Search

In this paper, we present and discuss the new experimental results that characterize the output parameters, and the overall performance of the recently developed 0.8 THz double-beam gyrotron operating at the second harmonic of the cyclotron frequency on the TE8,5 mode. Alongside with this design mode, several other resonances excited at both the fundamental and second harmonic operations in a wide frequency range have been studied. These experiments demonstrate that the usage of a double-beam electron-optical system opens more possibilities for realization of such regimes of operation that otherwise are not accessible in a conventional (single-beam) gyrotron due to a severe competition between the neighboring modes and the narrowness of the zones of a stable single mode operation. Most notably, the results prove that second harmonic operation at higher beam currents and respectively at higher levels of the output power are possible as expected from the preceding theoretical considerations and numerical studies. The results of this study are helpful for getting a better insight in the physics of the operation of the double-beam gyrotrons and suggest some possibilities for further optimization of both the design of the tube and the experimental setup.

Two resonators with different radii were separately excited in a complex cavity to demonstrate super multi-frequency operations of a gyrotron. A demountable gyrotron was constructed and driven by 15-kV, 0.4-A annular electron beams. The magnetic field strength was varied from 4 to 8 T to excite a series of transverse electric modes in each resonator. In the frequency range of 120 – 220 GHz, 13- and 9-mode oscillations at the fundamental harmonics were identified in the first and second resonators, respectively. Furthermore, backward wave oscillations of these modes allowed continuous frequency variation. Even if two resonators oscillate at the same time with different frequencies, most of them can be independently excited by regulating the velocity pitch and/or injection radius of the electron beam. As a result, 27% of frequencies from 120 to 220 GHz became available with a single gyrotron. This is the largest frequency occupation reported so far in this frequency range. The frequency gaps between neighboring modes were less than 10 GHz. The maximum output power obtained for each mode was more than 28 W.

Equi-distantly distributed multi-peak frequency spectrum was observed in a high-power 300-GHz band gyrotron for the case of strong reflection off the vacuum window. An oscillation mode different from the design mode can oscillate by adjusting the magnetic field in the cavity. However, a large fraction of the output wave is reflected by the vacuum window. The reflected wave returns to the cavity through an internal mode converter. The azimuthal rotating direction of the reflected wave is opposite to that of the oscillating mode. A theoretical calculation of mode competition between the oscillating co-rotating mode and the counter-rotating mode originating from the reflected wave very well reproduces the observed frequency spectrum. The multi-peak frequency spectrum stems from periodic amplitude oscillation caused by the mode competition between the co-rotating mode and the counter-rotating mode. The frequency interval between the peaks is not simply decided by the round-trip time between the cavity and the vacuum window but determined by the period of the amplitude oscillation.

A sub-THz, second harmonic, multifrequency gyrotron, Gyrotron FU CW GVII, which radiates Gaussian beams, was developed. This device was designed to change the frequency in a stepwise manner in the range from 270 to 420 GHz. In this paper, we present the first experiments conducted using this gyrotron. Nine modes were selected for this study. The oscillation frequency of the selected modes was measured using a heterodyne system. The profiles of the Gaussian beams converted from the modes were assessed using an infrared camera, and the power of these beams was measured using a power meter. The oscillations of the nine second harmonic modes were successfully measured, and Gaussian beams converted from seven oscillation modes were observed. Four second harmonic modes, TE8,5, TE6,5, TE5,4, and TE2,6, oscillated in a single mode. The power of the beams ranged from 0.2 to 77 W. Even in the case of spurious oscillation of fundamental modes, the beams converted from the second harmonic modes could be separated from the fundamental mode beams by taking advantage of the different radiation directions emitted through the gyrotron window.

