test chamber, we have to install the same shatter tube used in the current experiments and equip it with measuring devices to observe the fragmented pellets. We also have to switch the fabrication process of the pellets from manual to semi-automatic using a feedback pressure controller on the barrel pressure. In this paper, we will describe the new experimental environment for independent SPI testing and the future plan.

Status of the Deep Learning-Based Shattered Pellet Injection Shard Tracking at ASDEX Upgrade

Plasma disruptions pose an intolerable risk to large tokamaks, such as ITER. If a disruption can no longer be avoided, ITER’s last line of defense will be the Shattered Pellet Injection. An experimental test bench was created at ASDEX Upgrade to inform the design decisions for controlling the shattering of the pellets and develop the techniques for the generation of the fragment distributions necessary for optimal disruption mitigation. In an effort to analyze the videos resulting from the more than 1000 tests and determine the impact of different settings on the resulting shard cloud, an analysis pipeline, based on traditional computer vision (CV), was created. This pipeline enabled the analysis of 173 of the videos, but at the same time showed the limits of traditional CV when applied in applications with a highly heterogeneous dataset such as this. We created a machine learning-based (ML) alternative as a drop-in replacement to the original image processing code using a semantic segmentation model to exploit the innate adaptability and robustness of deep learning models. This model is capable of labeling the entire dataset quickly, accurately and reliably. This contribution details the implementation of the ML model and the current state and future plans of the project.

Multi-pellet injection into the NBI-heated phase of TJ-II plasmas

A pellet-induced enhanced confinement (PiEC) phase, with general characteristics similar to those reported for the stellarator W7-X, is observed after single pellet injection (>1019 H atoms) into the neutral beam injection heated phase of plasmas in the mid-sized heliac-type stellarator TJ-II. In addition to a step-like increase in density, plasma diamagnetic energy content rises significantly with respect to that of reference discharges, energy confinement time is similarly enhanced when compared to International Stellarator Scaling law predictions (Yamada et al 2005 Nucl. Fusion 45 1684) renormalized for TJ-II, and the triple product, n e · T i · τ E, exhibits a clear bifurcation towards an improved confinement branch when compared to the branch product predicted by the same law. In this work, multiple pellets are injected in series into NBI-heated plasmas in the TJ-II and post-injection plasma performance is reported and discussed. For instance, a charge-exchange recombination spectroscopy diagnostic reveals significantly increased core ion temperatures after pellet injection compared to temperatures achieved in comparable reference plasmas, this pointing to increased ion energy content and improved ion energy confinement during a PiEC phase. It is also found that enhanced performance is independent of whether co- or counter-NBI heating beam is employed. Finally, record stored diamagnetic energy content and plasma beta values are achieved when the largest available pellets are employed. The results indicate that pellet injections extend the operational regime well beyond limits previously achieved in TJ-II without pellets.

Interpretative 3D MHD modelling of deuterium SPI into a JET H-mode plasma

The pre-thermal quench (pre-TQ) dynamics of a pure deuterium ( D2 ) shattered pellet injection (SPI) into a 3MA / 7MJ JET H-mode plasma is studied via 3D non-linear MHD modelling with the JOREK code. The interpretative modelling captures the overall evolution of the measured density and radiated power. The simulations also identify the importance of the drifts of ablation plasmoids towards the tokamak low field side (LFS) and the impurities in the background plasma in fragment penetration, assimilation, radiative cooling and MHD activity in D2 SPI experiments. It is found that plasmoid drifts lead to an about 70% reduction of the central line-integrated density (compared to a simulation without drifts) in the JET D2 SPI discharge considered. Impurities that pre-exist before the SPI as well as those from possible impurity influxes related to the SPI are shown to dominate the radiation in the considered discharge. With inputs from JOREK simulations, modelling with the Lagrangian particle-based pellet code PELOTON reproduces the deviation of the SPI fragments in the direction of the major radius as observed by the fast camera. This confirms the role of rocket effects and plasmoid drifts in the considered discharge and reinforces the validity of the JOREK modelling. The limited core density rise due to plasmoid drifts and the strong radiative cooling and MHD activity with impurities (depending on their species and concentration) could limit the effectiveness of LFS D2 SPI in runaway electron avoidance and are worth considering in the design of the ITER disruption mitigation system.

