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Large particle accelerator projects like the European X-ray Free Electron LASER (XFEL), which is under construction at DESY in Hamburg (Germany), or like the planned International Linear Collider (ILC) favor MBKs for their ability to generate high rf output powers at moderate electron beam energies. For the XFEL, Deutsches Electronen Synchrotron (DESY) ordered a prototype of a horizontally oriented, high-power MBK operating at 1300 MHz. This is CPGammas second generation 10 MW, L-band MBK, designated the VKL-8301B. The klystron is required to provide at least 65% efficiency at 10 MW peak rf output and more than 3 MHz instantaneous -l dB bandwidth. Average power and rf pulse length are 150 kW and 1.5 ms, respectively. Our design uses six off-axis electron beams for low cathode current density and thus longer cathode life. For the rf cavities we are utilizing fundamental-mode ring resonators. This ensures sufficient beam separation while still keeping the overall diameter of the device small in order to reduce cost. A more in-depth report on our design process and the tools we used was presented at the 2008 ICOPS. At the time of submission of this paper, initial rf hot test data were taken at reduced duty and pulse width while operating into well-matched output loads. At 0.42% rf duty, a peak rf output power as high as 11.2 MW was achieved, which corresponds to a 74% efficiency. The electron beam interception at and close to saturation was fairly high. Preliminary measurements of body power and the high efficiency indicate that mostly low-energy electrons past the output cavity gaps are involved in the klystron body interception in that operating condition. Initial optimization of the electromagnet for lower interception resulted in a 70% reduction of the rf body current, but this was accompanied by a noticeable drop in rf output power, efficiency (down to 67%) and gain. The klystron performance was stable for all operating conditions encountered so far. Further opti- ization of the magnetic field balance still has to be carried out for low body current and high efficiency at all required operating conditions, including operation into an output mismatch up to 1.2:1 VSWR. A more complete set of data taken after optimization and tuning will be presented at the conference.