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A first-of-its-kind, inductively driven micro-particle (Pellet) accelerator and injector have been developed and operated successfully in ADITYA-U circular plasma operations, which may ably address the critical need for a suitable disruption control mechanism in ITER and future tokamak. The device combines the principles of electromagnetic induction, pulse power technology, impact, and fracture dynamics. It is designed to operate in a variety of environments, including atmospheric pressure and ultra-high vacuum. It can also accommodate a wide range of pellet quantities, sizes, and materials and can adjust the pellets' velocities over a coarse and fine range. The device has a modular design such that the maximum velocity can be increased by increasing the number of modules. A cluster of lithium titanate/carbonate (Li2TiO3/Li2CO3) impurity particles with variable particle sizes, weighing ∼50–200 mg are injected with velocities of the order of ∼200 m s−1 during the current plateau in ADITYA-U tokamak. This leads to a complete collapse of the plasma current within ∼5–6 ms of triggering the injector. The current quench time is dependent on the amount of impurity injected as well as the compound, with Li2TiO3 injection causing a faster current quench than Li2CO3 injection, as more power is radiated in the case of Li2TiO3. The increase in radiation due to the macro-particle injection starts in the plasma core, while the soft x-ray emission indicates that the entire plasma core collapses at once. [ABSTRACT FROM AUTHOR]

A first-of-its-kind, inductively driven micro-particle (Pellet) accelerator and injector have been developed and operated successfully in ADITYA-U circular plasma operations, which may ably address the critical need for a suitable disruption control mechanism in ITER and future tokamak. The device combines the principles of electromagnetic induction, pulse power technology, impact, and fracture dynamics. It is designed to operate in a variety of environments, including atmospheric pressure and ultra-high vacuum. It can also accommodate a wide range of pellet quantities, sizes, and materials and can adjust the pellets’ velocities over a coarse and fine range. The device has a modular design such that the maximum velocity can be increased by increasing the number of modules. A cluster of lithium titanate/carbonate (Li _2 TiO _3 /Li _2 CO _3 ) impurity particles with variable particle sizes, weighing ∼50–200 mg are injected with velocities of the order of ∼200 m s ^−1 during the current plateau in ADITYA-U tokamak. This leads to a complete collapse of the plasma current within ∼5–6 ms of triggering the injector. The current quench time is dependent on the amount of impurity injected as well as the compound, with Li _2 TiO _3 injection causing a faster current quench than Li _2 CO _3 injection, as more power is radiated in the case of Li _2 TiO _3 . The increase in radiation due to the macro-particle injection starts in the plasma core, while the soft x-ray emission indicates that the entire plasma core collapses at once.

Fabrication of the continuous pellet fueling system (CPFS) for long pulse operation of the Wendelstein 7-X (W7-X) stellarator is currently underway. Continuous fueling is accomplished by cutting pellets from the cross section of a continuous solid extrusion of desublimated fuel gas from a cryogenic twin-screw extruder coupled to a gas gun cutter mechanism supplied by the Oak Ridge National Laboratory (ORNL). The cryogenic cooling is provided by three Gifford–McMahon-type cryocoolers with a fixed cooling capacity. To maximize the pellet injection rate, the system heat loads must be minimized to allow the remaining cooling capacity to be used to desublimate the fuel gas. Viscous heating, caused by the shearing of the solid extrusion as it is forced through the screws and pumped through the nozzle, is the largest heat load uncertainty. A research and development extruder has been fabricated and tested at ORNL to reduce the viscous heating and verify the maximum achievable extrusion rate. This article will present the research and development of the extruder design, various screw and nozzle designs, and test results. [ABSTRACT FROM AUTHOR]

A continuous pellet fueling system (CPFS) is currently being designed at the Oak Ridge National Laboratory (ORNL) for the long pulse operation of the Wendelstein 7-X (W7-X) stellarator. The purpose of the CPFS is to provide deep continuous fueling for feedback-controlled high-density operation and mitigation of predicted hollow density profiles. The system will provide the capability to inject cylindrical pellets of solid hydrogen or deuterium into the plasma core, with flexibility to vary the pellet size, velocity, and injection frequency. Pellets are nominally of 3 mm in diameter and have a length between 1 and 4 mm. The heart of the CPFS is a vertically oriented, twin-screw extruder, cooled by three Gifford-McMahon cryocoolers in parallel, designed to form a continuous filament of hydrogen or deuterium. The filament width, which determines the pellet length, can be adjusted by means of a variable nozzle driven by a linear actuator at the base of the extruder. Coupled to the nozzle is a solenoid-operated gas gun and cutter assembly. A pneumatic propellant valve pulses a burst of ~60 bar helium to accelerate the cut pellet into W7-X. Three gaps in the guide tubes provide pumping locations to remove the helium propellant before it reaches the plasma. The maximum velocity of the pellet is limited by its ability to survive navigating the guide tube trajectory intact. A microwave cavity located within the guide tube provides the capability to measure the pellet size and velocity. The mechanical and thermal designs of the W7-X extruder, adjustable nozzle, and gun and cutter assembly design are described. A guide tube design and experimental pellet survivability test results are presented. [ABSTRACT FROM AUTHOR]

