Cryogenic Pellets Research Articles

Peridynamic modelling of cryogenic deuterium pellet fragmentation for shattered pellet injection in tokamaks

Abstract Shattered pellet injection (SPI) is a promising method for controlling plasma disruptions in tokamaks. In this study, we present numerical modelling of the fragmentation of cryogenic deuterium pellets within the context of SPI, using the peridynamic (PD) theory. A dedicated in-house code has been developed, leveraging the meshfree method and GPU parallelization. The mechanical properties of cryogenic solid deuterium are obtained from available literature, and calibrated based on the shatter threshold along with the remaining solid mass fraction after shatter. The results from the bond-based peridynamics (BB-PD) successfully reproduce the main experimental results reported in the literature, both qualitatively and quantitatively.

Production and launch studies of cryogenic pellets for the ITER disruption mitigation system

A support laboratory has been set up to study pellet production, launch and shattering of cryogenic protium, deuterium, and neon pellets for the ITER disruption mitigation system, which plans to use 28.5 × 57 mm (diameter × length) protium, neon an mixture pellets in the Shattered Pellet Injectors. Such large protium pellets have not been produced and launched before, therefore the desublimation and launch process have been studied in detail in two steps. First 19 mm diameter pellets were produced, followed by the demonstration of the final pellet size. Pellet desublimation recipes were established for all pellet types, and it was found that, under certain conditions, even the large neon pellets can be launched with a propellant gas pulse, without requiring a mechanical punch device. This is attributed to cryogenic snow formation on the surface of the pellet. Conditions for the snow formation are studied and tendencies are understood using simple calculations.

Acceleration of cryogenic pellets for Shattered Pellet Injection

Shattered Pellet Injection (SPI) is considered as a method to effectively mitigate the effect of severe disruptions in tokamaks. For the development of SPI technology in ITER, the Centre for Energy Research, in collaboration with H-ion Kft and VTMT Kft, designed, constructed and operates a Disruption Mitigation System (DMS) Support Laboratory. To simulate pellet acceleration, we developed a zero-dimensional model. The model uses the parameters of pellet material, propellant gas characteristics, dimension of the barrel and fast valve (FV), opening characteristics, temperature, and filling pressure of the FV in order to calculate the time dependent pellet position, pellet velocity, FV pressure and barrel pressure during the acceleration process. The goal of the acceleration model is to forecast velocities for parameters where no data is available as of yet. This paper describes the model and compares its results to two significantly different setups: the SPI measurements at the ASDEX Upgrade test laboratory and the ITER DMS Support Laboratory. We show that our model is capable of predicting tendencies and revealing the sticking stress of pellets to the barrel.

Successful Large Cryogenic Pellets Production and Acceleration for Shattered Pellet Injection for ITER Disruption Mitigation System

Successful Large Cryogenic Pellets Production and Acceleration for Shattered Pellet Injection for ITER Disruption Mitigation System

Development of a Fast Valve Assisted Mechanical Launcher for Cryogenic Pellets

Development of a Fast Valve Assisted Mechanical Launcher for Cryogenic Pellets

Instrumented fast valve for the ITER DMS support laboratory test bench

The ITER Disruption Mitigation System (DMS) Support Laboratory at the Centre for Energy Research, Budapest, Hungary aims at the characterization of the de-sublimation, acceleration and shattering of the cryogenic pellets, which will be fired upon request into the ITER plasma to mitigate the detrimental consequences of disruptions. The test bench includes a fast-acting valve for pellet acceleration, which releases high-pressure gas on a millisecond time scale. The valve is a new development since no valve on the market existed which would have met the requirements. The electromechanical modelling of the valve actuator has been validated against a laboratory experiment, the final system was built and thoroughly tested on a test bench to characterize the valve properties, and to demonstrate that the system fulfils the requirements.