For the double-beam THz gyrotron, an efficient operation under nonrelativistic acceleration voltages is experimentally demonstrated. The output power at the frequency of 0.4 THz was 60 W for an operating voltage of 5.4 kV, and the starting voltage was as low as 2.4 kV; that is in good agreement with calculations. The starting voltage can be further decreased, and the output power can be increased by increasing the beam pitch factor at low voltages. Simulations of three-electrode magnetron injection gun (MIG) show that it can be achieved by the optimized MIG geometry and by applying the positive voltage at the modulating anode.

Gas breakdown in the millimetre-wave frequency band is an interesting phenomenon in nonlinear dynamics such as self-organized structure formation. We observed the transition between two types of filamentary plasma arrays in air discharge driven by a 303-GHz millimetre wave. Plasma is ignited at a parabolic mirror’s focal point in the overcritical condition. One array parallel to the electric field vector appears with a spacing of λ/4 at the focal point. Filaments then separate into plasma lumps ~10 μs after ignition. At 20 μs, a new comb-shaped array grows in the subcritical condition. Filaments are parallel to the incident beam with spacing of 0.96 λ and elongate towards the incident beam. This comb-shaped array appears only in the electric field plane; bulk plasma with a sharp vertex forms in the magnetic field plane. This array is created by a standing wave structure generated by waves diffracted from the plasma surface. Filamentary plasma array formations can influence the energy absorption by the plasma, which is important for engineering applications such as beamed energy propulsion.

In the experimental study on the frequency tuning of a double-beam gyrotron, an unexpected abrupt increase of the output power was observed for some magnetic field values corresponding to the excitation of high-order axial modes (HOAM). This effect can be explained through an after-cavity interaction of the spent but bunched electron beam with the neighboring transverse mode which has the same azimuthal but the next (i.e., greater by one) radial index with respect to the operating mode. The calculations show that under certain conditions the cyclotron synchronism between the electron beam and the next radial mode occurs in the extended region of an output cone, while the bunching of the spent electron beam is responsible for the excitation of this mode. The influence of some additional factors such as reflections of both modes from the output window, electron velocity spread, and magnetic field profile was studied numerically. It was shown that reflections, mode transformation, and velocity spread can enhance the effect of an abrupt power increase and reduce the starting current of the operating mode. The discovered after-cavity interaction can be either useful for a power increase at the HOAM operation of the frequency-tunable gyrotron or harmful for high cyclotron harmonic operation due to an expansion of the zone of fundamental cyclotron resonance mode excitation.

A collective Thomson scattering (CTS) diagnostic with a ±3 GHz band around a 77 GHz gyrotron probe beam was developed to measure the velocity distribution of bulk and fast ions in high-temperature plasmas. We propose a new in situ calibration method for a CTS diagnostic system combined with a raytracing code. The method is applied in two situations for electron cyclotron emission in plasmas and in a CTS diagnostic with a modulated probe beam. Experimental results highlight the importance of refraction correction in probe and receive beams. The CTS spectrum is measured with the in situ calibrated CTS receiver and responds to fast ions originating from a tangential neutral beam with an energy of 170 keV and from a perpendicular beam with an energy of 60 keV, both in the large helical device. From a velocity space analysis model, the results elucidate the measured anisotropic CTS spectrum caused by fast ions. The calibration methods and analyses demonstrated here are essential for CTS, millimeter-wave diagnostics, and electron cyclotron heating required under fusion reactor conditions.

Two types of back scattering systems are utilized for high temperature fusion plasma turbulence study. W-band backscattering system is used for measuring the electron Larmor radius scale turbulence and around Q-band frequency comb Doppler backscattering system have been designed for measuring the ion Larmor radius scale turbulence.

The frequency step tuning with second harmonic oscillations in a frequency range from 240 to 360-GHz has been demonstrated. A two-cavity configuration was introduced in order to increase the number of operating frequencies and to decrease their frequency intervals. Each cavity has a series of oscillation modes. These modes can be selectively excited according to the changes in the magnetic field strength, electron beam radii and the velocity pitch factor in the cavity. The experimental results suggest further multi-frequency operation is possible with multiply connected cavities.

