Ryutaro Minami

Handling TecHnology of Mega-WaTT MilliMeTer-Waves for opTiMized HeaTing of fusion plasMas

Millimeter-wave components were re-examined for high power (Mega-Watt) and steady-state (greater than one hour) operation. Some millimeter-wave components, including waveguide joints, vacuum pumping sections, power monitors, sliding waveguides, and injection windows, have been improved for high power CW (Continuous Waves) transmission. To improve transmission efficiency, information about the wave phase and mode content of high power millimeter-waves propagating in corrugated waveguides, which are difficult to measure directly, were obtained by a newly developed method based on retrieved phase information. To optimize the plasma heating efficiency, a proof-of-principle study of the injection polarization feedback control was performed in the low power test stand.

Development of 28GHz and 77GHz 1MW Gyrotron for ECRH of Magnetically Confined Plasma

Abstract We are developing a new 28GHz 1MW and a 77GHz 1MW gyrotron for ECRH system of tandem mirror GAMMA10 and Large Helical Device (LHD), respectively. The detail design study of 28GHz 1MW gyrotron such as cavity, magnetron injection gun (MIG) has been done. We obtained the oscillation power of 1.37MW and the oscillation efficiency of 42.7% with the pitch factor of 1.2. Two 77GHz 1MW gyrotrons have been fabricated and tested. The maximum output power of 1.1MW was obtained. The pulse width with 0.46MW extended to 5s with the short aging time of only 65 hours. A plasma injection for LHD with MOU output of 0.81MW 3.6s was performed.

Development of multi-purpose MW gyrotrons for fusion devices

The latest development achievements in the University of Tsukuba of over-1xa0MW power level gyrotrons required in present-day fusion devices, GAMMA 10, Large Helical Device (LHD), QUEST, Heliotron J and NSTX, are presented. The obtained maximum outputs are 1.9xa0MW for 0.1xa0s on the 77xa0GHz LHD tube and 1.2xa0MW for 1xa0ms on the 28xa0GHz GAMMA 10 one, which are new records in these frequency ranges. In long-pulse operation, 0.3xa0MW for 40xa0min at 77xa0GHz and 0.54xa0MW for 2xa0s at 28xa0GHz are achieved. A new programme of 154xa0GHz 1xa0MW development has started for high-density plasma heating in LHD. On the first 154xa0GHz tube, 1.0xa0MW for 1xa0s is achieved. As a next activity of the 28xa0GHz gyrotron, an over-1.5xa0MW gyrotron is designed and fabricated to study the multi-MW oscillation. The possibility of 0.4xa0MW continuous wave and 2xa0MW level output in operations of a few seconds, after the improvements of output window and mode converter, is shown. Moreover, a new design study of dual-frequency gyrotron at 28 and 35xa0GHz has started, which indicates the practicability of the multi-purpose gyrotron.

Extension of high Te regime with upgraded electron cyclotron resonance heating system in the Large Helical Device

Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1u2009MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5u2009s, 0.5u2009MW/CW) was newly installed in 2012, and the total injection power of Electron cyclotron resonance heating (ECRH) reached 4.6u2009MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature of 13.5u2009keV with the line-averaged electron density ne_firu2009=u20091u2009×u20091019 m−3. The electron thermal confinement clearly improved inside the electron internal transport barrier, and the electron thermal diffusivity reached neoclassical level. The global energy confinement time increased with increase of ne_fir. The plasma stored energy of 530u2009kJ with ne_firu2009=u20093.2u2009×u20091019 m−3, which is 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved.

Study of a Miter Bend Polarizer for Central ECRH on GAMMA-10

Abstract The new polarizers with grooved mirrors were installed in ECRH system on the GAMMA 10 to get the optimal polarized wave. Numeric calculation and low power performance test of this pair of polarizers was agreed well. And it is found that the setting error of polarizer angle within +/- 5° gives negligible reduction of the X-mode ratio less than 1%. On the experiment in the GAMMA 10, it is shown that the fraction of X-mode has key role to heat plasma efficiently.

High power transmission performance of millimeter wave remote steering launcher

High power transmission experiments with a millimeter wave remote steering launcher, which consists of a square corrugated waveguide and a copper steering mirror at the inlet of the waveguide, were carried out to study the steering capability and the high power transmission performance as a millimeter wave antenna for plasma heating in nuclear fusion application. A Gaussian-like beam was radiated from the outlet of the launcher waveguide at 0.85 MW and with more than 95% efficiency over the steering angle range -12/spl deg/ to 12/spl deg/. The wave frequency was 170 GHz. The measured heat distribution along the waveguide length agreed with calculations.

