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.     

Development of 394.6 GHz CW Gyrotron (Gyrotron FU CW II) for DNP/Proton-NMR at 600 MHz

Gyrotron FU CW II with an 8 T liquid He free superconducting magnet, the second gyrotron of the THz Gyrotron FU CW Series, has been constructed and the operation test was successfully carried out. It will be used for enhancing the sensitivity of 600 MHz proton-NMR by use of Dynamic Nuclear Polarization (DNP). The designed operation mode of the gyrotron is TE_2,6 at the second harmonic. The corresponding frequency is 394.6 GHz. The real operation frequency is 394.3 GHz at TE₀₆ mode, because of fabrication error of the diameter of the cavity. The operation is in complete CW at the output power of around 30 W or higher at the TE₀₆ cavity mode. There are many other operation modes at the fundamental and the second harmonic. Typical output power of the fundamental and the second harmonic are higher than 100 W and 20 W, respectively. The highest frequency observed up to the present is 443.5 GHz at the second harmonic operation of TE_6,5 mode. The measured results are compared with the theoretical consideration.     

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.     

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.     

THE FIRST EXPERIMENT OF A THz GYROTRON WITH A PULSE MAGNET

A THz gyrotron with a pulse magnet has been designed, constructed and operated in FIR FU. It is developed as one of high frequency gyrotrons included in Gyrotron FU Series. The gyrotron has already achieved the first experimental result for high frequency operations whose radiation frequency exceeds 1 THz. In this paper, the design detail and the operation test results for sub-terahertz to terahertz range are described. The second harmonic operation is confirmed experimentally at the expected frequency of 1.005 THz due to TE_6,11 cavity mode at the magnetic field intensity of 19.0 T.     

Gyrotron FU CW VII for 300 MHz and 600 MHz DNP-NMR Spectroscopy

Gyrotron FU CW VII, one of the FU CW Series Gyrotrons, has been designed, constructed and completed operational tests successfully in the Research Center for Development of Far Infrared Region, University of Fukui (FIR FU). The gyrotron operates at around 200 GHz for the fundamental cyclotron resonances and at around 400 GHz for the second harmonics. These radiation frequencies will be applied to 300 MHz and 600 MHz DNP enhanced NMR spectroscopy.     

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.     

Performance Test of CW 300 GHz Gyrotron FU CW I

A 300 GHz CW gyrotron FU CW I has been developed and installed in the Research Center for Development of Far-Infrared Region, University of Fukui as a power source of a high frequency material processing system. Its performance was tested and the maximum power of 1.75 kW / CW has been attained at the beam voltage of 15 kV and the beam current of 1A. The maximum window power efficiency of 15.5% has been obtained at the cathode voltage slightly lower than 15 kV. This gyrotron is designed to deliver a Gaussian beam after mode conversion from the oscillation mode TE_22,8 in the cavity with a complex of an internal radiator and beam shaping mirrors. The detailed measurement with an infrared camera has confirmed that a Gaussian beam is radiated when the magnetic field strength B _c at the cavity is adjusted at a proper value. However, within a range of B _c , the output power is emerged into multiple directions, which suggests simultaneous oscillation of competing cavity modes.     

A Quasi-Optical System for Converting TE0n Mode Outputs of a Gyrotron into Gaussian Beams

In this paper we present a novel design of a quasi-optical system for conversion of gyrotron outputs into Gaussian-like beams. It consists of a quasi-optical antenna, two focusing mirrors and a filter which removes the side lobes of the beam. The system is appropriate as a transmission line for frequency tunable gyrotrons operating at TE_0n mode. As an illustration of our approach, we present results which demonstrate the applicability of the developed system for conversion of the radiation generated by the Gyrotron FU IVA. The examples include conversion of three TE_0n modes (TE₀₂, 223 GHz; TE₀₃, 323 GHz; TE₀₄, 423 GHz) into Gaussian- like beams.     

