K. Hayashi

Development of 170 GHz/500 kW gyrotron

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.

Stable, single-mode oscillation with high-order volume mode at 1 MW, 170 GHz gyrotron

A stable, 1.13 MW single-mode oscillation was obtained at 170 GHz in a short-pulse gyrotron with a high-order volume mode TE 31,8 . The maximum efficiency was 30%, and no power degradation due to mode competition was observed. This result indicates the potential for the development of a 170 GHz, 1 MW, CW gyrotron which is required for electron cyclotron heating and current drive of large fusion devices.

Wave-beam shaping using multiple phase-correction mirrors

This paper describes a scheme for shaping a given wave beam into the desired profile using multiple phase-correction mirrors. This mirror system was applied to a gyrotron internal converter to flatten the radiated beam profile at the window. The flat output beam is reconverted into HE11 mode, a basic propagation mode for corrugated waveguides, by another pair of phase-correction mirrors for transmission into a fusion reactor. In addition, a wave-beam splitting and combining technique is also presented.

The design of a tapered dimple-type mode converter/launcher for high-power gyrotrons

This paper presents the design method for a gyrotron-dimple-type mode converter that is axially up-tapered to prevent inside oscillation. In this up-tapered converter, the propagation constant of the traveling TE modes varies axially since the mean radius grows in the axial direction, requiring a new formulation of the mode coupling in the converter. In this paper, the axial variation of the helix pitch that describes the TE mode propagation is discussed first. Then, the coupling coefficients between TE modes are analytically derived, with which a trial design of a linearly tapered converter is made for a 170-GHz gyrotron.

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.

Development of 10 MW L-Band Multi-Beam Klystron (MBK) for european X-FEL project

A 10 MW L-band multi-beam klystron (MBK) has been developed and tested by Toshiba, Japan for the European XFEL and a future linear collider projects. The Toshiba MBK has six low-perveance beams operated at low voltage of 115 kV (for 10 MW) and six ring-shaped cavities to enable a higher efficiency than a single-beam klystron for a similar power. After the successful acceptance testing at the Toshiba Nasu factory in March 2006, attended by DESY staff, the final acceptance test was done at DESY laboratory in June 2006. In these tests, the output power of 10.2 MW, more than the design goal (10 MW), has been demonstrated at the standard beam voltage of 115 kV at the RF pulse length of 1.5 ms and the beam pulse of 1.7 ms at 10 Hz. The efficiency was 66%. The robustness of the tube was also demonstrated by being operated continuously more than 24 hours above 10 MW. Total time of operation on the test stand at DESY already exceeds 750 hours (up to date February 4, 2007). A horizontal version of the Toshiba MBK is now under construction.

Experimental coupling efficiency of shaping mirrors matching a 168-GHz gyrotron output wave to the HE/sub 11/ mode

This paper presents an experiment in which the phase- and amplitude-flattened output of a 168 GHz gyrotron was converted into the HE/sub 11/ mode, the basic transmitting mode in corrugated waveguides, by means of an external matching box (MBOX) comprising two curved-surface mirrors. In estimating the coupling efficiency between the gyrotron output wave and the HE/sub 11/ mode, an improved method was proposed in which the reconstructed phase of the gyrotron output wave at four distances are averaged. From the phase reconstruction with averaging, it was found that 76% of the gyrotron output coupled into the HE/sub 11/ mode in a corrugated waveguide, while a coupling efficiency of 85% was calculated for an ideal gyrotron output. A detailed discussion on the MBOX performance as well as the accuracy of phase reconstruction shows that this low coupling efficiency is due to the nonideality of the actual gyrotron output and that designing the MBOX mirrors based on the actual measurement at the gyrotron window can improve the coupling efficiency.

Development of 10MW L-Band MBK for European X-FEL Project

A 10MW L-band Multi-Beam Klystron (MBK) has been developed and tested by Toshiba, Japan for the European XFEL and a future linear collider projects. The Toshiba MBK has six low-perveance beams operated at low voltage of 115 kV (for 10MW) and six ring-shaped cavities to enable a higher efficiency than a single-beam klystron for a similar power. After the successful acceptance testing at the Toshiba Nasu factory in March 2006, attended by a DESY stuff, the final acceptance test was done at DESY laboratory in June 2006. In these tests, the output power of 10.2MW, more than the design goal (10MW), has been demonstrated at the standard beam voltage of 115 kV at the RF pulse length of 1.5 ms and the beam pulse of 1.7 ms at 10 Hz. The efficiency was 66%. The robustness of the tube was also demonstrated by being operated continuously more than 24 hours above 10MW. Total time of operation on the test stand at DESY already exceeds 750 hours (up to date February 4, 2007). A horizontal version of the Toshiba MBK is now under construction.

Design of a 1-MW, CW coaxial gyrotron with two Gaussian beam outputs

The design of a 170 GHz, 1 MW-CW gyrotron for electron cyclotron heating of nuclear fusion plasmas is presented. The designed gyrotron incorporates a coaxial cavity to reduce mode competition, and a coaxial electron gun to support the cavity inner conductor. A new mode converter splits the generated wave into two beams and radiates them in different directions. The radiated beams are transmitted to two output windows through two mirror systems, being transformed into Gaussian-like beams. A single-stage depressed collector improves the overall efficiency of the gyrotron and reduces the heat flux to the collector surface.