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A TE31,11 mode gyrotron has being developed to improve the output power for the ITER electron cyclotron (EC) system. Strong mode competition having counter-rotating modes in both the high and/or low field sides is, in general, the typical issue for high order mode oscillations. Active anode voltage control and beam radius control were introduced to overcome this issue and obtain stable 1 MW oscillation. The TE31,11 mode was oscillated in a hard-self excitation region and the efficiency was improved up to 50%. Long pulse operation of output power of 1 MW with total efficiency of 46% for 230 s was also performed. Oscillations of over–1 MW power of 1.1 MW and 1.2 MW for 2 s pulse were achieved.

A new fast directional switch for high power millimeter waves without frequency modulation was proposed. The principle is a triangle ring resonator of circular corrugated waveguides with a vibrating reflector. The switching frequency was obtained up to 300 Hz by a single vibration reflector. The deformed waveforms of the output powers were observed at large amplitude vibrations of a reflector.

For successful tokamak operation with adequate mode selection of the electron cyclotron wave, launching wave polarization should be accurately controlled. In this study, a new method is proposed to efficiently identify the response of polarizers using a geometric approach in the Stokes space, focusing on the fact that polarization can be represented as a vector in Stokes space and the response of a polarizer can be described as a rotation. The proposed methods are intended (i) to determine the response parameters of a polarizer or (ii) to determine the response parameters of two polarizers simultaneously. A simple measurement system was prepared to observe the polarizers’ output polarization in the 110/138 GHz transmission line. The determined response parameters of two polarizers can reproduce measured polarizations with sufficient accuracy above 99%. The direction of an output polarization circularity, the only feature of a polarization that could not be directly known from the prepared measurement, could also be determined with sufficient accuracy.

The ITER electron cyclotron system is designed to inject a 20 MW RF beam by using twenty-four 170 GHz/1 MW gyrotrons. JAEA is currently developing a gyrotron having a high-order mode (TE31,11) to reduce the heat load in the cavity resonator and achieve an output power greater than 1 MW. The measured radiation profile at the front of the diamond window agreed with the results of the calculation. In order to suppress RF loss in the equatorial and upper port launchers, a high-quality HE11 mode is required at the exit of the matching optics unit (MOU). An HE11 mode purity of 96% was achieved by finely adjusting the two mirrors in the MOU. During the oscillation start-up phase, mode competition with counter-rotating TE29,12 mode was observed on the higher magnetic field side which caused arcing and pressure increase in the gyrotron. To avoid the counter-rotating TE29,12 mode from being excited, a start-up scenario that controls the voltage between the anode and cathode electrodes at the initial phase of operation was introduced, which was able to achieve a stable start-up of TE31,11 mode. A 1.2 MW output power having a total electric efficiency of 43% was obtained in high-power experiments. In steady-state operation, a 1000 s oscillation length and output power of 0.51 MW was achieved.

Mechanical and quasi-optical design of an electron cyclotron heating/current drive launcher for JT-60SA is in progress. A full-scale mock-up of the steering structure, which enables linear and rotation motions of the first mirror of the launcher, has been fabricated for cyclic test of the bellows part. Moreover, an improved design enables easy replacement of the bellows for rotation for maintenance. Quasi-optical characteristics of the antenna mirrors have been studied to evaluate its transmission efficiency and beam focusing property. In calculation, it was found that the antenna is applicable to two frequency operation at 110 GHz and 138 GHz. It was quantitatively shown that the transmission efficiency of ∼99% (not including Ohmic loss) is obtained even with the higher order mode (LP11even) fraction of 10% by optimizations of the shape of the first mirror. These results contribute to optimization/finalization of the launcher design toward fabrication of the launcher for JT-60SA.

The design modification of the ITER equatorial electron cyclotron (EC) launcher to enhance off-axis current drive performance, changing the steering capability of injection beams from toroidal to poloidal, is described. The modification requirement impacts the design of all 3 millimeter wave beam paths transmitted through the port plug and blanket shield modules (BSMs) installed in front. The optimized configuration of the mm wave design consisting of both steering and fixed mirrors, beam ducts between the waveguide outlet and BSM openings are successfully developed. Assuming a 100% pure HE 11 mode propagated at the waveguide output, a 99% transmission efficiency from the waveguide outlet to the BSM output is obtained. This modification ensures that mm wave beams from both middle and bottom beam row pass through the same BSM opening and will lead to the increase of neutron shielding potential. The configurations of BSMs are also modified maintaining the structural design concept of each module. The preliminary result of electromagnetic analysis shows that induced force and momentum torque on the modified BSMs are below the mechanical criteria of the BSMs’ support structure.