A complex cavity was fabricated to improve the mode selection property of a gyrotron for the 400 GHz second-harmonic oscillation. A combination of the TE 8,3 and the TE 8,5 modes was selected to provide resonant coupling. In the experiment, many fundamental and several second-harmonic modes oscillated independently in either the first or the second cavity. However, no fundamental oscillation was observed in the operation magnetic field region of the TE 8,3 /TE 8,5 mode. The fabrication error of the cavity radii was successfully controlled to be smaller than 1 $\mu\mathrm{m}$ . The oscillation of the design mode was observed by the frequency measurements. However, a mode competition with the TE3,5 mode occurred.

In this study, an application of the millimeter-wave discharge to the wireless power transfer is proposed. The ionization front of the millimeter-wave discharge propagates backward to the incident beam. The electromotive force is generated by the ionization front propagation with a magnetic field applied perpendicularly to the propagation direction. The incident beam energy is converted into DC electric energy through the discharge without any rectifier circuit. Thus, the wireless power transfer system can be easily adapted to the high frequency, such as the millimeter-wave band. The energy conversion efficiency is estimated. A millimeter-wave discharge chamber is under development to verify the estimation.

Spectroscopic applications have stimulated strong interest in frequency tunable gyrotrons. One of the possibilities to tune gyrotron frequency is known as frequency pulling, which is based on the effect of the electron beam on the oscillation frequency. Recently, the linear theory of this effect was developed, and the results were obtained for a sequence of modes with different axial indices. This paper is intended to analyze saturation effects in such gyrotrons. The nonlinear theory describing the frequency pulling and the efficiency in gyrotrons is developed in the cold-cavity approximation. The results are obtained for the modes with different axial indices (from 1 to 4). The frequency shift and the efficiency are calculated in a wide range of the dimensionless parameters characterizing the beam current and the external magnetic field. These results related to the frequency pulling are compared with those of the linear theory. Also, comparison is made with the results of the self-consistent theory and experiments.

Complex-cavity resonator with mode conversion is a promising way of improving mode selection properties of a terahertz gyrotron. In this paper, enhancement of the frequency tunability of the second-harmonic 0.4-THz gyrotron by using the complex-cavity resonator is considered. Results of detailed numerical study of the cold-cavity electromagnetic parameters are presented. Using the results of cold-cavity simulations, start-oscillation currents for different modes are calculated.

We studied the transmission characteristics of hybrid modes in a corrugated circular waveguide above the Bragg frequency to develop a broad-band transmission line for millimeter waves. Millimeter waves at 294 GHz were transmitted into a straight waveguide. From observed power profiles in waveguide cross-sections, a high attenuation rate of 0.13 dB/m was obtained. To match a theoretical attenuation constant with the experimental one, we introduced an ad hoc coefficient of conventional surface reactance in the waveguide wall. This was necessary because the wall began to look like the surface with a decreasing anisotropic reactance owing to the frequency above the Bragg frequency. Using nonlinear optimization for mode content analysis, the observed power profiles in the waveguide cross-section were matched with theoretical profiles. There was good agreement between the calculated and observed centers of power profiles and attenuation rate along the waveguide. The theoretical analysis showed that the magnetic field at the waveguide wall increases and the substantial attenuation takes place. Above the Bragg frequency coupling to backwards propagating modes is a point of consideration. A combination of the backwards propagating EH1,26 and the forward propagating HE11 modes satisfied the Bragg condition at 294.7 GHz which was the nearest frequency of operating frequency. A strong attenuation of the incoming HE11 mode by Bragg resonance was not expected due to large difference of 0.7 GHz. It becomes clear that the observed high transmission loss outside of the Bragg resonance can be explained by a decrease in anisotropic surface reactance at the wall.

