Observations of Lower Hybrid Wave Absorption in the Scrape Off Layer of a

Diverted TokamakG. M. Wallace1, R. R. Parker1, P. T. Bonoli1, R. W. Harvey2, A. E. Schmidt1, A. P.

Smirnov2, D. G. Whyte1, J. R. Wilson3, J. C. Wright, and S. J. Wukitch1

1MIT Plasma Science and Fusion Center, 2Comp-X Corporation, 3Princeton Plasma Physics Laboratory

This work supported by the US DOE awards DE-FC02-99ER54512 and DE-AC02-76CH03073.

A

B

C

D

E

K

J

G

F

H

GH FullLimiter

J ICRF

antenna

AB SplitLimiter

Ip

K midplane

Limiter

D&E ICRF

antennas

LH coupler

C-Mod LHCD

System�f0=4.6 GHz�4X22

waveguide

phased array

�Peak of

launched n||

spectrum

variable from

1.5-3.5

HXR Camera�Measures

bremsstrahlung from non-thermal

electrons

�32 chords

�20-200keV photon

energy range

�Records time,

energy, and chord

for each incident

photon

HXR Emission Orders of Magnitude Lower than

Expected at High Density�Line integrated HXR emission drops much faster with

density than 1/n

�Precipitous drop in emission around 1X1020 m-3

�Higher toroidal field and plasma current increase count

rates at high density

�Changing antenna phasing (n||) has little effect

Parametric Decay Does Not

Explain Drop in HXR�3-wave coupling process results

in a downshifted LH wave and an

ion cyclotron wave as ω�2ωlh

�Effect should become more pronounced with higher ne, B

�PDI level (relative to

fundamental) is less than -20dB

in C-Mod data

�All data

from C-Mod

is for

ω/ωlh>3

Conclusions�We observe unexpected LH wave behavior at

densities above ne~1X1020 m-3

�Drop in HXR emission

�Current in the SOL increases as HXR drops

�Highly localized wave fields in SOL

�Accessibility criterion and Parametric Decay

Instability do not explain these behaviors

�Modeling shows rays are damping in the SOL at

high density

Future Work�Compare experimental results with

TORIC/COMSOL full wave simulations (See invited

talk by J. Wright and poster by S. Shiraiwa)

�Change plasma parameters (outer gap, topology)

to find high performance discharges while mitigating

deleterious effects on LHCD

�Examine implications for ITER and DEMO.

GENRAY/CQL3D with SOL Improves

Agreement with Experiment at High Density

�Simple SOL model added in GENRAY

�e-folding based on perpendicular distance from

LCFS

�Density scale length (σn [m]) a function of

poloidal angle

�Temperature scale length (σT [m]) constant for all poloidal angles

�SOL parameters chosen close to experimental

values

•σT = 0.005 m

•Tmin = 5 eV

•σn = 0.02-0.1 m

•nmin = 1X1011 m-3

�Addition of SOL decreases HXR emission slightly

at low density and significantly at high density

�Fall in HXR emission sensitive to particulars of the

SOL ne and Te profiles

�Damping due to collisions in SOL is strong

function of plasma density and temperature

�Need additional data from scanning probes

during high power LHCD to get SOL profiles

Conventional GENRAY/CQL3D Modeling

Disagrees with Experiment at High Density�GENRAY: Traces ray trajectories only inside last closed flux surface

�CQL3D: Fokker-Planck solver calculates wave

damping and current drive

�Predicts HXR emission α~1/n

�Many rays do not penetrate far into plasma above

1X1020 m-3

Measurements Show Driven Current in SOL�Measure Ignd on Langmuir probes in upper and lower

divertors to determine free flowing current

�Significant currents appear only with LH above 1X1020 m-3

�Current along field lines is closed through vacuum vessel

�Current driven is greater for higher n||

SOL Wave Fields Detected Away from Antenna�Horizontal Reciprocating RF probe located on A-Port

