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4th US-PRC Magnetic Controlled Fusion Collaboration Workshop, UT Austin, May 5-6, 2008
Recent ICRF Results in the Alcator CMod Tokamak
Presented by Yijun Lin
With contributions from S. Wukitch, A. Binus, A. Incecushman, E. Marmar, M. Reinke, and J. Rice
Alcator C-Mod Project
MIT, Plasma Science and Fusion Center
Cambridge, MA 02139, USA
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Outline
Introduction of Alcator C-Mod
Overview of ICRF program
Recent ICRF results:
• ICRF technology: fast ferrite tuning system
• Fast wave direct electron heating
• Mode conversion sawtooth modification
• Mode conversion flow drive
Collaboration areas
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C-Mod Unique in World and US
Among High Performance Divertor Tokamaks
Unique in the World:
• High field, high performance divertor tokamak
• Particle and momentum source-free heating
and current drive
• Equilibrated electron-ion coupling
• Bulk all high-Z plasma facing components
• ITER level (and beyond) Scrape-OffLayer/Divertor Power Density
• Approach ITER neutral opacity, radiation
trapping
• Highest pressure and energy density plasmas
Exclusive in the US :
• ICRF minority heating
• Lower hybrid current drive
• A premier major US facility for graduate student
training
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Present ICRF System
D & E antennas
J antenna
Frequency
80 MHz
40-80
MHz
Power
2 x 2 MW
4 MW
Antenna
2x2
Strap
4 Strap
Phase
fixed
variable
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ITER Relevance
C-Mod ICRF
ITER ICRF
• Fundamental and second
harmonic with access to
direct fast wave.
• Fundamental 3He and second
harmonic tritium.
• Both high and low single
• High single pass absorption.
pass absorption scenarios.
• Antenna power density is
~10 MW/m2.
• Antenna power density ~6-8
MW/m2.
• Utilize ICRF with high Z
first wall materials.
• Likely to have high Z first wall
materials.
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C-Mod ICRF Research Themes
RF system R & D:
• Antenna design, and real-time match for successful RF
operation.
RF edge-plasma interactions:
• Antenna coupling, dynamic loading, voltage and power
limitations, RF sheath, and impurity production.
Wave propagation and absorption:
• Fundamental minority heating, 2nd harmonic majority
heating, direct fast wave, and mode conversion absorption
regimes.
• To validate simulation codes, scalable to ITER and reactors,
to provide confidence in simulation codes used for discharge
analysis.
Plasma current and flow drive:
• To develop means to control plasma current profile and
affect stability of MHD modes (e.g., sawtooth modification).
• Investigate flow drive and application.
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RF Tech. R&D: Fast Ferrite Tuning System
• A transmitter needs to be
isolated from the antenna
through impedance matching
network.
• Antenna loading changes
with plasma conditions in
real-time.
• Ferrite material  varies vs.
ambient B field  length
variation in the line.
• Digital control on the tuner
current to achieve real-time
matching.
• Triple-stub design: one prematch stub, and two with
ferrite tuners.
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Fast Ferrite Tuning System
• Equivalent length
change up to 36 cm
at 80 MHz with
change of 300 A coil
currents.
• Power supply swing
capability: 75 A/ms
• Computation
iteration: 250 s
• Filled with SF6 for
higher voltage
handling
• Water cooled
• Optical arc detection
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Digital Controller and Power Supply
Linux server
PLC interfaces
RF power/phase detector
Power
Supply
Digital
Controller
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FFT Performance in L- and H-modes
• Under real-time digital
control, the FFT system
can maintain the power
reflection below 2%
under significant antenna
load variation.
• First installed as a
double-stub system in
2007, re-configured to
triple-stub in 2008, and
has been running
successfully in the entire
2008 campaign.
• Max net-power handled
1.85 MW in H-mode.
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Latest ICRF Physics Experiments
From 04/22 to 05/02/2008 (last two weeks), we ran experiments
with J-antenna at 50 MHz, and D/E antennas at 80 MHz.
In this setup we can access many scenarios other than the
normal D(H) minority ICRF heating:
• Bt ~ 5.3 T, D(H) plasma  J antenna fast wave direct
electron heating (no IC resonance), H-minority heating with
D/E antennas.
• Bt ~ 5.1 T, D(H) plasma with low 3He concentration  Hminority heating with D/E antennas and 3He-minority
heating from J antenna.
• Bt ~ 5.1 T, D(H) plasma with moderate 3He concentration
 H-minority heating with D/E antennas and D(3He) mode
conversion heating/current drive/flow drive from J antenna.
• Bt ~ 3.3 T,  H-minority heating with J antenna and also
2nd harmonic deuteron heating.
Really recent data: No detailed analysis, but only preliminary
intepretation.
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Fast Wave Electron Heating
Wplasma [kJ]
FWEH
Te [keV]
J (50 MHz) Power [MW]
J antenna at 50 MHz does
not have an ion cyclotron
resonance in normal Bt =
5.3 T D(H) plasmas.
Fast wave electron heating
was observed in good
confinement plasmas
(relatively high β) preheated with minority
heating.
D+E (80 MHz) Power [MW]
Data from 04/23
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Sawteeth Period vs. MCCD
50 MHz, D(3He), Mode
conversion near q = 1
surface
Co-Current CD, Average
sawtooth 10 ms
Counter-Current CD,
sawtooth 8 ms
Heating phase,
sawtooth 10 ms
Data from 04/22 and
04/25
D+E 1.7 MW
D/E 1.7 MW and J 1.7 MW
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Strong Rotation with Small Increase in Wp
Toroidal rotation (km/s)
Strong co-Current rotation
> 100 km/sec was
observed with only 50 kJ
increase in plasma energy
in some mode conversion
plasmas.
Stored energy (kJ)
A factor of 2 more than
normal intrinsic plasma
rotation scaling
ΔV [km/sec] ~ 0.9*
ΔW[kJ]/Ip[MA]
J 50 MHz (MW)
J antenna in mode
conversion regime, while D
and E in minority heating
regime.
D+E 80 MHz (MW)
First observed on 04/22/2008.
Surprising result.
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Mode Conversion Flow Drive
Toroidal rotation (km/s)
J-ant
D+E
Stored energy (kJ)
At the power level, J
antenna in mode
conversion regime
generated about twice
core rotation than D and
E antenna in minority
heating.
J 50 MHz (MW)
D+E 80 MHz (MW)
Data from 04/25
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Rotation Velocity vs. RF power
Blue circles: Mode conversion
flow scales with the RF power.
Black squares: Minority heating.
Data from 04/25 and 04/29
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Collaboration Areas
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•
•
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ICRF physics and technology
LHRF physics and technology
Diagnostics
MDS-plus
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