A subterahertz 303-GHz rectenna was developed and then assessed using a 300-kW-class gyrotron. Measurement of the antenna gain was conducted using finline, which is a waveguide-to-microstrip line (MSL) transducer: gain of 8.32 dBi was obtained. The input power was estimated by the application of the beam profile measured using an IR camera and the beam output power. Results show that the maximum dc output power and rectenna power density (dc output power per unit area) were 17.1 mW and 3.43 kW/ $\text{m}^{2}$ , respectively. The diode breakpoint was observed. The RF-dc conversion efficiency was obtained as 2.17% with 342-mW input power and 130- $\Omega $ dc load. This letter is the cutting edge of high-frequency operation in a microwave MSL-type rectenna.

Second-harmonic 0.4-THz gyrotron with complex-cavity resonator with TE 8,3 -TE 8,5 mode conversion is studied. Cold-cavity electromagnetic parameters are simulated and start-oscillation current is calculated. The numerical results are compared with the experimental measurements.

In this paper, we present and discuss the latest experimental results from the investigation of the operational performance of a 0.8 THz double-beam gyrotron, which has been developed and manufactured recently in the framework of an international research project led by IAP-RAS and FIR UF. After the initial tests, the current experiments have been focused not only on the study of the operation at the design mode TE 8,5 (at the second harmonic of the cyclotron frequency) but also of several other modes the generated radiation of which could be appropriate for various other applications besides the envisaged 1.2 GHz DNP-NMR spectroscopy. The obtained results give a deeper insight into the specifics of the operation of the double-beam gyrotron and especially on its capability to provide an improved mode selection and therefore a single mode second harmonic operation.

A sub-THz second harmonic multi-frequency gyrotron with Gaussian beam output, Gyrotron FU CW GVII, has been developed. This gyrotron was designed to change the frequency stepwise in the range from 270 to 420 GHz. Oscillations of seven second-harmonic modes were successfully observed and Gaussian beam radiations converted from six oscillation modes were observed. Two second harmonic modes; TE 6,5 and TE 8,5 ; oscillated in a single mode. Even in case of simultaneous oscillation of fundamental modes, radiation beams converted from second harmonic modes can be separated from the fundamental mode beams by making use of the different radiation directions emitted from the gyrotron window.

Equi-distantly distributed multi-peak frequency spectrum was observed in a high power 300 GHz band gyrotron for the case of strong reflection off the vacuum window. An oscillation mode different from the design mode can oscillate by adjusting the magnetic field in the cavity. However, a large fraction of the output wave is reflected by the vacuum window. A theoretical calculation shows that the multi-peak frequency spectrum stems from periodic amplitude oscillation that is caused by mode competition between the oscillating co-rotating mode and the counter-rotating mode originating from the reflected wave.

We developed GaN schottky barrier diode (SBD) for sub-terahertz rectenna to improve the RF-DC conversion efficiency and DC output power. Designed a GaN SBD using the device simulator and fabricated prototype. The similar 1-V curve was obtained between measurement and simulation. Furthermore, calculated the theoretical RF-DC conversion efficiency on the rectenna. As a result, diodes with series resistance 3.6 Ω, junction capacitance 0.035 pF and breakdown voltage 46.3 V were calculated. The RF-DC conversion efficiency of 78% was theoretically obtained at 303 GHz. This work showed RF-DC conversion efficiency can be greatly improved using GaN SBD for rectenna in sub-terahertz (herein 100 GHz ~ 700 GHz).

A 265 GHz rectenna was developed by microfabrication on Si wafer and then assessed using a 265 GHz gyrotron. Coplanar waveguide (CPW) has smaller transmission loss than Microstrip line (MSL) in the higher frequency band and high precision circuit patterns can be fabricated on the same plane by microfabrication. A coplanar slot antenna was designed and a gain of 6.14 dBi was obtained by simulation. The input power was estimated by application of the beam profile measured using an IR camera and the beam output power. As a result, the maximum DC output power and rectenna power density (DC output power per unit area) were 0.5 mW, 0.09 kW/m2 with 266 mV output voltage and $150 \Omega$ DC load. The RF-DC conversion efficiency was obtained as 0.07 % with 644 mW input power. This work is the cutting edge of high-frequency operation in a microwave CPW type rectenna.

