A Compact Two-Frequency Notch Filter for Millimeter Wave Plasma Diagnostics
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A Compact Two-Frequency Notch Filter for Millimeter Wave Plasma Diagnostics D. Wagner 1 M. Thumm 3
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& W. Kasparek & F. Leuterer & F. Monaco & T. Ruess & J. Stober &
Received: 18 March 2020 / Accepted: 6 May 2020/ # The Author(s) 2020
Abstract
Sensitive millimeter wave diagnostics in magnetic confinement plasma fusion experiments need protection from gyrotron stray radiation in the plasma vessel. Modern electron cyclotron resonance heating (ECRH) systems take advantage of multifrequency gyrotrons. This means that the frequency band of some millimeter wave diagnostics contains more than one narrow-band gyrotron-frequency line, which needs to be effectively suppressed. A compact standard waveguide notch filter based on coupled waveguide resonators with rectangular cross-section is presented which can provide very high suppression of several gyrotron frequencies and has low insertion loss of the passband. Keywords Electron cyclotron resonance heating . Magnetically confined fusion plasmas . Millimeter wave diagnostics . Notch filter
1 Introduction Modern multi-frequency megawatt-class gyrotrons can operate at several frequencies in the millimeter wave range [1–4]. These frequencies correspond to the various transmission maxima of their single-disk chemical vapor deposition (CVD) diamond window. In recent years, high-power gyrotrons became very attractive as sources for electron cyclotron resonance heating (ECRH) systems in thermonuclear fusion plasma experiments, since they allow for more flexibility with respect to the applied magnetic field in the magnetic confinement fusion devices [5]. Sensitive
* D. Wagner [email protected]
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Max-Planck-Institut für Plasmaphysik, Boltzmannstr.2, D-85748 Garching, Germany
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Institut für Grenzflächenverfahrenstechnik und Plasmatechnologie, Universität Stuttgart, Pfaffenwaldring 31, D-70569 Stuttgart, Germany
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Karlsruher Institut für Technologie, Institut für Hochleistungsimpuls- und Mikrowellentechnik, Kaiserstr. 12, D-76131 Karlsruhe, Germany
Journal of Infrared, Millimeter, and Terahertz Waves
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a (mm)
Fig. 1 Geometry of the stepped waveguide width (top), frequency characteristic of reflected (middle) and transmitted modes (bottom)
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millimeter wave diagnostic systems need protection against ECRH stray radiation [6]. When applying multi-frequency gyrotrons, more than one frequency must be suppressed. A specific problem concerning these filters is the frequency chirp of high-power gyrotrons due to cavity heating and expansion, specifically at the beginning of the pulse, which can range from tens to Fig. 2 Calculated transmitted power as a function of width ar and length lr of the waveguide cavity. The red lines mark the domains where the transmitted power is below − 23 dB. The arrow marks the point where the m
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