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The latest development achievements in the University of Tsukuba of over-1 MW power level gyrotrons required in present-day fusion devices, GAMMA 10, Large Helical Device (LHD), QUEST, Heliotron J and NSTX, are presented. The obtained maximum outputs are 1.9 MW for 0.1 s on the 77 GHz LHD tube and 1.2 MW for 1 ms on the 28 GHz GAMMA 10 one, which are new records in these frequency ranges. In long-pulse operation, 0.3 MW for 40 min at 77 GHz and 0.54 MW for 2 s at 28 GHz are achieved. A new programme of 154 GHz 1 MW development has started for high-density plasma heating in LHD. On the first 154 GHz tube, 1.0 MW for 1 s is achieved. As a next activity of the 28 GHz gyrotron, an over-1.5 MW gyrotron is designed and fabricated to study the multi-MW oscillation. The possibility of 0.4 MW continuous wave and 2 MW level output in operations of a few seconds, after the improvements of output window and mode converter, is shown. Moreover, a new design study of dual-frequency gyrotron at 28 and 35 GHz has started, which indicates the practicability of the multi-purpose gyrotron. © 2013 IAEA, Vienna.

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. © 2010 Springer Science+Business Media, LLC.

A 28 GHz 1 MW with TE 8,3 cavity for GAMMA10 tandem mirror and a 77 GHz 1.5 MW gyrotron with TE 18,6 for Large Helical Device (LHD) have been developed to upgrade their Electron Cyclotron Heating (ECH) systems. In the 28 GHz gyrotron, the maximum power of 1.05 MW was obtained, which is in agreement with its design target value. And the high efficiency of 40% without collector potential depression (CPD) was obtained with 0.8 MW. In the first and second 77 GHz gyrotrons, 0.8 MW 3.6 sec., 0.3 MW 60 sec operations have been achieved, but several issues due to the stray RF and velocity dispersion have been found, too. In consideration of these, the design improvement was performed to aim at 1.5 MW in the third tube. In the short pulse test, the maximum output power of 1.6 MW and the maximum total efficiency of 49.4% with CPD were obtained. In the long pulse test, the pulse length extended to 5 sec with 1 MW, 1800 sec with 0.1 MW, 1.6 sec with 1.5 MW. Total injection power of 3.1 MW to LHD plasma has been achieved. It is shown that the electron beam pitch factor α decreases with increasing of beam current, comparing with the experimental and calculation results. © 2010 Springer Science+Business Media, LLC.

We are developing a new 28GHz 1MW and a 77GHz 1MW gyrotron for ECRH system of tandem mirror GAMMA10 and Large Helical Device (LHD), respectively. The detail design study of 28GHz 1MW gyrotron such as cavity, magnetron injection gun (MIG) has been done. We obtained the oscillation power of 1.37MW and the oscillation efficiency of 42.7% with the pitch factor of 1.2. Two 77GHz 1 MW gyrotrons have been fabricated and tested. The maximum output power of 1.1 MW was obtained. The pulse width with 0.46MW extended to 5s with the short aging time of only 65 hours. A plasma injection for LHD with MOU output of 0.81MW 3.6s was performed.

In GAMMA 10, the upgrade program of high power plug and central ECRH have started for the study of the physics of plasma potential and the demonstration of high plasma performance proving the tandem mirror advantage. A new high power (upgrade) gyrotron has been developed in collaboration with JAEA from 2003 for this purpose and now three upgrade gyrotrons were installed in GAMMA 10. The first gyro torn in 2004 outputted about 500 kW power at the gyrotron window, corresponding to 400kW injection power to plasma. In 2005, the power of 570 kW at the window was obtained from the second one, which was improved mainly in mode converter. These gyrotron were applied to plug cells and a new record value of ion confining potential Φ c = 3 kV was obtained at 470 kW injection, which is four times higher than the previous value before 2003. For the central ECRH system, the upgrade of DC power supply was conducted using the DC power supply from JFT-2M ECH system and the system outputted 400kW at MOU out with the upgrade gyrotron. The central ECRH has been conducted with the combination of the new efficient antenna, transmission line and the upgrade gyrotron, and the initial experiment resulted Te ≥ 500eV.