A 90 GHz W-band millimeter-wave back-scattering system is designed and installed for measuring electron scale turbulence (k⊥ρs ∼ 40). A metal lens relay antenna is used for in-vessel beam focusing, and a beam diameter of less than 40 mm is achieved in the plasma core region. This antenna can be steered at an angle of 159° ± 6°, which almost covers the plasma radius. The estimated size of the scattering volume is ∼105 mm at the edge and 135 mm at the core, respectively. A 60 m corrugated waveguide is used to achieve a low transmission loss of ∼8 dB. A heterodyne detection system for millimeter-wave circuits with probing power modulation can distinguish the scattered signal from background noise.

Collective Thomson scattering (CTS) is one of attractive diagnostics for measuring locally and directly the fuel temperature and the velocity distribution of fast ions in fusion plasmas. A mega-watt class source of millimeter or sub-millimeter waves is required to detect a weak scattered radiation superimposed on background radiation owing to electron cyclotron emissions (ECEs) from plasmas. Based on electron cyclotron resonance heating (ECRH) system with the frequencies of 77 GHz and 154 GHz in the Large Helical Device (LHD), the CTS diagnostic system has been developed to measure bulk ion temperatures from a few keV to ~10 keV and fast ions originated from 180 keV-neutral beam injection in the LHD. The measured CTS spectra and their time evolutions are analyzed with the electrostatic scattering theory. The bulk ion temperatures obtained from CTS spectra increase with the neutral beam injections and decrease with the heating terminated. The velocity map of simulated fast ions explains that the bumps on tail of measured CTS spectra are caused by the co- and counter- fast ions. A new prescription for anisotropic velocity distribution function is proposed. As for 154 GHz bands, the CTS spectrum broadenings for D and H plasmas are distinguished reasonably at the same temperature, and its ion temperatures are comparable to those of the charge exchange recombination spectroscopy. As reactor-relevant diagnostics, a 300 GHz gyrotron and a corresponding receiver system have been implemented in LHD to access high density plasmas with low background ECEs. The recent progress for CTS diagnostics and their spectrum analysis with the probe frequencies of 77 GHz, 154 GHz, and 300 GHz in the LHD experiments is described.

Sub-Terahertz 303 GHz rectennas of Microstrip line (MSL) and Coplanar waveguide (CPW) was developed and measured using a 300 kW-class Gyrotron. Measurement of Microstrip patch antenna was carried out using finline which is MSL-to-waveguide transducer and gain of 8.32 dBi was obtained. CPW rectenna was developed by the photolithography process. As a result, the maximum output power and rectenna power density (output power per unit area) for MSL rectenna and CPW rectenna were 17.1 mW, 3.43 kW/m2, and 14.5 mW, 4.87 kW/m2, respectively. The RF-DC conversion efficiency was obtained 2.17 % with 342 mW input power and 130 Ω DC load [1].

A high-power gyrotron is under development to investigate the thrust generation by the incident microwave beam for the beamed energy propulsion rocket. The beam frequency is set at 94 GHz which is the window frequency of air. The target output power is 600 kW at maximum. A mode converter is installed to deliver a Gaussian beam. The electron gun of the gyrotron is a diode type. The electrodes were designed by using the EGUN code. The power supply is composed of a capacitor bank and a high voltage switching devise. This power supply can charge the capacitor bank at 80 kV at maximum. This study is the first gyrotron development in the field of the aerospace.

In this study, we present observations of the discharge structure in air breakdown using a high power millimeter wave beam at 303 GHz. The discharge is ignited by focusing the incident beam using a parabolic mirror. At the focal point, the discharge is under the overcritical condition. Plasma filaments extend along the electric filed vector of the incident beam. The pitch size of each filaments is one-quarter of the wavelength. After the ignition, the condition turns to be subcritical and the filaments begin to separate into granular plasmas. Filaments then extend in parallel to the incident beam in the E-k plane while in the H-k plane, a diffusive structure was formed. Dynamic evolutions of the complicated discharge structure were observed in both electric and magnetic field planes of the incident beam in high resolution and a clear difference between the structures in E-k field and H-k field planes was observed for the first time.