ORNL Progress in Disruption Mitigation Technology in Support of ITER

The mitigation of plasma disruptions in tokamaks has become a very important topic in magnetic fusion research, motived by the potential challenges that may occur in ITER disruptions due to the high magnetic field and high plasma current. Such disruptions can have a deleterious effect on the internal components due to the fast dissipation of the plasma thermal energy and the magnetic stored energy leading to large forces, as well as the possible formation of several megaamperes of energetic runaway electrons during the current quench. Oak Ridge National Laboratory has been developing and deploying technology to inject material into the plasma to rapidly radiate the thermal energy and start a fast plasma current ramp down to dissipate the magnetic stored energy. The choice of materials to inject and the injection technology have evolved over the past decades to arrive at the present systems planned for ITER based on cryogenic pellets of hydrogen-neon mixtures for thermal mitigation and hydrogen pellets for runaway electron mitigation. This scheme injects shattered cryogenic material into the plasma from pellets formed in situ in a pipe gun and fired onto angled metal surfaces at the end of the injection line just before entering the plasma. In this paper, we describe the evolution of schemes and technologies that have been employed for disruption mitigation and runaway electron prevention and dissipation, discuss how they have performed in present-day experiments, and give the outlook for the use of this technology in a burning plasma and how it may continue to evolve in the future.

Shattered pellet technology development in the ITER DMS test laboratory

A support laboratory for the ITER Disruption Mitigation System has been set up at the Centre for Energy Research, Budapest, Hungary. It consists of a cryogenic pellet injector capable of producing, launching and shattering 19 × 38 and 28.5 × 55 mm (d x L) cryogenic pellets. Up to now more than 300 pellets made of hydrogen, deuterium, neon and mixtures thereof have been formed and launched . Nearly all pellets flew through the 4 m long, 60 mm diameter flight tube, which represents the ITER flight path between the propellant recovery chamber and the shattering head. Initial analysis of the fragment plumes produced by an open shattering head indicate that they extend 3 times further parallel to the shattering plate than perpendicularly to it. The results show that the large pellets needed for the ITER SPI system can be produced and launched.

Simulation and developments for large pellet formation and acceleration for shattered pellet injection of the ITER DMS

Amongst the technological developments needed for ITER, the disruption mitigation system (DMS) is one of the most challenging since it has to mitigate the consequences of high-energy plasma disruptions to ensure the lifetime of ITER's structure and in-vessel components. ITER DMS is using Shattered Pellet Injection, whose technology needs to be developed for ITER specific requirements.In the framework of the “Technology Fundamental Studies for DMS” contract with ITER Organization, as part of the ITER DMS Task Force activities, and CEA, the DSBT (Low Temperature Systems Department) designed and operates a pellet injection test bench to produce and accelerate large cryogenic pellets with diameters from 10 to 28.5 mm and length/diameter ratios up to two. The objective of this activity is to study and to optimise the pellet formation and acceleration process using pure Protium, Deuterium, Neon and mixtures of these gases.Analytic and numerical studies were used to define the geometrical and cryogenic parameters for the pellet desublimation in the in-situ condensation cell of the cryostat. An analytical model has also been developed for the pellet acceleration in the single stage gas gun to validate the propellant valve design aiming at achieving velocities of up to 500 m/s for 28.5 mm protium pellets and 200 m/s for neon pellets.The complete test bench is composed of the gas propulsion system, the cryostat with its key component the pellet formation cell, the flight tube and the target chamber with viewing ports. The mechanical assembly is completed with specific instrumentation and fast data acquisition systems for the observation of the pellet formation and acceleration. The pellet integrity and trajectory are characterised by shadowgraphy with high resolution, high-speed cameras located at 1 m and 3 m after the barrel exit and with an accelerometer and a target foil in the target chamber.This document details the simulation studies, the selected test bench design and first obtained results.

A comparison of the influence of plasmoid-drift mechanisms on plasma fuelling by cryogenic pellets in ITER and Wendelstein 7-X

