Transcript Document

C-Mod Advanced Tokamak Program
DOE Review of C-Mod Five-Year Proposal
May 13, 2003
MIT PSFC
Presented by A. Hubbard
MIT Plasma Science and Fusion Center,
for the C-Mod Advanced Tokamak Task Force
C-Mod Advanced Tokamak Program
• Introduction
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What do we mean by “Advanced Tokamak”?
Why do we want it?
Why on C-Mod?
What tools will we use?
• Overview of Schedule and Goals
• Research Highlights and Five-Year Plans
– By topic, control parameter.
• Summary of Goals and Program Relevance.
“Advanced Tokamak”
•
Attractiveness of the tokamak as an ultimate fusion reactor
increases if it is steady state and produces power at lower cost.
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Steady state implies non-inductive current drive.
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Lower cost implies much of the current drive is self-generated, ie.
High bootstrap fraction.
(external drive is expensive, in $ and Watts!).
Also want high confinement, and high b for lower cost (all are
inter-related).
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Research aimed at demonstrating these features and optimizing the
tokamak configuration is a key near-term component of the US
fusion development plan.
Will they be achievable in CTF and/or DEMO?
Unique Features of C-Mod
• AT scenarios look very attractive in design studies (eg.
ARIES-RS,AT) and there are active experimental programs
on several tokamaks (eg. DIIID, JT60U, AUG, JET, others).
What will C-Mod contribute that is new?
• In physics terms, “Steady-state” current drive implies pulse
lengths >> current relaxation time tCR.
C-Mod can run 5 second pulses, tCR~ 0.2-1.4 s .
[tCR = 1.4 a2kTe3/2 /Zeff; Zeff=1.5; Te= 2-7.5 keV ]
• Most AT expts have Ti > Te, te-i > tE and use NBI for core
fuelling and rotation drive in ITBs. Reactor scenarios have
te-i << tE (Te >Ti), no core fuelling, RF heating and CD.
C-Mod can test feasibility of AT scenarios with all these
features simultaneously.
Normalized pulse length exceeds that
of all other present divertor tokamaks
1 .4 a k T e
2
t CR 
3/2
Z e ff
Assumed: Zeff=2,
Te=6 keV (ITER 19 keV).
Note: C-Mod has already
run 3 sec pulses. Some
pulse lengths are
upgrades.
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Allows us to be sure that current is fully diffused for most of the discharge.
Enables the study of transport and stability limits in steady conditions.
With full non-inductive current drive, current profile determined uniquely by RF
and self-generated (bootstrap) sources.
This is a strong test requiring good control and alignment of deposition and
pressure profiles, including current drive well off-axis.
Profile Control Tools
“The crucial distinguishing feature of an Advanced Tokamak over a
conventional tokamak is …the use of active control of the
current or shear profile, and of the pressure profile or transport
characteristics” (AT Workshop, GA, 1999)
Tools available or under development:
• Current profile:
– Lower Hybrid Current Drive. (2003. Upgrade 2005).
4 MW, 4.6 GHz, 2 launchers with independent phasing, N//.
– Mode Conversion Current Drive. (on-axis, tests 2003)
– Bootstrap current drive via pressure profile control.
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Density profile.
– Control of core transport, peaking.
– Cryopump controls edge source. (2005)
– D2 and Lithium pellet injectors.
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Temperature Profiles
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– 8 MW ICRH, 40-80 MHz, 2 independently variable deposition locations.
– 4 MW LHCD.
– Control of core transport via RF deposition, magnetic shear.
Shear Flow - MC flow drive.
C-Mod complements other AT programs
•
Each program has different tools, features, emphasizes areas
appropriate to these (table does not list all research!)
Tokamak
j(r) control tools
AT program emphasis
C-Mod
LHCD (high n), MCCD,
FWCD
Steady state. High pressure.
Double-barrier regimes; all RF driven,
te-i << tE. Transp. control.
DIII-D
tailored Ip ramp, ECCD,
NBI, FWCD
bN > no-wall limit, active RWM
stabilization, NTMs, ITBs.
JT60-U
tailored Ip ramp, NBI,
LHCD (low ne), ECCD
Full non-inductive CD, high bp, doublebarrier regimes. Current ‘holes’.
