Observation of Beam Current Fluctuation Extracted from an RF-driven if Ion Source

In the Japan Proton Accelerator Research Complex (J-PARC), peak if current of several tens of mA is produced from a cesiated hydrogen plasma generated by a solid-state radio-frequency (RF) amplifier with the frequency of 2 MHz. In case of the high-intensity if beam extracted from the ion source, the plasma density in the source chamber is so high that the ion sheath around the beam extraction area follows the RF oscillation. Because the ion plasma frequency defined by the ion density is much higher than the driving frequency. The potential fluctuation of the plasma is combined with the driving RF electric field and causes motion of charged particles in the plasma. As a result, the beam extracted from the source plasma also fluctuates. The beam current signal from a Faraday cup was measured by a spectrum analyzer. A powerful frequency component at 2 MHz which is the same as that of the RF amplifier was also observed after the acceleration of a radio-frequency quadrupole (RFQ) linac located downstream of the ion source.