C-Mod ICRF Research Program

C-Mod Pac Meeting

February 23-25, 2004MIT PSFC

Presented by Steve Wukitch

Outline:1. Overview of ICRF program2. Antenna progress and plans3. Mode conversion physics4. MCCD and MCFD5. ICRF Sawtooth control6. Fast ion driven Alfven modes.

ICRF Highlights

Demonstrated high power, phased antenna operation with performance independent of antenna phase.

Demonstrated sawtooth control via localized current drive and fast ions.• Observed first monster sawteeth on C-Mod.

Measured core localized Alfvén modes excited by minority ion tail in both current rise and high power ICRF experiments.

Initial flow drive experiments indicated the driven flow is sensitive to antenna phasing.

Context of the C-Mod ICRF Program

ITER initially plans to have 20 MW ICRF heating power.• Most critical element is the antenna.

» Must handle high voltages in the presence of plasma and» Needs to be load tolerant or robust matching.

• Provides means for burn control.» Heat bulk ions– no other means available.» Control sawtooth period.

• Provide peaked on-axis current drive during advanced scenario.

Physics understanding of energetic particles is of obvious importance for burning plasma experiments.• Core localized Alfvén modes are likely to be the most

unstable.• Fast particle transport in the presence of Alfvén modes is

another important issue.

ICRF Research Themes

Reliable antenna coupling with minimum negative impact on the plasma.• Further experiments with phased antenna operation to

specifically examine impurity and density influx.• Compare experimental antenna measurements with RF

antenna-plasma model (TOPICA) Examine weak single pass absorption scenarios.

• Is H-mode heated by D(3He) similar to D(H) scenario?• Does the impurity production rate increase?

Investigated mode conversion in ICRF experiments. • Compare PCI measurements with TORIC in the mode

conversion regime.

Research Themes Continued

Investigate mode conversion current drive (MCCD).• Specifically evaluate the impact of mode converting to ion

cyclotron waves in addition to ion Bernstein waves.Experimental exploration of mode conversion flow drive

(MCFD).• Theoretical calculations are difficult and • Experiments could provide important insight.

Investigate and develop sawtooth control.• Utilize both fast particle pressure and localized current drive

to control sawtooth period. Investigate Alfvén modes driven by minority ion population

in both current ramp and high power ICRF experiments.• Compare experimental observations with CASTOR code.• Compare measured fast ion distribution with TORIC-

FPPRF.

Tasks

RF Sources:• Upgrade ICRF transmitter control and system reliability.

Matching network:• Test and implement prototype fast ferrite matching network.• Investigate load tolerant antenna configuration.

Antennas:• Design a compact 4-strap ICRF antenna.• Test J-port voltage handling with shorter vacuum transmission lines.• Test screenless antenna operation.

Codes:• TOPICA 3-D modeling of ICRF antenna code (U. Torino)• TORIC is readily accessible on MARSHAL cluster (Sci-DAC)• Finite banana width Fokker Planck code with self consistent RF wave

fields. (proposed Sci-DAC initiative)Diagnostics:

• Upgrade PCI to higher k and perform proof of principle localization experiment. (DOE Diagnostic Initiative)

• Test and implement active charge exchange for H minority energy distribution.

• Increase number of poloidal B-dot coils for Alfvén mode identification (toroidal mode number).

Overview of the ICRF System

variablefixedPhase

4 Strap2 x 2 StrapAntenna

4 MW

40-80 MHz

J-port

2 x 2 MWPower

~ 80 MHzFrequency

D & E-port

Thru 2006

D-Port

J-port

Demonstrated 5 MW ICRF Discharge

To improve antenna power handling• Reduced or eliminated regions

where the RF E || B and• shielded the BN-metal interface

from the plasma.These modifications have allowed

• D and E-port antennas to routinely achieve 1.5 MW (10 MW/m2) and

• J-port antenna to achieve 3 MW (11 MW/m2) operation.

2

4

PRF (MW) 

WMHD (MJ)�

2

4

Te0 (keV)

2

4

RDD (x1013 s-1)

1

1.5

nline_04 (x1020 m-2)

1.5

2

0.15

0.05

1030605030 IP=0.8 MA, BT=5.4 T

0.8 1.0 1.2Time [sec]

Zeff

Comparable Plasma Response for +90˚ Phase

In L-mode (limited) discharge, 2.7 MW is coupled for ~0.4 sec.

