Recent progress in dust-trapping research at the photon factory advanced ring

For decades, electron storage rings have been afflicted by irregular phenomena that give rise to sudden decrease in the beam lifetime. These phenomena are ascribed to the trapping of positively ionized macroparticles ('dust'), and are still problematic to the stable operation at the Photon Factory Advanced Ring (PF-AR). Careful observations for many years led us to a hypothesis that two kinds of electric discharges in vacuum may trigger the dust trapping, and the measures we took on the basis of this hypothesis, namely, switching distributed ion pumps off and conditioning discharge sources with higher current beams, could significantly suppress the occurrence of the phenomena. Experimental demonstration of the hypothesis was also carried out at PF-AR, and proved that the electric discharges surely triggered the dust trapping. A remarkable result was additionally obtained in this experiment; a dust particle trapped by the beam was visually observed with video cameras. Visual observation of trapped dust would play a key role in further investigation of the dust-trapping mechanism. (author)