Pellet Speed Research Articles

Pellet fueling technology development leading to efficient fueling of ITER burning plasmas

Pellet injection is the primary fueling technique planned for core fueling of ITER [ITER Technical Basis 2002 ITER EDA Documentation Series (Vienna: IAEA)] burning plasmas. Efficient core plasma fueling with deuterium and tritium D–T is a requirement for achieving high fusion gain and it cannot be achieved with gas fueling. Injection of pellets from the inner wall has been shown on present day tokamarks to provide efficient fueling and is planned for use on ITER. Modeling of the fueling deposition from inner wall pellet injection using the Parks E×B drift model indicates that pellets have the capability to fuel well inside the separatrix. Gas fueling calculations show very poor neutral penetration due to the high density and wide scrape off layer. Isotopically mixed D–T pellets can provide efficient tritium fueling that will minimize tritium wall loading when compared to gas puffing. Currently the performance of the ITER inner wall guide tube design is under test with initial results indicating that pellet speeds in excess of 300 m/s will lead to fragmented pellets. The ITER pellet injection technology requirements and remaining development issues are discussed along with a plan to reach the design goal for employment on ITER.

Interaction of Impurity (Li, Be, B and C) and Hydrogen Isotope Pellet Injection with Reactor-relevant Plasmas

Based on the two-dimensional kinetic ablation theory of the hydrogen pellet ablation developed by Kuteev [B.V. Kuteev, Nuclear Fusion, 35 (1995) 431], an algorithm of erosion speed and ablation rate calculations for Li, Be, and B impurity pellets in reactor-relevant plasma has been derived. Results show compatibilities of lithium pellet injection used in α-particle diagnostics are positive in comparison with other solid impurity pellets (e.g. Be, B and C). Using the 2-D Kuteev lentil model, including kinetic effects, we find that currently existing pellet injection techniques will not meet core-fueling requirements for ITER-FEAT. A pressure as high as 254 MPa must be applied to a pellet accelerator with a 200 cm-long single-stage pneumatic gun, in order to accelerate a pellet with a radius rp0 = 0.5 cm to a velocity of νp0, 24 × 105 cm/s penetrating 100 cm into the ITER plasma core. Comparisons of pellet velocity- and radius-dependent penetration depth between the Neutral Gas Shielding and the Kuteev's models are made. However, we find that the isotopic effects can lead to a 33% lower pellet speed for solid DT, compared to an identical H2 pellet penetrating the same length in ITER-FEAT plasma, and our calculations show that HFS injection will much improve core fueling efficiency.

Gestural stability in vowels

In accordance with proper perception of linguistic sound units, past research has demonstrated some degree of acoustic and physiological stability. In contrast, articulatory stability has been thought to be inconsistent because articulations may vary so long as the vocal tract area function results in appropriate formant structure [Atal et al., J. Acoust. Soc. Am. 63, 1535–1555 (1978)]. However, if the area function for the constriction and its anterior region can maintain acoustic stability, articulatory stability should be observed in the relational behavior of four tongue pellets used in xray microbeam data. Previous work examined normalized pellet data in order to arrive at an average posture for each vowel [Hashi et al., J. Acoust. Soc. Am. 104, 2426–2437 (1998)]. But by assuming static (average) gestures, the research fell short of a correct postural characterization. This study of tongue pellet speed and normalized pellet displacement of front vowels spoken by ten microbeam database subjects reports that the tongue tip pellet speed maxima identify vowel edges (end of vowel onset, beginning of offset) while displacement of the three anterior pellets identify changes in formant structure (e.g., two stable regions in the Northern Cities English front low vowel).

