Method for measuring interaction impedance in helix TWT

A nonresonant method for measuring the interaction impedance in a helix TWT using a hairline conductive wire is studied. The theory of the interaction impedance measurements using a dielectric and/or a conductive perturbing rod with a finite diameter, becomes complex when the upper space harmonics of a slow wave structure are taken into account. This problem may be overcome if one uses a hairline conductive wire instead of a dielectric rod with a finite diameter. The diameter of this conductive wire placed precisely along the helix axis, is about 1-2% of the helix diameter. It is shown that in this case the change of the propagation constant characterises the interaction impedance of the fundamental space harmonic independently of the upper harmonics.