C-Mod ICRF Program

Alcator C-Mod PAC MeetingJanuary 24-26, 2006

MIT Cambridge MA

Presented by S.J. Wukitch

Outline:1. Overview of ICRF program2. Overview of ICRF systems and capabilities3. Details of ICRF program

• Antenna-Edge Physics• Core Physics• RF Tool Development

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 have been extensively benchmarked between codes and with experiments.

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 C-Mod’s ICRF Program

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

High and low single pass absorption scenarios are accessible

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

Antenna power density is ~10 MW/m2.

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

Utilize ICRF with high Z first wall materials.

ITER will utilize fundamental 3He and second harmonic tritium.

Standard scenarios are fundamental and second harmonic.

ITER plans to start with 20 MW ICRF power.ICRF provides the bulk auxiliary heating power on C-Mod.

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 drive to be suitable for sawtooth pacing.• Completed initial comparisons with new CNPA synthetic diagnostic in

AORSA/CQL3D code

Overall ICRF Strategy

Utilize flexible ICRF system in combination with sophisticated diagnostics and state of the art simulations to investigate ICRF issues.

Antenna-Edge Physics:• 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.

Core physics:• Assess performance of D(3He) and D(H) minority heating• Experimental verification and validation of simulation codes both in minority and

mode conversion regimes.• Examine interaction between ICRF and LH.

Tool development: • Utilize localized current drive, mode conversion and ion cyclotron current drive, for

sawtooth pacing.

Overview of the ICRF System

variablefixedPhase

4 Strap2 x 2 StrapAntenna

4 MW

40-80 MHz

J-Antenna

2 x 2 MWPower

~ 80 MHzFrequency

D & E-Antenna

Thru 2007

D & E Antenna

J antenna 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 Codes and Diagnostics

Codes:• TOPICA (on MARSHALL) 3-D modeling of

ICRF antenna code via Polytechnico di Torino.• TORIC (on MARSHALL) for wave propagation,

power deposition, and current drive calculations.» Coupled with Fokker Planck codes, DKE, and » CQL3D via (Sci-DAC) when completed.

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

Diagnostics:• 32 channel phase contrast imaging diagnostic to

measure density fluctuations primarily from mode converted waves.

• Active charge exchange neutral particle analyzer (50-350 keV) for H minority energy distribution.

• Developing an edge reflectometer with ORNL (G. Hanson) for measuring the density and density profile local to the antenna.

• Visible cameras monitoring D and J antennas.0.10 0.05 0.00 0.05 0.10

R-Raxis (m)

0.2

0.4

0.6

0.8

1.0

1.2

neL

[x

10

17

m-2

]~

Simulated Channel

Antenna-Edge: Boronization Erosion and Impurity Production

Status:• Enhanced erosion/impurity production

is localized to the active antenna.• Identified outer divertor tiles as most

important impurity source.• Antenna characteristics dominate

potential far-field effects.Plans:

• Installed marker tiles to further investigate localization and erosion rates.

• Assess what makes the shelf location the dominant core Mo contributor.

40 60 80 100R (cm)

0.05

0.1

WMHD (MJ)

0.7 0.8 0.9

1060421011+

12,2

0

1st (representative)2nd, D+E antennas

Time (s)

H-mode

2nd, J antenna

0.5

1

1.5Prad (MW)

Antenna-Edge: ELMs and Active Matching

Status:• Analyzed load variation associated

with ELMs and H->L transitions.• Typical ELMs are close to threshold

for causing an antenna fault.• Bench tested fast ferrite double stub

matching network.» For typical ELMs, system keeps

reflection coefficient H transitions on loading variation.• Develop fast edge reflectometry for density profile measurements.

Antenna-Edge: Antenna Performance

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.» Antenna susceptibility to discharge formation is

increased with high neutral pressure and B-field, decreases with lower secondary electron coef.

Plans:• Assess interaction between ICRF antenna

and LH coupler.• Continue benchmarking of TOPICA.• Investigate limitations on 6 MW discharges.• On test stand, we plan to investigate

» Different surface treatments and associated conditioning,

» 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

0.35 sec, 6 MW L-mode discharge achieved during machine conditioning.