Triboelectric Charging Between Polytetrafluoroethylene and Metals

Measurements were obtained on charge exchange processes occurring between small metal ballistic pellets and a polytetrafluoroethylene (PTFE) tube. Lead pellets of 5.5-mm diameter were propelled at a range of speeds through PTFE tubes of different lengths by compressed carbon dioxide. The charge incurred by the pellets and the tube was compared with the charge observed on copper and lead spheres rolled through the tube, driven by gravity. In all experiments, the charge on the moving pellet was measured with a Faraday cup. The experiments determined the effect of pellet speed on the magnitude of the charge accumulated on the pellet and on the PTFE tube. Sectioned shielding on the PTFE tube allowed the determination of surface charge distribution along its length by means of an electrometer. It was observed that the charge also depended on the work functions of the materials involved (lead, copper, PTFE). The charge on the pellets was found to range from +0.5/spl times/10/sup -8/ to +3.0/spl times/10/sup -8/ C, for pellet speeds from 10 to 80 m/spl middot/s/sup -1/. The reproducibility of results is discussed and comment provided on the degree of charge imbalance observed.

A system for cryogenic hydrogen pellet high speed inboard launch into a fusion device via guiding tube transfer

Particle deposition deep inside the hot target plasma column by cryogenic hydrogen pellet injection is required for efficient particle refueling of fusion devices such as tokamaks. As the ablation plasmoid is subject to a strong outward drift in hot plasmas, pellet launch from the tokamak inboard side is more useful than from the outboard. The depth of the pellet particle deposition depends on density and temperature of the target plasma, and on the pellet mass and velocity. Plasma operation determines density and temperature values, the maximum affordable density perturbation limits the pellet mass. Consequently, the pellet speed remains the only technically variable parameter allowing improvement of the refueling performance. To achieve this an inboard high-speed pellet injection system based on looping type geometry was designed and built at the midsize tokamak ASDEX Upgrade, and first fueling studies had validated the potential for the required injection velocity increase. Throughout the last two years experimental efforts focused on careful step-by-step optimization of the different system hardware components and the operational procedures. Introducing amongst other features a well pumped, rectangular guide track section, the feasibility for the inboard launch scheme up to an injection velocity of 1 km/s was successfully demonstrated. Detailed off-line tests have confirmed that pellets can withstand controlled mechanical and thermal impact even at this high speed, albeit for the sacrifice of increasing and significant mass losses. In a first application to plasma refueling deep penetration into hot target plasmas and hence, high fueling performance was achieved by deeper pellet born particle deposition and hence enhanced particle sustainment times.

A repetitive two-stage gas gun with porous cell cryostat pellet generator for continuous high-speed fuelling

In order to respond to the next generation Tokamak need, PELIN Laboratory at Saint Petersburg, CEA-DRFC at Cadarache and CEA-SBT at Grenoble started in 2000 a tripartite collaboration to promote new pellet injector technologies. The aim was to merge the laboratories knowledge to provide pellet injectors able to deliver pellets, either at low velocities (0.2–1 km/s) and high frequencies (up to 20 Hz) or at high velocities (>3 km/s) and low frequency (0.1 Hz) in steady state mode. Researches and developments are, respectively, carried out at Saint Petersburg and at Grenoble. This paper reports the first results obtained at Grenoble on a repetitive two-stage gas gun (TSGG) coupled to a porous cell pellet generator. Such a TSGG was developed in 1992 by SBT for the single shot mode. Several modifications have been made to reach the repetitive mode. The TSGG breech peak pressure ranges from 20 to 160 MPa. Therefore, porous cell design was modified to stand these pressure levels. Pellet speeds from 1600 to 3000 m/s were performed with a cycle period from 15 to 34 s. The first campaign has produced promising results, firstly on the porous cell pellet generator technology that can provide repetitive deuterium ice formation, and secondly on the TSGG used also in a repetitive mode. For further experiments, improvements on cryostat design and on remote control system should be undertaken. New cryostat design will give access to lower cell temperature, necessary to improve the ice resistance and thus to allow higher pellet speeds. A fully automatic remote control system will improve the overall reliability of the injector and ease the sequence setting for shorter period operation.

Control and data acquisition system for multi-barrel pellet injector

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.

