Linear and Non-Linear Inserts for Genuinely Wideband Continuous Frequency Tunable Coaxial Gyrotron Cavities
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Linear and Non-Linear Inserts for Genuinely Wideband Continuous Frequency Tunable Coaxial Gyrotron Cavities Zisis C. Ioannidis & Olgierd Dumbrajs & Ioannis G. Tigelis
Received: 23 January 2008 / Accepted: 5 February 2008 / Published online: 23 February 2008 # Springer Science + Business Media, LLC 2008
Abstract We consider two continuous frequency tunable CW coaxial gyrotron oscillators, one 330 GHz with 3 GHz bandwidth and output power 50 – 400 W for scientific applications and one 30 GHz with 0.4 GHz bandwidth and output power 40 – 140 kW for industrial applications. The continuous tuning of both gyrotrons is achieved by moving the linearly tapered inner conductor in the axial direction in combination with the proper adjustment of the operating magnetic field. We consider also a non-linear tapering, which makes it possible to reduce the length of the insert and to improve efficiency of the device. Keywords Coaxial resonators . Gyrotrons . Frequency tuning
1 Introduction Frequency step-tunable gyrotrons for Electron Cyclotron Resonance Heating (ECRH) of magnetically confined fusion plasmas and for Electron Cyclotron Current Drive (ECCD) in plasmas were considered in [1, 2]. In recent years, new exciting applications of gyrotrons have emerged beyond fusion plasma applications. For example, Dynamic Nuclear Polarization (DNP) to enhance signal to noise ratio in Nuclear Magnetic Resonance Z. C. Ioannidis : I. G. Tigelis (*) Department of Electronics, Computing, Telecommunications and Control, Faculty of Physics, National and Kapoditrian University of Athens, Building V, Panepistimiopolis, Zografou, 15784 Athens, Greece e-mail: [email protected] Z. C. Ioannidis e-mail: [email protected] O. Dumbrajs Department of Engineering, Physics and Mathematics, Helsinki University of Technology, Euratom-TEKES Association, 02015 Helsinki, Finland e-mail: [email protected] O. Dumbrajs Institute of Solid State Physics, University of Latvia, Riga LV-1063, Latvia
Int J Infrared Milli Waves (2008) 29:416–423
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(NMR) [3, 4] and Electron Spin Resonance (ESR) spectroscopy experiments [5] as well as material processing [6] in the industry. In all these applications, the possibility of tuning the gyrotron frequency is a great advantage. There are several possibilities to change the frequency of a gyrotron oscillator by changing: i) the accelerating and modulation voltages (electrical tuning), ii) the magnetic field (magnetic tuning) and iii) the physical dimensions of the oscillator using a split cavity structure or a movable piston (mechanical tuning) (see e.g. [1] and references therein). Recently, an operation in the higher order axial modes was proposed [7, 8] for a smooth variation of frequency within a particular axial mode and between adjacent axial modes. It should be emphasized that as long as the cavity geometry is not changed during the operation, the tuning is possible only in discrete steps, which are determined by the difference between the eigenvalues of the modes. The exception is the so-called frequency pulli
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