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High-density radio-frequency (RF) plasma sources have been produced by ring-shaped hollow-cathode discharge at various trench shapes. The influence of hollow-cathode trench shape on high-density plasma production has been investigated at a wide range of argon gas pressure and various input powers. The trench shapes are selected from well-typed, taper-typed, step-typed, and three improved step-typed trench shapes. It is revealed that, at the lower pressure less than 200 mtorr, the plasma density for the well-typed shape is the highest among the three shapes, while for the step-typed shape, it is the lowest. For the higher pressure more than 200 mtorr, the plasma density for the step-typed shape has the highest value. The results indicate that the plasma density for all typed shapes is almost proportional to RF input power. It is found that the improved step-typed trench shapes can attain a wide range of gas pressure sustaining higher plasma density.