Development of 170 and 110 GHz gyrotrons and their application to electron cyclotron heating systems are presented. A parasitic oscillation that degraded the electron beam quality was suppressed, and the performance of the gyrotron improved significantly. Up to now, 0.9 MW/9.2 s, 0.5 MW/30 s, 0.3 MW/60 s, 0.2 MW/132 s, etc, have been demonstrated at 170 GHz. At 110 GHz, 1.3 MW/1.5 s, 1.2 MW/4.1 s, 1 MW/5 s were obtained. It is found that the reduction of stray radiation and the enhancement of cooling capability are important for continuous wave operation. Four 110 GHz gyrotrons are under operation in the electron cyclotron heating and current drive system of JT-60U. Power up to approximately 3 MW/2.7 s was injected into the plasma through movable mirrors in the poloidal direction and contributed to electron heating and the suppression of the neo-classical tearing modes.

This paper presents the design method for a gyrotron-dimple-type mode converter that is axially up-tapered to prevent inside oscillation. In this up-tapered converter, the propagation constant of the traveling TE modes varies axially since the mean radius grows in the axial direction, requiring a new formulation of the mode coupling in the converter. In this paper, the axial variation of the helix pitch that describes the TE mode propagation is discussed first. Then, the coupling coefficients between TE modes are analytically derived, with which a trial design of a linearly tapered converter is made for a 170-GHz gyrotron.

A new 28GHz 500kW gyrotron was developed for the electron cyclotron resonance heating system of tandem mirror GAMMA10. The maximum output power of 570 kW was obtained by experimental optimization. The efficiency of the gyrotron is 40∼50% in the power range of 300∼500 kW output with the optimization in spite of the restriction of the present magnet. The non-desirable influences caused by he leakage field of the magnet system of GAMM10 were minimized by the adjustment of gyrotron coil parameters and the minor change of the gyrotron and magnetic shield design.

This paper describes a scheme for shaping a given wave beam into the desired profile using multiple phase-correction mirrors. This mirror system was applied to a gyrotron internal converter to flatten the radiated beam profile at the window. The flat output beam is reconverted into HE11 mode, a basic propagation mode for corrugated waveguides, by another pair of phase-correction mirrors for transmission into a fusion reactor. In addition, a wave-beam splitting and combining technique is also presented.

High efficiency coupling of the millimeter-wave output of the dual-frequency gyrotron with the transmission line was demonstrated. The dual-frequency gyrotron was design to produce similar beam profiles for two operation frequencies (170 GHz and 137 GHz). Using two RF beam reflecting mirrors in a matching optics unit (MOU), which are designed to transform the beam profile of the gyrotron output to the fundamental waveguide mode (HE(11) mode) at 170 GHz, high efficiency coupling was demonstrated for two frequencies. The measured mode purity of HE(11) mode was 96% at 170 GHz and 94% at 137 GHz operations with the identical mirrors at the fixed mirror position and angle. The results indicate that the significantly simple dual frequency system is realized by the gyrotron designed to output the similar beam profiles at different frequency operation.

The design of a 170 GHz, 1 MW-CW gyrotron for electron cyclotron heating of nuclear fusion plasmas is presented. The designed gyrotron incorporates a coaxial cavity to reduce mode competition, and a coaxial electron gun to support the cavity inner conductor. A new mode converter splits the generated wave into two beams and radiates them in different directions. The radiated beams are transmitted to two output windows through two mirror systems, being transformed into Gaussian-like beams. A single-stage depressed collector improves the overall efficiency of the gyrotron and reduces the heat flux to the collector surface.

A maximum pulse duration of 1.3 s was achieved with a power of 410 kW at a frequency of 110 GHz and a maximum power of 550 kW was obtained with short-pulse operations (ca. 2 ms). The oscillation mode of the gyrotron is the TE22,2 whispering gallery mode, which is transformed into a gaussian-like beam by a built-in quasi-optical mode converter. R.f. power is extracted through a sapphire double-disc window cooled by FC-75. In long-pulse operation, no damage was observed in the gyrotron.

