1/11
First Measurements of Parallel Wavenumber of Lower Hybrid Waves
S. G. Baek, G. M. Wallace, T. Shinya*, R. R. Parker, S. Shiraiwa, Y. Takase*
MIT Plasma Science and Fusion Center
*University of Tokyo
57th Annual Meeting of the Division of Plasma Physics
Savannah, Georgia
November 17, 2015
2/11
Motivation: Characterizing the LH wave propagation on the first pass is
crucial for predicting LHCD performance in future reactors.
• C-Mod LHCD system at 4.6 GHz operates at reactor-relevant conditions, except temperatures.
• In the strong single-pass damping regime (e.g., in ITER
and future reactors), parasitic loss mechanisms are
expected to be strongly mitigated.
• But, LH wave still needs to propagate across edge/SOL
plasma on the first pass (~ 15 cm in ITER as compared to
~ 2 cm in C-Mod.)
• Goal is to develop a diagnostic to characterize wave
propagation, in particular the parallel refractive index (n|| =
c k|| /ω0) of LH waves.
• With a new magnetic probe array, we carried out the first
proof-of-principle experiment.
See also:
I. Faust (Thu. Afternoon, VI2)
R. Parker (Monday CP12)
S. Shiraiwa (Monday CP12)
B. Mumgaard (Monday CP12)
1Wallace et al, Phys. Plasmas 17, 082508 (2010) 2Shiraiwa et al, RF Conference (2015)
O. Meneghini
MIT Ph.D Thesis S. Takahiro (Monday CP12)
Multi-Pass Regime in C-Mod
3/11
An array of three magnetic probes measures the dominant n|| of LH wave-
fields.
• Top row: 𝐵 ⊥ 𝐵0
• Bottom row: 𝐵 ||𝐵0
• Only the dominant single n|| can be obtained with the
three probes.
• Probe separation distance (Δx) = 6.5 mm: sensitive up to
n|| = 5.5.
1 cm Slit
Shield
Field Line Mapping Magnetic Probe w/ Shield Probe Head
20 cm
Δx = 6.5 mm
• Probe location below the midplane, and 108 deg
toroidally away from the launcher.
• Rprobe = RLCFS + 2~3 cm
108 deg.
1 cm Slit
Shield
4/11
Direct digitization of LH signals at 25 MHz allows performing cross-spectral
analysis.
Fixed LO
@ 275 MHz
CH1
Vari. LO
@ ~ 4.9 GHz
CH2
CH3
Dig
itiz
er
IF=
25 MHz
• Two-step frequency down-conversion from 4600 ± 25 MHz to 25
± 25 MHz.
• Digitizer is triggered every 10 msec at 100 MS/s.
• In each data segment, the data is further divided into 6 segments
to perform a cross spectral analysis.
- auto-power
- dominant n||
- magnitude squared coherence (γ2)
Spectrograms
5/11
In the standard (forward) field configuration, the probes detect coherent
wave-fields from the launcher.
C-Mod Tokamak Top View
Launcher
Probe Head
Location
Forward Field Reverse Field
Peak Power 1.3x10-2 ± 0.004 2.3x10-4 ± 0.0001
n|| 1.5 ± 0.05 1.1 ± 0.39
Coherence 0.94 ± 0.01 0.60 ± 0.10
Ray Trajectory
Ray Path in
Forward Field
Ray Path in
Reverse Field
6/11
Ray-tracing model predicts that wave-fields with low |n||| reach the probe
location.
• Ray have a limited radial penetration with
the decrease in the |n|||.
𝑣𝑔⊥
𝑣𝑔||≈
𝜔0
𝜔𝑝𝑒1 −
1
𝑛||2
ρ < 0.7 ρ > 0.9
Simulation Parameters:
𝑛 𝑒 ≈ 1.1 × 1020 m−3, 𝐼𝑝 = 1.0 MA.
7/11
The phase and amplitude responses as a function of applied n|| suggests that
the probes detects the wave-fields with |n||| ≈ 1.6.
• The dominant n|| is about 1.6, regardless of the
change in the applied peak n|| at the launcher.
Experimental parameters: 𝑛 𝑒 = 1.1 × 1020 m−3
• The decrease in the observed power is consistent
with the change in the antenna spectrum.
Exp.
Power at |n||| = 1.6
in Antenna
Spectrum
• In the applied spectrum from the LH launcher,
the spectral power content at |n|||= 1.6
decreases with the increase in the applied peak
|n|||.
8/11
• Qualitatively consistent with wave
propagation that shows a weaker radial
penetration with the increase in density.
As density increases, the probes detect wave-fields with higher |n||| as the
resonance cone shifts radially outward. .
• Applied peak n|| is fixed at 1.6. 𝑣𝑔⊥
𝑣𝑔||≈
𝜔0
𝜔𝑝𝑒1 −
1
𝑛||2
𝑛 𝑒 = 1.3 × 1020 m−3 𝑛 𝑒 = 1.5 × 1020 m−3
ρ < 0.9 ρ > 0.9
9/11
The observed power decrease correlates with the decrease in the spectral
power content at higher n||.
• The density dependence of the observe
power can be mapped to the n||
dependence.
• At higher density, the decrease in
coherence may implies a possible role of
wave-scattering1, also evidenced by the
broadened frequency spectrum (not
shown here).
1P. Bonoli, Phys. Fluids 25, 359 (1982)
10/11
• These waves1 are excited in front
of, or near the launcher at the
expense of source power at 4.6
GHz.
• The sideband power is lower by
an order of magnitude than the
main signal at 4.6 GHz.
Parametrically excited sideband LH waves are measured with a new
diagnostic.
• The measured n|| at 4.57 GHz tracks the n|| measured at 4.6
GHz.
• High n|| components might have propagated radially inward,
as seen in the previous ray-tracing results.
1M. Porkolab, Phys. Fluids 20, 2058 (1977).
11/11
Summary and Future Work
• The new diagnostic in SOL measures the coherence wave field from the launcher, in line with the ray-tracing
simulations.
• The observed power and n|| dependences is a combined effect of wave propagation and the probe location
being in SOL.
• Spectral broadening mechanisms are observed at high density, and we are continuing to investigate these
experimental results.
A preliminary design of a new
probe array Future Work:
• A new probe system is designed to be placed closer to the LH launcher by
another 36 deg.
• The increased number of probes will allow performing Fourier analysis to
evaluate the n|| spectrum (n|| = [ 0, 1.67, 3.35, 5.01])
12/11
Least Square Fitting
13/11
Dependence of the measured wave power on Ip
14/11
Spectral broadening is observed in the frequency spectra due to wave
scattering effect.
1P. Bonoli, Phys. Fluids 25, 359 (1982)
15/11
Ion cyclotron Sideband @ 4.57 GHz (Ip = 0.8 MA)