NEWSLETTERof the International Consortium“Development of High-Power Terahertz
Science & Technology”
October 2021

№ 19

CONTENT Editorial(Page 1)

How to contribute to the Newsletter Invited papers• M.Yu. Glyavin, G.G. Denisov, “Breakthrough to high-power continuous-wave terahertz generation:
a 250-Watts, 0.5-THz second-harmonic gyrotron"(Page 2)
• M.D. Proyavin, D.I. Sobolev, V.V. Parshin, V. I. Belousov, S.V. Mishakin, M.Yu. Glyavin, “Study of
3D-Printed Dielectric Barrier Windows for Microwave Applications”(Page 5)• E. Khutoryan, A. Kuleshov, S. Ponomarenko, K. Lukin, Y. Tatematsu, M. Tani, “Regimes of Hybrid
Modes in a THz Clinotron.”(Page 10)
 Special journal issues in the field of THz science(Page 13)
 List of selected recent publications and new books(Page 17)
 News from the Net(Page 26)

EDITORIAL: HOW TO CONTRIBUTE TO THE NEWSLETER
Dear Reader,
We are inviting contributions to the following rubrics:• Research highlights (annotations) presenting the projects pursued by the members of the Consortium.
• Short regular and invited papers.
• Proposals for collaborative research work.
• News from the participating institutions.
• Information about conferences, symposia, workshops, seminars.
• Programs and frameworks for an exchange of visits and mobility of researchers. Job opportunities(especially for young researchers, e.g. postdoctoral positions, specializations, internships).
• Annotations of books, conference proceedings, software and internet resources. Additions to the list of
the recent scientific publications and conference reports at the website of the Consortium(http://fir.ufukui.ac.jp/Website_Consortium/publist.html ).
• Information and announcements about awards and nominations.
• Short presentations of laboratories and research groups belonging to the participating institutions.
Please submit your contributions to the Newsletter as well as requests for information to:Professor Masahiko Tani
Director of FIR UF – Facilitator of
the International Consortium
FIR UFtani@fir.u-fukui.ac.jp

Dr. Svilen Sabchevski
Editor of the website and the Newsletter
Institute of Electronics of the Bulgarian
Academy of Sciencessabch@ie.bas.bg

INVITED PAPERS
Breakthrough to high-power continuous-wave terahertz generation: a 250-Watts, 0.5-THz
second-harmonic gyrotron
M. Yu. Glyavin, G. G. Denisov
Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, 603950, Russia
Abstract: To cover the so-called terahertz gap in powerful sources of coherent
electromagnetic waves, a second-harmonic gyrotron operating in a 10 T liquid helium-freesuperconducting magnet has been designed, manufactured, and tested. With an operating
voltage of 15 kV and a beam current of 0.6 A, the gyrotron generated coherent continuouswave radiation at a frequency of 0.526 THz with an output power of 250 W and an
efficiency of 2.7%. This result is based on the common efforts of IAP RAS/GYCOM teamand the full list of authors is presented in the paper published at Electr. Dev. Lett. Journal(doi: 10.1109/LED.2021.3113022).
The development of compact, simple and reliable sources of terahertz (THz) radiation is important fornumerous applications. Unfortunately, strong magnetic fields are required to ensure the conditions for cyclotron
resonance between rotating electrons and fast waves at THz frequencies: about 36 T is needed for thegeneration of 1 THz at the fundamental resonance. The typical magnetic field of commercially available cryomagnets with a sufficiently large warm bore (50–100 mm) does not exceed 15 T; therefore, operation at thesecond harmonic is required to achieve 0.5 THz. A number of second-harmonic gyrotrons developed forspectroscopy experimentally demonstrated an output power of 10 to 40 W in the 0.4–0.5 THz frequency range[1, 2]. Below we present the design, manufacture, and experimental testing of a gyrotron based on a 10 T
helium-free magnet Jastec JMTD 10T100, in which coherent radiation of 0.526 THz was obtained at a record
power level of 250 W.
A well-proven gyrotron of the sub-terahertz range at the fundamental harmonic with a radiation frequency of
0.263 THz [3] was taken as the basis for the development of a 0.526-THz tube at the second harmonic. The TE6,5 mode, which had been successfully used earlier for stable single-mode excitation at the second harmonic at
lower frequencies, was chosen as the operating mode. We also considered the whispering gallery modes such as
TE11,p and TE14,p, used in gyrotrons with similar frequencies [2,3], however, numerical calculations predicted
better conditions for the excitation of the TE-6,5 mode (in particular, a higher beam-wave coupling factor to
reduce the operating current). The radius of the guiding centers of the electron beam is 0.87 mm, with a cavity
radius of 1.988 mm. In this case, we were able to use the same triode magnetron injection gun developed for a
0.263-THz tube. The electrodes' geometry was selected so that the nominal parameters of the electron flux wereachieved in the diode mode (the insulated anode has a ground potential). By varying the anode potential, it ispossible to adjust the beam parameters in a wide range of magnetic fields and cathode voltages. The position of
the electrodes in the magnetic field was slightly changed compared with the 0.263-THz tube: the distance fromthe emitter to the center of the solenoid is 374 mm, in contrast to 354 mm in the initial version. Such a shift had
virtually no effect on the parameters of the electron beam: in accordance with numerical simulation, the pitch
factor of electrons (the ratio between rotational and axial electron velocities) is 1.4, and the root mean squarevelocity spread is about 8% with allowance for the initial thermal velocities and roughness of the emitter. These
values look good enough to achieve high efficiency of the interaction between the electron beam and the cavity
mode.
The cavity length was optimized to obtain efficiency close to the maximum for a range of pitch factors from
1.2 to 1.4. According to the calculations, a uniform section length of 20 mm was chosen for a nominal
accelerating voltage of 15 kV and a beam current of 0.4 to 0.6 A. The cavity was made of copper; its diffractive
and Ohmic Q-factors are estimated as 63000 and 10400, respectively. High precision (fractions of a micron) of
the cavity fabrication, high surface quality, and the absence of taper or ellipticity are required in the consideredfrequency range [4]The conventional mechanical processing techniques did not provide a sufficient manufacturing accuracy, andthe output power did not exceed several watts in the initial experiments. The most successful version of thecavity employed in the experiments was manufactured using ultra-precision turning technology using carbide
cutters.
A quasi-optical converter that transforms the operating mode into a Gaussian beam has been developed to extract
radiation from the vacuum volume of the gyrotron. The simulation results showed a conversion efficiency of 98.4%.
The mode converter consists of a Vlasov launcher, a parabolic mirror, and four correcting mirrors. The surface depthof the profiled mirrors is about 1 mm, which is acceptable for precision manufacturing. Photos of the experimental
setup with the tube inside a cryomagnet are shown in Figure 1.

Fig. 1. Photo of the gyrotron and an experimental setup with the tube inside a cryomagnet.
The output power of the gyrotron was measured with a water calorimeter. A study of the excitation of oscillationsin the vicinity of the operating mode is shown in Fig. 2a. The area of excitation of the operating mode at the second
harmonic is clearly seen; parasitic modes are excited at the fundamental harmonic on the right and left of this area.
By varying the magnitude of the magnetic field, the optimal conditions for stable excitation of single-mode
oscillation were found. We obtained a maximum output power of 250 W for a beam voltage of 15 kV and a beam
current of 0.6 A, which corresponds to an output efficiency of 2.7%. Considering the ratio of diffractive and Ohmic
Q-factors, this value corresponds to an interaction efficiency of about 30%, consistent with theoretical estimates fora reasonable value of the pitch factor about 1.2–1.3. The difference between interaction and output efficiency isdefined by the Ohmic losses in the cavity, which are about 85%. By varying the beam current, the output power ofthe gyrotron can be smoothly adjusted from maximum to tending to zero. Figure 2b shows the results of experiments
on measuring the radiation power at the second harmonic for an operating current of 590 mA. The conditions for
excitation of the operating mode were studied experimentally to determine the minimum starting current. Excitationof the mode at low beam currents was monitored by two methods, namely, observation of the temperature trace of
the wave beam passing in a continuous-wave mode through a dielectric plate and measurement of the signal from ahigh-frequency detector. The measured starting current is 20 mA, which agrees with the calculations and confirms
the sufficiently high accuracy of the cavity fabrication and acceptable parameters of the electron beam.
The amplitude-phase structure of the wave beam field was reconstructed using measurements of the intensity
distributions in several cross-sections with an infrared camera. The analysis showed that the content of the ТЕМ0,0
mode in the output radiation is 62% (see Fig. 3). The most probable reason for the distortion of the wave beam is theinsufficient accuracy of the mutual installation of the elements of the quasi-optical converter.
To correct the structure of the output radiation and focus it into a spot, we designed an additional matching opticalunit (MOU) consisting of two mirrors with synthesized profiles. According to the simulation results, this unit
converts the gyrotron radiation into a Gaussian beam with a calculated half-width at the focal point of 1.1mm /1.1mm (by the 1/e level of the intensity) and a ТЕМ0,0 mode content of more than 98%, while the energy loss forconversion does not exceed 4%.

