20 Years of Research on the Alcator C-Mod …psfc Years of Research on the Alcator C-Mod Tokamak...

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


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


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


Advantages of High-Field for Fusion There’s a Reason It’s Called Magnetic Fusion Energy


● 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


● 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


● 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



● BT up to 8T








● BT up to 8T






C-Mod – Internal Hardware Design Was Also Unique


● 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


● 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









– Disruptions



“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


● 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


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


● 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


● 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


– 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


● 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)


● 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


● 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


● 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


● 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


● 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


– 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:


● 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









– Disruptions



C-Mod Experiments Impacted Our Understanding of Global and Local Threshold For L-H Transition


● 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


● 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)


● 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)


● 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


● 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


● 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









– Disruptions



Established First Quantitative Relation Between Pedestal and Core Confinement


● 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


● 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


● 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


● 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


● 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









– Disruptions



C-Mod Has Driven The Development Of ICRF Physics And Technology


● 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


● 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


● 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


● 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


Field Aligned ICRF Antenna

B


Field Aligned ICRF Antenna

B

B

ICRF Impurity Generation Addressed By Field-Aligned Antenna


● 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









– Disruptions



Lower Hybrid Current Drive (LHCD) on C-Mod


● 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


● 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


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


● 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


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


● 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









– Disruptions



C-Mod Made Crucial Early Contributions To International Database On Disruptions


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


● 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


● 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


● 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









– Disruptions




● 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


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


● 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?


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)


● 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



● 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



● 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



● 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


END

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