Power squared

18 February 2013

Researchers in Germany present an 825 GHz traveling wave tube design with square helix slow wave structure, including input and output couplings. As well as being manufacturable, simulations suggest the design could boost solid-state THz source power outputs by more than an order of magnitude.

Squaring the circle

One of the bottlenecks in the development of terahertz technologies is producing powerful but compact sources. To date, solid-state electronic frequency multiplied sources can achieve 1 mW output at 1 THz. One way to increase the output power is to use a terahertz amplifier in the form of traveling wave tube (TWT).

TWTs are vacuum electronic amplifiers used widely in satellite communications and electronic countermeasures at microwave frequencies. They are known for their ability to produce high-power high-frequency signals. TWTs contain a slow wave structure (SWS), in which the input signal interacts with an electron beam, increasing the EM energy of the signal at the expense of the kinetic energy of the electrons.

Among the many SWS shapes that have been tried, the helix has proven best for operation bandwidth. However it is difficult to produce a conventional circular helix structure at the scale required for THz operation, where the diameter would be a few tens of micrometres. At this scale the normal fabrication method of winding wire around a mandrel is not feasible. On the other hand the two dimensional lithographic nature of microfabrication methods prevents the fabrication of circular shapes.

In this work, the Goethe University team at Frankfurt present a TWT design with a square helix SWS that should be achievable with existing microfabrication technologies and which, for the first time, includes the design and modelling of input and output couplers.

Power couplings

Author Mikko Kotiranta explained “The results presented allow the manufacturing and construction of the final tube, including all necessary coupling. Structure excitation through a rectangular waveguide has not been considered in the previous studies of the square helix SWS. A waveguide interface allows the coupling of the amplifier to other terahertz components or instruments, and its inclusion in the simulations makes the simulated structure more realistic.”

In their Letter the Goethe team include analysis of the broadband amplification behaviour and linearity of the amplifier with first-of-their-kind 3D particle-in-cell simulations. These show that the TWT design could  amplify the signal from an existing tunable solid-state source by more than an order of magnitude resulting in a power level between 10 and 100 mW.

“The simulated output power of over 25 dBm and bandwidth of 8% demonstrate the huge potential of TWTs for use in compact, powerful broadband sources in terahertz applications including communications, sensing and imaging” said Kotiranta.

Although the team note the potential benefits for THz applications the current work has been aimed at a submillimetre frequency (825 GHz) used in astronomic applications. The earth’s atmosphere strongly attenuates terahertz signals, but there are several frequency bands (including 800-900 GHz) in which the attenuation is less severe, and radio telescopes at dry, high-altitude locations exploit this.

Gains and losses

However, before any applications can benefit there are still challenges to overcome. Co-author Prof. Viktor Krozer said “The realisation of a TWT amplifier is an extremely advanced undertaking and many unexpected challenges are waiting to be overcome. Having said that, numerous important steps have been taken and lessons have been learned by our group during the development of a 1-THz cascade backward wave amplifier in the European OPTHER (Optically Driven Terahertz Amplifiers) project. In any case, the realisation of the SWS and the focusing and alignment of the electron beam inside it are certainly major challenges. We are cooperating directly with industrial TWT device suppliers to allow for a timely real-world implementation.”

The Goethe team are in contact with Centre National de la Recherche Scientifique (CNRS) and Thales in France, discussing adoption of their X-ray LIGA technology for the manufacturing of the square helix. Krozer noted that “During our collaboration in the OPTHER project, they demonstrated very impressive results on free standing copper pillars, which could serve as a starting point for the square helix structure.” Key issues that need to be addressed in moving from such pillars to the square helix structure include creating lateral connections between them and ensuring proper alignment during the fabrication process.

Since the work reported in their Letter the Goethe team have been investigating the coupling structures to further increase their bandwidth and realise more of the square helix’s potential. Future work will concentrate on the fabrication of the helix and measurement of the helix losses.

Krozer explained “The losses depend on the surface roughness through the skin depth which is about 72 nm for copper at 825 GHz. This was taken into account in the presented simulations by reducing the metal conductivity, but measurement will naturally have the last word on the topic.” The measured losses will influence the decision on the SWS length and that will ultimately decide just how large the amplifier gain can be.

Further reading

This article is based on the Letter: Design of 825-GHz square helix traveling wave tube (new window).

Project OPTHER: http://www.opther.eu (new window)

A PDF version (new window) of this feature article is also available.

Journal content

Cover of Electronics Letters, volume 50, issue 24

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