• H cyclotron resonance is:» ~0.67 m at 80 MHz» ~0.69 m at 78 MHz

• Magnetic axis is ~0.68 mJ-port heating efficiency appears is similar to D and E-port antennas.

• Stored energy is response is ~ identical.

• Central electron temperature and neutron rate are similar.

Impurity and density production are similar.

• Density evolution is the same.• Radiated power, average Zeff and

Dα traces are nearly identical.Performance is independent of phase and strength of single pass absorption.

12

0.04

0.06

2

3

2

4

0.8

1

0.51

Prad (MW)

1.5

2

0.6 0.8 1.2 1.4

4

6

PRF (MW) 

WMHD (MJ)

Te0 (keV)RDD (x10

13 s-1)

nline_04 (x1020 m-2)

1030710005 IP=0.8 MA, BT=5.2 T

Zeff

1.0

Dα main (AU)

J-port (+90∞) D and E-port

Time [sec]

ICRF Antennas Plans

Evaluate passive matching techniques.• Quarterwave transformers successfully used to limit VSWR

in matching network in D-port.• Load tolerant matching on E-port was corrupted by large

strap to strap coupling.» Plan to test on J-port where paired straps have lower coupling.» Coupling transmitter to transmitter may also be too large.» May not be compatible for current drive phasing.

Test and implement prototype ferrite tuners in E-port matching system.

Evaluate impact of short vacuum transmission line on J-port.Evaluate antenna operation without Faraday screen.Provide additional antenna measurements to compare with

antenna simulation (TOPICA) codes.• Antenna design, vacuum and plasma data is being used to

benchmark code.Design new 4-strap antenna.

Explore Weak ICRF Absorption Regimes

Assess D(3He) absorption at high power.

• Required for C-Mod BP target discharge.

• More sensitive to 3He concentration than expected.

• Investigate the effect of higher power density and plasma temperature.

• Is H-mode heated by D(3He) similar to D(H) scenario?

• Does the impurity production rate increase?

Series of experiments where a direct comparison of D(H) and D(3He) using 80 MHz and 50 MHz will be investigated.

ICRF Mode Conversion

In multi-species plasma, fast wave dispersion relation indicates possible mode conversion from fast wave to ion Bernstein (IBW) or ion cyclotron waves (ICW) including D-T plasmas.

Narrow deposition may provide tool for modifying or controlling pressure and current profiles.

Mode conversion can compete with other damping mechanisms.

• Detailed measurements provide stringent test for ICRF code predictions.

• Absorption details could affect current and flow drive predictions.

FW

ICWIBW

FW

-6     -4      -2       0       2       4       6

PCI chords

5< k <10 cm-1

( )( )2||

2||

2||2

nSLnRn

n−

−−=⊥

Cold plasma, fast wave dispersion relation:

R, L and S are Stix notation

Dispersion relation in MC region

Investigate Mode Conversion (MC) Physics

Study various scenarios D(3He), H(3He), and D(H).• PCI measures chord-integrated fluctuation

level.• High resolution (~0.7 cm) ECE (UT-FRC)

radiometer measures power deposition profile.

• First principle 2-D full-wave calculation using TORIC.

• Spectroscopic measure of minority concentration.

Experiment

TORIC

S (MW/m^3/MW_inc)

r/a

Deposition profile (FRCECE) compared to predicted profiles during D(H) MC experiments.

H-3He(D)PCI data

TORICSimulation

PCI: a Unique Diagnostic to Detect ICRF Waves

Detected mode converted ion cyclotron wave (ICW) and fast wave.• Measured wavenumbers are in good agreement with dispersion relation.

Diagnostic upgrade:• Upgraded from 12 to 32 channels. • Increased digitizing rate from 1 to 10 MHz with increased memory.• Plan to perform proof of principle experiment to utilize field alignment of

scattered wave to provide localization.

Dispersion curves near MC Region

FW

ICWIBW

FW

-6     -4      -2       0       2       4       6

PCI chords

5< k <10 cm-1

-10 -5 0 5 10kR , cm -1

320

340

360

380

∆f [

kHz]

80.47

80.50

80.53

f RF

[M

Hz]

phase velocitytowardsantenna

phase velocityaway from

antenna

Contour Plot of Fourier Analyzed PCI Data

Observed MC Electron Heating in Current Drive Phase

Strong localized electron heating observed in Ctr-CD phasing.Plasma response is independent of antenna phasing.

• Mode conversion efficiency is not strongly influenced by k-spectrum.

