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Developments of a two directional beam steering electron cyclotron heating/current drive launcher for JT-60SA have been progressed based on a linear motion antenna concept. A full-scale (˜7 m) mock-up of the mirror steering structure with two kinds of bellows structures and an in-vessel support structure has been fabricated and tested in vacuum successfully. The support structure consists of a pair of angular ball bearings made of Si3N4 for rotational motion and a linear guide assembled by a rail and a housing made of non-magnetic stainless steel and Si3N4 balls. Solid lubricant has been used for the linear guide and it results in the improvement of the life time by a factor greater than 10 with respect to the case without lubricant. Considering that the mock-up test results satisfied the requirements for the bellows and support structures (reliable 40 cm linear movement, 105 cycles and angular rotation 30°, 104 cycles), the final design of the launcher is underway targeting the start of its fabrication.

A research activity in the Institute for Plasma Science and Technology of National Research Council (ISTP-CNR, Italy, former IFP-CNR) and in L.T. Calcoli (LTC, Italy) is aimed at the design and construction of calorimetric loads for absorption and measurement of high power millimeter-waves in the electron cyclotron frequency range. Recently, two CW 170 GHz loads, one for the European ITER gyrotron test facility and the other for the FALCON test- bed, have been installed at the Swiss Plasma Center (SPC, Switzerland). One short pulse (2 s) load for 1 MW, designed and optimised to operate at two different frequencies (84 GHz and 126 GHz), was provided for testing and conditioning two new dual-frequency gyrotrons for the Tokamak a Configuration Variable (TCV, Switzerland). Two additional CW loads, designed for absorbing powers higher than 1 MW, have been delivered to the National Institutes for Quantum and Radiological Science and Technology (QST, Japan) and exploited for the acceptance tests and the conditioning of the prototype of the Japanese ITER gyrotron. The present status and the most recent experimental results achieved in the framework of this development activity are reported in the paper.

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.

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.

A diplexer to be used as a fast switching device of high power millimeter wave for an electron cyclotron current drive system has been developed. The operation frequency of the device is tunable by controlling the thermal expansion of the cavity using a precision chiller. The tunable range of the diplexer is about 225 MHz at the frequency band of 170 GHz. The temperature dependence of the resonant frequency shift in the ring resonator diplexer is −3.4 to −3.9 MHz/K, which agrees with the theoretical prediction. The switching operation is confirmed up to rf power of 260 kW, a pulse duration of 100 ms, and a frequency of 170 GHz.

The ITER EC system includes both an equatorial port launcher and upper port launchers as RF power injection devices, The waveguide switch in the transmission line (TL) is used to select the operating launcher. Its operation is required even during plasma operation, primarily for the mid-pulse switch operation. Changing the waveguide switch requires that the gyrotron stop RF power during the switch operation since the direction is changed by mechanical movement of the mirror position which takes a few seconds. Since the ITER EC system is based on a multi-subsystem concept, each subsystem has its own subsystem control unit (SCU). The EC main controller supervises all subsystem controllers. Hence, cooperative operation requires the sharing of the information of both RF power status and switch status between the gyrotron SCU and TL SCU via the main controller. Since the design of the inter-subsystem control scheme is a key issue for ITER EC system control, its evaluation is required. At QST, the gyrotron and ITER-relevant TL test stand were utilized for demonstration of mid-pulse switch operation. For this purpose, SCUs for each subsystem and the main controller were developed using the ITER-relevant control system. The operation of the mechanical switch during gyrotron pulse was demonstrated. During the 150 s operation of the high power gyrotron at 400 kW level, the waveguide switch in the TL was operated to change the direction of RF power. The time duration for the switch operation with inter-subsystem control scheme took 1.5 s in total. The synchronizing of RF power suspend and resume with switch motion has therefore been realized, and RF power direction control during the gyrotron operation was successfully demonstrated.

