prospects for a dominantly microwave-diagnosed magnetically … · 2016-04-11 · prospects for a...
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Prospects for a dominantly microwave-diagnosed magnetically confined fusion reactor
Francesco VolpeColumbia University
Presented at the 4th International Conference on Frontiers in Diagnostic TechnologiesFrascati (Italy), March 30 – April 1, 2016
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Apart from liquid metal work, group uses microwaves to form and heat stellarator plasma…
CNT Stellarator
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…and group uses microwaves to stabilize MHD instabilities and diagnose plasma
Locked Mode Stabilization
Metamat., Reverse Chromatic Aberration
More details:http://pl.apam.columbia.edu
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Prospects for a dominantly microwave-diagnosed magnetically confined fusion reactor
Francesco VolpeColumbia University
Presented at the 4th International Conference on Frontiers in Diagnostic TechnologiesFrascati (Italy), March 30 – April 1, 2016
Acknowledgement/Citation:based on previous works by R. Boivin, A. Costley, T. Dolan, T. Donné, F. Orsitto, G. Vayakis and conversations with F. Orsitto.
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Environment harsher and harsher from ITER, to DEMO and plant
[C. Kessel, FPA 2015]
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Outline
• Reactors will be challenging for some diagnostics:– Magnetic, Optical widely recognized
– Beam-based sometimes overlooked?
• m-wave and direct-line-of-sight diagnostics are robust– Some well-established
• Example: Electron Cyclotron Emission
– Some existing but not top choice • Example: ne from microwave scattering
– Some non-existing • New proposal: internal, local measurement of B based on
mode-conversion oblique reflectometry imaging
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A harsh environment for diagnostics• Heat load• Energetic particles, sputtering, erosion• Neutrons
– Nuclear heating ≫ 1 MW/m2
– Transmutations
• Radiation-induced – EMF due to Compton & photo-electrons,
along & across mineral-insulated coaxial cables,
(plastic insulation radiation-damaged too quickly)
– Conductivity in insulators
– Degradation
[Dolan 2014]
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A harsh environment for diagnostics (cont’d)• Damage to optics
– Sputtering (except W, Mo? Cleaning?)– Coating by impurities and polymers– Radiation-induced darkening– Radiation-induced luminescence
• Tritium retention in spectrometers• Vibrations, Electromagnetic loads• T-induced EMF, Thermal cycling
baking/operation
[Dolan 2014]
After ~100 hours cumulative exposure to DIII-D plasma
[McKee 2016]
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A harsh environment for diagnostics (cont’d)• Reduced neutral beam penetration in large plasma
might affect beam-based diagnostics:1. Beam Emission Spectroscopy
• Density fluctuations 𝑛𝑒2. Charge Exchange Recombination
• Rotation 𝑣𝜙and ion temperature 𝑇𝑖3. Motional Stark Effect
• Magnetic field helicity 1 𝑞
2 & 3 important in tokamak, less so in stellarator
[Volpe 2016]
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Magnetics, Bolometers, Neutron and Opt. Diagnostics most susceptible to this Harsh Environment
[Donné, NF 2012]
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Tokamak will have additional needs and challenges compared with stellarator reactor
• Disruptions• Runaway electrons• Lower aspect ratio Æ
reduced beam penetration• Importance of rotation and q profile
W7-X, Germany
11
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Several diagnostics from present tokamaks might have issues or be unavailable in a reactor
• MSE
• DISRAD
• CER
• ASDEXgauges
• ECE
• Thermocouple array• Langmuir probes
• Thomson scattering
• Neutrons• Reflectometers
• Phase contrast imaging
• IR cameras
•VUV cameras
• Langmuir probes
• Verticalscanning probe
• Radial scanning probe
• FIR & mw scattering
• Bolometers• Lithium beam spectroscopy
• Visible bremsstrahlung
• SXR• Filterscopes
• FIDA
• Fast ion collectors
• Fast wave reflectometer
• DiMES• Magnetics
• Interferometers
• MDS spectrometer
• SPRED
• Tile current array
• Fastframingcamera
•Visiblecameras• BES
• TALIF• CECE
• ECEI • NPAs2/2011
• Fast Ion Loss Detector• Gamma detectors
• DBS
• Coherence Imaging
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Solutions
• Use present diagnostics, but innovate:– New materials, resilient to radiation effects and transmut. (Pt vs. Au) – Harden mirrors (single crystals, liquid mirrors)– In-situ cleaning (laser, microwave)
• ...or protect them by brute force:– Shutters– Additional shielding– Additional cooling
• Use present FIR, THz, mm-wave, m-wave diagnostics and direct-line-of-sight (no lens, no mirrors) as much as possible
• Develop FIR, THz etc. diagnostics of observables currently inaccessible to these techniques
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Plasma diagnostics cover whole available spectrum.Issues at some f, l, E.