Fig. 2. Output power versus magnetic field: a) generation zones for the operating mode at the second cyclotron
harmonic TE-6,5 and the nearest parasitic mode, accelerating voltage U = 15 kV and beam current I = 0.4 A; b) TE-6,5
output power at U = 15 kV and I = 0.59 A.

Fig. 3. Transverse distributions of the gyrotron wave beam intensity at a distance of 216 mm from the window: a)measured and b) calculated. Aperture size is 50 x 50 mm.

Frequency measurements were the key to the experimental campaign. For the initial frequency estimation, weused a Fabry-Perot resonator, which allowed us to determine the frequency with accuracy on the order of agigahertz and to determine the harmonic number accurately. Next, a spectrum analyzer with an external mixer
was employed to refine the frequency more accurately and measure the signal spectrum. As an additional checkand determination of the presence of radiation impurities with a frequency corresponding to the first harmonic,
we used the scheme with scattering by a diffraction grating. The sawtooth-shaped corrugated aluminium platewith a period of 0.6 mm and angles of 30° and 60° at the base has more than 99% of 0.526 THz power reflected
into -1st order (reflection angle 14° to the normal) for an incidence angle of 45°. We found no significant
intensity peaks at other angles. Therefore, the operating frequency was determined as 0.526 THz without the
admixture of parasitic frequencies (parasitic modes).
Thus, in the experiment with a continuous-wave gyrotron at the second harmonic with a cylindrical cavity,
coherent radiation with a frequency of 0.526 THz and an output power of 250 W was obtained. This power
level looks almost an order of magnitude higher than in the previous experiments.
References[1] S.K. Jawla, R.G. Griffin, I.A. Mastovsky, M.A. Shapiro, and R.J. Temkin, “Second Harmonic 527-GHz
Gyrotron for DNP-NMR: Design and Experimental Results,” IEEE Transactions on Electron Devices, vol.
67, (2020) 328-334. DOI:10.1109/TED.2019.2953658.
[2] M. Blank, P. Borchard, S. Cauffman, K. Felch, M. Rosay, and L. Tometich, “High-Frequency Gyrotrons for
DNP-Enhanced NMR Applications,” IEEE International Vacuum Electronics Conference, IVEC 2014(2014). DOI: 10.1109/IVEC.2014.6857463.
[3] M.Y. Glyavin, A.V. Chirkov, G.G. Denisov, A.P. Fokin, V.V. Kholoptsev, A.N. Kuftin, A.G. Luchinin, G.
Y. Golubyatnikov, V.I. Malygin, M.V. Morozkin, V.N. Manuilov, M.D. Proyavin, A.S. Sedov, E.V.
Sokolov, E.M. Tai, A.I. Tsvetkov, and V.E. Zapevalov, “Experimental Tests of a 263 GHz Gyrotron forSpectroscopic Applications and Diagnostics of Various Media,” Review of Scientific Instruments, vol. 86(2015) 54705. DOI: 10.1063/1.4921322.
[4] A.S. Zuev, A.S. Sedov, E.S. Semenov, A.P. Fokin, and M.Yu. Glyavin, “Analysis of the Possibilities toControl Diffraction Quality Factors of the Cavities of Subterahertz Gyrotrons,” IEEE Transactions on
Plasma Science, vol. 48 (2020) 4037-4040. DOI:10.1109/TPS.2020.3025689.
More detail in: M.Yu. Glyavin, A.N. Kuftin, M.V. Morozkin, M.D. Proyavin, A.P. Fokin, A.V. Chirkov,
V.N. Manuilov, A.S. Sedov, E.A. Soluyanova, D.I. Sobolev, E.M. Tai, A.I. Tsvetkov, A.G. Luchinin, S.Yu.
Kornishin, G.G. Denisov, ”A 250-Watts, 0.5-THz continuous-wave second-harmonic gyrotron,” Electron
Device Letters, vol. 11 (2021). DOI:10.1109/LED.2021.3113022.

Study of 3D-Printed Dielectric Barrier Windows for Microwave Applications
M. D. Proyavin, D. I. Sobolev, V. V. Parshin, V. I. Belousov, S. V. Mishakin, M. Yu. Glyavin
Institute of Applied Physics of the Russian Academy of Sciences, 603950, Nizhny Novgorod, Russia

Abstract: 3D printing technologies offer significant advantages over conventional
manufacturing technologies for objects with complicated shapes. This technology provides
the potential to easily manufacture barrier windows with a low reflection in a wide
frequency band. Several 3D printing methods were examined for this purpose, and thedielectric properties of the various types of materials used for 3D printing wereexperimentally studied in the frequency range 26–190 GHz. These measurements show thatthe styrene-butadiene-styrene and polyamide plastics are suitable for broadband lowreflection windows for low-to-medium-power microwave applications. Two barrier windows
with optimized surface shapes were printed and tested. The results demonstrate that thestudied technique can fabricate windows with a reflection level below −18 dB in thefrequency band up to 160 GHz. The studied windows can be used for spectroscopic tasks
and other wideband microwave applications.

