Pellet Injection Research Articles

Neutral gas and plasma shielding (NGPS) model and cross-field motion of ablated material for hydrogen–neon mixed pellet injection

A neutral-gas and plasma shielding (NGPS) model is applied for cryogenic hydrogen–neon mixed pellet injection used for the mitigation of tokamak disruptions. The NGPS model is useful to evaluate the characteristics of ionized plasmoid (size, density, temperature, and radiation), while reproducing the scaling of the neutral gas shielding model. It is found that even if one takes into account the optical thickness for radiation, the energy loss due to line radiation associated with neon is strong enough to limit the temperature and the pressure of the ionized plasmoid for pure neon or hydrogen–neon mixed pellets in the early phase of the material homogenization. Consequently, the ionized plasmoid of the neon mixed pellets is expected to homogenize along the magnetic field line where the ablated material is released—without significantly making the cross-field drift motion—as compared to pure hydrogen pellets.

Overview of the COMPASS results * *In the future we will refer to the author list of the paper as the COMPASS Team.

COMPASS addressed several physical processes that may explain the behaviour of important phenomena. This paper presents results related to the main fields of COMPASS research obtained in the recent two years, including studies of turbulence, L–H transition, plasma material interaction, runaway electron, and disruption physics: Tomographic reconstruction of the edge/SOL turbulence observed by a fast visible camera allowed to visualize turbulent structures without perturbing the plasma.Dependence of the power threshold on the X-point height was studied and related role of radial electric field in the edge/SOL plasma was identified.The effect of high-field-side error fields on the L–H transition was investigated in order to assess the influence of the central solenoid misalignment and the possibility to compensate these error fields by low-field-side coils.Results of fast measurements of electron temperature during ELMs show the ELM peak values at the divertor are around 80% of the initial temperature at the pedestal.Liquid metals were used for the first time as plasma facing material in ELMy H-mode in the tokamak divertor. Good power handling capability was observed for heat fluxes up to 12 MW m−2 and no direct droplet ejection was observed.Partial detachment regime was achieved by impurity seeding in the divertor. The evolution of the heat flux footprint at the outer target was studied.Runaway electrons were studied using new unique systems—impact calorimetry, carbon pellet injection technique, wide variety of magnetic perturbations. Radial feedback control was imposed on the beam.Forces during plasma disruptions were monitored by a number of new diagnostics for vacuum vessel (VV) motion in order to contribute to the scaling laws of sideways disruption forces for ITER.Current flows towards the divertor tiles, incl. possible short-circuiting through PFCs, were investigated during the VDE experiments. The results support ATEC model and improve understanding of disruption loads.

Advances in physics and applications of 3D magnetic perturbations on the J-TEXT tokamak

In the last two years, three major technical improvements have been made on J-TEXT in supporting of the expanded operation regions and diagnostic capabilities. (1) The successful commission of the 105 GHz/500 kW/1 s electron cyclotron resonance heating (ECRH) system increasing the core electron temperature from 0.9 keV up to around 1.5 keV. (2) The poloidal divertor configuration with an X-point in the high-field side has been achieved. In particular, the 400 kW electron cyclotron wave has also been successfully injected into the diverted plasma. (3) A 256-channel electron cyclotron emission imaging diagnostic system and two sets of four-channel Doppler backscattering diagnostics have been successfully developed on J-TEXT, allowing detailed measurement of the electron temperature and density fluctuations for turbulence and MHD research. The locked mode (LM), especially the 2/1 LM, is one of the biggest threats to the plasma operation. Both the thresholds of 2/1 and 3/1 LM are observed to vary non-monotonically on electron density. The electrode biasing was applied successfully to unlock the LM from either a rotating or static resonant magnetic perturbation (RMP) field. In the presence of 2/1 LM, three kinds of standing wave (SW) structures have been observed to share a similar connection to the island structure, i.e. the nodes of the SWs locate around the O- or X-points of the 2/1 island. The control and mitigation of disruption is essential to the safe operation of ITER, and it has been systematically studied by applying a RMP field, massive gas injection (MGI) and shattered pellet injection on J-TEXT. When the RMP-induced 2/1 LM is larger than a critical width, the MGI shutdown process can be significantly influenced. If the phase difference between the O-point of LM and the MGI valve is +90° (or −90°), the penetration depth and the assimilation of impurities can be enhanced (or suppressed) during the pre-thermal quench (TQ) phase and result in a faster (or slower) TQ. A secondary MGI can also suppress the runaway electron (RE) generation, if the additional high-Z impurity gas arrives at the plasma edge before TQ. When the secondary MGI has been applied after the formation of the RE current plateau, the RE current can be dissipated, and the dissipation rate increases with the injected impurity quantity but saturates with a maximum of 28 MA s−1. The non-local transport is experimentally observed in the ion transport channel. The electron thermal diffusivity significantly increases with the ECRH power. Theoretical work shows that significant intrinsic current can be driven by electromagnetic turbulence, and the robust formation mechanism of the E × B staircase is identified from the Hasegawa–Wakatani system.

