T. Kariya

High power 170 GHz gyrotron with synthetic diamond window

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...

Development of 170 and 110 GHz gyrotrons for fusion devices

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.

High-Efficiency Oscillation of 170 GHz High-Power Gyrotron at TE31,8 Mode Using Depressed Collector

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.

1 MW and long pulse operation of Gaussian beam output gyrotron with CVD diamond window for fusion devices

Abstract A 110 GHz-Gaussian beam output gyrotron with chemical vapor deposition (CVD) diamond window was developed for electron cyclotron heating and current drive on JT-60U. A stable Gaussian output beam power of 1.0 MW for 2 s was obtained with depressed collector operation. The temperature at the center of the diamond window was stabilized at the Δ T ∼25 K. Gaussian beam output from the gyrotron remarkably improved the coupling efficiency to the HE 11 mode in the transmission waveguide. 94% of the gyrotron output power was coupled to the corrugated waveguide of 31.75 mm in diameter, via a matching optics unit with two mirrors. A combination of the Gaussian output and the diamond window are indispensable for high power gyrotron operation at more than 1 MW and efficient coupling to the transmission line.

Design and tests of 168-GHz, 500-kW gyrotrons and power supply system

Abstract 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.

High-power and long-pulse gyrotron development in JAERI

Abstract 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.

Upgrade Program of ECRH System for GAMMA 10

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 GAMMA10. The first gyrotorn 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 Φc = 3kV 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.

High power operation of Gaussian beam gyrotron with CVD diamond window for JT-60U

A 110 GHz-Gaussian beam output gyrotron with CVD (Chemical Vapor Deposition) diamond window was developed for the electron cyclotron heating and current drive on JT-60U. A stable Gaussian output beam power of 1.1 MW-0.1 s was obtained with the efficiency of 39% with depressed collector operation. Due to Gaussian beam output from the gyrotron, the coupling efficiency to HE11 mode in the mirror optical unit was 94%. Moreover, the transmission efficiency of 89% for HE11 mode was performed at the power level of 1 MW on the 40 m-transmission test line of φ31.751 mm corrugated waveguide and 8-miter bends included 1-pair of polarizer. The gyrotron and transmission line were installed into JT-60U system and 0.6 MW-0.3 s power was successfully injected into JT-60U plasmas as an initial operation.

Experimental study of high-power millimeter wave transmission of sapphire window for electron cyclotron range of frequencies system

The possibility of use of a sapphire disk as an output window of a high-power and long-pulse gyrotron was investigated using an RF test facility with high-power gyrotrons. The dielectric loss tangents, tan δ=1.8×10-4 (T/300)1.3±0.1 for 110 GHz and tan δ=2.6×10-4 (T/300)1.3±0.1 for 170 GHz (T: absolute temperature in K), were obtained in the temperature range of 300 K to 600 K. The experimental results showed that RF reflection from the window became large due to the permittivity change with increasing temperature, which limits the applicable power for the sapphire window rather than the thermal runaway. Moreover, it was found that a 170 GHz, 1 MW, CW gyrotron is feasible using a two-port window system consisting of sapphire double disks more than 18 cm in diameter with a flat RF power profile.

Experimental study of second harmonic electron cyclotron resonance heating in an axisymmetric end mirror or a tandem mirror

Second harmonic electron cyclotron resonance heating has been used for the production of hot electrons in an axisymmetric end mirror of the GAMMA 10 tandem mirror. Heating power is injected in the linearly polarized extraordinary mode into a limited area of the mirror field for the purpose of localized heating and temperature control. A ray tracing code and a Fokker-Planck code are used for a preparatory consideration of the heating configuration as well as for an interpretation of the experimental results. The path of the injected power flow is analysed with a simple model and the localized heating is confirmed. The electron temperature is controlled and the plasma in which the hot electrons form a substantial fraction of the total density (up to 0.8) is maintained macroscopically stable. The hot electron production rate is found to depend on the magnetic field strength, the target plasma density, the gas feed rate and the heating power. Several key factors that are important for successful results to be achieved are found. Experimental observations indicate that, essentially, the concept of quasi-linear diffusion of electrons in velocity space holds. It is also found that warm electrons play an important role in microwave absorption as well as in efficient hot electron production.