Experimental investigation of the dual- frequency effects in helicon plasmas on the PPT device

Investigation of dual-frequency (DF) effects in helicon plasmas was conducted on the PKU Plasma Test (PPT) device, integrating a 13.56 MHz helicon source with a 2 MHz auxiliary heating system under cylindrical and magnetic mirror configurations. Experimental results demonstrate that lower-frequency (LF, 2 MHz) auxiliary heating effectively elevates electron temperature ( T_\mathrme ) while reducing ion density ( n_\mathrmi ), thereby overcoming the limitation of single-frequency (SF) helicon systems in achieving efficient electron heating. The observed density reduction is attributed to the accelerated axial flow velocity, validated via 10-tip Mach probe diagnostics, and suppressed radial particle transport confirmed by 5-tip Langmuir probe array measurements. DF operation of helicon plasma in magnetic mirror field shows similar T_\mathrme elevation and lower density reduction amplitude. Moreover, the linear response of T_\mathrme to LF power offers a viable strategy for temperature adjustment.