The development of high-power gyrotrons needs matched loads with good performance capable of absorbing and measuring powers as high as 1 MW in the International Thermonuclear Experimental Reactor (ITER) and higher (up to two times) in the future devices, such as the DEMOnstrating fusion power reactor. At Istituto di Fisica del Plasma - Consiglio Nazionale delle Ricerche IFP-CNR (Milan), several spherical loads were developed, all with identical absorbing geometry but different heat removal systems for operations with pulselengths (PLs) from a few milliseconds to continuous wave (CW). A dummy load designed for 2-MW CW 170-GHz operations has been constructed in the frame of the development program of the European gyrotron for ITER. The first prototype hemisphere of this load, joined with an uncoated one, has been successfully tested with long pulses and under vacuum using the gyrotron for ITER of the National Institutes for Quantum and Radiological Science and Technology in Naka. The symmetry of the system is such that the equivalent power (EP) load of the absorbing (coated) hemisphere is doubled when the second one is reflective (uncoated), with an almost unchanged distribution with respect to a fully coated load. The tests demonstrate the capability of the IFP-CNR load to withstand and measure radiation at an EP in an excess of 1.5MW. Experiments involving gyrotron pulses with increasing power and PL have been performed in the load and are reported. The maximum EP load reached in the tests was $\approx 1.8$ MW for 15 s while the longest pulse was 300 s, at $\approx 1$ MW. During the experiments, the temperatures of either the load or the preload were monitored using thermocouples and multiple infrared cameras, while a pressure sensor and a hydrophone have been used for the monitoring of cavitation phenomena and water boiling in the cooling circuits. The results achieved in the tests are described in this paper.

The future EC systems will consist of several gyrotron sources providing MW-level millimeter wave power at a frequency around or above 170 GHz. The development of matched loads is necessary to test the new sources, the components for the transmission lines and the launchers, and must ensure high qualification for compatibility with the nuclear environment. The development at IFP-CNR and LTC of several compact high-power prototypes during the last decade led to a refinement of the overall design, resulting in the present low-reflectivity vacuum-compatible loads. The activity on loads is supported by F4E in view of the development of the EU gyrotron for ITER and required high-power tests at QST (Naka, Japan). Qualification is now supported by new tests at SPC (Lausanne, Switzerland) using the FALCON test-bed designed to test components and the EU gyrotron prototype for the EC system of ITER. These high-power tests, performed on the first CW prototype load (provided with 16 + 16 cooling channels in parallel) and shown in this paper, highlighted the need to improve the mechanical, vacuum and hydraulic design to reach the final goal of 1000s at ∼1 MW.

Background. Vonoprazan affords more clinical benefits than proton pump inhibitors (PPIs) during the healing of gastroduodenal ulcers. However, it remains controversial whether vonoprazan is more effective than PPIs when used to heal artificial ulcers arising after endoscopic submucosal dissection (ESD). Aim. This study investigated the effects of vonoprazan compared with esomeprazole on the healing of post-ESD artificial ulcers. Methods. Sixty patients who underwent gastric ESD between May 2015 and May 2017 were randomized to treatment with vonoprazan (V group) or esomeprazole (E group) for 8 weeks. Upper endoscopy was performed at 4 and 8 weeks after ESD, and drug effects were estimated based on the ulcer healing rates and shrinkage rates. Results. Fifty-three patients were analyzed. The respective 4- and 8-week ulcer healing rates did not differ significantly between V and E groups (8.0 versus 11.5%, P=0.669; 88.9 versus 84.6%, P=0.420). Similarly, the respective 4- and 8-week ulcer shrinkage rates did not differ significantly between V and E groups (96.8 versus 97.5%, P=0.656; 100 versus 100%, P=0.257). Conclusion. The healing of artificial ulcers after ESD did not differ using vonoprazan or esomeprazole. Both vonoprazan and esomeprazole were effective when used to promote artificial ulcer healing after ESD.

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.

Recent progress of QST high power high frequency gyrotrons is reported. Main motivation of the high power gyrotron is the application to the plasma heating and current drive and control of the magnetic confinement fusion devices. Using a multi-frequency operation, 1 MW Gaussian beam output was obtained at four frequencies; 203 GHz (TE37,13), 170 GHz (TE31,11), 137 GHz (TE25,9), 104 GHz (TE19,7). In particular, the achievement of 203 GHz 1 MW output at 50 % will give a strong impact on the DEMO (Demonstration power plant) design. In parallel, a high frequency oscillation up to 300 GHz was studied using a short pulse gyrotron under collaboration with University of Tsukuba. In this gyrotron, 300 GHz/ 0.6 MW (TE 31 , 18 ), 240 GHz/ 0.35 MW (TE 28 , 14 ) oscillations were demonstrated.

