20 Years of Research on the Alcator C-Mod Tokamak
Presented By Martin Greenwald
For the Alcator C-Mod Team November 2013 – APS/DPP
Acknowledgements – C-Mod Authors
Acedo p, Agostini m, Alex j, Alfier a, Allain jp, Allen aj, Allen s, Alper b, Andelin d, Anderl ra, Andrew y, Angioni c, Antar gy, Arai k, Ashbourn jma, Austin me, Aydemir ay, Ayyad a, Bader a, Baek sg, Bai b, Bajwa k, Bakhtiari m, Baldwin mj, Ballinger r, Barnard hs, Barnes m, Basse np, Bateman g, Batishchev ov, Batishcheva aa, Baumgaertel ja, Beals df, Beck wk, Becker h, Beek w, Beiersdorfer p, Belli em, Belo p, Bengtson rd, Bergerson wf, Bernabei s, Berry la, Bespamyatnov i, Beurskens mna, Biewer tm, Binus as, Blair a, Boedo ja, Boivin rl, Bombarda f, Bondeson a, Bonnin x, Bonoli pt, Borner p, Borras mc, Bortolon a, Bosco j, Bose b, Boswell cj, Brambilla m, Brandstetter s, Brandwein j, Bravenec rv, Bretz nl, Brill ju, Broennimann c, Brooks jn, Brower d, Brown gv, Brunner d, Budny r, Burke wm, Bush ce, Butner dn, Byford w, Caldwell d, Calisti a, Candy j, Carmack wj, Carreras ba, Carter md, Casey ja, Casper ta, Catto pj, Chan th, Chang cs, Chatterjee r, Childs r, Chilenski ma, Chin b, Choi m, Christensen c, Chung tk, Churchill rm, Cima g, Clementson j, Cochran w, Cochrane t, Connor jw, Conway gd, Coppi b, Coster dp, Counsell gf, Cziegler i, Daigle j, Danforth r, Darrow ds, Davis em, Davis-lee w, Decker j, Deichuli pp, Dekow g, Del-castillo-negrete d, Delgado-aparicio l, Demaria m, Diallo a, Diamond ph, Ding wx, D'ippolito da, Doerner rp, Domier cw, Dominguez a, Doody j, Dorland w, Dorris j, Drake jf, Duval bp, Edlund em, Eikenberry ef, Eisner ec, Elder jd, Ellis r, Elton rc, Ennever pc, Erents sk, Ernst dr, Evans te, Fairfax s, Fasoli a, Faust i, Ferrara m, Finkenthal m, Fiore cl, Fitzgerald e, Fournier kb, Fredd e, Fredian tw, Friedberg j, Gandy r, Gangadhara s, Gao c, Garnier d, Gates d, Gentle k, Goetz ja, Goldstein wh, Golfinopoulos t, Golovato sn, Gorelenkov nn, Graf a, Graf ma, Granetz rs, Graves t, Green dl, Greenough n, Greenwald m, Griem hr, Grimes m, Groebner rj, Grulke o, Gu mf, Gwinn d, Hahm ts, Hallatschek k, Hanson gr, Harra lk, Harrison s, Harvey rw, Hastie rj, Heard j, Hender tc, Hill kw, Hollmann em, Horne sf, Hosea jc, Howard nt, Howell df, Hsu t, Hubbard ae, Hughes jw, Humphreys da, Hutchinson dp, Hutchinson ih, In y, Ince-cushman a, Irby j, Izzo va, Jablonski d, Jaeger ef, Jernigan t, Johnson dk, Kamiya k, Kanojia ad, Karney cff, Keenan fp, Kesner j, Kessel c, Knoll da, Knowlton s, Ko js, Koert p, Kondo w, Kopon d, Kramer gj, Kumagai t, Kung cc, Kurz c, Labombard b, Lao ll, Lau c, Leblanc b, Leccacorvi r, Lee j, Lee sg, Lee wd, Liao kt, Lin l, Lin y, Lipschultz b, Liptac j, Lisgo s, Lo dh, Loarte a, Loesser gd, Luke t, Lumma d, Lynn a, Ma ch, Ma y, Macgibbon p, Maddison gp, Magee ew, Maingi r, Majeski r, Manabe t, Maqueda rj, Marmar es, Marr k, May mj, Mazurenko a, Mccann sm, Mccracken gm, Mcdermott rm, Meneghini o, Migliuolo s, Mikkelsen dr, Moos hw, Mossessian d, Moyer ra, Mumgaard r, Murray r, Myatt l, Myra jr, Nachtrieb r, Nazikian r, Nelson-melby e, Nevins wm, Niemczewski a, Ochoukov r, Ohkawa h, Oshea pj, Oyama n, Pablant n, Pace dc, Pappas da, Parisot a, Parker rr, Parkin w, Parks pb, Payne j, Pedersen ts, Petrasso rd, Pfeiffer a, Phillips ck, Phillips kjh, Phillips pe, Pigarov ay, Pinches sd, Pinsker r, Pitcher cs, Podpaly y, Porkolab m, Rachlewkallne e, Ram ak, Ramos jj, Reardon jc, Redi mh, Regan sp, Reinke ml, Reiter d, Rice je, Richards rk, Rogers bn, Rognlien td, Rokhman y, Ross dw, Rost jc, Rowan wl, Rushinski j, Russell da, Sampsell m, Schachter j, Schilling g, Schmidt ae, Schmittdiel d, Schneider r, Scott s, Scoville jt, Sears j, Shiraiwa s, Sigmar dj, Simakov an, Simon d, Skinner ch, Smick n, Smirnov ap, Smith d, Snipes ja, Snyder pb, Sohma m, Sorci j, Stangeby pc, Stek p, Stillerman ja, Stotler dp, Sugiyama l, Sung c, Takase y, Tang v, Taylor g, Terry dr, Terry jl, Theiler c, Tinios g, Titus ph, Tsujii n, Tsukada k, Ulrickson m, Umansky mv, Valeo e, Vieira r, Walk jr, Wallace g, Waltz re, Wampler wr, Wang y, Watterson r, Watts c, Weaver jl, Welch bl, Wesley jc, White ae, Whyte dg, Wilgen j, Wilson h, Wilson jr, Wilson m, Wising f, Wolfe sm, Wootton aj, Woskov p, Wright gm, Wright jc, Wukitch sj, Wurden ga, Xu p, Xu xq, Yamaguchi i, Youngblood b, Yuh h, Zaks j, Zhong x, Zhurovich k, Zweben sj
