Experimental Investigation of the Cs Behavior in the Cesiated H- Ion Source During High Power Long Beam Operation

The behavior of the Cesium (Cs) in the Cs-seeded negative ion sources has been investigated experimentally under the beam accelerations of up to 0.5 MeV. The pulse length was extended to 100 s to catch the precise variations of the Cs D2 emission, discharge power, negative ion current and temperatures in the ion source. The variations of the negative ions were estimated by the beam current and the heat loads in the accelerator. This experiment shows that the buildup of temperature in the chamber walls lead to the evaporation of deposited Cs to enter the plasma region and influence the H- ion production. The H- ion beams were sustained stably by reducing the temperature rise of the chamber wall below 50 ℃. A stable long pulse beam could be achieved through the temperature control of the surfaces inside the source chamber walls

Development of poloidal horseshoe limiter concept for JA DEMO

In the development of a DEMO fusion reactor, mitigating the heat load deposited on the breeding blanket at steady-state operation is one of the challenging issues because of the localized high heat flux brought by charged particles and low thermal conductivity of the Reduced Activation Ferritic/Martensite (RAFM) steel structural material. This study proposes a concept of limiter for JA DEMO; a horseshoe-shaped limiter which is continuous poloidally in the first wall except for the divertor area, and discretized toroidally. The limiter is protruded from the first wall to shade the blankets from the charged particles, which move along the magnetic field lines. To enhance the heat removal capability and neutron irradiation tolerance, a tungsten mono-block using RAFM heat sink was applied as the plasma-facing unit (PFU) on the limiters. The heat removal capability of the limiter’s PFU was evaluated as 4.6 MW/m2. The limiter shape satisfying this value was determined basing on the utilization of heat load evaluation with three-dimensional magnetic field line tracing. Three limiter shape parameters; protrusion height, protrusion width and the first wall set-back, were investigated to understand the variation of charged particle load distribution. Both cylindrical and box-shaped blanket concepts were considered, and design windows for the limiter shape were acquired to discuss the optimum parameter. The optimum limiter shape fulfilling the requirement of steady-state operation has been defined, and the limiter’s occupied area amounts to 1.2 % of the FW area for both blanket options

Analysis of the Cesium Distribution in the JT-60SA Negative Ion Sources for Steady Long-Pulse Operation

To realize stable negative ion beams for 100 s required in the neutral beam injector of JT-60SA, a physical model to control cesium (Cs) distribution inside the negative ion source has been developed in order to maintain the stable negative ion production at the plasma grid (PG) surface with Cs. In this work, to quantitatively evaluate Cs coverage on the PG, a three-dimensional Cs transportation code was introduced to consider the spatial Cs distribution in the source. The spatial temperature distribution of the chamber wall was also introduced in this model. As a result, the reasonable variation of the Cs coverage for 100 s was obtained, compared to that in the initial model. Based on the modified model, the operational temperature of the chamber wall was proposed to be less than 60 ℃ to suppress the desorption of Cs in the chamber wall and to sustain the stable negative ion production. In addition, it was also suggested that a slightly higher wall temperature before the operation leads to a decrease in the amount of Cs stored at the chamber wall, resulting in suppression of Cs consumption in the ion source

Optimization of the Accelerators for the ITER Neutral Beam Injector Project

A joint Japan-EU R&D activity is in progress to optimize the accelerator for the ITER NBI. The accelerator baseline design is based on a five grids system which can be adapted to operate with three grids for initial operations at low voltage (500 kV). Moreover, in order to speed up the test of the NBI system at the Test Facility, a negative ion source with extraction voltage up to 100 kV will be operated in parallel to the full injector. In this contribution the three accelerators mentioned above are presented discussing the procedure to optimize the grid geometry in order to assure optimum optics during operation when the grids undergo deformations and thermal stresses due to the particles that hit their surface