Tunable 35–200GHz,high power,gyrotron

The extended frequency tuning range of the Strathclyde University gyrotron is demonstrated. The gyrotron consisted of a field-immersed, field emission cold cathode and a shaped anode cavity. The cavity was oversized with an ill-defined boundary at one end. This system could therefore support the oscillation of a very large number of TE_pqs modes. The maser was thus highly tunable via differential mode excitation. A heterodyne technique was used to demonstrate the maser oscillation in the Ka-band (26.5–40GHz), with the maser oscillating at 35GHz in the TE₀₁ mode. Previous to this a quasi-optical diffraction grating spectrometer was used to show the maser was oscillating under different conditions respectively, single and multi-mode in the W-band (75–110GHz) and multi-mode in the G-band (140–220GHz). To control the output beam a quasi-optical mode converter has been successfully developed. This converts various TE_pq modes into a Gaussian beam. Studying the output of the maser, the mm-wave pulse has been shown to consist of a ～100ns envelope containing ～2ns pulses spaced ～5ns apart. Although this behaviour may have several explanations, some of the most recent code simulations predict this behaviour as a result of mode beating in the complex transient start-up of the maser oscillation. Since this gyrotron not only supports many transverse modes, but also several longitudinal modes, such beating is a likely consequence.     

Detailed Consideration of Experimental Results of Gyrotron FU CW II Developed as a Radiation Source for DNP-NMR Spectroscopy

A CW gyrotron for the sensitivity enhancement of NMR spectroscopy through dynamic nuclear polarization has been designed. The gyrotron operates at the second harmonic and frequency of 394.6 GHz with the main operating mode TE_0,6. Operating conditions of other neighboring cavity modes such as TE_2,6 at frequency of 392.6 GHz and TE_2,3 at frequency of 200.7 GHz were also considered. The experimental conditions of the gyrotron at low and high voltages are simulated. The output power of 56 watts corresponds to the efficiency of 2 percent at low voltage operation and frequency of 394.6 GHz is expected.     

Investigation of a Broadband Quasi-Optical Mode Converter for a Multi-Frequency 1 MW Gyrotron

A broadband quasi-optical (QO) mode converter for a multi-frequency gyrotron has been designed and tested at Forschungszentrum Karlsruhe (FZK). The launcher is optimized for the TE_22,8 mode at 140 GHz, but the radiated beams present an almost identically focused pattern for all 9 considered modes between 105 GHz (TE_17,6) and 143 GHz (TE_23,8). Combining with a beam-forming mirror system, which consists of a quasi-elliptical mirror and two phase-correcting mirrors with non-quadratic surface contour, further calculations show that efficiencies of more than 94% have been achieved for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode. Low power (cold) measurements show a good agreement with theoretical predictions. This QO mode converter can be used for the broadband operation of a multi-frequency 1 MW gyrotron.     

A 250 GHz, 50 W,CW Second Harmonic Gyrotron

A design concept of a 250 GHz, ≈ 50 W, CW, second harmonic gyrotron for spectroscopy will be presented in this paper. Mode competition, in particular with regard to competition with first harmonic modes, and mode selection procedures are carefully investigated by considering various candidate modes and the TE_6,2 and TE_6,3 modes are chosen as probable operating modes. Cold cavity and self-consistent calculations of power and efficiency are carried out to determine optimum parameters for these two modes. The results indicate that an output power of well over 50 W, CW can be achieved.     

CW Operation of a Submillimeter Wave Gyrotron (Gyrotron Fu IV) for High Stability of the Output Frequency

The first cw operation of our submillimeter wave gyrotron (Gyrotron FU IV) using a 12 T superconducting magnet has been successfully carried out. Output power is more than 20 W at a frequency of 301 GHz in the TE₀₃₁ resonant cavity mode. Time-resolved frequency measurement s shows that the frequency fluctuation of the gyrotron output is smaller than 2 MHz. This frequency fluctuation is mainly due to the fluctuation in the output voltage of the power supply.     

Design of a system converting an output radiation of frequency tunable gyrotron into a gaussian beam

A novel design of a versatile quasi-optical system for conversion of gyrotron radiation into collimated gaussian beams is presented and discussed. The proposed system consists of a quasi-optical antenna, two ellipsoidal mirrors and a spatial filter which truncates the sidelobe radiation. The system is appropriate as a transmission line for frequency tunable gyrotrons operating at TE_0n and TE_1n modes. As an illustration of our approach, we present results which demonstrate the applicability of the developed system for conversion of the radiation generated by the Gyrotron FU IVA. The examples include conversion of four TE_1n mode outputs (TE₁₂, 170 GHz; TE₁₃, 271 GHz; TE₁₄, 372 GHz; TE₁₅, 472 GHz) into gaussian-like beams and three TE_0n modes (TE₀₂, 223 GHz; TE₀₃, 323GHz; TE₀₄, 423 GHz) into bigaussian-like beams.     

