Pellet Speed Research Articles

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.

Impact of breech geometry and propellant flow on the release of large pellets for the ITER disruption mitigation system

Studies have been performed on the release mechanism for large pellets using high pressure gas in a shattered pellet injector. Typically, pellets are dislodged from the cryogenic surface and accelerated down a barrel using high pressure gas delivered by a fast-acting propellant valve. The pellets impact an angled surface which shatters the pellet into many small fragments before entering the plasma. This technique was initially demonstrated on DIII-D (Commaux et al 2016 Nucl. Fusion 56 046007) and is now deployed on JET, KSTAR, ASDEX-Upgrade, and other tokamaks around the world in support of ITER’s disruption mitigation system design and physics basis. The large hydrogen, 28.5 mm diameter, 2 length-to-diameter ratio, pellets foreseen for ITER SPI operation have low material strength and low heat of sublimation, which cause the pellets to be fragile and highly reactive to the impact of warm propellant gas. Due to the size of the pellets, significantly more propellant gas is required to dislodge and accelerate them. This creates a potentially significant propellant gas removal issue as 2–6 bar-L of gas is expected to be required for release and speed control. The research presented in this paper is an in-depth exploration of the parameters that are keys to reliable pellet release and speed control. Computational fluid dynamics (CFD) modeling of propellant flows through various breech designs was conducted to determine the force generated on the back surface of a pellet. These simulations assumed the use of the ORNL designed flyer plate valve. CFD modeling combined with experimental measurements provide adequate insight to determine a path to an optimal valve and breech design for ITER SPI pellet release and speed control while minimizing propellant gas usage.

MHD modeling of shattered pellet injection in JET

Nonlinear 3D MHD simulations of shattered-pellet injection (SPI) in JET show prototypical SPI-driven disruptions using the M3D-C1 and NIMROD extended-MHD codes. Initially, radiation-driven thermal quenches are accelerated by MHD activity as the pellet crosses rational surfaces, leading to a radiation spike, global stochasticization of the magnetic field, and a complete thermal quench. Eventually, current quenches, preceded by a current spike are seen as the Ohmic heating becomes equal to the radiative cooling. The results are qualitatively similar for both a single monolithic pellet, pencil-beam model, and a realistic shatter to represent the SPI plume. A scan in viscosity from 500 to 2000 m2 s−1 for MHD simulations finds that reducing viscosity increases MHD activity and decreases thermal quench time slightly. A realistic cloud of fragments modeling shows that mixed-D–Ne pellet travels deeper into the plasma core before the thermal quench. At the slow pellet speeds, the pellet is found to be moving slowly enough inward that even the 5% neon in the mixed pellet is enough to effectively radiate the thermal energy available. Radiation toroidal peaking is predicted to be at levels consistent with experimental observations and reduced as the pellet travels deeper into the plasma. These simulations lay the ground work for more-sophisticated validative and predictive modeling of SPI in JET using both M3D-C1 and NIMROD.

Initial Commissioning Test Results of the Wendelstein 7-X Continuous Pellet Fueling System

A continuous pellet fueling system (CPFS) for use on the Wendelstein 7-X (W7-X) stellarator has been fabricated and assembled with commissioning tests completed at the Oak Ridge National Laboratory (ORNL). Continuous fueling is accomplished by cutting pellets from the cross section of a continuous solid extrusion produced by a twin-screw extruder and accelerated by a gas gun cutter mechanism. The pellets travel through a series of straight guide tubes before entering the stellarator through a curved guide tube. The CPFS has an array of diagnostics that include the extruder torque, rotation rate, and thrust. A shock and pressure sensor provide verification of proper pellet cutting and acceleration. Two ORNL-developed microwave cavity diagnostics within the injection line guide tubes provide pellet speed and relative mass measurements. For commissioning, a high-speed camera has been positioned at the base of the extruder to verify extrusion speed and quality as well as the pellet cutting process, and a third microwave cavity has been mounted after the curved guide tube to verify pellet quality and size. Maximum injection rate, pellet speed, barrel and guide tube induced erosion, and pellet survivability data have been recorded. This paper presents the laboratory experimental setup and results of these commissioning tests.

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.

