Abstract
We investigate parametric decay instabilities (PDIs) occurring for gyrotron radiation near the upper hybrid resonance at the ASDEX Upgrade tokamak. The PDIs are observed through anomalous millimeter-wave scattering which is recorded using the high-resolution, fast acquisition collective Thomson scattering system installed at ASDEX Upgrade, and an experiment in which such observations are made during a scan of the toroidal magnetic field is performed. A previously published theoretical model is used to calculate the gyrotron power necessary to excite PDIs in the experiment; the theoretical model is capable of predicting whether or not PDIs will be observed at a given toroidal magnetic field with a high degree of accuracy.
Highlights
When high-power gyrotron beams are injected into magnetically confined fusion plasmas, the wave amplitude may in some cases become so large that the linear approximation breaks down and a parametric decay instability (PDI), which couples the incoming electromagnetic wave to two plasma waves, is excited
We seek to validate the theoretical model from [8], which gives the gyrotron power threshold, Pt0h, that must be exceeded in order to excite PDIs in the collective Thomson scattering (CTS) experiments at ASDEX Upgrade, by calculating the fraction of power converted to X-mode upon the reflection from the wall, the optical thickness of the ECR, the field enhancement near the upper hybrid resonance (UHR), and assuming decay of the X-mode wave to a warm lower hybrid (LH) wave and an electron Bernstein wave
We have investigated PDIs occurring for X-mode radiation near the UHR at ASDEX Upgrade
Summary
When high-power gyrotron beams are injected into magnetically confined fusion plasmas, the wave amplitude may in some cases become so large that the linear approximation breaks down and a parametric decay instability (PDI), which couples the incoming electromagnetic wave to two (electrostatic) plasma waves, is excited. Because of the frequency selection rule, the high-frequency wave will be down-shifted by approximately ωLH (∼ 2π×1 GHz in the main plasma of ASDEX Upgrade) from the gyrotron radiation, generally placing it outside the notch filters protecting the detectors used for millimeter-wave diagnostics such as ECE and collective Thomson scattering (CTS), causing it to hamper or potentially even damage these diagnostics. PDIs. In the present work, we seek to validate the theoretical model from [8], which gives the gyrotron power threshold, Pt0h, that must be exceeded in order to excite PDIs in the CTS experiments at ASDEX Upgrade, by calculating the fraction of power converted to X-mode upon the reflection from the wall, the optical thickness of the ECR, the field enhancement near the UHR, and assuming decay of the X-mode wave to a warm LH wave and an electron Bernstein wave. The paper is arranged as follows: in Section 2 the experimental setup and observations at ASDEX Upgrade are described; in Section 3 the results of the theoretical modelling are presented and compared with the observations; in Section 4 we draw our conclusions
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