Design of a Quasi-Optical Mode Converter for a Frequency Step-Tunable Gyrotron

The quasi-optical mode converter for a frequency step-tunable gyrotron which consists of a dimpled-wall antenna (Denisov-type launcher) and a beam-forming mirror system has been optimized for 9 modes from TE_17,6 at 105 GHz to TE_23,8 at 143 GHz. The first mirror is a large quasi-elliptical focusing one; the second and third are phase-correcting mirrors with a non-quadratic shape of the surface. The results of calculations show that for these modes the Denisov-type launcher has a well-focused beam with low diffraction losses, and the radiation pattern presents an almost identical field shape for all modes considered. A multi-mode optimization of the phase-correcting mirrors with two different methods has been tested. The simulations show that the phase-correcting mirrors can be used for broadband operation in the frequency range from 105 GHz up to 143 GHz in the various design modes. This quasi-optical mode converter can achieve efficiencies of 94%-98% for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode.     

Development and Applications of High—Frequency Gyrotrons in FIR FU Covering the sub-THz to THz Range

Powerful sources of coherent radiation in the sub-terahertz and in the terahertz frequency range of the electromagnetic spectrum are necessary for a great and continuously expanding number of applications in the physical research and in various advanced technological processes as well as in radars, communication systems, for remote sensing and inspection etc.. In recent years, a spectacular progress in the development of various gyro-devices and in particular of the powerful high frequency (sub-terahertz and terahertz) gyrotron oscillators has demonstrated a remarkable potential for bridging the so-called terahertz power gap and stimulated many novel and prospective applications. In this review paper we outline two series of such devices, namely the Gyrotron FU Series which includes pulsed gyrotrons and Gyrotron FU CW Series which consist of tubes operated in a CW (continuous wave) or long pulse mode, both developed at the FIR FU Center. We present the most remarkable achievements of these devices and illustrate their applications by some characteristic examples. An outlook for the further extension of the Gyrotron FU CW Series is also provided.     

High Efficiency Mode Converter for Low-Frequency Gyrotron

A high efficiency quasi-optical (QO) mode converter for high-power, low-frequency gyrotron have been designed and tested. For low-frequency gyrotrons, the scales of the mode converter are comparatively small on the wavelength scale, thus causing significant diffraction losses. Over-1 MW power gyrotron with TE_8,3 cavity at 28 GHz have been developed, which has a high efficiency mode converter designed by the use of numerical methods for launcher optimization. This calculation is sufficiently optimized to maximize the fractional Gaussian content of the far field. The total transmission efficiency from the mode converter to output window is 94.7%. For the experimental result of first tube, the output power of more than 1 MW has been obtained with about 40% efficiency and output burn pattern agrees fairly with the calculated profiles, which imply the design appropriateness. Besides, the frequency dependence for diffraction loss is discussed, and these results give the guiding design principle of the mode converter for high-power, low-frequency and long-pulse gyrotrons.     

A W-band Third Harmonic Gyrotron with an Iris Cavity

The design and experimental results of a W-band gyrotron operating at the third cyclotron harmonic are presented. The gyrotron is designed to operate at the TE₆₁ mode, which is significantly distinct from competing modes. An iris cavity is employed for the purpose of trapping the third harmonic mode more effectively and lowering its start current. In the experiment, the gyrotron is drived by a triode magnetron injection gun (MIG) which can produce a 45 kV, 3 A electron beam. When maximum axial magnetic field is 1.22 T, a single mode third harmonic gyrotron radiation is observed with the frequency of 94.86 GHz. The maximum output power is 5.5 kW, corresponding to an efficiency of 4%. Another third harmonic mode TE₀₂ is also detected at 88.8 GHz, with maximum output power of 1.5 kW.     

Numerical simulation of mode interaction in 170 GHz/1 MW gyrotrons for iter

The paper presents an advanced method for and results of calculating main parameters of CW 170 GHz/1 MW gyrotrons operating at the TE_28.7 and TE_31.8 modes for ITER. Parameters are optimized to achieve maximum efficiency of the gyrotron with an acceptable Ohmic load on the cavity. Numerical modeling of starting up a gyrotron with an optimized cavity and processes of mode interaction are discussed.