Modification of Current Profile, Toroidal Rotation and ... · Motivation ¾Use LHCD as actuator to broaden current profile for access to low shear, high performance, near-steady-state

Modification of Current Profile, Toroidal Rotation and Pedestal by Lower Hybrid Waves in Alcator C-Mod

R. Parker, P.T. Bonoli, O. Meneghini, M. Porkolab, A.E. Schmidt, S. Shiraiwa, G. Wallace, J. R. Wilson, A.E.

Hubbard, J.W. Hughes, J-S. Ko, R.M. McDermott, M.L. Reinke, J.E. Rice, S. Scott

18th Topical Conference on RF Power in Plasmas

Gent, Belgium

June 24-27 2009

Lower Hybrid waves are injected into Alcator C-Mod plasmas at 4.6 GHz via an 88-waveguide grill

Limiters

Probes used toMeasure edge density

4X22 Waveguide grill

5.5X60 mm2 waveguides, stainless steel

1.5 < n3.5 >׀׀, continuously and dynamically variable

12 klystrons, 250 kW each, CW

Maximum coupled power = 1.2 MW

Motivation

Use LHCD as actuator to broaden current profile for access to low shear, high performance, near-steady-state regimes relevant to ITER.

Similar field – BT = 5.4

Similar shape – Diverted

Similar density – n = 5x1019 – 2x1020 m-3

Similar frequency – f = 4.6 GHz

Develop understanding of LH physics for confident extrapolation to ITER

Outline

Modification of current profile by LHCD

Observation of LHCD-induced counter-rotation, comparison with injected wave momentum

Pedestal, rotation modification in H-Mode

Development of new FEM simulation tool

New coupler design and future plans

Summary

Outline

Modification of current profile by LHCD

Observation of LHCDObservation of LHCDObservation of LHCD---induced counterinduced counterinduced counter---rotation, rotation, rotation, comparison with injected wave momentumcomparison with injected wave momentumcomparison with injected wave momentum

Pedestal, rotation modification in HPedestal, rotation modification in HPedestal, rotation modification in H---ModeModeMode

Development of new FEM simulation toolDevelopment of new FEM simulation toolDevelopment of new FEM simulation tool

New coupler design and future plansNew coupler design and future plansNew coupler design and future plans

SummarySummarySummary

Time (s)

MSE signals show clear response to LHCD pulse

MSE traces at various major radii for discharge with MHD. Onset of tearing mode activity modifies current profile.

Time (s)

Ip (MA)

Vloop(V)

Ne_barX1019m-3

Te0(keV)

PLH(kW)

Trad,R=0.86(keV)

MHD

n׀׀ = 1.56

R=0.85

R=0.83

R=0.82

R=0.80

R=0.78

R=0.76

R=0.74

R=0.69

Arc

tan(

Bv/B

φ) (

degr

ees)

MH

D

׀ ׀ ׀

MSE-constrained EFIT reconstructions show broadened jφ profiles

For EFIT reconstruction P’ and FF’ are modeled by 2- and 3- term polynomials, respectively, and poloidal field on mid-plane is constrained by MSE measurements. Any structure in jφ(R) is smoothed by this approach.

Behavior of central q and internalInductance (from EFIT) are consis-tent with broadening of jφ profile.

Time(s)l i

Time(s)J φ(M

A/m

2 )

20

10

R(m)

Ohmic

nll=1.95

nll=1.56

• Analytic formula developed by Petty et al. (PPCF 47 (2005) 1077) is also used to directly derive Jφprofiles

• Analysis is limited to two n|| cases that have multiple shots for the same n|| for better statistics (d(ΔBV)/dR is involved).

• For n|| = 1.56 there appears to be more structure than in EFIT reconstructions.

• For n|| = 1.95, no noticeable structure. Good agreement with the EFIT Jφ profile.

1080320010, 1080320011, 1080320012, 10803200131080320017, 1080320019, 1080320029

Petty et al.

1.56 (60°)1.95 (75°)

n|| (Phase)

-0.2

-0.1

0.0

0.1

dBv

(T)

0

5

10

15

20

Jphi

(MA

/m2)

@1.025 sec

0.0 0.2 0.4 0.6 0.8 1.0r/a

Direct reconstruction from MSE data yields current profiles with some structure—but is it real?

0.6 0.8 1.0 1.2 1.4time (sec)

0.30

0.35

0.40

0.45

0.50

0.55

Ip (M

A)

n|| = 1.95

1080320017 (19,29)

LH ON

MSE + EFIT

MSE + Petty et al.

1080320013 (10,11,12)

0.6 0.8 1.0 1.2 1.4time (sec)

0.30

0.35

0.40

0.45

0.50

0.55

Ip (M

A)

LH ON

Off axis

central

n|| = 1.56

MH

D activity

Both EFIT and direct reconstructions show broadening of Jφ profile

LHCD causes a redistribution of current from “center” (r/a < 0.44)to “off-axis” (r/a > 0.44) – strongest for lowest n||

• Reference Ohmic phase taken from 0.6 to 0.7 sec.