Additive manufacturing technologies have great potential for industry, science, and technology. There are
a number of tasks that are difficult or almost impossible to implement with traditional fabrication methods. In
particular, 3D printing from dielectric materials is a highly convenient and cheap tool for prototyping andmanufacturing radiofrequency components. It creates a method of readily obtaining components with asophisticated surface shape. For low-reflection microwave windows, a subwavelength grating with a speciallydesigned shape at both sides of the window disk could significantly reduce the reflection coefficient of incident
radiation in a wide frequency band. This paper explores the possibility of using 3D printing to make millimeterwave barrier windows that can operate in a wide frequency range.
Currently, there are several areas in which windows with the broadband transmission of low-power
microwave radiation are used. These areas include, in particular, molecular gas spectroscopy and DNP/NMR
spectroscopy, measurements of fine positronium structure, radiometers and geophysical instruments used foratmospheric transparency studies. For all the mentioned applications, the radiation power does not exceed a few
watts, which allows one to use windows made of polymers without the risk of overheating.
There are different methods to obtain the broadband transmission of microwave radiation through thewindows. The almost reflectionless transmission of a linearly polarized wave can be achieved for the Brewsterangle disk, however, this output suffers from an inefficient use of space, which may be especially important fordivergent wave beams and some distortion of the transverse field structure. Polymers can be mixed with
nanoparticles to produce a multilayer dielectric coating. Metamaterial devices and gradient index photonic
structures are also used to reduce the reflection. Another known method considered in this paper is the use of
windows with a surface grating of a specially optimized shape, providing a significant reduction in reflection.
For the manufacture of such a surface, it is highly convenient to use 3D printing. Besides low cost and time
consumption, 3D printing has no restrictions on the curvature of the surface compared to Computerized
Numerical Control (CNC) machining. Furthermore, it has advantages in creating thin elements from relatively
soft and brittle materials.
Currently, many different materials for 3D printing are presented on the market, and their number isconstantly growing. Unfortunately, manufacturers usually do not provide information on the dielectric
properties of materials, especially in the microwave, millimeter, and terahertz regions. In order to design the
microwave components, the dielectric properties are of great importance, so the characterization of thematerials used for additive manufacturing is needed.
To study the loss tangent and dielectric constant of the plastics, two independent methods were used toincrease the reliability of the results. In the first method, a rod made of the investigated plastic was inserted intoa rectangular waveguide. The reflection and transmission coefficients were measured, and the dielectric
properties were calculated from the experimentally obtained frequency dependences. Formulas for reflection
and transmission of the dielectric waveguide plug can be found in [1]. The second method for measuring theproperties of dielectrics was to place samples in the form of flat disks between the mirrors of a high-quality
open two-mirror Fabry-Perot resonator. By measuring the quality factor of an empty cavity and a cavity with adielectric insert, it is possible to measure the properties of the test sample with high accuracy [2]. These
measurements were made for selected materials and frequencies up to 200 GHz.
For the experimental study of the broadband windows prototypes, we chose SLS printing from polyamide
due to low losses and high manufacturing accuracy, which allow the creation of small-scale structures suitablefor devices operating at frequencies of several hundred GHz. The sizes of the subwavelength antireflection
structures were chosen to produce fine details by the selected printing method adequately. These structures
should perform well when half of the wavelength is bigger than the period of the structure because there could
be no ±1st order diffraction scattering in these conditions. However, the performance at higher frequencies
might degrade faster or slower depending on the shape of the elements. This paper considers the two variants ofknown antireflection subwavelength gratings at window disks for additive manufacturing. The first variant of
the surface shape is a periodic array of pyramids with a base size smaller than the wavelength [3]. The shape of
the pyramidal grating is shown in Figure 1a. The advantages of such a surface are a weak dependence of thereflection coefficient on the frequency and polarization of the incident radiation. The base side of the pyramids
was chosen to be 1 mm, and the height was 2 mm. Simulations show that this corrugation applied to both
surfaces of the polyamide window disk and provided reflections of less than −20 dB in the frequency bandwider than one octave. For comparison, the flat polyamide disk had reflections of up to −9 dB in this band. The
second option is a one-dimensional periodic corrugation of a special shape, proposed in [4]. This profile is
optimized to minimize the reflection of the polarization with the direction of the electric field orthogonal to thegroove direction at the frequency range of 60–160 GHz. The shape of the grooves is shown in Figure 1b. The
period and depth of one-dimensional corrugation are 2 mm and 2.5 mm, respectively. The corrugation profile
was optimized for one linear polarization only, and the reflection coefficient was less than −20 dB for
frequencies below 105 GHz.
Three discs were printed: the first one had a flat surface on both sides, the other two had gratings of thetested shape on both sides. The reflection coefficients were obtained by two measurement setups by a vector
network analyzer with a step of 30 MHz in the bands of 75–110 GHz and 130–160 GHz, where the disks with
optimized surface shapes calculated reflection minima. The window disks were attached to the output of thecorrugated tapers providing the Gaussian wave beam flat phase front (Figure 2a). The 0-dB level of the setup
was set using the flat mirror closing the end of the taper, and the minimum sensitivity limit was set as thereflection from the open end of the taper.

Fig. 1. 3D-printed windows with (a) pyramids; (b) one-dimensional grating of special shape.

Valve
Valve
Directional
Coupler

VectorNetwork
Analyzer

Dielectric
Disk

Waveguide
Taper

Absorber

VectorNetwork
Analyzer

Valve

PTFE
Lens

Dielectric
Disk

PTFE
Lens
Waveguide
Taper

Waveguide
Taper

Valve

AbsorberFig. 2. Schematics of the setups used to measure the dielectric disks parameters: (a) reflection measurements;(b) transmission measurements.
Reflection measurement results are shown in Figures 3-5 for the flat-surface disk, disk with pyramids, anddisk with one-dimensional corrugation, correspondingly. The disk with a one-dimensional corrugation of thesurface was measured for both orthogonal linear polarizations. However, the results for the second polarization
are significantly worse than for the optimal polarization. The measured reflection coefficient is below −18 dBfor both corrugated disks in the frequency ranges of 75–110 GHz and 130–160 GHz. In contrast, the flat disk
has a much narrower band with low reflections (10 GHz at the level of −20 dB). Note that the option withpyramids is more advantageous if radiation of arbitrary polarization is required, while the option with onedimensional corrugation is better for specific linear polarization. The one-dimensional corrugation works betterfor lower frequencies but (due to its larger period) is worse for higher frequencies. Due to the high sensitivity ofthis shape to manufacturing tolerances, the measured reflection of one-dimensional corrugation significantly
deviates from the calculated one, which is also noted in [5].
To measure wideband transmission, we used a quasi-optical setup consisting of a vector network analyzer(VNA), a pair of tapers with PTFE lenses on adaptors, and an air gap between them (Figure 4b). Disks wereplaced in the center of the air gap, which is also the position of the Gaussian beam waist. The transmission
through the two thinner disks is presented in Figure 8. The flat disk has a thickness of 3.15 mm, and the averagethickness of the disk with pyramids is 3.33 mm. The low-reflection disk has a significantly better transmission
and is very close to the maximum transmission predicted using the measured loss tangent of the polyamide. The
disk with one-dimensional corrugation is not shown because it has high dielectric losses in this frequency banddue to its bigger average thickness of 7.5 mm. The oscillations of the flat-sided disk transmission are caused by
Fabry–Perot resonances inside the disk (period approximately 30 GHz) and resonances between the disk andone of the lenses (approximately 4 GHz). The reflection from the tapers causes fast oscillations (approximately
1 GHz).
To measure wideband transmission, we used a quasi-optical setup consisting of a vector network analyzer(VNA), a pair of tapers with PTFE lenses on adaptors, and an air gap between them (Figure 4b). Disks wereplaced in the center of the air gap, which is also the position of the Gaussian beam waist. The transmission
through the two thinner disks is presented in Figure 6. The flat disk has a thickness of 3.15 mm, and the average
thickness of the disk with pyramids is 3.33 mm. The low-reflection disk has a significantly better transmission
and is very close to the maximum transmission predicted using the measured loss tangent of the polyamide. The
disk with one-dimensional corrugation is not shown because it has high dielectric losses in this frequency banddue to its bigger average thickness of 7.5 mm. The oscillations of the flat-sided disk transmission are caused by
Fabry–Perot resonances inside the disk (period approximately 30 GHz) and resonances between the disk andone of the lenses (approximately 4 GHz). The reflection from the tapers causes fast oscillations (approximately
1 GHz).
0

Reflection, dB

-10

-20

-30

-40
80

100

120
Frequency, GHz

140

160

Fig. 3. Reflection from the 3D-printed disk with a flat surface on both sides. The black line corresponds to thenumerical simulation in CST Studio. The blue line is the measured reflection coefficient, and the red line is thelower sensitivity limit of the measurement setup.
Based on the obtained values of the real and imaginary parts of the dielectric permittivity, as well as themechanical and thermal parameters of the tested materials given in [20], it is possible to determine the losses
and temperature conditions of a window that can withstand atmospheric pressure, depending on its diameterand the supplied microwave power. When considering the heat problem, the transverse dependence of the
microwave radiation intensity on the radius was taken as a Gaussian wave beam with a width of 0.64 of thewindow radius. Numerical modeling was performed with the following parameters to represent many plastics
with similar properties: n = 1.5, emissivity ε = 0.9, and the thermal conductivity of plastic κ = 0.4 W·m−1K−1.
The dielectric loss tangent was taken as slightly larger than the best-tested plastic δ = 0.002. The temperature of
the cooled edge of the window (ambient temperature) was T0 = 20 °C, with a convection coefficient of h = 10
W·m−2·K−1. Several window diameters were considered between 1 cm and 10 cm with different thicknesses
from 1 mm to 10 mm, and the wavelength was set as λ0 = 3 mm, which corresponded to the 100 GHz basefrequency. Dielectric losses are proportional to the frequency; therefore, the maximum power for any otherfrequency can be calculated by multiplying the ratio of the base frequency to the target frequency. Numericalmodeling of the dielectric disk heating by a Gaussian beam was performed in COMSOL Multiphysics. The
reflections of the beam on disk surfaces were neglected, assuming the antireflection surfaces. We simulated
maximum beam power, which can be transmitted through the disk, given that the maximum stationarytemperature is 120 °C, so a disk with diameter 100 mm and thickness 10 mm can withstand approximately 150
W of CW transmitted power at a frequency of 100 GHz without convective cooling.
0

