Advances in physics and performance ofthe I-mode regime over an expanded
operating space on Alcator C-Mod*
A. E. Hubbard,
D. Brunner, S.-G. Baek, A. Creely, I. Cziegler1,J.W. Hughes,B. LaBombard, Y. Lin, Z. Liu 2,3, E.S. Marmar, M.L. Reinke4,J.E. Rice, B. Sorbom, C. Sung5 J.L. Terry, C. Theiler6, J. R. Walk,A.E. White, D.G. Whyte, S.M. Wolfe, S.J. Wukitch, X.Q. Xu3
and the Alcator C-Mod TeamMIT Plasma Science and Fusion Center
1York Plasma Institute, Univ.of York, UK, 2ASIPP, Hefei, China,
3LLNL, Livermore, CA, USA, 4ORNL, Oak Ridge ,TN, USA
5University of California, Los Angeles, CA, USA
6EPFL, Swiss Plasma Center, Lausanne, Switzerland*Supported by the U.S. Dept of Energy, Office of Fusion Energy Sciences

EX/1-3, 26th IAEA Fusion Energy Conference,Kyoto, Japan, October 19, 2016

AlcatorC-Mod

Outline

Alcator
C-Mod• Key features of the I-mode regime for fusionenergy.
• Expanded operating space on C-Mod.
– Magnetic field increased to 8 T.
– Near double null configurations.
• Particle and energy transport• Pedestal physics and transitions.
• Extrapolation of I-mode for fusion devices– Prospects, issues, and needed research.
A. Hubbard, FEC 2016 EX/3-1

2

I-mode regime combines the best
features of H-mode and L-mode

AlcatorC-Mod• Temperature pedestal andhigh energy confinement.
• L-mode density pedestal andlow particle confinement.
– Stationary, controlled densities.
– Avoids accumulation of high or
low Z impurities and need forroutine boronization.

• ELM-free, avoiding
damaging heat pulses.
– Pedestals are MHD stable.

• Highly attractive features forfusion energy. Key issues:– Can it be accessed in reactor
conditions?
– Will E will be high enough?
A. Hubbard, FEC 2016 EX/3-1

1.1 MA, 5.8 T, q95=3.4
LSN, upwards B×B

3

I-mode has now been achieved oververy wide ranges of parameters

AlcatorC-Mod• Multidevice studies on C-Mod, ASDEX-Upgrade, DIII-D
were reported at FEC 2014 Hubbard, Osborne, Ryter et al NF16•

I-mode is robust over wide ranges of dimensional parameters:
Ip 0.4-1.4 MA, BT 1.9-6 T, density 0.16-2.4x1020m-3, Ploss 1.5-4.1 MW
and dimensionless parameters: q95 2.4-5.2, *ped 0.17-4,
Heating with ICRH, NBI and/or ECH.

• In 2015-2016, 30% of C-Mod
operation has been in theconfiguration favouring I-mode, withion B×B drift away from X-point.
Emphasis on– Extending to even higher
magnetic field and current(8 T, 1.7 MA).
– Identifying and addressing key
physics issues for extrapolation.
A. Hubbard, FEC 2016 EX/3-1

2.0

C-Mod I-modes

20161.5I p(MA)

•1.0
2015

0.5
0.00246
B T(T)

84FEC 2014: weak dependence of L-I threshold,larger power range with higher BT
Ploss

0.8

C-Mod I-mode
C-Mod L-I
AUG I-mode
AUG L-I_neS-20(10 MW m )0.6

DIII-D I-mode
DIII-D L-I

H-mode0.2
L-modeq95= 3-4.3, n>nmin0• At <3 T, modest
increases in power
tend to give I-Htransitions.
• At >5 T, can remain
in I-mode to muchhigher powers.

0.4

0.0

Alcator
C-Mod

24− Some discharges
still have I-Htransitions.

6B T(T)
A. Hubbard, FEC 2016 EX/3-158 T I-modes confirm, extend thepromising trends with BT
Ploss

0.8_-20(10 MW m )neS

C-Mod I-mode
C-Mod L-I

• P(L-I)/ne~ BT0.25.
0.8Pmax, I ~B

0.6

availablepower

0.4
0.25

0.2
0.0
0

P(L-I)~B

L-mode2

AlcatorC-Mod

4
6BT (T)

8− At 8 T, we used D(He3)
ICRH, which has lower
single pass absorption
than usual D(H), hence
Ploss more uncertain.