Megawatt-class gyrotrons covering a wide frequency range (14 GHz?300 GHz) are in increasing demand for nuclear fusion. Recent electron cyclotron heating and electron cyclotron current drive experiments highlight a requirement of megawatt-scale gyrotrons at a relatively lower frequency (14?35 GHz) range of some plasma devices, like GAMMA 10/PDX of the University of Tsukuba, QUEST of Kyushu University, NSTX-U of Princeton Plasma Physics Laboratory, and Heliotron J of Kyoto University. Collaborative studies for designing a new 28 GHz/35 GHz dual-frequency gyrotron and a 14 GHz gyrotron have commenced. Operation above 1 MW of 28 GHz/35 GHz dual oscillation was demonstrated experimentally. Further in the design of dual-frequency gyrotron, operations with 2 MW 3 s and 0.4 MW CW (continuous wave) at 28 GHz, and power exceeding 1 MW for 3 s at 34.8 GHz have been shown to be feasible. The 14 GHz gyrotron is expected to operate above 1 MW. We are also developing higher frequency gyrotrons (77?300 GHz). The joint program of National Institute for Fusion Science and the University of Tsukuba developed two new 154 GHz gyrotrons for the large helical device after the demonstration of three 77 GHz gyrotrons. The 154 GHz gyrotrons achieved a maximum output power of 1.25 MW and quasi-CW operation of 0.35 MW for 30?min.

The authors present the design and initial operation of two 120-GHz gyrotrons under development: a 200-kW TE/sub 03/ mode gyrotron and a 500-kW, TE/sub 12.2/ whispering-gallery-mode gyrotron. The TE/sub 03/ mode gyrotron was tested up to 170 kW with 10-ms pulses. The transmission characteristics of the window proved to affect the mode map significantly for the TE/sub 03/, TE/sub 23/, and TE/sub 52/. For the whispering-gallery-mode gyrotron, a weak TE/sub 12.2/ oscillation was observed at a beam voltage of 50 kV and a beam current of 10 A, although the expected beam condition was not obtained because of a difficulty in the electron gun. >

A design and experimental results of a 110 GHz gyrotron with a single stage depressed collector are presented. By recovering a spent beam energy, efficiency of the gyrotron was improved from 30% to 50% with the RF output power of 610 kW and the pulse duration of 50 ms. A stable 5 sec oscillation was performed at 350 kW (output energy 1.75 MJ) with high efficiency of /spl sim/48% and the 420 kW-3.3 sec oscillation was also obtained. Conditioning time was short compared with the conventional gyrotron since a collector heat load was significantly reduced by the application of the beam retarding voltage. >

An experimental and design study has been commenced for a dual-frequency gyrotron (28 and 35 GHz). The target output power at 28 GHz is 2 MW. For a modified 28 GHz 1 MW gyrotron, an output power of 1.25 MW and operation of 0.6 MW for 2 s have been achieved at 28 GHz. For the 35.45 GHz oscillation test, a cavity oscillation power of 1.2 MW and an efficiency of 33.9% have been confirmed by considering the calculated transmission efficiency of 72%. These results support the development of dual-frequency gyrotrons for lower frequency ranges.

Stable 1.1 MW oscillation was achieved by a 170 GHz high power gyrotron. The oscillation mode capable of the CW operation is TE31,8. The efficiency was 32%, which was enhanced to 57% by a depressed-collector operation. The parasitic oscillation in a beam tunnel (a beam drifting tube between an electron gun and a cavity) that degraded the oscillation efficiency was suppressed by installing an RF absorber. This result has a large impact on the development of a 1-MW-long-pulse gyrotron that is required for fusion devices.

An improvement of electron cyclotron heating (ECH) system in the central cell of the GAMMA 10 tandem mirror is carried out by the use of two grooved mirrors. Two grooved mirrors consist of a twister and a circular polarizer. In case of the ECH system in the central cell region, fundamental ECH is examined and launched microwave beam from the antenna propagates through the two parabolic mirrors. The incident microwave is radiated from the second mirror at a position with magnetic field intensity B=2.0 T and with an incident angle of 35.1 degrees. Two grooved mirrors can generate suitable elliptical polarizations for both of ordinary mode (O-mode) and extraordinary mode (X-mode) in oblique launch with a slanted magnetic field line. The first results of the polarization control of the incident wave for high power ECH experiments are reported.