Numerical Optimization of Electromagnet Current Distribution in Superconducting Linear Acceleration System

An enhancement of the acceleration performance of a superconducting linear accelerator when injecting pellets into a helical fusion reactor has been investigated numerically. For this purpose, a numerical code using the finite element method was developed to analyze both the time evolution of a shielding current density in a high-temperature superconductor film and the dynamic motion of the film. In the present study, to optimize the shape of double magnets, a multi-objective optimization problem is solved using the non-dominated sorting genetic Algorithm-II. The results of the computations show that the optimization of the inner electromagnet is more effective in improving the pellet velocity than that of the outer electromagnet. In addition, we found that the pellet speed and energy efficiency have a trade-off relation. Therefore, it is possible to determine the preferred solution for the design of an acceleration magnet, whether focusing on the pellet speed or the energy efficiency. In other words, this relationship can be used to choose whether to emphasize velocity or efficiency in the design of SLA systems.

Development of the pellet injection system on the J-TEXT tokamak

Pellet injection is an attractive technology for core-fueling and magnetohydrodynamic study in magnetic-confinement fusion devices like tokamaks and stellarators. It can inject solid hydrogen/deuterium pellets into the plasma with deeper density deposition compared with other fueling methods, such as gas puffing. A three-barrel H2 pellet injection system was installed on the J-TEXT tokamak and experiments were carried out. The pellets are formed in three barrels cooled by a cryocooler and compressor system at around 9 K, and are 0.8 mm/1 mm diameter and 0.8 mm length. The pellet is launched by helium propellant gas and injected from the low-field side of the plasma. The normal range of pellet speed is 210–310 m s−1 for different propellant gas pressures. Due to the three-barrel structure, the number of injected pellets can be adjusted between one and three. Pellets can be launched sequentially with arbitrary time intervals, which enables flexible applications. The results of the experiments show that pellet fueling efficiency can reach 50%. The energy confinement time increased by about 7.5‒10 ms after pellet injection.

Study of Solid Hydrogen Pellet Speed in a Gas Gun–Type Injector

Based on the ideal gas gun theory (IGT) approximation, an analytical study of solid hydrogen pellet motion in a gas gun–type pellet injector has been performed. A parametric investigation has been conducted to study the pellet speed dependence on the gun characteristics and the propellant conditions. The calculations have been verified by applying various experimental data reported from the literature. Experimental results are within 70% to 90% of the ideal IGT and are in line with global predictions. Calculations indicate that the speed of the pellet has a strong dependence on the propellant pressure and its mass, and a weak dependence on the length of the gun barrel. In addition, the effects of shock waves due to the sudden opening of the propellant valve and some nonideal effects, such as the effect of friction at the propellant pellet–wall interface, have been studied. The results of the calculations have been verified by applying them to the experimental results.

ОПРЕДЕЛЕНИЕ РАБОЧИХ ПАРАМЕТРОВ ЛЕНТОЧНОГО ДРАЖИРАТОРА

Conventional methods of seed pelleting involve using pelletizers with a drum-type working unit. In these devices, pelleting is carried out in a cyclic mode, which makes operational restrictions and complicates the use of pelletizers in the technological line of complex pre-sowing seed treatment. The authors evaluate the effectiveness of belt-type pelleting machines ensuring the continuity of pelleting and significantly increasing the performance. The paper provides some basic requirements to continuous seed pelletizing. The conditions are determined to describe the pellets rolling down the inclined surface of the moving conveyor belt. Consequently, the analytical relationship is established between the inclination angle of the pelletizer belt and the friction coefficient, the pellet diameter and speed of their forward movement along the conveyor belt. The main operating parameters of the belt pelletizer are determined – length, speed, and the inclination angle of the conveyor belt. The obtained mathematical expressions are universal, since they allow analyzing the working parameters of the pelletizers when processing crop seeds with different initial size, weight, and tribotechnical properties. When studying the movement of the coated seeds along the moving belt of the inclined conveyor, the friction force acting on them, the reaction force of the support, the authors took into account the rolling force and the gravity force. Based on the analogy of the translational movement of seeds along an inclined plane to their movement along the inner surface of the drum of an infinitely large radius, the authors established the numerical values of the parameters of continuous pelleting. The paper finishes with a conclusion about possible reducing the dimensions of the belt-type pelletizer. The estimated length of the working area of the conveyor belt is suggested to be the sum of the lengths of several belts inclined to the ground in different planes.

Dispersive shell pellet injection modeling and validation for DIII-D disruption mitigation

Dispersive shell pellet injection for disruption mitigation on the DIII-D tokamak is modeled with the 3D MHD code NIMROD. The simulations attempt to reproduce as closely as possible a pellet velocity scan that was performed on the DIII-D tokamak [Hollmann, et al., Phys. Rev. Lett. 122, 65001 (2019)]. This scan involved injecting a hollow diamond shell filled with boron dust that is released in the core of the plasma. Trends that are reproduced in the simulation include improved mitigation, reduced plasma current spike amplitude, and increased likelihood of hot-tail runaway electron production with increased pellet speed. The simulations provide additional physics insight into these findings. Limitations of the present model and requirements for a predictive model are discussed.