ORNL mock-up tests of inside launch pellet injection on JET and LHD

In experiments on ASDEX-Upgrade and DIII-D tokamaks, the injection of D 2 pellets from the magnetic high-field side of the plasma resulted in deeper pellet penetration and improved fueling efficiency. Based on those successful experiments, fusion researchers at the Joint European Torus and the Large Helical Device decided to implement inside launch pellet injection. These injection schemes require the use of curved guide tubes to route the pellets from the acceleration devices to the inside launch locations, and the pellets are subjected to stresses from centrifugal and impact forces in traversing the tubes. Before the installations on the large experimental fusion devices, mock-ups of the guide tubes were constructed and tested at the Oak Ridge National Laboratory to determine the pellet speed limit for reliable operation without pellet fracturing. In laboratory testing of the mock-ups, it was found that the pellet speed had to be limited to a few hundreds of meters per second for intact pellets. In this paper, the test equipment and experimental results are described.

Dynamic consequences of differences in male and female vocal tract dimensions.

Phonetic differences between male and female speakers are generally considered in terms of the static acoustic and perceptual consequences of different articulatory dimensions. This article investigates the dynamic acoustic and articulatory implications of differences in mean male and female vocal tract dimensions. The temporal acoustic consequences of time-varying twin-tube resonators of different dimensions are explored, and the possible implications for human speech production are considered. Empirical support for the theoretical predictions is sought by investigating the kinematic and acoustic patterns in diphthong productions from 26 female and 22 male speakers in the University of Wisconsin X-ray Microbeam Speech Production Database. Aside from expected acoustic differences, the shape of male and female formant tracks plotted in Bark space is found to be very similar. Male and female patterns of tongue movement, however, are found to be very dissimilar. The mean male diphthong, defined by the tracks of four midsagittal pellets, is characterized by greater pellet excursions, higher pellet speed, and consistently larger dorso-palatal strictures than its female counterpart. The empirical findings suggest that gender-specific dynamic behavior could be an important factor in accounting for nonuniform vowel system differences, but at the same time having more wide-ranging implications for transitional phenomena and undershoot.

Speed limit of frozen pellets (H2, D2, and Ne) through single-loop and multiloop tubes and implications for fusion plasma research

Frozen pellets (H2, D2, and Ne at 8 K) of nominal 2.7 mm diam were shot through a coiled tube (single loop of ≈0.6 m diam and 8.5 mm bore), and the speed limit for survival was recorded for each pellet type. Intact H2 pellets were observed at speeds approaching 500 m/s; but neon pellets could not survive much more than 100 m/s. The speed limit for D2 pellets fell in the middle at ≈300 m/s. Some D2 pellets were also shot through a 30 m coiled tube consisting of 11 loops (average loop diameter of ≈0.8 m), and a speed limit of ≈100 m/s was observed. Injection of frozen H2 or D2 pellets is commonly used for core fueling of magnetically confined plasmas, and frozen neon pellets are sometimes used for impurity transport studies in similar experiments. The results from these tests add to a pellet database for injection lines with single- and complex multiple-curved guide tubes. All of the information to date suggests that frozen pellets can be delivered reliably from a pellet source to any accessible plasma location on a fusion device via “roller-coaster” tubes as long as the pellet speed is maintained below a threshold limit.

Improved core fueling with high field side pellet injection in the DIII-D tokamak

The capability to inject deuterium pellets from the magnetic high field side (HFS) has been added to the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. It is observed that pellets injected from the HFS lead to deeper mass deposition than identical pellets injected from the outside midplane, in spite of a factor of 4 lower pellet speed. HFS injected pellets have been used to generate peaked density profile plasmas [peaking factor (ne(0)/〈ne〉) in excess of 3] that develop internal transport barriers when centrally heated with neutral beam injection. The transport barriers are formed in conditions where Te∼Ti and q(0) is above unity. The peaked density profiles, characteristic of the internal transport barrier, persist for several energy confinement times. The pellets are also used to investigate transport barrier physics and modify plasma edge conditions. Transitions from L- to H-mode have been triggered by pellets, effectively lowering the H-mode threshold power by 2.4 MW. Pellets injected into H-mode plasmas are found to trigger edge localized modes (ELMs). ELMs triggered from the low field side (LFS) outside midplane injected pellets are of significantly longer duration than from HFS injected pellets.

