MSMW'13, Kharkov, Ukraine, June 23-28

DEVELOPMENT OF HIGH FREQUENCY GYROTRONS IN FIR FU COVERING SUB-THz TO THz RANGE

FOR APPLICATIONS TO HIGH POWER THz SPECTROSCOPY

Toshitaka Idehara and Svilen P. Sabchevski Research Center for Development of Far Infrared Region, University of Fukui (FIR FU)

Address: 3-9-1 Bunkyo, Fukui-shi 910-8507, Japan Tel. +81 776 27 8657, Fax +81 776 27 8657, E-mail idehara@fir.u-fukui.ac.jp

Abstract Powerful sources of coherent radiation in the sub-terahertz to terahertz frequency range are required for expanding number of applications in the physical research and in various advanced high power THz technologies. In recent years, a spectacular progress in the development of various gyro-devices and in particular, the powerful high frequency (sub-terahertz to terahertz) gyrotron oscillators has demonstrated a remarkable potential for bridging the so−called terahertz power gap and stimulated many novel and prospective applications. In this paper, we outline two series of such gyro-devices, namely the Gyrotron FU Series which includes pulsed gyrotrons and Gyrotron FU CW Series which consist of CW (continuous wave) or long pulse gurotrons. Both series are developed at Research Center for Development of Far Infrared Region, University of Fukui (FIR FU). We present the most remarkable achievements of these devices and illustrate their applications by some characteristic examples. An outlook for the further extension of the Gyrotron FU CW Series is also provided. 1. Introduction As the most powerful sources of continuous (CW) coherent radiation in the sub-terahertz and the terahertz frequency range, the gyrotrons have demonstrated a remarkable potential for bridging the so called T-gap (a.k.a. Terahertz power gap) in the electromagnetic spectrum opening the road to many novel applications in the fundamental scientific research and in the THz technologies [1−5]. There are two main streams of gyrotron development worldwide. For the first one the main target parameter is the output power, while for the second one the main goal is to achieve higher output frequency. The former tubes are well represented by a kilowatt class technological gyrotrons for advanced microwave materials treatment [6,7] and by megawatt class tubes used for electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) of magnetically confined plasma in various reactors (e.g. ITER) for controlled thermonuclear fusion [1]. The output frequency of these devices is in the millimeter wavelength region (typical frequencies for the technological gyrotrons are around 30 GHz and 140-170 GHz for fusion), the latter tubes operate at much higher frequencies but at considerably lower output powers (typically, ranging from several tens to several hundreds of watts), which however exceed significantly the output power of other devices (solid state, lasers) in this frequency range. A remarkable recent breakthrough demonstrated by the high-frequency gyrotrons has been the crossing of the symbolic threshold of 1 THz [8,9] between terahertz electronics and the terahertz photonics. The aim of this paper is to present the current status of high−frequency gyrotron development at FIR FU as well as to outline the main novel and prospective applications that are under study now. The paper is organized as follows. In the next section we depict two series of devices, namely Gyrotron FU and Gyrotron FU CW Series, developed at the FIR FU. Some of their characteristic applications are presented in Sec. 4. Finally, we draw some conclusions and present an outlook. 2. Gyrotron FU and Gyrotron FU CW Series The development of high−frequency gyrotrons at FIR FU started with FU I, which is the first member of the Gyrotron FU Series (see Table 1). This series includes nine tubes altogether that cover a broad frequency range from 38 GHz to 0.889 THz, operating on different modes at the fundamental, second, third and in the case of a large-orbit gyrotron (LOG) FU VI even fourth and fifth harmonic of the cyclotron frequency [10]. The frequency of 0.889 THz, achieved by the Gyrotron FU IVA with a 17 T magnet at a second harmonic operation was a long-time world record for the highest frequency demonstrated by a gyrotron until it was surpassed by a pulse gyrotron with a 21 T pulse magnet [8] (see below). The gyrotrons of this series have been used as radiation sources for several electron spin resonance (ESR) experiments (FU E, FU IVA), for plasma scattering measurements and X-ray Detected Magnetic Resonance (XDMR) (FU IA, FU II) as well as for study of such important physical phenomena as mode interaction (competition, cooperation mode switching) (FU II), frequency and amplitude modulation, frequency step switching, operation at high harmonics (FU VI) and with high mode purity (FU V), stabilization of the output parameters and operation for long periods of time (FU V)