Structure of pellet cloud emission and relation with the local ablation rate

The experimental reproduction of the conditions of pellet injection expected in future large devices being not possible in present day machines, it is mandatory to validate as thoroughly as possible the available ablation models. Among the different points still under discussion, there is the relation between the spectroscopic measurement of the ablation clouds and the local ablation rate. This relation is investigated by coupling an emission model to the time-dependent simulation of ablation clouds with the HPI2 pellet ablation/deposition code. The simulated quantities are the time-evolution of the cloud visible spectrum (λ = 400–700 nm) and images in different wavelength domains (e.g. Hα,Hβ or the continuum centered at λ=576nm ). It is found that the cloud emission is anisotropic, this is particularly the case for H α and H β lines, and that the relation between the cloud emission and the ablation rate depends not only on the conditions of pellet injection, but also on the direction of observation. It follows that, in general, it is not possible to estimate the ablation profile from that of an emission line (Hα or Hβ) . The code predictions are compared with corresponding measurements for a welldocumented pellet injected in LHD, showing a good agreement for global values and main trends. The reasons for observed discrepancies are discussed.

Safety factor diagnostic for tokamak core plasma from three-dimensional reconstruction of pellet ablation trail.

Using a stereo camera system, a new diagnostic for the safety factor of the core plasma based on the pellet ablation trail is applied on the Experimental Advanced Superconducting Tokamak (EAST). In EAST discharge No. 128 874, a shattered pellet injection system is applied to inject a shattered neon pellet into the EAST. Since the strong magnetic field in tokamaks binds the ablated pellet material, the orientation of the pellet ablation trail is the same as the local magnetic field direction. Thus, from the three-dimensional reconstruction result of the pellet ablation trail, the local safety factor q can be obtained. The motional Stark effect (MSE) diagnostic is applied to determine the safety factor q profile in this shot. The determined safety factor q results for this new diagnostic are in quantitative agreement with those from the MSE diagnostic with the mean relative difference of only 6.8%, confirming the effectiveness of this new diagnostic of the safety factor.

Progress of CO2 Dispersion Interferometer on EAST

A CO2 Dispersion Interferometer (DI) system on the Experimental Advanced Superconducting Tokamak (EAST) was successfully operated, providing plasma electron density measurements. The DI system utilizes a continuous-wave 9.3 μm CO2 laser source to measure line-averaged electron densities. This offers significant advantages, including the ability to minimize fringe jumps and insensitivity to mechanical vibrations. These characteristics are well-suited for future high-density, long-pulse plasma discharges. The DI system provides a real-time density feedback signal to the plasma control system for routine density control during long-pulse operation. Experiments with EAST demonstrated good agreement between the density obtained by the DI system and the preset densities. The DI system also exhibited stability during long-pulse discharge. Moreover, the DI system was stable during rapid density changes and high-density pellet injections. In shot No. 120594, the DI system exhibited stable density feedback during continuous projectile injection lasting over 50 seconds; the line-averaged electron density is approximately 4×1019 m-3. In contrast to the long-wavelength source interferometer, which may deflect light from the detector owing to excessive refraction angles in larger density-gradient discharges, the DI ensured accurate density measurements. The DI system on EAST is dependable for accurately measuring the electron density.

Study of the pellet ablation cloud using the tomography technique for two-directional simultaneous photography in GAMMA 10/PDX

The pellet ablation mechanism is an interesting subject for plasma fuelling in fusion plasmas. In GAMMA 10/PDX, pellet injection experiments for higher density plasma production are planned to conduct detached plasma experiments in the higher density plasma condition. We measured the pellet ablation cloud by using the two-directional simultaneous photography system in GAMMA 10/PDX. The tomography reconstruction technique was used for considering the pellet trajectory in the plasma and pellet ablation. The three-dimensional pellet trajectory and pellet ablation images in the plasma were clearly obtained for the first time, to the best of our knowledge.