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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.

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.

At the Plasma Research Center (PRC) in University of Tsukuba, development of Mega-Watt Gyrotrons is performed for fusion research. Development of a new 154 GHz 1 MW gyrotron for ECRH system of Large Helical Device (LHD) has been done. The design studies of electron gun (MIG), cavity oscillator, mode converter, transmission mirrors, collector and superconducting magnet (SCM) have indicated more than 1 MW output with efficiency ~33 % for cavity oscillation mode of TE28,8. In the short pulse test of the first test tube, the maximum output power of 1.06 MW and the maximum total efficiency of 35.2 % with CPD were obtained.

A practical multi-gyrotron oscillation system, using collector-potential depression, composed of six gyrotron tubes and 3 U of power supplies, was designed, fabricated and tested. This system was designed to generate power levels of 3 MW for pulse duration of 1-s at ≈168 GHz for electron cyclotron heating of LHD at the National Institute for Fusion Science. The all-solid-state power supply unit can drive a maximum of three gyrotrons by equipping the collector power supply with three pairs of the anode and body power supplies. The gyrotrons used a TE31,8,1-mode interaction cavity. A single-stage depressed collector with sweeping coils was employed to increase system efficiency and reduce the heat flux to the collector surface. An internal converter produced a flattened Gaussian profile at a single-disk silicon-nitride window. The output mode was reconverted into the HE11 mode by an MOU. We reconstructed a main circuit of the power supply unit because of stray capacitors in the actual circuit. There were some differences between the designed and measured output wave profiles. The tubes were tested for 1-s pulse with power levels of 500 kW; system efficiencies were 30% at the peak and 28% at the average and temperatures of the windows were ≈200°C.

A large sized synthetic diamond window assembly was installed on a 170 GHz gyrotron. An aperture and a thickness of the window are 83 and 2.23 mm, respectively, whose edge was directly cooled by water. Gyrotron performances of 520 kW at 6.2 s and 450 kW at 8 s were attained. A temperature increase of the window stabilized after ∼5 s, whereas the power had been limited to below 170 kW with conventional windows. This drastic improvement of a deliverable power was obtained from the outstanding properties of the diamond, that is, an extremely high thermal conductivity (∼1800 W/mK) and a low value of loss tangent (tan δ 1 s), but no damage nor trouble was found both on the gyrotron and the diamond window. These results give a prospect for a multimegawatt power output from the gyrotron, i.e., the diamond window gives a solution for the window problem which has been regarded as the most serious issue on the development of high powe...

Along the upgrade program of ECRH system in GAMMA10, the gyrotron of 500kW level at 28GHz has been developed. The first 500kW gyrotron delivered 500kW at the windows and contributed to produce high ion confining potential. During the operation, it is found that there were some points to be improved: 1) Saturation level of output is smaller than expected. 2) Body current is large. 3) Beam over current occurs frequently. The second gyrotron was redesigned to improve these three points for stable operation. The second one has been changed in the design of internal mode converter, the shape of structure in collector and the slope of internal top roof of the collector. As a Result of 3 improved points, the maximum output power increased from 516kW to 570 kW, body current decreased from 5mA to 2mA, and the rate of beam over current decreased from 7% to 4%.

A development of 170GHz/500kW level gyrotron was carried out as R&D work of ITER. The oscillation mode is TE31,8. In a short pulse experiment, the maximum power of 750kW was achieved at 85kV/40A. The efficiency was 22%. In the depressed collector operation, 500kW/36%/50ms was obtained. The maximum efficiency of 40% was obtained at PRF=470kW whereas the power decrease by the electron trapping was observed. Pulse extension was done up to 10s at PRF=170kW with the depressed collector operation. The power was limited by the temperature increase of the output window.

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.

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. >

A 170 GHz gyrotron with a single stage depressed collector was developed for high power and long pulse operation. Its oscillation mode is TE31,8 volume mode, and power profile at output window is flat to reduce heat deposition on the output window. At initial experiment, the output of 520 kW-0.6 sec and 230 kW-2.2 sec were obtained, and the maximum of improved efficiency is 38%.

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.

A gyrotron with an energy recovery system was developed and its stable operation was performed. The efficiency was improved from 30% to 50% at 610kW, 50msec, and the long pulse, high efficiency operation, 350kW, 5sec, 48% was demonstrated. These results should bring a large cost reduction of ECH system.

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.