APS-DPP November, 2013 2 20 Years of Research on Alcator C-Mod
Graduate Students Supported By C-Mod
APS-DPP November, 2013 3 20 Years of Research on Alcator C-Mod
Acedo, Pablo Adams, Mark Allen, Aaron Angelini, Sarah Antohi, Paul Arai, Keigo Arima, Kiroyuki Badar, Aaron Bai, Bo Bakhtiari, Mohammed Barnard, Harold Becker, Deborah Behlmann, Matt Besen, Matt Bespamyatnov, Igor Bonnin, X. Borras, Christina Bose, Brock Boswell, Chris Brookman, Mike Brunner, Daniel Bruno, Antonio Caldwell, Dwight Chilenski, Mark Christensen, Cindy Chung, Kyu Sun
Chung, Taekyun Churchill, Michael Colborn, Jeffrey Cziegler, Istvan Daley, Kathy Davis, Evan Deakins, David Decker, Joan Delaney, Michael Dominguez, Arturo Edlund, Eric Ennever, Paul Faust, Ian Ferrara, Marco Fournier, Kevin Fu, Xiangrong Gangadhara, Sanjay Gao, Chi Garnier, Darren Garot, Kristine Garrett, Michael Geelen, Paul Goh, Jonathan Golfinopoulos, Ted Graf, Michael Graves, Timothy
Haakonsen, Christian Habib, Youssef Hartwig, Zachary Howard, Nathan Hsu, Thomas Hughes, Jerry Humphreys, David In, Yongkyoon Ince-Cushman, Alex Jablonski, David Jennings, Thomas Jureidini, Imad Kasten, Cale Ke, Jian Kesler, Leigh Kiryaman, Tugrulby Ko, Jin-Seok Kohno, Haruhiko Koltonyuk, Michael Krasheninikova, Natalya Kube, Ralph Kurz, Christian Kwak, Joowan
Lau, Cornwall Lee, Davis Lee, Jungpyo Liao, K.T. Lin, Liang Lin, Yijun Liptac Jr, John Lo, Daniel Luke, Thomas Lumma, Dirck Lynn, Alan Lyons, Laurence Ma, Yunxing Marr, Kenneth Marshall, Eric May, Mark Mayberry, Matt Mazurenko, Alex McDermott, Rachael McLaughlin, James Meneghini, Orso Miller, Daniel R. Miller, Jody Mumgaard, Robert Murphy, John Muterspaugh,
Matt Nachtrieb, Robert Nelson-Melby, Eric Niemczewski, Artur Novack, John Ochoukov, Roman Ohkawa, Hana Olafsen, Jeff Olynyk, Geoffrey OShea, Peter Parisot, Alexandre Passian, Ali Patacchini, Leonardo Payne, Joshua Pedersen, Thomas Perkins, John Podpaly, Yuri Preynas, Melanie Reardon, James Reinke, Matt Robinson, Mareena Rost, Chris Ruiz Ruiz, Juan Safo, Jude Sallander, Jesper
Sampsell, M.B. Sarlese, Justin Schachter, Jeff Schmidt, Andrea Schmittdiel, David Sears, Jason Shinya, Takahiro Short, Michael Shuggart, Ashley Sierchio, Jennifer Smick, Noah Smith, Kelly Robert Sorbom, Brandon Sorci, Joe Soukhanovskii, Vlad Squire, Jared Stek, Paul Stoke, Kristy Sung, Choongki Tang, Vincent Tejero, Erik Thannickal, Varghese Thomas, Edward Thomas, Michael Thoms, Jon Tinios, Gerasimos
Tsui, Chi-Wa Tsujii, Naoto Tutt, Teresa E Umansky, Maxim Urbahn, John VanderHelm, Mark Veto, Balint Walk, John Wallace, Gregory Wang, Ling Wang, Ying Warburton, David Weathers, Lorretta Weaver, James Wei, Xuan Woller, Kevin Wong, Frank Wright, Graham Xu, Peng Youngblood, Brian Yuh, Howard Zhang, Jiexi Zhou, Chuteng Zhurovich, Kirill
Collaborating Institutions
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 4
Alfven Laboratory
ANL
ASIPP - Hefei
Auburn
Australian National University
Bagley Associates
Budker Institute, Novosibirsk
Cal Tech
CCFM
CEA - Cadarache
Chalmers University
CIEMAT
Colorado School of Mines
Columbia
Compx
CRPP-EPFL
Dartmouth College
Ecole Royale Militaire, Brussels
EFDA-Jet Joint Undertaking
Einhoven University
ENEA-Frascati
FNAL
FOM - Neiuwegein
General Atomics
Hebrew University
IGI Padua
Imperial College
IPP - India
Ioffe Institute
ITER Organization
JAEA Japan
Johns Hopkins University
Keldysh Institute
KFA Julich
KFKI-RMKI Budapest
KSTAR Korea
Kurchatov Institute
LANL
LBNL
Lehigh University
LLNL
Lodestar
Max Planck-IPP, Garching
Max Planck-IPP, Greifswald
NIFS
Notre Dame University
New York University
ORNL
Oxford University
Politecnico di Torino
PPPL
Princeton University
Purdue University
Riso National Laboratory
SNLA
Southeast Louisiana Univ.
