Pneumatic Pellet Injector Research Articles

Pellet injectors for steady state plasma fuelling

Successful steady state operation of a fusion reactor should be supported by repetitive pellet injection of solidified hydrogen isotopes in order to produce high performance plasmas. This paper presents pneumatic pellet injectors and its implementation for long discharge on the LHD and TORE SUPRA, and a new centrifuge pellet injector test results. All injectors are fitted with screw extruders well suited for steady state operation.

A mock-up of a tritium pellet injector for ITER

The ITER tokamak is to be equipped with a pellet injector for plasma fuelling. A tritium compatible pneumatic pellet injector with a screw extruder has been designed, tested with protium, and is under installation for testing at the experimental tritium loop in the Russian Nuclear Center in Sarov. The rectangular rod 3×4 mm 2 from condensed protium was extruded within 1500 s at rate of ∼15 mm/s. Pellets ∼3 mm in size formed from the rod were accelerated up to 500 m/s by helium under a pressure of 1 MPa with an injection frequency of 1 and 2 Hz. The main injector subsystems and the current status of the work are described.

Acceleration of large size deuterium pellets to high speeds using a small two-stage pneumatic gun

The technology of two-stage pneumatic pellet injectors represents by far the most reliable way to perform deep plasma fueling, with pipe gun devices capable of routinely launching small or medium size (up to 4 mm) D2 pellets at speeds in excess of 3 km/s, using rather small two-stage guns. It is still an open question, however, if scaling of the pellet size to the International Thermonuclear Experimental Reactor relevant values (6–8 mm) will or will not require a somewhat proportional increase in the physical size of the two-stage gun. In order to investigate this question, an extensive study was carried out at ENEA Frascati, using numerical simulation codes. It clearly indicated that a “compact” two-stage gun may have the potential to accelerate large size pellets at speeds up to 5 km/s. A low cost experiment was also scheduled. A spare pipe-gun cryostat of the single-shot two-stage pneumatic injector, previously used for high-speed pellet fueling of the Frascati tokamak upgrade, was modified in order to accommodate larger bore (up to 6 mm) launching barrels. In this article, we will mainly discuss the results of numerical simulations. A very early experimental campaign, carried out in 1996, will also be briefly reported, during which intact 6 mm D2 pellets were launched at speeds up to 2.5 km/s.

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 of the collaboration were met. Both performance and reliability of the system were improved, with the facility’s 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 of 43 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-stage light-gas gun and an ORNL deuterium extruder, equipped with a pellet chambering mechanism/gun barrel assembly. The main issues to be investigated were the strength of extruded deuterium ice as opposed to that produced by in situ condensation in pipe guns (hence the highest acceleration which can be given to the pellet without fracturing it), and the maximum repetition rate at which the system can operate without degradation in performance. Pellet velocities of up to 2.55 km/s have been achieved in joint experiments at ORNL. A new pressure tailoring valve was developed by the Frascati group for this application and proved to be a crucial component for good performance. Tests carried out in repeating mode, at frequencies of 0.2–0.5 Hz and speeds up to 2.2 km/s, indicate no significant degradation in performance with increasing repetition rate. Some preliminary tests using 3.7 mm pellets gave very encouraging results. The equipment and the experimental results are described in this article.

Small-bore (1.8-mm), high-firing-rate (10-Hz) version of a repeating pneumatic hydrogen pellet injector

Repeating pneumatic pellet injectors developed at the Oak Ridge National Laboratory (ORNL) were used for plasma fueling experiments on the Tokamak Fusion Test Reactor (TFTR) and the Joint European Torus (JET). For plasma fueling on the DIII–D tokamak, a small-bore (1.8-mm) injector has been developed and tested in the laboratory at pellet rates of up to 10 Hz and speeds of ≤1 km/s (for pulse lengths of up to 15 s). This performance represents the smallest pellet size and highest repetition rate demonstrated with an ORNL repeating pneumatic pellet injector. The design has been incorporated in the three-barrel injector that was previously used on JET; the injection system, equipped with nominal pellet sizes of 1.8-, 2.7-, and 4.0-mm diameter, has been installed on DIII–D and will be used in future plasma fueling experiments.

Pellet fueling system for ITER

Abstract The International Thermonuclear Experimental Reactor (ITER) will use an advanced, high-velocity pellet injection system to achieve and maintain ignited plasmas. Three pellet injectors are proposed. For ramp-up to ignition, a highly reliable, moderate-velocity (1- to 1.5-km/s) single-stage pneumatic injector and a high-velocity (1.5- to 5-km/s) two-stage pneumatic pellet injector that uses frozen hydrogenic pellets encased in sabots will be used. For the steady-state burn phase, a continuous, single-stage pneumatic injector is proposed; this injector will provide a flexible fueling source beyond the edge region to aid in decoupling the edge region (whose parameters are constrained by divertor requirements) from the high-temperature burning plasma. Because pellet fueling is more efficient than gas injection, it may be possible to use pellet fueling to provide all of the tritium and a portion of the deuterium needed to sustain the fusion reaction and to use deuterium gas injection to provide the required edge densities for optimum divertor conditions. In addition to decreasing the tritium throughput and raising the tritium burn fraction, this mode of operation allows a deuterium-rich boundary layer gas at the edge, which could significantly lower tritium inventories in plasma-facing components. All three pellet injectors are designed for operation with tritium feed gas. Issues such as performance, neutron activation of injector components, maintenance, design of the pellet injection vacuum line, gas loads to the reprocessing system, and equipment layout are discussed. Results and plans for supporting research and development in the areas of tritium pellet fabrication, continuous extruders, and high-velocity, repetitive two-stage pneumatic injectors are presented.