The measurement of the ion velocity distribution is especially important in reactor relevant plasmas. The collective Thomson scattering (CTS) is one of the most promising methods for evaluating the ion velocity distribution function. In spite of its potential, this method had long been suffered from the absence of adequate power sources. Recent development in the higher power, and higher frequency range of the gyrotron and the transmission technique enabled to realize the measurement of not only the bulk but also high energy component of the ion velocity distribution function. One of the newly installed 77 GHz gyrotrons in LHD that can generate more than 1 MW over 5 s and is connected to one of the Gaussian mirror beam launching antenna sets is used as a probing beam for CTS. A heterodyne receiver system of a fundamental mixer with fixed frequency local oscillator was installed on the upstream of the transmission line. The probe beam is 100% power modulated at 50 Hz to separate the scattering component from background ECE. Signals that can be attributed to the CTS were obtained and the analysis method of these data are developed (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of experimental configuration for the electron Bernstein wave (EBW) heating by using the existing electron cyclotron heating (ECH) antennas on LHD was performed. By using an antenna installed in the lower port, direct oblique launching of the extraordinary (X-) mode from the high magnetic field side (HFS) is available. Since the parallel component of the refractive index (N||) varies during propagation because of the inhomogeneity of the magnetic field, N|| can be zero when the launched X-mode crosses the fundamental electron cyclotron resonance (ECR) layer even N|| is noonzero initially. In such a condition, if the electron density is above a certain level the obliquely launched X-mode can pass the fundamental ECR layer without being damped out and can be mode-converted to EBW that is absorbed at the Doppler shifted ECR layer. By using an antenna installed in the horizontal port, oblique launching from the lower magnetic field side (LFS) toward the over-dense plasma is available. Excitation of EBW via the mode conversion process of ordinary mode(O)-extraordinary mode(X)-electron Bernstein wave (B) is expected with the O-mode launching toward an appropriate direction. The O-X-B mode conversion rate and the region of power deposition were surveyed by varying the magnetic field strength and the launching direction. The results of the survey suggest that efficient heating in the core region is difficult by using the existing antenna. Rearrangement of the final mirror of the launching antenna may be needed.

A short pulse high order mode gyrotron for 300 GHz oscillation was tested. The designed oscillation mode is TE 32,18 and the gyrotron is operated with a 13T superconducting magnet. The output RF power and its frequency was measured. In the preliminary experiment, the 520 kW power was obtained at 299.85 GHz which corresponds to designed oscillation mode.

A mode conversion miter-bend with angle controllable mirror was developed. By changing mirror angle, RF beam angle in the miter bend is controlled and transmission mode contents (LP01 and LP11 modes) are controlled. The miter bend was tested at 170 GHz mW-class low power RF system. The dependence of miter bend mirror angle and ratio of LP01 and LP11 modes was acquired. The concept of mode conversion system was validated.

The results of high power testing of new water-cooled ECH waveguide components for ITER are presented. The components are a precision-coupled 4.2 m waveguide assembly, a short expansion joint, and water-cooled waveguide for gyrotron commissioning. The testing was conducted at the QST Naka Fusion Institute using gyrotron pulses of 450 kW at 170 GHz for 300 s. Analysis shows that the power absorbed per unit length for the various waveguide components are dependent on location in the transmission line with respect to high order mode generators, such as miter bends. Additionally, larger-than-expected reflections from the load led to high absorption levels in the transmission line.

A short pulse gyrotron was designed, fabricated and tested for 300 GHz oscillation. The oscillation mode is TE 32, 18 and its neighborhoods and the power is output from the top of the gyrotron. The superconducting magnet has 110 mm room temperature bore diameter, and the maximum magnetic field is 13 T. In the preliminary experiment, the 300 kW power was obtained at ∼300 GHz.

A method is described to form a laminar electron beam for a high power gyrotron. An optimum potential profile is investigated to adjust each electron trajectory in between cathode and 1st anode. A high quality laminar beam is realized with a wide operation window.