Sydney University
UKAEA, Culham
University of Alaska
University of California Berkeley
University of California Davis
University of California San Diego
University of California Los Angeles
University of Colorado
University of Idaho
University of Maryland
University of Texas
University of Tokyo
University of Toronto
University of Utah
University of Washington
University of Wisconsin
University Tromso, Norway
Weizmann Institute
Alcator C-Mod Is The Third In A Series Of Compact High-Field Tokamaks At MIT
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 5
BT up to 8 T IP up to 2 MA R = 0.67 m a = 0.22 m
Outline
● High-field Approach to Fusion
– C-Mod – Concept Philosophy and Engineering
● Research Highlights
– Boundary Physics
– Edge Transport Barriers
– Core Turbulence and Transport
– ICRF - Ion Cyclotron Range of Frequencies
– LHCD – Lower Hybrid Current Drive
– Disruptions
● Future Directions
APS-DPP November, 2013 6 20 Years of Research on Alcator C-Mod
Advantages of High-Field for Fusion There’s a Reason It’s Called Magnetic Fusion Energy
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 7
● Practical fusion energy requires absolute plasma parameters
– The appropriate normalization for the nuclear physics is kTi/ENUCLEAR
– Thus, there is an optimum range for absolute temperature
– Engineering and economics set the required neutron loading (µn2)
● Operational limits in a tokamak all increase with field
– Maximum plasma current (MHD kink limit) IP µ B
– Maximum plasma pressure (MHD β limit) p µ B2
– Maximum plasma density (density limit) ne µ IP µ B
● Fusion Power µ (Reactor Cost µ )
● The path to fusion energy would be much more attractive if the next nuclear steps had significantly lower costs
23 4N R B
q β
3 2R B
Benefits of High-Field for C-Mod Experiment
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 8
● High performance at moderate size and cost
● Provides critical data for Burning Plasma experiments (ITER) and high-field fusion development path
● Relevance of parameters with respect to ITER or Reactor physics
– Boundary physics: Same absolute power and particle flux density, plasma pressure
– RF: Same ωc , ωp , ωRF ⇒ same wave physics
– Scaling: Breaks parameter covariances when combined with larger low-field experiments
C-Mod’s Physics Capabilities Flows From Its Unique Magnet Technology
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 9
● BT up to 8T
– Magnet current up to 225,000 A
– Copper TF and PF magnets, cooled by LN2
– Picture frame design with sliding joints at corners
– Joints slide under full current, full mechanical load
– Forces (up to 10,000 tons) taken by heavy external structure
C-Mod’s Physics Capabilities Flows From Its Unique Magnet Technology
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 10
● BT up to 8T
– Magnet current up to 225,000 A
– Copper TF and PF magnets, cooled by LN2
– Picture frame design with sliding joints at corners
– Joints slide under full current, full mechanical load
– Forces (up to 10,000 tons) taken by heavy external structure
C-Mod’s Physics Capabilities Flows From Its Unique Magnet Technology
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 11
● BT up to 8T
– Magnet current up to 225,000 A
– Copper TF and PF magnets, cooled by LN2
– Picture frame design with sliding joints at corners
– Joints slide under full current, full mechanical load
– Forces (up to 10,000 tons) taken by heavy external structure
C-Mod – Internal Hardware Design Was Also Unique
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 12
● All high-Z metal plasma facing components (PFCs)
– From day one (20 years experience)
– 7,000 solid Molybdenum and Tungsten tiles
● ICRF antennas and a good deal of diagnostic equipment is installed inside vacuum vessel on outer wall
● Disruption forces and power load to vessel, antennas and diagnostics can be substantial
Field-aligned ICRF Antenna
Toroidally-aligned ICRF 2-strap Antennas
LHCD Launcher
QCM Antenna
Nature of C-Mod Device Requires That We Address And Solve Critical, Relevant Scientific and Technological Challenges
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 13
● Startup with highly conductive vacuum vessel
● Large disruption forces
● Very high power loads on first wall
– Metal PFCs (Plasma Facing Components)
● All RF heating and current drive
– Very high power densities for ICRF heating and LH current drive launchers
– Current Drive at higher densities
Outline
● High-field Approach to Fusion
– C-Mod – Concept Philosophy and Engineering
● Research Highlights
– Boundary Physics
– Edge Transport Barriers
– Core Turbulence and Transport
– ICRF - Ion Cyclotron Range of Frequencies
– LHCD – Lower Hybrid Current Drive
– Disruptions
● Future Directions
APS-DPP November, 2013 14 20 Years of Research on Alcator C-Mod
“Right now everyone is worried about getting and keeping heat in. Eventually the main problem will be how to handle the heat coming out.” Stangeby, U of Toronto, 1983.
