A. Frattolillo

Development of a Two-Stage Pneumatic Repeating Pellet Injector for the Refueling of Long-Pulse Magnetic Confinement Fusion Devices

Next-step fusion devices, like the International Thermonuclear Experimental Reactor (ITER), and future fusion power plants will require a flexible plasma fueling system, including both gas puffing and high- and low-speed pellet injection. To sustain core plasma density, relatively large pellets penetrating beyond the separatrix will have to be provided at a repetition rate of ∼1 Hz for very long pulse operation. In the context of a cooperative agreement between the U.S. Department of Energy and the Euratom-ENEA Association, Oak Ridge National Laboratory (ORNL) has collaborated with ENEA Frascati to demonstrate the feasibility of a high-speed (2 to 3 km/s) repeating (≃1-Hz) pneumatic pellet injector for long-pulse operation. A test facility was assembled at ORNL that combined a Frascati repeating two-stage light-gas gun and an existing ORNL deuterium extruder, equipped with a pellet chambering mechanism/gun barrel assembly. It was operated in the course of three joint experimental campaigns between September 1993 and May 1995. The results of the first two campaigns appear in an earlier paper. Here, the results are reported of the third campaign, during which the original objectives ofthe collaboration were met. Both performance and reliability of the system were improved, with the facilitys being capable of delivering sequences of 2.7-mm deuterium pellets at a repetition rate of 1 Hz and velocities up to 2.5 km/s. The test facility was also briefly operated with neon pellets to explore the potential to produce fast killer pellets. Speeds of 1.7 km/s were easily achieved using a piston mass of43 g. Higher speeds should be achievable with a system specifically designed for neon or other high-Z gases.

High‐speed repeating hydrogen pellet injector for long‐pulse magnetic confinement fusion experiments

The projected fueling requirements of future magnetic confinement fusion devices [e.g., the International Thermonuclear Experimental Reactor (ITER)] indicate the need for a flexible plasma fueling capability, including both gas puffing and low‐ and high‐speed pellet injection. Conventional injectors, based on single‐stage pneumatic guns or centrifuges, can reliably provide frozen pellets (1‐ to 6‐mm‐diam sizes) at speeds up to 1.3 km/s and at suitable repetition rates (1 to 10 Hz or greater). Injectors based on two‐stage pneumatic guns and ‘‘in situ’’ condensation of hydrogen pellets can reliably achieve velocities over 3 km/s; however, they are not suitable for long‐pulse repetitive operations. An experiment in collaboration between Oak Ridge National Laboratory (ORNL) and ENEA Frascati is under way to demonstrate the feasibility of a high‐speed (≳2 km/s) repeating (∼1 Hz) pneumatic pellet injector for long‐pulse operation. A test facility has been assembled at ORNL, combining a Frascati repeating two‐st...

High-speed repetitive pellet injector for plasma fueling of magnetic confinement fusion devices

The projected fueling requirements of future magnetic confinement devices for controlled thermonuclear research [e.g., the International Thermonuclear Experimental Reactor (ITER)] indicate that a flexible plasma fueling capability is required. This includes a mix of traditional gas puffing and low- and high-velocity deuterium-tritium pellets. Conventional pellet injectors (based on light gas guns or centrifugal accelerators) can reliably provide frozen hydrogen pellets (1- to 6-mm-diam sizes tested) up to /spl sim/1.3-km/s velocity at the appropriate pellet fueling rates (1 to 10 Hz or greater). For long-pulse operation in a higher velocity regime (>2 km/s), an experiment in collaboration between Oak Ridge National Laboratory (ORNL) and ENEA Frascati is under way. This activity will be carried out in the framework of a collaborative agreement between the U.S. Department of Energy and European Atomic Energy Community-ENEA Association. In this experiment, an existing ORNL hydrogen extruder (equipped with a pellet chambering mechanism/gun barrel assembly) and a Frascati two-stage light gas gun driver have been combined on a test facility at ORNL. Initial testing has been carried out with single deuterium pellets accelerated up to 2.1 km/s with the two-stage driver; in addition, some preliminary repetitive testing (to commission the diagnostics) was performed at reduced speeds, including sequences at 0.5 to 1 Hz and 10 to 30 pellets. The primary objective of this study is to demonstrate repetitive operation (up to /spl sim/1 Hz) with speeds in the 2- to 3-km/s range. In addition, the strength of extruded hydrogen ice as opposed to that produced in situ by direct condensation in pipe guns can be investigated. The equipment and initial experimental results are described.

A new two-stage pellet injector for FTU

The most recent steps for developing injectors that meet the requirements of the Frascati Tokamak Upgrade are described. Details of a cryostat capable of building 1.6-mm-diameter deuterium pellets are given. Such pellets have been successfully accelerated up to 2.8 km/s with good repeatability, using a relatively large two-stage gun (0.5-m long, 3.5-cm diameter) as a propeller. A miniature version of the propeller was developed and tested using plastic pellets. Velocities of up to 3.2 km/s were obtained. The final goal is to build a complete miniature system capable of very high pellet velocities using reduced volumes of propelling gas.<<ETX>>

Experimental study of the propellant gas load required for pellet injection with ITER-relevant operating parameters

