Innovative Cesiation Deriving Incredible 145 mA Beam from J-PARC Cesiated RF-Driven H⁻ Ion Source

In NIBS2022, the stable 8 hours operation of the J-PARC cesiated RF-driven H⁻ ion source (IS) in a test-stand with a 69.9 keV 120 mA beam and a beam duty factor of 4 % (1 ms x 40 Hz) was reported. However, the Cesiation condition was produced after many times and rather large amount of Cs and H₂Os injections. The fluctuation of the H⁻ ion beam intensity (I_H⁻) in a pulse and the transverse emittances were rather larger than those for the previous 65 keV 110 mA operation. The plasma electrode temperature (T_PE) of 254 °C much higher than that not only for the usual J-PARC IS (about 70 °C) but also for the standard cesiated H⁻ ion sources (180 ∼ 200 °C) suggested the existence of novel H₂O mediated cesiation surviving against the high T_PE. In this paper, the J-PARC IS improvements based upon a hypothesis of the best cesiation constituted of sub monolayer H₂O (chemically bound with Mo) mediated cesiation and Cs half monolayer on remaining surface are presented. The innovative cesiation derived a 76.5 keV 145 mA beam stably with the small beam fluctuation and transverse emittances suitable for Radio Frequency Quadrupole LINACs of high energy LINACs. The measured results of the 145 mA / 83 mA beam, extraction electrode current and RF waveforms, parameter trends of an 8 hours 145 mA operation and the transverse emittances are presented. The optimum T_PE was increased from 70 °C to 230 °C. Furthermore, the beam intensity for the J-PARC IS operation energy of 52.5 keV was increased from 72 mA to 83 mA, which was consistent with the 1.5 power perveance law on the beam energy compared with 145 mA for 76.5 keV.