Space-Charge Effects on Transverse Emittance in the Extraction Region of the RIKEN 28-GHz ECRIS

Abstract

To confirm the significance of space-charge effects (SCEs) on the emittance growth in the “extraction region” of the RIKEN 28-GHz electron-cyclotron-resonance ion source (ECRIS), we reconstructed the phase-space distribution of argon (Ar) beams traveling through high-intensity nitrogen (N) beams and reaching at the end of “extraction region”. Here, the “extraction region” is defined as the low-energy beam transport region between the ECRIS puller electrode and a magnetic analyzer. The transverse phase-space distribution (PSD) of the Ar beams, which were exposed to the SCEs due to the high-intensity N beams, was measured after magnetic analysis using the pepper-pot emittance meter (PPEM). Because we suppressed each Ar beam current below 50 µA, the expansion in the transverse phase-space distribution due to the SCEs after the magnetic analyzer could be sufficiently reduced. Sufficient angular resolution was achieved by improving the image analysis of PPEM measurements with the “optical flow method”. Therefore, we obtained a transverse PSD at the endpoint of the extraction region of the ECRIS using the three dimensional backtracking. The obtained rms emittance of the Ar10,11,13+ were found to increas as a function of the extraction current I ext, while those of Ar16+ were found to be nearly constant. The reconstructed PSD at the end of the “extraction region” shows that the smaller the M/Q, in other words, the larger the charge state Q, the smaller both the positional and angular distribution for each I ext. To reveal the space charge effect on the beam trajectory passing though the “extraction region”, further experimental measurements and analyses are underway, including detailed trajectory simulation.