International Journal of Terahertz Science and Technology
Vol.8, No.3, September 2015. PP.85-128 (4)
date2015-09-29 22:45:51 Click No.1812

TST, Vol. 8, No. 3, PP. 85-100

(Invited paper) Towards a 0.24-THz, 1-to-2-MW-class gyrotron for DEMO

M. Thumm 1,2*, J. Franck 1, P.C. Kalaria 1, K.A. Avramidis 1, G. Gantenbein 1, S. Illy 1, I.G. Pagonakis 1, M. Schmid 1, C. Wu 1, J. Zhang 1, and J. Jelonnek 1,2,
Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany
1 Institute for Pulsed Power and Microwave Technology (IHM)
2 Institute of High Frequency Techniques and Electronics (IHE)
* Email:
manfred.thumm@kit.edu

(Received August 18, 2015 )

Abstract: Current design studies on electron cyclotron heating and current drive (ECH&CD) systems for a DEMO fusion reactor demand gyrotron frequencies of above 200 GHz for efficient CD and a total gyrotron efficiency above 60 % to achieve an efficient fusion power plant operation. Considering the mandatory total heating power for DEMO and the space and maintenance requirements per tube, gyrotrons with a unit power of above 1 MW will be needed. Furthermore, for plasma stability control using a simple fixed ECCD launcher, fast frequency tunability (in a few seconds) in steps of about 2-3 GHz is necessary. Such tubes require broadband or tunable output windows and their quasi-optical mode converter must support the conversion of the various modes to a fundamental Gaussian wave beam. Slow tunability of gyrotrons (within a few minutes) in leaps of about 30C40 GHz is considered advantageous (multi-frequency gyrotrons), e.g. for DEMO plasma start-up, for DEMO variants with a relatively low magnetic field or for multi-purpose use in other fusion devices such as ITER. A multi-frequency gyrotron can operate with a simple, single-disk synthetic-diamond output window which is transparent at the relevant frequencies.

Physical design studies towards DEMO-compatible 2 MW-class coaxial-cavity gyrotrons are being performed at KIT. A well suited coaxial-cavity mode series with good multi-frequency properties is:

TE35,21 (170 GHz) C TE42,25 (203.8 GHz) C TE49,29 (237.5 GHz) C TE56,33 (271.3 GHz).

At 237.5 GHz a coaxial-cavity design for the TE49,29 mode has been found and optimized with quite promising results (1.9 MW, 33% electronic efficiency, without depressed collector (DC)). This frequency has been chosen such that the respective mode TE35,21 exactly matches the ITER frequency of 170 GHz, allowing the gyrotron to be used for a later upgrade of the ITER ECH&CD system. The azimuthal neighboring modes of TE49,29 have a frequency separation of 2 GHz and are well-suited fast frequency tuning. The design of the magnetron injection gun (MIG) has been finished, based on a realistic 10 T gyrotron magnet system.

As a backup solution, conceptual design studies on a hollow-cavity 1 MW-class gyrotron have also been performed. The current design permits multi-frequency operation with the following modes:

TE31,11 (170 GHz) C TE37,13 (203 GHz) C TE43,15 (236.1 GHz) C TE49,17 (269.1 GHz).

Numerical simulations with realistic electron beam velocity spread and beam width show, that about 1 MW output power at 36 % electronic efficiency (without DC) can be achieved.

The development of electron guns with high beam quality and of multi-stage depressed collectors (MSDC) for energy recovery is necessary to achieve the required overall gyrotron efficiency of above 60 %. KIT is installing a new gyrotron high-voltage power supply which will allow the operation of high-power gyrotrons with MSDCs.

Keywords: DEMO, Electron cyclotron heating and current drive (ECH&CD), Multi-frequency gyrotrons, Step-tunable gyrotrons, Mode selection, High-order modes, Magnetron injection gun, Quasi-optical mode converter, Broadband synthetic diamond Brewster and tunable windows.

doi: 10.11906/TST.085-100.2015.09.09

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TST, Vol. 8, No. 3, PP. 101-112

(Invited paper) Terahertz spectroscopy of biomolecules

Dongshan Wei 1, Zhongbo Yang 1, Mingkun Zhang 1, Mingjie Tang 1, Shihan Yan 1, Changcheng Shi 1, Liangping Xia 1, Huabin Wang 1, Tianying Chang 1, 2, Chunlei Du 1, and Hong-Liang Cui 1*, 2
1
Chongqing Key Laboratory of Multi-scale Manufacturing Technology, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, 400714, China
2 College of Instrumentation Science and Electrical Engineering, Jilin University, Changchun, Jilin, 130061, China
*1 Email:
hcui@cigit.ac.cn

(Received August 18, 2015 )