Abstract A single barrel pellet injection system is developed for pellet injection related studies targeting SST-1 tokamak plasma. Conventional pipe gun concept is adopted for designing the injector. The hydrogen ice is formed in-situ in the gun barrel, and a high pressure light propellant gas is used to dislocate the pellet from the freezing zone and accelerate it to higher speed. This system uses a cryogen free, closed loop compact cryocooler, which provides ease of handling and operational reliability to the pellet freezing process. Pellet parameters such as size and speed are considered by using the density limit and neutral gas shielding (NGS) model based calculations, respectively. The designed size and speed (V p) of the pellet is fixed at (1.6 mm length [ℓ] × 1.8 mm diameter [φ]) and V p < 800 m/s, respectively. A programmable logic controller (PLC) based control system and a graphical user interface (GUI) is developed and installed to operate the injector remotely. This paper presents the design and test results of the injector such as pellet freezing capability, effect of temperature gradient along the barrel on pellet formation process and its speed control with propellant pressure. [ABSTRACT FROM AUTHOR]

Cryogenic deuterium pellet injection is one of important techniques for edge localized mode (ELM) mitigation for successful operation of ITER in the future. A high frequency, 50 Hz, pellet injection system (PI-50) based on screw extrusion forming ice and pneumatic acceleration has been built and developed for ELM mitigation for EAST. As velocity and reliability of launching frozen pellet are of importance to ELM mitigation, a diagnostic system including video cameras is installed on this pellet injection system to acquire the pellet's velocity and monitor the quality of pellet. With the help of this diagnostic system, the velocity and reliability of PI-50 system have been measured successfully. The reliability of PI-50 is more than 85% when the launching frequency is 50 Hz, which means this system can be used for ELM mitigation on EAST in the future. PI-50 system has been installed on EAST at present and is expected to be involved physics experiment in next campaign on EAST. [ABSTRACT FROM AUTHOR]

New fueling systems have been designed for WEST, a full tungsten wall tokamak with divertor configuration. They consist of standard gas injection, supersonic molecular beam injection and pellets injection systems, each being able to fuel continuously the plasma discharge up to a duration of 1000s. A massive gas injection system is also implemented to mitigate the impact of disruptions and runaway electrons. The standard gas system consists of 4 independent lines allowing the injection of up to 4 different gas species during a plasma discharge through 11 injections areas, 4 of them being located in the divertors and in the private flux region. The pellet injection system is able to inject up to 10 pellets per second through 4 tracks: one on the low field side, and the other 3 on the high field side. The supersonic molecular beam injection system consists of 3 injectors located close to the midplane, two on the high field side and one on the low field side. On trigger, the massive gas injection system can deliver in a single puff up 800 Pa.m3 of noble gas in order to stop the plasma discharge. All the fueling systems have been characterized and can be used routinely on plasma. [ABSTRACT FROM AUTHOR]

The paper presents high frequency pellet injectors developed for edge localized mode mitigation and plasma fuelling of the EAST and KSTAR tokamaks. Each pellet injector is able to inject solid deuterium or hydrogen pellets at steady state mode. Injectors consist of modules including screw extruder cooled by liquid helium and pneumatic punchers for pellet fabrication and acceleration which are optimized to reach maximal injection frequency for specified pellet parameters. The EAST pellet injector is capable of injecting 1.5 mm diameter and 1.2–1.8 mm length pellets at frequency up to 50 Hz with velocities 200–250 m/s using two injection modules working in an alternating mode at 25 Hz each. Injection reliability over 90% has been confirmed during several cycles of continuous D2 pellet injection at 50 Hz. The KSTAR pellet injector has been designed to inject 2 mm size pellets at frequency up to 20 Hz and velocity 200 m/s. Due to improved design of pellet fabrication and acceleration system the world record in continuous injection of 20,164 deuterium pellets (Ø2.0 × 1.5 mm) for 1039 s at 20 Hz with reliability 97% has been achieved. [ABSTRACT FROM AUTHOR]

A new high frequency pellet injector has been developed for edge localized mode mitigation and plasma fuelling of the EAST tokamak. The pellet injector consists of two independent modules operating at 1–25 Hz. Both modules are equipped with screw extruders cooled by liquid helium for solid deuterium ice production at steady state mode. An innovative pellet fabrication and acceleration system has been designed and applied to reduce gas load on the extruders in 4–5 times and to increase the pellet injection reliability. Pellets of 1.5 mm diameter and 1.5 mm length were injected at 250 ± 50 m/s velocity at 25 Hz with reliability over 93% using one module in preliminary tests. Two modules working in turn are under tests now. It is anticipated they will be able to inject pellets at 50 Hz after further technical improvements in near future. [ABSTRACT FROM AUTHOR]

