Beam Characteristics Study for the KSTAR neutral beam long pulse ion source
Highlights
► An attempt was made to simulate beam extraction using IGUN code considering the engineering restrictions in designing an accelerator. ► The second gap has to be reduced and the G2 voltage ratio has to be optimized so that a high optimum perveance value and improved beam optics can be attained. ► The IGUN code simulation results were in agreement with the experimental results. The code can be used to design a new accelerator.
Introduction
Neutral beam injection into tokamak plasma is considered as an effective method of plasma heating and as a viable means to create a noninductive current drive in high-density fusion plasma. A prototype ion source has been developed for the Neutral Beam Injection (NBI) system of the Korea Superconducting Tokamak Advanced Research (KSTAR) project [1]. The design goal is to provide a 10 s, 100 keV/2 MW deuterium neutral beam into the KSTAR tokamak. Considering the neutralization efficiency and beam transmission efficiency, the necessary beam power at the ion source exit must be about 70 A/100 keV of the hydrogen ion beam.
The neutral beam ion source is, mechanically and functionally, composed of two main parts: the plasma generator and the ion beam accelerator. The accelerator has four electrodes: plasma (G1), gradient (G2), deceleration (G3) and ground (G4) grid. Because the KSTAR NB ion source is basically designed to operate for a long pulse, all components of the ion source should have cooling channels. This limits the design of the accelerator in large current beam extraction experiments. In the course of its development, several design modifications of the ion source were done to obtain a beam current at the design level with a divergence angle of less than 1°. As a result, the plasma generator showed relatively satisfactory discharge characteristics [2], while the accelerator part required an upgrade to meet the beam power requirements for heating the KSTAR plasma [3]. The necessity was related to the beam characteristics under various acceleration parameters in the redesign of the accelerator. First, a simulation study was carried out by using the IGUN code [4], with the result showing that reducing the thickness of the G1 grid, the G2 grid, and the second gap (the distance between the G2 and the G3 grid) is beneficial during high-perveance (I/V3/2) beam extraction to maintain a low level of divergence [5].
The purpose of this paper is to investigate experimentally the beam characteristics while varying several parameters of the ion source. The experimental results provide opportunities for the validation of the code simulation results as well as advancing our understanding in this research area. All of the studies in this field have led to a better understanding of the physics of the accelerator, which has in turn allowed the design of the ion source to be advanced further to meet the requirements of the KSTAR project.
Section snippets
Experimental set up
The present prototype ion source, based on the long pulse system, consists of a magnetic bucket plasma generator (discharge bucket chamber) and a set of accelerator with circular apertures. Schematics of the plasma generator and the accelerator modules are shown in Fig. 1. The plasma discharge chamber, which was developed by the Japan Atomic Energy Agency (JAEA) and enlarged to the size of an ion source discharge chamber for JT-60, has a cross-section of 25 cm × 59 cm and a depth of 31 cm. It
Experimental results
The second gap distance could be shortened simply by inserting a thick spacer between the grid plate and the grid supporter without any additional manufacturing or fabrication of the accelerator. This, also has the advantage of facilitating an easy change of the gap distance through replacement of the spacer.
With the second gap distance decreased to 6.0 mm, the performance of the accelerator and the perveance scan results are shown in Fig. 2. The optimum perveance value, with beam at the
Conclusion and discussion
The design goal of the long pulse, high-current ion source being developed for the KSTAR NB system is to provide a 10 s, 100 keV, 70 A hydrogen ion beam. Earlier experiments with a prototype ion source in the NB test stand showed that the accelerator grid assembly requires additional upgrading to achieve the final goal. The accelerator should have good beam optics, a large optimum perveance value, and a sufficient cooling capacity. Considering the engineering restrictions in designing an
References (6)
- et al.
Rev. Sci. Instrum.
(2008) - et al.
Rev. Sci. Instrum.
(2008) - et al.
Rev. Sci. Instrum.
(2008)
Cited by (5)
Performance of 300 s-beam extraction in the KSTAR neutral beam injector
2012, Current Applied PhysicsCitation Excerpt :The first LPIS (LPIS-1) is composed of a plasma generator and a set of prototype tetrode accelerators. The plasma generator was developed by the Japan Atomic Energy Agency (JAEA) in Japan [9], and the prototype accelerator was developed by the Korea Atomic Energy Research Institute (KAERI) in Korea [10]. The LPIS-1 consists of a magnetic bucket plasma generator with multi-pole cusp fields, and a set of tetrode accelerators with copper circular apertures.
Modification to the accelerator of the NBI-1B ion source for improving the injection efficiency
2016, Review of Scientific InstrumentsPerformance of a new ion source for KSTAR tokamak plasma heating
2014, Plasma Science and TechnologyAssessment and modification of an ion source grid design in KSTAR neutral beam system
2014, Review of Scientific InstrumentsOperating characteristics of a new ion source for KSTAR neutral beam injection system
2014, Review of Scientific Instruments