Microwave tubes and applications

Microwave tubes (MWTs) continue to be important despite competitive incursion from solid-state devices (SSDs) [1–14]. Availability of GaAs, SiC, and GaN-based broadband and medium-power SSDs up to W-band frequencies greatly affected the domain of MWT (Table 1). For example, presently GaN devices are available as commercial products in S, C, X, and Ku bands, and newer configurations are promising for current gain with cut-off frequency up to 150 GHz and maximum oscillation frequency up to 230 GHz with ~100 nm gate length. GaN also provides solutions up to Ka band with linear power densities up to 5 W/mm. However, the single device performance of SSDs is no match to that of MWTs. High-power requirements in microwave, mm-wave, and terahertz frequencies are met by SSDs through the approach of power combining with the advantages of (i) redundancy in case of failure of some of the constituent elements if power combining is done in space (as in phased-array radar), though not so if it is done in device/module level, (ii) low cost and ease of operation having no constraints on the prime power, cooling, volume, and weight, though with the limitations of poor efficiency (high prime-power consumption), extremely high-power density (thermal management) and requirements of high currents (susceptible to EMI/EMC), and temperature compensation, and (iii) smaller volume if it is done in device/module level, though at the cost of the size of power supply and heat sinks [5,11–13].

Table 1. Comparative domain coverage of solid-state and microwave tube technologies.

Applications	Microwave and millimeter-wave bands
	L	S	C	X	Ku	K	Ka	Q	V	W	D
Educational, laboratory sources, EMI/EMC sources, medical LINACs, spectroscopy, radio-astronomy, communication, broadcasting, navigation, telemetry, sat-com ground stations, homeland security, medical imaging	×	×	×	×	×	×	×	×	×	×	×
Microwave heating	×	×	×	◊	◊	◊	◊	□	□	□	□
Accelerator, high-energy research	×	×	×	◊	◊	□	□	□	□	□	□
Electronic warfare	×	×	×	×	×	◊	◊
Narrow-band radars	×	×	×	◊	◊	◊	◊	□	□	□
Broadband imaging radars				◊	◊	□	□
Missile seekers (no cooling, high peak power)				◊	□	□	□	□	□	□
Space applications		×	×	◊	□	□	□	□	□	□
HPM/DEW/LIC	□	□	□	□	□					□
×SSD	◊ Competition					□ MWT

The superiority of MWTs over SSDs would be manifold; a few of those would be in terms of lesser heat generation due to collision in the bulk of the device, higher breakdown limit on maximum electric field (inside the device), smaller base-plate size (determined by the cooling efficiency), higher peak pulsed power operability, ultra-bandwidth (three-plus octave) performance above a gigahertz, etc. Further, unlike SSDs, MWTs are fabricated out of metals and ceramics, which make these inherently hardened against radiation, and more capable of withstanding temperature and mechanical extremes. Thus, for example, even though attempts were made in the middle of 1990’s to replace around 50% space traveling-wave tubes (TWTs)with SSDs in satellite qualified systems, such replacements declined within a few years to only ~10% in view of the requirement of 5 × 10⁶ h mean time before failure that could be met only by space-TWTs. However, the advantages of SSDs have been accrued in the microwave power module where a helix-TWT and a solid-state power amplifier co-exist, sharing gain equally in a compact system where an electronic power conditioner provides a built-in power supply. Further, microelectronic technology of SSDs has been used in fabricating vacuum microelectronic MWTs to realize high-power terahertz devices, such as micro-fabricated TWTs [1–5]. The MWT technology is thus still sustainable, and it provides useful solutions for various applications with comparative domain coverage of both the technologies (Table 1).

