James M McGill

Development of a Twin-Screw D 2 Extruder for the ITER Pellet Injection System

A twin-screw extruder for the ITER pellet injection system is under development at the Oak Ridge National Laboratory. The extruder will provide a stream of solid hydrogen isotopes to a secondary section, where pellets are cut and accelerated with single-stage gas gun into the plasma. A one-fifth ITER scale prototype extruder has been built to produce a continuous solid deuterium extrusion. Deuterium gas is precooled and liquefied before being introduced into the extruder. The precooler consists of a copper vessel containing liquid nitrogen surrounded by a deuterium gas filled copper coil. The liquefier is comprised of a copper cylinder connected to a Cryomech AL330 cryocooler, which is surrounded by a copper coil that the precooled deuterium flows through. The lower extruder barrel is connected to a Cryomech GB-37 cryocooler to solidify the deuterium (at 15 K) before it is forced through the extruder nozzle. A viewport located below the extruder nozzle provides a direct view of the extrusion. A camera is used to document the extrusion quality and duration. A data acquisition system records the extruder temperatures, torque, and speed, upstream, and downstream pressures. This paper will describe the prototype twin-screw extruder and initial extrusion results.

Alternative Techniques for Injecting Massive Quantities of Gas for Plasma-Disruption Mitigation

Injection of massive quantities of noble gases or D2 has proven to be effective at mitigating some of the deleterious effects of disruptions in tokamaks. Two alternative methods that might offer some advantages over the present technique for massive gas injection are ¿shattering¿ massive pellets and employing close-coupled rupture disks. Laboratory testing has been carried out to evaluate their feasibility. For the study of massive pellets, a pipe-gun pellet injector cooled with a cryogenic refrigerator was fitted with a relatively large barrel (16.5-mm bore), and D2 and Ne pellets were made and were accelerated to speeds of ~ 600 and 300 m/s, respectively. Based on the successful proof-of-principle testing with the injector and a special double-impact target to shatter pellets, a similar system has been prepared and installed on DIII-D, with preliminary experiments already carried out. To study the applicability of rupture disks for disruption mitigation, a simple test apparatus was assembled in the laboratory. Commercially available rupture disks of 1-in nominal diameter were tested at conditions relevant for the application on tokamaks, including tests with Ar and He gases and rupture pressures of ~ 54 bar. Some technical and practical issues of implementing this technique on a tokamak are discussed.

A compact flexible pellet injector for the TJ-II stellarator

A compact multi-barrel pellet injector system is being developed for the TJ-II stellarator. Its design is based on the system currently installed at the MST facility (Univ. Winconsin) and will provide maximum flexibility at minimal cost, while also allowing for future upgrades. It is a four-barrel system destined for use both as an active plasma diagnostic and as a plasma fueling source. In order to achieve both objectives it will be sufficiently flexible to allow frozen hydrogen pellets with diameters from 0.4 to 1 mm to be formed and accelerated to velocities between 100 and 1000 m s-1. However, floor space restrictions and nearest-neighbor considerations limits the overall length to <1.7 m (mechanical punch end to final guide tube interfaces). This will be done by redesigns of the MST gun barrel, vacuum coupling, gas dump and guide tube sectors. Finally, the system is completed by stand-alone instrumentation and controls, as well as Lab View controlled gas manifolds

Experimental Study of Pellet Delivery to the ITER Inner Wall through a Curved Guide Tube at Steady-State Pressure

Injection of solid hydrogen pellets from the magnetic high-field side will be the primary technique for depositing fuel particles into the core of International Thermonuclear Experimental Reactor (ITER) burning plasmas. This injection scheme will require the use of a curved guide tube to route the pellets from the acceleration device, under the divertor, and to the inside wall launch location. In an initial series of pellet tests in support of ITER, single 5.3-mm-diam cylindrical D2 pellets were shot through a mock-up of the planned ITER curved guide tube. Those data showed that the pellet speed had to be limited to ap300 m/s for reliable delivery of intact pellets. Also, microwave cavity mass detectors located upstream and downstream of the test tube indicated that ap10% of the pellet mass was lost in the guide tube at 300 m/s. The tube base pressure for that test series was ap10-4 torr. However, for steady-state pellet fueling on ITER, the guide tube will operate at an elevated pressure due to the pellet erosion in the tube. Assuming the present design values for ITER pellet fueling rates/vacuum pumping and a 10% pellet mass loss during flight in the tube, calculations suggest a steady-state operating pressure in the range of 10-20 torr. Thus, experiments to ascertain the pellet integrity and mass loss under these conditions have been carried out. Also, some limited test data were collected at a tube pressure of ap100 torr. No significant detrimental effects have been observed at the higher tube pressures. The new test results are presented and compared to the baseline data previously reported