Statistical assessment of ELM triggering by pellets on JET

This article investigates the triggering of ELMs on JET by injection of frozen pellets of isotopes of Hydrogen. A method is established to determine the probability that a specific pellet triggers a particular ELM. This method allows clear distinction between pellet-ELM pairs that are very likely to represent triggering events and pairs that are very unlikely to represent such an event. Based on this, the pellet parameters that are most likely to affect the ability of pellets to trigger ELMs have been investigated. It has been found that the injection location is very important, with injection from the vertical high field side showing a much higher triggering efficiency than low field side (LFS) injection. The dependence on parameters such as pellet speed and size and the time since the last ELM is also seen to be much stronger for LFS injection. Finally, the paper illustrates how improvements to the pellet injection system by streamlining the pellet flight lines and slightly increasing the pellet size has resulted in a significantly improved ability to deliver pellets to the plasma and trigger ELMs.

Analysis of the Shattered Pellet Injection Fragment Plumes Generated by Machine Specific Shatter Tube Designs

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. Pellets are fired, and before entering the plasma, strike a bent tube known as a shatter tube causing the pellet to shatter. The process of pellet fragmentation is a chaotic process that can be described in terms of fragment size distribution through a statistical model that incorporates the effects of the pellet material and impact characteristics. In addition to the fragment size distribution, the shatter plume has other characteristics of interest, such as a fragment velocity distribution and temporal mass evolution. The fragment velocity distribution is important because it is needed to accurately model the spread and location of the ablation and the deposition of impurities in the plasma over time. The temporal mass evolution is necessary to determine the time-resolved delivery of mass to the plasma. Due to installation constraints, the shatter tube currently installed on JET has a unique geometry with a modest S-bend followed by a 20-deg bend at the end of the tube. The DIII-D and KSTAR shatter tube design is a simple tube bent through an angle of 20 deg followed by a straight section. The resulting shatter sprays from the JET shatter tube and a 20-deg miter bend shatter tube were experimentally characterized for various pellet materials and speeds. Laboratory testing of these shatter tubes allows for the use of fast cameras to capture the fragment spray traveling through a large vacuum chamber. These high-speed videos of the shatter plumes allow the fragment size distribution, temporal mass evolution, and velocity distribution of the fragments within the plume to be determined. This paper presents a comparison of the unique geometry of the JET shatter tube to the miter bend geometries used for shattering and some insight into the variables that may be adjusted to produce the optimal shatter spray. The impact of entrained propellant gas on the resulting shatter spray was examined during testing.

FEM-simulation of superconducting linear acceleration system for pellet injection

In order to simulate the high-temperature superconducting (HTS) linear acceleration (SLA) system for the pellet injection, the integration method of the applied magnetic field generated from the acceleration coil has been proposed. To this end, the regularization technique is used in the evaluation of the improper integrals, and simultaneously, a FEM code is developed for analyzing the shielding current density in an HTS film. In addition, the SLA system has been simulated using the code. The results of the computations show that the accuracy of the applied magnetic field is considerably improved. In this sense, the regularization technique is a useful tool. Also by locating the outer coil, the acceleration time during which the pellet speed reaches 5 km/s is about 3.5 times shorter than that of the only use of the inner coil. These results mean that the outer coil is effective in the improvement of the acceleration performance for the SLA system.

Injection of high-speed solid D2 pellets using a “Direct-Line-of-Sight” (DLS) guide tube