[Boivin 2012]
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Most observables can also be probed at non-problematic f, l, E
Fusion alphas, Neutrons and gammas
Electron Temperature
Electron Density
[Boivin 2012]
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Some measurements traditionally rely on f, l, E that will create issues
Ion Temperature and rotation
Instabilities
Magnetic structure
[Boivin 2012]
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The dielectric tensor e contains information on n, T, B, v...
• Magnetic fusion plasmas are non-uniform, anisotropic, birefringent optical media
• wce, wpe≈ 10-100 GHz• Wave Emission, Interference, Reflection, Refraction, Scattering Æ
info on local or line-averaged eÆ info on n, T, B, v, …
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Collective Thomson Scattering (CTS) can measure ion distribution and related quantities
• Electrons respond to wake created by much slower ions
• Microwave scattering measures that wake and gains information on:– 𝑇𝑖– energetic ions (’s)– 𝑣𝜙?
kIncident radiation
Received scattered radiation
ks
kiResolved fluctuations
ReceiverProbe
Bindslev et al.
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Collective Thomson Scattering (CTS) can also measure impurity content
[Orsitto, Volpe 1998]
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Microwave scattering measures turbulent nefluctuations, through Doppler broadening
ETG Turbulenceand electron diamagnetic velocity
[Mazzucato 2009]
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Microwave diagnostics of magnetic field are line-integrated or sensitive to fluctuations• Polarimetry
– Faraday rotation (𝐤 ∥ 𝐁)– Cotton-Mouton effect (𝐤 ⊥ 𝐁)
• Cross-Polarization Scattering – 𝛿𝑛𝑒 preserves polarization OÆO, XÆX– 𝛿𝐵⊥ changes polarization OÆX, XÆO
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Special B-dependent view makes O and X modes degenerate and not evanescent at n-dependent location
(wp
/w)2
0
1
z
B
L
Slow X
O L
O-wave cut-off
x
sin2f=N2z,opt=Y/(Y+1), Y=wc/w
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2D angular scans confirmed that BXO-converted EBW emission is anisotropic Æ q-profile diagnostic in overdense plasmas
Btotal
ne
PlasmaO-mode
O-mode
Inclination of conversion contours at various f givesInclination of field line at various radial positions
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OX conversion of injected wave has advantages over BXO conversion of internally emitted EBWs• Instead of peak in transmissivity, “hole” in reflectivity
• More flexible: no need for EBW emission (arbitrary n and B)• SNR, time- and space-resolution are reflectometer-like!
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Imaging around reflectivity hole reinforces q-profile measurement and could add details on magnetic structure
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Full-wave modeling was performed at 10-20 GHz
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Full-wave modeling exhibits expected “hole” in reflected signal
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Conclusions
• Magnetic, optical and beam-based diagnostics will face challenges in magnetic fusion reactors
• Ongoing research on new materials, hardening, shielding, in-situ cleaning...
• In parallel with that, microwave diagnostics could provide an alternative route to reactor diagnosis– Existing and widely utilized– Existing but relatively under-utilized. Ex.: scattering Æ 𝑇𝑖, 𝑣𝜙, 𝛿𝑛𝑒…
• Complementary to BES and CER?– To be developed. Example:
• Mode-Conversion Oblique Reflectometry Imaging• Complementary to MSE?