Massive pellet and rupture disk testing for disruption mitigation applications

Injection of massive quantities of noble gases or D2 has proven to be effective at mitigating some of the deleterious effects of disruptions in tokamaks. Two alternative methods that might offer some advantages over the present technique for massive gas injection are “shattering” massive pellets and employing close-coupled rupture disks. Laboratory testing has been carried out to evaluate their feasibility. For the study of massive pellets, a pipe gun pellet injector cooled with a cryogenic refrigerator was fitted with a relatively large barrel (16.5 mm bore), and D2 and Ne pellets were made and were accelerated to speeds of ∼600 and 300 m/s, respectively. Based on the successful proof-of-principle testing with the injector and a special double-impact target to shatter pellets, a similar system has been prepared and installed on DIII-D and should be ready for experiments later this year. To study the applicability of rupture disks for disruption mitigation, a simple test apparatus was assembled in the lab. Commercially available rupture disks of 1 in. nominal diameter were tested at conditions relevant for the application on tokamaks, including tests with Ar and He gases and rupture pressures of ∼54 bar. Some technical and practical issues of implementing this technique on a tokamak are discussed.

The Compact, Multiple Barrel High Speed Pellet Injector for the Ignitor Experiment

Ignitor is a compact, high field tokamak (R0 = 1.32 m, BT = 13 T) designed to attain ignition in high density, relatively low temperature plasmas (ne0 = ni0 = 1021 m-3, Te0 = Ti0 = 11 keV), by Ohmic heating (or with small amounts of additional ICRF heating). Tailoring of the density profile peaking during the initial plasma current rise is important to optimize Ohmic and fusion heating rates. Therefore, a pellet injector has always been included in the Ignitor design. Simulations performed with the NGS ablation model, for the reference ignition plasma parameters in Ignitor, indicate that deuterium pellet of a few mm sizes (< 4 mm) injected at 3-4 km/s from the low field side should achieve sufficient penetration, particularly during the current ramp up. A four barrel, two-stage pneumatic injector for the Ignitor experiment has been built in collaboration between ENEA and Oak Ridge National Laboratory, featuring two innovative concepts: (i) the proper shaping of the propellant pressure pulse to improve pellet acceleration, and (ii) the use of fast closing (~10 ms) valves to drastically reduce the expansion volumes of the propellant-gas removal system. The ENEA sub-system, including four independent two-stage guns and pulse- shaping valves, the gas removal system, and the associated controls and diagnostics, has been extensively tested at CRIOTEC. The ORNL sub-system consists of the cryostat and pellet diagnostics, with related control and data acquisition system. Initial testing with D2 pellets at speeds of ~1 km/s, using ORNL single-stage propellant valves, are scheduled to be completed by June 2007. The ENEA two-stage drivers will then replace the ORNL propellant valves, and integrated testing at high speeds (>3 km/s) will be finally carried out. The NGS model was also used to assess the maximum ablation depth of D2 pellets, of the sizes and speeds produced by the Ignitor Pellet Injector, inside JET plasmas. A similar analysis is now extended to the Large Helical Device (LHD), which has recently obtained high density plasma discharges (up to 5times1020 m-3). Deep pellet penetrations can be achieved over a wide range of plasma parameters in LHD, even at its highest temperature, thanks to the high speed of the IPI pellets.A four barrel, double stage pellet injector for the Ignitor experiment is under construction in collaboration between the ENEA Laboratory at Frascati and the Oak Ridge National Laboratory. The goal is to reach pellet velocities up to 4 km/s. Innovative concepts at the basis of the injector design considerably reduce the requirements on the expansion volumes necessary to prevent the propulsion gas to reach the plasma chamber. The full four barrels, double stage gun and gas removal system have been built and tested, while the design and construction of the pellet injector vacuum chamber, cryogenic system, gun barrels and pellet diagnostics is underway at ORNL, where the final assembly and testing of the complete system will be carried out

