The methodology used this chapter, for the helix pitch profile design, was initially based on a paper by V. Strivastava [6]. This is the most recent published work on the design of a slow-wave structure. The method is based on the fundamental processes within the tube, these are: the growth of RF wave on the helix, bunching of the electron beam and power extraction from the beam. The helix pitch should be designed to exploit these processes as they happen along the tube. This methodology is next described.
In the region where the initial growth in RF signal takes place, the helix pitch p0 is designed for maximum signal growth. In the region where the bunching takes place, the pitch p1 is then adjusted to maximise this process i.e. achieve the tightest bunches. Towards the end of the tube, RF power is extracted from the electrons in the beam reducing the beam velocity. The helix in this region p2 should then be designed to optimise this power extraction process. These pitch values can be optimised using a single section with a uniform pitch.
The paper [6] states that with a positive step taper included, the input section length z0 with pitch p0 is designed for maximum RF current or bunch intensity. With a negative step added, z1 is then adjusted for maximum forward RF power in the output region (with pitch p2). The length of taper in the output region z2 is then adjusted for maximum forward power at the output. A summary is given below:-
For a single uniform section:-
adjust p0 for maximum small signal gain
adjust p1 for maximum bunching (maximum RF current) adjust p2 for maximum output power or efficiency Then:-
Adjust z0 for maximum bunching in centre region (with pitch p1) Adjust z1 for maximum forward power in output region (with pitch p2)
Axial distance Helix pitch p0 z2 z0 z1 p2 p1
Fig 6.1 Stepped helix pitch profile for optimised performance [6]
The strategy described above effectively optimises the signal gain, development of the bunch formation, capture of the bunch and power extraction. Hence, the strategy is ideal for gain and efficiency optimisation. The drawback with this method however is no consideration for the nonlinear performance or the broadband performance. The method needs to be developed therefore, by incorporating optimisation for a linear performance and, if possible, a broadband performance. A brief description of the important interaction processes and the modified criteria for a full TWT design is next explained. For this purpose, the TWT structure is divided into the three interaction regions: linear, bunching and output.
Helix TWT Processes and Design Criteria
Linear Region
In the region nearest to the tube input, the un-modulated beam is surrounded by a sinusoidal RF wave on the helix. The electrostatic influence of the AC wave on the beam causes a sinusoidal current to become established in the beam at the same frequency (or frequencies) as the RF signal. At this small-signal level, the phase of the electrons in the beam relative to the forward-wave on the helix is almost constant, the RF characteristics are approximately linear and the tube efficiency is very low. The design criteria for this region are thus: maximum signal growth rate at the band centre and for the small-signal gain variations to be minimal across frequency. These criteria can be achieved from the design of the slow-wave structure.
Bunching Region
In this region, strong electron beam bunching occurs due to the influence of the circuit field. The bunch becomes strong where the RF current component in the beam grows until a maximum is reached. This is where mutual repulsion between the electrons, which is due to the space-charge effect, becomes an important factor on the overall behaviour. An electromotive force is induced in the helix by this AC beam current component, which becomes limited when the amplifier is driven hard enough, resulting in a limited RF output power. Phase shifting of the RF signal in the slow- wave structure also occurs as a result of its interaction with the inherently nonlinear beam (as discussed in Chapter 4).
To enhance the bunching process, an increased phase velocity is required at a position before the beam phase becomes non-linear. The aim is to optimise the convergence of the electron trajectories so that the bunch has a maximum number of electrons captured. As a result, more RF power can be extracted from the bunch enhancing the conversion efficiency. In addition, less velocity spreading in the bunch often results from the optimised bunch formation. Careful design of the positive taper is necessary to optimise the AM/AM and AM/PM conversion. The circuit phase velocity in this region must be chosen to optimise the shapes of the transfer curves without sacrificing too much conversion efficiency.
Output Region
The output region of the TWT is beyond the point of maximum bunching. The RF beam current therefore declines in this region, while the forward circuit power continues to grow until it saturates. The beam velocity decreases rapidly and the non- linear behaviour of the signal is most profound.
To optimise the power extraction from the beam, the circuit phase velocity is lowered. For the electrons to lose most of their energy in this output region, it is desirable for them to remain in their maximum retarding phase. It is also desirable to confine the spread of the beam velocity, especially to limit the number of fast electrons. This is to improve the depressed collector efficiency, enhancing the overall efficiency.
A constant output power in overdrive is significant when the curvature of the power transfer curve is small enough [2]. The nonlinear performance in this region may therefore be improved if the RF power is retained without it being lost back to the beam. The broadband operation can be optimised if the reduction of beam phase in this tapered region is made consistent across a range of frequencies in the band. Signal reflections at the output terminal should also be avoided in the practical design.