## The analysis, simulation and testing of an experimental travelling- wave tube.

##### Abstract

As a design and analysis aid for the development of an
experimental TWT, a computer program is written which
allows the small-signal gain to be computed for various
operating conditions, such as various conditions of tube
bias (beam voltage and current) and frequency. In order to
arrive at a value for the gain, a number of parameters
need first to be defined or calculated.
Using the method (Approach II) of Jain and Basu [17] which
is applicable to a helix with a free-space gap between it
and circular dielectric support rods surrounded by a metal
shell, the dielectric loading factor (DLF) for the
structure is found and the dispersion relation then solved
to obtain the radial propagation constant y and axial
propagation constant B. The method is tested for a helix
with measured data and found to be acceptably accurate.
Helix losses are calculated for the low-loss input and
output sections of the helix, using the procedures
developed by Gilmour et al [14,18], from which values are
found for the helix loss parameter d. Another value for d,
obviously much larger, is also found for the lossy
attenuator section of the helix. Here measured data for
the attenuator is used as a basis for a polynomial which
models the attenuator loss as a function of frequency.
The Pierce gain parameter C is found using the well-known
equations of Pierce [21,22,26], and then the space-charge
parameter Q. Here knowledge of the space-charge reduction
factor F is required to find Q, and a simple non-iterative
method is presented for its calculation, with some
results. From the other parameters already calculated the
velocity parameter, b, is then found.
since sufficient information is now available, the
electronic equations are solved. These equations are in a
modified form, better accounting for the effects of space-charge
than the well-known standard forms. Results are
compared and slight differences found to exist in the
computed gain. Now that the x's and y's (respectively the
real and imaginary parts of the complex propagation
constants for the slow and fast space-charge waves) are
known the launching loss can be calculated. Launching
losses are found for the three space-charge waves, not
just for the gaining wave.
The gain of the TWT is not found from the asymptotic gain
equation but from a model which includes the effects of
internal feedback due to reflections at the ports and
attenuator. Values of reflection coefficients are modelled
on the results of time-domain measurements (attenuator)
and found by calculation (ports). This model permits the
unstable behaviour of the tube to be predicted for various
conditions of beam current and voltage and anticipates the
frequencies at which instability would be likely. Results
from simulations are compared with experimental
observations.
The need to pulse the experimental tube under controlled
conditions led to the development of a high-voltage solid state
pulse modulator providing regulated output pulses of
up to 5000V and 200mA directly, without the use of
transformers. The pulse modulator design embodies two
unusual features a) its operation is bipolar, delivering
positive or negative output pulses, depending only on the
polarity of the rectifier input, and b) the use of
multiple regulating loops and stacked pass elements to
achieve high-voltage operation. Some results are
presented.