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etc. All these tubes are being operated in a pulsed mode with pulse lengths of the order of milliseconds, repetition rate of the order of several Hz, and duty ratio of several percent. The successful development of all these high-frequency radiation sources culminated in the design and construction of the first pulsed terahertz gyrotron which operated at a frequency exceeding 1 THz. Such remarkable breakthrough has been achieved by a tube with a pulsed ice−protected magnet producing strong magnetic field with a maximum intensity about 21.4 T [8]. A block diagram of the experimental set up is shown Table 1. Gyrotron FU series with a 21 T pulse magnet [8] recently (see below). The gyrotrons of this series have been used as radiation sources for several electron spin resonance (ESR) experiments (FU E, FU IVA), for plasma scattering measurements and X-ray Detected Magnetic Resonance (XDMR) (FU IA, FU II) as well as for study of such in Fig. 1a. It includes a 300 kJ capacitor bank for powering the magnet, power supplies for the electron gun (triode MIG) and for the additional gun coils and a computer control of the experimental system. The tube is of a demountable type and has a cavity with a diameter and length of the regular cylindrical section of 3.9 mm and 10 mm, respectively. The output radiation is transmitted by a circular waveguide with an inner diameter of 28 mm to the output window made of alumina (3 mm thick) and then to an external waveguide of the same diameter. The traces of the measured signals in the breakthrough experiment are shown in Fig 1b. They correspond to a second harmonic operation at the cavity mode TE6,11 for which a frequency of 1.005 THz has been measured at field intensity of 19.0 T. The operational tests demonstrated that the tube can be operated at both the fundamental and the second harmonic resonances covering a wide frequency range from 387 GHz to 1.009 THz through step tunability. This first THz gyrotron developed at FIR FU has been used as a prototype for the development of the next CW THz gyrotron, FU CW III (see Table 2). It is well known, however, that many potential applications require continuous (CW) radiation for long periods of time. Such demand motivated the development of the next line of devices, namely the Gyrotron FU CW series. Since these devices are newer and the previous (FU) are well represented in the literature, we confine ourselves only to them in this paper. An inventory of the Gyrotron FU CW series is presented in Table 2, where the most characteristic features of each tube are mentioned.

Gyrotron Frequency, THz Characteristic features and achievements (The applications are distinguished by italic letters)

FU I 0.038−0.220

First high-frequency medium power gyrotron of the FU Series. Output power of 9 kW at 100 GHz.

FU E 0.090−0.300 Radiation source for the first experiment on ESR

FU IA 0.038−0.215 Radiation source for plasma diagnostic at WT−3

FU II 0.070−0.402

Studies on mode interactions (competition, cooperation etc.). Radiation source for plasma scattering measurements at CHS in NIFS. Radiation source for XDMR experiments at ESRF.

FU III 0.100−0.636 3rd harmonic single-mode operation. Amplitude modulation. Frequency step switching

FU IV 0.160−0.847 Frequency modulation. CW operation during long time periods with a high stability of the output power and frequency

FU IVA 0.160−0.889 Highest frequency at third harmonic (a world record for a long period). Radiation source for ESR experiments.

FU V 0.186−0.222 CW operation for a long time using He−free superconducting magnet. High stability of the frequency and the amplitude. High mode purity operation. FU VI

0.064−0.137 Large−orbit gyrotron (LOG) with a permanent magnet. High harmonic operation up to 5th harmonic of the cyclotron frequency. High purity mode operation at TEm1 modes.

