Design of a Peculiar TM35 Transverse Mode THz Extended Interaction Oscillator for Multibeam kW-Class Operation

A terahertz (THz) extended interaction oscillator (EIO) operating in a peculiar
${\text {TM}_{35}}$
transverse mode and
${2\pi }$
longitudinal mode has been proposed here. The aim is to generate a high-power THz radiation source with an enlarged cavity. This transverse
${\text {TM}_{mn}}$
mode (with
$m=3$
,
$n=5$
) has an exceptionally large cross Section which was unattainable with fundamental mode (i.e.,
${\text {TM}_{11}}$
) or even with higher order mode (HOM, i.e.,
${\text {TM}_{31}}$
) THz-EIOs. The overmoded structure (i.e.,
$n>1$
) makes this mode most suitable for three-beam operation by maintaining a sufficiently large separation of 1.0 mm between adjacent beam tunnels. The analytical formalism for setting up an initial ladder-like RF circuit has been discussed along with the field distribution analysis of the
${\text {TM}_{35}}$
mode. The stability analysis, which is crucial for this complex mode, shows a mode interval of 0.4 kV between the operating voltage ranges of the desired
${2\pi }$
-mode and the nearest competing
${\pi /16}$
-mode. Furthermore, an external circuit with an external quality-factor
$Q_{e} =333$
has been designed for a stable operation. To demonstrate the effectiveness of this mode in yielding a kilowatt-class THz radiation source, a 0.368 THz
${\text {TM}_{35}}$
three-beam EIO has been simulated both in computer simulation technology (CST)- particle-in-cell (PIC) and MAGIC-3D software. CST-PIC simulations predict an output power of 1.1 kW at a beam voltage of 18.1 kV at an overall beam current of 2.7 A (
$3\times0.9$
A). Furthermore, the skin and ohmic losses have been incorporated. These results are validated by MAGIC-PIC simulations which predict an output power of 1.2 kW with an efficiency of 2.45%. This design requiring a relatively small power supply is a potential candidate for compact high-power portable THz systems for present and future applications.