0.20.61.0 P_LH (MW)

0.0 0.5 1.0 1.5 2.0 sec

1.56 (60°)1.95 (75°)2.33 (90°)2.72 (105°)

n|| (Phase)

-0.2

-0.1

0.0

0.1

dBv

(T)

0

5

10

15

20

Jphi

(MA

/m2)

@1.025 sec

Raw MSE signals & reconstructed profiles show inverse nll dependence of efficiency

01

2

3

4

56

q

0.0 0.2 0.4 0.6 0.8 1.0r/a

0.0 0.2 0.4 0.6 0.8 1.0r/a

• Small spatially uniform diffusion coef-ficient with a linear velocity dependence is used .

n|| = 1.95

0

5

10

15

Jphi

(MA

/m2)

0.0 0.2 0.4 0.6 0.8 1.0r/a

MSE + EFIT + ECEMSE + Petty et al.

CQL3D with diffusionCQL3D without diffusion

1080320017 (19,29) @ 1.025 sec

Simulations using GENRAY/CQL3D show qualitative agreement with experiment but with discrepancies

Plans to resolve discrepancies:

Higher resolution j(r) measure-ments with MSE upgradeand polarimetry (new)

Direct detection of wave fields with reflectometer, possibly PCI detection of LH wave density

Incorporate pinch and diffusion from dynamic X-ray measurementsinto CQL3D

Improve simulations using full wave plus FP codes

Fokker Planck modeling using GENRAY/CQL3D

Outline

Modification of current profile by LHCDModification of current profile by LHCDModification of current profile by LHCD

Observation of LHCD-induced counter-rotation, comparison with injected wave momentum





LHCD generates counter rotation and negative radial electric field – cause or effect?

The rotation generally developson the resistive time scale and is largely confined to the core.

A. Ince-Cushman, PRL 102, 035002 (2009)J. Rice, Nucl. Fusion 49, 025004(2009)

Some profile peaking is also observed.

Rate of injected wave and particle momentum is sufficient to explain plasma momentum buildup

Rate of toroidal momentum buildup in plasma:

NxVolvnmP i3102.4 −=⋅= ϕϕ &&

Nx

Powerc

nvWdA

cn

skAdP g

3107.6

ˆˆ1

−=

=∫ •=•∫ •= ϕϕϕ ϕϕ

ωrrrr

&

Rate of momentum injected by wave1:

Rate at which fast current carrying electrons lose momentum:

Nxe

IRmnvdAmRP

SD

e

SD

faste 3107.52)(2 −==∫

=τ

πτ

πϕ&

1A. Bers in Plasma Physics – Les Houches 1972, G. DeWitt and J. Peyraud, eds, Gordon and Breach(1975)

Wave momentum transferred to trapped electrons also results in inward pinch

dtdP

Benv

dtdp

eTRdtRE

rr

eREemRdtd

ptr

ϕ

ϕϕ

ϕ

ψδψ

ψφ

1

2

≈

≈∫−=∂∂

≈Δ

=−

'

)( &

Numerically, m/s (inward) 7.0≈rv

LH pulse is square-wave modulated andBremsstrahlung is sorted into time andand energy bins

PLH

(t)

t

25 ms

5 10 15 20 25 300

0.5

1

1.5

2

2.5

3

3.5

4x 10

4

Viewing Chord

Cou

nt ra

te fo

r 40

42.5

keV

(s

1)

Integrated Chordal HXR Measurements

40 50 60 70 80 1.8

1.6

1.4

1.2

1v (m/s)

HXR Energy (keV)

)-

-

-

-

-

By analyzing buildup and decaywith a diffusion-pinch model, a pinch velocity is obtained:

Analysis of bremsstrahlung emission yields experimental evidence for inward pinch of fast electrons

Andréa Schmidt B 57Scan across plasma vertical cross-section

Outline



Pedestal, rotation modification in H-Mode




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

Cou

nt R

ate

(Ch

9−24

, 40−

200

keV

) [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

Greg Wallace B 60

At high densities, evidence for fast electrons and currentdrive diminishes – an issue for C-Mod H-modes

Hard X-Ray emission falls rapidly above n ~ 1x1020 m-3

Weak PDI (decay to IC harmonics) routinely observed at all densities

Density “limit” is well below that expected for decay to two LH waves (ω ~ 2ωLH)

Possible explanation: Propagation, damping in SOL

0.4 0.6 0.8 1 1.2 1.4 1.6

x 1020

103

104

105

106

107

108

ne [m−3]