Reflection, dB

-10

-20

-30

-40
80

100

120
Frequency, GHz

140

160

Figure 4. Reflection from the 3D-printed disk with pyramids on both sides. The black line corresponds to thenumerical simulation in CST Studio. The blue line is the measured reflection coefficient, and the red line is thelower sensitivity limit of the measurement setup.
0

Reflection, dB

-10

-20

-30

-40
80

100

120
Frequency, GHz

140

160

Fig. 5. Reflection from the 3D-printed disk with special one-dimensional corrugation on both sides. The blackline corresponds to the numerical simulation in CST Studio. The blue line is the measured reflection coefficient,and the red line is the lower sensitivity limit of the measurement setup.
Current 3D printing technologies allow easy-to-manufacture dielectric microwave components, such asbroadband barrier windows with low reflections. In this paper, the dielectric properties of the materials used invarious 3D printing methods were measured. The most suitable materials were found to be useful in microwave
systems with a frequency of up to several hundred gigahertz and a power of up to several tens of watts. The
analytical calculations and numerical simulations verify the use of the studied materials for microwave devices
with a power of about 100–200 watts.

Transmission, dB

0

-1

-2

-3
110

120

130

140
Frequency, GHz

150

160

170

Fig. 6. Transmission through the disks. The red line corresponds to the disk with pyramids on both sides, andthe blue line corresponds to the flat-surface disk. The black line shows dielectric losses calculated in the 3.33
mm thick polyamide disk with zero reflections.
Barrier windows with surface shapes specially optimized for low reflection in a wide frequency band
were printed and examined at low power. The printed windows provided a reflection coefficient below −18 dB
at frequencies of up to 160 GHz, making it possible to use in microwave devices that require thereception/transmission of a signal in a wide frequency range. We conclude that the current additivetechnologies are competitive with traditional manufacturing methods for dielectric microwave components at 1
millimeter and for longer waves.

References

1.
2.
3.
4.
5.

Rinkevich, A.B.; Perov, D.V.; Ryabkov, Y.I. Transmission, reflection and dissipation of microwaves inmagnetic composites with nanocrystalline finemet-type flakes. Materials 2021, 14, 3499.
Vlasov, S.N.; Koposova, E.V.; Mazur, A.B.; Parshin, V.V. On permittivity measurement by a resonancemethod. Radiophys. Quantum Electron. 1996, 39, 410–415.
Ma, J.Y.L.; Robinson, L.C. Night moth eye window for the millimetre and sub-millimetre wave region.
Opt. Acta Int. J. Opt. 1983, 30, 1685–1695.
Vlasov, S.N.; Koposova, E.V.; Kornishin, S.Y. Wideband windows for millimeter- and submillimeterwave vacuum devices. Radiophys. Quantum Electron. 2020, 63, 115–123. 2020.
Laureto, J.; Tomasi, J.; King, J.A.; Pearce, J.M. Thermal properties of 3-D printed polylactic acid-metalcomposites. Prog. Addit. Manuf. 2017, 2, 57–71.

More detail in: M.D.Proyavin, D.I.Sobolev, V.V.Parshin, V.I.Belousov, S.V.Mishakin, M.Yu.Glyavin, “Studyof 3D-Printed Dielectric Barrier Windows for Microwave Applications,” Electronics, vol. 10, n. 18 (2021) 2225.
DOI: 10.3390/electronics10182225.

Regimes of Hybrid Modes in a THz Clinotron
Eduard Khutoryan1, 2, Alexei Kuleshov1, 2, Sergey Ponomarenko1, Konstantin Lukin1, Yoshinori Tatematsu2,
Masahiko Tani2
1

O. Ya. Usikov Institute for Radiophysics and Electronics of NAS of Ukraine
12 ac. Proskura str., Kharkiv, Ukraine, 61085
2
Research Center for Development of Far-Infrared Region, University of Fukui,
3-9-1 Bunkyo, Fukui-shi, Fukui, Japan, 910-8507
e-mail: jeanalexkh@gmail.com

Existing and potential applications of THz radiation in physics, biology, chemistry, medicine, imaging,
spectroscopy, etc. require compact radiation sources with sufficient output power and frequency tuning range.
Although a gyrotron is powerful source in the THz range, slow-wave vacuum electron devices have a great
potential for such applications due to their compactness. However, advance of these devices in the THz range ishampered by a confinement of RF field near slow wave system, high-frequency ohmic losses, technologicallimits, necessity of high-density beams and high focusing magnetic fields, the decrease of the efficiency ofoutput waveguides, etc. Our previous study has showed that the so-called hybrid bulk-surface mode is very
promising to enhance the beam-wave interaction efficiency and the output power of THz BWOs and clinotrons.
The hybrid bulk-surface wave appears in the cavity with non-uniform (periodically modified) grating. One of
the eigenmodes of an open non-uniform grating is a leaky wave, which contains both the wave, radiating into
free space and the surface wave confined to the grating surface (surface plasmon polariton) and propagating
along a grating. Placing a metal wall above the grating causes reflections of the radiating wave back to thegrating with further coupling of the radiating wave to the surface plasmon polariton. Since the reflectedradiating wave is confined in the cavity volume (being transformed into a bulk cavity wave) and is coupled witha surface plasmon polariton, such hybrid mode is bulk-surface one. The results of theoretical study arepresented below and demonstrate classification of these hybrid regimes in the clinotron oscillator of 0.5-0.7
THz frequency range. To provide efficient output radiation, searching for optimized feedback loop, positionand dimension of the output waveguide, grating profile, etc. have been carried out by simulations using the PIC
code MAGIC2D.
The variety of oscillation regimes may be roughly described by mutual velocities of the bulk and surface
waves, total group velocity, hybridization factor, etc. From those dispersions shown in Fig. 1, one candistinguish several regimes that can be realized by the proper electron beam voltage and a set of the waveguideparameters:•(A) Bulk wave radiation angle is close to 90o to the grating, like in DRO and orotron. Due to the surface
wave, the area of vgr ≅ 0 is much wider than for pure bulk fast mode;•(B) Bulk wave is radiated at the angle 120-150o to the grating (i.e. propagates oppositely to a beam) andtherefore, a feedback is provided by the backward radiating harmonic;•(C) Similar to (B) but the radiating angle is close to 180o to the grating that resembles Zennek-Sommerfeld
wave;

•

(D) Surface wave propagates oppositely to a beam and the bulk wave; feedback is provided by the
backward surface wave.

Fig. 1. Dispersions of waveguides containing gratings with period l = 0.07 mm, and a regular groove depth h1 = 0.09
mm, D = 2 mm. (a) h3 = 1.3h1, h2 = h1; (b) h3 = 1.2 h1, h2 = h1. Areas A-D correspond to different regimes of thehybrid mode.

In the regime of bulk wave (-2nd spatial harmonic) the radiation is almost normal to the grating as in devices
based on Smith-Purcell radiation such as orotron. The feedback in such regime may be due to multiple
reflections from the upper wall (as in an open resonator of an orotron), due to reflections from side walls anddue to the backward direction of the bulk wave. Continuous frequency tuning range in this regime is defined by
the dispersion curve marked as A in Fig. 1 (a), where an increase of the oscillation frequency corresponds to anincrease of the radiation angle from normal and, hence, results in the excitation of higher order axial modes(HOAM).
In the Regime B, according to the dispersion, the radiation angle is within 120-150o. Here, the feedback isdue to the backward bulk wave that is demonstrated by field patterns for D = 8 mm (Fig. 2).

Fig. 2. RF field pattern at steady state demonstrating bulk wave and output radiation propagation.