A new four-barrel pellet injection system for the TJ-II stellarator

A new pellet injection system for the TJ-II stellarator has been developed/constructed as part of a collaboration between the Oak Ridge National Laboratory (ORNL) and the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT). ORNL is providing most of the injector hardware and instrumentation, the pellet diagnostics, and the pellet transport tubes; CIEMAT is responsible for the injector stand/interface to the stellarator, cryogenic refrigerator, vacuum pumps/ballast volumes, gas manifolds, remote operations, plasma diagnostics, and data acquisition. The pellet injector design is an upgraded version of that used for the ORNL injector installed on the Madison Symmetric Torus (MST). It is a four-barrel system equipped with a cryogenic refrigerator for in situ hydrogen pellet formation and a combined mechanical punch/propellant valve system for pellet acceleration (speeds ∼100 to 1000 m/s). On TJ-II, it will be used as an active diagnostic and for fueling. To accommodate the plasma experiments planned for TJ-II, pellet sizes significantly smaller than those typically used for the MST application are required. The system will initially be equipped with four different pellet sizes, with the gun barrel bores ranging between ∼0.5 to 1.0 mm. The new system is almost complete and is described briefly here, highlighting the new features added since the original MST injector was constructed. Also, the future installation on TJ-II is reviewed.

Twin-screw extruder development for the ITER pellet injection system

The ITER pellet injection system is comprised of devices to form and accelerate pellets, and will be connected to inner wall guide tubes for fueling, and outer wall guide tubes for ELM pacing. An extruder will provide a stream of solid hydrogen isotopes to a secondary section, where pellets are cut and accelerated with a gas gun into the plasma. The ITER pellet injection system is required to provide a plasma fueling rate of 120 Pa-m3/s (900 mbar-L/s) and durations of up to 3000 s. The fueling pellets will be injected at a rate up to 10 Hz and pellets used to trigger ELMs will be injected at higher rates up to 20 Hz.

A Compact Flexible Pellet Injector for the TJ-II stellarator

A compact multi-barrel pellet injector system is being developed for the TJ-II stellarator. Its design is based on the system currently installed at the MST facility (Univ. Winconsin) and will provide maximum flexibility at minimal cost, while also allowing for future upgrades. It is a four-barrel system destined for use both as an active plasma diagnostic and as a plasma fueling source. In order to achieve both objectives it will be sufficiently flexible to allow frozen hydrogen pellets with diameters from 0.4 to 1 mm to be formed and accelerated to velocities between 100 and 1000 m s-1. However, floor space restrictions and nearest-neighbor considerations limits the overall length to <1.7 m (mechanical punch end to final guide tube interfaces). This will be done by redesigns of the MST gun barrel, vacuum coupling, gas dump and guide tube sectors. Finally, the system is completed by stand-alone instrumentation and controls, as well as Lab View controlled gas manifolds

Twin-screw extruder and pellet accelerator integration developments for ITER

The ITER pellet injection system consisting of a twin-screw frozen hydrogen isotope extruder, coupled to a combination solenoid actuated pellet cutter and pneumatic pellet accelerator, is under development at the Oak Ridge National Laboratory. A prototype extruder has been built to produce a continuous solid deuterium extrusion and will be integrated with a secondary section, where pellets are cut, chambered, and launched with a single-stage pneumatic accelerator into the plasma through a guide tube. This integrated pellet injection system is designed to provide 5 mm fueling pellets, injected at a rate up to 10 Hz, or 3 mm edge localized mode (ELM) triggering pellets, injected at higher rates up to 20 Hz. The pellet cutter, chamber mechanism, and the solenoid operated pneumatic valve for the accelerator are optimized to provide pellet velocities between 200–300 m/s to ensure high pellet survivability while traversing the inner wall fueling guide tubes, and outer wall ELM pacing guide tubes. This paper outlines the current twin-screw extruder design, pellet accelerator design, and the integration required for both fueling and ELM pacing pellets.