measurement of lower-hybrid waves with microwave ... · a microwave scattering diagnostic is being...
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Measurement of lower-hybrid waves with
microwave scattering on TST-2
N. Tsujii, Y. Takase, A. Ejiri, H. Furui, H. Homma, K. Nakamura,W. Takahashi, T. Takeuchi, H. Togashi, K. Toida, T. Shinya,
M. Sonehara, S. Yajima, H. Yamazaki and Y. Yoshida
The University of Tokyo, School of Frontier Sciences
18th International ST Workshop/Princeton, November 3, 2015
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Outline
1 Introduction
2 Microwave scattering
3 Full-wave analysis of LH waves
4 Summary
5 Backup
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A microwave scattering diagnostic is being developed tomeasure the Lower-Hybrid (LH) waves directly and testnumerical simulations
BackgroundI Non-inductive plasma start-up with LH waves has been studied
on TST-2I The current drive efficiency in the actual experiment is much
lower than what is predicted by GENRAY-CQL3D
Microwaves in the range of 12-40 GHz have wavenumber similarto LH waves in TST-2 and suited for diagnosing these waves
The wave measurements will be compared with numericalsimulations to quantitatively understand the current drive physics
Previous direct LH wave measurementsI Microwave back-scattering with a reflectometer [Baek 2014]I CO2 laser scattering [Takase 1985]
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The TST-2 Machine
R = 36 cm
a = 23 cm
RF non-inductive start-up
Bt < 0.15 T(pulse length ∼80 ms)
ne < 1018 m−3
Ip < 25 kA
RF 400 kW @ 200 MHz
Ohmic (inductive) start-up
Bt < 0.3 T(pulse length ∼30 ms)
ne < 1019 m−3
Ip < 110 kA
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Non-inductive plasma start-up using LH waves is studiedon TST-2
t
Ip
ne
3©2©1©
LH NB 1 Plasma initiation (ECH, LH)
2 Ip ramp-up with LH waves
3 Further ramp-up with neutralbeam injection
The LH wave has one of the highest current drive efficiencyamong rf wavesSimilar scenario tested on JT-60U [Shiraiwa 2004]The plasma density must be kept low during Ip ramp-up← LH high-density limit
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LH waves @200 MHz can access 1018 m−3 for n‖ = 5-9
1014 1015 1016 1017 1018 1019 1020
ne [m-3]
0.00
0.05
0.10
0.15
0.20
0.25
0.30B
[T
]
ω = 2ωLH
accessible
5
MC
9 OH
RFP = 0
Density limit low at low field
ω > 2ωLH to avoid parametric decay instability6/22
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The experimentally achieved plasma current (< 25 kA) isan order of magnitude smaller than simulated
0 5 10 15 20ne [1017m-3]
0
100
200
300
400
Ip [
kA
]
0.05 0.10 0.15 0.20Bt [T]
0
5
10
15
20
ne [
10
17m
-3]
@h
alf I
p m
ax
MC (n||=6)
ne [1017m−3]
0.077 T0.1100.1650.220
(exp)
Simulation by GENRAY (ray-tracing) and CQL3D(Fokker-Planck solver)
The experimental density limit qualitatively consistent withsimulation
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Outline
1 Introduction
2 Microwave scattering
3 Full-wave analysis of LH waves
4 Summary
5 Backup
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A launched microwave beam can be Bragg-scattered bydensity fluctuations of the LH waves
digitizer
power divider
voltagecontrolledoscillator
plasma
focusing mirrors
mixer
LH
swept homodyne system
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Optimum scattering geometry was investigated from thesimulated typical wave trajectory on TST-2
-0.4
-0.2
0.0
0.2
0.4
Z [
m]
rays at toroidal angle=75°
-500 0 500 1000kR [m-1]
-1000-800
-600
-400
-200
0
200400
kZ
[m
-1]
10
100
fre
q [
GH
z]
parameters at phase match
-0.4
-0.2
0.0
0.2
0.4
xd [
m]
-40 -20 0 20 40incident beam angle θ [deg]
-0.15-0.10
-0.05
0.00
0.05
0.10
0.15Y
s [
m]
xdxd
1st pass2nd
3rd
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LH waves propagating in a wide range of poloidalcross-section can be detected with a microwave launchedfrom the midplane