• NO discharges at
7.8-8 T had I-Htransitions, up to 5 MW
available ICRF power(Ptot/S ≤ 0.63 MW/m2)
10 => power range is evenlarger than at 5-6 T.
‒ Some had I-L backtransitions.

A. Hubbard, FEC 2016 EX/3-1

6

High B I-modes exhibit typical features

0.08 T
40C-Mod shot 1160923003 8 T 1.35 MA

64 PRF
2 (MW)
020
2.0 --ne (10 m-3)
1.0(0)e(keV)

Frequency (kHz)

1.0 Te,ped (keV)
0.5
0.0
1.5
1.0 < P > (atm)
0.5
L-mode
0.0

400
300

I-modereflectometer (75 GHz)n~eAlcator
C-Mod

Example with 7.8 T,
1.35 MA, (q95= 3.7),
PRF=4.7 MW:• Central Te to 7.3 keV.
• Volume averaged <P> to1.4 atm (0.14 MPa).
• H98=1, assuming RF=80%.
• No ELMs• Weakly coherent mode at350 kHz (higher than atlower Ip and BT).
− Up to 450 kHz at 1.7 MA.

200
100
00.65

0.75

0.85

0.95 1.05
Time (s)A. Hubbard, FEC 2016 EX/3-1

1.15

1.25

7

Te, Ti pedestal and Er wellin 7.8 T, 1.35 MA I-mode
TekeV
1.5
1.01160923002 1160930014

2.00.5

AlcatorC-Mod• Steep Te gradient, 1 keV pedestal• L-mode density profile.
– Contrasts with EDA H-mode atsimilar B, I, Ploss.

020 -310 m

8 T EDA H-mode

4ne328 T I-mode

100.85

0.90

0.95

r/a

A. Hubbard, FEC 2016 EX/3-1

1.00

1.05
8

Te, Ti pedestal and Er wellin 7.8 T, 1.35 MA I-mode
2.01160923002 1140522022

Ti

keV
1.5
1.00.5

AlcatorC-Mod• Steep Te gradient, 1 keV pedestal• L-mode density profile.
– Contrasts with EDA H-mode atsimilar B, I, Ploss.
• CXRS shows Ti pedestal.

0100ErkV/m50
0-50-1000.85

8 T I-mode
2.8 T I-mode
0.90

0.95r/aA. Hubbard, FEC 2016 EX/3-1

1.00

1.05

• Er well 100 kV/m depth (Er,min -50).
– Much steeper than 2.8 TI-modes (at lower Ip and Ploss).
– In the range of typicalstationary H-modes,~30-100 kV/m.
9I-mode has also been extended to near
DN configurations, sharing heat flux
Near-DN I-mode, rsep -3 mm••••

A. Hubbard, FEC 2016 EX/3-1Alcator
C-ModI-mode is most robust with ion B×Bdrift away from X-point.
2016 experiments, at 5.4 T, foundI-mode could still be accessed with rsep
as small as 3 mm.
Can maintain I-mode with rsep 1-2 mm.
DN or drift towards X-pt give I-Htransitions; These H-modes are very highconfinement (H98>2), but transient.

10I-mode has also been extended to near
DN configurations, sharing heat flux
1.0

fraction of total power flux

0.9
0.8

I-modes

I-mode, 0.8 MA, 9e+19 m− 3, REV-B

Inner Upper
Outer Upper•0.7
0.6
0.5

I-H transitions
Outer Lower•0.4D. Brunner;
Details at APS 2016

0.3
0.2
0.1
0.0−4

•

Alcator
C-Mod

I-mode is most robust with ion B×Bdrift away from X-point.
2016 experiments, at 5.4 T, foundI-mode could still be accessed with rsep
as small as 3 mm.
Can maintain I-mode with rsep 1-2 mm.

• Small rsep shares power more evenly
between strike points.

Inner Lower−3−2−101δrsep(mm)234Power handling in reactor scenarios is still a key issue:• Lack of ELMs in I-mode eliminates transient heat load.
But, steady heat flux is still high, and detachment is challenging; tend to get
I-L transitions first.
• Advanced divertors are likely needed for reactors in any regime. Simulations
with X-Pt Target Divertor are very promising – see
Umansky TH/P6-32, Thurs. pm
A. Hubbard, FEC 2016 EX/3-1

11

Alcator
C-Mod

Particle and Energy Transportin I-mode

A. Hubbard, FEC 2016 EX/3-1

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I-mode has L-mode imp, H98,y2 0.7-1.3

