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Low-voltage operation of millimeter-wave folded waveguide traveling-wave tube (TWT) was investigated using a 12 kV linear electron beam. Backward wave oscillation operated at second space harmonic was observed with output power of 20 W, linear tunability of 6% within 0.01 dB/MHz, and voltage-frequency stability of 0.56 MHz/V. The measured frequency and output power are in a good agreement with the predicted values using a particle-in-cell (PIC) code. In addition, backward-wave interaction at second-space harmonic as an amplifier was observed with a measured linear gain of 15 dB and a bandwidth of 0.3%. For forward wave interaction, a linear gain of 25 dB, bandwidth of 17%, and efficiency of 7% were predicted for fundamental space harmonic using a PIC code. Improvement in efficiency was predicted for the forward-wave amplifier where it was operated as an oscillator employing a delayed feedback. Output power was increased by 10 dB in the delayed feedback oscillator comparing with the backward-wave oscillator. The effect of period doubling due to potential fabrication inaccuracies on the stopband was studied experimentally. The folded waveguide TWT fabricated using the Lighographie, Galvanoformung, Abformung (LIGA) process operated at much higher frequencies is discussed.