A table-top pellet injector (TATOP) has been designed to fulfill the following scientific aims: to study the ELM triggering potential of impurity pellets, and to make pellet injection experiments comparable over several fusion machines. The TATOP is based on a centrifugal accelerator therefore the complete system is run in vacuum, ensuring the compatibility with fusion devices. The injector is able to launch any solid material (stable at room temperature) in form of balls with a diameter in the 0.5–1.5 mm range. The device hosts three individual pellet tanks that can contain e.g. pellets of different materials, and the user can select from those without opening the vacuum chamber. A key element of the accelerator is a two-stage stop cylinder that reduces the spatial scatter of pellets exiting the acceleration arm below 6°, enabling the efficient collection of all fired pellets. The injector has a maximum launch speed of 450 m/s. The launching of pellets can be done individually by providing TTL triggers for the injector, giving a high level of freedom for the experimenter when designing pellet trains. However, the (temporary) firing rate cannot be larger than 25 Hz. TATOP characterization was done in a test bed; however, the project is still in progress and before application at a fusion oriented experiment. [ABSTRACT FROM AUTHOR]

Abstract: A new hydrogen/deuterium pellet injector has been developed for Experimental Advanced Superconducting Tokamak (EAST). The pellet injector based on a screw extruder is able to fire pellets (∅2mm×2mm; frequency 1–10Hz and velocity 150–300m/s) in steady state mode with reliability greater than 95%. An injection line was designed for pumping propellant gas and for diagnostic purpose also. A guide tube for magnetic high-field side (HFS) injection was developed and theoretical calculation has been done. After successful engineering commissioning, the injection system served at EAST 2012 campaign and first experimental results were obtained. [Copyright &y& Elsevier]

Abstract: Three innovative repetitive and multi-barrel deuterium ice pellet injectors have been designed for plasma investigations in fusion facilities. The pellet injector for JET can inject small pellets (variable volumes within 1-2mm3) at frequency up to 67Hz and velocity 80–200m/s for ELM (edge localised modes) control and large pellets (volume within 35–70mm3) at frequency up to 15Hz and velocity 100–300m/s for plasma fueling. A new extrusion nozzle has been designed and tested to improve the extrusion stability and injection reliability. The pellet injector for HL-2A has a piston type extruder with a variable cross-section. It allows pushing out deuterium ice rod from the extruder at velocity up to 100mm/s and injecting a limited number of pellets 1.3mm diameter and variable length in the range 1.3–1.7mm at frequency up to 30Hz for plasma fueling. The pellet injector for fueling of a facility with theta pinch plasma is capable to inject one pellet with variable length from 5 to 15mm and diameter 1mm with extremely low velocity of about 40–50m/s, followed by eleven pellets with variable length from 1 to 3mm and diameter 0.9mm at velocities 100–200m/s. A novel gas valve with in-built punch mechanism has been developed to simplify the injector design. The design and distinct features of these three injectors are presented. [Copyright &y& Elsevier]

Abstract: A main challenge for fusion research oriented pellet launcher systems is the control of the plasma edge region, especially of the possibly dangerous edge localized modes (ELMs) observed in high performance H-mode plasmas. In order to improve the experimental data base required for extrapolation, a novel cryogenic multi-pellet injection system was developed for ASDEX Upgrade, which is entirely dedicated to control and mitigation of type-I ELMs. Based on the blower gun principle, it is capable of combining high repetition rates up to 143Hz and low pellet sizes and velocities. Expected benefits are an enhanced tokamak operational area and the option for more sophisticated experiments investigating the underlying physical mechanisms. This paper reports on design criteria, construction, extensive laboratory tests and first injection into ASDEX Upgrade plasmas. [Copyright &y& Elsevier]

A control and data acquisition system has been developed for the Mega-Amp Spherical Tokamak (MAST) multi-barrel pellet injector to allow reliable operation with multiple pellet sizes. The system operates with one PC running Windows NT with four embedded PCI cards. All measurements and control functions are implemented on a PC. The system software consists of two programs. The first program provides pellet firing and registration of up to four fast signals associated with each pellet–pellet speed, mass, and other signals. The second program controls pellet preparation and all the pellet injector equipment. This program is realised as state machine—allowing a sequence of any number of states, it includes independent digital PID control of the temperature of each barrel. Both programs run conjointly using message exchange. The system is integrated and synchronised with the MAST control and data acquisition systems. [Copyright &y& Elsevier]