C-Mod Boundary Plasma Are Uniquely Reactor-Prototypical
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 15
● For long-term survival, the divertor must operate below a maximum absolute Te
– Relevant dimensionless parameter is kTe/EATOMIC (In plasma and in surface)
– Thus plasma pressure sets the operating point
– High field allows C-Mod to operate at reactor-like pressures, higher than any other tokamak, same heat and particle fluxes, BT, plasma Te and ne
o q
up to 1GW/m2
o High neutral and photon opacity
● C-Mod pioneered the vertical target divertor and high-Z plasma facing components
– Adopted by ITER and other devices
Key Features: Shallow field angle
with extended divertor leg,
recycled neutrals directed toward
divertor plasma providing natural
baffling
Reactors Will Need To Run With High-Z Metal Walls
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 16
inboard outboard
C-Mod Has Demonstrated That High-Z Metal Divertors Are Practical
C-Mod Highlighted The Advantages and Challenges of High-Z, Metallic PFCs
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 17
● Documented impact on operations, wall conditioning, disruption recovery…
– Now under intensive study by AUG, JET
● First measurement of reduced erosion in tungsten and migration from solid metal tiles
– Erosion yield ≈ 5x10-5 per incident ion
● Fuel retention in solid metals
– > 10-3 better than in low Z, but too high
– Need to run divertor at high temperatures
● Plasma, obviously, has much lower tolerance for high-Z impurities
– Impurity sources with ICRF are critical
● Plasma can dramatically modify surface - Tungsten nanostructures demonstrated for the first time in confinement device
nm Tungsten Redeposition
W tile Barn
ard,
JNM
201
1
Wri
ght,
NF
2012
First In-Situ, Time Resolved Measurements of Fuel Retention and Erosion In Plasma Facing Components
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 18
● Inject 1 MeV D+ Beam
● Steer via tokamak magnets onto PFC surfaces (between shots)
● Measure induced nuclear reactions with in-vessel detectors
D + D ⇒ He3 + n
(Measures fuel retention)
D + B11 ⇒ P + B12 + γ
(Measures erosion of boron surface coating)
See Hartwig – NI3.00003
Divertor Regimes and Detachment Physics At Reactor Level Fluxes: Investigations of Plasma on Open Field Lines
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 19
– Discovered importance of divertor geometry – confirmed advantage of vertical plate
– Discovered importance of volume recombination, neutral collisionality, Lyα opacity
– Discovered the critical dependence of this process on cross-field transport
● For a fusion reactor, net erosion
must be held below 10-6 atom
lost for every incident ion
● To meet this criterion, the
temperature of the plasma in
contact with the divertor must be
below 10 eV
● Even better – transfer plasma
momentum to neutrals, detach,
achieving Te < 2eV.
● C-Mod:
LaBo
mba
rd, P
oP 1
995
Conventional Wisdom For Boundary Turbulent Cross-field Transport Was Overturned By C-Mod Observations
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 20
● Turbulence and transport had been assumed to be Bohm-like (or as a free parameter, adjusted to match models)
– Observations found no dependence on B and strong dependence on collisionality
● Instead, profiles are held near critical gradients by marginal stability
– Turbulence levels adjust to heat and particle fluxes at essentially fixed pressure gradients
– Normalized pressure gradient decreases with collisionality
– Consistent with theoretical models of B. Rogers, B. Scott
LaBo
mba
rd, N
F 20
05
Turbulence And Transport Delineates Two Distinct Regions Of The Boundary (Scrape-Off Layer = SOL)
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 21
● Near SOL (Scrape-Off Layer)
– Steep gradients, fluctuations with “normal” statistics
– Marginally stable gradients drive turbulence which sheds blobs (filaments)
● Far-SOL: can’t be understood in terms of local instability
– Flatter gradients, order unity, intermittent fluctuations
– Blobs propagate radially due to polarization from un-cancelled particle drifts
oProvides mechanism for main chamber recycling – first recognized on C-Mod
LaBo
mba
rd, N
F 20
00
Studies Of Edge Turbulence Provided The Likely Mechanism For The Tokamak Density Limit
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 22
● The limit, which manifested as a disruption, has been known for more 40 years
– Conventional wisdom: The limit resulted from power losses from atomic radiation
● Experiments on C-Mod supported an alternate hypothesis
– As the density was raised, higher collisionality lowers the sustainable pressure gradient through turbulence - Seen in fluctuation measurements
– The loss of warm plasma is balanced by the input of cold neutrals
– The resulting heat loss leads to shrinkage of edge temperature and unstable current profiles
Edge temperature profiles from probes and Thomson Scattering
Te profile has already shrunk by 1.5 cm at 60% of the density limit.
Poloidally Asymmetric Transport Drives SOL Flows
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 23
● Measurements of boundary turbulence showed strong ballooning transport
● Plasma on low-field (good curvature) side is populated only by sonic parallel flows
Outer mid-plane probes
Vertical probes
Inner-wall probes
LaBo
mba
rd, N
F 20
04
Divertor Heat Footprint: Innovative Diagnostics Have Led To Better Understanding Of This Critical Issue
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 24
● C-Mod addressed ITER relevant diagnostic challenges
– High heat flux
– Vertical plate geometry
– Reflective metal PFCs – uncertainty in albedo
● Integrated instrumentation was deployed with novel in situ calibrations
– Langmuir probes (plasma Te, ne)
– Retarding Field Analyzer (plasma Ti)
– Surface thermocouples (instantaneous heat flux)
– Calorimeters (integrated heat flux)
– IR camera (surface temperature)
● Results anchored international database at large BT, BP, small R, high pressure and heat flux
Divertor Heat Footprint: Innovative Diagnostics Have Led To Better Understanding Of This Critical Issue
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 25
● C-Mod addressed ITER relevant diagnostic challenges
– High heat flux
– Vertical plate geometry
– Reflective metal PFCs
● Integrated instrumentation was deployed with novel in situ calibrations
– Langmuir probes (plasma Te, ne)
– Retarding Field Analyzer (plasma Ti)
– Surface thermocouples (instantaneous heat flux)
– Calorimeters (integrated heat flux)
– IR camera (surface temperature)
● Results anchored international database at large BT, BP, small R, high pressure and heat flux
Eich
, NF
2013
Te
rry,
JNM
201
3
C-Mod Demonstrated Dramatically Reduced Heat Loads On Tungsten-Molybdenum Divertor Via Impurity Seeding
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 26
– The concern is that reducing the heat flux across the separatrix will lead to a lower pedestal and thus lower confinement
– With the right choice of impurity and control of impurity level, this approach can succeed
● Heat load challenge: narrow deposition footprints
● One approach to addressing this challenge:
● Radiate a majority of the power near the plasma edge, thus spreading the heat over a larger surface area.