Abstract: Terahertz (THz) electromagnetic wave belonging to the frequency band from 0.1 to 10 THz (pundits narrow this range to 0.3-3 THz) has emerged as a powerful tool for investigating biomolecular systems. Since the energy level of THz wave largely coincides with that of the biomolecular low-frequency motions including vibration, rotation and translation of the molecular skeleton and that of the weak intermolecular interactions including hydrogen-bond and van der Waals etc., THz spectroscopy as a molecular detection technology has its unique advantages over some other existing ones. In the last several years, our group has focused on THz spectroscopy detection and spectral imaging of biomolecules, especially on the development of a THz near-field microscopy equipment for imaging of cells and even biomacromolecules. On the theoretical front, we have calculated and analyzed the characteristic spectra of polypeptides and investigated the effects of conformation and size of biomolecules on their THz spectra. Experimentally, microfluidic channels for THz spectroscopy tests were fabricated and the THz spectra of l-DNA were investigated. A scattering-type scanning near-field terahertz microscopy based on the atomic force microscopy and the THz spectrometer was built and initial results about the modulation of the THz near-filed signals were presented.

Keywords: Terahertz spectroscopy, Biomolecule, Absorption coefficient, Near-field microscopy, Molecular dynamics, Microfluidic channel.

doi: 10.11906/TST.101-112.2015.09.10

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TST, Vol. 8, No. 3, PP. 113-118

(Invited paper) A 330 GHz active terahertz imaging system for hidden objects detection

C. C. Qi *, G. S. Wu, Q. Ding, and Y. D. Zhang
China Communication Technology Co., Ltd., Baotian Road No. 1, Building 37, Chentian Industrial Zone,
Xixiang, Baoan District, Shenzhen, PR.China
* Email:
qichunchao@huaxunchina.cn

(Received August 18, 2015)

Abstract: A 330-GHz terahertz active imaging system has been designed for personal concealed objects detection. The study concerns both the optimization of a terahertz transceiver and the development of an optomechanical system. Unlike passive imaging systems, active imaging systems will emit a strong enough illumination source (generally approximating to 1 mW) to penetrate through thick clothes, and to generate a high signal-to-noise ratio (SNR) to overcome large signal clutter and speckle caused by a scene with a diversity of angles of incidence, surface roughness, and clothes layers. On the other hand, active imaging systems are insensitive to temperature change, and thus it is not easily impacted by ambient environment. Therefore the detector performance requirement is extremely lowered. In our active imaging systems, a heterodyne coherent detection technology as applied in others groups and a frequency-modulated continuous wave (FMCW) technology adopted from the radar field have been utilized to achieve a mm-scale resolution. In the emission end, the emission chain generates a fast chirp source of 20-21.25 GHz which is then doubled, amplified, 3 times passively doubled, and transmitted at 320-340 GHz by the horn antenna. The emission bandwidth of 20 GHz ensures sub-cm range resolution. In the receiving end, the reflected signal of 320-340 GHz plus a shift frequency proportional to the target range (for example 40 MHz) is mixed via a sub-harmonic mixer with a 160.48-170.48 GHz multiplier chain. In the sub-harmonic mixer end, an intermediate frequency (IF) signal of 960 MHz\40 MHz is generated and transmitted to in-phase and quadrature (I/Q) module. After frequency downconversion, a 40 MHz signal including phase and amplitude information of the detected objects is digitized. To obtain a scan area of 2.0 m (L) x 1.0 m (W) with about 7.5 mm resolution within 5 seconds, the tilt angle is designed as 2.875 degree with a rotation speed of 3768 rpm and a vibration speed of 5.6750/s. Both the rotation and tilt axes of the scanner are controlled by commercial motor drivers. About 26667 Image data are collected continuously during the scanning with the transceiver triggered by an optical encoder on the rotational axis. Using the Delaunay algorithm, a THz-image with gray or color maps is finally obtained. In short, the active terahertz imaging system can be applied in personal concealed weapon and contraband surveillance in the airport and customs.

Keywords: Terahertz imaging, Transceiver, Optomechanical scanning system, Multiplier chain

doi: 10.11906/TST.113-118.2015.09.11

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TST, Vol. 8, No. 3, PP. 119-128

(Invited paper) Investigation of the tapered waveguide structures for terahertz quantum cascade lasers

T. H. Xu, and J. C. Cao *
Key Laboratory of Terahertz Solid-State Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, 200050, China
* Email:
jccao@mail.sim.ac.cn

(Received August 18, 2015)

Abstract: The authors present a quasi-three-dimensional numerical simulation model based on the beam propagation method, which is suitable to simulate and design terahertz quantum cascade lasers with non-uniform axial waveguide structure. Using this simulation model, various kinds of tapered terahertz quantum cascade lasers were studied. According to our analysis, the normal tapered waveguide structure, although is simple, is a good choice to achieve good output beam quality and high output power simultaneously.

Keywords: Beam propagation method, Terahertz quantum cascade laser, Beam quality, Tapered, Numerical simulation

doi: 10.11906/TST.119-128.2015.09.12

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