| CPC H05H 7/04 (2013.01) [C22C 38/002 (2013.01); C22C 38/02 (2013.01); C22C 38/12 (2013.01); C22C 38/16 (2013.01); H01F 1/15308 (2013.01); H01F 1/15333 (2013.01); H01F 1/15341 (2013.01); H01F 41/0226 (2013.01)] | 14 Claims |

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1. A core that is a toroidal core for an acceleration cavity applicable to a frequency of 100 kHz or more, obtained by winding an Fe-based magnetic ribbon having crystals with an average crystal grain size of 1 μm or less, wherein
a space factor of the Fe-based magnetic ribbon is 40% or more and 59% or less and the toroidal core has a portion having a gap portion of 10 μm or more from an inner diameter to an outer diameter, the gap portion being sandwiched between a first portion of an inner diameter side of the Fe-based magnetic ribbon and a second portion of an outer diameter side of the Fe-based magnetic ribbon which is adjacent to the first portion,
at least one of surfaces of the Fe-based magnetic ribbon includes an insulating layer having a thickness within a range of 5% or more and 25% or less of a thickness of the Fe-based magnetic ribbon, and
a space factor of the gap portion is 5% or more and 40% or less.
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