ADVANCES IN LOWER HYBRID CURRENT DRIVE …...LH2 4-way split with toroidal bi-junction 25 G.M....
Transcript of ADVANCES IN LOWER HYBRID CURRENT DRIVE …...LH2 4-way split with toroidal bi-junction 25 G.M....
ADVANCES IN LOWER HYBRID CURRENT
DRIVE TECHNOLOGY ON ALCATOR C-MOD G.M. WALLACE,1 S. SHIRAIWA,1 J. HILLAIRET,2 M. PREYNAS,2 W. BECK,1 J.A.
CASEY,3 J. DOODY,1 I.C. FAUST,1 D.K. JOHNSON,1 A.D. KANOJIA,1 C. LAU,1 R.
LECCACORVI,1 P. MACGIBBON,1 O. MENEGHINI,1 R.R. PARKER,1 D.R. TERRY,1 R.
VIEIRA,1 J.R. WILSON,4 AND L. ZHOU1
1MIT PLASMA SCIENCE AND FUSION CENTER, CAMBRIDGE, MA 02139, USA 2CEA, IRFM, 13108 SAINT PAUL LEZ DURANCE, FRANCE 3ROCKFIELD RESEARCH, LAS VEGAS, NV 89135, USA 4PRINCETON PLASMA PHYSICS LABORATORY, PRINCETON, NJ 08543, USA
E-MAIL CONTACT OF MAIN AUTHOR: [email protected]
IAEA FEC 2012
San Diego, CA, USA
October 9, 2012
Improvements in LHCD technology allow
for longer pulses and higher power
Advanced transmitter protection system allows for long pulses (tpulse >> τR) without jeopardizing klystrons
Moveable integrated LH protection limiter reduces reflection coefficients, but at the cost of power handling
Design of new off mid-plane LH launcher will ~double available power and increase single-pass absorption
This work was supported by US Department of Energy collaborative agreements DE-FC02-99ER54512 and DE-AC02-09CH11466 and SBIR grant award DE-FG02-07ER84762.
G.M. Wallace, IAEA FEC, San Diego, CA USA
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LH waves launched by slow wave
structure at plasma edge
ω ~ 3 – 5 x ωLH
ωLH = ωpi(1+ ωpe2/ ωce
2)-1/2
LH launcher couples electrostatic slow mode
Waves launched preferentially in counter-current direction
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Current Drive
Counter-current
Waveguide array
LH waves transfer energy and parallel
momentum to fast electrons to drive current
LH waves Landau damp on
electrons at v|| ~ 3vte
Asymmetry in f(v||) results in
net current
[Fisch, Rev. Mod. Phys., 1987]
v||
f(v
||) Fast electrons
carrying current
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G.M. Wallace, IAEA FEC, San Diego, CA USA
LHCD system on C-Mod investigates
LH physics with ITER-like parameters
ne = 0.5-5x1020 m-3 (ITER = 0.5-1x1020 m-3)
BT = 3 – 8 T (ITER = 5 T)
Upper, lower, or double null diverted plasma configuration (ITER = lower null)
n|| = 1.5 – 3 co- or counter-current (ITER ~ 2)
fLHCD = 4.6 GHz (ITER = 5 GHz)
4 rows of 16 waveguides
PSource = 2.5 MW
X-mode
Reflectometer horns
Langmuir
probes
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G.M. Wallace, IAEA FEC, San Diego, CA USA
LH power provided by CPI/Varian
klystrons operating at 4.6 GHz
Vb = 50 kV
Ib ~ 12 A
PRF = 250 kW
Coolant flow rate 70 gpm
Minimum coolant pressure 16 psi
Grouped in “carts” of 4 klystrons
Carts connected in parallel to Thales HVPS
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G.M. Wallace, IAEA FEC, San Diego, CA USA
Transmitter Protection System (TPS)
upgraded to allow for longer pulses
Modeling by klystron manufacturer
Coolant should not boil during 5.0 s RF pulse at 250 kW
Coolant will boil after 1.2 s with no RF output
Boiling could result in damage to the klystron collector
Need to monitor coolant outlet temperature to prevent boiling
Administrative limit on pulse length of 0.5 s to avoid harming klystrons
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TPS Control Hardware
Development of TPS funded
by USDoE SBIR grant award
DE-FG02-07ER84762.
