Lower Hybrid Current Drive Experiments in Alcator C-Mod

Ron ParkerMIT PSFC

Principal Collaborators: Paul Bonoli, John Liptac, Andréa Schmidt, Greg Wallace, Randy Wilson1

Special thanks to: Stefano Bernabei1, Joan Decker2, Bob Harvey3, Amanda Hubbard, Joel Hosea1, Jim Irby, Earl Marmar, Alex Parisot, Yves Peysson2, Miklos Porkolab

Neville Greenough1, Dave Johnson, Atma Kanojia, Patrick MacGibbon, George MacKay, Dave Terry

Alcator C-Mod Physics and Engineering Groups

1Princeton Plasma Physics Laboratory2CEA, Cadarache3CompX, Del Mar, CA

Outline

1. Motivation

2. Lower Hybrid System Description

3. Coupling Measurements

4. Current Drive Results

5. GENRAY/CQL3D Comparisons

6. TRANSP Model of Current Diffusion

7. Summary and Conclusions

Outline - Highlights

1. Motivation

2.2.2. Lower Hybrid System DescriptionLower Hybrid System DescriptionLower Hybrid System Description

3. Coupling Measurements

4. Current Drive Results

5. GENRAY/CQL3D Comparisons

6. TRANSP Model of Current DiffusionTRANSP Model of Current DiffusionTRANSP Model of Current Diffusion

7. Summary and Conclusions

Goal is to use LHCD to supplement boot-strap current and produce high perform-ance steady-state regimes

~ 1 MW net coupled, Γ2 ~ 15%, vacuumgap needed for agreement with model.

Nearly 1 MA driven, n19IR/P ~ 3,sawtooth stabilization, central heating

Good agreement with total current, hardX-Ray and cyclotron emission syntheticdiagnostics. Experimental profiles narrower.

Experimental results in-line with require-ments for high performance SS operation.

Modeling of Lower Hybrid Current Drive in Alcator C-Mod1 Indicates Fully Steady-state High Performance Regimes are Accessible

Ip = 0.98 MA, IBS = 0.7 MA, ILH = 0.28 MAβn = 2.9, HITER-89 = 2.5 (assumed)

PLH = 2.4 MW, ηLHCD = 0.1, n|| = 2.75 PICRF = 4 MW

1P. T. Bonoli, M. Porkolab, J. Ramos, et. al., PPCF 39,(1997)223; P.T. Bonoli, R.R.Parker, M. Porkolab, et.al., Nucl Fus 40,(2000)1251

A 96-Waveguide Grill (4X24) has been Mounted in C-Mod to Implement LHCD Experiments

frontcoupler

vacuumwindow H-taper

3 dB splitter

E-taper

shortor

dump

loadsdiagnostic

probes

diagnosticprobes

C-Mod port flange

C

G

D

The 96 Waveguide Grill is Protected by Moly Limiters and Has ProbesImbedded in it for Measurement of Plasma Parameters at the Grill

Probes

Ti grill as installed in January 2005

This grill quickly deteriorated due torapid formation of titanium hydride.

Formation of TiH could be problematic for ITER.

Stainless steel grill installed in January 2006 as it looked at the end of Alcator C-Mod campaign in July.

Limiters

The LH System is Powered by 12 Klystrons Originally Used in Alcator C

The RF power is furnished by 12 well-traveled Klystronsoperating at 4.6 GHz and up to 250 kW for 5 sec.

Conventional microwave components split the RF power from each klystron 4-ways before entering coupler

The klystrons grouped in banks of 4 in C-Mod Cell

Splitter network used to divide klystronpower

Vector Modulators and I/Q Detectors are Used to control the RF Phases and Amplitudes in Each Pair of Waveguide Columns

Spectra derived from measured amplitudes and phases at the junction between forward and rear waveguide assemblies

Amplitude and Phase of each pair of waveguidecolumns is electronically controlled with responsetime of ~ 1 ms.

Phase between adjacent columns is set manually.

Dynamic Phase Control Allows Measurement of Reflection Coefficient in Single Shot

Reflection coefficients havebeen compared to Brambillacoupling code. (Next Slide)

Work is in progress to compare with SWAN code and TOPICAFor LH

0.65 0.7 0.75 0.8 0.85 0.90

50

100

150

60o 90o 120o

150o

180o

90o

0.65 0.7 0.75 0.8 0.85 0.90

1

2

3

0.65 0.7 0.75 0.8 0.85 0.90

1

2

3

4

0.65 0.7 0.75 0.8 0.85 0.90

1

2

3

60 80 100 120 140 160 1800.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

Dtop

Dmid

Dbot

Time (sec)

Time (sec)

Time (sec)

Time (sec)

Co

up

led

Pw

r (kW

)N

e (1

020 m

-3)

Ne

(102

0 m-3

)N

e (1

020 m

-3) Phase (Degrees)