The length of feedback Lfb depends on the radiation angle and the waveguide height D. Simulation
demonstrated that in the case when Lfb is longer than the grating length Lg: Lfb > Lg, oscillations drop (at Lg = 17mm and U = 19 kV it holds for D ≅ 10 mm). In our simulation the coupling hole is placed at the vicinity of z =0 due to the backward wave nature. The rays responsible for the feedback are only these being reflected fromthe upper wall to the grating (rays that are reflected to the side wall are idle). Therefore, removing the part of
the upper wall Lout = D tanα (from which rays fall on the side wall), does not have any effect on the beam-wave
interaction and this size of the coupling hole is close to the optimal value. The bulk wave is a reason of both
efficient feedback and effective radiation output, whereas surface wave is a reason of high beam coupling
impedance. This fact was proven in the simulation of the case when the middle part of the grating was
shortened but the oscillation still existed and resembled two-cavity klystron with coupling between cavities.
At the large radiation angle which is close to 180o (Regime C), there is almost no role of the upper reflector
for the feedback by the backward radiating harmonic. Thus, the interaction power Pint almost independent from
D that was proven by hot simulations. In simulations, 22 kV beam excited the oscillation with the interaction
power of about 200 W at 0.625 THz. The feedback here is also due to the backward radiating harmonic, which
has relatively low loss and provides quite uniform RF field distribution along grating, and, hence, all electron
layers effectively interact with the wave. The wave with radiation angle close to 180o matches well with
Zennek-Sommerfeld wave of the flat metal surface and this wave is responsible for a feedback. The consideredoscillator is non-resonance BWO. Resonance is possible when reflectors are placed at grating ends that createsan open (without upper wall) or closed resonator. The flat mirrors inclined 5o to grating enabled an increase of
the interaction power about 1.5 times.
Conclusion
The higher output power of the hybrid mode in comparison with pure surface or bulk modes is due to anincrease of the feedback and efficiency of the radiation output caused by radiating harmonic, and due to highcoupling factor caused by the surface harmonic. 2D PIC simulations predicted output power of dozens of Watts
in 0.5-0.7 THz range provided by the 0.2 A, 17 kV electron beam with the output efficiency of up to 1%. The
simulated frequency tuning range was about 5%. Mode competition has been discussed, in particular, it isshown that single or two mode operation, hysteresis effect, etc. are controlled by the waveguide height.
For more detail, please access the original publication (Khutoryan E., Kuleshov A., Ponomarenko S. et al.,“Efficient Excitation of Hybrid Modes in a THz Clinotron,” J Infrared Millimeter, and Terahertz Waves, vol. 42(2021) 671–683. DOI:10.1007/s10762-021-00800-y.) as well as the following research papers:

1. S. S. Ponomarenko et al., "Traveling-Wave Amplification in a Circuit With Nonuniform Grating," inIEEE Transactions on Electron Devices, vol. 68, n. 10 (2021) 5232-5237. DOI:
10.1109/TED.2021.3105951.
2. S. S. Ponomarenko et al., "Spectral Characteristics of THz CW Clinotrons," in IEEE Transactions on
Electron Devices, vol. 67, n. 12 (2020) 5766-5770. DOI:10.1109/TED.2020.3032940.
3. E. Khutoryan et al., "Efficient Regime of Hybrid Surface-Radiating Waves in a THz Clinotron," 2020
IEEE 21st International Conference on Vacuum Electronics (IVEC), 2020, pp. 155-156. DOI:
10.1109/IVEC45766.2020.9520601.
4. A. Likhachev et al., "THz Imaging System Based on Frequency-Tunable 140 GHz Clinotron and QuasiOptical Antenna," 2020 IEEE Ukrainian Microwave Week (UkrMW), 2020, pp. 946-949.
DOI:10.1109/UkrMW49653.2020.9252570.

SPECIAL JOURNAL ISSUES IN THE FIELD OF THZ SCIENCESpecial Issue "Terahertz Optical Elements: Science and Technology". For more detail, please follow thelink.

Special Issue on Microwave, Millimeter and Terahertz Techniques in Medical, Biological, andBiomedical Engineering. For details follow the link.

This Special Issue aims to present the recent development of advanced electromagnetics, microwave,
millimeter/terahertz techniques for medical, biological, and biomedical applications. Submissions areencouraged of original and high-quality review papers with scopes in state-of-the-art research related toelectromagnetic theory, techniques, and integration for medical and biological applications, including but notlimited to RF and microwave diagnostic and therapeutic systems; electromagnetic imaging; radiometry andthermography; computational methods for bio-electromagnetics; implantable and wearable diagnostic andtherapeutic systems; radar for remote or noninvasive monitoring and tracking; electromagnetic sensors anddevices or biomedical applications. The articles will include data from experiments, computational simulations,
and/or models.
Topic Areas:• Sources: Microwave devices, vacuum tube-based sources, free-electron lasers, synchrotrons, gas lasers,
quantum cascade lasers• Detectors: THz detectors, microwave antenna and antenna arrays, electromagnetic sensors, radar for remoteor noninvasive monitoring and tracking• Devices: Modulators, reflectors, filters, waveguides, metamaterial-based devices, RF ablation therapeutic
devices and instruments• Systems: Emitter-receiver systems, imaging systems, RF and microwave integrated circuits, diagnostic andtherapeutic systems, implantable and wearable diagnostic and therapeutic systems• Spectroscopy: Interaction between THz waves and all states of matter including THz spectroscopy on solids,
liquids, gases, and plasmas• Applications: Human physiological sensing and biomedical imaging, biological analyses, healthcare systems,
noninvasive monitoring and tracking, noninvasive medical diagnostic, biomedical sensing, RF ablation
therapeutics, pharmaceutical industry• Computational methods for bioelectromagnetics
Publication Target Dates
Paper submission deadline March 1, 2022
Initial review completed May 1, 2022
Special Issue publication date August 1, 2022
Guest Editors
Prof. Lulu Wang, Shenzhen Technology University, China, wanglulu@sztu.edu.cn
Prof. Alexei Kuleshov, O. Ya. Usikov Institute for Radiophysics and Electronics of National Academy of
Sciences of Ukraine, Ukraine, KuleshovAN@nas.gov.ua
Prof. Svilen Sabchevski, Institute of Electronics, Bulgarian Academy of Sciences, Bulgaria, sabch@ie.bas.bg

Special Issue: Sensing and Imaging with Electromagnetic Waves

Guest Editor
Dr. Reza K. Amineh
Department of Electrical and Computer Engineering, New York Institute of Technology, NY 10023, USA
Email: rkhalaja@nyit.edu
Manuscript Topics
Sensing and imaging with electromagnetic waves has grown rapidly in the past few decades thanks to thesignificant progress in the hardware components and processing algorithms. On the hardware side, theemergence of new and cost-effective sensors and their associated data acquisition boards has paved the way
toward numerous novel applications. On the processing side, the development of faster computing machines
has allowed for realization of more sophisticated algorithms that facilitate real-time or quasi real-time imaging
and sensing of the media. The range of applications is very broad and encompass frequencies from fractions ofHertz to tera-Hertz and beyond, including but not limited to: Eddy current inspections, inductive sensing ofconductive objects, capacitive sensing of dielectric objects, microwave imaging for biomedical andnondestructive testing applications, microwave sensing for material identification, radar imaging and remotesensing, tera-Hertz imaging and sensing, and optical imaging and sensing. The objective of this Special Issue isto provide an overview of the current research on methods and applications of electromagnetic waves inimaging and sensing of media, highlighting the latest developments and innovations in modern applications interms of sensors, data acquisition circuits, sampling methods, algorithms, and so on. We will also try to identify
new challenges and opportunities for new applications.
Instructions for authors
http://www.aimspress.com/electreng/news/solo-detail/instructionsforauthors
Please submit your manuscript to online submission system
https://aimspress.jams.pub/

Paper Submission
All manuscripts will be peer-reviewed before their acceptance for publication. The deadline for manuscript
submission is 15 May 2022.

