Plural spline shafts are arranged around columnar plasma, and plural spline shafts are coaxially aligned in a direction that the columnar plasma extends within a vacuum chamber. The plural coaxially aligned spline shafts are positioned so that axial gap is formed between the respective male spline sections. The axial gap of the plural male spline sections is positioned at a center of the columnar plasma in the direction that the columnar plasma extends.