Overview of Research Results from theAlcator C-Mod Tokamak*
Earl Marmar
on behalf of the Alcator C-Mod TeamMIT Plasma Science and Fusion Center and CollaboratingInstitutions
OV/2-4, 27th IAEA Fusion Energy Conference,Ahmedabad, India, October 22, 2018

*Supported by the U.S. Department of Energy, Fusion Energy Sciences

Compact, High B Tokamak Physics• C-Mod Completed Operations in 2016
– 23 highly productive years– Many groundbreaking physics results– Analysis ongoing

E. Marmar, FEC 2018 OV/2-4• Pointing to a high field path for future– High Temperature, High FieldSuperconductors enable compact
burning plasma and reactor concepts

2Achieved World Record Core and Pedestal Plasma Pressures
High Performance: n≤1.5x1021; T≤10keV; P≤2.1 atm, Pped≤0.8 atm
Pedestal Pressure to 90% of ITER Target[1]X All C-ModEDA H-mode
Super H-mode
I-mode

X

X
XXX
X
X
X
X
X

Compact/High B: R=0.67m; a=0.22m; ≤1.8; B0≤8T•All RF Auxiliary Heating & Current Drive– ICRF: P≤6.5MW, 50 MHz to 80 MHz– LHCD: P≤1.5MW, 4.6 GHz

E. Marmar, IAEA FEC 2018 OV/2-4••High Power Density: PB/R ~ 100 MW-T/m,
q||~3 GW/m2
High-Z Plasma Facing Components– Vertical plate divertor[1]Snyder, et al., IAEA FEC 2018 EX/2-4

3

ELM Suppressed I-mode High Energy Confinement Regime:
Operating Window Widens at High B• I-mode, primarily studied on C-Mod, hasmany attractive features:– High energy confinement, lowparticle/impurity confinement– Weak confinement degradation withpower– No ELMs to challenge the divertor• May be particularly attractive for the high B
approach to reactors– H-mode threshold increases with B,suppressing the I- to H- transition athigh power• Some of the remaining challenges– Power handling and robustness todetachment– Scaling to burning plasma conditions
E. Marmar, IAEA FEC 2018 OV/2-4(BxB away from X-point)

H-modeL-modeAlso see Wilks, et al., IAEA FEC 2018, EX/P6-19; Happel, et al., EX/2-3

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Super H-Mode: C-Mod Extends Validation of Predictions toITER Relevant Pedestal Pressure, in All-Metal Machine[1]• Low *, high temperature peelinglimited pedestal region predicted bytheory[2], observed on DIII-D[3]• Explored on C-Mod by transitioning
to H-mode from low density, low
impurity content I-mode target• Excellent agreement between EPED
predictions and experiment– Extended pedestal pressures to80 kPa on final day of C-Mod
operation; Te-ped = 2 keVRecord Pedestal Pressure Shot (Ip=1.4MA, q95=3.2)
EPED Solutions:Super H-mode
Near Super H-mode

Te-ped = 2 keVConventional
H-modeExperimental points shownat 3 times (in msec) during
the discharge[2]Snyder, et al., Nucl. Fusion 2015
[3]Solomon, et al., Phys. Plasmas 2016
E. Marmar, IAEA FEC 2018 OV/2-4[1]Hughes, et al., Nucl. Fusion 2018; Snyder, et al., IAEA FEC 2018, EX/2-45

Non-Inductive Lower Hybrid Current Drive isChallenging at High Density• C-Mod results[1] show
anomalously low current drive
efficiency and sharply decreased
production of fast electrons athigh density, high Greenwald
fraction (fG)
• Increasing Ip at fixed density(and thus lowering fG) reduces
the anomaly– Current drive efficiency
matches model[1]Wallace, et al., Phys. Plasmas 2010
E. Marmar, IAEA FEC 2018 OV/2-46• C-Mod results[1] show
anomalously low current drive
efficiency and sharply decreased
production of fast electrons athigh density/high Greenwald
fraction (fG)
• Increasing Ip at fixed density(and thus lowering fG) reduces
the anomaly• Directly correlates with
reduction in edge turbulence
driven particle flux at low fG

