Yeong-Shin Park

Characterization of plasma ion source utilizing anode spot with positively biased electrode for stable and high-current ion beam extraction.

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.

Brightness enhancement of plasma ion source by utilizing anode spot for nano applications.

Anode spots are known as additional discharges on positively biased electrode immersed in plasmas. The anode spot plasma ion source (ASPIS) has been investigated as a high brightness ion source for nano applications such as focused ion beam (FIB) and nano medium energy ion scattering (nano-MEIS). The generation of anode spot is found to enhance brightness of ion beam since the anode spot increases plasma density near the extraction aperture. Brightness of the ASPIS has been estimated from measurement of emittance for total ion beam extracted through sub-mm aperture. The ASPIS is installed to the FIB system. Currents and diameters of the focused beams with∕without anode spot are measured and compared. As the anode spot is turned on, the enhancement of beam current is observed at fixed diameter of the focused ion beam. Consequently, the brightness of the focused ion beam is enhanced as well. For argon ion beam, the maximum normalized brightness of 12,300 A∕m(2) SrV is acquired. The ASPIS is applied to nano-MEIS as well. The ASPIS is found to increase the beam current density and the power efficiency of the ion source for nano-MEIS. From the present study, it is shown that the ASPIS can enhance the performance of devices for nano applications.

Operating conditions for the generation of stable anode spot plasma in front of a positively biased electrodea)

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.

Investigation of helium ion production in constricted direct current plasma ion source with layered-glows.

Generation of helium ions is experimentally investigated with a constricted direct current (DC) plasma ion source operated at layered-glow mode, in which electrons could be accelerated through multiple potential structures so as to generate helium ions including He(2+) by successive ionization collisions in front of an extraction aperture. The helium discharge is sustained with the formation of a couple of stable layers and the plasma ball with high density is created near the extraction aperture at the operational pressure down to 0.6 Torr with concave cathodes. The ion beam current extracted with an extraction voltage of 5 kV is observed to be proportional to the discharge current and inversely proportional to the operating pressure, showing high current density of 130 mA/cm(2) and power density of 0.52 mA/cm(2)/W. He(2+) ions, which were predicted to be able to exist due to multiple-layer potential structure, are not observed. Simple calculation on production of He(2+) ions inside the plasma ball reveals that reduced operating pressure and increased cathode area will help to generate He(2+) ions with the layered-glow DC discharge.

Effect of ambient plasma properties on anode spot in an inductively coupled plasma

Summary form only given. 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.

Numerical study on space-charge-limited bipolar current flow in spherical electron sheath

Effect of bipolar current flow on the characteristics of spherical electron sheath has been studied by numerically solving Poissons equation with iterative process. Contrary to the previous studies, simulations include not only the double layer, but also the transition state of the electron sheath with bipolar current flow. Simulation results show that the bipolar current flow due to the introduction of ion current increases the space-charge-limited electron current between fixed boundaries, owing to partial neutralization. The ratio of electron current increment depends on both radius ratios of outer to inner spheres and the fraction of ion to electron current. The maximum value of this ion current fraction, the Langmuir condition, is found to be proportional to the radius ratio and higher than the value of planar sheath. Correspondingly, maximum ratios of the electron current increment in spherical boundary are higher than that in planar electron sheath, 1.86. Based on the current enhancement at the fixed spherical boundary, increase of sheath thickness by the bipolar current flow is determined for the spherical electron sheath with movable boundary, considering area expansion of the sheath boundary. Depending on the initial boundary sphere radius ratios, sheath thickness increases until the ion current fraction reaches to the Langmuir condition. A remarkable result is that the maximum increment ratios of sheath thickness are identical to 1.364, the value of the planar sheath, regardless of the initial sheath thickness and boundary radius ratio. Since the current density, which determines the thickness of the electron sheath at fixed sheath voltage, is limited by the plasma properties, the sheath expansion ratio might be expected to be the same at fixed plasma condition, regardless of sheath geometry. This study will help to understand transition of spherical electron sheath to spherical double layer in the presence of bipolar current flow more clearly.

Breakdown Conditions of Local Sheath Discharge in Front of Positively Biased Electrode Immersed in Inductively Coupled Plasma

Summary form only given. Increasing ion beam current is one of the most challenging issues in plasma ion source. It was reported that ion beam current was drastically enhanced by generating additional plasma, called local sheath discharge, near the extraction hole located at center of the electrode by biasing positively.The breakdown condition and discharge mechanism, however, were not elucidated sufficiently. In this study, breakdown conditions for the local discharges have been experimentally investigated by varying the discharge conditions such as gas pressure, power for generating background plasma, and geometry of the electrode. An electrode screened by dielectric material except small circular area, is immersed in inductively coupled plasma (ICP) and biased with positive voltage with respect to the space potential of the plasma. A bright plasma bubble is observed locally just above the exposed part of the biased electrode with abrupt increase of current to the electrode when the bias voltage exceeds a certain threshold, breakdown voltage. The breakdown voltage decreases as the gas pressure is raised. Radio frequency power for generating ICP as well as the electrode geometry influences on the breakdown voltage too. These experimental results indicate that ambient plasma properties such as density, temperature, and space potential play an important role in generating local sheath discharges.

Tokamak equilibrium solver by neural network algorithm

Summary form only given, as follows. Tokamak equilibrium analyses have been routinely carried out by solving the Grad-Shafranov equation with various conventional numerical techniques. As a new approach for the equilibrium analysis a neural network equilibrium solver has been proposed from the functional approximation capabilities of the artificial neural network. The neural network equilibrium solver does not have to perform finite differences and coordinate transformations, so its manipulation becomes easy and simple. It may also save computing time significantly for smoothly evolving equilibria from the equilibrium information of the previous time step. Moreover, the implicitly embedded interpolation function of the neural network solver can make it applicable to real-time equilibrium analysis with the aid of on-line learning algorithms. With comprehensive structural variance and manipulation, characteristics of the neural network solver - the structural dependency on external source profile variations, the number of reasonable iterations for the stable convergence, and computational speed etc. - will be presented and elucidated in view of real-time equilibrium analyses.

Plasma position control simulation with a simple model for new control coil system of KSTAR tokamak

Summary form only given, as follows. Summary form only given. In-vessel coils are to be used for the fast plasma position control, field error correction (FEC), and resistive wall mode (RWM) feedback stabilization in the Korea Superconducting Tokamak Advanced Research (KSTAR) device. Recently, a new configuration that incorporates toroidal segmentation concept, has been adopted. The new coil system is found to allow a wider range of plasma control flexibility satisfying the KSTAR advanced physics requirements for the plasma position and FEC/RWM control capability, in addition to engineering advantages. With the modified coil structure, plasma position control has been simulated by using a simple linear /sup 1/RZIP (R, Z, I/sub p/) model. The RZIP model is developed as a simple circuit equation representing a linear plasma response model. This model can be easily constructed without solving plasma, equilibrium and is more explicit in the quantities that define the plasma response. By applying step response simulation and random perturbation simulation for the vertical position control, PID controller gain and power supply limit are compared with those of the previous axisymmetric internal control coils. Results from the simple model are benchmarked with the /sup 2/TSC simulation for axisymmetric internal control coils.