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

Apart from liquid metal work, group uses microwaves to form and heat stellarator plasma…

CNT Stellarator

…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

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.

Environment harsher and harsher from ITER, to DEMO and plant

[C. Kessel, FPA 2015]

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

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]

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]

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]

Magnetics, Bolometers, Neutron and Opt. Diagnostics most susceptible to this Harsh Environment

[Donné, NF 2012]

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

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

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

Plasma diagnostics cover whole available spectrum.Issues at some f, l, E.

[Boivin 2012]

Most observables can also be probed at non-problematic f, l, E

Fusion alphas, Neutrons and gammas

Electron Temperature

Electron Density

[Boivin 2012]

Some measurements traditionally rely on f, l, E that will create issues

Ion Temperature and rotation

Instabilities

Magnetic structure

[Boivin 2012]

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, …

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.

Collective Thomson Scattering (CTS) can also measure impurity content

[Orsitto, Volpe 1998]

Microwave scattering measures turbulent nefluctuations, through Doppler broadening

ETG Turbulenceand electron diamagnetic velocity

[Mazzucato 2009]

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

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

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

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!

Imaging around reflectivity hole reinforces q-profile measurement and could add details on magnetic structure

Full-wave modeling was performed at 10-20 GHz

Full-wave modeling exhibits expected “hole” in reflected signal

0.35�

0.0�X(m)�

Z(m)�

0.35�

0.0�

X(m)�

0.35�

0.0�

X(m)�

1.0�0.0��

Ex

Ey

Ez

(a)�

(b)�

(c)�

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?