This paper presents a summary of recent progress pertaining to the manufacturing and inspection of ITER gyrotrons and the operation system in QST. Major achievements are as follows. (i) The final design of the ITER gyrotron was accomplished and manufacturing of two ITER gyrotrons was completed. Then operation test in QST prior the shipment to ITER has started with ITER relevant high voltage power supply configuration. The 1st ITER gyrotron has achieved 1.05 MW operation for 300 s with 51% efficiency. Measured cooling channel waveform of 300 s pulse demonstrated thermally stable condition representing sufficient cooling performance for 1 MW CW operation; (ii) 5kHz modulation operation was demonstrated up to 200 s at with agt;0.8 MW at flat top of pulses; (iii) 300 s / 1 MW operation was repeated for 20 shots with successful 19 shots which demonstrating agt;95% of reliability requirement. These results lead to success of ITER EC HaCD system construction and commissioning toward first plasma.

We describe a (1)H polarization enhancement via dynamic nuclear polarization (DNP) at very low sample temperature T≈30 K under magic-angle spinning (MAS) conditions for sensitivity-enhanced solid-state NMR measurement. Experiments were conducted at a high external field strength of 14.1 T. For MAS DNP experiments at T90 K, a new probe system using cold helium gas for both sample-cooling and -spinning was developed. The novel system can sustain a low sample temperature between 30 and 90K for a period of time10 h under MAS at ν(R)≈3 kHz with liquid He consumption of ≈6 L/h. As a microwave source, we employed a high-power, continuously frequency-tunable gyrotron. At T≈34 K, (1)H DNP enhancement factors of 47 and 23 were observed with and without MAS, respectively. On the basis of these observations, a discussion on the total NMR sensitivity that takes into account the effect of sample temperature and external field strength used in DNP experiments is presented. It was determined that the use of low sample temperature and high external field is generally rewarding for the total sensitivity, in spite of the slower polarization buildup at lower temperature and lower DNP efficiency at higher field. These findings highlight the potential of the current continuous-wave DNP technique also at very high field conditions suitable to analyze large and complex systems, such as biological macromolecules.

In this paper we present results that demonstrate the utility of a continuously frequency-tunable 0.4 THz-gyrotron in a dynamic nuclear polarization (DNP)-enhanced solid-state NMR (SSNMR) spectroscopy at one of the highest magnetic fields, B0 = 14.1 T (600 MHz for 1H Larmor frequency). Our gyrotron called FU CW VI generates sub-mm wave at a frequency near 0.4 THz with an output power of 4–25 W and a tunability over a range of more than 1 GHz by sweeping the magnetic field at the gyrotron cavity. We observed overall down shifting of the central frequency by up to ~1 GHz at high radiation duty factors and beam current, presumably due to the cavity thermal expansion by a heating, but the tunable range was not significantly changed. The frequency tunability facilitated the optimization of the DNP resonance condition without time-consuming field-sweep of the high-resolution NMR magnet, and enabled us to observe substantial enhancement of the SSNMR signal (eDNP = 12 at 90 K).

Trials of steady-state operation (SSO) in the Large Helical Device (LHD) were started when a continuous wave (cw) gyrotron with the output power up to 0.2 MW was introduced to the electron cyclotro...

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)

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.

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.

Electrode biasing experiments under electrode current control were carried out in the Tohoku University Heliac and the Compact Helical System to examine the role of an ion viscosity on a transition to a high-confinement regime and to investigate the dependence of the ion viscosity on magnetic structure. Observations included (a) an increase of electron density, (b) an increase of electron stored energy, (c) a formation of the steep gradient of electron density, and (d) a formation of a negative electric field in both devices during electrode biasing negatively. The dependence of the ion viscosity normalized by the ion pressure on the poloidal Mach number qualitatively agreed with the neoclassical theory based on the Shaing model. This result supported the transition mechanism of the neoclassical theory based on ion viscosity, which advocates that the transition to a high-confinement mode is the bifurcation phenomenon resulting from the existence of local maximum in ion viscosity.

