Alcator C-Mod ICRF Research Program

Peer Advisory Committee MeetingMIT Plasma Science and Fusion Center

February 6-7, 2008

Overall Themes1. Demonstrate ICRF heating and current drive actuators to optimize

high performance H-mode and AT plasmas.

2. Contribute to the development and validation of advanced simulation tools scalable to ITER and reactors.

Outline:1. Context of C-Mod ICRF program2. Overview of ICRF system and capabilities3. Proposed research

Motivation for ICRF Program

ICRF heating has been demonstrated to be effective.

• Achieved record stored energy and high performance.

• Only heating method that heats ions directly.

Primary challenge is antenna performance.

• Sources are 2 MW steady state and most efficient of all auxiliary heating power sources.

• Core wave physics can be complicated but

» Sophisticated simulation codes exist

» And extensive experimental validation of codes is underway.

2

4 PRF (MW)

0.1

0.2

0.3WMHD (MJ)

2

3

4

BT=5.4 T, IP=1 MA 1050426022

2

4

1

2 Rneut (x1014 s-1)

0.6 0.8 1 1.2 1.4

1

2

3PRad (MW)

6

Te0 (keV)

ne (x1020 m-3)

Time (s)

Context of the C-Mod ICRF Program

Primary goal of ICRF physics program is to:• provide first principle understanding of ICRF physics including antenna

coupling and wave absorption

• such that it is a reliable heating and/or current drive actuator that can be utilized to optimize overall plasma performance, in C-Mod and future reactors, with minimum negative impact on plasma.

ICRF provides bulk auxiliary heating power in C-Mod.• Fundamental minority, mode conversion, and second harmonic minority

ion cyclotron scenarios are extensively investigated and plan to investigate Fast Wave.

Emphasize validating physics and computational models thru comparison of experiments and simulations.

• Access to wide range of RF absorption scenarios and a flexible RF systems, diagnostics, and advanced simulation codes.

We investigate and develop solutions to technological and physics issues associated with the antenna/coupler and operations to enable successful RF operation.

Context of C-Mod’s ICRF Program

Standard scenarios are fundamental and second harmonic with access to direct fast wave.

ITER will utilize fundamental 3He and second harmonic tritium.

High and low single pass absorption scenarios are accessible

ITER expected single pass absorption is similar to H minority case in C-Mod

Antenna power density is ~10 MW/m2.

ITER power density is expected to be 6-8 MW/m2.

Utilize ICRF with high Z first wall materials.

ITER and future burning plasma devices are likely to utilize high Z first wall materials

Proposed research is well aligned with FESAC planning committee recommendations.

• Make contributions to model validation (Gap 1), • develop and test ICRF antennas (Gap 7), and • ICRF compatibility with first wall materials(Gap 9).

Research Themes

RF-plasma edge interactions:• Includes antenna coupling; issues of dynamic loading; voltage and power

limitations; and impurity production.• Plan to emphasize ICRF related impurity production – develop

understanding of underlying physics and means to mitigate or reduce impurity production.

• Operational emphasis is on real time matching to maximize coupled power and minimize discharges lost to poor match.

Wave propagation and absorption:• Emphasize validating simulation codes, scalable to ITER and reactors, to

provide confidence in simulation codes used for discharge analysis• Validate simulation codes against minority, 2nd harmonic, direct fast wave,

and mode conversion absorption regimes.Current drive:

• Maximize driven current and develop driven current profile control for control on plasma current profile and affecting stability of MHD modes.

Present ICRF System

A

B

C

D

E

K

J

G

F

H

GH FullLimiter

D & Eantennas

J antenna

AB SplitLimiter

Ip

K midplane

Limiter

LH

Coupler

D & E Antenna

J antenna

Frequency 80 MHz 40-80 MHz

Power 2 x 2 MW 4 MW

Antenna 2 x 2 Strap 4 Strap

Phase fixed variable

ICRF System FY09-FY10

New 4-strap antenna installed in J port.

D antenna removed for diagnostics displaced from B port.

Available source power reduced to 6 MW from 8 MW.

Real time matching (double stub FFT system ) on E antenna only.