An existing pipe gun test facility at ORNL was used for an experimental study of propellant gas loads required for ITER-relevant pellet injection, with the key objective of determining the minimal amount of gas required for optimal pellet speeds. Two pellet sizes were tested, with nominal 4.4 and 3.2 mm diameters comparable to pellets planned for fueling and ELM pacing in ITER, respectively. A novel scheme was used to freeze solid pellets from room temperature gas; this facilitated operations at higher temperatures (14.5 to 16.5 K, similar to those planned for extruder operations for ITER pellet injectors) and thus lower pellet breakaway pressures and gas loads. Most of the single-shot D2 pellet tests were carried out with a relatively low H2 propellant gas load of ~0.0133 bar-L. Some limited testing was also carried out with a mixed propellant gas that consisted mostly of D2, which is more representative of the gas that will be used for ITER pellet injection. In testing it was found that this reference gas load resulted in pellet speeds in close proximity to a speed limit (~300 m/s) previously determined in a series of tests with D2 pellets shot through a mock-up of the curved guide tubes planned for the ITER installation (for pellet fueling from the magnetic high-field side). The equipment, operations, and test results are presented and discussed, with emphasis on the relevance for ITER operations.

New simple method for fast and accurate measurement of volumes

A new simple method is presented, which allows us to measure in just a few minutes but with reasonable accuracy (less than 1%) the volume confined inside a generic enclosure, regardless of the complexity of its shape. The technique proposed also allows us to measure the volume of any portion of a complex manifold, including, for instance, pipes and pipe fittings, valves, gauge heads, and so on, without disassembling the manifold at all. To this purpose an airtight variable volume is used, whose volume adjustment can be precisely measured; it has an overall capacity larger than that of the unknown volume. Such a variable volume is initially filled with a suitable test gas (for instance, air) at a known pressure, as carefully measured by means of a high precision capacitive gauge. By opening a valve, the test gas is allowed to expand into the previously evacuated unknown volume. A feedback control loop reacts to the resulting finite pressure drop, thus contracting the variable volume until the pressure exac...

Acceleration of neon pellets to high speeds for fusion applications

The injection of impurity pellets into the plasmas of tokamak fusion reactors has been proposed as a technique to lessen the deleterious effects of plasma disruptions. Equipment and techniques that were previously developed for pneumatic hydrogen pellet injection systems and used for plasma fueling applications were employed for a limited experimental study with neon pellets. Isotopic hydrogen pellets doped with neon have previously been used for injection into fusion plasmas to study impurity particle transport, and pure neon pellets are applicable for disruption studies. Using a repeating pneumatic injector in the laboratory, it was found that the formation and acceleration of 2.7‐mm‐diam neon pellets were relatively straightforward; reliable operation was demonstrated with both a single‐ and a two‐stage light gas gun, including velocities of ∼700 m/s with a single‐stage injector and up to 1740 m/s with a two‐stage injector. Based on the operating sequences and successful tests demonstrated in the labor...

IMPROVED TWO-STAGE GUN FOR PELLET INJECTION

The injection of high speed (over 2000 m/s) hydrogen pellets in plasma machines is a difficult task, especially due to the poor mechanical strength of solid hydrogen, which imposes limits on the acceleration of the pellets. In this work a method of shaping the acceleration law using a two-piston pneumatic injector is described. Numerical and experimental runs with plastic pellets have been performed. Also, some preliminary experimental results with deuterium pellets are presented.

Selectable flight tube design developments for iter fueling, ELM pacing, and impurity pellets

ITER fueling and Edge Localized Mode (ELM) pacing pellets will be created by punching and chambering pellets with a solenoid operated cutter from an extrudate formed by a twin-screw extruder. The cut pellets are then accelerated down a flight tube by a fast pneumatic valve. The impurity pellets are formed in-situ with a pipe gun technique and accelerated down a flight tube with the same pneumatic valve design. The pellets are directed into three possible plasma locations by the flight tube selector. The pellets exit the selector and travel through flight tubes that penetrate the cryostat and vacuum vessel before entering the plasma on either the outer diameter (low field side) or inner diameter (high field side). The geometries of the flight tubes have been optimized to maximize bend radii over 800 mm to ensure intact pellets traveling at >200 m/s. A proof of concept selector was built and tested to verify pellet survivability through the selector inlet and output tubes as well as the two internal selector funnels. The ENEA/ORNL highspeed pellet injection system was adapted for selector testing to provide both 4.4 mm and 3.2 mm diameter deuterium pellets as fueling and ELM pacing pellet surrogates respectfully. The pellets were accelerated with a limited quantity of propellant gas of ~13 mbar-L to mimic the expected ITER pellet speeds. The pipe gun barrel had to be kept near 16 K to lower the pellet shear strength to enable the pellets to release from the barrel when fired.

Simple automatic device for real time sampling of gas production by a reactor

An innovative automatic device, allowing periodically drawing samples of the gases produced by a generic reactor, is presented. The gases evolving during the reaction are collected in a storage manifold, equipped with a variable volume consisting of a stainless steel bellow, whose expansion or contraction is driven by a linear step motor. A capacitive gauge monitors the pressure inside the storage manifold, while a feedback control loop reacts to any pressure change adjusting the variable volume (by means of the step motor) in such a way to keep the pressure at a desired set point P0. As long as the reaction proceeds, the gas production results in a progressive expansion of the variable volume, whose instantaneous value is constantly monitored by means of a slide potentiometer, whose lever is rigidly connected to the bellow’s moving extremity. Once the bellow’s expansion has reached a predetermined volume increment ΔV, which means that an amount of gas P0ΔV has been produced and collected in the storage c...