Gyrotron FU CW Research Articles

Clinical Course of High-Frequency Millimeter-Wave (162\xa0GHz) Induced Ocular Injuries and Investigation of Damage Thresholds

The objective was to investigate the clinical course of ocular damage and the healing process in eyes exposed to 162 GHz millimeter wave (MMW). As a 162-GHz electromagnetic wave source, a gyrotron FU CW GV was used. An ocular damaged rabbit model with good reproducibility for 162 GHz MMW exposure was developed, and damage occurrence threshold values were obtained. The right eyes of pigmented rabbits were exposed to 162 GHz from a spot-focus-type lens antenna, and the non-exposed contralateral eyes were used as control eyes. Slit-lamp examination 1 day after MMW exposure revealed a round area of opacity, characterized by fluorescein staining indicating damaged epithelial cells in the central pupillary zone. Corneal edema, indicative of corneal stromal damage, peaked 2–3 days after exposure, with thickness gradually subsiding to normal by 9 days after exposure. Power densities of 162 GHz MMW causing ocular damage with probabilities of 10, 50, and 90% obtained by probit analysis were 173, 252, and 368 mW/cm2, respectively.

Development of Gyrotron FU CW GVII: a Second Harmonic, Multifrequency Gyrotron that Radiates Gaussian Beams

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.

Simultaneous Stabilization of Gyrotron Frequency and Power by PID Double Feedback Control on the Acceleration and Anode Voltages

In this paper, we present the results of simultaneous stabilization of both the frequency and the output power by a double PID feedback control on the acceleration and anode voltages in the 460-GHz gyrotron FU CW GVI, also known as “Gyrotron FU CW GO-1” (according to the nomenclature adopted at Osaka University). The approach used in the experiments is based on the modulation of the cyclotron frequency and the pitch factor (velocity ratio) of the electron beam by varying the acceleration and the anode voltages, respectively. In a long-term experiment, the frequency and power stabilities were made to be better than ±10−6 and ±1%, respectively.

Stabilization of Gyrotron Frequency by PID Feedback Control on the Acceleration Voltage

The results of frequency stabilization by proportional-integral-derivative (PID) feedback control of acceleration voltage in the 460-GHz Gyrotron FU CW GVI (the official name in Osaka University is Gyrotron FU CW GOI) are presented. The experiment was organized on the basis of the frequency modulation by modulation of acceleration voltage of beam electrons. The frequency stabilization during 10 h experiment was better than 10−6, which is compared with the results of the frequency deviation in free-running gyrotron operation.

High-Speed Frequency Modulation of a 460-GHz Gyrotron for Enhancement of 700-MHz DNP-NMR Spectroscopy

The high-speed frequency modulation of a 460-GHz Gyrotron FU CW GVI (the official name in Osaka University is Gyrotron FU CW GOI) was achieved by modulation of acceleration voltage of beam electrons. The modulation speed f m can be increased up to 10 kHz without decreasing the modulation amplitude δf of frequency. The amplitude δf was increased almost linearly with the modulation amplitude of acceleration voltage ΔV a. At the ΔV a = 1 kV, frequency spectrum width df was 50 MHz in the case of f m < 10 kHz. The frequency modulation was observed as both the variation of the IF frequency in the heterodyne detection system measured by a high-speed oscilloscope and the widths of frequency spectra df measured on a frequency spectrum analyzer. Both results well agree reasonably. When f m exceeds 10 kHz, the amplitude δf is decreased gradually with increasing f m because of the degradation of the used amplifier in response for high-speed modulation. The experiment was performed successfully for both a sinusoidal wave and triangle wave modulations. We can use the high-speed frequency modulation for increasing the enhancement factor of the dynamic nuclear polarization (DNP)-enhanced nuclear magnetic resonance (NMR) spectroscopy, which is one of effective and attractive methods for the high-frequency DNP-NMR spectroscopy, for example, at 700 MHz. Because the sensitivity of NMR is inversely proportional to the frequency, high-speed frequency modulation can compensate the decreasing the enhancement factor in the high-frequency DNP-NMR spectroscopy and keep the factor at high value. In addition, the high-speed frequency modulation is useful for frequency stabilization by a PID control of an acceleration voltage by feeding back of the fluctuation of frequency. The frequency stabilization in long time is also useful for application of a DNP-NMR spectroscopy to the analysis of complicated protein molecules.

Development of the Multifrequency Gyrotron FU CW GV with Gaussian Beam Output

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.

The Development of 460 GHz gyrotrons for 700 MHz DNP-NMR spectroscopy

Two demountable gyrotrons with internal mode converters were developded as sub-THz radiation sources for 700 MHz DNP (Dynamic Nuclear Polarization) enhanced NMR spectroscopy. Experimental study on the DNP-NMR spectroscopy will be carried out in Osaka University, Institute for Protein Research, as a collaboration with FIR UF. Both gyrotrons operate near 460 GHz and the output CW power measured at the end of transmission system made by circular waveguides is typically 20 to 30 watts. One of them named Gyrotron FU CW GVI (we are using “Gyrotron FU CW GO-1” as an official name in Osaka University) is designed to have a special function of high speed frequency modulation δf within 100 MHz band. This will expand excitable band width of ESR and increase the number of electron spins contributing to DNP. The other gyrotron, Gyrotron FU CW GVIA (“Gyrotron FU CW GO-II”) has a function of frequency tunability Δf in the range of wider than 1.5 GHz, which is achieved in steady state by changing magnetic field intensity. This function should be used for adjusting the output frequency at the optimal value to achieve the highest enhancement factor of DNP.

