APS-DPP Milwaukee, WI, Oct. 23, 2017

Research was supported by US DOE OFES

Measurements of Heat-flux Footprints at the Inner Divertor Target of Alcator C-Mod J.L. Terry, D. Brunner, B. LaBombard, S.M. Wolfe

Background & motivation for interest in I-mode power exhaust

•  I-mode is an attractive stationary regime of high energy confinement (H-mode-like energy confinement)

•  L-mode particle and impurity confinement •  No ELMs

•  We seek to measure the heatflux widths (λq) at the inner target and see how they compare with those on the outer target and the “Eich scaling” for λq (which is based on outer target λq in attached H-mode plasmas)

MAST J. Harrison

Whyte NF (2010) & Hubbard IAEA (2012)

Background & motivation for interest in I-mode power exhaust

•  Exhaust power handling in I-mode remains a challenge (as it does for H-mode plasmas)

•  Lower densities and low impurity density result in lower Prad and higher Pouter target(compared to H-mode)

•  We have tried to detach I-mode plasmas, so far without success

•  Fewer ideas for power handling on inner divertor than ideas for outer divertor power handling (e.g. Super X or X-point Target or Li Vapor Box Div.)

•  Near-DN should provide a way to mitigate power to inner target •  We also seek to measure the heat-flux on the inner target in near-DN

configurations and examine the power sharing between inner and outer targets

MAST J. Harrison

Minimum heat-flux widths on outer target follow Eich-scaling for both I- and H-modes

MAST J. Harrison

Eich, et al., NF (2013) &

Brunner (this meeting)

I-modeEDA

H-mode

new C-Mod data

ITER15 MA

Minimum for I-mode heat-flux widths on inner target consistent with Eich-scaling

MAST J. Harrison

C-Modinner target

widthsfor I-mode

•  IR thermography of inner target

•  q-flux footprint profiles fit to Eich-function

•  qparallel inferred from geometry

•  mapped to outer midplane, ρ

shot

115

0922

006:

t=0.

745

s

-6 -4 -2 0 2 4 6 8! (mm)

0

100

200

300

400

para

llel h

eatflu

x (MW

/m2 )

"q=1.0 mm- - Eich fit

Significant scatter observed on inner target heat-flux widths

MAST J. Harrison

•  Significant scatter also observed on the outer target widths (LaBombard PoP (2011) & Brunner (this meeting))

C-Modinner target

widthsfor I-mode

(~x3 variability)

Search for hidden variables affecting λq is on-going, but as yet unsuccessful

MAST J. Harrison

•  Inner target heat-flux widths observed to evolve in time, typically to larger values, but with no change observed for global parameters

•  Some scatter in λq due to profile shapes with long fall-offs into PFZ, i.e. when

SEich ~ λq

shot

115

0922

009:

t=1.

298

s

-6 -4 -2 0 2 4 6! (mm)

0

50

100

150

200

para

llel h

eatflu

x (MW

/m2 )

"q=1.34 mm- - Eich fitSEich=

1.24 mm

However, profile-FWHMs still show approx. same variability/scatter

-4 -2 0 2 4 6 8! (mm)

0

100

200

300

400

500

600

para

llel h

eatflu

x (MW

/m2 ) t=0.716-0.724 s

shot

115

0923

025:

S_E

ich(0

.72)

=0.3

7 mm

, S_E

ich(0

.89)

=0.6

8 mm

"q=1.33 mm, SEICH=0.37 mmfit to Eich funct.

fit to Eich funct.

t=0.89 s

"q=2.37 mmSEICH=0.68 mm

For I-mode plasmas Podiv is ~1x to 4x Pidiv

MAST J. Harrison

Inner target q-flux measurements made using IR thermography

Outer target measurements made using TC’s to get Eodiv

tot, then dividing by Δticrf 0 1 2 3 4 5 6Pinput (MW)

0.0

0.2

0.4

0.6

P div

/Pinp

ut

Outer divertorInner divertor

Investigated near-DN I-mode plasmas as way to mitigate inner target heat-flux

•  Experiment: modulate SSEP (“δRsep”) in near-DN I-modes

•  Pidiv responds to distance to 2nd sepx, while Pidiv

peak does not

0

50

100

150

kJ

WMHD

0

1

2

3

4

MW

PICRF

0.6 0.8 1.0 1.2 1.4seconds

0.00.1

0.2

0.3

0.40.5

MW

Pidiv

0100

200

300

400

500

MW m

-2

Idiv Qpara Peak-4-3-2-101

mm SSEP

The movie of the SSEP modulation that was played in the original Power Point version can be found at

http://www-internal.psfc.mit.edu/~terry/Terry_APS_2017_SSEP_modulation.pptx

Summary •  Minumum heat-flux widths, λq, on inner target in I-mode are ~1 mm

and are consistent with Eich-scaling, λq~Bpol-1.

•  Significant scatter (~x3) in inner target heat-flux widths, but similar to scatter seen for outer target widths. To date, “hidden” variables responsible for scatter have not been found.

•  Power-sharing between inner/outer targets for I-mode plasmas is such that Pidiv/Pin ≾ Podiv/Pin by roughly 1x-4x.

•  Near-DN operation reduces Pidiv, but doesn’t reduce qidivpeak

significantly until SSEP (“δRsep”) > 0.

•  Power handling in I-mode at inner target remains a significant challenge.