Nuclear Instruments and Methods in Physics Research A 483 (2002) 331–335 Investigation of bunch repetition rate deviations in FIR FEL driven by a magnetron-based microtron Grigori M. Kazakevitch a, * ,1 , Young Uk Jeong a , Byung Cheol Lee a , Jongmin Lee b a Laboratory For Quantum Optics, Korea Atomic Energy Research Institute, P.O. Box 105, Yusong, Taejon 305-600, South Korea b Kwangju Institute of Science and Technology(K-JIST), 1 Oryong-dong, Kwangju 500-712, South Korea Abstract The stability of the bunch repetition rate in a FIR FEL driven by a 2.8 GHz magnetron-based microtron was investigated using a heterodyne method with a low Q-factor straight-flight measuring cavity. The measuring cavity is located in the straight section of the FIR FEL injection beam line and is excited by the passage of electron bunches. The RF signal from the measuring cavity coupling loop was mixed with a precise heterodyne signal with a frequency difference of several MHz. The beat frequency was analyzed to obtain the temporal distribution of the bunch repetition rate deviation during the macro pulse of the electron beam. The time resolution and the accuracy of measurements are approximately 100 ns and a few kHz, respectively. Based on this data, we could determine the level and shape of the magnetron current and the initial frequency shift between magnetron and accelerating cavity for the FEL operation in the wavelength range 100–300 mm. r 2002 Elsevier Science B.V. All rights reserved. PACS: 41.60.Cr Keywords: Microtron; Frequency pulling; Magnetron; Free electron laser; Deviations. 1. Introduction Investigation of the frequency stability of a RF field in the accelerating cavities becomes very important if the accelerator is used as an injector for a free electron laser (FEL). In case of a microtron-FEL injector driven by a conventional magnetron, the frequency stability of the RF accelerating field is one of the most important but rather complicated problems. For FEL opera- tion the next requirements: a high current of the accelerated beam and a high frequency stability of the RF field in the accelerating cavity should be satisfied simultaneously. In the magnetron-driven microtron the accelerating field frequency stability depend on the beam-accelerating cavity interaction and on the frequency stability of the magnetron generator, which usually is not so high. To investigate the RF accelerating field instability of the high current microtron-FEL injector we have developed a direct method of the bunch repetition rate deviations measurements using a monitoring straight-flight cavity excited by the passage of the electron bunches [1,2]. Using this method the developed principle of the accelerating field frequency stabilization has been investigated to *Corresponding author. Tel.: +82-42-868-8253; fax: +82- 42-861-8292. E-mail address: kaza@kaeri.re.kr (G.M. Kazakevitch). 1 Permanent postal address: Budker Institute of Nuclear Physics, Academician Lavrentyev, 11, Novosibirsk 630090, Russia. 0168-9002/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0168-9002(02)00338-8