Frequency 80 MHz 40-80 MHz

Power 2 MW 4 MW

Antenna 2 Strap 4 Strap

Phase fixed variable

A

B

C

D

E

K

J

G

F

H

GH FullLimiter

D & Eantennas

J antenna

AB SplitLimiter

Ip

K midplane

Limiter

LH

Coupler

ICRF System FY11-13

New 4-strap antenna installed at E port.

Available source power is 8 MW up from 6 MW.

Variable phasing available for both antennas.

Real time matching available in FY12 and FY13.

Simplify views

A

B

C

D

E

K

J

G

F

H

GH FullLimiter

J antenna

AB SplitLimiter

Ip

K midplane

Limiter

LH

Couplers

E antenna

Frequency 80 MHz 40-80 MHz

Power 4 MW 4 MW

Antenna 4 Strap 4 Strap

Phase variable variable

ICRF Simulation Tools

Codes:• TOPICA: 3-D modeling of ICRF antenna code

with full wave plasma model (TORIC) in collaboration with Polytechnico di Torino and RF Sci-DAC (CSWPI).

• TORIC for wave propagation, power deposition, and current drive calculations.

» Coupled with Fokker Planck codes, DKE, and CQL3D.

• Access to finite banana width Monte Carlo code with self consistent RF wave fields through RF Sci-DAC.

Synthetic Diagnostics:• Synthetic phase contrast imaging diagnostic to

model measured density fluctuations in TORIC.• Synthetic charge exchange neutral particle

analyzer (50-350 keV) implemented in CQL3D.• Plan to implement synthetic charge exchange

recombination spectroscopy for fast ions in CQL3D.

-2

-1

0

1

2

MC layer

0.64 0.66 0.68 0.70 0.72 0.74 0.76R (m)

0

1

2

ExperimentalSynthetic

Re(∫ ne dl)~

|∫ ne dl|~

[a.u.]

[a.u.]

10-3

1

IBW FW

LCFS

ICW

ICW

MC

-1

Diagnostics

Present status:• 32 channel PCI diagnostic for

density fluctuations associated with RF waves.

• 4-channel CNPA (compact neutral particle analyzer)

Plans:• Upgrade CNPA to 8 channels.• Implement charge exchange

recombination spectroscopy for fast ions (UTexas).

• SOL reflectometers in new 4-strap antennas (ORNL).

• Edge probes at RF limiters and plasma limiters: emissive probes, RF magnetic probes and retarding energy analyzer.

A

B

C

D

K

J

G

F

H

GH FullLimiter

J antenna

AB SplitLimiter

Ip

K midplane

Limiter

LH

Couplers

E antennaEmissive, RF, andretarding energy analyzer probes

CNPACXRS-FIPCI

SOL Reflectometer

Antenna-Edge: Boronization Erosion and Impurity Production

Goals:• Identify location of RF related impurity

sources.• Develop understanding of underlying physics.• Develop antenna designs to minimize RF

sheaths.• Assess mitigation techniques.

Status:• Enhanced erosion/impurity production is

localized to the active antenna.• Characterized ICRF sheath dependence on

ICRF power and confinement.Plans:

• Implement RF and emissive probes and retarding energy analyzers to characterize the plasma potential.

• Measure SOL density profile to monitor RF impact on density profile and provide detailed input to simulation codes

• Imaging to identify impurity sources and measure rates.• Characterize new antenna sheath characteristics for range of RF absorption

scenarios.• Develop analysis tools for investigation of sheath mitigation techniques through

collaboration with RF-Sci DAC CSWPI.• Assess what makes RF sources the dominant core Mo contributor.

H-mode

L-mode

H-mode with

BN tiles

1.51.00.50

150

100

50

Pla

sma

Po

tenti

al [

V]

RF Power [MW]

1000516014-20, 22, 1031120015

∼P1/2

ICRF Coupling and Active Matching

Goals:• Maintain maximum power delivered over wide

range of plasma conditions• Minimize discharges lost to poor match.

Status:• Analyzed load variation associated with ELMs

and H->L transitions and variation is near antenna fault level.

• Operation with prototype fast (~1 msec) ferrite tuners was successful (~1.5 MW).

Plans:• Measure local density profile and • Change figure to match comparison

compare with TOPICA as part of ongoing model validation.• Continue testing of prototype fast ferrite tuners and implement on other antennas.

