Discharge Characteristics of Large-Area High-Power RF Ion Source for Positive and Negative Neutral Beam Injectors

A large-area high-power radio-frequency（RF） driven ion source was developed for positive and negative neutral beam injectors at the Korea Atomic Energy Research Institute（KAERI）. The RF ion source consists of a driver region, including a helical antenna and a discharge chamber, and an expansion region. RF power can be transferred at up to 10 kW with a fixed frequency of 2 MHz through an optimized RF matching system. An actively water-cooled Faraday shield is located inside the driver region of the ion source for the stable and steady-state operations of high-power RF discharge. Plasma ignition of the ion source is initiated by the injection of argongas without a starter-filament heating, and the argon-gas is then slowly exchanged by the injection of hydrogen-gas to produce pure hydrogen plasmas. The uniformities of the plasma parameter,such as a plasma density and an electron temperature, are measured at the lowest area of the driver region using two RF-compensated electrostatic probes along the direction of the shortand long-dimensions of the driver region. The plasma parameters will be compared with those obtained at the lowest area of the expansion bucket to analyze the plasma expansion properties from the driver region to the expansion region.

Development of data acquisition and over-current protection systems for a suppressor-grid current with a neutral-beam ion source

Neutral beam injection is one of the effective auxiliary heating methods in magnetic-confinementfusion experiments. In order to acquire the suppressor-grid current signal and avoid the grid being damaged by overheating, a data acquisition and over-current protection system based on the PXI（PCI e Xtensions for Instrumentation） platform has been developed. The system consists of a current sensor, data acquisition module and over-current protection module. In the data acquisition module,the acquired data of one shot will be transferred in isolation and saved in a data-storage server in a txt file. It can also be recalled using NBWave for future analysis. The over-current protection module contains two modes： remote and local. This gives it the function of setting a threshold voltage remotely and locally, and the forbidden time of over-current protection also can be set by a host PC in remote mode. Experimental results demonstrate that the data acquisition and overcurrent protection system has the advantages of setting forbidden time and isolation transmission.

Experiment and analysis of the neutralization of the electron cyclotron resonance ion thruster

An electron cyclotron resonance ion thruster must emit an electron current equivalent to its ion beam current to prevent the thruster system from being electrically charged. This operation is defined as neutralization. The factors which influence neutralization are categorized into the ion beam current parameters, the neutralizer input parameters, and the neutralizer position. To understand the mechanism of neutralization, an experiment and a calculation study on how these factors influence thruster neutralization are presented. The experiment results show that the minimum bias voltage of the neutralizer was -60 V at the ion beam current of 80 mA for the argon propellant, and a critical gas flow rate existed, below which the coupling voltage increased sharply. Based on the experiment, the neutralization was analyzed by means of a onedimensional calculation model. The computation results show that the coupling voltage was influenced by the beam divergence and the negative potential zone near the grids.

Influence of a centered dielectric tube on inductively coupled plasma source: Chamber structures and plasma characteristics

A high-density RF ion source is an essential part of a neutral beam injector. In this study, the authors attempt to retrofit an original regular RF ion source reactor by inserting a thin dielectric tube through the symmetric axis of the discharge chamber. With the aid of this inner tube, the reactor is capable of generating a radial magnetic field instead of the original transverse magnetic field, which solves the E × B drift problem in the current RF ion source structure. To study the disturbance of the dielectric tube, a fluid model is introduced to study the plasma parameters with or without the internal dielectric tube, based on the inductively coupled plasma（ICP） reactor. The simulation results show that while introducing the internal dielectric tube into the ICP reactor, both the plasma density and plasma potential have minor influence during the discharge process, and there is good uniformity at the extraction region. The influence of the control parameters reveals that the plasma densities at the extraction region decrease first and subsequently slow down while enhancing the diffusion region.