Analysis of oscillation characteristics and optimal conditions for high power operation of Gyrotron FU CW GIII

The oscillation characteristics of Gyrotron FU CW GIII and its wave frequency and output power dependences on the magnetic field strength, the gun coil current, and the anode voltage were investigated experimentally. The experimental results were analyzed theoretically using a self-consistent code that included the electron properties in the cavity, corresponding to the actual operating conditions in the experiments. As a result, it was found that the variation in frequency with the magnetic field strength was related to an axial profile change in the electromagnetic wave in the cavity. In addition, the optimal condition that gives the maximum output power was found to be determined by the pitch factor rather than by the electron beam radius under the given operating conditions.

Characteristics of the mode converter of Gyrotron FU CW GII radiating Gaussian beams in both the fundamental and second harmonic frequency bands

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.

27aAX-7 Gyrotron FU CW GIAの発振特性評価(27aAX プラズマ科学・核融合プラズマ(ジャイロトロン・放電物理・イオン源),領域2(プラズマ))

27aAX-7 Gyrotron FU CW GIAの発振特性評価(27aAX プラズマ科学・核融合プラズマ(ジャイロトロン・放電物理・イオン源),領域2(プラズマ))

Development of second harmonic gyrotrons, Gyrotron FU CW GII and Gyrotron FU CW GIII, equipped with internal mode converters

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.

Structural and Microwave Properties of Silica Xerogel Glass-Ceramic Sintered by Sub-millimeter Wave Heating using a Gyrotron

In this paper, we present and discuss experimental results from a microwave sintering of silica glass-ceramics, produced from amorphous silica xerogel extracted from sago waste ash. As a radiation source for a microwave heating a sub-millimeter wave gyrotron (Gyrotron FU CW I) with an output frequency of 300 GHz has been used. The powders of the amorphous silica xerogel have been dry pressed and then sintered at temperatures ranging from 300 °C to 1200 °C. Microwave absorbing properties of the sintered samples were investigated by measuring the dielectric constant, the dielectric loss, and the reflection loss at different frequencies in the interval from 8.2 to 12.4 GHz. Furthermore, the characteristics of the formation process for producing silica glass-ceramics were studied using a Raman Spectroscopy and a Scanning Electron Microscopy (SEM). The results indicate that the samples sintered at 1200 °C are characterized by lower reflection losses and a better transparency due to the formation of a fully crystallized silica glass- ceramic at sufficiently high temperature.

Development of a Compact sub-THz Gyrotron FU CW CI for Application to High Power THz Technologies

For application of high frequency gyrotron to high power THz technology, Gyrotron FU CW series is being developed in FIR FU. Gyrotron FU CW CI is developed as one of sub-THz gyrotrons included in the series. The advantage of the gyrotron is compactness using a compact superconducting magnet and compact power supply system, which makes the accesses of the gyrotron to applied large-scale devices easier and extends the applications of gyrotron to wider fields. The designed frequency and cavity mode are 394.5 GHz and TE26 mode for application to the 600 MHz DNP-NMR spectroscopy. As the operation results, the frequency and the output power were 394.03 GHz and around 30 W, respectively, which are available for the application to the 600 MHz DNP-NMR measurement. In addition, this gyrotron can operate at many other frequencies and cavity modes for application to high power THz technologies in wide fields. In this paper, the design and the operation results including long pulse or CW mode are presented.

Development of a kW Level-200 GHz Gyrotron FU CW GI with an Internal Quasi-optical Mode Convertor

Development of gyrotrons with an internal mode convertor has started in Research Center for Development of Far-Infrared Region, University of Fukui (FIR FU). As the first gyrotron of such a kind, we have designed and manufactured Gyrotron FU CW GI. It operates at 203 GHz at fundamental cyclotron resonance. We have designed a cavity and a mode convertor under some constraints such as reuse of an electron gun and small diameter of a magnet bore. Designed output power is about 1 kW. We have succeeded in observation of a circular radiation pattern. The maximum observed output power is 0.5 kW for the setting cathode voltage of 20 kV and the beam current of 0.5 A. This success makes gyrotron development in FIR FU to proceed to a new stage.

