D. T. Fehling

Pellet fuelling, ELM pacing and disruption mitigation technology development for ITER

Plasma fuelling with pellet injection, pacing of edge localized modes (ELMs) by small frequent pellets and disruption mitigation with gas jets or injected solid material are some of the most important technological capabilities needed for successful operation of ITER. Tools are being developed at the Oak Ridge National Laboratory that can be employed on ITER to provide the necessary core pellet fuelling and the mitigation of ELMs and disruptions. Here we present progress on the development of the technology to provide reliable high throughput inner wall pellet fuelling, pellet ELM pacing with high frequency small pellets and disruption mitigation with gas jets and shattered pellets. Examples of how these tools can be employed on ITER are discussed.

Tritium pellet injector results

Injection of solid tritium pellets is considered to be the most promising way of fueling fusion reactors. The Tritium Proof‐of‐Principle experiment has demonstrated the feasibility of forming and accelerating tritium pellets. This injector is based on the pneumatic pipe‐gun concept, in which pellets are formed in situ in the barrel and accelerated with high‐pressure gas. This injector is ideal for tritium service because there are no moving parts inside the gun and because no excess tritium is required in the pellet production process. Removal of 3He from tritium to prevent blocking of the cryopumping action by the noncondensible gas has been demonstrated with a cryogenic separator. Pellet velocities of 1280 m/s have been achieved for 4‐mm‐diam × 4‐mm‐long cylindrical tritium pellets with hydrogen propellant at 6.96 MPa (1000 psi).

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.

Comparison of deuterium pellet injection from different locations on the DIII-D tokamak

Deuterium pellets have been injected into plasmas in the DIII-D tokamak from the inner wall, top, and outer midplane port locations to investigate fuelling efficiency, mass deposition and interaction with edge localized modes (ELMs). Pellets injected from the outer midplane port (low field side (LFS)) show a large discrepancy in the mass deposition profile and fuelling efficiency from conventional pellet ablation theory, while the penetration depth compares favourably with theory. The mass deposition from pellets injected from inner wall and top locations is deeper than expected from ablation theory. The profile measurements indicate that pellet mass is deposited inside the measured penetration radius, thus verifying that a drift of the pellet ablatant is occurring in the major radius direction during the toroidal symmetrization process. The scaling of the measured drift magnitude in DIII-D is found to depend strongly on the pellet size and plasma pedestal temperature. Extrapolation to a burning plasma configuration on ITER is favourable for inner wall pellet fuel deposition depth well beyond the separatrix. Pellets injected into H-mode plasmas from all locations trigger ELMs with much larger ELM events induced by the outside midplane injected pellets. This suggests that the LFS is more sensitive to ELM triggering and may be the preferred location to inject very small pellets to trigger frequent small ELMs and thus minimize ELM induced damage to the divertor material surfaces.

ATF heavy ion beam probe: Installation and initial operation

Installation of the HIBP on ATF began in the summer of 1988. All of the major hardware components have now been installed. The initial operation of the diagnostic has begun amid the final stages of testing and control system integration. The existence of significant magnetic fields and gradients outside of the main plasma volume and fully three‐dimensional particle trajectories have raised several interesting issues during the design, assembly, alignment, and operation of the beamline and analyzer. The diagnostic must function in a challenging environment. It must perform satisfactorily despite electrical interference from several nearby sources, pressure excursions caused by gas puffing, and UV/plasma loading.

Development of a two-stage light gas gun to accelerate hydrogen pellets to high speeds for plasma fueling applications

The development of a two‐stage light gas gun to accelerate hydrogen isotope pellets to high speeds is under way at Oak Ridge National Laboratory. High velocities (>2 km/s) are desirable for plasma fueling applications, since the faster pellets can penetrate more deeply into large, hot plasmas and deposit atoms of fuel directly in a larger fraction of the plasma volume. In the initial configuration of the two‐stage device, a 2.2‐1 volume (≤55 bar) provides the gas to accelerate a 25.4‐mm‐diam piston in a 1‐m‐long pump tube; a burst disk or a fast valve initiates the acceleration process in the first stage. As the piston travels the length of the pump tube, the downstream gas (initially at <1 bar) is compressed (to pressures up to 2600 bar) and thus is driven to high temperature (≊5000 K). This provides the driving force for acceleration of a 4‐mm pellet in a 1‐m‐long gun barrel. In preliminary tests using helium as the driver in both stages, 35‐mg plastic pellets have been accelerated to speeds as high as ...

A high throughput spectrometer system for helium ash detection on JET

Acquiring information about helium ash production and transport is fundamental for future burning plasma devices, such as International Thermonuclear Experimental Reactor, since the helium ash must be continuously removed from the plasma to prevent the dilution of the deuterium–tritium (DT) fuel. This diagnostic for future JET DT operation uses charge-exchange recombination spectroscopy (CXRS) in conjunction with the JET neutral heating beam to measure the helium density at 20 radial locations across the JET plasma via the 4686 A He+ line and an array of heated 1 mm quartz fibers. The CXRS diagnostic utilizes a high throughput short focal length spectrometer with f/1.8 input optics, two entrance slits, a holographic transmission grating, and refractive optics. The detector is a thinned back-illuminated charge coupled device that has high quantum efficiency, a 10 MHz readout speed, and a time resolution of 5 ms.

New ORNL Pellet Injection System and Installation/Initial Operations on MST

Abstract A compact pellet injection system that was recently developed at the Oak Ridge National Laboratory has been installed on the Madison Symmetric Torus (MST) at the University of Wisconsin and used in initial plasma fueling experiments. The system, referred to as a “pellet injector in a suitcase,” is a pipe gun device with a four-barrel capability (presently equipped with two 1.0-mm-bore barrels), and it uses a cryogenic refrigerator for in-situ hydrogen pellet formation (typically, D2 pellets). This new, portable, stand-alone pellet injection system was developed to provide a flexible means of plasma fueling on a wide variety of magnetic confinement devices, with relatively low costs for installation and operation. The injector has already been used to produce useful results with pellets on MST plasmas, including significant and rapid increases (almost 100%) in the line average density, and effectively depositing fuel in the plasma core (central densities of [approximately equal to ≈ 1.4 × 1019 m−3). In this paper, the injection system, its performance, and reliability will be described, and results from some initial MST pellet experiments will be highlighted.

Plasma source development for fusion-relevant material testing

Plasma-facing materials in the divertor of a magnetic fusion reactor have to tolerate steady state plasma heat fluxes in the range of 10 MW/m2 for ∼107 s, in addition to fusion neutron fluences, which can damage the plasma-facing materials to high displacements per atom (dpa) of ∼50 dpa. Materials solutions needed for the plasma-facing components are yet to be developed and tested. The material plasma exposure experiment (MPEX) is a newly proposed steady state linear plasma device designed to deliver the necessary plasma heat flux to a target for testing, including the capability to expose a priori neutron-damaged material samples to those plasmas. The requirements of the plasma source needed to deliver the required heat flux are being developed on the Proto-MPEX device which is a linear high-intensity radio-frequency (RF) plasma source that combines a high-density helicon plasma generator with electron- and ion-heating sections. The device is being used to study the physics of heating overdense plasmas i...