�Electrode connected through 4.6 GHz band pass filter to

RF diode

�Localization of RF electric field in scrape off layer near

radius of the LH launcher (R=~91cm)

�Damage to LH Launcher

is in bands tilted along the field lines

�Similar pattern of

visible light observed

on cameras during

high power operation

�Mismatch of plasma

shape and launcher

shape create leading

edges on the waveguide

septa�q|| of ~1.4X107 W/m2

necessary to melt septa

in 0.5 s achieved at 20

eV and 1X1019 m-3

B-Field

C-Mod Cross Section

D=123 cm d=40 cm

ac=5 mm ad=5 mm

HXR Diagnostic

Motivation�Lower Hybrid Current Drive (LHCD)

efficiency is predicted to scale as ~1/ne

�LHCD at high density on other tokamaks

has shown a “density limit” as ω�2ωlh

�H-mode plasmas on C-Mod are close to

but below this density limit

�Effects of SOL profiles on LH waves are well

known but poorly understood, particularly in

diverted tokamaks

�Prior modeling of LH waves in tokamaks has

treated core and SOL separately

�SOL effects will be of particular importance

for non-inductive burning plasma experiments

due to long distance between antenna and

separatrix

Accessibility Criterion for LH Waves Is

Satisfied�n|| will upshift proportional to 1/R due to toroidicity

�n|| will also shift due to poloidal curvature

�Close to accessibility limit on the midplane for

n||=1.9 at ne~1.5X1020 m-3

�Discriminant at local max where rays turn around

� not limited by accessibility

0.4 0.6 0.8 1 1.2 1.4 1.6

x 1020

0

0.5

1

1.5

2

2.5

3

3.5

4x 10

6

ne [m

−3]

Count R

ate

(C

h 9

−24, 40−

200 k

eV

) [s

−1] Line Integrated HXR Count Rate

n

||=1.9, 5.4T, 800kA

n||=2.3, 5.4T, 800kA

n||=1.9, 7.0T, 800kA

n||=2.3, 7.0T, 800kA

n||=1.9, 5.4T, 1.1MA

n||=2.3, 5.4T, 1.1MA

0.4 0.6 0.8 1 1.2 1.4 1.6

x 1020

103

104

105

106

107

ne [m

−3]

Count R

ate

(C

h 9

−24, 40−

200 k

eV

) [s

−1] Line Integrated HXR Count Rate

n||=1.9, 5.4T, 800kA

n||=2.3, 5.4T, 800kA

n||=1.9, 7.0T, 800kA

n||=2.3, 7.0T, 800kA

n||=1.9, 5.4T, 1.1MA

n||=2.3, 5.4T, 1.1MA

0.4 0.6 0.8 1 1.2 1.4 1.6

x 1020

103

104

105

106

107

108

ne [m

−3]

Count R

ate

(C

h 9

−24, 40−

200 k

eV

) [s

−1] Line Integrated HXR Count Rate

n||=1.9, 5.4T, 800kA

n||=2.3, 5.4T, 800kA

n||=1.9, 7.0T, 800kA

n||=2.3, 7.0T, 800kA

n||=1.9, 5.4T, 1.1MA

n||=2.3, 5.4T, 1.1MA

1 1.02 1.04 1.06 1.08 1.10

0.2

0.4

0.6

0.8

1

RF

Pro

be S

ignal [V

]

Time [s]1 1.02 1.04 1.06 1.08 1.1

0.86

0.88

0.9

0.92

0.94

0.96

Pro

be R

adiu

s [m

]

Time [s]

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

x 1020

−55

−50

−45

−40

−35

−30

−25

−20

3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6−55

−50

−45

−40

−35

−30

−25

−20

ω/ωLH

PD

I le

ve

l [d

B]

4.4 4.45 4.5 4.55 4.6 4.65 4.7

x 109

−80

−70

−60

−50

−40

−30

−20

−10

Frequency [Hz]