An international pellet ablation database

The contents are described of an international pellet ablation database (IPADBASE) that has been assembled to enable studies of pellet ablation theories that are used to describe the physics of an ablating fuel pellet in a tokamak plasma. The database represents an international effort to assemble data from several tokamaks of different magnetic configurations and auxiliary heating methods. In the initial configuration, data from JET, Tore Supra, DIII-D, FTU, TFTR, ASDEX Upgrade, JIPP T-IIU, RTP and T-10 have been included. The database contains details of measurements of deuterium and hydrogen pellet ablation, including pellet mass and speed, plasma electron density and temperature profiles, and pellet ablation light emission. A summary of the database contents and a scaling analysis of the data are presented

Development and implementation of a rail current optimization program

Efforts are underway to automate the operation of a railgun hydrogen pellet injector for fusion reactor refueling. A plasma armature is employed to avoid the friction produced by a sliding metal armature and, in particular, to prevent high-Z impurities from entering the tokamak. High currents are used to achieve high accelerations, resulting in high plasma temperatures. Consequently, the plasma armature ablates and accumulates material from the pellet and gun barrel. This increases inertial and viscous drag, lowering acceleration. A railgun model has been developed to compute the acceleration in the presence of these losses. The model suggests that, depending on the rail and insulator materials used, there is a point of diminishing returns. Namely, for a given current, there is an acceleration time beyond which little or no increase in pellet speed is produced. The optimal pulse width was determined by identifying the time at which the acceleration decreased to zero. In order to quantify these losses, the ablation coefficient, /spl alpha/, and drag coefficient, C/sub d/, must be determined. These coefficients are estimated based on the pellet acceleration. The sensitivity of acceleration to /spl alpha/ and C/sub d/ has been calculated using the model. Once /spl alpha/ and C/sub d/ have been determined, their values are applied to the model to compute the appropriate current pulse width. An optimization program was written in LabVIEW software to carry out this procedure. This program was then integrated into the existing code used to operate the railgun system. Preliminary results obtained after test firing the gun indicate that the program computes reasonable values for /spl alpha/ and C/sub d/ and calculates realistic pulse widths.

5529587 Fludidized oxydesulfurization of coal: Diver John R, Lake Forest, IL, United States

A conceptual design for a pellet injection system will be worked out, capable to support key missions of the new tokamak device JT-60SA. For exploitations in view of ITER and to resolve key physics and engineering issues for DEMO, several tasks were assigned to this system. Physics investigations aim at operation at high density in ITER and DEMO relevant plasma regime above Greenwald density, power exhaust techniques with radiation layers, particle balance studies, and ELM control and mitigation. The postulated engineering requirement is to quantify pellet actuation on electron density for application within the advanced real-time control scheme by controlling density gradients. Our intended pellet system comprises three major components: pellet source, accelerator, and guiding system. The guiding system must be installed inside the torus vessel already under construction; hence still possible launch geometry was pursued first. Three different options have been identified: inboard, outboard, and top launch. The first one is most promising with respect to fuelling performance but will impose pellet speed restrictions to about 470 m/s for adequate pellet sizes. Both others offer headroom for significantly higher injection speed but under less favourable physics boundary conditions. In order to evaluate expected performances for all relevant plasma scenarios, detailed modelling efforts for every launch geometry option have been made. For a suitable pellet source covering all requirements, several options are at hand including commercial providers. For the accelerator, the high speed option up to about 4000 m/s could be covered by a two-stage gas gun. Single-stage gas guns and centrifuges can cover the speed range up to about 1000 m/s for the basic work load since both fulfil the requirements for pellet size and speed. Due to a higher speed precision resulting in less timing jitter, a centrifuge would be better suited for control requirements.