» Measure present voltage limitations of the tuners.» Optimize tuner characteristics for frequency range utilized in experiments.

• Test methods for edge density and density profile control.» Local gas puffing is an obvious choice for affecting the far scrape-off density.» Edge pedestal modification through magnetic geometry and pumping.

Antenna-Edge: Antenna Power and Voltage Limits

Goal:• Obtain high power over wide range of plasma

conditions.Status:

• Achieved antenna power densities of ~10 MW/m2.

• Current ICRF pulse length are 0.35 sec at 6 MW and 1 sec at 5 MW.

• Discharge formation is responsible for neutral pressure limit.

Plans:• Assess interaction new ICRF antenna limits.• Continue benchmarking of TOPICA.• Investigate limitations on 6 MW discharges.• On test stand, we plan to investigate

» Different surface treatments, conditioning, materials» Initial RF trip at neutral pressure limit, and» Influence B-field has on high voltage breakdown.

2

4

6

0.05

0.1

2

4

0.6 0.8 1.2 1.4

0.6

0.8

1

1.2

1.4

PRF

[MW]

Te0 [keV]

nline [x1020 m-2]

Time [sec]

1031106026 BT

=5.2 T IP

=0.8 MA

WMHD [MJ]

1.0

Experimental Validation of ICRF Simulations

Goal:• Validate simulations against wide range

of experimental regimes.Status:

• PCI diagnostic calibration improved• TORIC field strengths have been verified

AORSA.• Developed 3-channel compact neutral

particle analyzer (CNPA).• Implemented synthetic CNPA diagnostic

in AORSA-CQL3D.Plans:

• In mode conversion regime, compare PCI data to validate simulation amplitudes.

• Upgrade CNPA to multi-channel and measure tail energy in fundamental and second harmonic H scenario to test upgraded simulation capability.

• Utilize target discharges with Te0>5 keV to investigate direct electron heating with fast wave.

» Measure power deposition profile with high resolution ECE.

» Validate TORIC simulations.

100 150 200 250 300

1012

1013

1014

1015

Ch

3 10

51

20

600

2

F(E

) (

A.U

.)

Energy(keV)

Charge Exchange Spectra

measured

av_sim

nφ = +10nφ = -10

Wave Propagation: D(3He) Minority Heating

Goal: Optimize heating efficiency in low single pass absorption scenario.Status:

• Confirmed previous greater sensitivity to 3He concentration than D(H).

• Found L-mode heating effectiveness and H-mode threshold were similar to D(H) heating.

• Conflicting results relating to erosion and impurity generation.

Plans:• Direct comparison of D(H) and D(3He) H-mode discharges.• Alternately move additional resonances in and out of the plasma and compare

performance.• Investigate high power, high performance at 8 T with D(3He) minority heating

under optimized conditions.• Compare impurity generation characteristics as function of single pass

absorption strength.

0.5 0.6 0.7 0.8 0.9

fc3HefcD

Rmajor [m]

f=50 MHz, BT=5 T, IP=1 MA

0.4 0.5 0.6 0.7 0.8-0.4

-0.2

0.0

0.2

0.4

fcH

Rmajor [m]

f=80 MHz, BT=5.4 T, IP=1 MA

fc11B

ICRF Current Drive

Goals:• Tailor current profile to

mitigate or control sawtoothperiod.

• Provide central seed current for optimizing current profile in AT plasmas.

• LH is primaryStatus:

• Observed sawtooth period control with mode conversion current drive (MCCD).

• Obtained target discharge (Te> 5 keV) to allow evaluation of fast wave (FWCD) and ion cyclotron current drive (ICCD).

• table

2

4 PRF (MW)

0.1

0.2

0.3WMHD (MJ)

2

3

4

BT=5.4 T, IP=1 MA 1050426022

2

4

1

2 Rneut (x1014 s-1)

0.6 0.8 1 1.2 1.4

1

2

3PRad (MW)

6

Te0 (keV)

ne (x1020 m-3)

Time (s)

ICRF Current Drive: Mode Conversion CD

Status:• Inefficient for bulk current drive in C-

Mod• Ehst-Karney parameterization

overestimates drive current due to incorrect wave polarization.

• Good candidate for sawtooth pacing in C-Mod

» Clear sawtooth variation with phase and deposition location.