• Core fueling of EU-DEMO tokamak requires pellet injection from the High Field Side. • Pellet speeds ≳ 1 km/s are necessary for efficient core fueling of reactor plasmas. • At such high speeds, the use of conventional curved guide tubes might be hampered. • Injection of very high-speed pellets along oblique inboard trajectories is proposed. • Ability of 2.7 km/s pellets to survive in a Direct-Line-of-Sight guide demonstrated. Extensive investigations, within the EUROfusion Work Package "Tritium, Fuelling and Vacuum", indicate that sufficiently deep fuel deposition inside H-mode plasmas of the EU-DEMO tokamak requires injection of fuel pellets from the High Field Side (HFS) at speeds ≳ 1 km/s. To implement this, two different approaches are being pursued: one makes use of “conventional” curved guide tubes, featuring large bend radii (≳ 6 m), to transport 1 km/s pellets to the HFS while trying to preserve their mass and integrity; the other explores the feasibility of injecting high-speed (≳ 2 km/s) pellets from the HFS, along "Direct-Line-of-Sight" (DLS) paths. This paper focuses on the latter approach. Recent tests with an existing ENEA-ORNL high-speed injector have confirmed that the trajectories of free-flight pellets, travelling under vacuum at speeds up to 2.4 km/s, spread within an angle ≲ 0.68°. Despite their small scatter cone, free-flight pellets may require too much cut off volume of the Breeding Blanket (BB), due to the large distance between the injector and the plasma. The introduction of a straight DLS guiding tube transporting the high-speed pellets, to avoid significant loss of BB material, has been investigated. The existing ENEA-ORNL injector has been modified to accommodate a 10 mm i.d. DLS guide tube, and intact pellets have been consistently delivered downstream of the guide at speeds up to 2.6 km/s, with remarkably reduced scatter cone, thus showing the viability of this innovative approach.

Shattered pellet injection technology design and characterization for disruption mitigation experiments

The technology to form and shoot high-Z cryogenic solid pellets mixed with deuterium using a gas gun that are shattered upon injection into a plasma has been developed at Oak Ridge National Laboratory for mitigating disruptions. This technology has been selected as the basis for the baseline disruption mitigation system on ITER. The development of shattered pellet injection systems has progressed to be able to accelerate large pellets of pure argon and neon with or without including deuterium. Impact studies have been carried out at shallow angles to determine funnel performance in guiding pellets from multiple barrels into a common injection line and across pumping breaks. The characterization of the shattered spray has also progressed with fragment size measurements as a function of pellet speed showing a strong inverse relationship. Results of these studies are reported with implications for applications on existing and future tokamak devices.

A Flexible Pellet Injection System for the Tokamak JT-60SA: The Final Conceptual Design

The research plan of the JT-60SA, a superconducting tokamak device currently under construction, requests a powerful pellet injection system for its particle fueling and edge-localized-mode (ELM) pacing experiments. These investigations, foreseen to answer basic questions with respect to the operation of ITER and a future fusion power plant like DEMO, need pellets with flexible parameters delivered precisely and reliably for control purposes. Here, we present a conceptual design of this system based on classical pellet technology. Analysis showed pellets will show the best performance for fueling and most likely also for ELM pacing when injected from the torus inboard side, despite the limited maximum pellet speed caused by this approach. This is due to constructional constraints rising from the fact the JT-60SA vacuum vessel is already under construction, enforcing inboard injection via a multibend guiding-tube system and limiting the maximum pellet speed to about 470 m/s. To match this boundary condition and fulfill the need for precise control, a centrifuge accelerator has been chosen. Based on the stop cylinder principle and equipped with a double accelerator arm, it can host up to six steady-state ice extruders working simultaneously for pellet production. This way, all system requirements expressed in the research plan can be well covered, providing even some headroom for better flexibility during the planned investigations. Details of our design and the reasoning for the layout chosen are provided in this paper.

Pellet-Injector Technology—Brief History and Key Developments in the Last 25 Years

High-speed injection of solid fuel was first proposed in 1954 as a possible solution to the problem of transporting fresh fuel across the confining magnetic fields into the plasma of a fusion reactor. While it took a few decades, the use of cryogenic pellets (typically H2 and D2) on fusion experiments became common place; most tokamaks and stellarators are now equipped with a pellet injector(s). These devices operate at low temperatures (~10 to 20 K) and most often use a simple light gas gun to accelerate macroscopic-size pellets (~0.4- to 6-mm diameter) to speeds of ~100 to 1000 m/s. Before the advantages of pellet injection from the magnetic high-field side (HFS) of a tokamak were recognized in 1997, development focused on increasing the pellet speed to achieve deeper plasma penetration and higher fueling efficiency. The HFS injection technique typically dictates slower pellets (~100 to 300 m/s) to survive transport through the curved guide tubes that route the pellets to the plasma from the inside wall of the device. Two other key operating parameters for plasma fueling are the pellet-injection repetition rate and time duration—a single pellet is adequate for some experiments and a steady-state injection rate of up to ~50 Hz is appropriate for others. In addition to plasma fueling, cryogenic pellets have often been used for particle transport and impurity studies in fusion experiments (most often with neon pellets). During the past two decades, a few new applications for cryogenic pellets have been developed and used successfully in plasma experiments: (1) one for edge-localized mode mitigation, (2) one for plasma disruption mitigation (requires large pellets that are shattered before injection into the plasma), and (3) another in which pure argon pellets are used to trigger runaway electrons in the plasma for scientific studies. In this paper, a brief history and the key developments in this technology during the past 25 years are presented and discussed.

