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Senstius MG, Freethy SJ, Nielsen SK. Nonlinear degradation of O-X-B mode conversion in MAST Upgrade. EPJ WEB OF CONFERENCES 2023. [DOI: 10.1051/epjconf/202327701009] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/25/2023] Open
Abstract
Spherical tokamaks like the MAST Upgrade device are often operated in an overdense regime. As a consequence, conventional electron cyclotron resonance heating (ECRH) and current drive (ECCD) are typically not possible. MAST Upgrade is planned to investigate a mode coupling scheme known as O-X-B at high power, which may allow gyrotrons to heat and drive current in overdense plasmas by coupling electromagnetic waves to electrostatic electron Bernstein waves (EBWs) at the upper hybrid (UH) layer. However at the gyrotron beam intensities planned for MAST Upgrade, several nonlinear effects may degrade the linear mode coupling into EBWs. Using particle-in-cell simulations, parametric decay instabilities (PDIs) and stochastic electron heating (SEH) are investigated in the region near the UH layer. It is found that nonlinear effect could have a substantial impact on the O-X-B scheme in MAST Upgrade at high gyrotron intensities.
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Hansen S, Nielsen S, Stober J, Rasmussen J, Salewski M, Willensdorfer M, Hoelzl M, Stejner M. Parametric Decay Instabilities during Electron Cyclotron Resonance Heating of Fusion Plasmas, Problems and Possibilities. EPJ WEB OF CONFERENCES 2023. [DOI: 10.1051/epjconf/202327701002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/25/2023] Open
Abstract
We review parametric decay instabilities (PDIs) expected in connection with electron cyclotron resonance heating (ECRH) of magnetically confined fusion plasmas, with a specific focus on conditions relevant for the ITER tokamak. PDIs involving upper hybrid (UH) waves are likely to occur in O-mode ECRH scenarios at ITER if electron density profiles allowing trapping of UH waves near the ECRH frequency are present. Such PDIs may occur near the plasma center in ITER full-field scenarios heated by 170 GHz O-mode ECRH and on the high-field side of half-field ITER plasmas heated by 110 GHz or 104 GHz O-mode ECRH. Additionally, 110 GHz O-mode ECRH of half-field ITER scenarios may have low ECRH absorption, due to the electron cyclotron resonance being located on the high-field side of the main plasma. This potentially allows PDIs driven by a significant amount of ECRH radiation reaching the UH resonance in X-mode to occur, as X-mode radiation can be generated by reflection of unabsorbed O-mode radiation from the high-field side wall. The occurrence of PDIs during ECRH may damage microwave diagnostics, such as the electron cyclotron emission and low-field side reflectometer systems at ITER, as well as complicate the calculation of heating and current drive characteristics. However, if PDIs are induced in a controlled manner, they may provide novel diagnostic tools and allow the generation of a moderate fast ion population in plasmas heated only by ECRH.
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Senstius MG, Freethy SJ, Allen J, Nielsen SK. A radiometer to diagnose parametric instabilities during linear excitation of electron Bernstein waves in the Mega Amp Spherical Tokamak (MAST) Upgrade. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2022; 93:103522. [PMID: 36319379 DOI: 10.1063/5.0101785] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/03/2022] [Accepted: 09/07/2022] [Indexed: 06/16/2023]
Abstract
Highly overdense magnetically confined fusion plasmas, such as the Mega Amp Spherical Tokamak (MAST) Upgrade, cannot easily be heated using conventional electron cyclotron resonance heating because high density cutoffs block microwave access to the plasma core. Instead, electromagnetic waves can be coupled to electron Bernstein waves (EBWs) through the O-X-B mode coupling scheme, and the EBWs can then be absorbed at higher densities. The excitation of EBWs occurs at the upper hybrid (UH) layer where nonlinear wave interactions, called parametric decay instabilities (PDIs), are known to occur at reduced power thresholds. We present a design for a radiometer to detect PDIs during O-X-B in MAST Upgrade. The radiometer will aid in determining at what power levels PDIs become important as well as inferring various parameters about both electrons and ions near the UH layer. We estimate a gyrotron power density threshold for PDI and expected frequency shifts to be produced in them. The design allows for shifts from several decays involving lower hybrid (LH) waves to be observed.
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Affiliation(s)
- Mads G Senstius
- Department of Physics, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark
| | - Simon J Freethy
- CCFE, UKAEA, Abingdon, Oxfordshire, OX14 3DB, United Kingdom
| | - Joe Allen
- CCFE, UKAEA, Abingdon, Oxfordshire, OX14 3DB, United Kingdom
| | - Stefan K Nielsen
- Department of Physics, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark
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Donnelly SM, Lopez NA, Dodin IY. Steepest-descent algorithm for simulating plasma-wave caustics via metaplectic geometrical optics. Phys Rev E 2021; 104:025304. [PMID: 34525672 DOI: 10.1103/physreve.104.025304] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2021] [Accepted: 07/20/2021] [Indexed: 11/06/2022]
Abstract
The design and optimization of radiofrequency-wave systems for fusion applications is often performed using ray-tracing codes, which rely on the geometrical-optics (GO) approximation. However, GO fails at wave cutoffs and caustics. To accurately model the wave behavior in these regions, more advanced and computationally expensive "full-wave" simulations are typically used, but this is not strictly necessary. A new generalized formulation called metaplectic geometrical optics (MGO) has been proposed that reinstates GO near caustics. The MGO framework yields an integral representation of the wavefield that must be evaluated numerically in general. We present an algorithm for computing these integrals using Gauss-Freud quadrature along the steepest-descent contours. Benchmarking is performed on the standard Airy problem, for which the exact solution is known analytically. The numerical MGO solution provided by the new algorithm agrees remarkably well with the exact solution and significantly improves on previously derived analytical approximations of the MGO integral.
