LAUSR

A novel expression for the form factor of a disc-loaded circular cylindrical interaction structure has been derived, which is helpful in finding optimum beam position, so that maximum beam–wave interaction can be achieved in a disc-loaded gyro-TWT amplifier. The penetration of the propagating waves in different regions of the structure was observed using CST Microwave Studio, and it has been found that RF fields are mostly concentrated in the disc-free free-space region. Only a small percentage ($$\sim $$∼ 5%) of the travelling wave enters in disc-occupied free-space region. The dependence of the form factor variation on the disc parameters, namely disc-hole radius, disc repetition, and disc thickness of the structure excited in azimuthally symmetric $$\text {TE}_{01}$$TE01 mode, was observed. Maximum coupling of the beam and RF wave was found to be at the ratio of hollow electron beam radius to structure wall radius $$\left( = 0.44 \right) $$=0.44 for $$\text {TE}_{01},\left( = 0.39 \right) $$TE01,=0.39 for $$\text {TE}_{02}$$TE02, and $$\left( = 0.35 \right) $$=0.35 for $$\text {TE}_{03}$$TE03 mode.