Gyrotron FU CW GIII has been developed as an improved version of Gyrotron FU CW GII. A new electron gun has been designed to improve the quality of an electron beam. After careful set up of FU CW GIII, the maximum oscillation power of 420 W was obtained, which is more than 5 times larger than that obtained on Gyrotron FU CW GII.

Long pulse operation of a dual frequency gyrotron for JT-60SA, which can oscillate both 110 GHz and 138 GHz waves, was started. Oscillations at ~0.4 MW for 2 s (both frequencies) and 0.7 MW for 1 s (110 GHz) were obtained, so far. Temperature increase in the cavity, the output window and some components, which absorb stray radiation in the gyrotron, were measured and cavity loss power and the dielectric loss tangent of the output window were evaluated. Heat loads in the gyrotron measured so far were acceptable for long pulse operation at an output power of 1 MW.

A table-top gyrotron with an internal mode convertor was designed and fabricated at FIR FU. The length of the gyrotron is as small as 860 mm. The output power higher than 1 kW at 203.4 GHz is expected. The oscillation of TE 3,3 mode was confirmed in an initial test operation.

Competitions between fundamental and second harmonic modes are observed in high power sub-THz gyrotron. The combination of simultaneously oscillating modes sensitively depends on the operation conditions, and sometimes, second harmonic modes nonlinearly excite a fundamental mode.

A high-power gyrotron is developed for use in the collective Thomson scattering diagnostic of fusion plasma. It operates at fundamental harmonic with a frequency of 295 GHz. An electron gun with an intense laminar electron beam is designed, and the oscillation power of more than 200 kW is expected.

Gyrotron FU CW GIII with an internal mode convertor for a Gaussian beam has been developed in FIR FU. It is an extension of Gyrotron FU CW GII. A new electron gun has been designed to improve the quality of an electron beam, and cooling systems for the window and inside mirrors are newly installed for completely CW operation.

Two gyrotrons FU CW GI and GII with an internal mode convertor to a Gaussian beam have been developed in Univ. Fukui. Oscillation modes have been carefully selected considering their coupling with electrons, compatibility with the convertors. Gaussian-like beams have been successfully observed from both gyrotrons.

The Gytrotron FU CW III using a 20 T superconducting magnet has been developed as a radiation source in the THz region. Gyrotron radiations were observed in the magnetic field intensity region higher than 19.25 T, The frequency of the radiation is expected to exceed 1 THz. The highest measured frequency is 670.62 GHz using the frequency measurement system consisting of a harmonic mixer and a spectrum analyzer.

A tabletop gyrotron is under development at FIR FU. A magnetron injection gun, a cavity resonator and an internal mode converter were newly designed. As the results of new design, the cavity oscillation power is expected to achieve 0.5 ∼ 1 kW at 203.4 GHz and the output wave will be converted to Gaussian beam.

For direct measurement on hyperfine structure of positronium, high power sub-THz radiation sources, Gyrotrons FU CW V and FU CW VIII have been developed. FU CW V is a frequency fixed gyrotron operating at 203.3 GHz for the proof-of-principle experiment and FU CW VIII a frequency continuously tunable gyrotron to measure directly the energy difference between orth- and para states of positronium.

A numerical code has been developed to treat the mode conversion process between electromagnetic and electron Bernstein waves for arbitrary propagation angle and polarization considering real experimental configurations on various devices. Robustness of any heating/detection condition with change of the plasma parameters or the frequency can be also estimated with this numerical code in a short time.

The collective Thomson scattering (CTS) is one of the most promising methods for evaluating the ion velocity distribution function.

A gyrotron enabling high-power, long-pulse oscillations at both 110 and 138 GHz has been developed for electron cyclotron heating (ECH) and current drive (CD) in JT-60SA. Oscillations of 1 MW for 100 s have been demonstrated at both frequencies, for the first time in the world as a gyrotron operating at two frequencies. The optimization of the anode voltage, or the electron pitch factor, using a triode gun was a key to obtain high power and high efficiency at two frequencies. It was also confirmed that the internal losses in the gyrotron were sufficiently low for expected long pulse operation at the higher power level of ~1.5 MW. Another important result is that an oscillation at 82 GHz, which enables use of fundamental harmonic waves in JT-60SA while the other two frequencies are used as second harmonics waves, was demonstrated up to 0.4 MW for 2 s. These results of the gyrotron development significantly contribute to enhancing the operation regime of the ECH/CD system in JT-60SA.