Oscillation at tri-frequency 170 GHz, 137GHz and 104GHz with more than 1MW were demonstrated with a high-order mode (TE 31,11 ) gyrotron developed for ITER. The successful modification of the ITER equatorial launcher design to enhance off-axis current drive performance and to ensures that mm wave beams from both middle and bottom beam row pass through the same BSM opening that leads to the increase of neutron shielding potential is performed.

High-power, long-pulse operations of a gyrotron for JT-60SA (Super-Advanced) have been carried out at 110 GHz (1 MW/100 s) and 138 GHz (1 MW/100 s). These results fully satisfied the requirements of the electron cyclotron heating and current drive (ECH/CD) system in JT-60SA. It was experimentally shown that the higher power operation at each frequency is expected to be acceptable for this gyrotron from the viewpoint of heat load at the cavity resonator, collector, and stray radiation absorbers. An oscillation of 1 MW for 1 s at 82 GHz has been demonstrated as an additional frequency of the same gyrotron. Experiments toward 1.5 MW or higher at 110 GHz and 138GHz are ongoing.

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.

For the Electron cyclotron resonance heating and current drive (ECH&CD) system on fusion DEMO reactor, the high power mm-wave of 200 GHz ∼300 GHz is expected. To increase the gyrotron frequency, extremely high-order oscillation mode should be adopted to suppress the Ohmic loss on the resonator wall less than ∼20 MW/m2. Here, we designed and fabricated a 300 GHz gyrotron of TE 32,18 mode oscillation and started a short pulse experiment to investigate the oscillation characteristics of the high order mode at ∼300 GHz. As a preliminary result, we demonstrated the power generation of ∼0.5 MW at ∼300 GHz oscillation at the oscillation mode of TE 32,18 , and other modes.

In JAEA, development of high power gyrotron is underway. One activity is a 1 MW/170 GHz gyrotron development for ITER application and another is a dual frequency (138GHz/110GHz) gyrotron for JT-60U. The advanced mode TE 31,11 was adopted for high power generation and multi-frequency operation. Up to now, 1.24 MW/2sec was obtained with DC input of 50 kV×55A, which is available by ITER power supply. In the multi-frequency operation, 1MW oscillation was obtained at three frequencies: 170 GHz, 137 GHz and 104 GHz. And, high efficiency long distance power transmission from the gyrotron window to the end of the 40 m transmission line were demonstrated: 91 %, 89 % and 84 %, respectively. On the other hand, in the dual frequency gyrotron development, the power output of 1 MW/100 s, which is the target of the JT-60SA, were achieved for both frequencies (138 GHz/110 GHz).

The diamond window assembly is part of the ITER primary vacuum boundary and acts as the first tritium barrier and therefore it is classified as Safety/Protection Important Component (SIC/PIC). It consists of an ultra-low loss CVD diamond disk mounted in a system of metallic parts (copper/steel) and has to fulfil adequate transmission capability for high power mm-waves. High power RF experiments with a 1st window prototype had shown parasitic heating due to small gaps in the housing. After a design optimization directed to the mm-wave properties, the parasitic excitations of oscillations have been avoided in a 2nd prototype. This one is equipped with inserted waveguide structures, which cover gaps in the metallic structure of the window housing. From high power RF-measurements with a 0.86 MW/100 s pulse a loss tangent of 7.1 × 10−6 could be estimated, corresponding to an increase of temperature of only 120 mK between inlet and outlet of the cooling system. The diamond window assemblies cannot be entirely covered by codes and standards. To comply with the French safety regulations, instead an ad-hoc qualification programme is required, being developed in the framework of a contract between KIT and F4E. A new prototype (3rd) will be built, which is designed to fit to the single HELICOFLEX sealed waveguide structures of the ex-vessel mm-system of the EC upper launcher (UL). The testing programme ranges from mechanical to vacuum tests up to dielectric loss measurements at low and high power. A clear definition of the testing requirements and of the acceptance criteria is necessary as well as a complete documentation of the process.

