

I. Introduction A
microfabricated ladder type TWT has been modeled, simulated, and analyzed for
50 GHz Vband operation. Millimeterwave TWT devices are used for high power
applications, including scientific research, telecommunications, and military
and security. TWTs have been constructed out of various shapes. A helix shape
is common in TWT amplifiers, but presents problems as frequency increases. At
higher frequencies, the device dimensions become smaller, causing the helix
design to be difficult to fabricate. The ladder type slow wave circuit has a
symmetric configuration that can easily be microfabricated at high
frequencies in a clean room environment. The characteristics of a ladder
include wide bandwidth, fundamental forward interaction (high rate gain),
high interaction impedance (high dispersion and low group velocity), and low
cost assembly. II. Analysis Cold test and hot
test simulations have been performed. For the cold test, HFSS (High Frequency
Structure Simulator) based on the finite element method has been employed. VORPAL
simulation software has been used for the hot test. VORPAL is a PIC simulator
that uses the CFDTD method. The ladder circuit was simulated using VORPAL
4.0.0 on a Linux system with two quadcore 2.33 GHz processors. Using open
MPI, up to eight cores were scheduled. Simulation time can take ten days on
this system. A compute cluster was also used. When scheduled with thirtytwo
nodes, runs took four days. Figure 1 shows the three dimensional model of the
ladder structure in the ridged waveguide. An oval shaped beam tunnel
supported by dielectric chips was used. Iterative simulation analyses
optimized geometries including major and minor radius ratio of the oval
tunnel, rug gap, and ridge height. The calculated cutoff frequency, in
general, increases as the beam tunnel narrows and as the rug gap decreases.
In order to maximize the bandwidth, the Pierce impedance and dispersion data
were observed using the small signal analysis. The dispersion curve of the
Vband ladder slowwave circuit predicted by the HFSS simulations employing
periodic boundary conditions is shown in Figure 2. The lower band is the
symmetric ladder mode and the upper band is the antisymmetric ladder mode.
The electron beam line operating at 22 kV is also shown. The parameters of a
basic beam optics design are listed in Table 1. The gain dependence on the
number of ladder periods is shown in Figure 3. The circuit gain was
calculated for a 68period ladder structure to save the computation time. The
perfectly matched layer (PML) boundary condition was applied for the last
8period to prevent reflections. However, the length of the ladder circuit
will not be limited to 68period in reality. The input power was varied from
0.1 to 100 W for gain calculation. The results are shown in Figure 4. As the
input power is increased, gain significantly dropped off near 100 W of input
power, signifying the saturation region had been reached. Figures 5 (a) and
(b) show particle velocity versus axial position at linear and saturation
regions, respectively. In the linear region (Figure 5(a)), electrons are
velocity modulated but do not bunch into groups. In the saturation region
(Figure 5(b)), the faster electrons catch up the slower electrons and they
bunch into groups. Successful modeling, simulation, and analysis of the
ladder type TWT at 50 GHz have been performed. This model of the ladder
circuit, using VORPAL, can now easily be modified for use at other
frequencies for various applications. *
Acknowledgements: This work has been supported in part by the University of
Colorado at Colorado Springs. 