Normalized Hard X-Ray Flux

Non-Inductive Lower Hybrid Current Drive isChallenging at High Density[1]Wallace, et al., Phys. Plasmas 2010
E. Marmar, IAEA FEC 2018 OV/2-4

Baek, et al., IAEA FEC 2018, EX/P6-287

Non-Inductive Lower Hybrid Current Drive isChallenging at High Density• C-Mod results[1] show
anomalously low current drive
efficiency and sharply decreased
production of fast electrons athigh density/high Greenwald
fraction (fG)
• Increasing Ip at fixed density(and thus lowering fG) reduces
the anomaly• Directly correlates with
reduction in edge turbulence
driven particle flux at low fG[2][1]Wallace, et al., Phys. Plasmas 2010
E. Marmar, IAEA FEC 2018 OV/2-4

0.1
0.2
0.3
0.4
Normalized Greenwald Fraction (fG)
[2] LaBombard, et al., Phys. Plasmas 20088

Direct Measurement of Lower Hybrid Wave ElectricField Confirms Scattering near the Outboard Midplane[1]
LH wave absorption negligible in SOL
near the launcher• Measurements of ELH using Polarization
Spectroscopy of Stark-split D
spectrum confirm that LH waves are
not being absorbed in the SOL
E. Marmar, IAEA FEC 2018 OV/2-4

SOL density fluctuations predict LH
wave scattering• The waves are scattered nearoutboard midplane• Full-wave 3D modeling consistent withscattering due to density fluctuations[1]Martin, et al., IAEA FEC 2018, EX/8-2

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Low Greenwald Fraction is not attractive for Power Reactors
Possible Solution: High Field Side Launch• High-field side SOL is quiescent:[1]HFS Launcher[1]Smick, et al., Nucl. Fusion 2013
E. Marmar, IAEA FEC 2018 OV/2-4• Wave accessibility anddamping also expected toimproved with HFS
launch[2]• Plan to test on DIII-D[3][2]Bonoli, et al., Nucl. Fusion, 2018[3]Wukitch, et al., EPJ Web of Conf. 2017; Wallace et al., IAEA FEC 2018 FIP/3-310

C-Mod Results Show that “Eich” Scaling[1] for SOL Power WidthContinues to the ITER Poloidal Fieldq database Extended with Operation at 8 Tesla[2]Implies ITERwill have
q=0.5 mm

E. Marmar, IAEA FEC 2018 OV/2-4[1]Eich, et al., Nucl. Fusion 2013[2]Brunner, et al., Nucl. Fusion 2018; Brunner, et al., IAEA FEC 2018, EX/P6-9 11

Original q Scaling Developed for H-mode
C-Mod Results[1] show q  Pressure-1/2 for L-mode, I-mode and H-mode
Each confinement regime gives q  1/Bp, butwith different constants of proportionalityE. Marmar, IAEA FEC 2018 OV/2-4

Plasma Pressure unifies all the C-Mod results[1]Brunner, et al., Nucl. Fusion 2018; Brunner, et al., IAEA FEC 2018, EX/P6-9

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Divertor Solutions Needed For Reactor Regimes• Current experiments(and ITER) push tothe limits of
conventional verticalplate divertor• New solutions will beneeded for the ~5xbigger challenge
anticipated inreactors

E. Marmar, IAEA FEC 2018 OV/2-4

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Advanced Divertor Concepts are being Developed:
Must be Tested at Ultra-High Power Density
Modeling shows great promise for thelong-leg X-point target concept[1]ADX is a DTT concept that can test many
advanced divertor configurations[2]Prad and Te contours with Psol= 6 MW (PB/R = 70 MW-T/m)

E. Marmar, IAEA FEC 2018 OV/2-4[1]Umansky, et al., IAEA-FEC 2018 (TH/7-2)[2]LaBombard, et al., Nuclear Fusion 201514

Advanced Divertor Concepts Compatible with Reactor Constraints
Modeling shows great promise for the
long-leg X-point target concept[1]Long-Leg Divertor Compatible withReactor Concepts (e.g. ARC[2])Prad and Te contours with Psol= 6 MW (PB/R = 70 MW-T/m)

