High-speed emittance measurements for beams extracted from J-PARC RF ion source

Oscillation of emittance and Twiss parameters in the negative ion (H−) beam from the J-PARC 2MHz RF ion source is measured by application of a double-slit emittance monitor located at the RFQ entrance. The emittance monitor is equipped with a newly-developed 60 MS/s data acquisition system, so that beam current oscillations at a few MHz can be observed with enough time resolution. From the measurement, it is shown that the beam phase space consists of: (1) a Direct Current component in the beam core; (2) a 2 MHz oscillating component which takes place in the diverging halo; (3) a doubled RF frequency (4 MHz) oscillation which faintly exists in the beam halo. The major component is the 2 MHz component, which resultantly decides the beam emittance oscillation frequency. A typical value of the beam emittance in the present experiment is 0.33 π mm-mrad, while the amplitude of the 2 MHz oscillation is around 0.04 π mm-mrad. The results indicate that the high-frequency oscillation component occupying about ten-percent of the beam from the RF source travels a few meters passing through a magnetic lens focusing system.