Loar
te, P
oP 2
011,
H
ughe
s, N
F 20
11
C-Mod Measurements Show That Turbulent Cross-field Transport Controls Critical SOL/Boundary Phenomena,
Including:
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 27
● Main chamber recycling: Particles are not all exhausted through divertor
● Detachment threshold and dynamics:
● Density limit:
● Generation of SOL flows:
● Heat load footprint:
⇒Prediction of plasma boundary and plasma-wall phenomena requires deeper understanding of cross-field transport
Outline
● High-field Approach to Fusion
– C-Mod – Concept Philosophy and Engineering
● Research Highlights
– Boundary Physics
– Edge Transport Barriers
– Core Turbulence and Transport
– ICRF - Ion Cyclotron Range of Frequencies
– LHCD – Lower Hybrid Current Drive
– Disruptions
● Future Directions
APS-DPP November, 2013 28 20 Years of Research on Alcator C-Mod
C-Mod Experiments Impacted Our Understanding of Global and Local Threshold For L-H Transition
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 29
● An edge transport barrier (H-mode) is a critical performance requirement for ITER
● As C-Mod was under construction, predictions for the L-H power threshold ranged from 100kW to 10MW
– Actual result was Order(1 MW)
⇒ Impact of C-Mod data on database conditioning and extrapolations
● C-Mod carried out first experiments that characterized the threshold as a critical local temperature (or gradient)
● Demonstrated local hysteresis in transition
– Showing stable and unstable operating regions on the bifurcation curve.
Hub
bard
, PPC
F 19
98
Hub
bard
, PPC
F 20
02
Boundary and Core Flows Shed Light on Long-Standing Mystery Impact of Magnetic Geometry on L-H Threshold
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 30
● Poloidally asymmetric turbulent transport leads to geometry dependent boundary flows
– The sense of the flow (co- or counter-current) is correlated with an offset in the power threshold for all combinations of field direction and x-point geometry
– These flows are mirrored by flows deeper in the plasma
● Connects to a broad body of work that strongly suggests sheared flows as the key to understanding the creation of transport barriers
Asdex 1989
Bx∇
B
Bx∇
B
High-Performance Regimes w/o Edge Localized Modes (ELM)
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 31
● A fusion reactor will need the good energy confinement provided by H-modes but…
– Particle transport is too good (density, impurity build-up)
– Edge pressure gradient is too steep; produces unacceptable ELMs
● C-Mod has developed two regimes that combine good energy confinement without impurity build-up and without large ELMs
– EDA H-Mode: High collisionality, ELMy H-mode-like particle transport
– I-Mode: Low collisionality, L-mode-like particle transport (see Walk – UI2.00003)
Gre
enw
ald,
NF
1997
High-Performance Regimes w/o Edge Localized Modes (ELM)
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 32
● A fusion reactor will need the good energy confinement provided by H-modes but…
– Particle transport is too good (density, impurity build-up)
– Edge pressure gradient is too steep; produces unacceptable ELMs
● C-Mod has developed two regimes that combine good energy confinement without impurity build-up and without large ELMs
– EDA H-Mode: High collisionality, ELMy H-mode-like particle transport
– I-Mode: Low collisionality, L-mode-like particle transport (see Walk – UI2.00003)
Why
te, N
F 20
10
Pedestal Regulation Via Short-Wavelength EM Modes
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 33
● Quasi Coherent Mode in EDA H-mode
– (50-150 kHz, δf/f≈ 10%, k≈ 1.5 cm-1)
– Particle diffusivity scales with QC amplitude
– (see Golfinopoulos YI2.00005 & LaBombard YI2.00006)
● Weakly Coherent Mode in I-mode
– (200-300 kHz, δf/f≈ 20-100%, k≈ 1.5 cm-1)
– Particle diffusivity scales with WCM amplitude
– Particle and energy barriers associated with different fluctuation frequency ranges
● Similar fluctuations between ELMs in ELMy H-mode
– Evidence for Kinetic Ballooning Mode predicted to regulate pedestal between ELMs.
Hub
bard
, PoP
201
1 G
reen
wal
d, P
oP 1
999
Pedestal Regulation Via Short-Wavelength EM Modes
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 34
● Quasi Coherent Mode in EDA H-mode
– (50-150 kHz, δf/f≈ 10%, k≈ 1.5 cm-1)
– Particle diffusivity scales with QC amplitude
– (see Golfinopoulos YI2.00005 & LaBombard YI2.00006)
● Weakly Coherent Mode in I-mode
– (200-300 kHz, δf/f≈ 20-100%, k≈ 1.5 cm-1)
– Particle diffusivity scales with WCM amplitude
– Particle and energy barriers associated with different fluctuation frequency ranges
● Similar fluctuations between ELMs in ELMy H-mode
– Evidence for Kinetic Ballooning Mode predicted to regulate pedestal between ELMs.