Scan the incident beam angle θ
Scan the beam frequency
Detect the scattered light at multiple poloidal locations (xd)
The source frequency swept at 1 kHz→ the whole wavenumber spectrum is measured every 1 ms
Incident beam angle needs to be scanned on a shot-to-shot basis
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The O-mode microwave scattering system
Microwave bandsI Ku band (WR-62):
12.4-18.0 GHzI K band (WR-42):
18.0-26.5 GHzI Ka band (WR-28):
26.5-40.0 GHz
Incident beam from themidplane horizontal port
4-antenna arraybelow the plasma
Small toroidal tilt tocompensate wave kφ
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Outline
1 Introduction
2 Microwave scattering
3 Full-wave analysis of LH waves
4 Summary
5 Backup
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Full-wave simulation of LH waves was performed withAORSA
E‖ ne
ne = 5× 1017 m−3, Te = 500 eV, Maxwellian, Prf = 1 MW14/22
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The expected signal level is within the detectable range
0.2 0.3 0.4 0.5 0.6R [m]
-0.4
-0.2
0.0
0.2
0.4
Z [
m]
-2 -1 0 1 2kV/m
0.2 0.3 0.4 0.5 0.6R [m]
-4 -2 0 2 41016 m-3
0.2 0.3 0.4 0.5 0.6R [m]
-4 -2 0 2 41016 m-3
nphi=-15nphi=-15 φ=-105°φ=-105°nphi=-15nphi=-15
neE‖ ne
φ ∼ reλneL
∼ 2.8× 10−15m · 0.01m · 2× 1016m−3 · 0.05m = 0.0315/22
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Outline
1 Introduction
2 Microwave scattering
3 Full-wave analysis of LH waves
4 Summary
5 Backup
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The microwave scattering system was designed fordirectly measuring LH waves on TST-2
The current drive efficiency on TST-2 is lower than what ispredicted by GENRAY-CQL3D
A microwave scattering diagnostics at 12-40 GHz is designedand being fabricated
The LH fluctuation level estimated with AORSA is within thedetectable range
Future workI Detect LH wavesI Investigate changes in the location of LH waves and their
wavenumber spectrum for various parametersI Test GENRAY-CQL3DI Test full-wave codes (AORSA, TORLH)
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References
S. G. Baek, et al., Phys. Plasmas 21, 012506 (2014)
Y. Takase, et al., Phys. Fluids 28, 983 (1985)
S. Shiraiwa, et al., Phys. Rev. Lett. 92, 035001 (2004)
J.C. Wright, et al., Phys. Plasmas 16, 072502 (2009)
R.W. Harvey and M.G. McCoy, Proceedings of the IAEATechnical Committee Meeting on Simulation and Modeling ofThermonuclear Plasmas, Montreal, Canada, 1992
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Outline
1 Introduction
2 Microwave scattering
3 Full-wave analysis of LH waves
4 Summary
5 Backup
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Strong correlation of plasma current and density leads toupper limit in the driven current at a given magnetic field
0.04 0.06 0.08 0.10 0.12 0.14 0.16Bt [T]
0
5
10
15
20
25
30
Ip [
kA
]
0 2 4 6 8 10ne [1017m-3]
0
20
40
60
80
100
PLH [
kW
]
(n||=5)(n||=5)MC (n||=6)
Density limit substantially lower than expected at higher field?
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LH accessibility condition
LH ↔ FW mode conversion condition:ωpi
ω= N‖Y ±
√1 + N2
‖ (Y 2 − 1),
Y 2 =ω2
ωceωci.
In terms of N‖,
N‖ =ωpi
ωY ±
√1 +
ω2pi
ω2(Y 2 − 1).
Launching the wave from the low-density side, the parameter isaccessible if
N‖ > Na =ωpi
ωY +
√1 +
ω2pi
ω2(Y 2 − 1).
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Full-wave analysis will be performed
TORLH [Wright 2009]
Finite Larmor radius code(k⊥ρL < 1)
θ: spectral ansatz
ψ: finite element
Coupled with CQL3D[Harvey 1992]
→ Non-thermal electrondistribution
Weak absorption,multi-pass regime
→ Full-wave effect may beimportant
-0.3 -0.2 -0.1 0.0 0.1 0.2R-R0 [m]
-0.2
0.0
0.2
Z [m
]
0.01 0.10 1.00|E||| (a.u.)
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