AUG
tailored Ip ramp, NBI, ECCD Hybrid H-mode/AT scenarios. ITBs.
High bN, NTMs
JET
tailored Ip ramp, NBI,
LHCD (low ne)
Tore-Supra tailored Ip ramp,
FTU
Internal transport barriers
D-T operation. Current ‘holes’.
Long pulse,ITBs, L-mode edge. Limited,
LHCD (low ne)
circular plasmas.
LHCD (high ne)
ITBs. Limited, circular plasmas.
Main goals of the AT physics
program
1. Demonstrate and model current profile control using LH and
ICRF waves, at high densities (>1020 m-3).
2. Understanding, control and sustainment of Internal Transport
Barriers, with coupled ions and electrons, te-i << tE (Te~Ti ) and
without momentum input (RF only). Global confinement H89P > 2.5.
3. Achieve full non-inductive current drive (70% bootstrap) and
extend pulse length to near steady state (5 sec, 4-6 tCR)
- divertor power handling and wall particle issues.
4. Attain and optimize no-wall b limits, with bn of at least 3, and
explore means of achieving higher values
Program involves all physics areas (RF, transport, divertor, MHD)
and has broad participation from the C-Mod team. More
details in each of topical science presentations.
Example of an AT target scenario
meeting performance target.
•
One of many optimized
scenarios modelled with
ACCOME.
– Ip=860 kA, non-inductive.
– ILH=240 kA
– IBS=600 kA (70%)
– bN=2.9
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Double transport barrier
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BT=4 T
ICRH: 5 MW
LHCD: 3 MW, N//0=3
ne(0)= 1.8e20 m-3
Te(0)=6.5 keV (H=2.5)
Scenarios without barrier, or
only an ITB, have similar
performance.
P. Bonoli, Nucl. Fus. 20(6) 2000.
Overview of 5-year schedule for
Advanced Tokamak Program
Current Profile Control:
Lower Hybrid Current Drive system
2003
2005
Frequency 4.6 GHz
4.6 GHz
Source
Power
3 MW
4 MW
Antenna
1 grille
(4x24
guides)
2 grilles
(4x24
guides
each)
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Designed for well controlled spectrum.
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Each antenna will have flexible N//, variable over range 2-4.
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Variable between or during discharges using phase shifters.
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2 launchers can have different spectra.
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Allows us to tailor spectrum for desired wave accessibility (depending
on n(r), B), and to control deposition and current drive profiles,
including CD far off axis.
LH System nearly complete
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RF sources, power supplies,
WG prepared by MIT.
12 Klystrons (3 MW) installed,
tested in the C-Mod cell.
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LH Coupler and splitter
fabricated by PPPL
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All components complete.
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Vacuum hardware delivered to
MIT.
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Will be installed following tests.
Target Plasma Development and
Scenario Modelling closely linked.
•
Modelling is used to assess
wave accessibility, damping,
and CD efficiency, and guide
target plasma development
toward more optimal
scenarios.
Experimental
target profiles
New experiments,
Optimized parameters
•
Exploring several different
regimes:
– Rampup,
– L-mode,
– H-mode and
– Double-barrier.
Modelling
w RF, LHCD
Sensitivity studies
How should target
be improved?
Example: Double Barrier Regime
6x10 20 m-3
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Density profile taken from an ITB
discharge with off-axis ICRH. Te has
been raised to 3 keV .
Good LH penetration even with
typical H-mode pedestal.
• However, deposition profile and
efficiency were not optimal.
• Experiments in 2002 succeeded in
producing lower EDA density Hmodes, better LH target plasmas.
ACCOME predicts 60% bootstrap
current (470 kA)
• But, at radius smaller than
optimum.
• 2003 experiments aim to expand
barrier, increase Te.
Ip=750 kA, ILH=110 kA fBS=0.68
2003: Begin LHCD Experiments
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Complete testing, install launcher, splitter.
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Fall 2003: Begin LH Experiments (details in RF talk).
– Assess power handling.
What are the limits for short, long pulses?
– Focus on LH coupling, wave physics studies.
– Measure coupling efficiency, reflectivity vs edge density,
launcher and limiter position.
– Begin measurements of current drive profile and
efficiency.
– Key diagnostics:
• MSE for j(r) (commissioned in 2002).