» Heating phase ST behavior could be a result of inherent asymmetry due to mode conversion.

Plans:• Utilize MCCD to pace sawteeth in the

presence of energetic ions driven by ICRF (MDC-5).

1

2

0.7 0.75 0.8

3

3.5

0.7 0.75 0.8

PRF (MW)

D-port D+J-port D-port D+J-port

Time [sec] Time [sec]

ctr-CD co-CD

Te0 (keV)

ICRF Current Drive: FWCD

Status:• Identified suitable target discharges to begin initial deposition studies and

current drive experiments.

Plans:• Measure power deposition profile variation with antenna phasing and

target discharge temperature and density.• Compare counter and co- current drive phasing with full compliment of

diagnostics.• Benchmark TORIC simulation against measured driven current and power

deposition profiles.

ICRF CD: Ion Cyclotron Current Drive

Status:• Have capability to investigate two regimes predicted to have opposite effects

on sawtooth period.• Differentiate FO and Fisch

Plans:• In L-Mode low density discharges, access both FO and classical regime.• Evaluate ICCD in H-mode discharges in classical regime and assess efficacy.• Use one antenna in CD or CTR-CD phasing with other antenna in heating

phase to maintain plasma parameters as similar as possible.• Scan BT to sweep cyclotron resonance through q=1 surface on both low and

high field side.• Monitor changes in ST period and CNPA for tail energy.

Classical (Fisch-Karney) regime: Finite orbit (FO) regime:Ohkawa effect?

Current is carried by passing particles.

Peak efficiency near Ecritical (~15Te for H)For co-current drive phasing, is stabilizing ST for a cyclotron resonance at the q=1 on the low field side of magnetic axis.

Current is generated by trapped particle orbits.

Efficiency increases with particle energy.For co-current drive phasing, FO-ICCD is stabilizing for a cyclotron resonance at the q=1 on the high field side of magnetic axis.

ICRF and LHRF Interactions: Coupling

Goal:• Demonstrate compatibility between ICRF and LHRF to enable tokamak

performance optimization.Status:

• Coupled LH power into H-mode and L-mode ICRF heated discharges.• Observed fractional reflected power signal fluctuations and LH faults are

significantly increased with neighboring ICRF antenna operation.Plans:

• Examine interaction of ICRF modification of the SOL density and density profile.

• Integrate ICRF and LHRF phase and amplitude control into active feedback control for discharge optimization.

IC and LH Waves Interactions: IBW and LH

Goal:• Utilize ICRF to localize LH wave damping off-axis.

Background:• IBW waves have been suggested to provide a seed electron population

that results in damping of LH waves away from location predicted if electrons have a Maxwellian distribution.

• Mode conversion heating is at sufficient high power density that the distribution function is likely modified.

• Can MC heating modify the location at which LH waves are damped?Plans:

• Utilize MC heating off axis and LHCD at n|| =3.5 (120o phase). D+E on-axis minority heating.

• Measure hard X-ray profile to evaluate the effect on the LH current profile and PCI to monitor MC waves.

• TORIC/CQL3D modeling using self-consistent distribution functions.

IC and LH Waves Interactions: Fast ions and LH

Goal:• Evaluate fast ion absorption of LH waves.

Background:• LH source frequency in ITER (~ 5 GHz) is driven by the need to avoid

parasitic absoprtion of the LH wave on fast fusion α-particles• Can fast ions significantly absorb LH waves?

Plans:• Inject fixed LHRF power, scanning n|| from (1.5 – 3.5) on different

discharges until an interaction is observed with the minority tail:• Measure hard X-ray profile to evaluate the effect on the generation of fast

electrons and CNPA to assess impact on fast ion distribution.• Model process with GENRAY – CQL3D.• Experimental parameter scans include:• Scan ICRF power to vary the minority tail energy.• Change B to move the ICRF resonance position.• Vary the LH wave n|| to change the LH wave phase speed.

Summary

Recent Progress:• Identified erosion is associated with active antenna and primary source is

on the outer divertor shelf.• Analyzed ELM impact on antenna loading and matching.

» Bench tested active tuning (FFT) and » established it would be ameliorate ELM impact on antenna.