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Fig.1 a) Block diagram of the experimental setup of the breakthrough experiment with the first pulse THz gyrotron; b) traces of the measured signals (Vk – cathode voltage in kV; P − output power in arbitrary units; and B – magnetic field intensity in T). Table 2 Gyrotron FU CW series 3. Recently developed gyrotrons, members of the FU CW C and FU CW G series For many applications (especially for the spectroscopic studies) the gyrotron used as a radiation source has to be embedded in a complex system and its radiation effectively coupled and delivered without significant losses to the irradiated area. Bulky and heavy devices like conventional gyrotrons are not suitable for such systems. This motivated us to develop a concept of a compact gyrotron [11,12], which is based on the utilization of compact liquid He-free superconducting magnets and slim tubes with a special design allowing its insertion in the relatively small diameter bore of the solenoid. The mentioned special design envisages also development of

Gyrotron Frequency range, THz

Power, kW

Max. B , T

Characteristic features and achievements

FU CWI 0.300 2.3 12 Advanced materials processing. Novel medical technologies.

FU CW II FU CW IIA

0.110−0.440 20−206 8 DNP−NMR at 600 MHz for protein research at Osaka University. Heating of Si substrate. Preliminary experiment on Bloch oscillation.

FU CW III 0.130−1.080 10−220 20 High−power THz technologies

FU CW IV 0.131−0.139 5−60 10 DNP-NMR at 200 MHz for analysis of polymer surface.

FU CW V 0.2034 100−200 8 Accurate direct measurement of the hyper−fine splitting (HFS) of positronium. Radiation source for novel medical technologies.

FU CW VI 0.393−0.396 50−100 15 DNP−NMR at 600 MHz for protein research at Osaka University.

FU CW VII 0.2037; 0.3953 200 50 CW

9.2 DNP−NMR at 300 and 600 MHz at Warwick University.

FU CW VIIA 0.1315 0.395 200 8 ESR echo experiment in the sub-THz region.

FU CW VIII 0.100−0.350 100 8 Pump and probe technique for XDMR at ESRF

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an optimized low−voltage and low−current MIG with reduced (minimized) radial dimensions. This concept has been realized recently in one of the newest gyrotrons, namely FU CW CI [13], which has been built using an 8 T magnet (see Fig. 2). It delivers radiation with a frequency around 394.6 GHz, operating on the mode TE2,6+ at the second harmonic resonance for a magnetic field intensity of 7.2 T. Its total length is only 1.02 m. Such a highly portable (almost table-top) gyrotron is very convenient for integration in an experimental infrastructure for 600 MHz DNP-NMR spectroscopy. Fig. 2 Gyrotron FU CW CI Fig. 3 Gyrotron FU CW GI Almost all applications of sub-terahertz and terahertz radiation require a well−collimated wave beam with linear polarization, while the one produced in the resonant cavity is a waveguide mode (usually a high-order one). This makes it necessary to use either an external mode converter and transmission line after the output window or an internal mode converter. Since the latter has many advantages and significantly simplifies the usage of the device in a complex experimental environment we have begun the development of such gyrotrons. The first compact tubes with internal mode converters are FU CW GI and FU CW GII [14] ( Fig 3). FU CW GI has an internal mode converter which consists of helically−cut Vlasov launcher and four mirrors. It forms a nice Gaussian−like beam with almost circular cross section. The measured output power at 202.55 GHz (fundamental operation on the counter-rotating mode TE5,2 ) is 500 W. The next device, FU CW GII has a similar design but will operate at the second−harmonic resonance. The design of the next compact gyrotron with an internal mode converter, FU CW CII, has already been completed. Its preliminary operational tests have been carried out. At the present, even in short pulse operation, we have achieved fairly high power operation exceeding 1 kW at the designed mode TE03 with the frequency of 203 GHz band. This operation condition is adjusted for experiments on the direct measurements of the hyperfine structure of positronium (See Sec. 4.4 below). Before reviewing the applications of the gyrotrons belonging to the FU CW series we end up this section with Fig. 4, which summarizes their most important operational parameters. Also shown on the chart are several new tubes that are under development at present or are in the process of a computer aided design (CAD) using our problem-oriented software package for modeling and simulation of gyrotrons GYROSIM. It can be seen that the series FU CW occupies a wide area in the parameter space and offers powerful frequency tunable sources of coherent radiation for many new sub-terahertz and terahertz technologies. 4. Novel and prospective applications of sub-terahertz and terahertz gyrotrons of the FU CW series 4.1 DNP−NMR spectroscopy As it has already been mentioned in the previous section, most of the gyrotrons of the FU CW series are developed as radiation sources for a proton NMR spectroscopy at high magnetic fields with a signal enhancement by a dynamic nuclear polarization (DNP) [15]. The principle of this technique is to increase the signal to noise ratio (S/N) and thus the overall sensitivity by transferring the large electron spin polarization to the nuclear (proton) spin system irradiating the studied sample with a microwave radiation with a frequency equal or close to the electron paramagnetic resonance (EPR) also known as electron spin resonance (ESR) at cryogenic temperatures. Since the enhancement is proportional to η1/η2=657, whereη1andη2 are the electron

