Japan Atomic Energy Agency
Japan Atomic Energy Agency (JAEA; President: Toshio Kodama) succeeded in development of a key control technique to suppress the beam divergence in an ion source of the neutral beam injector (NBI) during a long-pulse operation in a joint experiment with Korea Atomic Energy Research Institute (KAERI) and the National Fusion Research Institute of Korea (NFRI). This technique has ensured a higher-power and longer-pulse neutral beam injection (2.0 MW for 100 seconds) than one required for the JT-60SA (1.9 MW for 100 seconds), a superconducting tokamak device for nuclear fusion, which is under construction at JAEA Naka Fusion Institute.
In order to realize the ion source for the JT-60SA device, it was necessary to develop a technique to control a high–power and high-density beam for 100 seconds, more than 3 times longer pulse duration than the past limit (30 seconds). JAEA has an international collaborative agreement with KAERI and NFRI in terms of fusion research and development. Thereby, prior to the JT-60SA operation, the joint experiment to validate the long-pulse capability of the ion source was held at KAERI’s test stand.
In the case of the long-pulse operation over 30 seconds, it was found that the unexpected heat loads onto the acceleration grids, generated by larger beam divergence, restricted the beam generation time. Therefore, in order to avoid this beam divergence, an improved technique to control the pressure of the discharge gas and the applying acceleration voltages, which determines the beam divergence angle, was developed. With this improvement, the production of 2.0 MW ion beam for 100 seconds, which was more powerful than the requirement for the JT-60SA device (1.9 MW for 100 seconds) was achieved for the first time.
This technique can be widely applied to control other ion sources such as ones for ITER and DEMO, where further longer pulse operation (more than 1,000 seconds) is required. Moreover, it is expected to contribute to improve the economic efficiency of the ion sources for industrial use such as semi-conductor production and for large-scale accelerators.
The NBI is widely used as a plasma heating system. In an ion source of NBI, ion beams are converted to neutral beams through a gas cell, as shown in Fig. 1. In the JT-60SA project, the NBIs will be used for the plasma heating up to a high temperature, where fusion reactions effectively occur, and for the plasma current drive in order to maintain high pressure plasmas.
Fig. 1 Schematic view of the NBI system
Neutral beams are produced by an ion source and a gas cell in the NBI system. The ion source consists of an ion production unit and an electrostatic accelerator, and accelerates high energy ion beams. In the ion production unit, ions are produced by an arc discharge driven by filaments. In the accelerator, a high voltage is applied to accelerate ions through 4 acceleration grids with 1,020 apertures.
The JT-60SA device has a big advantage that it will be able to inject neutral beams to plasma in various timings and directions according to the plasma characteristics, using a total of 13 NBI units, as shown in Fig. 2. A single NBI unit has 2 ion sources each. A total of 24 positive ion sources and 2 negative ion sources will be employed for these NBIs.
This positive ion source was developed for the JT-60 tokamak, the former fusion device, in 1986. A single ion source was designed to accelerate 75 keV, 35 A ion beams for 10 seconds from a small extraction area which size was almost a half of ones developed for the large fusion devices in the world. Since this ion source was modified several times, the available pulse duration has been extended up to 30 seconds by now. However, the JT-60SA device is planned to have NBIs which realize a pulse operation for 100 seconds. Thus, the longer-pulse operation technique of the ion source was required to be established prior to the JT-60SA operation start. For this purpose, JAEA planned to have the joint experiment to demonstrate the longer-pulse operation of this ion source in collaboration with KAERI and NFRI.
Fig. 2 JT-60SA device and the ＮＢＩ systems
Total 13 NBI units will be installed around the JT-60SA device. Each unit has 2 ion sources, and injects neutral beams to plasma for 100 seconds.
Fig. 3 shows the ion source which consists of a discharge chamber for ion production and an accelerator with 4 acceleration grids having 1,020 apertures. These ion sources were installed to KAERI’s test stand for the joint experiment.
The long-pulse capability of the ion source is strongly correlated with the heat loads on the acceleration grids. In addition, the beam divergence angle is determined by the rate between the beam current and the intermediate voltage of the acceleration voltage.
In the joint experiment, the beam current was found to be degraded according to the accumulation of the discharge gas for ion production, with a long time constant over 30 seconds. Due to this degradation, the beam current deviated from the amount to keep optimum relationship with the intermediate voltage, and the beam divergence angle became larger accordingly. As a result, a part of some beams contacted with the electro-nodes in the ion source, and generated additional heat loads onto the acceleration grids. In conclusion, it was found that this unexpected heat loads limited the pulse duration of the neutral beams for the first time. Therefore, the pressure of the discharge gas was precisely controlled to suppress the time variation of the beam current. Moreover, the intermediate voltages were adjusted according to the beam current. In order to achieve the higher-power and longer-pulse beams, a technique to control these 2 parameters simultaneously was developed. After these modifications, the current measured at the intermediate grid became lower and the temporal constant, which means the low heat loads, was achieved and maintained during a long-pulse operation, as shown in Fig. 4.
Fig. 3 Installation to the KAERI test stand
The JT-60 ion source was installed to KAERI’s test stand for the joint experiment.
As a result, the ion beam of 2.0 MW for 100 seconds was achieved, which was more powerful beam than the required value for the JT-60SA device (1.9 MW for 100 seconds), as shown in Fig. 5. Because this technique can be applied to the long-pulse operation of the ion sources for the Korean Superconducting Tokamak Advanced Research (KSTAR), this achievement by an international collaboration is useful for the KSTAR at NFRI, a participant in this joint experiment, as well.
Fig. 4 Schematic view of the long-pulse operation technique (Top),Current measured at the intermediate grid (Bottom)
Fig. 5 Progress of the beam pulse duration of the ion source.
The ion source developed in this joint experiment will be used for the JT-60SA device, where the plasma heating and current drive over 100 seconds will need to be demonstrated. The results will be submitted to “Fusion Engineering and Design”, a science journal, as well.
The NBI is one of the plasma heating systems being utilized in nuclear fusion research. It injects powerful neutral beams to the plasmas. At first, ion beams are produced in an ion source of NBI. Then, in a gas cell, the ion beams are converted to neutral beams to be injected into the magnetic field in a tokamak device.
The JT-60SA project is the ITER satellite tokamak program in the Broader Approach (BA) activity, which is a collaborative program between Japan and Europe. The JT-60SA device is under construction at JAEA Naka Fusion Institute located in Naka city of Ibaraki prefecture, Japan.
The KSTAR is a research project using a superconducting tokamak device constructed at NFRI, located in Daejeon city of Korea.
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