Am

plit

ude [dB

] PDI Level

3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.810

3

104

105

106

107

Line Integrated HXR Count Rate

ω/ωLH

Count R

ate

(C

h 9

−24, 40−

200 k

eV

) [s

−1]

n||=1.9, 5.4T, 800kA

n||=2.3, 5.4T, 800kA

n||=1.9, 7.0T, 800kA

n||=2.3, 7.0T, 800kA

n||=1.9, 5.4T, 1.1MA

n||=2.3, 5.4T, 1.1MA

−1 −0.5 0 0.510

3

104

105

106

107

Line Integrated HXR Count Rate

n||crit

−n||launch

Count R

ate

(C

h 9

−24, 40−

200 k

eV

) [s

−1]

n

||=1.9, 5.4T, 800kA

n||=2.3, 5.4T, 800kA

n||=1.9, 7.0T, 800kA

n||=2.3, 7.0T, 800kA

n||=1.9, 5.4T, 1.1MA

n||=2.3, 5.4T, 1.1MA

0.6 0.8 1

−0.5

−0.4

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4

R [m]

z [

m]

0 0.2 0.4 0.6 0.810

0

105

C22−

4C

4C

0

Poloidal Distance Along Ray [m]

0 0.2 0.4 0.6 0.80.85

0.9

0.95

1

ρ

0.65 0.7 0.75 0.8 0.85 0.9

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

3.2

Shot = 1080513016 Time = 1.0769 [s]

Major Radius [m]

n|| c

rit

Local n||,crit

n||=1.94

n||=2.33

ce

pe

ce

pepi

n

ccc

cncnc

ω

ω

ω

ω

ω

ω

ε

εε

++−=

+≥

=−⇒

=++

×⊥

⊥⊥

2

2

2

2

||

2

||

04

2

2

0

2

2

4

4

1

04

0

Moderate Damage to LH Launcher due to Increase in Local Temperature and Density During LHCD

0.6 0.8 1

−0.5

−0.4

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4

ne = 5×10

19 [m

−3]

0.6 0.8 1

−0.5

−0.4

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4

ne = 1.5×10

20 [m

−3]

0.4 0.6 0.8 1 1.2 1.4 1.6

x 1020

103

104

105

106

107

ne [m

−3]C

ount R

ate

(C

hord

s 9

−24, 40−

200 k

eV

) [s

−1]

Line Integrated HXR Count Rate

n||=1.9, 5.4T, 800kA

n||=2.3, 5.4T, 800kA

n||=1.9, 7.0T, 800kA

n||=2.3, 7.0T, 800kA

n||=1.9, 5.4T, 1.1MA

n||=2.3, 5.4T, 1.1MA

GENRAY/CQL3DNo SOL

GENRAY/CQL3DWith SOL

1017

1018

1019

1020

100

101

102

103

ne [m

−3]

Te [eV

]

log10

(q||)

4

5

6

7

8

9

10

0.4 0.6 0.8 1 1.2 1.4 1.6

x 1020

0

1

2

3

4

5

6

7x 10

5

ne [m

−3]

SO

L C

urr

ent D

ensity [A

/m2]

75°, 5.4T, 800kA

90°, 5.4T, 800kA

75°, 7.0T, 800kA

90°, 7.0T, 800kA

75°, 5.4T, 1.1MA

90°, 5.4T, 1.1MA

0.6 0.7 0.8 0.9 1 1.1 1.2 1.30

500

1000

LH

Net P

ow

er

[kW

] Lower Null SOL Current

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3−0.4

−0.2

0

Oute

r 08 I

gnd [A

]

0.6 0.7 0.8 0.9 1 1.1 1.2 1.30

0.2

0.4

Inner

03 I

gnd [A

]

Time [s]

Equal/Opposite

Currents on

Inner/Outer

Divertor Probes

0.6 0.8 1

−0.5

−0.4

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4

ne = 1.5×10

20 [m

−3]

ne [m-3]