Analysis of Plasma Impedance in the Linac4 H- Source
A new linear accelerator Linac4  is being commissioned at CERN as a part of the upgrade of their accelerator chain. A radio frequency (RF) driven type negative hydrogen H- source is used as an injector of Linac4. The Linac4 H- source must deliver 40-50 mA, 45 keV H- beam. The power transfer efficiency between the RF generator and the ion source plasma is one of the important parameters that determine the extracted H- beam current. In order to achieve efficient power supply, it is required to match the impedance between the RF system and plasma loading. In order to match the impedance, it is essential to analyze the plasma impedance.
In this study, we analyze the plasma impedance by using the numerical simulation code. The model consists mainly two parts. One is the two-dimensional model of RF electromagnetic field, which calculates the electromagnetic fields numerically by solving Maxwell equations using the Finite Difference Time Domain (FDTD) Method. The other part is the particle dynamics model with the three-dimension in real and velocity space. The equations of motion for the charged particles are numerically solved. The particle velocity is also changed by collision process which is modeled by Monte Carlo method.
We are now analyzing the plasma resistance and inductance under the steady state. As the first step, the absorbed power to plasma is calculated by using Poynting theorem. The next step, the plasma resistance and inductance are modeled from the absorbed power to plasma. Also, we have developed the impedance prediction model which is the theoretical model  assuming Maxwellian distribution. The comparison between the results of the numerical simulation and those of the theoretical model will be presented.
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