@article{oai:repo.qst.go.jp:00080729,
 author = {Zen, Heishun and Ohgaki, Hideaki and Hajima, Ryoichi and Ryoichi, Hajima},
 issue = {10},
 journal = {Applied Physics Express},
 month = {Oct},
 note = {The highest extraction efficiency (9.4%) of a free electron laser (FEL) oscillator has been achieved at the midinfrared FEL facility of Kyoto University. Because of the interaction between the electron beam and FEL electromagnetic field, a maximum electron energy decrease of 16% was observed. The measured energy decrease was consistent with the measured FEL spectrum. An FEL micropulse energy of ~100 μJ with the expected few-cycle pulse duration at a wavelength of 11 μm was observed. This result is an important milestone for the high-extraction-efficiency FEL oscillator and will contribute to the strong-field physics of atoms and molecules},
 title = {Record high extraction efficiency of free electron laser oscillator},
 volume = {13},
 year = {2020}
}