Takahiro Shinya

Accurate reflectivity data of millimeter wave absorber materials are essential for predicting heat-flux distributions and optimizing the design of dummy loads (DLs) used in electron cyclotron resonance heating (ECRH) systems. However, conventional millimeter wave reflectivity measurements typically exhibit uncertainties of approximately ±5%, which are insufficient for reliable design. In this study, a high-accuracy reflectivity measurement was developed and applied to thermally sprayed TiO2 coatings. Measurements were performed at 110 GHz, corresponding to the operating frequency of the JT-60SA ECRH system. To improve measurement accuracy, an HE11-mode launcher was employed to suppress beam divergence during free-space propagation, and a two-dimensional scanning measurement was performed to capture the full reflected-wave intensity distribution. In addition, a reference-based calibration method using a copper reflector eliminated the need for direct estimation of the incident wave intensity and reduced the influence of source-power fluctuations. Reflectivities were measured for incident angles between 30° and 65° under both S- and P-polarized conditions, achieving an uncertainty of less than ±0.8%. The measured data were successfully approximated using a Fresnel-based model with a single fitting parameter. The resulting reflectivities were implemented in ray-tracing and thermal analyses and validated through comparisons with infrared-camera and calorimetric measurements obtained during 500 kW high-power experiments. Good agreement was obtained for temperature-rise distributions, temporal temperature evolution, and absorbed-power fractions. These results demonstrate that the proposed method provides reliable reflectivity data for predicting heat-flux distributions and power-absorption characteristics and serves as a powerful tool for the design and optimization of low-reflection DLs.

Gyrotrons having a high output-frequency of more than 200 GHz will be necessary for future fusion devices, such as the DEMO reactor. At 170 GHz, gyrotrons for ITER achieved a high power of 1 MW with a high electrical efficiency of approximately 50% (oscillation efficiency of approximately 30%) in a long pulse operation. At frequencies higher than 200 GHz, no gyrotrons have achieved ITER-like performance, which this study attempted to demonstrate using a prototype multi-frequency gyrotron for 170 GHz, 203 GHz, and 236 GHz. At 203 GHz, an output power of 0.87 MW was achieved at the gyrotron window with an oscillation efficiency of 25% and an electrical efficiency of 41% in a long pulse operation of 100 s. Although this performance is lower than that of the ITER gyrotrons at 170 GHz, this study represents the first demonstration that ITER-like performance is achievable at frequencies above 200 GHz. In addition, short 1 ms pulse tests at 236 GHz achieved an output power of 1 MW.

High-power (1 MW), long-pulse operations up to 300 s were demonstrated by multi-frequency oscillations at 170 GHz, 137 GHz, and 104 GHz. A multi-frequency gyrotron based on the design of the ITER gyrotron has been developed and studied. By redesigning the built-in mode convertor and internal mirrors of the ITER gyrotron, the internal scattered power of the gyrotron was reduced while maintaining the same output beam size and beam direction for the three frequencies. In addition, the mirrors of a matching optics unit were designed to efficiently couple the RF beam at 170 GHz, 137 GHz, and 104 GHz oscillations to a waveguide 50 mm in diameter, the same size that will be used in ITER. An HE ₁₁ mode content of ∼94% was achieved for the three frequencies. Output powers of 1 MW at 170 GHz and 137 GHz, and 0.9 MW at 104 GHz were demonstrated up to 300 s. These oscillation frequencies can facilitate in a wide range of toroidal magnetic field operations at ITER, including plasma start-up and electron heating and current drive.

Design, development, manufacturing, and test activities of the RF power system (RFPS) for Linear IFMIF Prototype Accelerator (LIPAc) were completed in Europe. Installation and commissioning activities were carried out at the International Fusion Materials Irradiation Facility-Engineering Validation and Engineering Design Activities (IFMIF/EVEDA) site in Rokkasho, Japan. Challenging IFMIF requirements led to a number of innovations during design and development. Commissioning required a major effort on calibration and fine setting and led to development of new functionalities. The RFPS was validated under full power and 125-mA deuteron beam loading conditions, in the radio frequency quadrupole (RFQ), demonstrating good performance. This article is an overview about how technical challenges impacted the prototype RFPS design and its functional evolution during commissioning, cavity conditioning, and beam operation of the RFQ.

The eight-chain RF system and radio frequency quadrupole (RFQ) linac were successfully integrated at the facility of the Linear IFMIF Prototype Accelerator (LIPAc). We achieved an unprecedented high current deuteron beam up to 55 mA in pulse operation (300 μs, 1 Hz). The keys of the success were (1) understanding reflected power from RFQ to RF system, (2) tuning each RF chain individually, (3) intensive RF conditioning for holding the stable RF power in the RFQ, and (4) optimizing the beam loading compensation control. Interestingly, the reflection at the breakdown in RF conditioning was higher than an estimation and reached up to 600 kW so far. In order to avoid damage on tetrodes by the high reflection, the impedance of each circulator was accurately adjusted.