Hub
bard
, PoP
201
1
Outline
● High-field Approach to Fusion
– C-Mod – Concept Philosophy and Engineering
● Research Highlights
– Boundary Physics
– Edge Transport Barriers
– Core Turbulence and Transport
– ICRF - Ion Cyclotron Range of Frequencies
– LHCD – Lower Hybrid Current Drive
– Disruptions
● Future Directions
APS-DPP November, 2013 35 20 Years of Research on Alcator C-Mod
Established First Quantitative Relation Between Pedestal and Core Confinement
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 36
● Folk wisdom always connected good core confinement with higher pedestals
● C-Mod provided the first experiments that made this connection quantitative
– Connected to theoretically expected dependence on critical gradient length (R∇T/T = R/LT ) for stability of drift waves
Profile self-similarity robustly observed across many confinement regimes
(I-Mode)
100 Randomly chosen shots (OH, L, H) IP = 1 MA; BT = 5.4 T
Gre
enw
ald,
NF
1997
Gre
enw
ald,
FS&
T 20
07
C-Mod Presents Unique Features For Core Transport Research
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 37
● Main heating method provides no external torque, no core particle sources
– Good platform for studies of intrinsic rotation and particle transport
● Dominated by electron heating
● Higher BT/R (and BP) leads to higher densities, higher collisionality, reduced neutral effects
– Electrons and ion better equilibrated than in other experiments
– Breaks covariance between ν*, νeiτE , n/nG and λo/a
Multi-Scale GYRO simulation of C-Mod core plasma showing electron and ion scale features
C-Mod Pioneered Studies of Intrinsic Rotation
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 38
● With no neutral beams to provide torque: C-Mod observed large, co-current toroidal rotation - M ≈ 0.2 – 0.3 (Vφ ≈ 150 km/s; ω > 2x105 rad/s)
● A scaling was derived: ∆V µ ∆W/IP (Pressure gradient? Temperature gradient?)
– Self generated flows large enough reduce transport and aid in formation of ITB
● Momentum transported from edge into core
– Opportunity to test emerging gyrokinetic models for momentum transport
Rice
, NF
1999
Simultaneous Validation of Particle and Energy Transport Simulation C-Mod Provides A Unique Platform
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 39
● Density peaking seen in H-mode at low collisionality with ICRF heating
– With AUG and JET, breaks covariance between ν* and n/nG
● Unambiguous prediction: density profile peaking for ITER
● GS2 simulations found that particle pinch driven by higher-k modes
● Laser blow-off and X-ray imaging diagnostics enable measurement of impurity particle transport and reliable estimate of uncertainties
● Simultaneous matched GK predictions (GYRO) for impurity transport (D, V) and ion thermal diffusivity
H-Mod Density Profiles Impurity Transport In L-modes
Gre
enw
ald,
NF
2007
How
ard,
NF
2012
Validation – Simulation Shortfall Is Not Universal Result
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 40
● Well documented, significant discrepancy in ion and electron heat flux between GK simulations and experiments has been reported (C. Holland PoP 2009)
● Working with the same code, GYRO, no shortfall observed in either channel for similar shots on C-Mod
– Electron thermal channel can be matched for conditions where TEMs drive significant low k turbulence (i.e. those that can be resolved in our simulations)
● At low power, discrepancy seen in electron channel (but not ion channel)
– Work proceeding on multi-scale simulations that extend to much higher k
Low Power, Low Te High Power, High Te
How
ard,
PoP
201
2
Outline
● High-field Approach to Fusion
– C-Mod – Concept Philosophy and Engineering
● Research Highlights
– Boundary Physics
– Edge Transport Barriers
– Core Turbulence and Transport
– ICRF - Ion Cyclotron Range of Frequencies
– LHCD – Lower Hybrid Current Drive
– Disruptions
● Future Directions
APS-DPP November, 2013 41 20 Years of Research on Alcator C-Mod
C-Mod Has Driven The Development Of ICRF Physics And Technology
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 42
● C-Mod system: 8 MW of source power, as much as 6 MW coupled to plasma
– RF sources run from 40 – 80 MHz
– Most work done with H minority heating in D plasmas at 5.4 T, 80 MHz
– D(He3) minority, mode conversion D/H, D/He3 and 2nd harmonic H also utilized
● The principal motivation for ICRF research was operational requirements
– Reliable, high power-density ICRF underlies the C-Mod research program
– Science and technology challenges had to be faced and solved
● Along the way, we’ve learned a lot about ICRF physics and technology
– Driven significant evolution of RF launcher technology
– Pioneering comparisons with simulations
C-Mod Carried Out Extensive Testing of ICRF Models For Minority and Mode Conversion Heating
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 43
● Testing different predictions of ICRF codes can strengthen confidence in underlying models
– Full wave models for minority and mode conversion deployed and tested (TORIC, AORSA)
● First observation of IC wave in experiments and simulations – Predicted theoretically decades before
wave fields fast velocity distribution local power deposition
Bader, NF 2012
Tsuj
ii, P
oP 2
012
Lin,
PPC
F 20
05
Robust ICRF Flow Drive – Demonstrated in C-Mod
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 44
● Motivation: Stabilization of macro (MHD) and micro-instabilities
– Rotation drive from beams are weak in ITER and absent in reactor
– Intrinsic rotation may not be sufficient
– Can we control stability and transport with RF driven flows?