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and the proton gyro-magnetic ratios, theoretically, enhancements of more than two orders of magnitude are possible., In practice however, the results depends on many factors and most notably on the quality of the Fig. 4. Some characteristic operational parameters of the gyrotrons belonging to the Gyrotron FU CW Series. instrumentation in which the operational performance of the gyrotron is of paramount importance. Some of the challenging requirements to the gyrotrons used for DNP−NMR have already been mentioned above, e.g. (i) output power greater than 15-20 W at the output window and at least several watts at the irradiated sample; (ii) stable output power (fluctuations less than ± 0.5%); (iii) frequency stability of the order of 2 MHz and less; (iv) frequency tunability in a wide frequency range (1 GHz and more); (v) high mode purity (>90%), and additionally, (vi) all these output parameters and spectral characteristics must be maintained in a CW mode of operation during long running times (>24 hours). The state-of-the-art of the gyrotron development for DNP-NMR spectroscopy is well represented in the recent publications. [16] Several FU CW gyrotrons have demonstrated output parameters that meet the requirements listed above and have been used or are in a process of preparation for DNP−NMR experiments at FIR FU Center, in the Institute of Protein Research at Osaka University and in the Magnetic Resonance Center at Warwick University. More specifically, the gyrotrons FU CW II and FU CW VI have already been installed in a system for 600 MHz DNP−NMR spectroscopy at Osaka University, while the gyrotron FU CW VII is prepared for both 300 MHz and 600 MHz DNP−NMR at Warwick University. As an example, here we will mention a recent DNP experiments at 14.1 T for solid-state NMR [17]. They have been carried out using the gyrotron FU CW II as a radiation source at a frequency of 394.5 GHz (second−harmonic operation). A circular waveguide with a diameter of 28 mm has been used to transmit the radiation from the gyrotron to the bottom of the MAS (Magic Angle Spinning) NMR probe. In these experiments a tenfold increase of the intensity of the signal has been observed for a 13C-labeled organic compound with an estimated power of the sub-millimeter wave of 0.7 W at the sample (see Fig. 5). It is worth noting that this study is the first one in which a high−field DNP under static magnetic field with an intensity of 14.1 T has been carried out successfully. At the present, the experiments continue using the frequency tunable gyrotron FU VI, which allowed us to increase the enhancement factor significantly, achieving