Development of DNP-Enhanced High-Resolution Solid-State NMR System for the Characterization of the Surface Structure of Polymer Materials

A dynamic nuclear polarization (DNP)-enhanced cross-polarization/magic-angle spinning (DNP/CP/MAS) NMR system has been developed by combining a 200 MHz Chemagnetics CMX-200 spectrometer operating at 4.7 T with a high-power 131.5 GHz Gyrotron FU CW IV. The 30 W sub-THz wave generated in a long pulse TE \( _{{41}}^{{(1)}} \) mode with a frequency of 5 Hz was successfully transmitted to the modified Doty Scientific low-temperature CP/MAS probe through copper smooth-wall circular waveguides. Since serious RF noises on NMR signals by arcing in the electric circuit of the probe and undesired sample heating were induced by the continuous sub-THz wave pulse irradiation with higher powers, the on-off sub-THz wave pulse irradiation synchronized with the NMR detection was developed and the appropriate setting of the irradiation time and the cooling time corresponding to the non-irradiation time was found to be very effective for the suppression of the arcing and the sample heating. The attainable maximum DNP enhancement was more than 30 folds for C1 13 C-enriched D-glucose dissolved in the frozen medium containing mono-radical 4-amino-TEMPO. The first DNP/CP/MAS 13 C NMR spectra of poly(methyl methacrylate) (PMMA) sub-micron particles were obtained at the dispersed state in the same frozen medium, indicating that DNP-enhanced 1H spins effectively diffuse from the medium to the PMMA particles through their surface and are detected as high-resolution 13 C spectra in the surficial region to which the 1H spins reach. On the basis of these results, the possibility of the DNP/CP/MAS NMR characterization of the surface structure of nanomaterials including polymer materials was discussed.

Rapid Sintering of Silica Xerogel Ceramic Derived from Sago Waste Ash Using Sub-millimeter Wave Heating with a 300 GHz CW Gyrotron

In this paper, we present and discuss experimental results from a microwave sintering of a silica-glass ceramic, produced from a silica xerogel extracted from a sago waste ash. As a radiation source for the microwave heating a sub-millimeter wave gyrotron (Gyrotron FU CW I) with an output frequency of 300 GHz has been used. The powders of silica xerogel have been dry pressed and then sintered at temperatures ranging from 300°C to 1500°C. The influence of the sintering temperature on the technological properties such as porosity and bulk density was studied in detail. Furthermore, X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy have been used in order to study the structure of the produced silica glass-ceramics. It has been found that the silica xerogel crystallizes at a temperature of 800°C, which is about 200°C lower than the one observed in the conventional process. The silica xerogel samples sintered by their irradiation with a sub-millimeter wave at 900°C for 18 minutes are fully crystallized into a silica glass-ceramic with a density of about 2.2 g/cm3 and cristobalite as a major crystalline phase. The results obtained in this study allow one to conclude that the microwave sintering with sub-millimeter waves is an appropriate technological process for production of silica glass-ceramics from a silica xerogel and is characterized with such advantages as shorter times of the thermal cycle, lower sintering temperatures and higher quality of the final product.

Development of High Frequency Gyrotrons and Application to High Power Thz Technologies

Development of high frequency cw gyrotrons (Gyrotron FU CW Series) as high power THz radiation sources was advanced in FIR FU on the basis of the success in the development and application of the previous Gyrotron FU series [1]. In this paper, we would introduce the development of high frequency gyrotrons in FIR FU and applications to high power THz technologies

Frequency Tunable Gyrotron FU CW VA for Measuring Hyperfine Split of Positronium

New uptapered cavity is designed for the gyrotron FU CW VA enhancing its tunability and enabling its use for measurements of the hyperfine split of positronium.

Design of a Compact Sub-Terahertz Gyrotron for Spectroscopic Applications

In this paper we present the initial design of a novel and versatile high frequency gyrotron with parameters suitable for application to various spectroscopic studies that require coherent radiation in the subterahertz frequency range (such as NMR/DNP spectroscopy, ESR spectroscopy, spectrometer based on the X-ray detected magnetic resonance etc.). The most characteristic feature of the design is that it utilises a compact, cryogen-free 8 T superconducting magnet. As a result, the overall dimensions of the entire device are considerably reduced in comparison with the previously developed tubes belonging to the Gyrotron FU and Gyrotron FU CW series. This makes the novel gyrotron highly portable to diverse laboratory environments and easily embeddable to different measuring systems. The electron-optical system (EOS) of the tube is based on a compact low-voltage magnetron injection gun (MIG), which has been specially designed and optimized together with the resonant cavity using our problem-oriented software package GYROSIM for CAD of gyrotrons. The tube operates at the second harmonic of the cyclotron frequency and generates a radiation with an output power of about 100 W and a frequency tunable up to around 424 GHz, respectively.

Continuously Frequency Tunable High Power Sub-THz Radiation Source—Gyrotron FU CW VI for 600 MHz DNP-NMR Spectroscopy

A high frequency gyrotron with a 15 T superconducting magnet named Gyrotron FU CW VI has achieved continuous frequency tuning through the relatively wide range of 1.5 GHz near 400 GHz. The operation is at the fundamental cyclotron resonance of the TE06 cavity mode with many higher order axial modes. The output power measured at the end of the circular waveguide system ranges from 10 to 50 watts at the low acceleration voltage of 12 kV for beam electrons. The beam current is also low. It is around 250 mA. This gyrotron is designed as a demountable radiation source for the 600 MHz DNP-NMR spectroscopy. The design and operation results of the gyrotron FU CW VI are presented.