• Hard X-ray camera – Fall 2003
2004-05: j(r) control via LHCD
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Measure LHCD and heating
efficiency and deposition
profile vs density, N//. Both on
and off-axis CD.
Combine LHCD and ICRH.
Raising Te will increase LH
efficiency.
Explore L-mode, H-mode and
barrier regimes.
Intermediate goal is 50%
non-inductive with 1st
launcher.
Eg. ACCOME modelling of initial
experiments with L-mode targets:
Te0 = 5 keV,
ne0 = 1.5 x 1020 m-3
PLH=2 MW, N//=2.75
PICRH=1.8 MW
Ip = 690 kA
Predicts:
ILH = 250 kA. (35%), at r/a~0.6.
Te
R
2005-6: LHCD Upgrade.
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2nd antenna allows 4 MW source,
3 MW coupled for modest power density, 5 sec pulse.
Add new 4-strap ICRF antenna.
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Use flexibility of two launchers to create spectrum with two N// peaks.
– Modelling and other experiments (eg ASDEX, JFT2M) show that a high
N// component increases off-axis absorption and localization.
ASDEX, 90+180o.
Soldner, Leuterer et al,
Nucl. Fusion 34(7)1994.
Current profile control via MCCD
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Fast waves mode convert to IBW and/or ICW.
Mode-converted waves, and localized electron heating,
have been observed in C-Mod experiments.
(details in RF talk).
MCCD demonstrated on TFTR.
Recently improved TORIC modelling (through Sci-DAC),
gives good agreement with heating experiments.
Model predicts MCCD of ~ 70 kA on-axis.
– Exceeds ohmic j(r)
– Good complement to LHCD, may provide ‘seed current’.
– Off-axis CD also possible (lower efficiency).
2003: Initial tests of MC with current drive phasing.
2004: If successful, combine with LHCD. Synergism??
Control of core transport, profiles
(Internal Transport Barriers)
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ITB’s routinely triggered on C-Mod
by off-axis ICRH, at r/a ~0.5. (high
or low-field side)
– Width depends on B, not Rdep.
Core barriers co-exist with edge
pedestal (EDA H-mode.)
Also seen in ohmic H-mode.
Reversed shear not needed.
Stable conditions were reached
for ~15 tE, through addition of
modest on-axis ICRH.
We can control the degree of
transport within the barrier!
S. Wukitch, APS 2001, PoP 2002.
Energy and particle transport barrier
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TRANSP analysis shows strong
decrease in ceff as well as D
when barrier forms.
•
With central heating, core ceff and
D increase somewhat but are still
less than without barrier. Te(0),
Ti(0) increase.
– Can control barrier
strength, avoid impurity
accumulation as well as MHD
limits.
•
Localized energy transport barrier
is also seen clearly in sawtooth
heatpulse propagation.
Emerging understanding of ITB
mechanism from GS2 simulations
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Formation starts with decrease in
ITG mode (note low he , R/LT).
– At transition time, ExB shear
does not appear dominant.
Ware pinch peaks ne, pe.
– ne gradient then further
stabilizes ITG (positive
feedback), but can drive weak
TEM in barrier.
When on-axis heating is applied,
TEM increases (lower n*).
– Nonlinear simulations show
enough transport to balance
Ware pinch, arrest peaking.
– Too much heating erodes the
barrier.
Preliminary Picture; need many
tests in models, experiments!
D. Ernst, Sherwood 2002, APS 2002
M. Redi, TTF 2002, EPS 2002
C. Fiore, PoP 2001, TTF 2002
Transport and Pressure Profile
Control: Near-term Plans
2003 (planned expts)
• Improve understanding of ITBs
in ohmic plasmas and with offaxis ICRH.
Threshold condition? Hysteresis?
Role of rotation? Detailed profiles,
time behaviour of c, D.
R/LT , h variation, BT ramps, heat and
impurity pulses.
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Time-dependent TRANSP
model of proposed expt, used
to guide planning:
Ip rampdown increases IBS.
Barrier location control.
Want to expand for more attractive
AT scenario - lower B, f
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Improving performance.
Maximize energy confinement,
bootstrap current. Does regime
extend to higher T, lower n*?
– Higher power, Ip rampdown.
V. Tang, R. Parker, APS 2002
Transport and Pressure Profile
Control: Long-term Plans
2003-7
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Investigate influence of magnetic shear on ITB formation, location and
transport profiles.