Though MWTs offer varieties of applications, accounting all remains too tedious in this note. Instead, only some of those are highlighted here not only for the electromagnetics community but also for readers belonging to other relevant fields of expertise. For instance, applications in military radar, electronic warfare, missile guidance and tracking and active denial system would be interesting to note. The virtual cathode oscillator (VIRCATOR) and magnetically insulated line oscillator (MILO) are of the high-power MWT kinds for directed energy weapon (DEW) and information warfare (IW) to make the enemy electronic systems either faulty or non-operational. The applications of MWTs in civilian radar encompass weather detection, highway collision avoidance, air-traffic control, remote sensing, imaging in atmospheric and planetary science, space debris phased-array mapping, analysis of cloud, life detection to sense heart beats and breathing under earthquake rubbles, etc. In the terahertz regime, the applications include imaging, security inspection, enhanced sensitivity spectroscopy, and dynamic nuclear polarization enhanced nuclear magnetic resonance. Some of the peaceful applications of MWTs would be accounted as waste remediation, breaking of rock and concrete, tunnel boring, soil treatment, and fusion plasma heating for controlled thermonuclear reactor, besides industrial heating covering paper, leather, textile, tea, etc. [7,8]. The advent of millimeter-wave tube, namely, gyrotron made it possible to implement volumetric and selective heating in material processing, thereby allowing to develop ceramic sintering and joining, and production of new (stronger and less brittle) composite ceramics. Consequently, lightweight ceramic engines for aircraft and automobiles as well as strong, long-lasting ceramic walls for thermonuclear power reactors could be developed [6–8].

MWTs also dominate a major segment of the medical area; for example, in medical diagnosis and treatment using dielectric heating. To be more explicit, one can talk about (i) hyperthermia implemented using phased-array antenna that locally heats tumor cells selectively and raptures their membrane, leading to the destruction of cancerous cells, without harming healthy ones, and (ii) ablation that dries up or desiccates tumor with localized application of heat for the removal of unwanted tissues [14–16].

Other scientific applications of MWTs include RF linear accelerators, plasma diagnostics and chemistry, and non-linear spectroscopy. Some of the unconventional ones may be named as satellite power station, artificially created ionized layers for the extension of radio range, city lighting, nitrogen-fertilizer raining, and environmental control by both ozone generation and atmospheric purification of admixtures that destroy ozone layer, and so on [7,8,17,18].

R&D work on MWT design, simulation, analysis, and development offers opportunities for innovations in established sectors as well as inventions of new and unconventional applications. Looking at the current global R&D trend and keeping in view some of the directions of ongoing/possible investigations, MWTs could be earmarked as follows:

(i)	Improved performance conventional MWTs, such as wideband EW-TWTs, high-efficiency, long-life, light-weight space-TWTs, and high-power compact multi-beam klystrons;
(ii)	MWTs accruing the advantages of both the vacuum and SSDs/microelectronics, such as folded waveguide TWT, and reflex klystron, in the context of terahertz generation/batch production;
(iii)	Intensive relativistic electron beam driven MWTs, such as VIRCATOR, MILO, relativistic backward-wave oscillator, relativistic klystron (RELTRON), and OROTRON, which could be used in developing HPM MWTs/electromagnetic bomb for IW and DEW;
(iv)	Fast-wave MWTs, such as gyrotron, gyro-TWT, gyro-klystron, and cyclotron auto-resonance maser (CARM), which fill up the mm-wave technology gap in the high-power domain;
(v)	Plasma-assisted MWTs, such as plasma-filled TWT, pasotron, and gyrotron, which have scopes for enhancing space-charge limiting currents (and hence, the beam currents too), thereby increasing beam powers for conversion into large device RF output powers.

We are overwhelmed by the sheer number of authors who chose to submit papers for this Special Issue. Gilmour, in his paper, shares more than four decades of experience of authoring three world famous books in the MWT area and presenting a 100 courses to over 2000 scientists, engineers, and technicians, thereby bridging the gap between the needs of universities and industries. Beaudoin et al. report, in their paper, various aspects of the development of a highly efficient megawatt-class RF source – a modified version of the inductive output tube – required by a mobile heater for ionospheric modification studies in high-frequency active auroral research program. Tripathi et al. present the limitations of the existing pre-distortion techniques and propose a digitally supported signal injection linearization method for the simultaneous reduction of 3IM and 5IM distortions in TWTs for applications in transponders and ground terminals in satellite communication. In another paper, Barik et al. report anew design of the screen of an ion thruster, which can give a thrust as high as 0.363 mN with efficiency as high as 74.66%.