Core Physics: D(3He) Minority Heating

Primary ICRH scenario for 8 T discharges.• May anticipate greater impurity production

due to lower single pass absorption for D(3He) than D(H).

• Additional cyclotron resonances are present in the plasma.

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.

• Limited number of H-mode discharges and performance is on low side.• Conflicting results relating to erosion and impurity generation.

Plans:• Direct comparison of D(H) and D(3He) H-mode discharges.• Move additional resonances in and out of the plasma and compare performance.• Investigate between-shot-boronization maintains optimal plasma performance.• Investigate high power, high performance at 8 T with D(3He) minority heating

under optimized conditions.

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

Core Physics: Experimental Benchmarking of Simulations

Status:• Synthetic PCI density fluctuation profile

agrees with experiment.• Improved absolute amplitude agreement

after finding code renormalization problem.

• Implemented synthetic CNPA diagnostic and good agreement with Fokker-Planck simulation.

Plans:• Implement improved absolute calibration

of experimental data and compare with simulation amplitudes.

• Use CNPA to measure tail energy for second harmonic H scenario to test upgraded simulation capability.

-2

-1

0

1

2

MC layer

-2

-1

0

1

2

0.64 0.66 0.68 0.70 0.72 0.74 0.76R (m)

0

1

2

ExperimentalSynthetic

Re(∫ ne dl)~

Im(∫ ne dl)~

|∫ ne dl|~

[a.u

.][a

.u.]

[a.u

.]

Core Physics: Interaction between IC and LH Waves

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.

Tool Development: Mode Conversion Current Drive

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).

A. Parisot et al., PPCF 2007.

Tool Development: Ion Cyclotron Current Drive

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

sawtooth period.Plans:In L-Mode low density discharges, access both FO and classical regime.

• Use D and E antenna to keep plasma parameters 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.

Evaluate ICCD in H-mode discharges in classical regime.• Scan BT to sweep cyclotron resonance through q=1 surface.• Monitor changes of ST period and CNPA data.

For co-current drive phasing, is stabilizing for a cyclotron resonance at the q=1 on the low field side of magnetic axis.

Peak efficiency near Ecritical (~15Te for H)

Current is carried by passing particles.

Classical (Fisch-Karney) regime:

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.

Efficiency increases with particle energy.

Current is generated by trapped particle orbits.

Finite orbit (FO) regime:

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 code

Plans:• 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.

C-Mod ICRF

DIII-D and NSTX concentrated on fast wave current drive at 4-8 cyclotron harmonic.JET has done significant work on ion cyclotron current drive

Localized mode conversion for sawtooth controlInclusion of Fokker Planck solver to predict driven currentFast wave below all cyclotron frequenciesInvestigate ion cyclotron current drive

Current drive

JET role of phasing and multiple frequency on heating effectivenessNSTX and DIII-D have concentrated upon investigating high harmonic fast waves absorption on electrons and fast ions at high harmonics.

Details of mode conversionCore RF wave diagnostics: phase contrast imaging and compact neutral particle analyzerCompatibility of strong and weak single pass absorption with high performanceBenchmark full wave and finite banana width Fokker Planck codes

Propagation and absorption(heating)

NSTX: parametric decay and surface wavesOther tokamaks are dominated by ELMsDIII-D: double layer screenJET: load tolerant antenna designASDEX-U has program to investigate RF with metallic walls and breakdownTore Supra concentrates on long pulse issues (excellent set of IR cameras)

Investigate antenna operation with metallic limiters.RF role in erosion of boronization layer and enhanced sputtering.Influence of neutrals and B-field on antenna voltage and performance.Benchmark antenna analysis code.

Antenna (coupling)

Other tokamaksC-Mod Research Area

Priorites Compare Favorably with ITPA and Scientific Priorities

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

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

MDC5- Sawtooth stabilization for NTM control.Continue experiments to investigate sawtooth stabilization and control and test developing codes.

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.

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

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

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

ITPA

Scientific Priorities