Frac

tio

n P

ow

er R

efle

cted

Reflection Coefficient vs. PhaseLH Power and Probe Densites

Btop

Bmid

Bbot

Comparison of Measured Reflection Coefficient With Brambilla Coupling Code

G. Wallace QP1.00049

0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 1012

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Probe Density cm−3

Fra

ctio

n P

ower

Ref

lect

ed0.08cm Vacuum Gap dn/dx=6*1012cm−4

0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 1012

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Probe Density cm−3 or (dn/dx)*.15 cm−4

Fra

ctio

n P

ower

Ref

lect

ed

Variable Density Gradient with No Vacuum Gap

0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3

3.5x 10

12

Den

sity

cm

−3

Distance from Grill cm

Density Profile No Gap

Cutoff Density

Probe Density

0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3

3.5x 10

12

Den

sity

cm

−3

Distance from Grill cm

Density Profile with Gap

Cutoff Density

Probe Density

60o

90o

120o

60o

90o

120o

Ip (MA)

Vloop (V)

ne

(1019 m-3)

Te0 (keV)

PLH (kW)

Vloop (V)

PLH (kW)

Time (sec) Time (sec)

Shot 106061521 Shots 1060615: 5,7,13,21

Increasing Lower Hybrid Power Decreases the Voltage Required to Maintain Constant Current

Change in Loop Voltage (Normalized to Ohmic Value) vs Current Drive Parameteris Nearly Linear Over Range of Densities, Currents and Power

3.5 < ne < 7x1019 m-3

0.5 < Ip < 1 MA

120 < PLH < 830 kW

0

0.2

0.4

0.6

0.8

1

1.2

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

700kA500kA

Plh/neIpR0(Wm2/Ax1019)

ΔV/V

1 MA

Plh/n19IpR0 (Wm2/A)

The Current Drive Efficiency η = n19I(A)R(m)/P(W) ≈ 3

Ip = IOhmic + Ilh + Ihot

IOhmic = V/ROhmic

ILH = η0Plh/ne19R0

Ihot = V/Rhot

Fit: y = (η0 + η1)x/(1 + η1x)

η0 = n19IR/P =3.1 ± 0.1

η1 = 0.25 ± 0.25

=ROhmicneIpR0/RhotPlh

1Giruzzi, G.,et.al.,NuclearFusion, 37 (1997) 673

R. Wilson, J01.00002

Using method of Giruzzi1:

Note: ne = density @ Te = 2.2 keV

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.1 0.2 0.3

Δ V

/ V

PLH / ne Ip Ro (w m2 / 1019 A)

Loop voltage reduction versus normalized PLH

fully non-inductive

Ip > 700 kA

ne = 3.5 - 7 ×1019 m-3

120 < PLH< 830 kW

n|| = 1.6

Plh/n19IpR0 (Wm2/A)

Major Radius (cm)

n e (1

020

m-3

)

During LH pulse (t=1.19s)During LH pulse (t=1.21s)

Before LH pulse (t=0.58s)

TCI nebarThomson

During LH pulse (t=1.19s)Before LH pulse (t=0.58s)

Major Radius (cm)

T e (e

V)

Example of Discharge in Which Sawteeth Were Stabilized and Te0 Increased

Shot 106072029

Phase n||

60º 1.6

90º 2.3

120º 3.1

The Current Drive Efficiency Improves with Decreasing n||

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.05 0.1 0.15 0.2 0.25

60 degree90 degree120 degree

Plh/n19IpR0 (Wm2/A)

ΔV

/V

0.6 0.8 1.0 1.2 1.40.0

0.2

0.4

0.6

0.8

1.0

1.2

60º90º

Loop

Vol

tage

(V)

Time (sec)

ne = 5e19 m-3

Ip = 720 kWPLH = 480 kW

LHCD

Two Shots Have Been Modeled with GENRAY and CQL3D

Time (sec) Time (sec)

Ip (MA)

Vloop (V)

ne (1019 m-3)

Teo (keV)

PLH (kW)

1060728011 1060728014

GENRAY and CQL3D Are Used to Model the LHCD Experimental Results

Poloidal and Toroidal Projection of Rays Launched by Antenna (GENRAY) in Ohmic plasma:

rfr

rrp

pfeEppfC

pfpD

ptf e

Fse

ee

rfe

∂∂χ

∂∂δ

∂∂

∂∂

∂∂

∂∂ 1)(+),,()( //

//////

////

//

+Γ++= ⊥

Self-consistent damping of the rays is determined by Fokker-Planck equation containing quasi-linear RF term:

20 40 60 80 100-40-30-20-10

010

20

30

R(cm)

Z(cm)

-100 -50 0 50 100

-50

050

X(cm)Y(cm)

QL RF term FP term E-Field term

Electron Spatialdiffusion

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

r/a

Pow

er D

ensi

ty (

W/c

m3 )