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values around 60. We have just installed the gyrotron FU CW GII for high−quality DNP-NMR experiment by adjusting an optimum condition of the polarization. Fig. 5 The 1H polarization enhancement factor ε observed for a 13C-labeled compound with nitroxyl biradical TOTAPOL. 4.2 ESR spectroscopy In recent years, the ESR spectrometers utilizing gyrotrons as radiation sources have demonstrated their advantages in the field of high-frequency spectroscopy and have been used successfully for studying the magnetic properties of various materials [18]. An advanced ESR spectrometer with a pulse magnet with maximum field intensity of up to 40 T has been developed using the gyrotrons of the FU series as radiation sources. It is illustrated schematically in Fig. 6. An important advantage of the spectrometers based on gyrotrons is the sufficient microwave power, which makes a single pulse measurements possible. This spectrometer has been used for investigation of the magnetic properties of different materials, notably Fe-SiO2 granular films, anomalous magnetization in CsFeCl3, high-field spectra in powder spectra of BaCu2(PO4)2 and SrCu2(PO4)2 compounds as well as in anti-ferromagnetic single crystal MnF4. The experience gained in the preceding studies is helpful for the development of novel advanced spectroscopic techniques. One of them is the pulse ESR (EPR) which (as differentiated from the CW ESR) is also referred to as FT-ESR (Fourier Transform ESR). It offers significant gains in the sensitivity and reduction of the spectrum acquisition time. In the FT-ESR a short but very powerful microwave beam irradiates the sample and the signal coming from it is digitalized and Fourier transformed in order to obtain the ESR spectrum in the frequency domain. In the electron spin-echo envelope modulation (ESEEM) techniques, for example, one measures the intensity of the electron spin-echo resulting from the application of several microwave pulses as a function of the time interval between the pulses. Other well-known pulse ESR techniques are DEER (Double Electron−Electron Resonance), and ELDOR (Electron Double Resonance). Preparation for ESR spin echo experiments using FU CW VIIA in FIR FU is in progress now. A pulse-forming system has been developed utilizing light controlled semiconductor shutters [19]. It produces nanosecond sub-THz wave pulses with an arbitrary delay time of the order of microseconds that are appropriate for planned spin echo experiments. Since the pulse length required in a pulsed ESR system is inversely proportional to the square of the intensity of the microwave radiation, a power of several kilowatt is sufficient for a pulse duration of the order of nanoseconds. Therefore, such equipment and technique can be used for characterization of materials with short relaxation times. 4.3 XDMR spectroscopy X-ray detected Magnetic Resonance (XDMR) is a novel advanced pump and probe spectroscopic technique in which X-ray Magnetic Circular Dichroism (XMCD) is used to probe the resonant precession of either spin or orbital magnetization components pumped by the magnetic field of a strong sub-terahertz wave produced by a gyrotron in a plane perpendicular to the static bias magnetic field. A distinguishing feature of the XDMR is that it is element− and edge−selective method that can be used for detailed study of the precession dynamics of orbital and spin magnetization components of different nature and origin [20]. For preparation of such experiments the gyrotron FU II has been installed at the beam line ID12 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The same approach can be extended also to the investigation of a rich variety of new X-ray electro−optical or magneto−electric effects. Increasing the pumping frequency to the THz frequency range could transform XDMR

