New progress of the miniaturized microwave ion source at Peking University

The generation of plasma in a microwave ion source involves confining electrons using a static magnetic field and energizing them with an electromagnetic field that transmitted into the plasma chamber. However, according to electromagnetics theory, there is always a cut-off size in circular wave guides for a given frequency. For a 2.45 GHz microwave, this dimension is 72 mm, which should theoretically prevent transmission of the microwave into the discharge chamber and no plasma can be generated. Since 2006 Peking University (PKU) has successfully developed a series of permanent magnet 2.45 GHz microwave ion sources (PKU PMECRs) with a discharge chamber less than 50 mm, capable of delivering tens of mA beams for accelerators. To explain this anomalous phenomenon, a hybrid discharge heating (HDH) mode that combines surface wave plasma and electron cyclotron heating has been proposed. This HDH mode not only successfully explains PKU PMECRs, but also predicts that the optimized inner diameter of the plasma chamber is 24 mm, which is confirmed by experiments involving different liners in the miniaturized microwave ion source.