Hiromi Takahashi

The Development of a 77-GHz, 1-MW ECRH System for the Large Helical Device

Abstract A 77-GHz, 1-MW gyrotron is being newly installed in the Large Helical Device not only to enhance the total heating power but also to increase the possibility of controlling the local plasma parameters. Our progress in installing the new gyrotron and evaluating its properties is discussed. We have already finished the installation of the peripheral components, including the transmission line, and conducted a test at 1 MW for a short pulse. Our plan is to operate this gyrotron at a power of up to 1 MW for 5 s. The conditioning of the gyrotron has been smoothly conducted, and a gyrotron output power up to 810 kW for 3.6 s has been achieved so far. The total injected power of electron cyclotron resonance heating to the plasma reached a value of [approximately]2.5 MW.

LH transition by a biased hot cathode in the Tohoku University Heliac

In the Tohoku University Heliac (TU-Heliac), a helical axis stellarator, an electron injection electrode using a hot cathode made of LaB6 was developed and the transition mechanism to an improved mode has been intensively studied. In the electrode current-sweep mode of biasing experiments, a bifurcation phenomenon, i.e. a negative resistance feature in the electrode characteristics was observed accompanied with transition to an improved mode (H-mode) or transition from H-mode to L-mode, in helium plasma discharges of a wide range of collisionality. The ion viscous damping force was estimated from the J ? B driving force for poloidal rotation. The local maxima in viscosity were found at the poloidal Mach number around ?Mp ~ 1?3, as predicted by neoclassical theory. It was also found that the negative resistance (a bifurcation phenomenon) was observed when the poloidal viscosity showed a local maximum.

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.

Improvements of data quality of the LHD Thomson scattering diagnostics in high-temperature plasma experiments

In Large Helical Device (LHD) experiments, an electron temperature (T(e)) more than 15 keV has been observed by the yttrium-aluminum-garnet (YAG) laser Thomson scattering diagnostic. Since the LHD Thomson scattering system has been optimized for the temperature region, 50u2002eV≤T(e)≤10u2002keV, the data quality becomes worse in the higher T(e) region exceeding 10 keV. In order to accurately determine T(e) in the LHD high-T(e) experiments, we tried to increase the laser pulse energy by simultaneously firing three lasers. The technique enables us to decrease the uncertainties in the measured T(e). Another signal accumulation method was also tested. In addition, we estimated the influence of high-energy electrons on T(e) obtained by the LHD Thomson scattering system.

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.

Measurement of electrostatic potential fluctuation using heavy ion beam probe in large helical device

Heavy ion beam probe (HIBP) for large helical device (LHD) has been improved to measure the potential fluctuation in high-temperature plasmas. The spatial resolution is improved to about 10 mm by controlling the focus of a probe beam. The HIBP is applied to measure the potential fluctuation in plasmas where the rotational transform is controlled by electron cyclotron current drive. The fluctuations whose frequencies change with the time constant of a few hundreds of milliseconds and that with a constant frequency are observed. The characteristics of the latter fluctuation are similar to those of the geodesic acoustic mode oscillation. The spatial profiles of the fluctuations are also obtained.

Hysteresis during transition into improved mode on biasing experiment under the electrode current control in Tohoku University Heliac

Electrode bias experiments were carried out in Tohoku University Heliac (TU-Heliac). This paper presents a comparison of the plasma behaviour in discharge of the electrode current ramped up with that ramped down to investigate the bifurcation and hysteresis phenomena. Bifurcation of the electrode current against the bias voltage, i.e. negative resistance and hysteresis between (1) electron stored energy and input power from the electrode, (2) fluctuation level of ion saturation current and radial electric field and (3) poloidal momentum damping force and poloidal Mach number were observed. The driving force in the region where the hysteresis was observed corresponded to those in the negative resistance phase and those were located at local maximum of the ion viscosity. Hysteresis between the poloidal Mach number and the poloidal damping force was observed. However, the difference in ion viscosity, which was obtained by subtracting the friction term from the total poloidal damping force, between the electrode current ramped up/down in the transition region was rather small. The data in the negative resistance region corresponded to those in the regime where the viscosity decreased against the poloidal Mach number.

Suppression of spurious mode oscillation in mega-watt 77-GHz gyrotron as a high quality probe beam source for the collective Thomson scattering in LHD.

Collective Thomson scattering (CTS) diagnostic requires a strong probing beam to diagnose a bulk and fast ion distribution function in fusion plasmas. A mega-watt gyrotron for electron cyclotron resonance heating is used as a probing beam in the large helical device. Spurious mode oscillations are often observed during the turning on/off phase of the modulation. The frequency spectra of the 77-GHz gyrotron output power have been measured, and then one of the spurious modes, which interferes with the CTS receiver system, is identified as the TE(17,6) mode at the frequency of 74.7 GHz. The mode competition calculation indicates that the increase of the magnetic field strength at the gyrotron resonator can avoid such a spurious mode and excite only the main TE(18,6) mode. The spurious radiation at the 74.7 GHz is experimentally demonstrated to be suppressed in the stronger magnetic field than that optimized for the high-power operation.

Development of a Heavy Ion Beam Probe for Measuring Electrostatic Potential Profile and Its Fluctuation in LHD

A heavy ion beam probe (HIBP) using a 3-MV tandem accelerator has been installed on large helical device (LHD). Electrostatic potential in core plasma can be measured under the toroidal magnetic field strength of up to 3 T. By using the HIBP, the transition of potential profiles from electron-root to ion-root is observed in core plasmas during ramp-up of the electron density. Potential fluctuations are also measured electron cyclotron current drive (ECCD). Two kind of characteristic fluctuations are observed. One is a reversed-shear-induced Alfven eigenmode (RSAE), whose frequency varies during the evolution of the rotational transform profile, and the other is with a constant geodeisc acoustic mode (GAM) frequency.

Electrode biasing experiment in the Large Helical Device

The transition to the improved confinement mode by electrode biasing was observed for the first time in the Large Helical Device (LHD). A negative resistance was observed in the confinement mode sustained by the cold electrode biasing. The electrode current showed a clear decrease against an increase in the electrode voltage and was characterized by a hysteresis in the transition phenomena. The decrease in the electrode current suggested an improvement in the radial particle transport. An increase in the energy confinement time and remarkable suppression of density fluctuations, which correspond to the transition, were also observed. These results indicated that the electrode biased plasma in LHD showed similar improvements in confinement to those in the H-mode plasmas in tokamaks and stellarators.