Tritium Pellet Injector for TFTR

The tritium pellet injector (TPI) for the Tokamak Fusion Test Reactor (TFTR) will provide a tritium pellet fueling capability with pellet speeds in the 1- to 3-km/s range for the TFTR deuterium-tritium (D-T) phase. The existing TFTR deuterium pellet injector (DPI) has been modified at Oak Ridge National Laboratory (ORNL) to provide a four-shot, tritium-compatible, pipe-gun configuration with three upgraded single-stage pneumatic guns and a two-stage light gas gun driver. The TPI was designed to provide pellets ranging from 3.3 to 4.5 mm in diameter in arbitrarily programmable firing sequences at speeds up to approximately 1.5 km/s for the three single-stage drivers and 2.5 to 3 km/s for the two-stage driver. Injector operation is controlled by a programmable logic controller. The new pipe-gun injector assembly was installed in the modified DPI guard vacuum box, and modifications were made to the internals of the DPI vacuum injection line, including a new pellet diagnostics package. Assembly of these modified parts with existing DPI components was then completed, and the TPI was tested at ORNL with deuterium pellet. Results of the limited testing program at ORNL are described. The TPI is being installed on TFTR to support the D-D run period in 1992.more » In 1993, the tritium pellet injector will be retrofitted with a D-T fuel manifold and secondary tritium containment systems and integrated into TFTR tritium processing systems to provide full tritium pellet capability.« less

Acceleration of small, light projectiles (including hydrogen isotopes) to high speeds using a two-stage light gas gun

Small, light projectiles have been accelerated to high speeds using a two-stage light gas gun at Oak Ridge National Laboratory. With 35-mg plastic projectiles (4 mm in diameter), speeds of up to 4.5 km/s have been recorded. The ‘‘pipe gun’’ technique for freezing hydrogen isotopes in situ in the gun barrel has been used to accelerate deuterium pellets (nominal diameter of 4 mm) to velocities of up to 2.85 km/s. The primary application of this technology is for plasma fueling of fusion devices via pellet injection of hydrogen isotopes. Conventional pellet injectors are limited to pellet speeds in the range 1–2 km/s. Higher velocities are desirable for plasma fueling applications, and the two-stage pneumatic technique offers performance in a higher velocity regime. However, experimental results indicate that the use of sabots to encase the cryogenic pellets and protect them from the high peak pressures will be required to reliably attain intact pellets at speeds of ≊3 km/s or greater. In some limited tests, lithium hydride pellets were accelerated to speeds up to 4.2 km/s. Also, repetitive operation of the two-stage gun (four plastic pellets fired at ≊0.5 Hz) was demonstrated for the first time in preliminary tests. The equipment and operation are described, and experimental results and some comparisons with a theoretical model are presented.

Performance of a pneumatic hydrogen-pellet injection system on the Joint European Torus

A pneumatic-based, hydrogen isotope pellet injector that was developed at the Oak Ridge National Laboratory (ORNL) has been used in recent plasma fueling experiments on the Joint European Torus (JET). The injector consists of three independent machine-gun-like mechanisms (nominal pellet sizes of 2.7, 4.0, and 6.0 mm in diameter) and features repetitive operation (1–5 Hz) for quasi-steady-state conditions (>10 s). An extensive set of injector diagnostics permits evaluation of parameters for each pellet shot, including speed, mass, and integrity. Pellet speeds range from 1.0 to 1.5 km/s. Over 3700 pellets have been fired with the equipment at JET, with about 1500 pellets shot for plasma fueling experiments. In recent experiments, the system performance has been outstanding, including excellent reproducibility in pellet speed and mass, and a reliability of >98% in delivery of pellets to the plasma.