LIST OF SELECTED RECENT PUBLICATIONS
Bibliography and links to selected recent publications on topics related to the research field of theInternational Consortium and published after June 2021, i.e. after issuing the previous Newsletter #18.
This cumulative list is in chronological order as collected from various bibliographical and alert services
A. Publications by authors from the institutions participating in the International Consortium
E. Khutoryan, A. Kuleshov, S. Kishko, S. Ponomarenko, M. Glyavin, I. Bandurkin, V. Manuilov, A. Fedotov, I.
Zotova, S. Sabchevski, Y. Ishikawa, M. Fukunari, T. Saito, Y. Tatematsu, S. Mitsudo, T. Idehara, “Increase of
Gyrotron Output Power at High-Order Axial Mode Through an After-Cavity Excitation of the Next Transverse
Mode,” J Infrared Millimeter, and Terahertz Waves, vol. 42 (2021) 684–700. DOI:10.1007/s10762-021-007983.
https://link.springer.com/article/10.1007/s10762-021-00798-3
Khutoryan E., Kuleshov A., Ponomarenko S., Lukin K., Tatematsu Y., Tani M., “Efficient Excitation of Hybrid
Modes in a THz Clinotron,” J Infrared Millimeter and Terahertz Waves, vol. 42 (2021) 671–683.
DOI:10.1007/s10762-021-00800-y.
https://link.springer.com/article/10.1007/s10762-021-00800-y
Vodopyanov A.V., Sorokin A.A., Sintsov S.V., et al. “Tungsten Carbide Nanopowder Synthesis under the
Influence of Microwave Electromagnetic Radiation on a W–C System Nanocomposite Produced in a Thermal
Plasma,” Inorg. Mater. Appl. Res., vol. 12(2021) 735–739. DOI:10.1134/S2075113321030382.
https://link.springer.com/article/10.1134/S2075113321030382
Semenov E., Zapevalov V., Zuev A. “Methods for Simulation the Nonlinear Dynamics of Gyrotrons”. In:
Balandin D., Barkalov K., Gergel V., Meyerov I. (eds) Mathematical Modeling and Supercomputer
Technologies. MMST 2020. Communications in Computer and Information Science, vol. 1413 (2021).
Springer, Cham. DOI:10.1007/978-3-030-78759-2_4.
https://link.springer.com/chapter/10.1007%2F978-3-030-78759-2_4
G. Aiello, K.A. Avramidis, T. Franke, G. Gantenbein, J. Jelonnek, et al., "Large Area Diamond Disk GrowthExperiments and Thermomechanical Investigations for the Broadband Brewster Window in DEMO," in IEEE
Transac tions on Electron Devices, DOI:10.1109/TED.2021.3088077.
https://ieeexplore.ieee.org/document/9462126
S.V. Egorov, A.G. Eremeev, V.V. Kholoptsev, I.V. Plotnikov, K.I. Rybakov, A.A. Sorokin, S.S. Balabanov,E.Ye. Rostokina, Yu.V. Bykov, “Rapid microwave sintering of zinc oxide-based varistor ceramics,” Journal of
the European Ceramic Society, (2021). DOI:10.1016/j.jeurceramsoc.2021.06.028.
https://www.sciencedirect.com/science/article/pii/S0955221921004404
Furuya T., Muldera J., Bakunov M., Kitahara H., Ozaki T., Estacio E., Tani M., “Terahertz generation in a thin
GaAs slab in a tapered parallel plate waveguide by femtosecond laser excitation at 1560 nm,” Japanese Journalof Applied Physics, (2021). DOI: 10.35848/1347-4065/ac0f48.
https://iopscience.iop.org/article/10.35848/1347-4065/ac0f48
Zapevalov V.E., Zuev A.S., Kuftin A.N., Plankin O.P., Semenov E.S., “On Designing the Electron-Optical
System of a Multibarrel Gyrotron,” Radiophys Quantum El (2021). DOI:10.1007/s11141-021-10086-8.
https://link.springer.com/article/10.1007/s11141-021-10086-8
A.V. Samokhin, N.V. Alekseev, M.A. Sinayskiy, A.G. Astashov, A.V. Vodopyanov, A.A. Sorokin, S.V.
Sintsov, “Microwave assisted synthesis of WC nanopowder from nanosized multicomponent system W-C

produced in thermal plasma reactor,” International Journal of Refractory Metals and Hard Materials, (2021)
105618. DOI:10.1016/j.ijrmhm.2021.105618.
https://www.sciencedirect.com/science/article/pii/S0263436821001505
S. Stanculovic, R. Difonzo, A. Allio, K.A. Avramidis, P. Brücker, G. Gantenbein, S. Illy, J. Jelonnek, P.C.
Kalaria, M. Misko, T. Rzesnicki, L. Savoldi, “Calibration of the KIT test setup for the cooling tests of agyrotron cavity full-size mock-up equipped with mini-channels,” Fusion Engineering and Design, vol. 172(2021) 112744. DOI:10.1016/j.fusengdes.2021.112744.
https://www.sciencedirect.com/science/article/pii/S0920379621005202
T. Franke, G. Aiello, K. Avramidis, C. Bachmann, B. Baiocchi, C. Baylard, A. Bruschi, D. Chauvin, A. Cufar,
R. Chavan, C. Gliss, F. Fanale, L. Figini, G. Gantenbein, S. Garavaglia, G. Granucci, J. Jelonnek, G. Suárez
López, A. Moro, M. Moscheni, N. Rispoli, M. Siccinio, P. Spaeh, D. Strauss, F. Subba, I. Tigelis, M.Q. Tran, C.
Tsironis, C. Wu, H. Zohm, “Integration concept of an Electron Cyclotron System in DEMO,” Fusion
Engineering and Design, vol. 168 (2021) 112653. DOI:10.1016/j.fusengdes.2021.112653.
https://www.sciencedirect.com/science/article/abs/pii/S0920379621004294?via%3Dihub
J.P. Ferrolino et al, “Spintronic terahertz emission from Ni/Pt bilayer grown on MgO,” J. Phys.: Conf. Ser., vol.
1943 (2021) 012035. DOI: 10.1088/1742-6596/1943/1/012035.
https://iopscience.iop.org/article/10.1088/1742-6596/1943/1/012035
Sabchevski S., Glyavin M., Mitsudo S., Tatematsu Y., Idehara T., “Novel and Emerging Applications of the
Gyrotrons Worldwide: Current Status and Prospects,” J Infrared Millimeter, and Terahertz Waves, vol. 42(2021) 715–741. DOI:10.1007/s10762-021-00804-8.
https://link.springer.com/article/10.1007/s10762-021-00804-8
R.M. Rozental, S.V. Samsonov, A.A. Bogdashov, I.G. Gachev and N.S. Ginzburg, "High-power tunable sourceof chaotic radiation based on a Ka-band helical gyro-BWO," in IEEE Electron Device Letters, (2021).
DOI:10.1109/LED.2021.3100605.
https://ieeexplore.ieee.org/document/9499092
Ginzburg N.S., Zotova I.V., Kocharovskaya E.R. et al., “Self-Induced Transparency Solitons and Dissipative
Solitons in Microwave Electronic Systems,” Radiophys Quantum El (2021). DPI:10.1007/s11141-021-10092-w.
https://link.springer.com/article/10.1007/s11141-021-10092-w
Adilova A.B., Ryskin N.M., “Influence of the Delay on Mutual Synchronization of Two Coupled Gyrotrons,”
Radiophys Quantum El (2021). DOI:10.1007/s11141-021-10091-x
https://link.springer.com/article/10.1007%2Fs11141-021-10091-x
Y.K. Kalynov, I.V. Osharin and A.V. Savilov, "Stable Excitation of Higher Axial Modes in the TravelingWave-Tube Regime in Gyrotron Cavities With Additional Loss Elements," in IEEE Transactions on Electron
Devices. DOI:10.1109/TED.2021.3099765.
https://ieeexplore.ieee.org/document/9499665
R.M. Rozental, A.S. Sergeev, V.P. Tarakanov, I.V. Zotova, S.R. Rozental, and N.S. Ginzburg, "Conditions ofrogue-wave generation in gyrotrons," Physics of Plasmas, vol. 28(2021) 083302. DOI:10.1063/5.0057013.
https://aip.scitation.org/action/showCitFormats?type=show&doi=10.1063%2F5.0057013
I.O. Girka, M. Thumm, “Azimuthal Surface Waves in Low-Density Plasma Loaded Coaxial Helix TravellingWave-Tube-Like Waveguides,” ISSN 1562-6016, ВАНТ, № 4 (134) ( 2021) 25. DOI: 10.46813/2021-134-024.
http://dx.doi.org/10.46813/2021-134-024
Muldera J.E., Afalla J.P.C., Furuya T. et al. “Creating terahertz pulses from titanium-doped lithium niobatebased strip waveguides with 1.55 μm light,” J Mater Sci: Mater Electron (2021). DOI:10.1007/s10854-021-

