Lower Hybrid Wave Neutral Excitation, Ionization and SOL Power Loss of the Alcator C-Mod TokamakI.Faust, J.L. Terry, G.M. Wallace, S. Shiraiwa, M.L. Reinke, R. R. Parker, S.G. Baek, B. LaBombard, J. W. Hughes, J.R. Walk, and D.G. Whyte1Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA

AbstractThe efficiency of Lower Hybrid Current Drive (LHCD) in AlcatorC-Mod discharges diminishes precipitously in high density(line-averaged ne > 1020[m−3]), diverted plasmas as seen by the lackof indicative hard X-ray (HXR) bremsstrahlung and reduction in loopvoltage. VUV, Visible and infrared light, as well as measurements ofne,Te in the SOL show significant change in the high density regimewith the application of Lower Hybrid power. Poloidal dependence ofLHCD-induced hydrogen Lyman-alpha emission in high densityplasmas was investigated using a filtered poloidally-viewing pinholecamera. Due to limitations in the camera radial resolution, a prioriassumptions of the emission region were used to extract global emissionvalues. Correlations of total Lyman-alpha power are made versusdensity. Lyman-alpha emission are correlated to various experimentalparameters for the dependency of power loss. The measurementsindicate that Lyman-alpha power is enhanced globally by LHCD.

Lower Hybrid System of C-Mod

Lower Hybrid grill with fixed limiter

10 4.6 GHz klystrons power a4x16 waveguide grill [3]which launches theelectrostatic Lower Hybridwave which is asymmetricallyabsorbed by the electrondistribution throughlandau-damping.

System is capable of coupling900kW forward power to theplasma. The launching systemgenerates fully non-inductivescenarios in low averagedensity plasmas(ne < 5 · 1019[m−3]).

The system was designed to assist in generating advanced sce-nario, reversed-shear plasmas. It also will allow for extendedlength discharges in the near future.

Plasma Density Limits CD Efficiency

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

x 1020

102

103

104

105

106

107

108

Line Averaged ne [m−3]

Cou

nt R

ate

(Ch

1−32

, > 4

0 ke

V)

[s−

1 ]

Line Integrated HXR Count Rate, Neutron Subtracted

800kA, n||=1.9, 5.4T

800kA, n||=2.3, 5.4T

800kA, n||=1.9, 7T

800kA, n||=2.3, 7T

1.1MA, n||=1.9, 5.4T

1.1MA, n||=2.3, 5.4T

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

x 1020

20

40

60

80

100

120

140

160

180

200

Line Averaged ne [m

−3]

Ph

oto

n T

em

pe

ratu

re T

ph [

ke

V]

Average Photon Temperature Tph

versus density

800kA, n

||=1.9, 5.4T

800kA, n||=2.3, 5.4T

800kA, n||=1.9, 7T

800kA, n||=2.3, 7T

1.1MA, n||=1.9, 5.4T

1.1MA, n||=2.3, 5.4T

1120710020

• High density (ne > 1 · 1020[m−3]) diverted plas-mas exhibit negligible current drive with appliedLHCD power. [1]

• Limited discharges recover expected current drivedensity dependency (η ∼ n−1e )

• Hypothesized causes include parametric decay in-stability (PDI), density fluctation scattering, edgecollisional absorption, and refractive n‖ upshifts.

• Shape of HXR emission is rigid with changing den-sity [8].

• Results show little to no variation at low density.Statistical error of fit dominates above 1 · 1020.

LHCD Evaluated with X-ray Camera

32

1

• The Hard X-ray Camera (HXR)measures e− − e− and e−−ionbremsstrahlung for photon energies40keV<Eph < 250keV [2]

• 32 Poloidally-viewing CdZnTeradiation detectors compare toforward-modelled fast-electrondistributions

• Measured X-ray count rate is usedas a proxy for driven current andfor the total fast electron population

The model LH electron distribution [5] display the strong anisotropy of Hard X-ray Emission

Lyα Emission Varies with LHCDAverage Ly-α Emissivity versus radius

0.86 0.88 0.90 0.92 0.94midplane radius [m]

0

10

20

30

40

50

emis

sivi

ty [a

.u.]

