L. Svensson

R&D progress of the high power negative ion accelerator for the ITER NB system at JAEA

At JAEA, as the Japan Domestic Agency (JADA) for ITER, a MAMuG (multi-aperture multi-grid) accelerator has been developed to perform the required R&D for the ITER neutral beam (NB) system. As a result of countermeasures to handle excess heat load to the ion source by backstreaming positive ions, H− ion beam current was increased to 0.32 A (the ion current density of 140 A m−2) at a beam energy of 796 keV. This high power beam acceleration simulated the ITER operation condition maintaining the perveance (H− ion current density/beam energy3/2) of the ITER accelerator. After the high power beam operation, the pulse length was successfully extended from 0.2 to 5 s at 550 keV, which yielded a 131 mA H− ion beam as an initial test of the long pulse operation. A test of a single-aperture single-gap (SINGAP) accelerator was performed at JAEA under an ITER R&D task agreement. The objective of this test was to compare two different accelerator concepts (SINGAP and MAMuG) at the same test facility. As a result, the MAMuG accelerator was defined as the baseline design for ITER, due to advantages in its better voltage holding and less electron acceleration. In three-dimensional beam trajectory analyses, the aperture offset at the bottom of the extractor was found to be effective for compensation of beamlet deflection due to their own space charge. It has been analytically demonstrated that these compensated beamlets can be focused at a focal point by adopting the aperture offset at the final grid of the accelerator.

Development of 1 MeV H− Accelerator at JAEA for ITER NB

This paper reports the recent activities at JAEA for the development of 1 MeV H− ion accelerator toward the ITER NBI. For the development of MAMuG accelerator, a 320 mA H− ion beam was successfully accelerated as the highest record in the world at the MeV class energy (796 keV). This was achieved by protecting the H− ion source from the high heat load by the backstream positive ions, which was produced during the high current H− ion acceleration. The SINGAP accelerator was tested at JAEA to compare the performance of the SINGAP and the MAMuG. This paper also reports the results of the SINGAP test and the comparison between the SINGAP and the MAMuG is discussed.

Results of the SINGAP Neutral Beam Accelerator Experiment at JAEA

IRFM (CEA Cadarache) and JAEA Naka have entered into a collaboration in order to test a SINGAP [1] accelerator at the JAEA Megavolt Test Facility (MTF) at Naka, Japan. Whereas at the CEA testbed the acceleration current was limited to 0.1 A, at JAEA 0.5 A is available. This allows the acceleration of 15 H‐ beamlets in SINGAP to be tested and a direct comparison between SINGAP and MAMuG [2] to be made. High‐voltage conditioning in the SINGAP configuration has been quite slow, with 581 kV in vacuum achieved after 140 hours of conditioning. With 0.1 Pa of H2 gas present in the accelerator 787 kV could be achieved. The conditioning curve for MAMuG is 200 kV higher. SINGAP beam optics appears in agreement with calculation results. A beamlet divergence better than 5 mrad was obtained. SINGAP accelerates electrons to a higher energy than MAMuG. Measurements of the power intercepted on one of the electron dumps have been compared with EAMCC code [3] calculations. Based on the experiments described here, electron ...

1-MV Vacuum Insulation for the ITER Neutral Beam Injectors

The ITER is an international project which aims to develop an experimental reactor as a step to realize fusion energy. To inject 33 MW of 1 MeV D0 neutral beams for heating and current drive in ITER plasmas, D- ions are accelerated to 1 MeV. The electrostatic accelerator consists of five acceleration stages, and each acceleration gap has to withstand 200 kV. Gaps between the accelerator and the vacuum vessel, which is at ground potential, must sustain voltages up to 1 MV, and high-voltage vacuum insulation is a key issue. A minimum gap length of >; 900 mm was selected for the l-MV insulation distance. Development of a high-voltage bushing (HVB) which is a bulkhead and a feedthrough between the gas insulated HV transmission line and the beam source in vacuum is ongoing. The HVB consists of a stack of five large bore ceramic rings (1.56 m diameter), each 0.29 m in height. The five-stage insulation concept was applied to both the accelerator and the HVB for better voltage holding with multiple short gaps compared to fewer longer gaps. R&D on a single-stage HVB ceramic ring with the associated electrostatic screens was carried out, and voltage holding of -203 kV dc for 5 h was confirmed.

Aperture Size Effect on Extracted Negative Ion Current Density

This paper discusses experimental results obtained at the 1 MV testbed at CEA Cadarache that appear to show a higher extracted D− current density from small apertures. Plasma grids with different shapes have been installed and tested. All grids had one single aperture. The tests were done in volume operation and in caesium operation. We tested four grids, two with O/14 mm, one with O/11 mm and one with O/8 mm apertures. No aperture size effect was observed in volume operation. In caesiated operation the extracted current density for the O/8 mm aperture appears to be significantly higher (∼50%) than for the O/14 mm aperture. Simulations with a 3D Monte Carlo Trajectory Following Code have shown an aperture size effect of about 20%. Finally, as byproducts of the experiments, data on backstreaming positive ions and the temperature of the plasma grid have been obtained.