– Use LHCD and MCCD to control j(r).
– Produce ITBs with reversed shear, central ICRH.
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Study effect of flow drive on barriers.
– Depends on MCIBW flow drive tests.
– Can it be an active barrier control tool?
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Adjust heating profile to modify Te(R)
– Two ICRF frequencies.
– LH Heating
– Also need to modify transport; C-mod profiles are “stiff” without barriers.
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Optimize density, temperature, bootstrap profiles for compatibility with
LHCD, maximum non-inductive CD scenario.
– Goal is 50% non-inductive in 2005 (1 LH Launcher)
– 100% non-inductive by 2007 (2 Launchers)
TRANSP simulation of full non-inductive
CD with ITB, L-mode edge, with LHCD.
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Input ne(r) with barrier at r/a=0.5.
Input c(r) taken from analysis of C-Mod ITB experiments.
Te, current profiles evolve in time.
PICRF=3 MW
PLH=1.65 MW
N//=2.75
J. Liptac,
APS 2002
C-Mod Transport Barrier Research is highly
relevant to Burning Plasma Physics needs
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eg ITPA Transport and Internal Transport Barrier Physics
Topical Group, #1 High Priority Goal:
“Improve experimental characterization and understanding of
critical issues for reactor-relevant regimes with ITBs,
specifically with Te~ Ti, low toroidal rotation speed,
high density (n/nG=0.6-0.8), flat density profile (n(0)/<n> < 1.5),
Zeff <2, and moderate safety factor (q95=3.5-5), including:
– ITB formation and sustainment conditions.
– Impurity accumulation (low- and high-Z),
– Compatibility with divertor requirements (nsep/nG > 0.3)”
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This is one example of the interconnection of the ‘Advanced
Tokamak’ and ‘Burning Plasma’ thrusts.
Flow Profile Control
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Shear flow is known to affect transport,
barriers; an active RF control tool is of
great interest to all experiments.
Large toroidal rotation seen on C-Mod,
(up to 120 km/s), without momentum input
(ohmic or RF).
– Momentum transport covered in talk
by Greenwald.
Magnitude and profile vary with
confinement regime.
2003-4
• Improve Vf, Vq diagnostics (X-ray and
CXRS).
• Will look for evidence of localized
poloidal flow drive by mode converted
IC waves.
• If flow drive proves significant, will later test
influence on transport, ITBs.
Toroidal rotation profiles
Density Profile Control
Density is critical for LHCD accessibility, efficiency and
deposition profile!
•
Plans for cryopump:
– Experiments in 2002 varied Ssep.
High upper neutral pressure
(5-10 mTorr) when close to DN.
– Results used for design of upper
cryopump.
– Plan to use cryopump for
active density control in 2005.
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Transport control will be the best
tool for density peaking.
– Also have Li, D pellet injectors.
Will assess impurity accumulation,
wall saturation effects for long
pulses.
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Power Handling in Long Pulse AT
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In 2001, demonstrated 3 second pulses, but with PRF ~ 1 MW.
Divertor/SOL power handing will be a major challenge as power, pulse
length increase:
Parallel flux is already up to 0.5 GW/m2 with 3-4 MW ICRH !
Requirement for fairly low edge ne (1-2x1020 m-3) for LHCD makes radiative
divertor difficult.
C-Mod experience will be highly relevant to AT scenarios on ITER.
2003-5:
– 6 MW ICRH + 2 MW LH (coupled)
– ~3 second pulses.
– Add IR cameras to monitor LH antenna, hot spots
– Try strike point sweep.
2005-7:
– 6 MW ICRH+3 MW LH
– 5 second pulses.
– Upgrade outer divertor, plus other areas as required.
MHD Stability of non-inductive plasmas
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Expect core MHD stability to be
more important for C-Mod as
power, b raised.
Ideal no-wall limit bn~3.
– With optimized p(r), j(r).
– Strong shaping.
New antennas for active core
MHD spectroscopy can measure
linear growth rates.
– Plan to feedback on power,
profiles to avoid limit.
Study ELM, core MHD interaction.
Try stabilization of NTMs using
LHCD and/or MCCD.
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Plan to carry out a
design/feasibility study of
active stabilization methods to
allow b > no-wall limit
May install such a system ~ 2007.
