Mode-converted Electron Bernstein Waves
Transcript of Mode-converted Electron Bernstein Waves
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Talk at IAE, Kyoto University July 30, 2012 “Mode-converted Electron Bernstein Waves” (F. Volpe) 1
Mode-convertedElectron Bernstein Waves
Francesco Volpe
currently at Dept of Applied Physics and Applied Mathematics
Columbia University, New York
Presentation prepared while at Engineering Physics Dept
University of Wisconsin, Madison
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Talk at IAE, Kyoto University July 30, 2012 “Mode-converted Electron Bernstein Waves” (F. Volpe) 2
Outline
Motivation: “Someone likes it hot” and overdense
Principle of EBWs and Mode Conversions
EBWs in
Summary & Conclusions
This Talk Madison World
Stellarator W7-AS HSX CHS, Heliotron, LHD, WEGA
Sph.Tokamak MAST Pegasus Mini-RT, NSTX
Tokamak TCV DIII-D?
RFP RFX-mod MST
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Motivation for Electron Bernstein Waves
Several MCF plasmas are too dense or have too low magnetic field
for conventional Electron Cyclotron Resonant Heating (ECRH)
EBWs do not suffer from upper density limits
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Electron Bernstein waves
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Marine waves are another example of gyrophase organization
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OXB-Scheme: special view makes modes degenerate and not evanescent
(wp
/w)
2
0
1
z
B
L
Slow X
OL
O-wave cut-off
x
In the real space In the phase space
x
sin2f=N2z,opt=Y/(Y+1), Y=wc/w
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OXB-Scheme: O-X conversion
Merging of O and SX waves
when their cutoffs overlap:
SXOAiry
even
odd
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SX-B mode conversion
Finite Larmor radius
6th order correction to Booker quartic
Deflection from UHR and
Propagation at X>1 (overdense), until
Cyclotron damping
Conversion efficiency can be limited by:
- Collisional damping (if edge cold)
- Back-tunneling SX-FX (if barrier thin)
- Parametric decay (at high power)
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FX-(SX)-B “Conversion” is a Tunnel Effect
X-wave damped
Unlike OXB, evanescent layer always has finite thickness.
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Mode conversions are ubiquitous wave phenomena at medium interfaces. Example: seismic waves
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Mode conversions are ubiquitous wave phenomena at medium interfaces. Example: seismic waves
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EBW Emission, Heating & CD
in the W7-AS Stellarator
(with H.P. Laqua)
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Antenna
Max Transmission
Min Doppler effects
Pure O-mode detection
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Oblique view elliptic polarization phase-shifterneeded
polarization rotatorl/4 phase shifter
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Broadband phase shifter
Requirements:
Phase shift f=90080 (2% intensity)
Frequency band f=66-78GHz
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Test quasi-optical components
Antenna
Polarizer
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Heterodyne radiometer
After several amplifiers
and Power-splitters...
24 channel ECE heterodyne radiometer, f=63-78GHz +
16 channel „Zoom“ Df=4GHz, fLO adjustable +
8 new fixed channels
Independent acquisition for 70 and 140GHz radiometer
Overlap: ECE (X2) ne<1.2 ·1020 m-3
EBE (B1) ne> 6 ·1019 m-3
BIF=300MHz
spatial resolution ≥1cm
Bvideo≤1MHz
temporal resolution≥2ms
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Ray tracing
wwww
/
r/N
/
N/r
D
D
d
d
D
D
d
d
5th order Runge-Kutta
OX: 1) optimization of injection angle,
2) symmetries in narrow evanescent layer
XB: Ridder‘s method to correct N
Full stellarator geometry
Poloidal component
Off-midplane launch
Toroidal curvature
Broken axisymmetry
Parallel computing on 16 workstation cluster
D=Det(eij+NiNj-N2dij), e=hot diel.tensor
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EBE Spectrum and Profile
ne from TS and Li-Beam
B from equilibrium code TRANS} mapping f r
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Heat Waves: EB-diagnostic during EB-heating
EBH
Overlapped thermal spectrum cannot be
measured at the same harmonic
Non-thermal peaks fpumpnfLH (n=1,2,...)
Parametric decay
Problem:
EBH 2nd harm. & EBE 1st harm.
Plasma has to be O2-overdense
(at least ne >2.4·1020 m-3,
recently ne 4 ·1020 m-3)
Solution:
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Confinement Transition
H-Mode Threshold often
exceeds ECE cutoff:
EBE edge
EBE core
Ha
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Edge-localized Modes
Cross-correlation with Ha :
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Radiative Collapse
Hysteresis ?
T
fq~nenimp
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Due to oblique injection, OXB heating also drives current !
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Angle is fixed by OXB, but co/ctr-CD can be selected and N|| adjusted by varying the magnetic configuration
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EBW Emission & Heating
in the MAST Spherical Tokamak
(with V. Shevchenko)
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2D-steerable, multi-beam antenna allowed simultaneous EBW emission and heating
F Volpe, “EBW Studies on MAST”,
St.Petersburg, 27 January 2004
Mirror 5
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Emission in a spherical tokamak is “banded” as a consequence of low aspect ratio
R-cutoff
L-cutoff
UHR
Plasma
Resonance
5wce
4wce
3wce
2wce
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EBE evidenced fast electrons at sawtooth crashes
Da
Soft-X Rays
EBE
60.5GHz
62.5GHZ
63.5GZ
66.5GHz
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2D angular scans of EBW emission indicated optimal direction for heating at MAST spherical tokamak
-7 -6 -5 -40.0
0.2
0.4
0.6
0.8
1.0
f=60.5GHz, f = -7deg
BX
O-E
BW
Sig
nal (
a.u
.)