Authors

Authors

Development of repeating pneumatic pellet injector

A repeating pneumatic pellet injector has been constructed to experiment with the technique of continuous injection for fueling fusion reactors. This device is composed of a cryogenic extruder and a gun assembly in (among others) a high-vacuum vessel, diagnostic vessels, LHe, fuel-gas and propellant-gas supply systems, control and data acquisition systems, etc. The performance tests, using hydrogen, have proved that the device provides the function of extruding frozen hydrogen ribbons at the speed of 6 mm s −1, chambering pellet at the rate of 5 Hz, and injecting pellet at the speed of 900 m s −1, as planned.

Simplified eight-shot pneumatic pellet injector for plasma fueling applications on the Princeton Beta Experiment and on the Advanced Toroidal Facility

Plasma fueling via injection of solid hydrogenic pellets has expanded the operating range for tokamaks and stellarators to higher densities than attainable with gas puffing. Pellet injection has also resulted in improved plasma energy confinement in tokamak discharges for which the pellet or pellets penetrate deep into the plasma core. The eight-shot pneumatic pellet injector described herein has been developed for use on the Princeton Beta Experiment and on the Advanced Toroidal Facility for routine plasma fueling and for confinement optimization studies. The injector is based upon the so-called ‘‘pipe-gun’’ concept, which generates deuterium and hydrogen pellets by direct condensation in the gun barrel tubes, segments of which are cooled below the hydrogen triple-point temperature by contact with a liquid-helium-cooled block. Control of the pellet length is achieved both by regulating the deuterium fill pressure and by establishing temperature gradients along the barrel tubes. This injector features eight independent gun barrel assemblies mounted around the perimeter of a single cold block, each coupled to an ORNL-designed fast propellant valve. Thus, the injector is capable of injecting arbitrarily programmable sequences of up to eight pellets of sizes ranging from 1 to 3 mm at speeds up to 1500 m/s.

Design considerations for single-stage and two-stage pneumatic pellet injectors

Performance of single-stage pneumatic pellet injectors is compared with several models for one-dimensional, compressible fluid flow. Agreement is quite good for models that reflect actual breech chamber geometry and incorporate nonideal effects such as gas friction. Several methods of improving the performance of single-stage pneumatic pellet injectors to muzzle velocities ranging from 2 to 2.25 km/s in the near term are outlined. Two-stage pneumatic injectors have the potential for much higher muzzle velocities (4–6 km/s) because of the higher gas pressures and temperatures achievable with the second-stage compression. The design and performance of two-stage pneumatic pellet injectors are discussed, and initial data from the two-stage pneumatic pellet injector test facility at Oak Ridge National Laboratory are presented. Finally, a concept for a repeating two-stage pneumatic pellet injector is described.

A three-barrel repeating pneumatic pellet injector for plasma fueling of the Joint European Torus

A three-barrel repeating pneumatic pellet injector has been developed for plasma fueling of the Joint European Torus (JET) tokamak. The versatile device consists of three independent machine-gun-like mechanisms that operate at cryogenic temperatures (10–20 K). Individual extruders provide a continuous supply of solid hydrogen isotope to each gun assembly, where a reciprocating breech-side cutter forms and chambers cylindrical pellets from the extrusion; the deuterium pellets are then accelerated in the gun barrels with compressed hydrogen gas (pressures up to 105 bar) to velocities ≤1.5 km/s. The injector features three nominal pellet sizes (2.7, 4.0, and 6.0 mm) and repetitive operation (5, 2.5, and 1 Hz, respectively) for quasi-steady-state conditions (>10 s). The design allows the gun barrels to be aligned mechanically for accurate aiming. A remote stand-alone control and data acquisition system is used for injector operation. The injector system has been installed on JET. The design features, operation, and performance characteristics of the injector are described.

Pellet injection on the ZT-40M reversed field pinch

The four barrel pneumatic pellet injector originally used on the Alcator C tokamak at MIT has been reconfigured for use on the ZT-40M reversed field pinch (RFP) at Los Alamos National Laboratory. The lower temperature and density in this device require operation at velocities significantly lower than on Alcator. Allowing a 2-s warmup prior to firing the injector has enabled operation with pressures as low as 40 psig and velocities ranging as low as 200 m/s with H2 pellets, without major cryostat modification. Initial injection experiments in the ZT-40M plasma have verified that pellet refueling is effective for density control, but the net particle inventory increase is less than might be expected based on simple models. Indications of nonoptimal penetration and curvature of the pellet trajectory, in addition to comparable time scales for particle confinement and pellet transit, may account for this.

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.

Repeating pneumatic hydrogen pellet injector for plasma fueling

A repeating pneumatic pellet injector has been developed for plasma fueling applications. The repetitive device extends pneumatic injector operation to steady state. The active mechanism consists of an extruder and a gun assembly that are cooled by flowing liquid-helium refrigerant. The extruder provides a continuous supply of solid hydrogen to the gun assembly, where a reciprocating gun barrel forms and chambers cylindrical pellet from the extrusion; pellets are then accelerated with compressed hydrogen gas (pressures up to 125 bar) to velocities ≤1.9 km/s (1.6 km/s for deuterium pellets). The gun assembly design can accommodate different pellet sizes and barrel lengths. Steady-state rates of 2 s−1 have been obtained with 2.1- , 3.4- , and 4.0-mm-diameter pellets. The present apparatus operates at higher firing rates in short bursts; for example, a rate of 6 s−1 for 2 s with the larger pellets. These pellet parameters are in the range applicable for fueling large present-day fusion devices such as the Tokamak Fusion Test Reactor (TFTR). Experimental results are presented, including effects of propellant pressure and barrel length on gun performance.