CQL3D Power Depostion Profiles

10607280111060728014

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1−500

0

500

1000

1500

2000

2500

3000

r/a

Cur

rent

Den

sity

(A

/cm

2 )

CQL3D Modeled Current Profiles

10607280111060728014

Experiment Model

Shot/Time Vloop IP(MA) Vloop IP(MA)

106072811t = 1100 ms

0.2 0.53 0.2 0.543

106072814t = 1000 ms

~ 0 1.03 0.1 0.90

Good Agreement Between Measured and Modeled Current and Voltage

A. SchmidtQP1.00050

The Modeled Distribution Function @ r/a =0.6 for 1 MA Shot Has a Plateau Extending to γv||/c ~ 1.2

−1 −0.5 0 0.5 1 1.5 20

0.2

0.4

0.6

0.8

1

Contour plot of distribution function

γ v||/c

γ v pe

rp/c

0 1 2 3 4 5 6 710

9

1010

1011

1012

1013

1014

1015

1016

1017

1018

1019

γ v/c

f

Cuts of f(v) at Constant Pitch Angle

θ=0θ=π/4θ=π/2θ=3π/4θ=π

Model includes residual E-field,however its effect on plateau is small.

γv||/c ~ 1.2 corresponds to n|| =1.3,near minimum n|| of launched spectrum.

An Imaging X-Ray Spectrometer Records Fast Electron Bremsstrahlung Spectrum

D=123cm d=40cm

Spatial resolution:Estimation: Δr ~ 2a / #chords=1.4cm

Geometry: Δr = (1 + D/d)ac ~ 1.7cm

ac=5mmad=5mm

C-Mod Cross section with pinhole camera

B-port

Photon pulses from all 32 channels are stored for post processing

5 10 15 20 25 300

2

4

6

8

10

12

14

16

x 104 Modeled and Measured HXR Spectra

Channel Number

Cou

nt R

ate

(s−

1 keV

−1 )

24 keV (cql3d)34 keV (cql3d)44 keV (cql3d)24 keV (experiment)34 keV (experiment)44 keV (experiment)

X-Ray Profiles and ECE Spectrum Compare Favorably with CQL3D/GENRAY Model

CQL3D/GENRAY profiles are narrow, consistent with narrow current deposition.

X-Rays suggest a broader current profile –spatial diffusion?

A. SchmidtQP1.00050

100 200 300 400 500 600 7000

5

10

15

20

Predicted and Measured ECE Spectra

frequency (GHz)

equi

vale

nt r

adia

tion

tem

pera

ture

(ke

V)

nonthermalfeature

2nd harmonic, R=R0

Measured ECE SpectrumCQL3D ECE Spectrum/5

What is Significance of Negative Loop Voltage in 1 MA Shot?

Time (sec) Time (sec)

Ip (MA)

Vloop (V)

ne (1019 m-3)

Teo (keV)

PLH (kW)

1060728011 1060728014

TRANSP Modeling of Shot 1060728014 Indicates Negative VloopResults From Overdrive In Outer Region of Discharge

RF current overdrive causesa negative E-Field whichdiffuses rapidly to edge,resulting in transient surfacevoltage.

The time for steady-state to develop depends on relativealignment of inductive and RF current profiles.

A. Parisot QP1.00051

0.0 0.2 0.4 0.6 0.8 1.005

101520253035

Cur

rent

den

sity

[MA

m-2]

0.0

0.5

1.0

1.5

2.0

2.5

Loop

vol

tage

[V]

0.4 0.6 0.8 1.0 1.2 1.40

200

400

600

800

LH p

ower

[kW

]

Time [s]

1 MA ohmicbefore LH

Test LH driven current profiles

750 kA

1 MA

Experiment

TRANSP simulation

r/a

Summary and Conclusions (1)

Lower Hybrid Current Drive has been implemented on Alcator C-Mod:

Goal is to use LHCD as a tool to supplement bootstrap current and enable performance optimization studies leading to attractive steady-state regimes.

Results should also inform LHCD decision for ITER

So far ~ 1 MW has been coupled and current drive approaching 1 MA at n ~ 5e19 m-3

has been achieved.

First results indicate current drive efficiency is in line with expectations based onstate-of-the–art models and sufficient to achieve target regimes, although highern|| (lower efficiency) for accessibility and additional power will be required at higher density.

Good general agreement between experiment and both X-Ray and ECE CQL3D-GENRAY synthetic diagnostics. There are indications that j(r) is broader than code predicts – supported by TRANSP modeling.

Summary and Conclusions (2)

In next Alcator C-Mod campaign, emphasis will be on raising absorbed power toward 1.5 MW, increasing density above ne = 1e20 m-3 and exploring interactions with ICRH antenna.

Meanwhile, a second phase incorporating a second launcher and upgrading the source power to 4 MW is underway – operation scheduled for 2009.

With the second phase, sufficient source power will be available for development oftarget AT scenarios.