S. Maruyama

Pellet fuelling and control of burning plasmas in ITER

Pellet injection from the inner wall is planned for use in ITER as the primary core fuelling system since gas fuelling is expected to be highly inefficient in burning plasmas. Tests of the inner wall guide tube have shown that 5 mm pellets with up to 300 m s−1 speeds can survive intact and provide the necessary core fuelling rate. Modelling and extrapolation of the inner wall pellet injection experiments from present days smaller tokamaks leads to the prediction that this method will provide efficient core fuelling beyond the pedestal region. Using pellets for triggering of frequent small edge localized modes is an attractive additional benefit that the pellet injection system can provide. A description of the ITER pellet injection systems capabilities for fuelling and ELM triggering is presented and performance expectations and fusion power control aspects are discussed.

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.

Disruption mitigation technology concepts and implications for ITER

Disruptions on ITER present challenges to handle the intense heat flux, the large forces from halo currents, and the potential first wall damage from energetic runaway electrons. Injecting large quantities of material into the plasma during the disruption can reduce the plasma energy and increase its resistivity to mitigate these effects. Assessments of the amount of various mixtures and quantities of the material required have been made to provide collision mitigation of runaway-electron conversion, which is the most difficult challenge. The quantities of the material required (~0.5 MPa·m3 for deuterium or helium gas) are large enough to have implications on the design and operation of the vacuum system and tokamak exhaust processing system.

Fuelling and disruption mitigation in ITER

The ITER tokamak is to be fueled mainly by pellet injection and gas puffing to control discharge parameters. The ITER pellet injection system (PIS) will be the main plasma density control tool for fuelling ITER and also provides ELM pacing functionality. The gas injection system (GIS) provides gas fuelling for plasma and wall conditioning operation, and H2 and D2 gases to NB injectors. The fuelling system also serves the critical function of disruption mitigation, including the suppression of runaway electrons resulting from the mitigation. This paper presents an overview of the ITER fuelling system design and development, the requirements that the disruption mitigation system (DMS) must satisfy and the development strategy to ensure that a reliable DMS is in place for the start of ITER operations.

Fast acting eddy current driven valve for massive gas injection on ITER

Tokamak plasma disruptions present a significant challenge to ITER as they can result in intense heat flux, large forces from halo and eddy currents, and potential first-wall damage from the generation of multi-MeV runaway electrons. Massive gas injection (MGI) of high Z material using fast acting valves is being explored on existing tokamaks and is planned for ITER as a method to evenly distribute the thermal load of the plasma to prevent melting, control the rate of the current decay to minimize mechanical loads, and to suppress the generation of runaway electrons. A fast acting valve and accompanying power supply have been designed and first test articles produced to meet the requirements for a disruption mitigation system on ITER. The test valve incorporates a flyer plate actuator similar to designs deployed on TEXTOR, ASDEX upgrade, and JET [1-3] of a size useful for ITER with special considerations to mitigate the high mechanical forces developed during actuation due to high background magnetic fields. The valve includes a tip design and all-metal valve stem sealing for compatibility with tritium and high neutron and gamma fluxes.

Manifold concept design for ITER Gas Injection System

The main functions of ITER Gas Injection System (GIS) are to provide gas fuelling for plasma, wall conditioning operation, and neutral beam injectors. Dedicated manifold, which contains independent tubes for H<inf>2</inf>/D<inf>2</inf>, H<inf>2</inf>, T<inf>2</inf>, ⁴He/³He, N<inf>2</inf>/Ne, Ar and evacuation, is the key part of gas injection lines. It shall deliver gases from the tritium plant to the various fuelling systems. This paper presents an overview of GIS manifold design, especially introduces the solution of penetration structure and routing of manifold from concept design point of view.

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

Manifold Concept Design for ITER Gas Injection System

The main functions of the ITER gas injection system (GIS) are to provide gas fueling for plasma, wall conditioning operation, and neutral beam injectors. The dedicated manifold, which contains independent tubes for H₂/D₂, H₂, T₂, ⁴He/³He, N₂/Ne, Ar, and evacuation, is the key part of the gas injection lines. It shall deliver gases from the tritium plant to various fueling systems. This paper presents an overview of the GIS manifold design, particularly introducing the solution of the penetration structure and the routing of the manifold from the concept design point of view.

Pellet Injection Technology and Its Applications on ITER

Cryogenic pellet injectors for use in fusion research have been under development at Oak Ridge National Laboratory for over 30 years. The original application of the technology was to add fuel to magnetically confined plasmas to replace D-T ions that are consumed in the fusion reactions or lost due to transport out of the confining magnetic fields. This application is still the primary use for pellet injection and is planned for implementation on the ITER burning plasma experiment. More recently, there have been additional applications for the injection of cryogenic pellets in the areas of disruption and edge-localized mode mitigation. Injectors for these applications are also being implemented for ITER, which require refinements of the technology for production and shattering of very large pellets and production of very small high repetition rate pellets, respectively. Details of these applications and injection system designs are presented.

Preliminary results of glow discharge cleaning on SWIP test bench

In order to explore the characteristic of glow discharge cleaning (GDC) electrode of ITER and to support ITER GDC conceptual design, GDC tests are performed on SWIP GDC test bench. The setup of the GDC test bench is described. The tests focus on the breakdown, volt-ampere characteristics and heat load of helium and hydrogen glow discharge with recessed electrode, which is based on current ITER GDC conceptual design. Some results of such a recessed electrode and some unclear phenomena are shown.