Yasuyuki Itoh

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.

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.

Axisymmetric optical system for quasi-optical gyrotron

An analysis is carried out with a profile of hollow Gaussian radiation beam propagating in an optical system composed of coaxial annular curved mirrors. The result will be a useful tool in designing an axisymmetric quasi-optical gyrotron oscillator (ASQUOTRON).

Quasi-optical waveguide-mode converters using a pair of phase correction mirrors

This paper describes a quasi-optical method for the conversion of modes transmitted through highly oversized circular waveguides. A waveguide-mode is radiated once from a waveguide cut in the form of a radiation beam, which is then properly shaped by two curved mirrors and directed back into the waveguide. The curved mirror shapes are iteratively and automatically determined for given propagation distances using the design technique for phase correction mirrors. The proposed method gives favorable results in designing a waveguide expander/reducer, a TE01-TE02 mode converter, and a TE01-HE11 mode converter.

AXISYMMETRIC QUASI-OPTICAL GYROTRON OSCILLATOR

An axisymmetric quasi-optical gyrotron (ASQUOTRON) is considered to realize a 10 MW, 150 GHz, CW oscillator required for an electron cyclotron resonance heating of a fusion plasma. The gyrotron has an axisymmetric mirror to be used as its optical cavity. It is shown that the axisymmetric mirror of relatively small radius (∼20 cm) can be used in producing the 10 MW continuous wave with a tolerable mirror heat load (∼0.5 kW/cm2). Considerations are also made on wave transmissions through the mirror and to a target.

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.

Experimental study of a quasi-optical cavity with slotted output mirror

This paper describes an experimental study of a quasi-optical cavity to be used in qyrotrons. The cavity configuration was designed so that its output wave could pass into a quasi-optical transmission line. The test cavity was coaxially composed of a pair of annular mirrors, one of which was a curved conical mirror and the other was a plane mirror having a radial slot array for wave output. A mode excited in the cavity at 56.4 GHz was observed by detecting the radiation wave that leaked out through the slotted mirror. It was shown that the present slotted mirror served as a partially and uniformly transparent mirror to radiate the output wave with the same mode as in the cavity.

A 120 GHz Axisymmetric Quasi Optical Gyrotron

An axisymmetric quasi-optical gyrotron was designed, constructed, and operated. Its cavity resonator was composed of a conical mirror and a plane slotted mirror. The observed oscillation frequencies were in a range from 114 to 123 GHz. Output waves were transmitted quasi-optically to a water load. The measured output power was more than 20 kW at a frequency of 120 GHz.

Termination of Plasma Discharge by High-Z Liquid Impurity Jet Injection in a Tokamak Reactor

Abstract A feasibility study is presented of fast tokamak plasma terminations by means of high-Z impurity liquid jet injections in order to reduce the technological requirements of such terminations. The calculation was carried out by combining models described for the jet ablation and the current termination and taking into account the ionization of the jet material exposed to generated runaway electrons. The liquid jet was assumed to fragment and thus to deposit more massive impurity ions in the plasma. Although argon or krypton jet injection generates the runaway electron current, it decays in several hundred milliseconds with ionization of the residual jet material. These high-Z impurity jet injections would also be applicable for terminating or reducing the runaway electron current tails generated by major plasma disruptions.