AlcatorC-Mod• Extensive studies with injected Ca or Mo show imp is at L-modelevels (15-40 ms),10-100X lower than H-mode [Howard RSI’11. Rice NF’15]• Range in H98 indicates H-mode-like confinement, but some differencesin E scaling. One key difference is more favourable power scaling.
200WMHDStored energy increases nearlylinearly with power – thus H98
increases with power.
• Prior regression on 200 I-mode
slices supports this:E,Imode=Ip0.68 BT0.77 ne0.02 PL-0.29– Contrasts with ITER98p ~ PL-0.7(kJ)

I-modes,
Ip=1.15 MA

150100500I-mode
power range0123• As with other empirical scalings, covariances might affectdependences, eg on Ip, BT, ne.
A. Hubbard, FEC 2016 EX/3-1

456Loss Power (PRF,abs+Poh-dW/dt, MW)

Walk, MIT
Ph.D. 201413

Single-parameter scans done in 2016
for more direct assessment of E trends
0.06

0.05

τ E (s)• Initial analysis suggests τ E (s)some differences,
0.04
eg positive density
scaling, weaker Ip
0.02 ne scan,
Ip=1.1 MA,
scaling.
0.00
0.0

0.41
fit ne0.04

1.0

fit I p0.37

0.03
0.02
0.01

PRF=3 MW
0.5Alcator
C-Mod

20

1.5-3

ne (10 m )

2.0

0.00
0.0

Ip scan,
ne=1020 m-3
PRF=3 MW
0.5

1.0Ip(MA)

1.5• Also did recent scans of dimensionless parameters, *, *, , keeping
other parameters near const. Can be used to constrain size scaling.
– Detailed analysis is in progress. B E dependence on is more favourable
than H-mode; * dependence is weak.
• Multidevice study, similar to L-mode and H-mode confinement
databases, would give best scaling and extrapolation to future devices,
including direct size scaling.
– Include existing data from AUG, DIII-D, and results from other tokamaks
as they become available. Planned ITPA activity.
A. Hubbard, FEC 2016 EX/3-1

14

Validating core transport models in
I-mode regime
0.15
Coherence

Comparing L, I, plasmas at
5.4 T, moderate power, findin expt:• Strong (~2x) reduction incore e.
• Low k Te fluctuations(CECE) are reduced.

0.12

AlcatorC-Mod

Te fluctuations
ρtor = 0.84

L-modeI-mode~L:TT 1.17%

0.09
0.06
0.03

White PoP 2015
Creely NF 201600100 kHz

200300Nonlinear GYRO simulations and measurements of e,inc indicate:• ExB shear suppression of the ITG is larger in I-mode vs L-mode• ITG can account for observed low-k turbulence but not transport.
=>High-k electron scale (ETG) turbulence is important.
• I-modes are more ‘stiff’, resembling H-mode – hence Tped is key.
Details in APS invited, A. Creely, VI2:4 .
A. Hubbard, FEC 2016 EX/3-1

15

Alcator
C-Mod

Pedestal physics and transitions.

A. Hubbard, FEC 2016 EX/3-1

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Pedestal flows and turbulence arekey to L-I transitions

Alcator
C-Mod

Several simultaneous changes occur in the pedestal region at L-I transitions:• Reduction in mid-freq turbulence, correlated with eff.
• Fluctuating flow at GAM frequency (10’s of kHz) .
• Weakly Coherent Mode – broad fluctuation at few 100 kHz

GPI turbulence spectra
0.1000P(f) (A.U.)

Cziegler
APS
2015

Ohmic
pre-I
0.0100I-mode0.00100.00011

df/f=0.1
df/f=0.710f (kHz)100

Te(0) 4(keV) 2
0.8
Te,ped(keV) 0.6
I-mode0.4
V fluctnfreq60 GAM(kHz)20
freq 300 density fluctn WCM
(kHz)
1000

1.1

1.2

1.3

Time (s)1.4• Has become clear in last 2-3 years that GAM is very important inbroadening WCM, and likely in I-mode access, particle and energy
transport. Consistent on C-Mod (Cziegler PoP 2013), AUG (Manz NF 2015).
A. Hubbard, FEC 2016 EX/3-1

Manz EX/P6-32, Thurs pm

17Transfer from turbulence to zonal flows is 2x lower withB×B away from X-pt, opening an I-mode power window

