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Summary form only given. A second-harmonic TE/sub 21/ gyro-TWT amplifier with an axis-encircling beam is being constructed at UCD that is predicted to double the efficiency of our previous 200 kW, 12% efficient MIG TE/sup (2)//sub 21/ gyro-TWT. The new device will avoid the loss in efficiency due to off-axis electrons interacting with a linear polarized mode. The 70 W, 3.5 A axis-encircling beam with v/sub /spl perp///v/sub 2/ = 1.2 will be produced by a Cusp electron gun delivered by Northrop Grumman. The amplifier is predicted by our large signal code to produce 50 KW in Ka-band with 20% efficiency, 30dB saturated gain and 3% saturated bandwidth. An important advantage of harmonic gyro-TWTs is that they are capable of higher power than at the fundamental cyclotron harmonic because they are more stable to the absolute instability at cutoff of the operating mode. Due to the weaker strength of the harmonic interactions, the start-oscillation current is significantly higher for higher harmonic operation. This allows higher harmonic gyro-TWTs to operate stably with appreciably higher beam current. The second-harmonic TE/sub 21/ gyro-TWT amplifier is predicted to have a start oscillation current of 5 A for v/sub /spl perp///v/sub 2/, = 1.2, Bo/Bg = 0.99 which yields a safety margin 30% for our planned operating parameters. The device will employ a sliced mode-selective interaction circuit to destroy the odd order TE/sub ml/ modes. The circuit is sliced with two orthogonal slices through the axis to interrupt the wall currents and is surrounded by a lossy cylinder to absorb radiated power. Also, loss will be added to the interaction circuit to suppress gyro-BWO. The interaction circuit will have a wall resistivity 2300 times copper to yield a stable interaction length of 220 r/sub w/ for the strongest gyro-BWO threat, the TE (4)41 mode. The last 11.5 cm of the 42 cm circuit is not loaded to avoid attenuating the wave after it reaches high power levels. Identical multi-h- le 0 dB input and output couplers were designed with the BFSS code using the same design as the previous UCD experiment. The directional coupler contains an array of slots connecting the narrow wall of the TE/sub 10/ rectangular input waveguide to the TE/sub 21/ circular interaction waveguide. HFSS simulation results predict that the coupler will have less than 0.3 dB insertion loss over a bandwidth of 10%. All modes are matched in the coupler due to upstream termination. BFSS simulation results predict that the return loss will be greater than 19 dB for the TE/sub 11/ circular mode and 45 dB for the TE/sub 21/ circular mode over the 10% bandwidth.