A study on very high-power megawatt-level magnetron reported in a paper by Hu et al. reveal that the introduction of an asymmetry in its resonant cavity can yield a pure frequency spectrum at 2.9938 GHz with 3.21 MW output power and 43.23% conversion efficiency of the device. This makes it usable in a medical linear accelerator (LINAC). Liu (C) et al. have achieved 20-kW CW power in an injection-locked magnetron source, and also, power up to 33.1 kW with greater than 93% efficiency by coherently combining powers of two magnetrons, to meet the requirement of microwave energy industry. In yet another paper of relevance to LINAC of the European Spallation Source, Costanza et al. present a time-domain analysis, giving a semi-analytical method to calculate the transient power and energy dissipated in the walls of elliptical cavities of the accelerating structure excited in higher order modes by a sequence of bunches of charged particles.

Those who are interested in HPM devices for applications, such as DEWs, may like to read a review by Nallasamy et al. on advances and trends in MILO, who also give their optimized design of an S-band MILO of 2 GW output power at 2.72 GHz, operating at a beam voltage 600 kV and beam current 35 kA. A study on another HPM device, namely, X-band relativistic backward-wave oscillator (RBWO) with relativistic mass factor of two and accelerating voltage of 500 kV is reported by Min et al.

A new Lagrangian code for very fast simulation of non-azimuthally symmetric double-corrugated waveguide slow-wave structure for a mm-wave (225 GHz) TWT, proposed by Waring and Paoloni, is going to arouse interest among mm-wave-tube developers. In another study in the mm-wave regime, Wang et al. give an account of their design and development of a 140 GHz folded-waveguide TWT in a relatively larger circular electron beam tunnel, and logically explained the departure of the simulated device gain and power from their experiment. Liu (H) et al. give complete design of a Ka-band, sheet-beam TWT including its three-slot staggered-ladder coupled-cavity slow-wave structure and electron gun. The paper reports formation of a 19.6 kV, 5 A, 3 mm × 0.52 mm sheet beam of excellent transmission. Further, Ding et al. add to the study on mm-wave TWTs by proposing a modified dielectric-embedded microstrip meander line slow-wave structure for enhanced interaction impedance and electronic efficiency. They predict that the structure can yield 75.85 W saturated output power of a TWT at 35 GHz with a gain of 28.8 dB and an electronic efficiency of 6.32% using a 6 kV, 200 mA sheet beam.

Two papers in the area of fast-wave MWTs appear here – one by Kumar et al. on a magnetron injection gun (MIG) to be employed in a 200 GHz frequency-tunable gyrotron for 300 MHz DNP-NMR spectroscopy, and the other by Kesari et al. on a TE_6,2-to-Gaussian mode launcher studied with the help of two different software tools, namely, Launcher Optimization Tool (LOT) and Surf3d.

Stating in brief (as above) the authors’ contributions to the Special Issue – Microwave Tubes and Applications – of the Journal of Electromagnetic Waves and Applications, we feel honored being given the opportunity to guest-edit the Issue. Our job was made considerably easier by the expert and timely guidance of Prof. P. K. Choudhury, who, along with his companion Editor-in-Chief, Prof. M. Abou El-Nasr, recognized that MWTs could be one of the demanding areas, and a Special Issue of the Journal should be brought out [19]. Their recognition, in our opinion, is most appropriate in the premise of the Journal, since not only time-independent but also time-dependent field concepts are involved in the understanding of MWTs and their parts. For instance, the understanding of electron guns and that of interaction structures as well as beam-wave interaction mechanism of MWTs are developed using time-independent and time-dependent field concepts, respectively. We are, indeed, very happy to see interests in potential contributors for submissions, which took place from several countries. The timely completion of review process remains one of the greatly important issues to realize the volume; we extend our sincere thanks to expert reviewers for spending their precious time. It is our hope that the papers in this Issue would arouse curiosity in the global MWT community, and fulfill the aims and objectives of the Journal.

B. N. Basu and S. K. Datta
Guest Editors