Numerical simulation of fueling pellet ablation and transport in the EAST H-mode discharge

To understand the effect of injected deuterium (D) pellets on background plasma, the ablation of D pellets and the transport of D species in both atomic and ionic states in the EAST device are simulated using a modified dynamic neutral gas shield model combined with the edge plasma code SOLPS-ITER. The simulation results show that there is a phenomenon of obvious atomic deposition in the scrape-off layer (SOL) after pellet injection, which depends strongly on the injection velocity. With increasing injection velocity, the atomic density in the SOL decreases evidently and the deposition time is relatively shortened. Possible effects for triggering of edge localized modes (ELMs) by D and Li pellets are also discussed. With the same pellet size and injection velocity, the maximum perturbation pressure caused by D pellets is obviously higher. It is found that the resulting maximum perturbed pressure is remarkably enhanced when the injection velocity is reduced from 300 m/s to 100 m/s for a pellet with a cross section of 1.6 mm, which indicates that the injection velocity is important for ELM pacing. This work can provide reasonable guidance for choosing pellet parameters for fueling and ELM triggering.

Overview of the FTU results

Since the 2018 IAEA FEC Conference, FTU operations have been devoted to several experiments covering a large range of topics, from the investigation of the behaviour of a liquid tin limiter to the runaway electrons mitigation and control and to the stabilization of tearing modes by electron cyclotron heating and by pellet injection. Other experiments have involved the spectroscopy of heavy metal ions, the electron density peaking in helium doped plasmas, the electron cyclotron assisted start-up and the electron temperature measurements in high temperature plasmas. The effectiveness of the laser induced breakdown spectroscopy system has been demonstrated and the new capabilities of the runaway electron imaging spectrometry system for in-flight runaways studies have been explored. Finally, a high resolution saddle coil array for MHD analysis and UV and SXR diamond detectors have been successfully tested on different plasma scenarios.

On the role of preexisting MHD activity for the plasma response to massive deuterium injection

As part of a reliable disruption mitigation system (SPI) for ITER, pure deuterium shattered pellet injection (SPI) has been proposed as a way of avoiding hot tail runaway electron generation. It offers the possibility of diluting the plasma and, thereby, cooling it down by a large factor without immediately triggering a thermal quench (TQ). However, the reliability of this and similar SPI approaches could be reduced by preexisting MHD modes, which are usually present during the pre-TQ phase, when the disruption mitigation scheme is being triggered. To address this question, this theoretical study investigates massive deuterium injection into an MHD active ASDEX Upgrade plasma using the non-linear MHD code JOREK. Cases with and without preexisting 2/1 islands are studied. Scans are performed in the preexisting island size, the number of atoms injected, and the relative phase of the injection location with respect to the island. Realistic values of resistivity and heat diffusion anisotropy are considered. This provides insights into the physical mechanisms at play and the relevant time scales involved. Results largely indicate that plasma dilution by deuterium also seems to work reliably in the presence of preexisting MHD activity. Nevertheless, when injecting in phase with the X-point of a large preexisting island, the TQ can occur earlier than without. Altogether, simulations increase confidence in the reliability of plasma dilution by deuterium injection and its applicability to ITER.

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.

Progress of engineering design of CFETR vacuum systems

Vacuum systems for the China Fusion Engineering Test Reactor (CFETR), including pumping system, fueling system, wall conditioning, as well as their control systems, are required to achieve high vacuum, effective plasma fueling, and good wall condition for steady-state plasma operation and to facilitate remote flexible control with high reliability to ensure safe operation. In the last few years, based on the requirements of vacuum systems and subsystems of CFETR, their features and challenges have been analysed, basic parameters have been defined, and preliminary designs have been developed. Herein, nine cryopumps with pumping speed of 68 m³/s for D2 in a molecular flow regime are proposed for use in a torus vacuum chamber to achieve vacuum pressure <1.0 × 10−5 Pa before plasma operation. The throughput of the torus pumping system is designed to be >290 Pa•m³•s−1 to meet the requirement of steady-state operation of CFETR discharge. Pellet injectors and the gas injection system for seeding the fuel and impurity gas and wall conditionings, including baking, GDC, and ICRF-DC, for removal of hydrogen isotopes/impurities of the torus vacuum are also designed. Moreover, integrated design of vacuum systems and space layout of all subsystems are also investigated.

