TST, Vol. 8, No. 1, PP. 1-18
(Invited paper) Development of high power gyrotron and related technologies
Keishi Sakamoto *, Yasuhisa Oda, Ryosuke Ikeda, Takayuki Kobayashi, Ken Kajiwara, Koji Takahashi and Shinichi Moriyama
Japan Atomic Energy Agency, 801-1 Mukoyama, Naka, Ibaraki, Japan 311-0193
* Email: sakamoto.keishi@jaea.go.jp
(Received March 25, 2015)
Abstract: The research and development activities of high-power gyrotrons in JAEA for the use in nuclear fusion applications are reported. ITER requires 170 GHz 1 MW gyrotrons which are currently under development. High order modes of a cylindrical resonator to serve as oscillation mode candidates for 1 MW/ 170 GHz CW operation are studied. TE31,8 mode has shown the best performance operation, e.g., 1 MW/ 800 s/ 55% and 0.8 MW/1 hr/57%. Even higher modes are attempted to increase power. At the present stage, in TE31,11 mode, 1.24 MW/2s and 0.5 MW/1000 s have been achieved. This 170 GHz/TE31,11 mode has excellent characteristics for multi-frequency operation. By controlling resonant magnetic field and pitch factor of the electron beam, Gaussian power output was demonstrated at 203 GHz, 170 GHz, 137 GHz and 104 GHz. An anode control method was studied as a possible advantage operation scenario. A 5 kHz 1 MW full-power modulation for the plasma instability suppression was successfully demonstrated at 170 GHz by the fast of-off control of the anode voltage. A fast frequency switching of the gyrotron was studied for the power deposition profile control in the tokamak plasma, and the switching between 170 GHz and 167 GHz was proved with 3.5 s using the super-conducting sweeping coil. Furthermore, dual frequency 138 GHz/110 GHz gyrotron was developed for the application in the EC heating and current drive system of the JT-60SA tokamak. Power outputs of 1 MW/100 s were demonstrated at both frequencies. High power experiments of the transmission line and launcher were carried out using the gyrotron as a power source.
Keywords: High power gyrotron, Multi-frequency, Fast frequency switching, Power modulation.
doi: 10.11906/TST.001-018.2015.03.01
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TST, Vol. 8, No. 1, PP. 19-24
(Invited paper) THz Signal processing concepts based on graphene
H. L. Hartnagel *
Institut für Mikrowellentechnik und Photonik, Technische Universität Darmstadt, Darmstadt, Germany
* Email: hartnagel@mew.tu-darmstadt.de
(Received October 01, 2014)
Abstract: A number of THz signal-processing components are proposed. Firstly, a ballistic resonator is simulated for a graphene semiconductor structure. Graphene semiconductors have the advantage of exhibiting energy gaps depending on the width of the graphene sheet. Therefore loss free reflection of ballistic electrons as established by heterojunction epitaxy, can be realized by shaping the graphene widths. The design of corresponding THz oscillators is presented. Then quantum cascaded THz emitters are described. Finally, a number of basic components such as Schottky diodes on graphene transistors are discussed.
Keywords: THz wave, Ballistic electrons, Electron reflection, Semiconductor heterostructures, Quantum cascades in graphene, THz rectification by graphene FETs
doi: 10.11906/TST.019-024.2015.03.02
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TST, Vol. 8, No. 1, PP. 25-34
Evaluating the microwave performance of a two domain GaN Gunn diode for THz applications
Smita Francis *, and Robert van Zyl
Cape Peninsula University of Technology, French South African Institute of Technology, Department of Electrical, Electronic and Computer Engineering, Symphony Way, Bellville, 7535, South Africa.
* Email: sfrancis@polytechnic.edu.na
(Received October 2, 2014)
Abstract: The microwave performance of a two domain GaN Gunn diode is investigated at a fundamental frequency of 0.175 THz and its second and third harmonics through Monte Carlo particle simulations. Simulation results show that the two domain diode is feasible and that a significant increase in output power is achieved compared to that of a single domain diode. Furthermore, the microwave performance of the two domain diode can be enhanced by appropriate engineering of the transit region doping profiles. Three doping profiles are considered, namely nominally flat, exponentially increasing and exponentially decreasing, towards the anode contact regions. The exponentially increasing doping profile yields the highest output power, with 5 W, 514 mW and 87 mW generated at 0.175 THz, 0.350 THz and 0.525 THz, respectively. Thermal heating is generally significant in Gunn diode operation, but especially so for GaN diodes. Hence, thermal modeling is incorporated in the simulations consistently with the dynamic evolution of electrons through the device.
This investigation concludes that a narrow pulsed bias voltage is preferred in the simulation of the GaN Gunn diodes to overcome the adverse effect of thermal heating. It is reported that the narrow pulsed bias voltage enhances the performance of the two domain diode. Under these conditions the two domain GaN Gunn diode exhibits a maximum operational frequency limit of 0.525 THz.
Keywords: Gunn diode, Graded transit region, Negative differential resistance, Multi domain, Monte Carlo simulation.
doi: 10.11906/TST.025-034.2015.03.03
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TST, Vol. 8, No. 1, PP. 35-40
Identification of terahertz fingerprint spectra extracted from Gas-Fat coal
Li Zhaoxin 1, 2*, and Li Dehua 2
1 Terahertz Science and Technology Research Center, University of Electronic Science and Technology of China, Chengdu 610054, China
2 Key Laboratory of Terahertz Technology, Shandong University of Science and Technology, Qingdao 266590, China
*2 Email: lizx@sdust.edu.cn
(Received March 20, 2015 )
Abstract: The absorption spectra of gas-fat coal (GFC) in the frequency range 1 to 9 THz have been studied by experiment and computer simulation. The Gaussian lineshape fingerprint spectra were exfoliated from the Terahertz absorption curve by a fast method of curve fitting, and the corresponding low frequency vibrational modes were identified with the aid of Gaussian , one piece of quantum chemical computer software. It is found that there are 2 absorption peaks by experiment, which is in good agreement with those by computer simulation, and both of them point to bending vibrations in the plane of aromatic rings. It indicates that the in-plane bending energy is less affected by interactions between the basic structural units (BSUs) of coal. On the other hand, there are 2 pairs of discrepant peak positions by experiment and computing, which respond to out-of-plain bending vibrations of methyl and hydroxyl. It is believed to derive from interactions between the BSUs, which are not be considered in computer simulation. The sensitivity of out-of-plain bending energy to entire molecule structure and environment is essential for fingerprint identification of macromolecules.
Keywords: Terahertz, Coal chemistry, Gaussian lineshape, Fingerprint spectra.
doi: 10.11906/TST.035-040.2015.03.04
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