Tolerance Investigation on a Helically Corrugated Interaction Circuit of a GyroTWA

Gyrotron traveling wave amplifiers (gyro-TWAs) are used for broadband, high-power applications, like dynamic nuclear polarization (DNP), earth orbit radar and communications. Gyro-TWAs are vacuum electron devices, where an electron beam transfers a part of its perpendicular energy to an electromagnetic wave. Conventionally, the electron beam-wave interaction takes place in an ordinary waveguide, which limits the achievable bandwidth to a few percent. To increase bandwidth, other interaction circuits are employed. For instance, the helically corrugated interaction region
¹
(HCIR), which has helical corrugations along the propagation direction. The radius is given by r($\theta$, z=r
0
+r
1
$\cos \left(m_{B} \theta+2 \pi z / d\right)$. Here, r
0
is the mean radius, r
1
the corrugation depth and d the axial corrugation period. Additionally, m
b
gives the number of azimuthal perturbations, which is 3 in this case to couple the TE
11
and TE
21
circular waveguide modes. The manufacturing of this interaction region is challenging for high frequencies, because of the small inner dimensions, which restrict access to machine the inner perturbated wall structures. Therefore, an aluminum mandrel is milled, electroplated with copper and subsequently dissolved
²
. Concerning tolerances, Donaldson at al.
²
considered the change of the dispersion as a means to define the limits. However, this assumes a constant deviation of r
0
, r
1
or d along the entire interaction region. In this work, the influence of tolerances for only a few corrugation periods at different positions along the HCIR are studied. This investigation shows a reduced influence on the performance, if the deviations are restricted to such a small number of periods. As a result, larger manufacturing tolerances can be tolerated. This is significant considering the frontier to higher frequencies and smaller structure sizes required.