Ten hertz bead pellet injection and laser engagement

A laser inertial fusion energy (IFE) reactor requires repetitive injection of fuel pellets and laser engagement to fuse fusion fuel beyond a few Hz. We demonstrate 10 Hz free-fall bead pellet injection and laser engagement with γ-ray generation. Deuterated polystyrene beads with a diameter of 1 mm were engaged by counter illuminating ultra-intense laser pulses with an intensity of 5 × 1017 W cm−2 at 10 Hz. The spatial distribution of free-fall beads was 0.86 mm in the horizontal direction and 0.18 mm in the vertical direction. The system operated for more than 5 min and 3500 beads were supplied with achieved frequencies of 2.1 Hz for illumination on the beads and 0.7 Hz for γ-ray generation; these frequencies were three times greater than with the previous 1 Hz injection system. The duration of operation was limited by the pellet supply. This injection and engagement system could be used for laser IFE research platforms.

Physics of runaway electrons with shattered pellet injection at JET

Runaway electrons (REs) created during tokamak disruptions pose a threat to the reliable operation of future larger machines. Experiments using shattered pellet injection (SPI) have been carried out at the JET tokamak to investigate ways to prevent their generation or suppress them if avoidance is not sufficient. Avoidance is possible if the SPI contains a sufficiently low fraction of high-Z material, or if it is fired early in advance of a disruption prone to runaway generation. These results are consistent with previous similar findings obtained with Massive Gas Injection. Suppression of an already accelerated beam is not efficient using High-Z material, but deuterium leads to harmless terminations without heat loads. This effect is due to the combination of a large magnetohydrodynamic instability scattering REs on a large area and the absence of runaway regeneration during the subsequent current collapse thanks to the flushing of high-Z impurities from the runaway companion plasma. This effect also works in situations where the runaway beam moves upwards and undergoes scraping-off on the wall.

Turbulence mitigation in maximum-J stellarators with electron-density gradient

In fusion devices, the geometry of the confining magnetic field has a significant impact on the instabilities that drive turbulent heat loss. This is especially true of stellarators, where the density-gradient-driven branch of the ‘trapped electron mode’ (TEM) is predicted to be linearly stable if the magnetic field has the maximum-J property, as is very approximately the case in certain magnetic configurations of the Wendelstein 7-X experiment (W7-X). Here we show, using both analytical theory and simulations, that the benefits of the optimisation of W7-X also serve to mitigate ion-temperature-gradient (ITG) modes as long as an electron density gradient is present. We find that the effect indeed carries over to nonlinear numerical simulations, where W7-X has low TEM-driven transport, and reduced ITG turbulence in the presence of a density gradient, giving theoretical support for the existence of enhanced confinement regimes, in the presence of strong density gradients (e.g. hydrogen pellet or neutral beam injection).

First Results From Simulations of Rapid Shutdown With Neon Deposition in J-TEXT Rotating Plasmas

Encapsulated payload pellet injection (EPPI) has been demonstrated to be an effective method of disruption mitigation system (DMS) through a number of experiments and simulations in DIII-D and JET. This article investigates the effects of plasma rotation on impurity spreading under neon EPPI. Our analysis of simulations shows that the magnetic surface stochastization has a decisive influence on the core-oriented spread of impurity. The MHD instabilities increase with impurity ionization, especially of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n = 1$ </tex-math></inline-formula> component, leading to the enhancement of the local transport of impurities along the stochastic field lines. In the rotating plasma, the toroidal rotation can suppress the magnetic perturbation, and then, the boundary-oriented spreading of impurities is reduced. When the pellet source is toroidally localized at the magnetic axis, the rotation can quickly flatten the toroidal distribution of impurities, resulting in a smaller maximum radiated power and a lower toroidal peek factor (TPF).

Ultrasound-guided percutaneous implantation of rabbit VX2 carcinoma, using a coaxial technique and gelfoam pellet injection combination to establish a rabbit liver tumor model.