G.M. Wallace, IAEA FEC, San Diego, CA USA
Collector Over Temperature System
(COTS) models coolant temp in real time
COTS models average outlet water temperature based on integrating heat equation:
𝑑𝑇𝑜𝑢𝑡
𝑑𝑡=
𝐼𝑏𝑉𝑏 − 𝑃𝑅𝐹 − 𝑄𝐶𝐻2𝑂(𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛)
𝐶0
Integrated at 1 kHz
Beam power, RF power, and coolant flow are directly measured
Collector heat capacity, C0, determined empirically by time constant of outlet temp
Max coolant temperature proportional to Tout – Tin based on CPI model data
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Klystron
VbIb
PRF
QCH2OTin QCH2O
Tout
TPS shuts off HVPS if a fault is
detected on any klystron
Independent TPS hardware for
each cart
Fault conditions:
COTS temperature exceeds
threshold
Electron beam remains on for
integrated time of > 400 ms with
no RF output power
VSWR at klystron output window
too high
Optical arc detector indicates arc
at klystron window
Klystron body current
G.M. Wallace, IAEA FEC, San Diego, CA USA
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FAULT!
COTS calculation agrees closely
with downstream coolant temp
Downstream coolant temp measurement lags actual temperature
Model outlet temperature increase agrees with downstream measurement
Best fit for time constant with C0 = 35 kJ/K
Time constant is slower than predicted by CPI simulations (C0 ~10 kJ/K)
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G.M. Wallace, IAEA FEC, San Diego, CA USA
TPS upgrade allows for longer LH
pulses into plasma
5.0 s pulses into dummy load
1.0 s pulses into plasma
Typical C-Mod Ip flattop lasts
for 1.0 s
Long (tpulse = 1.0 s >> τR) LH
pulses allow other
parameters (ne, Te) to reach
new equilibrium after current
profile changes
G.M. Wallace, IAEA FEC, San Diego, CA USA
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Moveable integrated LH protection
limiter installed in fall of 2011 12
Fixed position
limiter
Installed ‘04
Removed Fall ‘11
Reinstalled Spring ‘12
Moveable, integrated
limiter
Installed Fall ‘11
Removed Spring ‘12
Moveable integrated limiter allows for
wider range of operating positions
Short connection length between LH protection limiters steep density gradient
Can only operate LH launcher in narrow range of radial positions (0 < RLH – Rlim < 0.5 mm) with fixed limiter
Reflection coefficients very high for RLH – Rlim > 0.5 mm
Launcher exposed to damaging plasma heat flux for 0 > RLH – Rlim
Integrated limiter moves with LH launcher at a fixed value of RLH – Rlim = 0.25 mm
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G.M. Wallace, IAEA FEC, San Diego, CA USA
Moveable integrated limiter reduces reflection
coefficients as compared to fixed limiter
Moveable
integrated limiter
reduces Γ2 from
~30% to ~20%
Small number of
discharges with high
reflections due to
insufficient density
(Γ2>40%) remain
G.M. Wallace, IAEA FEC, San Diego, CA USA
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Power handling with integrated
limiter significantly reduced
Arcing became problematic after first few run days, even at low power
Max launcher retraction 1.5 cm with integrated limiters vs. 3 cm with fixed limiters
Buildup of boron coating on launcher surfaces
Melting damage due to insufficient protection from plasma
Electrical isolation of LH limiter tiles
G.M. Wallace, IAEA FEC, San Diego, CA USA
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BZN
BZN
BZN
Shots w/ arcing o
Shots w/o arcing *
Inspection during manned access
shows damage to launcher 16
Arc tracks
extend
5cm into
guides
Boron film
Launcher refurbished following
removal of integrated limiter
Boron film removed
Alumina vacuum windows grit-blasted to remove contamination
Arc tracks buffed out from waveguide walls
