Amplification occurs as a result of the electrons within the beam forming bunches, induced by the electromagnetic (EM) field through which they are propelled. As the bunches reach a point of saturation, close to the output of the traveling wave tube, energy is transferred from the electrons onto the slow wave structure (SWS)—the result of which is an increase in the amplitude of the RF signal at the output of the tube relative to the level of the signal at the input of the tube. Coupledcavity TWTs, generally physically larger than helix TWTs, incorporate a series of coupled cavities arranged axially along the beam. Although this approach can achieve higher power levels than with a helix, it is also more limited in frequency bandwidth.

Due to higher levels of integration and need for higher efficiency, military system designers seek higher power densities from TWTs and have employed compact solutions such as tube-based microwave power modules (MPMs) in some systems. They have also sought smaller TWTs capable of multiple-octave frequency coverage. Mini-TWTs have been developed for a variety of applications, including in phasedarray radars, EW systems, and electroniccountermeasures (ECM) systems.

Mini-TWT development programs at e2v have focused on maintaining small size and low weight, but increasing bandwidth and output-power capabilities. Those programs have yielded some impressive results, including a mini-TWT capable of 100 W minimum output power from 4.5 to 18 GHz and a single device operating from 2 to 18 GHz. Availability of such devices allows system integrators to use a single tube in a design that might have relied on two or three TWTs previously. They also offer the high power and light weight that is suitable for decoy and unmanned aerial vehicle (UAV) applications. Both designs leverage technology developed for the firm’s model N10172 mini-TWT (from the late 1990s). The new tubes borrow the electrostatic and mechanical design of the multistage depressed collector from the N10172 but incorporate an all-new electron gun, with an option for focus electrode switching. The new tubes offer collector efficiency of better than 60 percent, and are being optimized for even higher efficiency. Pre-production testing of the 2-to-18-GHz model N20180 mini-TWT has already demonstrated that a helix TWT with greater than three-octave bandwidth is a reality.

e2v Technologies plc, 106 Waterhouse Lane, Chelmsford, Essex CM1 2QU, UK; +44 (0) 1245 493493, +44 (0) 1245 493492, Internet: www.e2v.com.

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