A sub-THz second harmonic multi-frequency gyrotron with Gaussian beam output is under development. This gyrotron can change the frequency stepwise in the range from 270 to 420 GHz. For multi-frequency Gaussian-beam output, all design modes should be converted to Gaussian beams with one mode converter. To achieve this, nine oscillation modes with close values of the transverse propagation angle were selected. Oscillation test of the selected modes was carried out with a linear type gyrotron equipped with the designed cavity. Oscillations of seven second-harmonic modes were successfully observed among the nine selected modes. Operation condition for each mode was experimentally investigated. An internal mode converter to radiate Gaussian beam from each mode was designed.

For measuring a turbulence in a high-temperature nuclear fusion plasma, two backscattering systems have been developed in the Large Helical Device (LHD). The Ka-band and U-band Doppler backscattering system have been designed for measuring the ion Larmor radius scale turbulence. Because of the turbulence profile measurements, a microwave frequency comb is used as a source. The W -band backscattering system is designed for measuring the electron Larmor radius scale turbulence.

A collective scattering method using the sub-THz gyrotron radiation is planned to apply to the direct detection of the electron Bernstein wave in the QUEST where the electron Bernstein wave plays one of the main heating/current drive roles in sustaining steady state spherical tokamak configuration.

A two-cavity configuration was introduced in order to increase the number of operating frequencies and to decrease their frequency intervals in a multi-frequency gyrotron. Each cavity has a series of oscillation modes, which can be chosen with the cyclotron frequencies of injected electron beams. The experimental observations have shown that the different oscillation modes for each cavity can be selectively excited according to the changes in the magnetic field strength in the cavity. The frequency step tuning with fundamental harmonic oscillations in a frequency range from 180 to 220-GHz has been demonstrated. The obtained results suggest a further multiple-frequency operation is possible with multiply connected cavities.

Equipment for sub- THz collective Thomson scattering diagnostics in the Large Helical Device has been developed. The probe beam source is a 303 GHz Gyrotron. It operates in a pulse mode. No parasitic mode is excited during the whole pulse width including the turn-on and turn-off phases. Single mode oscillation at several frequencies has been confirmed. For power transmission, a 3.5 inch corrugated waveguide line installed for electron heating with lower frequency gyrotrons will be used. The test with the 303 GHz gyrotron has shown a sufficiently low transmission loss.

Usually the gyrotron is associated with a very high power radiation source, for example, intended for heating plasma in thermonuclear projects. There are a number of promising applications of THz and sub-THz gyrotrons, such as nonlinear microwave spectroscopy, diagnostics of various media, remote detection of radioactive materials etc., for which extremely high power is not required, but the fast and flexible control of output parameters (frequency and power) is very important. The report provides an overview of the latest results obtained in IAP RAS related to control of gyrotrons output parameters.

Sub-Terahertz 303 GHz rectenna was developed by the photolithography process and measured using a 300 kW-class Gyrotron. As a result, the maximum output power and rectenna power density for the microstrip line (MSL) and coplanar waveguide (CPW) were 17.1 mW, 3.43 kW/m2, and 14.5 mW, 4.87 kW/m2, respectively. The RF-DC conversion efficiency was obtained 2.17 % with 342 mW input power and 130 $\Omega$ DC load. The results reveal the highest operation frequency except the photovoltaic converter.

