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Description
Oscillation of emittance and Twiss parameters in the negative ion $(\text{H}^-)$ beam from the J-PARC 2MHz RF ion source is measured by applications of a double-slit emittance monitor located at the RFQ (Radio Frequency Quadrupole) entrance. The emittance monitor is equipped with a newly-developed 60 MS/s data acquisition system, so that beam current oscillation in 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 DC component in the beam core, (2) a 2MHz oscillating component which takes place both in the beam core and the halo and (3) a doubled RF frequency (4 MHz) oscillation which slightly 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 0.34 $\pi$ mm-mrad, while the amplitude of the 2MHz oscillation is around 0.04 $\pi$ 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.