1

2

2.5

3

3.5Te0 (keV)

0.6

0.7

0.8

4

5

6

0.6 0.7 0.8

0.5

1

1.5

2

1030716019 [0,π,π,0]

nline [x1020 m-2]

[0,π/2,π,3π/2]

Prad (MW)

Dα (AU)

0.6 0.7 0.8Time [sec] Time [sec]

1030716016

PRF (MW)

D-port D+J-port  D-port D+J-port 3

3.4

3

3.4

3

3.4

2.4

2.8

0.985 0.995 1.005

1

2

Time [sec]

PRF (MW) 

Rmaj=0.66 m

Rmaj=0.69 m

Rmaj=0.72 m

Rmaj=0.75 m

1030716016 IP=0.8 MA, BT=8.0 T

T e [keV]

T e [keV]

T e [keV]

T e [keV]

Sawtooth Changes Suggest Local Driven Current

Performed series of L-mode, D(3He) discharges at 8 T to investigate MCCD.• Power absorbed by

electrons is ~0.3 MW, ~20% of total power.

• Simulation suggest RF driven current is ~10 kA.

With the deposition peaked near the sawtooth inversion radius, increases with Ctr-CD phasing and decreases with Co-CD phasing.

For Ctr-CD phasing, the sawtoothperiod increases for deposition near the q=1 but unchanged with the deposition peaked away from q=1.

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)

CTR CD

Co-CD

ρ

Sab

s [M

W/m

3]

rinv

Identified Scenario with High MCCD

Identified a series of both L and H-mode discharges to maximize MCCD.

• On-axis MCCD j(r) equals local ohmic current density.

• Use li, loop voltage, and MSE measurements to estimate driven current.

Move MCCD off axis and investigate CD efficiency.

• Investigate influence of plasma temperature, deposition location, and plasma current on driven current.

• Modeling will be critical given the complicated nature of mode conversion. (TORIC through Sci-DAC)

Modeled target discharge:• BT=5.4 T, • Ip=0.8 MA,• ne0 = 1.4 x1020 m-3, • Te0 = 5 keV, and• 65% D, 15% 3He, 5% H• J-port @ 50 MHz (MCCD)• D and E-port @80 MHz

0.0 0.2 0.4 0.6 0.8 1.0r/a

0

5

10

15

j (M

A/m

2)

RF power : 3 MWTotal: 96 kA

Investigate MC Flow Drive

A critical physics issue for AT and BPX is active control of pressure profile and ITB trigger and location.

• Theoretical calculations are difficult.• Experiments could provide insight.

Unique combination of diagnostics and simulation tools to investigate MCFD.

• Measure RF power deposition profile via power modulation.

• Identify RF mode with PCI.• Poloidal flow with HIREX.• Measure effect on transport via

sawtooth heat pulse propagation.• Simulate RF fields with TORIC and

import to flow drive calculation.

Initial experimental results indicate flow scaled with RF power and was sensitive to antenna phasing.

Utilize D(3He) discharges to investigate influence of antenna phasing, plasma current, 3He fraction, and deposition location.

Slope = -18(4) km/s/MW

Sawtooth Period Varies with Antenna Phasing in Both Minority and Mode Conversion Scenarios

Sawtooth period is • 16 msec with +90˚,• ~5 msec with –90˚, and • 8-15 msec with heating phase.

Infer sawtooth period is being modified by fast minority ions.

Sawtooth period• Increases with Ctr-CD and • Decreases with Co-CD.

Possible explanation is the local shear is modified by the driven current.

2

2.5

3

2

2.5

3

0.86 0.88 0.9 0.92 0.94

2

2.5

3

Te0

(keV

)

Time [sec]

+90°

-90°

Heating

Te0

(keV

)T

e0 (

keV

)

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)

Sawtooth Control Experimental Plans

Identified two mechanisms:• Fast particle pressure control

through antenna phase and• Localized MCCD.• MCCD appears to more

efficient.Fast particle pressure may have resulted in first monster sawteeth in C-Mod.Plan to investigate fast particle effects by scanning resonance position and plasma current.

• Develop finite banana width Fokker Planck code with self consistent RF wave fields. (proposed Sci-DAC initiative)

• Measure ion tail using active charge exchange.

Alfvén Mode Interaction with Fast Particles

Develop a physics understanding of energetic particles.• Core localized Alfvén modes are likely to be the most

unstable for future burning plasma experiments.• Fast particle transport in the presence of Alfvén modes is

another important issue. Diagnostic coverage is complementary.