Pellet injection is the most promising technique to achieve efficient plasma core fuelling, key for attaining stationary scenarios in large magnetic confinement fusion devices. In this paper, the injection of pellets with different volumes and speeds into standard plasma scenarios in ITER (tokamak) and Wendelstein 7-X (stellarator) is studied by modeling the pellet ablation and particle deposition, focusing on the evaluation of the expected differences in pellet plasmoid drifts in tokamaks and stellarators. Since the efficiency of the damping-drift mechanisms is predicted to depend on the magnetic configuration, device-specific characteristics are expected for the temporal evolution of the plasmoid drift acceleration. For instance, plasmoid-internal Pfirsch–Schlüter currents dominate the drift damping process for stellarators, while plasmoid-external currents are more relevant for tokamaks. Also, relatively larger drifts are in principle expected for W7-X due to higher field gradients in relation to machine dimensions. However, shorter plasmoid-internal charge reconnection lengths result in the drift damping due to internal Pfirsch–Schlüter currents being more effective than in a tokamak. Therefore, the average relative drift displacement during the whole plasmoid homogenization may a priori be comparable in both magnetic configurations. Moreover, High Field Side (HFS) injection is expected to be highly advantageous to maximize pellet particle deposition in ITER, whereas it may only be beneficial in medium to high β environments in W7-X. Finally, there may be means for the optimization of pellet injection configurations in both ITER and W7-X for the considered plasma scenarios despite the sizeable differences in the relative importance of the mechanisms of plasmoid drift acceleration and deceleration in play.

Development of a Shattered Pellet Injector test bench for the ITER DMS support laboratory

In the ITER Shattered Pellet Injection system large, Ø28.5x57mm cryogenic pellets have to be created and preserved in a barrel, until the control system detects a forthcoming disruption event and triggers the propellant gas valve to launch the pellet. The high-pressure propellant gas accelerates the pellet, which shatters on an angled plate near the edge of the plasma. The ablation of the resulting pellet fragments is intended to mitigate the consequences of the disruption and to prevent the generation of runaway electrons. To support the Shattered Pellet Injector technology for the ITER Disruption Mitigation System, a new support laboratory has been built at the Centre for Energy Research. The aim of the laboratory is to develop a test bench with ITER relevant dimensions, produce d=28.5mm Hydrogen, Neon and mixture pellets, observe the pellet acceleration and shattering with automated control. This contribution describes the cryogenic pellet injector of the test bench.

In-situ pellet growth and quality monitoring diagnostics for the ITER DMS Support Laboratory

The ITER disruption mitigation system is based on shattered pellet injection. The ITER DMS requires the use of large, 19 × 38 and 28.5 × 57 mm (diameter x length) cryogenic pellets, made of hydrogen, neon, or a mixture of those. The production of such large pellets, especially protium, is not straightforward, therefore the freezing process is monitored in real-time. A barrel view camera diagnostic was developed at the Centre of Energy Research for this purpose: the pellet is illuminated from one side of the barrel via a special vacuum adaptor featuring a high-pressure rated window, while observing it from the other side using a digital camera. It was found that the growth of the pellet can be followed easily, and issues such as bad heat contact between the cold head and the barrel can clearly be observed as well. Once the pellet is launched, its integrity and other parameters can be determined in an optically assessable vacuum chamber, using a shadowgraphy diagnostic. The pellet size can routinely be determined to sub-mm precision, and voids (or other non-solid material regions) can be identified as dark areas on the captured shadowgraphs.

Fragment plume diagnostics for cryogenic pellet shattering studies: Development and first experimental results

Shattered pellet injection is the baseline technology for the ITER Disruption Mitigation System (DMS) [1]. ITER DMS requires the use of large, 28.5 × 57 mm (diameter x length) cryogenic pellets, made of hydrogen, neon, or a mixture of those gasses and accelerated to several hundred m/s velocity. Beside the production, acceleration and transfer of such ITER-size pellets, the characterization of the shattering is one of the main task of the ITER DMS Support Laboratory at center of Energy Research as part of the ITER DMS Task Force program to validate the design choices of the DMS [2]. For this purpose, a fragment plume diagnostic chamber and two optical diagnostics using fast framing cameras, namely the laser curtain at two distances from the shattering head and a shattering observation were designed and implemented. Preliminary experiments for different pellet material and velocities at 20⁰ and 30⁰ shattering angle showed a strong pellets sublimation during shattering process. The fragments seem to slide along the shattering plate, no significant total reflection was detected. No qualitative difference between 20⁰ and 30⁰ shattering angle was observed, but clear differences for different materials and or pellet speeds.