Gas Gun Model and Comparison to Experimental Performance of Pipe Guns Operating with Light Propellant Gases and Large Cryogenic Pellets

Injection of multiple large (~10 to 30 mm diameter) shattered pellets into ITER plasmas is presently part of the scheme planned to mitigate the deleterious effects of disruptions on the vessel components. To help in the design and optimize performance of the pellet injectors for this application, a model referred to as “the gas gun simulator” has been developed and benchmarked against experimental data. The computer code simulator is a Java program that models the gas-dynamics characteristics of a single-stage gas gun. Following a stepwise approach, the code utilizes a variety of input parameters to incrementally simulate and analyze the dynamics of the gun as the projectile is launched down the barrel. Using input data, the model can calculate gun performance based on physical characteristics, such as propellant-gas and fast-valve properties, barrel geometry, and pellet mass. Although the model is fundamentally generic, the present version is configured to accommodate cryogenic pellets composed of H2, D2, Ne, Ar, and mixtures of them and light propellant gases (H2, D2, and He). The pellets are solidified in situ in pipe guns that consist of stainless steel tubes and fast-acting valves that provide the propellant gas for pellet acceleration (to speeds ~200 to 700 m/s). The pellet speed is the key parameter in determining the response time of a shattered pellet system to a plasma disruption event. The calculated speeds from the code simulations of experiments were typically in excellent agreement with the measured values. With the gas gun simulator validated for many test shots and over a wide range of physical and operating parameters, it is a valuable tool for optimization of the injector design, including the fast valve design (orifice size and volume) for any operating pressure (~40 bar expected for the ITER application) and barrel length for any pellet size (mass, diameter, and length). Key design parameters and proposed values for the pellet injectors for the ITER disruption mitigation systems are discussed.

SPINS-IND: Pellet injector for fuelling of magnetically confined fusion systems.

Using a Gifford-McMahon cycle cryocooler based refrigeration system, a single barrel hydrogen pellet injection (SPINS-IND) system is indigenously developed at Institute for Plasma Research, India. The injector is based on a pipe gun concept, where a pellet formed in situ in the gun barrel is accelerated to high speed using high pressure light propellant gas. The pellet size is decided by considering the Greenwald density limit and its speed is decided by considering a neutral gas shielding model based scaling law. The pellet shape is cylindrical of dimension (1.6 mm ℓ × 1.8 mm φ). For pellet ejection and acceleration, a fast opening valve of short opening duration is installed at the breech of the barrel. A three-stage differential pumping system is used to restrict the flow of the propellant gas into the plasma vacuum vessel. Diagnostic systems such as light gate and fast imaging camera (240 000 frames/s) are employed to measure the pellet speed and size, respectively. A trigger circuit and a programmable logic controller based integrated control system developed on LabVIEW enables to control the pellet injector remotely. Using helium as a propellant gas, the pellet speed is varied in the range 650 m/s-800 m/s. The reliability of pellet formation and ejection is found to be more than 95%. This paper describes the details of SPINS-IND and its test results.

State of the art and perspective of high-speed pellet injection technology

The injection of cryogenic pellets from the low field side (LFS) has long been in use for core fuelling of fusion devices, but injection from the high field side (HFS) has proved to provide a more effective core particle deposition, despite the severe limitations imposed to the pellet speed (≤300m/s) by inboard accessibility. In the future, an alternative approach may be that of injecting high-speed pellets from the HFS, through suitable “free-flight” paths, eliminating curved transfer systems. Furthermore, the expected length of the plasma discharges will require steady-state repetitive systems. ORNL and ENEA have been collaborating on high-speed injectors since 1990; they successfully realized a high-speed repeating pellet injector (2.55km/s at 1Hz). Since then, good progress has been achieved on both fronts of steady-state extruders, and operation and reliability of two-stage guns. A comprehensive R&D program is therefore proposed to investigate how far speed limits and repetition rates of combined two-stage guns and steady-state extruders technologies can be extended. Simulations results are presented showing pellet penetration for several injection locations on a tokamak under construction such as JT60-SA, on the basis of one set of design plasma parameters.