Abstract-The design and experimental study of a 35-GHz gyrotron-traveling-wave tube (gyro-TWT) amplifier operating in the circular TE 0 i mode at the fundamental cyclotron harmonic are presented. The interaction circuit in this experiment consisted of a new type of ceramic loading that provided the required loss for stable operation. A saturated peak power of 137 kW was measured at 34.1 GHz, corresponding to a saturated gain of 47.0 dB and an efficiency of 17%, with a -3-dB bandwidth of 1.11 GHz (3.3%). Peak output powers in the range of 102.1 to 148.6 kW with -3-dB bandwidths of 1.26 and 0.94 GHz, respectively, were measured by varying the operating parameters. The gyro-TWT was found to be zero-drive stable at these operating points, demonstrating that ceramic loading is a highly effective means of suppressing spurious oscillations in gyro-TWTs. This type of ceramic loading has the added advantage of being compatible with high average power operation.

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Microfabrication techniques have been developed using ultraviolet photolithography (UV-LIGA) with SU-8 photoresists to create advanced sheet beam amplifier circuits for the next generation of vacuum electron traveling wave amplifiers in the 210-220 GHz (G-band) frequency regime. We describe methods that have led to successfully fabricated millimeter wave circuits, including applying ultra-thick SU-8 photoresist layers on copper, copper electroforming solutions, and the challenging removal of the SU-8 photoresists. A table of experimental liquid SU-8 removal chemistries and results is also presented.

Vacuum electron devices require electron beams to be transported through hollow channels that pass through an electromagnetic slow-wave circuit. These electron 'beam tunnels' are shrinking toward sizes smaller than traditional techniques can manage as the operating frequencies push toward the THz. A novel technique is described and experimentally demonstrated that uses polymer monofilaments of arbitrary cross-sectional shape combined with ultraviolet photolithography (UV-LIGA) of SU-8 photoresists. This combination of monofilaments and SU-8 structures comprises a 3D mold around which copper is electroformed to produce high-quality beam tunnels of arbitrary length and size along with the electromagnetic circuits. True round beam tunnels needed for upper-millimeter wave and THz vacuum electron devices can now be fabricated in a single UV-LIGA step. These techniques are also relevant to microfluidic devices and other applications requiring very small, straight channels with aspect ratios of several hundred or more.