Extraction of single-ion beams from helicon ion source in high plasma density operation mode: Experiment and simulation

Abstract

Tests of a high plasma density high-brightness helicon ion source for nuclear microscopy applications are underway. Experiments were performed with hydrogen, helium, and argon. Different extraction structures of the helicon rf ion source were investigated with an imposed external magnetic field. We present measurements of the extracted current as a function of the extraction voltage and rf power. The source has been diagnosed by a microwave interferometer. Plasma densities in the vicinity of the emission hole of up to 7.2× 1012 cm-3 (for argon), 1.6× 1012 cm-3 (for helium), and 6.0× 1011 cm-3 (for hydrogen) were obtained for working gas pressure of <5 mTorr and rf power <350 W (frf =27.12 MHz). The ion current density was 80 mA cm2 with high percentage of protons in the beam (∼80%). In the extraction structure, the cathode channel has a 3 mm length and a 0.6 mm diameter. The phase set degradation due to aberrations in the extraction structure of the helicon rf ion source was simulated for a fourth-order approximation in series expansion of the electrostatic potential and third-order approximation in series of the magnetic fields, by the matrizant method. The calculations were performed involving experimental data on ion energy spread, average ion energy, and plasma density of the helicon rf ion source with permanent magnets. © 2006 American Institute of Physics.