PURPOSE We aimed to investigate the safety and tumor seeding rate of a coaxial implantation technique combined with injection of a gelfoam pellet in establishing a VX2 liver tumor model in rabbits. METHODS A VX2 liver tumor model was established in 60 male New Zealand white rabbits, which were randomly divided into 3 groups (20 in each group) based on implantation technique (all performed under ultrasound guidance): group A, single needle only; group B, single needle with injection of a gelfoam pellet; or group C, coaxial technique with injection of a gelfoam pellet. The rates of liver tumor formation and tumor seeding to extrahepatic tissues were compared 2 weeks after implantation. Data were also collected regarding procedure time, number of punctures, occurrence of complications, and mortality rate. RESULTS A VX2 liver tumor model was established in all 60 rabbits (100%, 60/60). Ectopic implantation rate was 70% (14/20) in group A, 35% (7/20) in group B, and 5% (1/20) in group C, with significant difference among the groups (p < 0.001). Post hoc analysis showed significant difference between group A and group C (p < 0.001). However, there were no significant differences between group B and group A or group C (p = 0.027, p = 0.048, respectively). There were no significant differences among the groups in terms of procedure time (p = 0.405) or number of punctures (p = 0.612). No complications or deaths occurred. CONCLUSION A coaxial technique with injection of a gelfoam pellet is an effective and safe method for VX2 liver tumor implantation in rabbits, and this technique can reduce the risk of tumor seeding to the abdominal wall and omentum.

Cell morphologies, mechanical properties, and fiber orientation of glass fiber-reinforced polyamide composites: Influence of subcritical gas-laden pellet injection molding foaming technology

Advanced materials and new lightweighting technologies are essential for boosting the fuel economy of modern automobiles while maintaining performance and safety. A novel approach called subcritical gas-laden pellet injection molding foaming technology (SIFT) was performed to produce foamed polyamide/glass fiber (PA/GF) composite. Gas-laden pellets loaded with nitrogen (N2) were produced by introducing sub-critical N2 into PA/GF composite during compounding using a twin-screw extruder equipped with a simple gas injection unit. Compared to the commercial microcellular injection molding (MIM) technologies, gas-laden pellets enable the production of foamed parts with a standard injection molding machine, which is more cost-effective and easier to implement. To the best of our knowledge, this is the first attempt that the SIFT technology is being used for the PA/GF composites for making foamed parts. The tensile strength, fiber orientation, cell morphology, and densities of foamed PA/GF parts were investigated, and the shelf life of N2-laden PA/GF pellets was examined. Results showed that the N2-laden pellets still possessed good foaming ability after one week of storage under ambient atmospheric conditions. One week is a noticeable improvement compared to those N2-laden neat polymer pellets without glass fibers. With this approach, the weight reduction of foamed PA/GF parts was able to reach 12.0 wt. %. Additionally, a nondestructive analysis of the fiber orientation using micro-computed tomography suggested that the MIM and SIFT samples exhibited a less degree of fiber orientation along the flow direction when compared to the solid samples and that the tensile strength of both technologies was very close at a similar weight reduction. Cell size increased and cell density decreased as the shelf life increased. These findings showed that this processing method could act as an alternative to current commercial foam injection molding technology for producing lightweight parts with greater design freedom.

Pellet triggering of edge localized modes in low collisionality pedestals at DIII-D

Edge localized modes (ELMs) are triggered using deuterium pellets injected into plasmas with ITER-relevant low collisionality pedestals, and the resulting peak ELM energy fluence is reduced by approximately 25%–50% relative to natural ELMs destabilized at similar pedestal pressures. Cryogenically frozen deuterium pellets are injected from the low-field side of the DIII-D tokamak at frequencies lower than the natural ELM frequency, and heat flux is measured by infrared cameras. Ideal MHD pedestal stability calculations show that without pellet injection, these low collisionality pedestals were limited by their current density (peeling-limited) rather than their pressure gradient (ballooning-limited). ELM triggering success correlates strongly with pellet mass, consistent with the theory that a large pressure perturbation is required to trigger an ELM in low collisionality discharges that are far from the ballooning stability boundary. For sufficiently large pellets, both instantaneous and time-integrated ELM energy deposition measured by infrared cameras is reduced with respect to naturally occurring ELMs at the inner strike point, which is the position where it is largest for natural ELMs. Energy fluence at the outer strike point is less effected. Cameras observing both heat flux and D-alpha emission often find significant toroidally asymmetric striations in the outboard far scrape-off layer resulting from ELMs that are triggered by pellets. Toroidal asymmetries at the inner strike point are similar between natural and pellet-triggered ELMs, suggesting that the reduction in peak heat flux and total fluence at that location is robust for the conditions reported here.