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Fig.6 A versatile ESR spectrometer based of FU gyrotron series into a unique spectroscopy which allows one to investigate the dynamics of the Van Vleck orbital paramagnetism for example. Moreover, it is believed that even XDMR measurements in the millimeter wavelength range would make it possible to study paramagnetic systems routinely and investigate optical modes as well as acoustic modes in ferrimagnetic/antiferromagnetic systems. It is expected also that high−field XDMR will become even more powerful method by detecting the high−frequency normal modes associated with magnetically coupled sublatices. 4.4 Study of the energy levels and HFS of positronium Another novel application of gyrotrons as powerful radiation sources is in the studies of the energy levels and hyperfine splitting (HFS) of positronium (Ps), which is a bound state of an electron and a positron that forms an exotic Hydrogen-like atom [21]. Ps is an excellent object to study and, presumably, to check (i.e. to prove or disprove) the theory of quantum electrodynamics (QED). The positronium exists in two states, namely a triplet (3S1 ortho−positronium (o−Ps)) and a singlet (1S0 para−positronium (p−Ps)). The energy splitting between o−Ps and p−Ps, i.e. HFS is about 203.4 GHz. The recent theoretical predictions that take into account higher order corrections have shown that there is a significant discrepancy between calculated and measured values by 15ppm (3.8σ). In all previous experiments, however, the HFS has been obtained indirectly from the measured Zeeman splitting in a static magnetic field. Such measurements are prone to significant systematic errors due to the possible nonuniformity of the magnetic field strength. In order to avoid such errors a new direct method has been proposed recently. For this method a powerful radiation source with an output frequency band near 203 GHz is needed because the cross−section of the hyperfine transition is extremely small. In order to provide it, the gyrotron FU CW V has been developed (see Table 2). This device has been embedded in a measuring system, which includes a mode converter and a transmission system, a high-finesse Fabry-Perot (FP) cavity, gas chamber and positron source as well as a system of detectors and electronic control modules. The whole experimental setup is illustrated schematically in Fig.7. The FP cavity consists of a metal-mesh mirror through which the gyrotron output radiation is coupled to the resonator and a convex copper mirror. Its length is controlled with a great precision (with 20 nm resolution) by a piezo stage carrying the copper mirror. The mesh mirror is made of gold and is deposited on a disc of fused silica. A small hole with a diameter of 0.6 mm at the center of the copper mirror couples the cavity to the pyro−electric power monitor which is used to register the resonances of the cavity. A finesse greater than 628, which corresponds to less than 1% round-trip losses has already been obtained experimentally and observed by the monitor. The positronium is formed in the cavity using a 22Na source of positrons and nitrogen mixed by iso-Butane as a stopping target. Under the irradiation by a 203 GHz wave some of the o−Ps (decaying into 3-photons) change into p-Ps (decaying into 2-photons) and thus the ratio of 2-photon

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events increases and they can be monitored by the photon detectors (LaBr3(Ce) scintilators) that are located around the cavity. In the measurements the frequency of the gyrotron is varied within ~2 GHz to observe a Breit-Wigner resonance of the transition. At the present, all components of the experimental setup are in a process of optimization and final testing preparing for the decisive experiments. The current status of this study has been reviewed in [22]. Fig. 7 Experimental setup for measuring of the HFS of positronium. 4.5 Bloch oscillator Bloch oscillating structures such as super-lattices (SL) are considered as promising candidates for realization of novel tunable solid−state radiation sources that provide broad band gain at terahertz frequencies [23]. So far however, the formation of high field domains in the semiconductor super−lattices has been hindering the realization of a Bloch oscillator proposed by Esaki and Tsu for more than 40 years. Attempting to solve this problem recently a series of experiments have been carried out for investigation of the current suppression in superconductor super−lattices driven by intense sub-THz radiation from a gyrotron [24]. In the experimental setup a 3 µm−thick GaAs/AlAs super−lattice sandwiched by n++−GaAs contact layers on a semi−insulating GaAs substrate has been used. The barrier and the well thickness of the SL were, respectively, 0.56 nm and 8.49 nm, while the SL was doped uniformly with Si up to nSi=1×1016 cm-3. The wafer was produced as a 10×10 µm2 small mesa diode structure and, then λ/2−dipole antennas were formed in such a way that the electrodes of the antenna are connected separately with the top and the bottom n++ contact layers (see Fig. 8). In order to examine the possibility of large amplitude operation in such coupled SL-antenna device I-V characteristics under the irradiation with intense sub-THz waves with a frequency of 400 GHz from a gyrotron have been registered and investigated. The gyrotron has been operated in a pulse mode with pulses of 50 µs duration and duty ratio 5%. The most important results obtained in this study are presented in Fig. 9. They show that when the incident power is increased from zero to 68 W the current is suppressed to up to 20 %. Such observation is in agreement with the theory of the large amplitude operation mode. It is believed that the physics of many similar structures (super−lattices, quantum wells, and nano−structured materials) can also be studied using experimental techniques that involve application of terahertz radiation. 4. Conclusions and outlook: High quality gyrotron development for advanced THz spectroscopy In recent years the gyrotrons have demonstrated a remarkable potential for bridging the THz power gap and have been used in a great and continuously increasing number of applications. Due to the limited space in this paper, we have confined ourselves only to the Gyrotron FU and FU CW series developed at the FIR FU Center. In parallel with us, however, many research institutions have contributed to the remarkable progress in both the theory and practice of gyrotron research. Among them are the Institute of Applied Physics of the RAS in Nizhni

