Direct plasma injection scheme with beam extraction in a radio frequency quadrupole linac cavity

Direct plasma injection scheme (DPIS) is a recently developed idea about beam injection into a radio frequency quadrupole (RFQ) linac to obtain high current pulsed heavy ion beam. Since a laser ion source is directly connected to an RFQ linac, a low energy beam transport is not needed. An ion beam is extracted at the edge of the cavity. We shifted the extraction position in the RFQ cavity to improve the beam loss problem due to beam diverging before arriving at the entrance of the RFQ electrode region. In this scheme, ion beam is extracted at the entrance of the RFQ electrode region and the extracted beam is immediately captured by a rf quadrupole electric field. A plasma electrode is inserted in the accelerator resonant cavity. It is required to avoid serious effect on the acceleration characteristics of the RFQ linac. The rf properties were measured using a network analyzer to verify the effect. The shifts of resonant frequency were negligibly small by inserting the electrode. The measurements of the electric field distribution along the beam axis in the RFQ linac showed that the plasma electrode had little effect on it. The result of beam acceleration experiment showed that the beam current was increased by about 20%. It was confirmed that the beam loss was reduced by this scheme.