• Edge B-dot probes measure low n modes (global).• PCI measures high n modes (core localized)• Ion tail energy will be measured through active charge

exchange.Have access to sophisticated modeling codes.

• Currently have TORIC coupled to FPPRF.• Recently obtained CASTOR code for modeling Alfvén

spectrum.• Develop finite banana width Fokker Planck code with self

consistent RF wave fields. (proposed Sci-DAC initiative)

Alfvén Cascades Observed During Current Ramp

Global and core modes are measured.

• Some modes are measured by both diagnostics.

Plan to upgrade edge B-dot coils.

• Install additional coils on D, E, and J-port antennas.

• Install inner wall probes.Additional experiments are planned to vary the ion tail characteristics and measure its influence on modes.

• Scan antenna phase (+90º, -90º, heating).

• Scan deposition location.

B-dot

Observe Modes Post Sawtooth Crash

Te

[keV

]

f [

kH

z]

Time [sec]

1031125024

BT=5.4 T, IP=0.8 MA

Appear to be multiple modes during single sawtooth.Modes appear to be suppressed by MHD mode.Mode kR = 1-2 cm-1 and remains constant in time.

• initially symmetric about kR=0 • become asymmetric about kR=0.• Unable to identify with edge B-dot coils.

Log amplitude contour plot with central ECE temperature channel overlaid.

Planned Experiments for Core Localized Modes

Improve diagnostics.• Increase PCI spatial coverage for low k resolution.• Active charge exchange diagnostic for measuring minority

tail.

Piggyback on additional experiments where the goal is to vary the ion tail characteristics and measure its influence on modes.• Scan antenna phase (+90º, -90º, heating).• Scan deposition location.• Compare discharges with varying current profiles (requires a

degree of current profile control).

Analyze and model data with CASTOR.

Summary

Demonstrated high power, phased operation.• Examine phased antenna operation over range of plasma conditions.• Investigate load tolerant configurations and impact of short VTL.

Examine weak (D(3He)) single pass absorption scenarios.• Direct comparison with strong single pass scenario.

Develop physics understanding of ICRF mode conversion through comparison of experiments and simulations.

Investigate mode conversion current drive (MCCD).• Specifically evaluate the impact of mode converting to ICW waves.

Experimental exploration of mode conversion flow drive (MCFD).

Investigate and develop sawtooth control.Investigate Alfvén modes driven by minority ion population in

both current ramp and high power ICRF experiments.Measure fast ion distribution and compare with TORIC-FPPRF.

IPPA SummaryGoal 1.3 (Wave-particle interactions):

• Have sophisticated RF diagnostics to measure RF wave propagation and power absorption and

• Have available sophisticated codes (Sci-DAC) to simulate and predict wave propagation, absorption, and mode conversion.

• Have diagnostics capable of detecting core localized Alfven modes and obtained Castor code for Alfven mode simulations.

Goal 3.3.1: (Assess profile control methods) Can RF waves maintain and control desirable confinement in long-pulse or reactor-scale plasmas?• 3.3.1.1 Current profile control:

» Assess current profile control capabilities using MCCD. • 3.3.1.2 Pressure profile control:

» Utilize localized ion cyclotron and mode conversion heating to modify pressure profile.

• 3.3.1.3 Plasma flow profile:» Experiments are planned to investigate flow drive.

Goal 3.4.1.1: LH and IC systems• Test fast ferrite matching network during plasma operation• Assess load tolerant antenna matching

Context in the World Fusion Program

Alcator C-Mod has unique capabilities in antenna operation, diagnostics, and simulation codes:• Flexible, high power density antennas.• Access to sophisticated simulation codes (e.g.,TORIC, TOPICA, CASTOR).• Phase contrast imaging diagnostic(PCI) provides a unique core measurement

of RF waves.• Active charge exchange provides information on minority distribution.

RF heating and current drive in some other machines:• DIII-D: Resumed fast wave heating and current drive for ω > 2ωcD.• NSTX: High harmonic fast wave (ω ~10ωcD)heating and current drive

is the primary RF system.• JET: Flexible ICRF system with emphasis on fast particle issues; Load

tolerant antenna design• JT-60U: ICRF system focusing on 2nd harmonic heating and fast particle

issues.• Tore Supra: antenna-plasma edge issues and long pulse issues in circular

plasmas for ICRF.• ASDEX-U: ICRF minority and mode conversion heating.