Shattered pellet injection experiments at JET in support of the ITER disruption mitigation system design

A series of experiments have been executed at JET to assess the efficacy of the newly installed shattered pellet injection (SPI) system in mitigating the effects of disruptions. Issues, important for the ITER disruption mitigation system, such as thermal load mitigation, avoidance of runaway electron (RE) formation, radiation asymmetries during thermal quench mitigation, electromagnetic load control and RE energy dissipation have been addressed over a large parameter range. The efficiency of the mitigation has been examined for the various SPI injection strategies. The paper summarises the results from these JET SPI experiments and discusses their implications for the ITER disruption mitigation scheme.

Post-thermal-quench shattered pellet injection for runaway electron seed depletion in ITER

The possibility of using shattered pellet injection after the thermal quench of an ITER disruption in order to deplete runaway electron (RE) seeds before they can substantially avalanche is studied. Analytical and numerical estimates of the required injection rate for shards to be able to penetrate into the forming RE beam and stop REs are given. How much material could be assimilated before the current quench (CQ) becomes too short is also estimated. It appears that, if hydrogen pellets were used, the required number of pellets to be injected during the CQ would be prohibitive, at least considering the present design of the ITER disruption mitigation system (DMS). For neon or argon, the required number of pellets, although large, might be within reach of the ITER DMS, but the assimilated fraction would have to be very small in order not to shorten the CQ excessively. This study suggests that other injection schemes, based for example on small tungsten pellets coated with a low Z material, may be worth exploring as an option for an upgrade of the ITER DMS.

Numerical prediction of the operating point for the cryogenic twin-screw hydrogen extruder system

An extruder-type cryogenic pellet injector system for plasma fuelling is under research at Institute for Plasma Research (IPR), India. A twin-screw extruder is a most stable device in the form of closed volumes of hydrogen between screw threads and promising process due to continuous production of hydrogen pellet for plasma reactor. In the present investigation, a non-Newtonian, non-isothermal CFD model was developed which modeled the shear rate and temperature-dependent viscosity in the Herschel Bulkley model. Brinkman number (Br) varies from 0.15 to 0.46 at 5 and 15 rpm respectively. Due to increasing the screw rotation speed for a given throughput, the shear rate increases (therefore, also the Br) and results in larger viscous dissipation. Solid hydrogen temperature is predicted at the location of the flight gap and intermeshing zone for the different barrel temperatures under varying screw rotation speeds (5–15 rpm). Prediction of hydrogen temperature at the intermeshing zone of the extruder system plays a vital role to decide the barrel temperature. Due to more number of moving surfaces at the intermeshing zone of the extruder system, the temperature of solid hydrogen is higher and increases above the melting point temperature. The effect of barrel wall temperature and screw rotation speeds (5–15 rpm) on the viscous dissipation rate and pressure development at required throughput was also examined for an extruder system. It is important to estimate the barrel wall temperature to get an operating point and corresponding viscous dissipation rate, which determines the cooling load required for extruder-die set-up. The results show that as the die wall temperature increases, the viscous dissipation, as well as the pressure drop decreases in the die. The investigation showed that the die’s wall temperature and screw rotation speed affect the operating point of the extruder-die system.

Fast visible camera diagnostic for dual shattered pellet injections at KSTAR

Direct imaging from fast visible cameras can provide intuitive information for the detailed dynamics of a fusion plasma. Images with different toroidal positions help our understanding of the 3D structure of the plasma phenomena. Fast-Visible camera (FVC) system was installed in KSTAR to investigate the assimilation process during plasma disruptions induced by Shattered Pellet Injections (SPIs). Since these events are highly non-axisymmetric and good toroidal coverage and high spatial and temporal resolution is required, the field of view of each camera is oriented towards the injection tube of each of the two SPIs, which are located toroidally opposite to each other. To investigate fragment penetration and ablation using images from a FVC, all components of the system should be able to support a fast frame rate due to the speed of shattered fragments of several hundreds of meter per second. The KSTAR FVC system typically uses a complementary metal-oxide-semiconductor (CMOS) with 10,000 frame per second at a resolution of 832 × 600 pixel typically for SPI experiments and its maximum frame rate is up to 600,000 frames per second with 64 × 8 pixels resolution. A hybrid optical system that consists of 2.04 m long relay optics mounted in the viewport cassette and a 4.5 m long imaging fiber bundle secures sufficient distance to reduce the harmful effects on the sensitive fast camera from radiation, neutron impact, and strong magnetic field. This paper reports the installation of the FVC system on KSTAR is introduced and semi three-dimensional observations of disruption mitigation processes during SPIs.

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.