E. Marmar, IAEA FEC 2018 OV/2-4[1]Umansky, et al., IAEA-FEC 2018 (TH/7-2)[2] Kuang, et al., Fus. Eng. Des. 201815

REBCO High Temperature/High Field Superconductor:
Game-Changer for High B/Compact Fusion Energy Path
• Conventional superconductor(Nb3Sn) limits maximum on-axis
B to about 5 tesla (R/a ~ 3)
• Development of High-Temp
Superconductors (HTS) opens
the window for increased B– Field limit is no longer B at
the coil, but engineering
stresses instead– Higher T (~20 K) operationalso has engineering
advantages, and may allow
for jointed coils
E. Marmar, IAEA FEC 2018 OV/2-4

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SPARC: Concept for Compact, High-Field Burning
Plasma Facility
SPARC technical objectives:

Size of DIII-D/ASDEX-U, B0=12 Tesla• Burn D-T fuel
• Q > 2 (with headroom)• Pfusion > 50MW
• Pulsed with 10s flattop burn (about2x CR)
• ~1,000 D-T pulses, >10,000 D-D
full-power pulses• ~1 hr D-T pulse repetition rate
• ~15 minutes between D-D shots
Desired Schedule:
3 yrs R&D (already started) + 4 yrs construction
E. Marmar, IAEA FEC 2018 OV/2-417

The H98=1 Confinement Projection Puts SPARC within
the Footprint of the Existing Tokamak Database

E. Marmar, IAEA FEC 2018 OV/2-418

Compact High Field Pilot Plant Concept (ARC)[1]With Advanced Divertor[2]• Recent design concept for a compact,high field, reactor using HTS magnets– incorporates long-leg X-point
target divertor for power handling• Joints in TF coil could dramaticallyease maintainability• About the size of JET, but at B0=9.2 T– Pfusion ~ 525 MW– Pelectric ~200 MW

E. Marmar, IAEA FEC 2018 OV/2-4[1]Sorbom, et al., Fus. Eng. Des. 2015[2]Kuang, et al., Fus. Eng. Des. 201819

Exciting Path for Fusion Energy

High-Field, High Power Density Plasma
Science Experiments
ADX/DTT

ARC Pilot Plant

Alcator C-Mod

SPARC

Magnet and Fusion Technology, Burning Plasma
E. Marmar, IAEA FEC 2018 OV/2-4

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Related Presentations at 2018 IAEA FEC
Tuesday Morning• E. Tolman: Conceptual design study for heat exhaust management in the ARC fusion pilot plant, FIP/P1-22
Wednesday Morning• P.B. Snyder: High Fusion Performance in Super H-Mode Experiments on Alcator C-Mod and DIII-D, EX/2-4
Wednesday Afternoon• T. Tala: Core Density Peaking Experiments in JET, DIII-D and C-Mod in Various Operational Scenarios Driven by Fuelling orTransport, EX/4-4
Thursday Afternoon• D. Brunner/M.V. Umansky: Extending the Boundary Heat Flux Width Database to 1.3 Tesla Poloidal Magnetic Field in theAlcator C-Mod Tokamak, EX/P6-9• T.M. Wilks: Access Requirements for Stationary ELM-Suppressed Pedestals in DIII-D and C-Mod Plasmas, EX/P6-19• R.S. Granetz: Machine Learning for Disruption Warning on Alcator C-Mod, DIII-D, and EAST Tokamaks, EX/P6-20• S.G. Baek: Observation of Efficient Lower Hybrid Current Drive at High Density on Alcator C-Mod, EX/P6-28
Friday Morning• M.R.K. Wigram: Performance assessment of tightly-baffled long-leg divertor geometries in the ARC reactor concept, TH/P7-20
Friday Afternoon• E.H. Martin/G.M. Wallace: Experimental Evidence of Lower Hybrid Wave Scattering in Alcator C-Mod Due to Scrape Off Layer
Density Fluctuations, EX/8-2• M.V. Umansky: Study of Passively Stable, Fully-Detached Divertor Plasma Regimes Attained in Innovative Long-Legged
Divertor Configurations, TH/7-2• M.G. Dunne: Impact of Impurity Seeding on Pedestal Structure in ASDEX Upgrade and Alcator C-Mod, EX/P8-2
E. Marmar, IAEA FEC 2018 OV/2-421