Gyrotron FU CW GV has been developed as a multifrequency gyrotron for operation over the frequency range from 162 to 265 GHz at frequencies separated by steps of approximately 10 GHz. The oscillation modes were selected; the radii of the caustic surfaces for the electromagnetic waves of the modes had similar values in the waveguide, and it was therefore expected that these modes would be converted into Gaussian beams by a mode converter. In reality, more than ten modes oscillated and the Gaussian-like beams were radiated. A double-disk window with variable spacing maintains the transmittance through the window at a high level over a wide range of frequencies. Using this window, output powers of more than 1 kW were observed for almost all the expected modes.

In the present study, double sintered nanocrystalline silica xerogel (NSX) bulk material has been produced from amorphous silica xerogel (ASX) powders extracted from sago waste ash. The ASX powders have been sintered at 1200°C, milled, pressed and then re-sintered at temperatures ranging from 1200° to 1600°C. The crystallization process of the NSX has been examined by thermal analysis (DSC/TGA) and X-ray diffraction (XRD), while the functional groups and the structure have been identified by infrared and Raman spectroscopy, respectively. From the DSC analysis it has been found that the tetragonal phase of α-cristobalite transforms to the stable phase of SiO2, namely cubic β-cristobalite in the samples re-sintered at 1600°C. This observation has been corroborated by an analysis of the XRD patterns, infrared and Raman spectra. The obtained data suggest that such novel material could be considered as an appropriate alternative for deposition of ceramic coatings on various machine and tool parts, e.g. compone...

A second-harmonic frequency step-tuning gyrotron is under development. The oscillation frequencies are separated by approximately 15-GHz steps in a range from 270 to 420-GHz. The operation modes were selected to convert into Gaussian like radiations by one internal mode converter. A triode electron gun was designed in order to operate in a wide range of magnetic fields. The oscillation tests were initiated with a demountable-type axial output tube with no mode-converter. All modes were successfully excited with the designed frequencies.

Frequency tunability in multi frequency bands was experimentally demonstrated in Gyrotron FU CW XA. Frequency tunable modes were searched for with measurement of the radiation pattern outside the vacuum window of the gyrotron with no mode converter. The frequency variation is due to the gyro-BWO mechanism and the same radiation pattern is kept within a finite range of the magnetic field for each oscillation mode. Five transverse modes showing the frequency variation more than 1 GHz were obtained.

A 300 GHz pulse gyrotron has been developed for collective Thomson scattering diagnostics in the Large Helical Device with a careful design consideration. Single-mode oscillation of more than 320 kW was attained. The frequency spectrum was very narrow and stable across the pulse width up to 80 μs. No parasitic mode was observed during the whole pulse width including the turn-on and turn-off phases. No-parasitic-mode oscillation is likely due to a very fast rise time of the beam voltage as well as due to adoption of a whispering gallery mode.

Gyrotron FU CW GII, the second gyrotron of the Gyrotron FU CW G-series with an internal quasi-optical mode converter, was recently developed at the Research Center for Development of Far-Infrared Region, University of Fukui (FIR FU) [Y. Tatematsu et al., J. Infrared, Millimeter, Terahertz Waves 35, 169 (2014)]. The design oscillation mode of this gyrotron is TE8,3 with a corresponding frequency of 393.4 GHz in the second harmonic cyclotron resonance. We observed oscillations of different modes including fundamental harmonic modes in the 200-GHz band upon varying the strength of the magnetic field in the cavity. Radiation patterns corresponding to these modes emitted from the window were Gaussian although the internal mode converter was designed for the TE8,3 mode. The directions of the radiated Gaussian beams were found to depend on the oscillation modes with a relation similar to that obtained for another gyrotron that was also equipped with a mode converter. This relation is explained based on geometrical optics. Moreover, numerical calculation with the electromagnetic wave code developed in FIR FU confirmed the same relation. The observed features of the Gyrotron FU CW GII show that it can serve as a unique power source with two frequency bands at 200 and 400 GHz. This result provides a way of developing multiple-frequency gyrotrons.