Measurement of effect of electron cyclotron heating in a tandem mirror plasma using a semiconductor detector array and an electrostatic energy analyzer

Temporally and spatially resolved soft x-ray and end-loss-electron analyses of the electron cyclotron heated plasmas are carried out by using a semiconductor detector array and an electrostatic energy analyzer in the GAMMA 10 tandem mirror. The flux and the energy spectrum of the end loss electrons are measured by a multi-grid energy analyzer. Recently, the electron cyclotron heating power modulation experiments have been started in order to generate and control the high heat flux and to make the edge localized mode-like intermittent heat load pattern for the divertor simulation studies by the use of these detectors for electron properties.

Code development for the calculations of time-dependent multimode oscillations in the cavity of the future high-power gyrotrons

A computational code for the electromagnetic field in the cavity of high-power gyrotrons is developed. The gyrotron mechanism is described by a self-consistent set of equations taking into account the dependence of the axial structure of microwave oscillations on the relativistic electron beam in the cavity. The parameters of the electron beam are calculated using the EGUN electron trajectory code, and the effects of the space charge and the velocity spread of the electron beams are considered. The computational code includes a time-dependent description of the electromagnetic field and a self-consistent analysis of the electron beams. The equations are solved using a second-order predictor-corrector scheme. The calculations of the present study are carried out for a 1-MW gyrotron developed for the GAMMA10/PDX tandem mirror (80u2005kV, operating mode: TE94 mode, 35.45u2005GHz). The calculations reveal that, using a triode-type electron gun, stable excitation of the desired mode can be realized in standard startup...

Study of sub-terahertz high power gyrotron for ECH&CD system of DEMO

Summary form only given. 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/m<;sup>2<;/sup>. Here, we designed and fabricated a 300 GHz gyrotron of TE<;sub>32,18<;/sub> 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<;sub>32,18<;/sub>, and other modes. A height of the test gyrotron is ~2 m. The electron gun is diode type magnetron injection gun (MIG), to maximize the diameter of an electron emitter (d=74 mm). The resonator is a conventional open cavity. The diameter is 31.6 mm, which corresponds to the 300 GHz oscillation with TE32,18 mode. This mode is capable of 0.5 MW/300 GHz at CW operation. The distance from the emitter to the resonator is ~520 mm. The oscillation power is transmitted by the collector as a waveguide, and outputted as the oscillation mode through the sapphire window of 102 mm in diameter. A super-conducting magnet (SCM) is a 13 T liquid-He-free-magnet, which has a room temperature bore diameter of 110 mm. A dummy load is put on the top of the gyrotron to absorb and measure the output power. In the experiment, we demonstrated the power generation of ~0.5 MW at ~300 GHz oscillation at the TE32,12 mode. The applied beam voltage is 80 kV, and the beam current is 36.8 A. Pulse duration is ~2 msec. A MIG field is optimized to oscillate the target mode. The power peak was around 12.0 T, which corresponds to the TE32,18 mode. By decreasing the magnetic field to ~11.73 T, lower adjacent mode TE31,18 appeared. Detailed studies of the mode competition, etc., will be carried out.

Development of 28 GHz Gyrotron for Cooperative ECH Study

Abstract At the Plasma Research Center at University of Tsukuba, development of megawatt gyrotrons is being performed as a collaborative electron cyclotron heating (ECH) study with some research organizations. A 28 GHz 1 MW 1 s gyrotron has been developed to upgrade the GAMMA 10/PDX ECH systems. To improve the oscillation efficiency in high current regions, the magnetron injection gun (MIG) of the 28 GHz gyrotron has been modified. Output power of 1.25 MW has been achieved with this gyrotron. For the first step of the collaborative research between Tsukuba University and Kyushu University, the Tsukuba 28 GHz gyrotron was adapted to the Q-shu University Experiment with Steady-State Spherical Tokamak (QUEST) ECH system, and the plasma heating and current drive effect were demonstrated. We obtained successful results, including an electron cyclotron–driven plasma current of 66 kA in the QUEST plasma experiment. For the next step of the collaborative research, the design targets of a 2 MW 3 s and 0.4 MW continuous wave have been achieved in a design study of a new 28 GHz gyrotron.