We present initial experimental results from the successful operation of a 600-kW peak, fundamental-mode multiple-beam klystron (MBK). The eight-beam device operates at a cathode voltage of ～45 kV and a total beam current of ～32 A with an axial guiding magnetic field of 1.8-2.2 kG. In the absence of radio-frequency (RF) drive, the measured beam transmission is in excess of 99%; at a driven frequency of 3.25 GHz, the measured beam transmission at saturation is ≥97%, where the four-cavity circuit generates a peak power of ～600 kW with an electronic efficiency of 40%. The measured beam transport and RF performance are in excellent agreement with predictions made by the three-dimensional gun/collector code, MICHELLE, and the large-signal klystron code, TESLA. The accuracy of the design codes enabled the achievement of a working device in a single hardware design pass.
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