An active phase compatible protection system to prevent excessive neutral beam shinethrough during JET plasmas

The JET Fast Beam Interlock System (FBIS) removes the neutral beam power with a fast response (<20 ms) under a diverse range of fault conditions. The configuration of FBIS for the tritium or active phase of operation of JET is described, in particular its expansion to include inputs from a new interlock designed to prevent excessive neutral beam shinethrough. This new system is based on visible bremsstrahlung emission from the plasma and is called the Bremsstrahlung Beam Interlock (BBI). Its main features are detailed here, including heated fibre-optics and a compensation fibre which make the interlock compatible with the active phase. In addition, the calibration of the interlock using JETs infra-red interferometer is described as well as operational experience with deuterium-tritium plasmas.

The JET active phase gas introduction systems for neutral beam injection and beamline commissioning and operation with tritium

During the first part of the active phase of JET, the Deuterium and Tritium Experiment 1 (DTE1), the tritium was delivered to the plasma mainly by neutral beam injection. T/sub 2/ and D/sub 2/ were delivered from U beds in the active gas handling system (AGHS) and were introduced to the Positive Ion Neutral Injectors (PINIs) at ground potential via a new specially developed Tritium and Deuterium Gas Introduction System (TDGIS). This paper describes the function of the TDGIS, the installation and the commissioning of the system. It also describes the operation of the neutral beam system up to 160 kV with tritium beams.

Heating neutral beams for ITER: Present status

The heating neutral beam (HNB) systems at ITER are designed to inject a total of 33 MW of either 1 MeV D0 or 870 keV H0 beams into the ITER plasma using two injectors with a possible addition of a third injector later to increase the injected power to ~50 MW. The injectors become radioactive due to the neutron flux from ITER and, in order to avoid the resulting complex remote maintenance, the design, choice of materials and the manufacturing process of each component of the injector is, wherever possible, such that they survive the life time of ITER. To ensure a smooth operational phase of neutral beams at ITER a neutral beam test facility (NBTF) is under construction at Consorzio RFX, Padova, (hereinafter referred to as RFX), which consists of 2 test beds, the 100 kV “SPIDER”, and a 1 MV “MITICA” facilities, which will be used to optimize the source operation for H and D beams. MITICA is essentially a full scale ITER prototype injector for the ITER beam parameters. The manufacturing and operation of the facility will allow validation of the operational space of the injectors and provide valuable information about the manufacturing processes applicable to HNB components. Operation of the two facilities is expected to begin in 2016 and 2019 respectively. Currently experiments on the ELISE facility with a half ITER sized RF beam source are underway. ITER relevant parameters for the H beams have almost been achieved. Efforts are underway to optimise the same with D beams. The experimental database from ELISE will be an important input for establishing the ITER relevant parameter space on the SPIDER source. This paper discusses the present status of the design and development of the injectors for ITER and the progress on the test facilities.

Manufacturing experience of beam dump for SPIDER facility

The SPIDER facility is undergoing installation & commissioning in the Neutral Beam Test facility at RFX, Padova. SPIDER Beam Dump (SBD) is mandated to diagnose calorimetrically an ion beam of power up to 6 MW accelerated from the SPIDER Beam Source. SBD utilizes Hypervapotron based Heat Transfer Elements (HTEs) as cooling elements. HTEs are made of Cu-Cr-Zr alloy, designed for removing a steady state heat flux of 10MW/m2. 62 HTEs are arranged in two halves, mounted on a support structure ensuring the required alignment. Diagnostics provisions include independent calorimetric measurement of the coolant temperature at the exit of each of the HTEs and multiple thermocouples embedded in the edges of the HTEs. SBD is designed w.r.t ITER Structural Design Criteria for In Vessel Components (SDC-IC) and is manufactured to meet ITER codes & standards requirements and specific requirements related to vacuum components as specified by ITER Vacuum Handbook. Several challenges experienced in the manufacturing include: i) Production of CuCrZr alloy with specific mechanical properties in aged condition, ii) EB welding of dis-similar material combination like CuCrZr - Ni, Ni -SS 316L and iii) Joining CuCrZr to CuCrZr in lap joint configuration, where an acceptance basis for the allowable extent of ground pores in the lap configuration of EB weld seam has been cast. The complete execution of this activity has conformed to ITER quality requirements. The procurement has been managed by ITER-India and has completed the manufacturing, factory inspection and has been delivered as the first mechanical equipment for the SPIDER facility. This paper will present an overview of the experience gained during SPIDER Beam Dump manufacturing including the identification of technical issues and their resolution and provide important inputs to the technical management of production of similar mechanical components for ITER requirements.

Reliability study of the JET neutral injection system

The JET (Joint European Torus) neutral beam injection (NBI) system, has demonstrated a very high degree of reliability and availability. Data on the reliability and availability of the beamlines and the associated subsystems (e.g. power supplies, cooling and cryogenic supplies, computerized control, etc.) have been recorded over the past three years. These data are used to quantify the overall reliability of the NBI systems and to identify problem areas. The overall reliability (energy injected into JET/energy requested) and availability for the 1990 experimental campaign are both better than 80% and improve significantly during routine operation to nearly 90% after the system has been fully commissioned.<<ETX>>