The Importance of Glide-Symmetry in Broadband Sheet-Beam Traveling-Wave Tubes

Microfabricated traveling-wave tubes (TWTs) are often utilized for amplifying millimeter-wave signals to high power levels over broad bandwidths. One of the crucial components of the TWT which determines its bandwidth is the periodic slow-wave structure (SWS) that is utilized to velocity-synchronize and exchange energy between guided electromagnetic (EM) waves to the space-charge waves in a linear electron beam. Interestingly, many of the “space-harmonic” SWSs found in literature, such as the serpentine, coupled-cavity, and staggered-vane have one geometric feature in common that enables broadband operation: glide-symmetry (GS). A periodic structure is glide-symmetric if it remains invariant under the glide operation, which consists of a reflection in the “glide-plane,” followed by a translation by half of its period
¹
. The modes on the
$\omega-k$
dispersion diagram of GS SWSs have intersections between spatial harmonics which cross, but do not form stopbands. This feature permits beam-wave velocity synchronization with a continuous guided forward wave over broad frequencies
²
with reduced risk of backward-wave oscillations. On the other hand, periodic SWSs (with period d) that do not exhibit glide-symmetry, either by design or due to fabrication errors such as beam tunnel offsets, will exhibit stopbands every
$k=(2n+1)\pi/d$
on the modal dispersion diagram
³
, which can lead to oscillations and non-optimal bandwidth.