Development of a hydrogen electrothermal accelerator for plasma fueling

We have developed a prototype high-velocity pneumatic pellet injector that uses hydrogen plasma propellant generated in a high-current arc discharge. A single-barrel pneumatic pellet gun has been fitted with a cylindrical arc chamber interposed between the hydrogen propellant inlet valve and the gun breech. The chamber incorporates a ceramic insert for generating vortex flow in the incoming gas stream, which provides azimuthal arc stabilization. The arc is initiated after the propellant valve opens and the breech pressure starts to rise; a typical discharge lasts 150–300 μs with peak currents up to 2 kA. The gun has been operated with 4-mm-diam, 6- to 11-mm-long deuterium and hydrogen pellets. At 100-bar plenum pressure (hydrogen propellant), the arc characteristics are 〈V〉=350–800 V, 〈I〉=600 A, so that 60–150 J of electrical power is dissipated. Pellet speeds increase by 300 to 600 m/s depending on the projectile mass, which typically represents a 10-J increment in the pellet kinetic energy. Velocities up to 1.7 km/s for deuterium pellets and 2.0 km/s for hydrogen pellets have been achieved. Comparing these data to muzzle velocities calculated from idealized one-dimensional compressible flow gun theory demonstrates that substantial propellant heating, resulting in increased propellant sound speed, has been achieved.

Development of protective layers on high-temperature alloys against hydrogen permeation

For TEXTOR a multiple pellet injector system is in preparation which consists of a) nine freezing cells for the pellet formation accommodated in a common cryostat and nine gas guns being built at CEA, Service des Basses Temperatures in Grenoble, b) a system for the propellant-gas handling together with the pellet diagnostics being built at KFA Julich. The pellets have a cylindrical shape with a size of 1.3 mm in diameter and a length of 1.5 mm. The guns are operated with a propellant gas pressure of up to 200 bar and the pellets are accelerated up to velocities of 1.4 km/s. The propellant gas valve of each gun is optimized to release as little gas as possible; nevertheless the gas release amounts to 0.3 bar·l. This amount of gas (typically hydrogen) has a mass more than a factor of hundred larger than that of one pellet, but only an amount of one percent of the pellet mass can be tolerated. The conventional way of handling the propellant gas is either to feed the pellets through a capillary guiding tube or by using two or three differential pumping chambers. The first scheme may cause pellet ruptures at velocities higher than 1 km/s and the second one requires large expansion volumes and strong pumping. In the TEXTOR pellet injector we are using a combination of a muzzle brake, three small expansion volumes and two fast closing valves for each gun. The muzzle brake diffuses the stream of the propellant gas immediately after the end of the barrel. Most of the gas is directed into two expansion chambers which have volumes of only 0.5 l each. Within 1 ms after the passage of the pellet, a newly developed valve closes the first two expansion volumes from TEXTOR, thus reducing the residual propellant gas to less than 0.5 mbar·l. A second combination of an expansion volume of 1 l and a fast valve reduces the propellant gas further down to about 10−2 mbar·l, which is less than 1 % of the pellet mass (typically 4 mbar·l). Three IR-light barriers are integrated in each pellet path; their photodiodes provide signals for the pellet speed determination and trigger signals for the fast closing valves. They also give the open/closed status of the valves. The mass of the individual pellet is measured by a microwave system consisting of a μW-cavity, a wave guide, a gunnplexer oscillator and a detection circuit operating at a frequency of 10.3 GHz.

Development of Hydrogen Pellet Injectors at ORNL

Pellet injectors that produce and accelerate frozen hydrogen isotope pellets are being developed at Oak Ridge National Laboratory (ORNL) for fueling of present and future plasma fusion devices. The development has focused primarily on two types of injectors: (1) gas guns, which utilize a pneumatic approach to accelerate pellets in a barrel with compressed helium or hydrogen propellant, and (2) centrifuge-type injectors, in which pellets are accelerated by centrifugal forces in a highspeed rotating track. In a single-pellet pneumatic injector, pellet speeds up to 1.4 km/s have been achieved. Three multipellet injection systems (ORNL four-pellet pneumatic design) are now functional, one each on the Poloidal Divertor Experiment (PDX), Alcator-C, and the Impurity Study Experiment (ISX-B). Currently, two repetitive devices (one of each injector type) are in operation to demonstrate steadystate fueling systems in the reactor-relevant parameter ranges of 1-km/s pellet velocity, variable pellet sizes up to 2 mm, and feed rates up to 10-40 pellets/s. The injector designs are described and operating characteristics discussed.