The operating conditions of a rf plasma ion source utilizing a positively biased electrode have been investigated to develop a stably operating, high-current ion source. Ion beam characteristics such as currents and energies are measured and compared with bias currents by varying the bias voltages on the electrode immersed in the ambient rf plasma. Current-voltage curves of the bias electrode and photographs confirm that a small and dense plasma, so-called anode spot, is formed near an extraction aperture and plays a key role to enhance the performance of the plasma ion source. The ion beam currents from the anode spot are observed to be maximized at the optimum bias voltage near the knee of the characteristic current-voltage curve of the anode spot. Increased potential barrier to obstruct beam extraction is the reason for the reduction of the ion beam current in spite of the increased bias current indicating the density of the anode spot. The optimum bias voltage is measured to be lower at higher operating pressure, which is favorable for stable operation without severe sputtering damage on the electrode. The ion beam current can be further enhanced by increasing the power for the ambient plasma without increasing the bias voltage. In the same manner, noble gases with higher atomic number as a feedstock gas are preferable for extracting higher beam current more stably. Therefore, performance of the plasma ion source with a positively biased electrode can be enhanced by controlling the operating conditions of the anode spot in various manners.

High current mode has been discovered and investigated in a constricted dc plasma ion source. As discharge currents exceed a certain threshold, voltage to sustain the constricted dc plasma suddenly falls down to almost half of the value. In this sense, constricted dc plasmas can be sustained at much higher current than in conventional mode operation at a fixed discharge voltage. Phenomenally, several discrete layered-glows are created between an anode glow and a cathode glow. The layers are thin and divided by dark spaces where charged particles can be accelerated. In this high current mode, ion beam current density is about 100 times higher than in conventional mode at the same voltage. It is noteworthy that lower gas pressure is desirable to sustain the layered-glow mode, which is also profitable for ion source in terms of differential pumping. Ion current density exceeds 300 mA/cm(2) at low discharge power of 175 W where ion density of plasma ball is estimated to be over 3.7x10(12) cm(-3).

Stability of an anode spot plasma, which is an additional high density plasma generated in front of a positively biased electrode immersed in ambient plasma, is a critical issue for its utilization to various types of ion sources. In this study, operating conditions for the generation of stable anode spot plasmas are experimentally investigated. Diagnostics of the bias current flowing into the positively biased electrode and the properties of ambient plasma reveal that unstable nature of the anode spot is deeply associated with the reduction of double layer potential between the anode spot plasma and the ambient plasma. It is found that stability of the anode spot plasma can be improved with increasing the ionization rate in ambient plasma so as to compensate the loss of electrons across the double layer or with enlarging the area of the biased electrode to prevent electron accumulation inside the anode spot. The results obtained from the present study give the guideline for operating conditions of anode spot plasmas as an ion source with high brightness.

Anode spot in front of a positively biased electrode immersed in an inductively coupled plasma has been investigated in terms of ambient plasma properties. As varying operating conditions of the ambient inductively coupled plasma, the anode spot properties are measured by retarding field energy analyzer and Langmuir probe, and compared with numerical simulation based on the double layer theory. Diagnostic results show that the anode spot contains two groups of electrons: thermal electrons generated in the anode spot and drifted electrons from the ambient plasma. The drift electrons have the same thermal electron temperature with electrons in the ambient plasma while their drift energies are analogous to the potential difference between the anode spot and the ambient plasma. Both electrons are observed to contribute to the current flowing to the positively biased electrode, showing measured electron energy distribution with drifting components. The electron density of the anode spot as well as the bias electrode follows the ambient plasma density. Measured density of the anode spot is always higher than that of the ambient plasma, which has not been explained by double layer theory with Langmuir condition considering drift electrons and drift ions. However, numerical simulation including the thermal electrons in the anode spot shows that the density ratio of the anode spot to the ambient plasma could be higher than unity. The present study describing the anode spot properties and its correlation with ambient plasma will contribute to utilize the anode spot in various applications.

Microwave plasma ion source with rectangular cavity resonator has been examined to improve ion beam current by changing wave launcher type from single-port to double-port. The cavity resonators with double-port and single-port wave launchers are designed to get resonance effect at TE-103 mode and TE-102 mode, respectively. In order to confirm that the cavities are acting as resonator, the microwave power for breakdown is measured and compared with the E-field strength estimated from the HFSS (High Frequency Structure Simulator) simulation. Langmuir probe measurements show that double-port cavity enhances central density of plasma ion source by modifying non-uniform plasma density profile of the single-port cavity. Correspondingly, beam current from the plasma ion source utilizing the double-port resonator is measured to be higher than that utilizing single-port resonator. Moreover, the enhancement in plasma density and ion beam current utilizing the double-port resonator is more pronounced as higher microwave power applied to the plasma ion source. Therefore, the rectangular cavity resonator utilizing the double-port is expected to enhance the performance of plasma ion source in terms of ion beam extraction.