● Flow drive with ICRF waves predicted but with substantial uncertainty in magnitude
● Experiments on C-Mod demonstrated robust flow-drive by mode-converted ICRF waves
● Flow shear approaches levels required to affect transport
Lin,
PRL
200
8 Li
n, N
F 20
11
High-Z Impurity Build-Up Observed On C-Mod With Strong ICRF Heating
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 45
● Significant high-Z impurity sources are unacceptable when combined with the good particle confinement seen in H-mode
– Boronization provides a temporary fix, but a more fundamental solution must be found
● The problem is believed to arise from rectification of RF fields by the plasma sheath
– Ions are accelerated by the increase in plasma potential and impact the first wall with much higher energies
● Changes in edge potential with RF power have been measured and may modify both source (through sputtering) and transport (via convective cells)
Gre
enw
ald,
NF
1997
Wuk
itch
JNM
200
9,
Czie
gler
, PPC
F 20
12
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 46
Field Aligned ICRF Antenna
B
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 47
Field Aligned ICRF Antenna
B
B
ICRF Impurity Generation Addressed By Field-Aligned Antenna
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 48
● Concept: Reduce parallel E-Field in front of antenna by symmetrizing antenna and surrounding structure
● Result: Hi-Z content dropped by about an order of magnitude.
● But: Many features of RF modeling were not borne out by experiments
– Much work remains to be done…
Field Aligned Antenna Toroidally Aligned Antenna
Wuk
itch,
PoP
201
3
Outline
● High-field Approach to Fusion
– C-Mod – Concept Philosophy and Engineering
● Research Highlights
– Boundary Physics
– Edge Transport Barriers
– Core Turbulence and Transport
– ICRF - Ion Cyclotron Range of Frequencies
– LHCD – Lower Hybrid Current Drive
– Disruptions
● Future Directions
APS-DPP November, 2013 49 20 Years of Research on Alcator C-Mod
Lower Hybrid Current Drive (LHCD) on C-Mod
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 50
● Tokamak reactor will need efficient, off-axis, noninductive current drive
– Driver technology must be steady-state and reactor relevant
● LHCD among the very few viable options
– Well demonstrated at low to moderate densities
● How does the physics and technology extrapolate to higher densities?
● C-Mod experiments at MW level
– ONLY experiment to study LHCD at the ITER field, RF frequency, plasma density and magnetic geometry.
High LHCD Effective At Moderate Densities
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 51
● Replaced 100% of Ohmic
current at <ne> = 0.5x1020 / m3
● Parallel E-Field brought to zero
and maintained for several
current relaxation times
Lower Hybrid Used To Drive Current Off-Axis
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 52
Reducing electron transport and creating thermal transport barrier
Modified magnetic shear with off-axis current drive (ne≈ 0.6 x 1020 /m3)
J(r)
q95(r) Sh
iraiw
a, N
F 20
11
Decrease in LHCD Efficiency at High Density
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 53
● At higher densities ( > 1x1020 /m3), required for higher performance, higher
bootstrap plasmas, driven current and fast electron populations drop below
simple predictions
Not linear wave
accessibility issue for
ne < 1.5x1020 /m3
Wal
lace
, PoP
201
0
Insight Gained From First Full-Wave Simulations of LH Waves Wave Trajectories Are Shallow And Interact Strongly With
Plasma Edge
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 54
Full wave + Fokker Planck, LHEAF/VERD code using finite element methods
First full-wave simulation of LH waves, using TORIC & spectral methods
Wri
ght,
PoP
200
9
Men
eghi
ni, F
S&T
2011
Understanding Emerging For Loss of Efficiency At High Density
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 55
● Mechanisms connected to shallow wave trajectory and low single-pass absorption
– Collisional wave damping in edge
– PDI on high-field (inboard) side
– Loss of fast electrons created near plasma edge
● So far, all LH experiments run in low single-pass regime
● Solution is to run with higher single-pass absorption (as it will be in ITER or Reactor)
– Higher Te
– Change in poloidal location of launcher
Shira
iwa,
PoP
201
1
Baek
, PPC
F 20
13
Outline
● High-field Approach to Fusion
– C-Mod – Concept Philosophy and Engineering
● Research Highlights
– Boundary Physics
– Edge Transport Barriers
– Core Turbulence and Transport
– ICRF - Ion Cyclotron Range of Frequencies
– LHCD – Lower Hybrid Current Drive
– Disruptions
● Future Directions
APS-DPP November, 2013 56 20 Years of Research on Alcator C-Mod
C-Mod Made Crucial Early Contributions To International Database On Disruptions
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 57
Motivation: At reactor scale, the mechanical and thermal stresses are generally unacceptable
● C-Mod with its high field and plasma current, forces on the vacuum vessel from halo currents > 600 kN ≈ 120,000 lbs (Challenge for internal hardware as well)
● Also on C-Mod, halo currents were directly measured and found to be non-axisymmetric and rapidly rotating (⇒3D geometry and dynamics of disrupting plasma)
Mfit sequence
If Some Disruptions Are Unavoidable, Mitigate The Worst Effects
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 58
● A Variety of Disruption Mitigation Techniques Were Explored
– Silver-doped solid pellets
– Massive D2 cryogenic pellets
– Massive Gas injection (MGI)
Pictures of stochastic field lines?
● MGI results are encouraging
– Initial concern over gas penetration – but impurities don’t need to penetrate as neutrals; MHD drives mixing
– Most (90%) energy removed as radiation
– Under some conditions, fast electrons generated in disruption are lost due to break up of flux surfaces see Granetz BI2.00003
First modeling that combined extended MHD + radiation (NIMROD ⇒ NIMRAD)
Izzo
, PoP
200
8
C-Mod Showed Radiation Asymmetry In Mitigated Disruptions
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 59
● Measured radiation patterns are spatially varied and highly dynamic
● Critical for ITER with its lower Surface/Volume and Beryllium walls
– Any local peaking of emission much larger than twice average would melt Beryllium first wall
– US is responsible for ITER disruption mitigation hardware
Oly
nyk,
NF
2013
It Will Be Hard To Control Radiation Symmetry Even With Multiple MGI Valves
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 60
● Modeling confirms complex spatial patterns of radiation even with uniform distribution of gas around torus
– Rotation is not yet captured by modeling:
– Can we predict MHD mode rotation speed for ITER during mitigated disruptions?