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Affiliation(s)
- Sean M Donnelly
- Department of Physics & Astronomy, Iowa State University, Ames, Iowa 50011, USA
| | - Nicolas A Lopez
- Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA
| | - I Y Dodin
- Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA.,Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
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Hansen S, Nielsen S, Stober J, Rasmussen J, Stejner M. Observation and Modelling of the Onset of Parametric Decay Instabilities during Gyrotron Operation at ASDEX Upgrade. EPJ WEB OF CONFERENCES 2019. [DOI: 10.1051/epjconf/201920302007] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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.
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Taylor G. Recent Developments in Electron Cyclotron Emission Research on Magnetically Confined Plasmas. FUSION SCIENCE AND TECHNOLOGY 2017. [DOI: 10.13182/fst07-a1491] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Affiliation(s)
- Gary Taylor
- Princeton University Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543
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Zohm H. Recent Experimental Progress in Electron Cyclotron Resonance Heating and Electron Cyclotron Current Drive in Magnetically Confined Fusion Plasmas. FUSION SCIENCE AND TECHNOLOGY 2017. [DOI: 10.13182/fst07-a1492] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Affiliation(s)
- Hartmut Zohm
- Max-Planck-Institut für Plasmaphysik, EURATOM Association D-85748 Garching, Germany
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Shevchenko VF, De Bock M, Freethy SJ, Saveliev AN, Vann RGL. Two-Dimensional Studies of Electron Bernstein Wave Emission in MAST. FUSION SCIENCE AND TECHNOLOGY 2017. [DOI: 10.13182/fst10-137] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Affiliation(s)
- V. F. Shevchenko
- EURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon, Oxon OX14 3DB, United Kingdom
| | - M. De Bock
- EURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon, Oxon OX14 3DB, United Kingdom
| | - S. J. Freethy
- EURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon, Oxon OX14 3DB, United Kingdom
- University of York, Heslington, York YO10 5DD, United Kingdom
| | - A. N. Saveliev
- Ioffe Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia
| | - R. G. L. Vann
- University of York, Heslington, York YO10 5DD, United Kingdom
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Shevchenko V, Baranov Y, Bigelow T, Caughman J, Diem S, Dukes C, Finburg P, Hawes J, Gurl C, Griffiths J, Mailloux J, Peng M, Saveliev A, Takase Y, Tanaka H, Taylor G. Long Pulse EBW Start-up Experiments in MAST. EPJ WEB OF CONFERENCES 2015. [DOI: 10.1051/epjconf/20158702007] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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O'Brien M. Electron Bernstein Waves In Magnetic Fusion Plasmas. FUSION SCIENCE AND TECHNOLOGY 2014. [DOI: 10.13182/fst13-699] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Affiliation(s)
- Martin O'Brien
- EURATOM/CCFE Fusion Association, Culham Centre for Fusion Energy United Kingdom
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Volpe F. A spinning mirror for fast angular scans of EBW emission for magnetic pitch profile measurements. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2010; 81:10D905. [PMID: 21033937 DOI: 10.1063/1.3475786] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
A tilted spinning mirror rapidly steers the line of sight of the electron Bernstein wave (EBW) emission radiometer at the Mega-Amp Spherical Tokamak (MAST). In order to resist high mechanical stresses at rotation speeds of up to 12,000 rpm and to avoid eddy current induced magnetic braking, the mirror consists of a glass-reinforced nylon substrate of a special self-balanced design, coated with a reflecting layer. By completing an angular scan every 2.5-10 ms, it allows one to characterize with good time resolution the Bernstein-extraordinary-ordinary mode-conversion efficiency as a function of the view angles. Angular maps of conversion efficiency are directly related to the magnetic pitch angle at the cutoff layer for the ordinary mode. Hence, measurements at various frequencies provide the safety factor profile at the plasma edge. Initial measurements and indications of the feasibility of the diagnostic are presented. Moreover, angular scans indicate the best launch conditions for EBW heating.
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Affiliation(s)
- Francesco Volpe
- Department of Engineering Physics, University of Wisconsin, Madison, Wisconsin 53706, USA.
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Diem SJ, Taylor G, Caughman JB, Efthimion PC, Kugel H, LeBlanc BP, Phillips CK, Preinhaelter J, Sabbagh SA, Urban J. Collisional damping of electron bernstein waves and its mitigation by evaporated lithium conditioning in spherical-tokamak plasmas. PHYSICAL REVIEW LETTERS 2009; 103:015002. [PMID: 19659154 DOI: 10.1103/physrevlett.103.015002] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/28/2008] [Indexed: 05/28/2023]
Abstract
The first experimental verification of electron Bernstein wave (EBW) collisional damping, and its mitigation by evaporated Li conditioning, in an overdense spherical-tokamak plasma has been observed in the National Spherical Torus Experiment (NSTX). Initial measurements of EBW emission, coupled from NSTX plasmas via double-mode conversion to O-mode waves, exhibited <10% transmission efficiencies. Simulations show 80% of the EBW energy is dissipated by collisions in the edge plasma. Li conditioning reduced the edge collision frequency by a factor of 3 and increased the fundamental EBW transmission to 60%.
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Affiliation(s)
- S J Diem
- Oak Ridge National Laboratory, Oak Ridge 37831, Tennessee, USA
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Erckmann V. Steady state ECRH for next step superconducting fusion devices. FUSION ENGINEERING AND DESIGN 2009. [DOI: 10.1016/j.fusengdes.2008.11.105] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
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