Investigation of the compatibility of pellet fueling with ELM-free H-mode plasmas in EAST tokamak

Experiments on pellet fueling have been carried out in edge localized mode (ELM)-free high-confinement mode (H-mode) plasmas with q 95 ∼ 6 in the EAST tokamak. Cryogenic deuterium pellets were injected into the ELM-free plasmas at a frequency of 10 or 5 Hz from ∼45 cm above the mid-plane on the low-field side. It is found that the ELM-free H-mode plasmas are still sustained even if both the edge and core plasma are impacted by the pellet injections (PIs). Several small ELMs would appear and the edge coherent mode accompanying the ELM-free phase fades or even disappears just after the pellet events, but the plasma would rapidly recover to the ELM-free state. Although the ELMing phase is very short, it may be an issue that still needs to be resolved in the future. Meanwhile, the 3/2 tearing mode often appearing in ELM-free discharges would be stabilized by the PIs, and the high-Z impurity concentration would be reduced during the PIs. All these results will be meaningful to International Thermonuclear Experimental Reactor and future fusion reactors.

Study of solid molecular deuterium D2 growth under gas pressure

The injection of high-speed cryogenic pellets made of frozen hydrogen-isotopes, represents to date the most effective method to fuel magnetically confined thermonuclear fusion plasmas. Moreover, the injection of very large pellets composed of cryogenic solid of some suitable impurity (typically a noble-gas such as H2, Ne, or H2/Ne, D2/Ne mixtures), shattered in relatively small fragments just before entering the plasma, seems to be the most promising method to reduce the damage risks for the plasma-facing components in case of a plasma disruption. This technology, known as "Shattered Pellet Injection" (SPI), allows to spread out the plasma energy and mitigate possible damage to the in-vessel components, as well as to densify the plasma to suppress the formation of runaway electrons, and/or dissipate their energy. Several techniques to produce and launch cryogenic pellets have been investigated in the past decades. "Pipe gun" injectors are reliable and relatively simple devices are still commonly used today. They make use of single- or two-stage pneumatic light-gas guns to accelerate the pellet at high speeds. In these injectors, the cryogenic pellets are formed “in situ” (i.e., inside the launching barrel), by de-sublimating them directly from the gas phase, i.e., at temperatures and pressures below those of the triple point. The simplest case is pure deuterium pellets (T < 18.7 K, P < 171.3 hPa). The production of good quality solid deuterium, capable of withstanding the mechanical stress during the acceleration of the pellets, is a key issue. To this end the phase transition of deuterium from gas to solid (and vice versa) is modeled with extensive molecular-dynamics (MD) simulations. Moreover, the solid growth from the gas phase is simulated in an ample range of temperatures and pressures, to find the best compromise between growth velocity and mechanical properties of the resulting solid system.

Actuator Development Step by Step: Pellet Particle Flux Control for Single- and Multiple-Source Systems

Fuel injection by means of solid cryogenic pellets is expected to provide a sound and efficient tool. Hence, the installation of a pellet launching system will be a necessity. Yet, pellets are considered as a serviceable actuator for integrated supplementary functions as, e.g., fast and efficient delivery of seeding gas, or in case needed, the pacing of edge-localized modes. Consequently, a control scheme has to be developed that is capable of mastering the simultaneous actuations covering different tasks. Our scheme relies on pellet launching by a centrifuge accelerator, providing the option for precisely predictable pellet injection sequences. In order to develop a suitable actuator control scheme, as a first step the central part was brought into service at ASDEX Upgrade. It proved operational for feedback particle flux control of a single pellet source. In a subsequent step, it is now upgraded to enable multitasked control of the JT-60SA multipellet source currently under construction. In its finally designated configuration, this control scheme provides a potential solution for a reactor-grade system.

Design and performance of shattered pellet injection systems for JET and KSTAR disruption mitigation research in support of ITER *

Shattered pellet injection (SPI) systems that form cryogenic pellets in a pipe-gun for injection of material to mitigate disruptions have been fabricated and installed for use in thermal mitigation and runaway electron (RE) dissipation experiments on JET and KSTAR. These systems are to support disruption mitigation research for ITER and are based on an ORNL three-barrel design for flexibility in pellet size selection and variable pellet composition studies. The SPI systems for JET and KSTAR have a common feature of the barrels being collimated into a single injection line that enters the vacuum vessel. The pellets are shattered in bent stainless steel tubes that are mounted inside the vacuum vessel of the tokamak, vertically on JET and horizontally on KSTAR. The JET installation has the unique feature of vertical SPI mounting and injection with the shatter plume aimed toward the inner wall to intercept known RE beam locations generated from argon gas injection induced disruptions. The KSTAR SPI installation has two identical SPIs that are mounted on the midplane 180 degrees apart with identical injection lines and shatter tubes aimed at the plasma magnetic axis. Installation and operation of these SPI systems has provided useful lessons learned in the implementation of this SPI technology and valuable experience in optimizing the formation and firing of the pellets to optimize the physics performance.