Drips ground off of waveguide septa
G.M. Wallace, IAEA FEC, San Diego, CA USA
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Before repair After repair
Power handling recovered with
reinstallation of fixed limiter
398 discharges with
LH since reinstallation
of fixed limiter
47 discharges with
arcing in the launcher
No lasting decrease
in performance
observed after arc
“clusters”
G.M. Wallace, IAEA FEC, San Diego, CA USA
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Shots w/ arcing o
Shots w/o arcing *
Independent, moveable protection
limiter would provide ideal solution
Moveable limiter did allow for lower reflection
coefficients and concept should not be abandoned
Ability to retract LH launcher behind local
protection limiters is necessary to shield launcher
from boronization and disruptions when LH is not in
use
Independent, moveable limiter could be grounded
to vacuum vessel wall, eliminating buildup of charge
on limiter tiles intersection fast electrons from LH
G.M. Wallace, IAEA FEC, San Diego, CA USA
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Additional LH launcher will ~double
available LH power on C-Mod
Currently use 1 launcher powered by 10 klystrons
Columns 1-6 and 11-16 fed pair-wise by split klystrons
Columns 7-10 fed by individual klystrons
Direct feed klystrons generally operated at 50% power
Plan to add 2nd launcher with each powered by 8 klystrons (16 klystrons total)
All columns of both launchers fed pair-wise by split klystrons operating at full power
Available LH power will increase to ~ 2 MW with additional launcher
G.M. Wallace, IAEA FEC, San Diego, CA USA
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Poloidal upshift of n|| along rays launched above
mid-plane results in stronger single-pass absorption
n|| upshifts along ray path when waves are launched above mid-plane
Landau damping becomes strong when vph ~ 3vth
Deformation of distribution function at 3vth results in stronger absorption of waves at higher phase vph
G.M. Wallace, IAEA FEC, San Diego, CA USA
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Off mid-plane “LH3” launcher designed
to increase driven current at high ne
LH2 + LH3 drive up to 200 kA when considered individually
LH2 + LH3 drive up to 300 kA when phase space synergy is considered
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With Synergy
Without SynergyRay P
ow
er
100%
0%
Envelope of LCFS from all 2010-2011
discharges defines shape of launcher 23
Analytic equilibrium approximates
envelope and existing limiter well
Analytic equilibrium with R, a, κ, and δ [Hakkarainen et al, Phys. Fluids B 2 (7), 1990.]:
𝑅 = 𝑅0 + 𝑟 cos 𝜃
𝑧 = 𝑟 sin 𝜃 𝑟 =
𝑎{1 −𝜅−1
2cos 2𝜃 − 1 +
𝛿
4cos 3𝜃 − cos 𝜃 }
R=0.67m
a=0.241m
κ=1.4
δ=0.45
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LH3 design based on combination of
LH2 4-way split with toroidal bi-junction 25
G.M. Wallace, IAEA FEC, San Diego, CA USA
Toroidal 90° bi-junction
Poloidal 4-way splitter
8-way splitter parameters optimized based
on ALOHA plasma scattering matrix
Optimization parameters:
Bi-junction length
Poloidal differential phase
Distance from bi-junction to 4-way splitter
Input phasing
G.M. Wallace, IAEA FEC, San Diego, CA USA
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ALOHA
64x64
COMSOL
9x9 Input
Set parameters:
Bi-junction phasing (90°)
Operating n|| range (2-3)
Plasma edge density = 2-8xncutoff
Vacuum gap distance (1 mm)
9x9
9x9
…
…
RF design of 8-way splitter evaluated
with COMSOL FEA simulation software 27
G.M. Wallace, IAEA FEC, San Diego, CA USA
S11 = -18.5 dB
Sn1 = -8.97 to
-9.35 dB
∆φ = -86.95° to 87.95°
Construction of first 8-way splitter
test setup in progress
G.M. Wallace, IAEA FEC, San Diego, CA USA
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