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The design and experimental study of a Ka-band gyrotron traveling-wave tube (gyro-TWT) amplifier operating in the circular TE01 mode at the fundamental cyclotron harmonic are presented. A periodic dielectric loaded structure is adopted in the interaction circuit of the gyro-TWT to suppress spurious modes TE11 and TE21. A saturated peak power of 156 kW is obtained with a saturated gain of 44 dB, an efficiency of 23.3% in an operating voltage of 67 kV, an electron beam current of 10 A, and a -3-dB bandwidth of 1.8 GHz (~5.2%). The gyro-TWT is found to be zero-drive stable at these operating points and to be characteristic of the excellent purity of the signal frequency spectrum in the operating frequency range.