AlcatorC-Mod• Prior work has shown L-H transition occurs when energy transfer rate
into ZF exceeds turbulent drive. [Manz PoP12, Yan PRL14, Cziegler PoP 14,NF15]• Transfer rate in the configuration with B×B away from X-pt(“unfavourable”) is only half the rate towards X-pt(“Favourable”) =>higher H-mode threshold!
• In the I-mode window, energy is also transferred to GAMs
I. Cziegler, YorkAPS 2015,
PRL in prep

A. Hubbard, FEC 2016 EX/3-1

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Alcator
C-Mod

Simulations of I-mode pedestal••••Simulations of a high density, 5.8 T I-mode pedestal have been carried out
with BOUT++. Details in Poster TH/P2-28, Z. Liu (Tues).
Nonlinear simulations find a mode with many features of WCM(n=20, 350 kHz, electron diamagnetic direction)
Predicts larger particle diffusivity than thermal, consistent with the key
feature of I-mode. Predicted e is close to experimental eff.
Extensions to include zonal flows and GAMs are in progress.
0.07

Reflectometer
Simulation ψ =0.988

0.06

ψ=0.98 (n=20 10x)0.05

m2 / sAmplitude (a.u.)

0.08

0.04
0.03
0.02
0.01
0•100200 300
400
Frequency (kHz)

500

1.6
1.4
1.2
10.8
0.6
0.4
0.20Electrons
IonsDerχerDirχirExpt χeff 0.2 m /s
20.9

0.92 0.94 0.96 0.98
ψ1

Other groups are working on gyrokinetic simulations of I-mode pedestal,and of L-I transitions.

A. Hubbard, FEC 2016 EX/3-1

19

Alcator
C-Mod

Extrapolation of I-mode forfusion devices

A. Hubbard, FEC 2016 EX/3-1

20

Key features and issues for ITER, DEMO

Alcator
C-ModKey features:• Naturally ELM-free– Do not need external control tools such as RMP coils or pellets.

• L-mode particle confinement has major operational
advantages compared to H-mode:– High Z impurities do not accumulate. Compatible with metal PFCs
(Mo on C-Mod, W on AUG).
– Do not need (or use on C-Mod) frequent boronization, which isimpractical in a reactor.
– Performance recovers well from any injections during a discharge.
– Density is readily controlled, not set by a particle barrier.

Key Issues:• Can I-mode be accessed with Pext? (L-I threshold)• Can it be maintained, once fusion power increases?
(I-H threshold, density range)
A. Hubbard, FEC 2016 EX/3-1

21

Expanded power range with BT ispositive for ITER, high B DEMOs
Ploss

0.8_-20(10 MW m )

neS

0.6

C-Mod I-modes

Pmax, I~B

0.8

available
power

Alcator
C-Mod

Scaling C-Mod P/ne by surface area,
predict for ITER:• P(L-I) ~ 70 MW at 5x1019m-3.
o NBI with rev Ip is biggest issue.

• If accessed, should be able tomaintain to full Pfus at higher ne.
0.25
P(L-I)~B
0.2For ARC (BT=9.2 T, R=3.3 m):
ARCL-modeITER• P(L-I) ~ 46 MW at 5x1019m-3
0.00246810 • Could maintain to full Pfus (525BT (T)MW) at ne ~1.3x1020 m-3.
0.4

JET

For information on ARC, seeDennis Whyte FIP/P7-6, Friday am•

I-mode profiles were used for steadystate scenarios [Sorbom FED 2015].

• Multi-device scalings of thresholds, density range as well as
E are needed for more confident extrapolation.
Experiments at larger size (JET, JT60-SA) will be extremely valuable.
A. Hubbard, FEC 2016 EX/3-122Progress in both operating space andphysics of I-mode on C-Mod

AlcatorC-Mod• I-mode regime, with high energy confinement, low particle
confinement, no ELMs, has now been achieved over very
wide parameter ranges. (BT 2.8-8 T, Ip 0.55-1.7 MA, 1.5-5 MW).
‒ Weak dependence of L-I threshold on BT.
‒ Upper power range increases with BT; at 8 T no I-H transitions areobserved up to Ptot/S= 0.63 MW/m2)‒ I-modes extended to near DN configuration for better flux sharing.

• GAM and weakly coherent mode in pedestal are key toI-mode physics, thermal transport barrier.
− Simulations are beginning to predict these features, as well as coretransport.
− Transfer rate from turbulence to zonal flows varies with magnetic drift
direction, explaining L-H dependence and opening up I-mode window.

• Experiments on other tokamaks are now critical to continue
research on this promising regime for fusion, and provide a
firm basis for extrapolation to future fusion devices.
A. Hubbard, FEC 2016 EX/3-1