Issues in Formation of Cryogenic Pellets for Fusion Applications

Cryogenic pellets are used for injection into fusion plasmas to add fuel to build up density and replace the ions lost from fusion reactions and imperfect confinement in the plasma. These pellets are formed at cryogenic temperatures with pure hydrogenic isotopes or mixtures of the isotopes. Technology to make these pellets and inject them into plasmas has been under development for many years, and various methods using freezing or desublimation have been shown to produce high-quality solid pellets suitable for injection. The throughput needed and possible impurity content from the necessary recirculation of fusion exhaust gases are two of the key issues to overcome for fusion pellet fueling systems in long-pulse burning plasmas. Here, we describe the technical challenges associated with these issues and the capability of pellet formation extruders to overcome them. Cryogenic pellets of deuterium, neon, and argon are also used in fusion tokamak devices for disruption mitigation in the form of large pellets that can be injected on demand to quickly dissipate the plasma thermal energy through radiation and add significant density in order to prevent runaway electron formation. Here, the issue is not throughput as with the fueling pellets but rather is the time it takes to form pellets of the size needed and the ability to dislodge them immediately on demand when needed to mitigate a disruption. The method used to make these pellets by desublimation is described, and examples related to how pellet size and input gas parameters affect the formation time are provided.

Shear Strength and Release of Large Cryogenic Pellets from the Barrel of a Shattered Pellet Injector for Disruption Mitigation

Shattered pellet injection (SPI) has been chosen as the baseline disruption mitigation system on ITER due to its ability to rapidly inject material deep into the plasma to greatly increase the plasma density and radiate the thermal energy. SPI utilizes a mechanical punch or high-pressure gas to release and accelerate a pellet that has been cryogenically desublimated in the barrel of a pipe gun. Various material injection combinations could possibly be implemented during different phases of a disruption event to radiate plasma energy, reduce electromagnetic loads on machine components, avoid the formation of runaway electrons, or to dissipate runaway electrons that form. Each injection phase could possibly utilize combinations of deuterium, neon, or argon. In this paper we outline experimental measurements of pellet material shear strength at SPI operating temperatures to understand the force needed to release SPI pellets. Deuterium, neon, argon, and deuterium-neon mixture pellets with diameters of 8.5, 12.5, and 15.7 mm are formed at a range of relevant gas pressures and temperatures and dislodged from the cold zone with a slow-moving piston driven by a motor. The slow-moving piston is kept above the triple point temperature of the material while the pellet is forming, then cooled to below the triple point temperature before contacting the pellet to minimize any thermal conduction to the pellet. The piston incorporates a load cell to measure the force applied when the pellet breaks away from the cold zone in the barrel. The ability of the gas and punch methods to exceed the shear strength of the studied pellet materials for release has been analyzed. High-pressure gas delivered by fast-opening valves produce pressure shock to the pellet due to supersonic expansion of the propellant gas. Pressure (and therefore, force) oscillations are present due to transverse density propagation throughout the breech volume. Mechanical punches deliver an impact force through a high-kinetic energy impact. The effect of the mechanical shock on the pellet has been explored and is presented in this paper. Scaling to larger ITER-size SPI pellets will be described.

Targeting a Versatile Actuator for EU-DEMO: Novel Control Scheme for Multisource Pellet Injector

Reactor plasma core fueling requires the injection of cryogenic pellets, most probably composed of a mixture of D2 and T2. Likely, pellet injection will be the most important actuator for plasma core density control. Therefore, pellet injection systems must be developed further that are capable of acting as actuator for density control. A novel control scheme is developed based on a centrifuge acceleration system. This scheme considers every available pellet launching slot and compares the current particle flux with the requested one. The response time is within the granularity of the available launching slots, in this case between 7 and 12 ms. First plasma experiments in feedforward mode showed excellent results, providing a good basis for upcoming plasma core density feedback control development activities.