TST, Vol. 3, No. 1, PP. 1-20
2.2 MW Record Power of the 0.17 THz European Pre-Prototype Coaxial-Cavity Gyrotron for ITER
Manfred Thumm1,2*, Tomasz Rzesnicki1, Bernhard Piosczyk1, Jens Flamm2, Gerd Gantenbein1, Stefan Illy1, Jianbo Jin1, Stefan Kern1, Andrey Samartsev1, Andreas Schlaich2
Karlsruhe Institute of Technology (KIT), Association EURATOM-KIT
1Institute for Pulsed Power and Microwave Technology (IHM)
2Institute of High Frequency Techniques and Electronics (IHE),
Kaiserstr. 12, 76131 Karlsruhe, Germany
* Email: manfred.thumm@kit.edu
Abstract: A 2 MW, CW, 0.17 THz coaxial-cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within an European Gyrotron Consortium (EGYC*), a cooperation of several European research institutions. To support the development of the industrial prototype of a CW gyrotron, a short pulse tube (pre-prototype) is used at KIT Karlsruhe (former FZK, Karlsruhe) for experimental verification of the design of critical components, like the electron gun, beam tunnel, cavity and quasi-optical (q.o.) mm-wave output coupler. Recently a significant progress has been achieved which will be reported here after a general introduction to the principles of gyrotrons. In particular, an output power of up to 2.2 MW with 30 % output efficiency (non-depressed collector operation) has been obtained in single-mode operation at 0.17 THz. Further on, with a newly designed mm-wave output coupler an efficient conversion of the generated mm-wave power into a fundamental Gaussian output beam, with a Gaussian mode content of almost 96 %, has been successfully performed. Recently, the narrow-band fused silica output window has been replaced by a broadband SiN Brewster window in order to be able to study the excitation of additional modes in the frequency range between 0.13 and 0.17 THz (tunable multi-frequency gyrotron). In first experiments an output power of 1.8 MW with an efficiency of 26% has been obtained in the TE28,16 mode at 0.1413 THz. In addition, in the next azimuthal neighbor mode TE29,16 at 0.1433 THz, an output power of 1.25 MW has been generated with an efficiency of 23%. Measurements of the power profiles of the mm-wave output beams have confirmed numerical simulations resulting in a very high efficiency and good mode purity of the q.o. output coupler also for these other frequencies and modes.
Keywords: Gyrotron, coaxial cavity, quasi-optical output coupler, frequency step tuning, parasitic oscillations, electron cyclotron heating and current drive of thermonuclear fusion plasmas
doi: 10.11906/TST.001-020.2010.03.01
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TST, Vol. 3, No. 1, PP. 21-25
The THz Kerr Effect in Liquids and Ferroelectric Crystals
Matthias C. Hoffmann, Harold Y. Hwang, Ka-Lo Yeh, Nathaniel C. Brandt, Keith A. Nelson*
Massachusetts Institute of Technology, Department of Chemistry
77 Massachusetts Ave., Cambridge, MA 02139, USA
* Email: kanelson@mit.edu
Abstract: We have observed THz-pulse-induced optical birefringence in liquids and ferroelectric crystals. The polarization modulation is dependent on the square of the applied THz pump field which may exceed 100 kV/cm levels. As in many all-optical Kerr effect measurements, there is a fast electronic response which may be followed by a slower reorientational response. The electronic response may be used to map out the intensity profile of the THz pulse. THz Kerr effect measurements may enable study of dipole dynamics in relaxor ferroelectric crystals.
Keywords: Terahertz Spectroscopy, Kerr Effect, nonlinear spectroscopy, molecular reorientation
doi: 10.11906/TST.021-025.2010.03.02
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TST, Vol. 3, No. 1, PP. 26-32
Anisotropic Terahertz Dynamics of Highly-Aligned Single-Walled Carbon Nanotubes
L. Ren1, C. L. Pint2, A. K. Wόjcik3, T. Arikawa1, Y. Takemoto4, K. Takeya4, I. Kawayama4,
A. A. Belyanin3, M. Tonouchi4, R. H. Hauge2, and J. Kono1*
1Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA
2Department of Chemistry, Rice University, Houston, Texas 77005, USA
3Department of Physics, Texas A&M University, College Station, Texas 77843, USA
4Institute of Laser Engineering, Osaka University, Yamadaoka 2-6, Suita, Osaka 565-0871, Japan
* Email: kono@rice.edu
Abstract: This paper summarizes our recent advances in the spectroscopy of single-walled carbon nanotubes (SWNTs) in the terahertz (THz) range. Using polarization-dependent time-domain THz spectroscopy, we have studied the anisotropic conduction properties of free carriers in a thin film of highly aligned SWNTs. When the THz polarization was parallel to the nanotube alignment direction, there was strong absorption, while virtually no absorption was observed when the THz polarization was perpendicular to the nanotube axis. Through a proper model, the THz complex dynamic conductivity of the SWNTs was extracted and showed a non-Drude-like frequency dependence, with the real part monotonically increasing with increasing frequency and exhibiting a peak around 4.5 THz. Furthermore, the reduced linear dichroism was calculated to be 3, which demonstrates the nematic order parameter to be 1. This indicates that the alignment of the nanotubes is “perfect” and any misalignment must have characteristic length scales much smaller than the wavelengths used in these experiments (1.5 mm – 150 μm).
Keywords: Terahertz spectroscopy, carbon nanotubes, dynamic conductivity, anisotropy
doi: 10.11906/TST.026-032.2010.03.03
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TST, Vol. 3, No. 1, PP. 33-54
Mm/sub-mm Bolometer Based on Electron Heating in Narrow-gap Semiconductor
V. Dobrovolsky, F. Sizov*, V. Zabudsky, N. Momot
Institute of Semiconductor Physics of the Ukrainian National Academy of Sciences,
Kiev-03028, Nauki Av., 41,
Tel: +380-44-5256296, Fax: +380-44-5251810,
* Email: sizov@isp.kiev.ua.
Abstract: Direct detection mm/sub-mm wave warm-carrier bipolar narrow-gap Hg1-xCdxTe semiconductor bolometers that can be used as picture elements in THz sensitive arrays, are considered. The response of Hg1-xCdxTe warm-electron bolometers was measured in ν=0.037-1.54 THz frequency range at T=68-300 K. Bipolar semiconductor warm-electron bolometer theoretical model was developed too. In the bolometer considered the electromagnetic wave propagates in semiconductor waveguide, heats electrons and holes there, creates their excess concentrations, as well as, the electromotive forces. These effects cause the bolometer response voltage. Experimental results confirm the model main conclusions. Because of response time defined by carrier recombination time in HgCdTe layers (τ~10-8-10-6 s) and the noise equivalent power that can reach NEP300 K ~ 4×10-10 W/Hz1/2 in mm-wave region, the arrays on the base of HgCdTe bolometers can make them promising option for active fast frame rate sensitive applications.
Keywords: Mm/sub-mm radiation, warm-carrier effect, narrow-gap semiconductor bolometer
doi: 10.11906/TST.033-054.2010.03.04
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