NIMRAD Simulation (vs toroidal angle)
● Phenomena are better understood, but results are decidedly mixed
– In thermal quench, MHD effects dominate, radiation patterns rotate
– What matters is number of rotation periods during quench
– Minimum rotation rate is required to average out power loading
Outline
● High-field Approach to Fusion
– C-Mod – Concept Philosophy and Engineering
● Research Highlights
– Boundary Physics
– Edge Transport Barriers
– Core Turbulence and Transport
– ICRF - Ion Cyclotron Range of Frequencies
– LHCD – Lower Hybrid Current Drive
– Disruptions
● Future Directions
APS-DPP November, 2013 61 20 Years of Research on Alcator C-Mod
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 62
● C-Mod operates at the right power and particle flux, right plasma pressure and density, right magnetic field, divertor geometry and materials
– Next: Right temperature ≈ Tungsten at >900˚K (Requirement for FNSF, Demo)
Solid tungsten tiles, divertor temperature controlled to 900oK
Toroidally continuous target, precision aligned, no leading edges
Future Work – Advanced Divertor Ready For Fabrication - But Shelved By Project Shutdown
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 63
Advanced Divertor: Enables Cutting-Edge Plasma Boundary/Divertor/PMI Research In C-Mod
Physics Mission – Support ITER/DEMO - Divertor physics, plasma optimization at extreme q// q// ~ 1.5 GW m-2 @ 1s pulse with no melt damage - Tungsten plasma-surface interactions at reactor temperatures, plasma heat/particle fluxes
State-of-the-art diagnostics/tool set - In-situ 1MeV D+ ion beam – key PMI diagnostic - Visible/IR endoscopes; pinhole bolometers - High heat flux Langmuir probes; thermal sensors - Very tight gas baffling, fuel/seeding delivery system
Future Work – Planned But on the Shelf – Advanced LHCD Launcher
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 64
● Off-Midplane LH launcher
● Double input power
● First high single-pass absorption LHCD system
– Higher efficiency
– Velocity-space synergy with midplane launcher
What Might A High-Field Fusion Development Path Look Like?
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 65
ADX (8 tesla)
● Advanced magnetic divertors (X-point target, SXD, etc) at reactor nTe, q//
● Core plasma optimization
● Reactor-relevant LHCD and ICRF (inside-launch, low PMI) ARC - High-field (9 Tesla)
pilot plant
C-Mod (8 tesla)
● High-temp vertical target divertor at extreme q//, tungsten PMI
● Advanced LHCD launchers
Summary of Signature Achievements (1)
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 66
● C-Mod is the highest field, diverted tokamak in the world with operation at 8 T and 2 MA of plasma current
● Advanced the state-of-the-art of plasma and PMI diagnostics
● Demonstrated tokamak initiation and control with a solid conducting vessel and structure.
● Has world-record P/S power densities of ~ 1 MW/m2, producing reactor-level SOL parallel heat flux densities exceeding 0.5 GW/m2
● Demonstrated the feasibility of very high-power tokamak operation with high-Z divertor and plasma facing components, including measurement of erosion and fuel retention rates
● Invented and established the vertical plate divertor as most favorable for power and particle handling and explored divertor regimes at reactor like plasma parameters including neutral and photon opacity
Summary of Signature Achievements (2)
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 67
● Discovered the “main-chamber recycling” phenomenon in C-Mod’s diverted plasmas and revealing intermittent, non-diffusive transport in the scrape-off layer as the underlying cause
● Demonstrated controlled divertor detachment and drastic reduction of divertor power load using seeded impurities at high power density
● Uncovered evidence for the marginal stability paradigm for SOL turbulent transport with a critical βP gradient decreasing at higher collisionality
● Identified edge plasma transport and its scaling with collisionality as the controlling physics ingredient in the tokamak density limit
● Demonstrated that spatial asymmetries in turbulence and transport drive near-sonic parallel plasma flows in the plasma edge, imposing a toroidal rotation boundary condition for the confined plasma – suggesting a mechanism for the ∇B drift asymmetry in the L-H threshold
● Carried out the first experiments that characterized the L-H threshold as a critical local temperature or temperature gradient
Summary of Signature Achievements (3)
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 68
● Demonstrated two stationary ELM-free regimes, EDA H-mode and I-Mode, where particle and impurity confinement were controlled by continuous, short wavelength electromagnetic modes in the pedestal
● First demonstrated the quantitative link between pedestal height and core performance across a wide range of operating conditions, validating the theoretically predicted dependence of turbulence on R/LT
● Discovered and explored large self-generated flows in the core plasma
● Validated gyrokinetic models simultaneously for energy and particle transport
● Proved experimentally that impurity asymmetry on flux surfaces occurs through mechanisms other than centrifugal force.
● Operated ICRF systems routinely at power densities above 10MW/m2
● Validated full-wave ICRF models by comparison with measured wave fields, fast particle distributions and local heating
Summary of Signature Achievements (4)
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 69
● Demonstrated unequivocal RF flow drive via ICRF mode conversion.
● Pioneered the field aligned-antenna concept that dramatically reduced high-Z impurity levels in ICRF heated plasmas
● Demonstrated efficient off-axis current drive with lower hybrid
● Developed the first full-wave LH codes, using these to explain the decrease in current drive efficiency at high densities
● Showed the importance of spatial asymmetries and fast dynamics for disruption halo currents and disruption mitigation radiation
● Trained over 170 graduate students in fusion science and plasma physics
APS-DPP November, 2013 20 Years of Research on Alcator C-Mod 70
END