Watanabe, K. JACoW Geneva, Switzerland 978-3-95450-187-8 10.18429/JACoW-eeFACT2016-WET3BH1 English 192-194 WET3BH1 ion klystron electron cavity radiation Contribution to a conference proceedings 2017 2017-07 https://doi.org/10.18429/JACoW-eeFACT2016-WET3BH1 http://jacow.org/eefact2016/papers/wet3bh1.pdf A high power RF system for the particle accelerators needs large electrical power in the operation. An improvement of efficiency is also always required as a technology component for the energy saving. To improve efficiency of a high-power source, the CPD (Collector Potential Depression) method already was applied a Gyrotron to recovery the electrical energy form the collector loss. The CPD is an energy-saving scheme that recovers the kinetic energy of the spent electrons after generating rf power. A CPD klystron (E37703 CPD) was fabricated at 2013, to recycle an existing klystron of Toshiba E3786. The purpose of our study is to demonstrate the proof-of-principle of the CPD method to apply a klystron. A plane of R&D of CPD klystron at KEK will be reported in this meeting.