.1s average after LH

.1s average during LH

LCFS

1080

1230

03

SOLCore

Ly-α emissivity shift vs LH Net Power

0 200 400 600 800 1000LH Net Power [kW]

0

2

4

6

8

10

Ly-α

em

issi

vity

mid

plan

e ra

idal

shi

ft [m

m] (

∆R)

High Density shots,

<10% density variation

• AXUV-based system using bandpass filters for the VUVnature of the Lyα line (121.6 nm). [4]

• Previous results have shown that Deuterium Lymanαemissivity profiles at the edge change in high densitywith applied LH power. (move outward from seperatrix)

• The Lyα profiles change on a fast (∼1ms) timescale andcorrelate to the applied LH power, suggesting an edgeeffect relating to LH.

Lyα and Hα Measure Edge DynamicsBP-LY Camera View • The Gas Puff Imaging diagnostic

(GPI), the new B-port Lymansystem, and other systems measurecommon main species lines. [6]

• B port Lyman Camera (BP-LY)views wide poloidal slice in orderto test connected field-line effects.

• VUV measurements unaffected byreflections, absolute measurementsare accurate with a fast timeresponse.

Global Power CalculationThe total power from the B-port Lyman Camera can-not be calculated using typical inversion principles.

PLyα,chord =

∫V

εdV = 〈A〉ε∫r

εdr ≈ 〈w〉ε〈lk〉ε∫r

εdr

The measurements are made with angular dependence, whichcan be converted to a path integral.

PLyα,meas =

∫Ω

∫V

ε

4πdV dΩ ≈ G

4π

∫r

εdr

The measured chord power revolved around the vessel(PLyα,annulus = 2πR

〈w〉εPLyα,chord), and summing the chords yieldsthe total power

PLyα,tot =∑k

8π2 〈Rklk〉εGk

PLyα,meas,k

ε is the Lyα emissivityG represents the diode to pinhole etendueA is the cross sectional area of the view

0.40.50.60.70.80.91.0

0.6

0.4

0.2

0.0

0.2

0.4

0.6

R is the major radius〈〉ε is the emission weighted averagePLyα,tot is the total Lyα power

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1R [m]

0.6

0.4

0.2

0.0

0.2

0.4

0.6

Z [

m]

SOL volumes shot 1120612007

Right is the Flux derived, mapped viewing areas repre-senting the Lyα emission regions. Each chord area is usedto derive the emissivity weighted height (〈l〉ε)

Total Lyα power calculatedMethod is used to calculate the total power as describedbefore. Response in total power is prompt (< 10−3s).

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8time [s]

10

20

30

40

50

60

70

80

Lym

an a

lpha P

ow

er

[kW

]

shot 1120710008 global Lyman-alpha power

High density ne ≈ 1.2 · 1020[m−3]

LH off at t = 1.3s

Moderate density ne ≈ 1.0 · 1020[m−3]

LH off at 1.1 and 1.3s

0.2 0.4 0.6 0.8 1.0 1.2time [s]

0

50

100

150

Lym

an a

lpha P

ow

er

[kW

]

shot 1120517010 global Lyman-alpha power

Ohmic Discharge ne ≈ 1.0 · 1020[m−3]

no LH off

Low Density ne ≈ 1.0 · 1020[m−3]

LH off at 1.3s

• Calculated total power typically on the order of 100kW.

• Changes are similar in magnitude and time responseto previous Lyα results.

• Calculations assume toroidal symmetry.• The loss of energy (as determined by electron cool-

ing rates) will be on the order of 2∼3 times the ly-man energy loss value.

• Most LHCD results of this type are from L-modeplasmas.

Te,BT affects LH Density Limit

LowBT ≈ 5.4T and Te ≈ 2keV. HighBT ≈ 8T and Te ≈ 4keV

• Applied LH power shifted the emission profile outward.These results were seen both in D and He Plasmas.

• For plasmas with Te ∼ 2 keV, the He-I emissivity profileincreased and shifted outward faster than energy confine-ment with applied LHCD power.

Lyα increase with LHCD in H-modes

0.0 0.5 1.0 1.5 2.00

1•1020

2•1020

3•1020

4•1020

TS core Ne [m-3]

0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

TS Core Te [keV]

0.0 0.5 1.0 1.5 2.00

200

400

600

800

LH Net Power [kW]

0.0 0.5 1.0 1.5 2.00.0

0.5

1.0

1.5

Lower Divertor View Brightness [a.u.]