A high power sub-THz gyrotron for use in the collective Thomson scattering diagnostics of fusion plasmas is under development. It operates at the fundamental harmonic with a frequency of 295 GHz. In recent experiments, we have succeeded in over 200 kW oscillation of this gyrotron.

This paper describes the design of a triode magnetron injection gun for use in a 200 kW, 300 GHz gyrotron. As power and frequency increase, the performance of the gyrotron becomes quite sensitive to the quality of the electron beam. Formation of a laminar electron flow is essential for the realization of a high quality beam with a small velocity spread. In this study, a new method is developed for a quantitative evaluation of the laminarity and is applied to optimize the electrode design. The laminarity depends not only on conventional design parameters such as the cathode slant angle but also on the spatial distribution of the electric field along the beam trajectory. In the optimized design, the velocity pitch factors, α, larger than 1.2 are obtained at 65 kV, 10 A with spreads, Δα, less than 5%.

New power records of second harmonic gyrotron oscillation have been demonstrated in the sub-THz band. The first step gyrotron of demountable type had succeeded in oscillation with power more than 50 kW at 350 GHz and nearly 40 kW at 390 GHz [T. Notake et al., Phys. Rev. Lett. 103, 225002 (2009)]. Then, the second step gyrotron of sealed-off type was manufactured. A cavity mode was carefully selected to avoid mode competition with a neighboring fundamental harmonic mode. Matching of the selected mode with the electron gun was also circumspectly considered. The second step gyrotron has attained higher power radiation than the first gyrotron. The maximum single mode power was 62 kW at 388 GHz. Then, the electron gun was modified for use of a different cavity mode with a higher coupling coefficient than that for the 62 kW mode. The new mode proved single mode oscillation power of 83 kW at about 389 GHz. These results are new second-harmonic-oscillation power records for sub-THz gyrotrons. The present study constitutes foundations of development of high power second harmonic sub-THz gyrotron for application to collective Thomson scattering measurement on fusion plasmas, especially on high-density plasmas such as those produced in LHD [N. Ohyabu et al., Phys. Rev. Lett. 97, 055002 (2006)]. This paper reports the design consideration to realize high power single mode gyrotron oscillation at second harmonic and the examination of oscillation characteristics of the gyrotron.

High-power sub-terahertz pulse gyrotrons are under development in FIR FU for application to collective Thomson scattering (CTS) measurement on fusion plasmas, especially on high-density plasmas such as those produced in LHD. Recently, we achieved a new power record of 62 kW at approximately 388 GHz with second-harmonic (SH) oscillation. Following this result, we modified the electron gun of the gyrotron to couple the electron beam more strongly to another oscillation mode that has a peak coupling coefficient two times as large as that of the 62 kW mode. Oscillation tests with the new mode attained higher power of 83 kW at about 389 GHz. These results constitute new second-harmonic-oscillation power records for sub-terahertz gyrotrons.

This paper describes the design of a magnetron-injection gun for a 100 kW, 300 GHz gyrotron. With an increase in power and frequency, performance of the gyrotron becomes quite sensitive to the quality of the electron beam. Formation of a laminar electron flow is essential for the realization of a high quality beam with small velocity spread. In this study, a new method is proposed for the evaluation of the laminarity, and applied to the design optimization of the electrodes. It is found that the laminarity depends not only on the conventional design parameter of the cathode slant angle, but also on the spatial distribution of the electric field inside the beam.

Collective Thomson scattering (CTS) is a promising candidate for direct measurement of the velocity distribution function of ions in fusion devices. The relevant frequency for a CTS probing beam source is in the millimeter- to submillimeter-wave range. The scattering volume of CTS is an important parameter for determining the intensity of the scattered power and the spatial resolution of the measurement. The conventional method of estimating the scattering volume is to simply calculate the geometrical volume of the probing and receiving beams. This calculation is valid only when the beams are diffraction free. The effective scattering volume is defined and applied to a CTS system using strongly focused beams and its validity is examined experimentally.