Ryosuke Ikeda

Generation of high power sub-terahertz radiation from a gyrotron with second harmonic oscillation

New power records of second harmonic gyrotron oscillation have been demonstrated in the sub-THz band. The first step gyrotron of demountable type had succeeded in oscillation with power more than 50 kW at 350 GHz and nearly 40 kW at 390 GHz [T. Notake et al., Phys. Rev. Lett. 103, 225002 (2009)]. Then, the second step gyrotron of sealed-off type was manufactured. A cavity mode was carefully selected to avoid mode competition with a neighboring fundamental harmonic mode. Matching of the selected mode with the electron gun was also circumspectly considered. The second step gyrotron has attained higher power radiation than the first gyrotron. The maximum single mode power was 62 kW at 388 GHz. Then, the electron gun was modified for use of a different cavity mode with a higher coupling coefficient than that for the 62 kW mode. The new mode proved single mode oscillation power of 83 kW at about 389 GHz. These results are new second-harmonic-oscillation power records for sub-THz gyrotrons. The present study constitutes foundations of development of high power second harmonic sub-THz gyrotron for application to collective Thomson scattering measurement on fusion plasmas, especially on high-density plasmas such as those produced in LHD [N. Ohyabu et al., Phys. Rev. Lett. 97, 055002 (2006)]. This paper reports the design consideration to realize high power single mode gyrotron oscillation at second harmonic and the examination of oscillation characteristics of the gyrotron.

Helium-cooling and -spinning dynamic nuclear polarization for sensitivity-enhanced solid-state NMR at 14 T and 30 K.

We describe a (1)H polarization enhancement via dynamic nuclear polarization (DNP) at very low sample temperature T≈30 K under magic-angle spinning (MAS) conditions for sensitivity-enhanced solid-state NMR measurement. Experiments were conducted at a high external field strength of 14.1 T. For MAS DNP experiments at T<<90 K, a new probe system using cold helium gas for both sample-cooling and -spinning was developed. The novel system can sustain a low sample temperature between 30 and 90K for a period of time >10 h under MAS at ν(R)≈3 kHz with liquid He consumption of ≈6 L/h. As a microwave source, we employed a high-power, continuously frequency-tunable gyrotron. At T≈34 K, (1)H DNP enhancement factors of 47 and 23 were observed with and without MAS, respectively. On the basis of these observations, a discussion on the total NMR sensitivity that takes into account the effect of sample temperature and external field strength used in DNP experiments is presented. It was determined that the use of low sample temperature and high external field is generally rewarding for the total sensitivity, in spite of the slower polarization buildup at lower temperature and lower DNP efficiency at higher field. These findings highlight the potential of the current continuous-wave DNP technique also at very high field conditions suitable to analyze large and complex systems, such as biological macromolecules.

Formation of a laminar electron flow for 300 GHz high-power pulsed gyrotron

This paper describes the design of a triode magnetron injection gun for use in a 200 kW, 300 GHz gyrotron. As power and frequency increase, the performance of the gyrotron becomes quite sensitive to the quality of the electron beam. Formation of a laminar electron flow is essential for the realization of a high quality beam with a small velocity spread. In this study, a new method is developed for a quantitative evaluation of the laminarity and is applied to optimize the electrode design. The laminarity depends not only on conventional design parameters such as the cathode slant angle but also on the spatial distribution of the electric field along the beam trajectory. In the optimized design, the velocity pitch factors, α, larger than 1.2 are obtained at 65 kV, 10 A with spreads, Δα, less than 5%.

Status of high power gyrotron development in JAEA

Recent results of a multi-frequency gyrotron and a high frequency power modulation experiment using 170 GHz gyrotron are reported. Three frequency oscillations and their Gaussian beam outputs are demonstrated at 170 GHz, 137 GHz and 104 GHz. At present, 0.9 MW/ 70 sec, 1.1 MW/ 5 sec were obtained at 170 GHz. An experiment of 203 GHz oscillation will be conducted from April 2013 in addition to further power increase and pulse extension. In parallel, 5 kHz full power modulation was demonstrated by an anode voltage modulation using a double anode switching.