• Shown Mode conversion current to be suitable for sawtooth pacing.• Completed initial comparisons with new CNPA synthetic diagnostic in

AORSA/CQL3D codePlans:

• Evaluate compatibility of high power ICRF with metallic first wall and high plasma performance.

• Investigate active matching components for maintaining match between antenna and transmitters.

• Assess interaction of ICRF antenna and LH coupler.• Assess performance of D(3He) and D(H) minority heating• Experimental benchmarking of simulation codes both in minority and mode

conversion regimes.• Examine interaction between ICRF and LH.• Utilize localized current drive, mode conversion and ion cyclotron current drive,

for sawtooth pacing.

Reference Material: Summary of Tasks

RF Sources:• Upgrade ICRF transmitter control and system reliability.• Monitor tube lifetime issues.

Matching network:• Test and implement prototype fast ferrite matching network.

Antennas:• Begin manufacturing of new 4-strap ICRF antenna.• Neutral pressure limit with modified SEC material.• Reduce RF sheaths through antenna design.

Codes:• TOPICA (on MARSHALL) 3-D modeling of ICRF antenna code (U. Torino).• TORIC (on MARSHALL) coupled with CQL3D Fokker Planck code for current

drive calculation. (Sci-DAC)• Finite banana width Monte-Carlo code with self consistent RF wave fields. (Sci-

DAC initiative)Diagnostics:

• Upgrade absolute calibration of PCI.• Develop and procure edge reflectometer diagnostic• Design multi-channel CNPA.

Priorites Compare Favorably with ITPA and Scientific Priorities

Experimentally test TORIC’s physics kernel and computational algorithm for both heating and current drive.

Develop real time current profile control using heating and CD actuators: assess predictability, inparticular for off-axis CD.

Continue experiments to investigate sawtooth stabilization and control and test developing codes.

MDC5- Sawtooth stabilization for NTM control.

•Test TORIC’s physics kernel and computational algorithm thru comparison of wave measurements with simulations.•Investigate MCCD, FWCD, ICCD.

Understanding the propagation of waves and their nonlinear interactions with plasmas will lead to new techniques to control plasma behavior, and will be key to optimizing the conditions for burning of the fusion fuel.

•Identifying operational space under which monster and small sawteeth are obtained.•Identify physics that sets sawtooth period.•Compare experimental sawtooth data with sawtooth models

The interaction of the energetic particle population with the background plasma is a complex process, and a basic understanding of this interaction is critical to practical applications of fusion.

ITPA

Scientific Priorities

ICRF Program Schedule

2009 201220112010

Alcator C-Mod ICRF

2013

6 MW ICRF, 2 Antennas 8 MW ICRF, 2 Quad Antennas

10 kW solid state IPA

FFT on E antenna FFT for all antennas

Sources

RF-Plasma

Edge Inter.

Core Physics

Current Drive

Antenna evaluation

Characterize RF sheaths Dependence on phasing + single pass absorption

Evaluate mitigation techniques

Comparison with TOPICA and other edge codes

MCCD for sawtooth pacing

ICCD

FWCD for central seed current

FWH and comparison with simulation

MC flow drive and MC studies

FWCD (fRF < fci)

D(3He) H-mode performance

Comparison of H minority, 2nd Harmonic with simulation

LH/IC Interactions - LH absorption on energetic ions and IBW generated electrons

Alcator C-Mod ICRF Research ProgramMotivation for ICRF ProgramContext of the C-Mod ICRF ProgramContext of C-Mod’s ICRF ProgramResearch ThemesPresent ICRF SystemICRF System FY09-FY10ICRF System FY11-13ICRF Simulation ToolsDiagnosticsAntenna-Edge: Boronization Erosion and Impurity ProductionICRF Coupling and Active MatchingAntenna-Edge: Antenna Power and Voltage LimitsExperimental Validation of ICRF SimulationsWave Propagation: D(3He) Minority HeatingICRF Current DriveICRF Current Drive: Mode Conversion CDICRF Current Drive: FWCDICRF CD: Ion Cyclotron Current DriveICRF and LHRF Interactions: CouplingIC and LH Waves Interactions: IBW and LHIC and LH Waves Interactions: Fast ions and LHSummaryReference Material: Summary of TasksPriorites Compare Favorably with ITPA and Scientific PrioritiesICRF Program Schedule