The development of high power gyrotrons needs matched loads with good performance capable of absorbing and measuring powers as high as 1 MW in ITER and higher (up to two times) in future devices like DEMO. At IFP-CNR (Milano), several spherical loads were developed with identical absorbing geometry but different heat removal systems for operation with pulse lengths from a few ms to CW. A dummy load designed for 170 GHz/2 MW has been constructed in the frame of the development programme of European gyrotron for ITER. The first prototype hemisphere of this load, joined with an uncoated one, has been successfully tested with long pulses and under vacuum using the JAEA gyrotron for ITER in Naka. The symmetry of the system is such that the equivalent power load of the absorbing (coated) hemisphere is doubled when the second one is reflective (uncoated), with almost unchanged distribution with respect to a fully coated load. The tests demonstrate the capability of the IFP-CNR load to withstand and measure radiation at an equivalent power in excess of 1.5 MW. Gyrotron pulses with increasing power and pulse length have been performed in the load and are reported. The maximum equivalent power load reached in the tests was ∼ 1.8 MW for 15 s while the longest pulse was 300 s, at ∼ 1 MW. During the experiments, the temperatures of either the load or the preload were monitored using thermocouples and multiple infrared cameras. The results of the tests are described in the paper.

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.

An initial test of a multi-frequency gyrotron with 170GHz, 137GHz and 104 GHz is successfully performed in JAEA RF gyrotron test stand. More than 700 kW output power is achieved for all frequencies. A test of a prototype power supply for ITER is started. The feature of the power supply is a double anode switch for the 5 kHz modulation with a DC Anode Power Supply and a DC Body Power Supply. The initial gyrotron oscillation test by using the power supply is successfully finished.

Development of two 460 GHz second harmonic gyrotrons for 700 MHz DNP-NMR spectroscopy is presented. One gyrotron is frequency-fixed and the other is frequency tunable in the bandwidth of 1.5 GHz. In addition, the former has a function of frequency modulation.

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.

Recent progress of JAEA high power long pulse gyrotron is reported. Multi-frequency oscillations and their Gaussian beam outputs are demonstrated at 170 GHz, 137 GHz and 104 GHz. For the full power modulation, clear 5kHz modulation was demonstrated using double anode switching. Furthermore, novel anode power supply system was fabricated to simplify the gyrotron power supply system.

A Broadband frequency tunable gyrotron operating at fundamental resonance for 600 MHz DNP-NMR has been developed. A cavity resonator and a magnetron injection gun are newly designed. In the preliminary experiments, the continuously frequency tuning bandwidth was 1.3 GHz and the output power was increased up to 100 W.

Beam current of gyrotron was stabilized from 17 % to 0.5% by the feedback control to adjust the heater voltage of electron gun. The fluctuation level of output power was decreased from 15 % to 5 %. More effective stabilization of the output power up to 2% was attained by feedback control to adjust heater voltage.

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.

Gyrotron FU CW CI is developed as one of sub-THz gyrotrons included in the FU CW 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 40 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 are presented.

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.

Continuously frequency tunable gyrotron, FU CW VIB, which is of sealed-off type has been developed for 600 MHz DNP-NMR at FIR FU. The frequency tuning bandwidth reached 1.4 GHz. The output power was ranged from 15 to 130 W.

Dynamic mode interaction between fundamental and second-harmonic modes has been observed in high-power sub-terahertz gyrotrons [T. Notake et al., Phys. Rev. Lett. 103, 225002 (2009); T. Saito et al. Phys. Plasmas 19, 063106 (2012)]. Interaction takes place between a parasitic fundamental or first-harmonic (FH) mode and an operating second-harmonic (SH) mode, as well as among SH modes. In particular, nonlinear excitation of the parasitic FH mode in the hard self-excitation regime with assistance of a SH mode in the soft self-excitation regime was clearly observed. Moreover, both cases of stable two-mode oscillation and oscillation of the FH mode only were observed. These observations and theoretical analyses of the dynamic behavior of the mode interaction verify the nonlinear hard self-excitation of the FH mode.

A 300 GHz pulsed gyrotron with a power exceeding 100 kW has been under development. This paper describes the design optimization of a magnetron injection gun for this gyrotron. An optimum geometry of the electrodes is investigated to provide a laminar electron flow with a small velocity spread.