Second harmonic gyrotrons, Gyrotron FU CW GII and Gyrotron FU CW GIII, were developed at the Research Center for Development of Far-Infrared Region, University of Fukui, Japan to achieve two goals. Each gyrotron was equipped with an internal quasi-optical mode converter. Using Gyrotron FU CW GII allowed the design of the cavity and mode converter to be validated, which was the first goal. After that, Gyrotron FU CW GIII, which is an improved version of Gyrotron FU CW GII, allowed us to achieve a high power output of up to 420 W, which was the second goal, with a cathode voltage setting of −21 kV and a beam current of 0.57 A. This was achieved using a newly developed electron gun and with the careful sitting of the gyrotron on the magnet.

A novel type of ceramic material was produced by mixing sago waste ash from the sago processing industry in Indonesia with clay. The composition was prepared by adding 50 %wt amount of sago waste into the clay, then a series of samples was milled for 6 h, 12 h, 24 h and 48 h, respectively. The samples were dry pressed and sintered at temperatures ranging from 800?C to 1200?C. The influence of the sintering temperature and the milling time on bulk density, firing shrinkage, water adsorption, and hardness was studied in detail. The results demonstrate that the low water absorption of less than 0.5% and the highest hardness of 5.82 GPa were obtained for the sample sintered at 1100?C and milled for 48 h. The investigation of the absorptive properties of such ceramics indicates that they could be recommended as a promising material for manufacturing of unglazed floor tiles.

A high power sub-THz pulsed gyrotron with a frequency of 303 GHz has been developed for use as the power source of the collective Thomson scattering (CTS) diagnostics in the Large Helical Device (LHD) at the National Institute for Fusion Science. Based on careful design considerations, a whispering gallery mode TE22,2 was adopted to avoid mode competition. More than 320 kW was attained with single-mode oscillation. The spectrum of the oscillation frequency is very narrow and stable during the entire pulse duration for pulse widths up to 100 μs. No parasitic mode is excited at the turn-on and turn-off phases nor during the steady state of the oscillation pulse. In addition to the adoption of the whispering gallery mode, a fast but finite rise time of the beam voltage is an important role for the realization of single-mode oscillation. Self-consistent calculations show that, for the finite voltage rise time corresponding to the present experiment, gradual evolution to the high power regime is attained from backward wave oscillation. This high power regime corresponds to the hard self-excitation region in the case of instantaneous voltage rise.

Plan of collective Thomson scattering (CTS) experiment for the Large Helical Device (LHD) in NIFS with a 303 GHz gyrotron is under way. Use of a sub-THz gyrotron expands the CTS-applicable region of plasma parameters. In LHD, sub-THz CTS can be applied to the high density operation region, plasmas with impurity hole, etc. Moreover, sub-THz CTS is expected to be free from ECE noise. Its “collective” use with 77 GHz and 154 GHz CTS will compose a powerful diagnostic system. A high power sub-THz gyrotron with a frequency of 303 GHz has been developed. Its maximum power is 320 kW. It oscillates in pulse mode and the maximum pulse width is around 100 μs, which is sufficient for use in CTS experiments. A whispering gallery mode TE22,2was adopted for this gyrotron to avoid mode competition. Careful frequency measurement has proved purely single mode oscillation of the TE22,2mode including turn-on and turn-off phases of the oscillation pulse. This is consistent with mode competition calculations taking account of a finite voltage rise time. A low loss transmission line is necessary for CTS. We have two possibilities. One is a new line with 1.25 inch corrugated waveguides that are optimized for the 300 GHz band. Transmission test with the 303 GHz gyrotron has been carried out and a sufficiently low loss coefficient has been confirmed. The other is to use an existing line with 3.5 inch corrugated waveguides for lower frequencies such as 77 GHz and 154 GHz. Transmission test has been carried out with the 303 GHz gyrotron and a sufficiently low loss coefficient has been confirmed also for 3.5 inch corrugated waveguides. An existing line with 3.5 inch corrugated waveguides will be used in the initial phase of 303 GHz CTS experiment.