Poloidal angle (deg)
-11 -10 -9 -8 -7 -6 -5
0.0
0.2
0.4
0.6
0.8
1.0f=60.5GHz, = -5.6deg
BX
O-E
BW
Sig
na
l (a
.u.)
Toroidal angle (deg)
2.5o FWHM 1.2o FWHM
Identified optimal launch direction for
first OXB heating in a spherical tokamak
In spite of non-optimal frequency (60GHz)
and alignment issues
Time, sec
60 kJ (~3 MW)
7 kJ (~ 0.25 MW)
250 kW
EC
RH
Po
we
rTo
tal
En
erg
y
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Btotal
ne
PlasmaO-mode
O-mode
2D angular scans confirmed that emission is anisotropic New q-profile diagnostic concept
Inclination of conversion contours at various f gives
Inclination of field line at various radial positions
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3.3585
tonnes
3.3585
tonnes
Self-balanced design spins at 12,000rpm
First measurements carried out.
Analysis under way.
Special design allows tilted mirror to spin at 12,000rpm and scan EBW emission within the same discharge
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EBW Heating
in the TCV Tokamak
(with A. Mϋck, L. Curchod, A. Pochelon et al.)
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Angular Scan of EBW injection shows min stray (max conversion) close to calculated optimum
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Stray radiation correlates with ELMy phases, consistently with reduced conversion efficiency
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SXR during pulsed EBWH was complicated by sawteeth
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Long, more central (r~0.4) EBWH resulted in clear bigger-than-sawtooth Te increase
Te builds over e~50ms
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Modeling of EBW Current Drive
in the RFX-mod RFP
(with R. Bilato and A. Köhn)
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For RFX-mod values of ne and Te at EBW deposition for 28GHz,
100kW absorbed should drive ~10kA, sufficient to improve confinement
Steady state
Localization
Position control
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Harmonics of ~11GHz are best for Current Drive at reversal layer of 1MA RFX-mod plasmas
Analytic Equilibrium:
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Magnetic configuration of RFP has some peculiarities
1. |B| almost poloidally symmetric, doesn’t scale as 1/R
Any launching position is Low Field Side (LFS).
No High Field Side (HFS) schemes, e.g. no SX-B.
2. low B low wce low w long vacuum wavelength λ
cutoffs and resonances close to each other within few λ tunneling
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FX-B is unsuitable for RFX-mod
Equilibrium at 1MA f>11GHz, e.g. f=28GHz l=1cm
Ln=2cm
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Injection along a vertical port, slightly tilted, is optimal
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Further waist optimization leads to 58% efficiency
Limiting factors to higher efficiency:
curvature mismatch (included)
fluctuations (not included)
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ne and B fluctuations reduce conversion efficiency
OXB
ne fluctuations at O-mode cutoff
a) cutoff is corrugated incidence on cutoff locally non-optimal reduced OX conversion efficiency
b) cutoff moves optimal angle changes with time mode conversion efficiency fluctuates in time
B fluctuations at O-mode cutoff
optimal angle fluctuates according to Nz,opt2 = wce/(wce+w)
FX-B
ne and B fluctuations at FX-mode cutoff and UHR their positions and thus thickness of evanescent layer in between fluctuates
Solutions; f/back on direction or, easier, on w
longer wavelength for easier tunneling
higher frequency for deeper cutoff, in less turbulent region
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“Alf’s” full-wave code
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Full-wave code reconstructs OXB conversion and follows EBWs until their wavelength can be resolved by grid
R or FX cutoff
UH resonance
P or O cutoff
L or SX cutoff
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Multiple reflections by graphite wall re-inject some power and increase overall OXB efficiency to 67%
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Summary and Conclusions
• Stellarators need EBWs
- overdense
+ Mode-converted EBWs benefit from Stellarator environment
- steep gradients, external control of configuration, tolerable ne and B fluctuations
= A marriage!
• EBW heating, emission and CD all proved in W7-AS stellarator
• In spite of 60GHz being too high, EBW heating succeded also in MAST,
- after scenario optimization (steep gradients)
- and guidance from EBW emission (optimal direction).
• “Spinning mirror” provided angular scans of OXB conversion efficiency every 10ms possible q-profile diagnostic.
• EBW heating also proved in TCV tokamak
• EBW CD at edge of RFX-mod RFP plasma might trigger tranport barrier
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Long-term (power-plant and CTF) applications of EBWs
Stellarators: compensation for undesired bootstrap current at high b .
Sph.Tokamaks: Non-inductive start-up and growth of a target for NBI.
Current profile tailoring.
Tokamaks: Te diagnosis of ITER divertor region? Not overdense, but Upper Hybrid resonant layer (thus, EBWs) exists.
RFPs: Improved confinement by suppressing MHD and triggering transport barriers at reversal layer.