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Nowgorod (Russia), JAERI (Japan), MIT Plasma Science and Fusion Center, the University of Maryland Energy Research Center, CPI (USA), Institute for Pulse Power and Microwave Technology at KIT (Germany), CRPP-EPFL, Lausanne (Switzerland) as well as many other research institutions and groups around the world. We hope that the progress in the field of the development and application of sub-terahertz and terahertz gyrotrons will be continued and accelerated in the coming years. Therefore, we plan to extend further the series of FU CW gyrotrons in two directions. The first one is to continue to design novel experimental (pilot) devices for emerging new fields, trying to satisfy the needs of the researchers in powerful coherent sources of sub-terahertz and terahertz waves as well as continuing our theoretical and experimental studies on the physics of gyrotrons. The second direction is to produce optimized versions of the pilot designs and on this basis develop standard designs which are compliant with the industrial standards and can be used for producing high-performance (turn−key quality) devices suitable for commercial application in various specialized system (DNP-NMR spectroscopes, therapeutic systems etc.). In order to approach such an ambitious and challenging goal we have begun the development of a concept of innovative CAD, based on our previous experience and on the available software tools for modeling and simulation of gyrotrons. Some of the most characteristic features of the innovative designs for the next generation of high performance gyrotrons are: (i) use of cryo-free superconducting magnets for compact gyrotrons with a simplified maintenance and easy operation; (ii) EOS based on high-performance magnetron injection gun, which forms high-quality electron beams with appropriate parameters for sufficient efficiency and stability of the operation at low accelerating voltages and lower thermal loading of the emitter; (iii) development of the tube together with the transmission line and the auxiliary units of the experimental equipment (e.g. cavity resonator of the spectrometer); (iv) system approach in which both the design and the optimization loops of different subsystems (EOS, the electro-dynamical system, the quasi-optical system and so on) are carried out successively and iteratively in the course of the development of the entire tube; (v) carefully designed resonant cavities that stipulate a broad tunability band. Various techniques for frequency control (continuous tunability; step-tunability; frequency modulation) that are being implemented in the innovative designs are discussed in [25, 26]. A promising opportunity for widening of the tunability band, which is based on the use of up-tapered resonators has been explored recently [27]. Acknowledgements This work was supported partially by the Special Fund for Education and Research from Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan and by SENTAN Project of Japan Science and Technology Agency (JST).

Fig. 8. Top-view microscope image of a coupled antenna−superlattice device. The inset shows a blow up of a small SL mesa diode. The sub-THz driving field and the dc bias field were applied parallel to the growth direction of the SL.

Fig 9. Current-voltage characteristics under the irradiation of 400 GHz radiation with various powers. (left) I-V curves measured at 300K. (right) Simulation calculated by the large-amplitude model.

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12. O. Dumbrajs, T. Idehara, S. Sabchevski, Design of an Optimized Resonant Cavity for a Compact Sub-Terahertz Gyrotron, J. Infrared Millimeter and Terahertz Waves, 31(2010) 1115−1125.

13. J. Mudiganti, T. Idehara, Y. Tatematsu, R. Ikeda, Y. Yamaguchi, T. Saito, I. Ogawa, S. Mitsudo, T. Fujiwara, Y. Matsuki, K. Ueda, Design of a 394.6 GHz Compact Gyrotron FU CW CI for 600 MHz DNP-NMR Spectroscopy, Proc. 36 Int. Conf. on Infrared, Millimeter and Therahertz Waves, IRMMW-THz 2011 (2−7 Oct, 2011, Houston, USA) 1−2.