06802-9.
https://link.springer.com/article/10.1007%2Fs10854-021-06802-9
D. Wagner, M. Thumm, "Improvement of the Output Mode Purity of a Complex-Cavity Resonator for a
Frequency-Tunable Sub-THz Gyrotron," in IEEE Transactions on Electron Devices, (2021). DOI:
10.1109/TED.2021.3105955.
https://ieeexplore.ieee.org/document/9524331
K.A. Avramidis et al., "Multifaceted Simulations Reproducing Experimental Results from the 1.5-MW 140GHz Preprototype Gyrotron for W7-X," in IEEE Transactions on Electron Devices, vol. 68, n. 6 (2021) 30633069. DOI:10.1109/TED.2021.3075653.
https://ieeexplore.ieee.org/document/9439205
S.S. Ponomarenko et al., "Traveling-Wave Amplification in a Circuit With Nonuniform Grating," in IEEE
Transactions on Electron Devices, (2021). DOI:10.1109/TED.2021.3105951.
https://ieeexplore.ieee.org/document/9524835
L.A. Yurovskiy, A.V. Nazarovsky, N.S. Ginzburg, "Generation of Short Microwave Pulses by Compression of
Chirped Signals Obtained by Raman Backscattering on Electron Beam with Variable Energy," in IEEE
Electron Device Letters, (2021). DOI:10.1109/LED.2021.3107980.
https://ieeexplore.ieee.org/abstract/document/9523545
N. Li, G. Link, M. Engler, J. Jelonnek, "Small-Size Coaxial Resonant Applicator for Microwave Heating
Assisted Additive Manufacturing," in IEEE Transactions on Microwave Theory and Techniques, (2021).
DOI:10.1109/TMTT.2021.3105431.
https://ieeexplore.ieee.org/abstract/document/9523629
J. Schlundt et al., "Conception and Development of a Pulsed Microwave Applicator for Exposure of Fresh
Microalgae Biomass," in IEEE Transactions on Plasma Science, (2021). DOIi:10.1109/TPS.2021.3101943.
https://ieeexplore.ieee.org/document/9531597
Y.S. Oparina, A.V. Savilov, , "Coherent super-radiative undulator emission of ultra-short THz wave pulses",
Physics of Plasmas, vol. 28 (2021) 093302. DOI:10.1063/5.0058758.
https://aip.scitation.org/doi/full/10.1063/5.0058758
M.Y. Glyavin et al., "A 250-Watts, 0.5-THz continuous-wave second-harmonic gyrotron," in IEEE Electron
Device Letters, (2021). DOI:10.1109/LED.2021.3113022.
https://ieeexplore.ieee.org/abstract/document/9539195
Shcherbinin V.I., Avramidis K.A., Pagonakis I.G. et al., “Large Power Increase Enabled by High-Q DiamondLoaded Cavities for Terahertz Gyrotrons,” J Infrared Milli Terahz Waves (2021). DOI:10.1007/s10762-02100814-6
S.V. Samsonov, G.G. Denisov, A.A. Bogdashov, I.G. Gachev, "Cyclotron Resonance Maser With Zigzag
Quasi-Optical Transmission Line: Concept and Modeling," in IEEE Transactions on Electron Devices, (2021).
DOI:10.1109/TED.2021.3114141.
https://ieeexplore.ieee.org/abstract/document/9552432
Glyavin M., Gashturi A., Malkin A., Sergeev A., Zheleznov I., Tatemtsu Y., Zotova I., “Investigation of Mode
Interaction in Harmonic Sub-THz Gyrotron,” J Infrared Millimeter, and Terahertz Waves, vol. 42 (2021) 843–
850. DOI:10.1007/s10762-021-00818-2.
https://link.springer.com/article/10.1007%2Fs10762-021-00818-2

B. Publications by other authors worldwide
Mealy T., Abdelshafy A.F., Capolino F., “High-Power $X$-Band Relativistic Backward-Wave Oscillator with
Exceptional Synchronous Regime Operating at an Exceptional Point,” Phys. Rev. Applied, vol. 15, n. 6 (2021)
064021. DOI:10.1103/PhysRevApplied.15.064021.
https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.15.064021
Artem’ev K.V., Batanov G.M., Davydov A.M. et al. “Characteristics of a Subthreshold Microwave Discharge
in a Wave Beam in Air and the Efficiency of the Plasma-Chemical Reactor,” Plasma Phys. Rep., vol. 47 (2021).
498–502. DOI:10.1134/S1063780X21050019.
https://link.springer.com/article/10.1134/S1063780X21050019
Artem’ev, K.V., Batanov, G.M., Berezhetskaya, N.K. et al. “Subthreshold Discharge in a Microwave Beam as
the Basis of a Plasmachemical Reactor for Cleaning Urban Air from Excess Hydrogen Sulfide,” Plasma Phys.
Rep., vol. 47 (2021) 403–406. DOI:10.1134/S1063780X21040036.
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Constable D.A., Phelps A.D., Whyte C.G., He W., Cross A.W., Ronald K., “Numerical and Experimental
Validation of the Passive Performance of a Co-Harmonic Gyro-Multiplier Interaction Region,” IEEE
Transactions on Microwave Theory and Technique, (2021).
https://eprints.lancs.ac.uk/id/eprint/156256/
S.J. Hosseini, M. Dahmardeh, “Compact smile-like mode converter antenna with high power capacity level,”Electromagnetics, (2021). DOI:10.1080/02726343.2021.1928371.
https://www.tandfonline.com/doi/abs/10.1080/02726343.2021.1928371?journalCode=uemg20
R. Moro et al., "28 GHz Microwave Power Beaming to a Free-Flight Drone," 2021 IEEE Wireless Power
Transfer Conference (WPTC), 2021, pp. 1-4, DOI:10.1109/WPTC51349.2021.9458030.
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F. Fanale, A. Bruschi, O. Darcourt, D. Farina, L. Figini, F. Gandini, M. Henderson, R. Hunt, C. Lechte, A.
Moro, P. Platania, B. Plaum, “Design validation of in-vessel mirrors and beam dump for first plasma operations
in ITER,” Fusion Engineering and Design, vol. 172 (2021) 112717. DOI:10.1016/j.fusengdes.2021.112717.
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V.I. Shcherbinin, "Multifunctional Coaxial Insert With Distributed Impedance Corrugations for Cavities of
Broadband Tunable Second-Harmonic Gyrotrons," in IEEE Transactions on Electron Devices, vol. 68, n.
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Baghel G.S., Kartikeyan M.V., “Output System of a 42/84 GHz, 0.5 MW, Dual Regime Gyrotron,” Defence
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T. Song et al., "Experimental Investigations on a 500 GHz Continuously Frequency-tunable Gyrotron," in IEEE
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Barh A., Varshney R.K., Agrawal G.P., Rahman B.M.A., Pal B.P., “Specialty Optical Fibers for THz
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Yang X., Du C., Zhang Z., Zhu J., Huang T., Liu P., “Linearly Polarized High-Purity Gaussian Beam Shaping
and Coupling for 330 GHz/500 MHz DNP-NMR Application,” Electronics, vol. 10 (2021) 1508.
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Francik A., Jaworski, G., Nowak M., Nowak K., “Vlasov Launcher Diagrammatic Design Using the RT
Method,’ Journal of Telecomunications and Information Technologies, (2021). DOI: 10.26636/jtit.2021.150321.
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Yang Sh, Tang Ch., Wang Sh.,Wang Z.,Zhang P., Yang Y., Lu Z., Gong Y., “Novel Mechanism for Terahertz
Radiation by Oblique Colliding of Two Electron Beams in Plasma,” Journal of Physics D: Applied Physics,(2021). DOI: 10.1088/1361-6463/ac10da.
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Guan X., Zhang J., Fu W., Lu D., Yang T., Yan Y., Yuan X., “Frequency Tuning Characteristics of a HighPower Sub-THz Gyrotron with Quasi-Optical Cavity,” Electronics, vol. 10 (2021) 526. DOI:
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NEW BOOKS
Dattoli G., Di Palma E., Sabchevski S.P., Spassovsky I.P., “High Frequency Sources of Coherent Radiation for
Fusion Plasmas,” (IOP Publishing, 2021). DOI:10.1088/978-0-7503-2464-9. Online ISBN: 978-0-7503-2464-9,
Print ISBN: 978-0-7503-2462-5.
https://iopscience.iop.org/book/978-0-7503-2464-9.pdf
Dattoli G., Di Palma E., Sabchevski S.P., Spassovsky I.P., “Magnetically confined plasma for fusion energy,”
Chapter 1 in the book “High Frequency Sources of Coherent Radiation for Fusion Plasmas,” (IOP Publishing,2021). DOI: 10.1088/978-0-7503-2464-9ch1.
https://iopscience.iop.org/book/978-0-7503-2464-9/chapter/bk978-0-7503-2464-9ch1
Dattoli G., Di Palma E., Sabchevski S.P., Spassovsky I.P., “MHD models, plasma equilibrium and instabilities,”
Chapter 2 in the book “High Frequency Sources of Coherent Radiation for Fusion Plasmas,” (IOP Publishing,2021). DOI: 10.1088/978-0-7503-2464-9ch2.
https://iopscience.iop.org/book/978-0-7503-2464-9/chapter/bk978-0-7503-2464-9ch2
Dattoli G., Di Palma E., Sabchevski S.P., Spassovsky I.P., “Plasma additional heating and Tokamak
engineering issues,” Chapter 3 in the book “High Frequency Sources of Coherent Radiation for Fusion Plasmas,”(IOP Publishing, 2021). DOI: 10.1088/978-0-7503-2464-9ch3.
https://iopscience.iop.org/book/978-0-7503-2464-9/chapter/bk978-0-7503-2464-9ch3
Dattoli G., Di Palma E., Sabchevski S.P., Spassovsky I.P., “Undulator based free electron laser,” Chapter 4 inthe book “High Frequency Sources of Coherent Radiation for Fusion Plasmas,” (IOP Publishing, 2021). DOI:
10.1088/978-0-7503-2464-9ch4.
https://iopscience.iop.org/book/978-0-7503-2464-9/chapter/bk978-0-7503-2464-9ch4
Dattoli G., Di Palma E., Sabchevski S.P., Spassovsky I.P., “An overview of the gyrotron theory,” Chapter 5 inthe book “High Frequency Sources of Coherent Radiation for Fusion Plasmas,” (IOP Publishing, 2021). DOI:
10.1088/978-0-7503-2464-9ch5.
https://iopscience.iop.org/book/978-0-7503-2464-9/chapter/bk978-0-7503-2464-9ch5Dattoli G., Di Palma E., Sabchevski S.P., Spassovsky I.P., “CARM theory and relevant phenomenology,”
Chapter 6 in the book “High Frequency Sources of Coherent Radiation for Fusion Plasmas,” (IOP Publishing,2021). DOI: 10.1088/978-0-7503-2464-9ch6.
https://iopscience.iop.org/book/978-0-7503-2464-9/chapter/bk978-0-7503-2464-9ch6
Dattoli G., Di Palma E., Sabchevski S.P., Spassovsky I.P., “Plasma heating with coherent FEL-like sources,”
Chapter 7 in the book “High Frequency Sources of Coherent Radiation for Fusion Plasmas,” (IOP Publishing,2021). DOI: 10.1088/978-0-7503-2464-9ch7.
https://iopscience.iop.org/book/978-0-7503-2464-9/chapter/bk978-0-7503-2464-9ch7
Fundamentals of Terahertz Devices and Applications, Dimitris Pavlidis (Ed.), 2021 John Wiley & Sons Ltd.
Print ISBN:9781119460718 |Online ISBN:9781119460749. DOI:10.1002/9781119460749.