0.0 0.5 1.0 1.5 2.00.0

0.1

0.2

0.3

0.4

0.5

Upper Divertor View Brightness [a.u.]

0.6 0.8 1.0 1.2 1.42•10

43•104

4•104

5•104

6•104

7•104

8•1049•104

Channel 1

0.6 0.8 1.0 1.2 1.41.5•10

42.0•104

2.5•104

3.0•104

3.5•104

4.0•104

4.5•1045.0•104

Channel 5

0.6 0.8 1.0 1.2 1.46.0•10

38.0•103

1.0•104

1.2•104

1.4•104

1.6•104

1.8•1042.0•104

Channel 9

0.6 0.8 1.0 1.2 1.405.0•10

31.0•10

41.5•10

42.0•10

42.5•10

43.0•104

Channel 13

0.6 0.8 1.0 1.2 1.44.0•10

36.0•103

8.0•103

1.0•104

1.2•104

1.4•104

Channel 17

0.6 0.8 1.0 1.2 1.45.0•10

3

1.0•104

1.5•104

2.0•104

Channel 21

B-port Lyman profile exhibitsdrop in brightness at top ofplasma ([Wm−2]) chord 21,and enhancement in divertor(chord 1)

• Results show that improvements in confinementwhen LHCD is applied to H-Mode [7]

• LH waves do not penetrate much past the LCFS• Confinement improvement seen with mitigation of

edge turbulence [J. Terry Talk CO4.00013]• Lyα brightness in divertor increases substantially• Lyα drops on field above midplane• Similar Results are found in Thompson back-

ground, which measures continuum infrared light(1060nm)

Lyα emission changes globally

0 5 10 15 20 25Channel Number (Lower to Upper Divertor)

0.00

0.05

0.10

0.15

0.20

Brig

htne

ss [a

.u]

Lyman Alpha Brightness vs. LH

Before LH (t=.88s)During LH (t=1.05s)Before LH (t=.88s)During LH (t=1.05s)

1120

7100

20

• Poloidally viewing Lyαsystem viewing magneticfield lines connected tothe LH launcher showsimilar enhancement ofemission for connectedversus unconnected field-lines.

Lyα power correlation to ne

0.90 0.95 1.00 1.05 1.10 1.15line average density [m^-3] 1e20

0

5

10

15

20

25

Change in L

ym

an a

lpha g

lobal pow

er

[kW

]

density vs change in Lyman alpha

• Data taken from end ofLHCD pulses measures arelative change in emit-ted Lyα power. Totalchange of emitted Lyαpower is ≈ 50kW. It fol-lows a threshold behavioras seen in the total HXRemission.

Future Work and Conclusions• Reduced sensitivity of HXR camera affects analysis

of the measured profile shape or Tph, other quantifi-able parameters must be found for representing fastelectrons.

• Calculations of the total lyman alpha power loss be-gins the quantification of SOL dynamics as they re-late to the LHCD density limit.

• Correlation to density displays threshold effect,through at a low emitted total power.

• Future correlations to other parameters key for de-termining this power loss mechanism.

AcknowledgementsThis work is supported by the US DOE awards DE-FC02-99ER54512and DEAC02-76CH03073. Thanks go out to the entire PSFC and theAlcator C-Mod team for their assistance on this work.

References[1] G. M. Wallace, et. al., Physics of Plasmas 17, 2508 (2010).[2] J. Liptac, et. al. Review of Scientific Instruments 77, 3504 (2006).[3] J. Wilson, et. al. Nuclear Fusion 49, 5015 (2009).[4] R. L. Boivin, et. al. Review of Scientific Instruments 72, 961 (2001).[5] V. Fuchs, et. al. Physics of Fluids 17, 3619 (1985).[6] S. J. Zweben, et. al. Review of Scientific Instruments 72, 1981 (2000).[7] J.W. Hughes, et. al. Nuclear Fusion 50 064001 (2010)[8] A.E. Schmidt, (2010) Doctoral Dissertation, MIT