Cou

nt R

ate

(Ch

9−24

, 40−

200

keV

) [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

C-m

od H

-mod

es

Large Circles:GENRAY-CQL3D

T e (k

eV)

pe

(kPa

)

R - RLCFS (mm)

ne

(102

0 m-3

)

Edge pedestal: LH off / LH on

ONLH

LH ON

However in high density H-modes, LH Waves modify pedestal and rotation

Pedestal density decreases, profile broadens

Pedestal temperature increases, leading to increased temperature and stored energy

01

2

PICRF (MW)PLH (MW)

1.52.02.5

1.0 1.1 1.2 1.3 1.4 1.5time (s)

0.51.01.5

Prad (MW)

123

Te,0 (keV)

ne (1020

m-3)

0.20.4

0.6Te, r/a~0.85

1.0 1.1 1.2 1.3 1.4 1.5304560 WMHD (kJ)

1.0 1.1 1.2 1.3 1.4 1.5

0.500.751.00

Vsurf (V)

(a)

(b)

(c)

(d)

(e)

(f)

(g)

In this H-Mode, LH induces substantial change in rotation-additive to ICRF induced rotation

0.8 1.0 1.2

0.6 1.2

-20

0

-40ΔV

φ (k

m/s

)

0.6

-20

0

20

ΔV

φ (k

m/s

)

0.6

Pedestal toroidal rotation change(B+4)

Pedestal & near SOL density

Line-integrated density

PLH

Central toroidal rotation change (Ar+16)

Pedestal rotation change occursbefore change in core rotation

LH also increases SOL density, decreases density in pedestal and core

SOL density profile broadens

In H-mode discharge LHCD modifies pedestal density and edge rotation

These H-modes will be a focus forLHCD studies in the next campaign!

Outline




Development of new FEM simulation tool



A new, finite element full-wave code for LH waves has been developed (1)

Uses commercial FEM software package COMSOL Multiphysics to iteratively solve for fields in spatial domain:

Syun’ichi Shiraiwa B 58

0)()())(( 20 =⋅−×∇×∇ rErkrE rrrrrε=

)'()'('2ˆˆ

)(Im)(

)'()'('2ˆˆ

11

1

zEzzdzzzE

i

kFTz

zEzzdzzzE

i

NzLN

zcold

N

zzzL

zLz

cold

−−

−

−−

+=

=

−−

+=

∫

∫

επ

εε

εε

επ

εε

(z is coordinate along field line)

=

= =

=

Nth iteration:

The code agrees with ray-tracing in the case of strong single pass damping

An attractive feature is that it seamlessly treats fields from source to dissipative sink in plasma,including propagation through waveguides, grill, SOL and pedestal

Adaptable to quasi-linear Fokker-Planck treatment of .)(vf r

A new, finite element full-wave code for LH waves has been developed (2)

Alcator C-Mod plasma: n0 = 5x1019m-3,Te0 = 2.5 keV, n׀׀ = 2.5. An 800 kA, BT = 5.4 T EFIT equilibrium was used.

MaxwellianzE 1-D Fokker-PlanckzE

Field profiles for Maxwellian and 1-D Fokker-Planck f(v) show significant differences





New coupler design and future plans


Outline

A new coupler designed for more efficient coupling and higher power handling capacity is being fabricated

Design is based on 4-way, H-planesplitter – load tolerant

Fed by standard C-Band waveguides,matched with step transformer

Design was carried out with integrated simulation based on CST MicrowaveStudio and coupling code TOPLHA

Orso Meneghini A 57

Reflection coefficient at waveguide

Reflectometer WG’

Future Plans

Hardware:

Install new coupler this summer

Expect to increase maximum coupled power to ~ 1.5 MW

Construct additional coupler and install in 2011

Add 7-8 more klystrons to bringtotal source power to 4 MW, ~ 3 MW to plasma in 2011

Main research topics:

Operation at high density, H-modedensity limit

Physics of rotation

Further development of full wavecodes1,2 and experimental validation

1Syun’ichi Shiraiwa B 582John Wright I 16






Summary

Outline

Lower hybrid current drive has been shown to broaden the currentprofile in Alcator C-Mod plasmas – useful tool to explore AT operation

Spatial deposition of jφ measured with MSE, in qualitative agreement with ray tracing, Fokker-Planck simulations, but detailed questions remain

Strong counter current rotation accompanies LH wave injection – wave momentum input sufficient, but details need further exploration

Current drive efficiency falls off rapidly with density, well below the 2ωLH limit – why?

A new full wave FEM simulation has been developed:

Seamless treatment from source to sink

Good agreement with ray-tracing in single pass regime, discrepancies in multi-pass case

Summary

Modification of Current Profile, Toroidal Rotation and ... · Motivation ¾Use LHCD as actuator to broaden current profile for access to low shear, high performance, near-steady-state

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