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Experiments have been performed on a relativistic magnetron driven at e-beam accelerator peak parameters: voltage = -0.4 MV, current = 16 kA, and pulselength = 0.5 μs. The magnetron is a six-vane device operating at about 1 GHz with extraction from two cavities. For equal power in both extraction waveguides, the peak microwave power of this device is between 200 and 300 MW. Microwave pulse-shortening limits pulselengths to the range of 10-100 ns. Time-frequency analysis of microwave emission indicates operation at about 1.03 GHz, close to the pi mode frequency identified from cold tests and the three-dimensional MAGIC code. Two cold cathodes were tested: 1) an emitting aluminum knob in the vane region with no endcap and 2) an extended cathode with a graphite fiber emission region in the vanes and endcap outside the vanes. Electron endloss current has been measured for the two cathodes. With no endcap, the cathode exhibited endloss current fraction up to 50% of the total; with one endcap, the cathode reduced the endloss current fraction to as little as 12%. Both cathodes produced peak total-electronic efficiency in the range of 14%-21%.