Integrated Scenario Modelling.
Modelling is critical to assess wave damping and CD, confinement
and stability in various regimes, and guide discharge development
toward more optimal scenarios.
Currently available models include:
– ACCOME: LH, ICRH and bootstrap Ip. Consistent MHD equilibria.
No transport; temperature, density specified.
– CQL3D (R. Harvey): Self-consistent 2-D velocity space Fokker-Planck.
(20-30% higher LHCD than ACCOME).
– TRANSP: Time dependent, predictive or simulation mode.
+ LSC for LHCD. Limitations in reverse N//, 1 poloidal launch point.
– TORIC: Full wave field solver for ICRH, mode conversion.
Plans for AT-related modelling:
RF near term
• Improve ACCOME LH model to reflect recent benchmarking of ACCOME
and CQL3D (ITER modelling, ITPA activity, ) P. Bonoli
• LHCD distribution simulations, X-ray diagnostic design (Y. Peysson,
Cadarache).
• RF Flow Drive (J. Myra, Lodestar, E.F. Jaeger, D. Batchelor, ORNL)
Plans for AT-Related Modelling
near term, longer term
RF (con’t)
• Full Wave simulations of LHCD (1-D and 2-D) and IBW (2-D)
(C.K. Phillips, PPPL, J. Wright, P. Bonoli, MIT, E.F. Jaeger, D. Batchelor,
ORNL - SciDAC).
• 2-D (Vperp, V//) Fokker Planck simulations of LHCD; Couple CQL3D to
ACCOME (R.W. Harvey – CompX, P. Bonoli)
TRANSPORT
• Coupled current drive and transport modelling using TRANSP in predictive
mode: extend range of scenarios. (J. Liptac, P. Bonoli).
• More gyrokinetic analysis (GS2) of ITB discharges. (M. Redi, PPPL, D.
Ernst. MIT)
• Couple LHCD model from ACCOME to TRANSP. (MIT, PPPL)
• Use evolving capabilities of TRANSP for more theory-based predictive
modelling. Eg. assessing wExB vs gITG.
• Develop and incorporate improved particle transport modelling (critical for
ITB simulations).
MHD
• Low n and ballooning stability analysis of modeled scenarios with PEST-2,
Keldysh code and MARS (J. Ramos, MIT PSFC).
• Assess active stabilization designs with VALEN (Columbia).
Integrated Advanced Tokamak Goals
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A successful AT demonstration must combine all of the control
tools and physics/technology areas discussed.
– Eg. LHCD and high bootstrap and high b and long-pulse divertor.
– Integration and parameter optimization will be an important part of
the program from the beginning. For example, tradeoffs
necessary in Ip, density. Not possible to separate the various parts
of the program (eg current, transport, density control).
– With so many tools, regimes to explore and exploit, increased
run-weeks will be essential.
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Five-year goal is:
– Fully non-inductive current drive of 0.85 MA, from LHCD plus
bootstrap current,
– bN=3.0 (or higher), for
– 5 second pulse length (~6 tCR at 5 keV).
– Core transport barrier with H89P > 2.5
•
Intermediate objectives established in each area to guide program,
assess progress in both performance and scientific understanding
(IPPA goals).
Goal 1: Current Profile Control
Demonstrate and model current profile control using
LH and ICRF waves, at high densities (>1020 m-3).
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Produce some optimized targets for LH studies
2003.
Commission LHCD, measure coupling, power handling
2003.
Measure profile of fast electrons with hard X-ray camera
2004.
Measure LHCD efficiency and localization in L, H, and ITB regimes
and determine effective upper density limit.
2005.
Demonstrate modification of current profile with changing N// 2005.
Compare to ACCOME, CQL3D, and full wave models.
2004-06.
Commission 2nd LH launcher, assess CD localization with
multiple N// spectra.
2006.
Goal 2: Core Transport Barriers
Produce, understand and control core transport barriers in LH and
ICRF driven regimes with strongly coupled electrons and ions.
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Measure core c, D with off and on-axis ICRH, over a range
of Ip, BT, and ne.
2003.
Conduct gyrokinetic simulations of several cases, compare with
experiment.
2004.
Assess influence of reduced or reversed shear (produced by LH)
on ITB formation, location.
2005.