https://onlinelibrary.wiley.com/doi/book/10.1002/9781119460749
Karmakar, Santanu, and Jagadish C. Mudiganti. "Gyrotron: The most Suitable Millimeter-Wave Source for
Heating of Plasma in Tokamak." (2021) Book chapter in Plasma Science and Technology. DOI:
10.5772/intechopen.98857.
https://www.intechopen.com/online-first/78592
Jie Yang, Shouxi Xu, Yong Wang, and Xiaoyan Wang, "Theoretical analysis of a 0.22 THz multistage confocal
waveguide gyro-TWT with circle-sector-shaped electron beam," AIP Advances, vol. 11(2021) 095217.
DOI:10.1063/5.0063867.
https://aip.scitation.org/doi/full/10.1063/5.0063867“Terahertz Technology and Its Applications,” Victor Pacheco Peña (Ed.) Pages: 162. ISBN 978-3-0365-0996-9(Hbk); ISBN 978-3-0365-0997-6 (PDF). DOI:10.3390/books978-3-0365-0997-6 Published: August 2021 by
MDPI Books (Open Access).
https://www.mdpi.com/books/pdfview/book/4100

NEWS FROM THE NET (OUR BROADER HORIZONS)The gyrotrons and the Future of Electric Vehicles (EVs)
In a recent posting by Mark Alexander in RICOCHET a new concept of superfast-charging electric vehicles
using gyrotrons has been announced. The author writes” Imagine a world where EVs can be
HyperfastCharged™ 1,000 times faster than current technology. Instead of charging your Tesla or other EV
for an hour every 200 miles or so, you can simply drive over a “charging zone” and get a full charge inseconds. In small towns, charging zones could be located at stop signs or service stations. This would relieve
range anxiety for many potential EV customers.As you may know, within the next couple of years, autonomous,
self-driving long-haul freight trucks will be a reality. Imagine autonomous driverless trucks driving freeways in
an EV lane with a HyperfastCharged™ zone every 100 miles. Long-haul trucks could be driving 24 hours perday, minus loading and unloading stops.“ According to Mark Alexander “The key material component is the
gyrotron (1), a powerful beam-based technology that can transmit incredible bursts of energy. For the working
prototype, a $500,000+ gyrotron will be purchased. A custom transmitter plate and a receiver plate/rectenna
will be created (2) (3). The gyrotron and transmitter plate will be placed below the road surface, creating theHyperfastCharged™ charging zone, and the receiver plate installed on a modified EV, connected to special
capacitors (4) that would charge any standard EV battery (5).”

Schematics of the technology. Figure courtesy RICOSHET

Finally, Andersen concludes: “This technology offers another approach that, when demonstrated, can be agame-changer. For those of you interested in the research related to the viability of using a gyrotron in thisway, see the research review on “Wireless power transfer via Subterahertz-wave” written in 2018 by Sei
Mizojiri and Kohei Shimamura.

Scientists Obtain Magnetic Nanopowder for 6G Technology
News Wise reports: “Material scientists have developed a fast method for producing epsilon iron oxide anddemonstrated its promise for next-generation communications devices. Its outstanding magnetic properties
make it one of the most coveted materials, such as for the upcoming 6G generation of communication devices
and for durable magnetic recording. The work was published in the Journal of Materials Chemistry C, a journalof the Royal Society of Chemistry.

Future prospects for microwave drilling in geothermal exploration
Recently, there is a growing interest in the microwave drilling (including such based on gyrotrons) for
geothermal energy exploration. Some news on the topic can be found in the following publications:• “U.S. drilling technology startup Quaise Inc. has started testing of its potentially disruptive drilling
technology initially developed at MIT.” (follow the link)• “New technology closer to reaching superhot geothermal energy sources.” (follow the link).
• “Progressing towards geothermal energy’s holy grail.” (follow the link)

Gyrotron Technology Inc. (GTI) Files Patent for Novel Method of Compacting Airbags
GTI, an industrial process technology company, has filed a patent application for a novel method of compacting
airbags. This new method utilizes the gyrotron, a very powerful source of high-frequency microwave energy.
The method allows direct heating of the cushion material without heating the mold, resulting in very significant
shortening of cycle time and substantial reduction of energy usage.
For more details, please visit the link.