Progress Toward Steady-State Operation in LHD Using Electron Cyclotron Waves

Abstract Trials of steady-state operation (SSO) in the Large Helical Device (LHD) were started when a continuous wave (cw) gyrotron with the output power up to 0.2 MW was introduced to the electron cyclotron heating (ECH) system on LHD in 2003. During the first trial of SSO in the seventh LHD experimental campaign in 2004, severe temperature increase on the waveguide transmission line and, as a result, intense pressure increase in the evacuated waveguide occurred, which terminated the operation at 756 s. Additional pumping sections and cooling structures on the transmission line worked well, allowing a 3900-s sustainment of plasma with ne = 0.15 × 1019 m−3 and Te0 = 1.7 keV by 0.1 MW injection power in 2005. The improvement of the ECH system by introducing cw gyrotrons with higher power for further improvement of plasma performance in SSO is in progress. Investigations on electron cyclotron current drive (ECCD) physics have been advanced a few years after the proof of ECCD in LHD. By obliquely injecting second-harmonic X-mode EC waves in toroidal direction, stable current up to 5.5 kA is driven, which was evaluated as a difference in plasma currents of the co- and counter-ECCD discharges with 0.1-MW EC wave power. It takes a few seconds for the driven current to saturate. Change in profile of rotational transform by ECCD and profile of driven current density are investigated by use of motional Stark effect measurement. Peaked and localized driven current profile at the plasma core region was confirmed for on-axis second-harmonic ECCD discharges.

Investigation of Experimental Configuration for Electron Bernstein Wave Heating on LHD

Investigation of experimental configuration for the electron Bernstein wave (EBW) heating by using the existing electron cyclotron heating (ECH) antennas on LHD was performed. By using an antenna installed in the lower port, direct oblique launching of the extraordinary (X-) mode from the high magnetic field side (HFS) is available. Since the parallel component of the refractive index (N||) varies during propagation because of the inhomogeneity of the magnetic field, N|| can be zero when the launched X-mode crosses the fundamental electron cyclotron resonance (ECR) layer even N|| is noonzero initially. In such a condition, if the electron density is above a certain level the obliquely launched X-mode can pass the fundamental ECR layer without being damped out and can be mode-converted to EBW that is absorbed at the Doppler shifted ECR layer. By using an antenna installed in the horizontal port, oblique launching from the lower magnetic field side (LFS) toward the over-dense plasma is available. Excitation of EBW via the mode conversion process of ordinary mode(O)-extraordinary mode(X)-electron Bernstein wave (B) is expected with the O-mode launching toward an appropriate direction. The O-X-B mode conversion rate and the region of power deposition were surveyed by varying the magnetic field strength and the launching direction. The results of the survey suggest that efficient heating in the core region is difficult by using the existing antenna. Rearrangement of the final mirror of the launching antenna may be needed.

High power long pulse tests of the IFP-CNR dummy load for the European ITER gyrotron

The development of high power gyrotrons needs matched loads with good performance capable of absorbing and measuring powers as high as 1 MW in ITER and higher (up to two times) in future devices like DEMO. At IFP-CNR (Milano), several spherical loads were developed with identical absorbing geometry but different heat removal systems for operation with pulse lengths from a few ms to CW. A dummy load designed for 170 GHz/2 MW has been constructed in the frame of the development programme of European gyrotron for ITER. The first prototype hemisphere of this load, joined with an uncoated one, has been successfully tested with long pulses and under vacuum using the JAEA gyrotron for ITER in Naka. The symmetry of the system is such that the equivalent power load of the absorbing (coated) hemisphere is doubled when the second one is reflective (uncoated), with almost unchanged distribution with respect to a fully coated load. The tests demonstrate the capability of the IFP-CNR load to withstand and measure radiation at an equivalent power in excess of 1.5 MW. Gyrotron pulses with increasing power and pulse length have been performed in the load and are reported. The maximum equivalent power load reached in the tests was ~ 1.8 MW for 15 s while the longest pulse was 300 s, at ~ 1 MW. During the experiments, the temperatures of either the load or the preload were monitored using thermocouples and multiple infrared cameras. The results of the tests are described in the paper.

Gyrotrons FU CW V and FU CW VIII for measurement of hyperfine structure of positronium

For direct measurement on hyperfine structure of positronium, high power sub-THz radiation sources, Gyrotrons FU CW V and FU CW VIII have been developed. FU CW V is a frequency fixed gyrotron operating at 203.3 GHz for the proof-of-principle experiment and FU CW VIII a frequency continuously tunable gyrotron to measure directly the energy difference between orth- and para states of positronium.

Development of continuously frequency tunable gyrotrons FU CW VI and FU CW VI A for application to 600 MHz DNP-NMR spectroscopy

A high power THz radiation source is needed to analyze the structures of complex proteins by DNP-NMR spectroscopy. For this objective, we have been developing gyrotrons having boardband frequency tunability using a long cavity. Prototype gyrotron was achieved with a tuning range of 1.6 GHz and exceeded output power of 10 W.

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.