Test whether barriers form spontaneously with reversed shear, with on-axis
ICRH. Compare ce, ci and D and assess impurity accumulation.
2005.
Optimize profiles and transport for steady state, with maximum
off-axis bootstrap current (target 70%).
2004-06.
Goal 3: Long Pulse Current Drive
Demonstrate, and develop predictive models for, full noninductive CD using LH and ICRF waves, in a high n regime
(> 1020 m-3) for pulse lengths >> tCR.
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As part of LH profile control studies, measure LHCD efficiency,
in L, H, and ITB regimes.
2005.
Demonstrate 50% CD, possibly at reduced ne, for 3 sec pulses.
2005.
Compare to ACCOME, CQL3D, and full wave models.
2004-06.
Maximize and document LH driven current vs target ne(r).
2006
Combine LHCD with bootstrap current from ITBs, to maximize
total non-inductive CD.
2006-08.
Extend pulse length to 5 seconds.
2007.
Goal 4: Increase Beta
Attain and optimize no-wall b limits, with bn of at least 3, and
explore means of achieving higher values.
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Use ICRH power to increase b in inductive discharges, and
measure MHD growth rates using active MHD antennas.
2003-04.
Compare observed growth rates with stability codes.
2003.
Explore MHD properties of reversed shear discharges
2005.
Use control of current and pressure profiles to optimize no-wall
beta limits, guided by modelling.
2006-07.
Conduct feasibility and design studies of conformal wall and
active MHD stabilization coils.
2004-05.
If studies show feasibility and if warranted by demonstrated high b,
fabricate and install MHD control hardware.
2006-08.
Full exploitation would likely extend to next five-year period.
C-Mod AT Program is closely aligned with the goals
of the Integrated Program Planning Activity.
MFE Goal 3: Advance understanding and innovation in highperformance plasmas.
General Specific Topic
Area
C-Mod Contributions
3.3.1
Profile
Control
3.3.1.1 Current
Profile
Lower-hybrid and bootstrap current drive,
MSE, x-ray measurements,
Current drive modelling.
3.3.1.2 Pressure
Profile
Two-F, Localized ICRF heating. Profile
diagnostics. Test effect of shear modification.
Models of heating and transport.
3.3.1.3 Flow Profile
Measure rotation profiles without direct input.
Test Mode-conversion flow drive.
3.3.1.4 Transport
Profile
Active control of core transport via heating
deposition, current profile control.
Measure momentum transport.
Compare with models of turbulence and transport.
3.3.1.5 Low n
Divertor Op’n
Develop power dissipation techniques at high
SOL powers, compatible with AT regimes.
Integrated Program Planning Activity contributions
(cont).
General
Area
Specific Topic
C-Mod Contributions
3.3.2
High
Beta
Stability
3.3.2.1 RWM
Control
Apply models of RWM stability and stabilization to
C-Mod. Assess feasibility of active control.
3.3.2.3 Active
Profile Control to
Avoid Boundaries.
Active MHD antennas to measure mode
proximity.
Control of heating, current profile to avoid limits.
3.3.3
3.3.3.1
Burning Coordinated and
Plasmas Joint Experiments
Study core barriers, AT regimes in BP-relevant
regimes.
Joint experiments via ITPA (ITB and SS) groups.
MFE Goal 1: Advance understanding of plasma and enhance
predictive capabilities.
Many contributions in transport, stability, wave-particle interaction
and plama-wall interfaces are related to AT experiments, will be
discussed in topical group areas.
Summary
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Advanced Tokamak thrust will be an increasingly important part of
the C-Mod program.
Focusses on RF control of current, transport and pressure
profiles in high density regime, for t >> tCR, to make unique
contributions to the world AT program.
We have succeeded in modifying core transport without momentum
input or reversed shear.
Initial LHCD system is nearly complete - 1st experiments in 2003.
Long term program leads progressively to a non-inductive, steady
state, high confinement advanced tokamak demonstration in a
unique regime highly relevant to ITER and the steps beyond
(CTF, DEMO).
– All RF drive, BT = 4-8 T, Ti~Te, ne~1-5 x 1020 m-3.
– Plenty of activities, and exciting physics, for the next 6+ years!
C-Mod program will be a critical part of the US effort in configuration
optimization, required early in the fusion development path.