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FIR Center Report FIR FU-102 February 2010 Design of a Compact Sub-Terahertz Gyrotron for Spectroscopic Applications S. Sabchevski and T. Idehara Research Center for Development of Far-Infrared Region University of Fukui Bunkyo 3-9-1, Fukui 910-8507, Japan Tel 81 776 27 8657 Fax 81 776 27 8770 Design of a Compact
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     FIR Center Report FIR FU-102February 2010 Design of a Compact Sub-Terahertz Gyrotronfor Spectroscopic ApplicationsS. Sabchevski and T. Idehara  Research Center for Development of Far-Infrared RegionUniversity of Fukui Bunkyo 3-9-1, Fukui 910-8507, Japan  Tel 81 776 27 8657Fax 81 776 27 8770      Design of a Compact Sub-Terahertz Gyrotron for SpectroscopicApplications S. Sabchevski 1, 2 and T. Idehara 11  Research Center for Development of Far-Infrared Region, University of Fukui,Fukui 910-8507, Japan 2  Institute of Electronics of the Bulgarian Academy of Sciences, Sofia 1784, Bulgaria Abstract In this paper we present the initial design of a novel and versatile high frequency gyrotron withparameters suitable for application to various spectroscopic studies that require coherentradiation in the subterahertz frequency range (such as NMR/DNP spectroscopy, ESRspectroscopy, spectrometer based on the X-ray detected magnetic resonance etc. ). The mostcharacteristic feature of the design is that it utilises a compact, cryogen-free 8 T superconductingmagnet. As a result, the overall dimensions of the entire device are considerably reduced incomparison with the previously developed tubes belonging to the Gyrotron FU and Gyrotron FUCW series. This makes the novel gyrotron highly portable to diverse laboratory environments andeasily embeddable to different measuring systems. The electron-optical system (EOS) of the tubeis based on a compact low-voltage magnetron injection gun (MIG), which has been speciallydesigned and optimized together with the resonant cavity using our problem-oriented softwarepackage GYRSIM for CAD of gyrotrons. The tube operates at the second harmonic of thecyclotron frequency and generates a radiation with an output power of about 100 W and afrequency tunable up to around 424 GHz, respectively. Key words: compact   gyrotron, cryogen-free superconducting magnet, sub-terahertzspectroscopy  1. Introduction As the most powerful sources of coherent radiation in the sub-terahertz and the terahertzfrequency range, the gyrotrons are being widely used in many fields of the fundamental physicalresearch and in the technologies [1,2]. Their notable applications include, but are not limited to:electron cyclotron resonant heating (ECRH) and electron cyclotron current drive (ECCD) of magnetically confined plasma in the reactors for controlled thermonuclear fusion; processing of advanced materials (ceramic sintering, welding, annealing, thermal treatment of semiconductors,glasses and plastics, polymer coating and curring of adhesives and so on); communication andradar systems. Among the recently emerged applications are also various types of high frequencyspectroscopy like electron spin resonance (ESR) or electron paramagnetic resonance (EPR)spectroscopy and nuclear magnetic resonance (NMR) spectroscopy with enhancement of thesignal by a dynamic nuclear polarization (DNP/NMR) at high magnetic fields [3]. For the latterapplications a number of gyrotrons have been developed or are under development now in US[4-7], Russia [8-9], Switzerland [10,11], Germany, India [12] as well as at the FIR FU ResearchCenter in Japan [13-18].Since the gyrotron used as a radiation source is only a part of the whole spectroscopic systemto which it must be integrated, its dimensions are a critical issue and influence the overallarrangement and functionality of the equipment. It should be noted also, that besides being bulkythe gyrotrons that use conventional superconducting magnets are difficult to maintain and operatedue to the considerable time for preparation of the cryogenic system (filling with liquid heliumwhich is both expensive and difficult to handle). An appealing alternative, which can solve theseproblems, is offered by the availability of compact cryogen-free (aka ―liquid helium - free‖) andcompact superconducting magnets based on the Gifford-McMahon (GM) closed cyclerefregerators. The use of such magnets simplyfies the operation of the radiation sourcesignificantly and makes it attractive to wider comunity of researchers.In this paper we present the initial design of a novel compact high frequency gyrotronoperating at the second harmonic of the cyclotron frequency and delivering a continuous wave of a frequency around 424 GHz and output power about 100 W. The article is organised as follows.First we formulate the targeted design goals and outline the main features of the conceptualdesign. Then we depict the main components, namely the magnetic system, the electron-opticalsystem (EOS) and the resonant cavity and discuss the design choices (type and configuration of the magnetron injection gun, shape and dimension of the resonant cavity, selection of theoperating mode etc. ). Finally we make some conclusions, evaluating the current design andformulating some further tasks directed forwards the optimization of the device. 2. Goals and basic features of the design The primary aim of this project is to develop a compact, sub-terahertz high performance gyrotronwhich takes advantage of the beneficial properties of a 8 T liquid cryogen-free superconductingmagnet and is suitable for embedding as a powerful radiation source in various high frequencyspectroscopic systems. The main requirements regarding such sources are: (i) high stability of   both the output power (at levels of several tens of Watt) and the frequency (typically of the orderof 10 ppm) in a CW mode of operation during long periods of time; (ii) frequency tunability in a wide range (preferably of the order of ± 0.5 GHz ); (iii) convenient output and transmissionsystem which delivers the radiation to the spectrometer in the form of a well collimated Gaussianbeam; (iv) ease of maintenance and operation.Bellow we present the results of an iterative process of a computer aided design (CAD)performed using the problem oriented software package GYRSIM [20], developed at FIR FU andused for study and optimization of the gyrotrons belonging to two series of devices, namelyGyrotron FU and Gyrotron FU CW [2]. 3. Magnetic system of the gyrotron The main component of the magnetic system is a compact 8 T liquid cryogen-free (also called ― cryo-cooled ‖ ) superconducting magnet. Its basic dimensions are shown in Fig. 1 and itsspecification is given in the Table 1. It can be seen that it is indeed a table-top unit and has aconfiguration which enables both simple installation and operation. Additionally the powersupply and the control system are characterized by small weight and dimensions too. Since thereis no liquid cooler around the coils the magnet can be installed not only vertically but in differentpositions too. Other benefits of such magnet are low operating cost (no expenses for storage andtransport of liquid helium and nitrogen) and ease of use (no need for special safety measures andtraining of the personal).The main constraint imposed by the construction of the magnet which influences the overalldesign of the tube is the inner bore diameter of 52 mm and length of 338 mm. It makes itnecessary to develop a very slim EOS which can be inserted inside the magnet.Table 1. Specification of the superconducting magnetType 8T52, Liquid He-free conduction cooled (or, ―cryo - cooled superconducting magnet‖ according to the recommendation of theCryogenic Association of Japan)Wire material of the superconducting coils NbTiMaximum central magnetic field 8 TOperating current of the magnet 72.2 ARoom temperature bore size 52 mmRoom temperature length of the bore 338 mmThe measured distribution of the magnetic field on the axis of the magnet at the nominalexcitation current (72.2 A) as well as the field profiles calculated by the COILS code (fromGIRSYM package) and POISSON/SUPERFISH code are shown in Fig.2. These data are used inthe numerical experiments for simulation of the EOS and the resonant cavity. For a fine tuning of the magnetic field intensity in the region of the magnetron injection gun (MIG) a set of twoadditional coils is used together with the superconducting magnet.
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