14. Y. Tatematsu, Y. Yamaguchi, T. Idehara, T. Ozeki, N. Yamada, R. Ikeda, T. Kawase, J. Aiba, H. Kato, T. Kanemaki, I. Ogawa, T. Saito, Development of Gaussian beam output gyrotrons FU CW GI and FU CW GII, Proc. 36 Int. Conf. on Infrared, Millimeter and Therahertz Waves, IRMMW-THz 2011 (2−7 Oct, 2011, Houston, USA) 1−2.

15. R.G. Griffin, T.F. Prisner, High field dynamic polarization−the renaissance, Phys. Chem. Chem. Phys., 12 (2010) 5737−5740.

16. S.−T. Han, R. G. Griffin, K.−N. Hu, C.−G. Joo, C. D. Joye, J. R. Sirigiri, R. J. Temkin, A. C. Torrezan, P. P. Woskov, Spectral Characteristics of a 140−GHz Long−Pulsed Gyrotron, IEEE Trans. PlasmaSci., 35 (2007) 559−564.

17. Y. Matsuki, H. Takahashi, K. Ueda, T. Idehara, I. Ogawa, M. Toda, H. Akutsu, T. Fujiwara, Dynamic nuclear polarization experiments at 14.1 T for solid-state NMR, Physical Chemistry Chemical Physics, 12 (2010) 5799−5803.

18. S. Mitsudo, T. Higuchi, K. Kanazawa, T. Idehara, I. Ogawa, High-field ESR measurements using Gyrotron FU series as radiation sources, J. Phys. Soc. Japan, 72 Suppl. B (2003) 172−176.

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MSMW'13, Kharkov, Ukraine, June 23-28

19. S. Sabchevski, T. Idehara, Design of a Compact Sub-Terahertz Gyrotron for Spectroscopic Applications, J. Infrared Millimeter and Terahertz Waves, 31(2010) 934−948.

20. A. ROGALEV, J. GOULON, F. WILHELM, G. GOUJON, X-RAY DETECTED MAGNETIC RESONANCE: A NEW TOOL TO PROBE MAGNETIZATION DYNAMICS, J. Infrared Millimeter and Terahertz Waves, 33(2012) 777−793.

21. Suehara, A. Ishida, T. Namba, S. Asai, T. Kobayashi, H. Saito, M. Yoshida, T. Idehara, I. Ogawa, S. Kobayashi and S. Sabchevski, The First Direct Measurement of the Hyperfine Splitting in Positronium, Journal of Physics: Conference Series, 194, (2009) 152010

22. S. Asai, T. Yamazaki, A. Miyazaki, T. Suehara, T. Namba, T. Kobayashi, H. Saito, T. Idehara, I. Ogawa, S. Sabchevski, Direct Measurement of Positronium Hyperfine Structure: ~A New Horizon of Precision Spectroscopy Using Gyrotrons~, J. Infrared, Millimeter and Therahertz Waves, 33 (2012) 766-776.

23. K. Hirakawa, N. Sekine, Carrier dynamics and dispersive terahertz Bloch gain in semiconductor superlattices , Physica E, 32 (2006) 320-324.

24. Y. Sakasegawa, T. Idehara, Y. Yamaguchi, S. Mitsudo, K. Hirakawa, Current suppression in semiconductor superlattices driven by intense sub-Thz radiation from a gyrotron, Proc. 17th Int. Conf. on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures EDISON 17 (7−12 August, 2011, Santa Barbara, California) P1.18.

25. S. Sabchevski, T. Idehara, S. Mitsudo, T. Fujiwara, Conceptual Design Study of a Novel Gyrotron for NMR/DNP Spectroscopy, J. Infrared and Millimeter Waves, 26 (2005) 1241−1264

26. S. Sabchevski, T. Idehara, Resonant Cavities for Frequency Tunable. Gyrotrons, J. Infrared Millimeter and Terahertz Waves, 29 (2008) 1−22.

27. O. Dumbrajs, T. Idehara, S. Sabchevski, Design of an Optimized Resonant Cavity for a Compact Sub-Terahertz Gyrotron, J. Infrared Millimeter and Terahertz Waves, 31(2010) 1115−1125.

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