Modulation of absorption manner in helicon discharges by changing profile of low axial magnetic field

The modulation of absorption manner in helicon discharge by changing the profile of low axial magnetic field is explored experimentally in this work. The experiments are carried out in Boswell-type antenna driven by 13.56-MHz power source in 0.35-Pa argon environment. The peak of the external non-uniform magnetic field (B_ex) along the axis is observed in a range from 0 Gs to 250 Gs (1 Gs = 10⁴ T), where the electron density varies from 0.5×10¹⁶ m^-3 to 9×10¹⁶ m^-3. When B_ex is located near the tube upper end sealed by a dielectric plate, or near the tube bottom end connected with a diffusion chamber, the plasmas are centralized in the tube in the former case while the strong luminance appears between the edge of the tube and the axial line in the latter case. When Bex is located in the middle of the antenna, moreover, an effective resistance (R_eff) peak appears apparently with increasing magnetic field. The glow moves toward first the edge of the tube and then the two antenna legs as the magnetic field increases. The discharge in this case is caused by the absorption of Trivelpiece-Gould (TG) wave. It is suggested that Bex is located in the middle of the antenna to obtain a higher efficiency of power transfer. 2017 Chinese Physical Society and IOP Publishing Ltd.