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Assessment of a Field-Aligned ICRF Antenna* 20th Topical Conference on Radio Frequency Power in Plasmas Sorrento, Italy June 25-28, 2013 S.J. Wukitch, D. Brunner, P. Ennever, M.L. Garrett, A. Hubbard, B. Labombard, C. Lau, Y. Lin, B. Lipschultz, D. Miller, R. Ochoukov, M. Porkolab, M.L. Reinke, J.L. Terry, A. Tronchin-James, N. Tsujii, D. Whyte and the Alcator C-Mod Team Key Results: 1. Novel, Field aligned ICRF antenna has reduced impurity contamination, impurity sources, and RF enhanced heat flux. 2. Field aligned antenna has greater load tolerance. 3. Measured plasma potentials for Field Aligned antenna are not remarkably different from plasma potentials associated with toroidally aligned (TA) antenna. 4. Low heating effectiveness with monopole phasing is a result of poor wave penetration. I2.2 - Wukitch 1 20th Topical Conf. RF Power in Plasmas *Work supported by US DoE awards

US DoE...Field Aligned Antenna Antenna straps, septa, and side protection tiles are normal to the total magnetic field, ~10 . Guiding Design Principle is Field Line Symmetry I2.2 -

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  • Assessment of a Field-Aligned ICRF Antenna*

    20th Topical Conference on Radio Frequency Power in PlasmasSorrento, Italy June 25-28, 2013

    S.J. Wukitch, D. Brunner, P. Ennever, M.L. Garrett, A. Hubbard, B. Labombard, C. Lau, Y. Lin, B. Lipschultz, D. Miller, R. Ochoukov, M. Porkolab, M.L. Reinke, J.L. Terry, A. Tronchin-James, N.

    Tsujii, D. Whyte and the Alcator C-Mod Team

    Key Results:

    1. Novel, Field aligned ICRF antenna has reduced impuritycontamination, impurity sources, and RF enhanced heat flux.

    2. Field aligned antenna has greater load tolerance.3. Measured plasma potentials for Field Aligned antenna are not

    remarkably different from plasma potentials associated with toroidallyaligned (TA) antenna.

    4. Low heating effectiveness with monopole phasing is a result of poorwave penetration.

    I2.2 - Wukitch 120th Topical Conf. RF Power in Plasmas

    *Work supported by US DoE awards

  • Outline

    Description of field aligned antenna.

    Impurity contamination and sources

    RF enhanced heat flux to antenna and PFCs

    Characterize RF enhanced plasma potentials.

    Comparison of Dipole and Monopole heating effectiveness

    I2.2 - Wukitch 220th Topical Conf. RF Power in Plasmas

  • Field Aligned AntennaAntenna straps, septa, and side

    protection tiles are normal to thetotal magnetic field, ~10.

    Guiding Design Principle is Field Line Symmetry

    I2.2 - Wukitch 320th Topical Conf. RF Power in Plasmas

    Field AlignedAntenna

  • Field Aligned AntennaAntenna straps, septa, and side

    protection tiles are normal to thetotal magnetic field , ~10.

    Antenna is helical to conform toplasma shape.

    Guiding Design Principle is Field Line Symmetry

    I2.2 - Wukitch 420th Topical Conf. RF Power in Plasmas

    Field AlignedAntenna

    B

    flux surface

  • Field Aligned AntennaAntenna straps, septa, and side

    protection tiles are normal to thetotal magnetic field , ~10.

    Antenna is helical to conform toplasma shape.

    Toroidally Aligned AntennaAntenna straps, septa, and side

    protection tiles are normal to thetoroidal magnetic field.

    Antenna is cylindrical

    Field Aligned Antenna is Normal and Conformal

    I2.2 - Wukitch 520th Topical Conf. RF Power in Plasmas

    Field AlignedAntenna

    Toroidally AlignedAntenna

  • Field Aligned Antenna is Normal and Conformal

    I2.2 - Wukitch 620th Topical Conf. RF Power in Plasmas

    Field AlignedAntenna

    Toroidally AlignedAntenna

    Field Aligned AntennaAntenna straps, septa, and side

    protection tiles are normal to thetotal magnetic field , ~10.

    Antenna is helical to conform toplasma shape.

    Faraday rods are parallel to the totalmagnetic field.

    Toroidally Aligned AntennaAntenna straps, septa, and side

    protection tiles are normal to thetoroidal magnetic field.

    Antenna is cylindrical.Faraday rods are parallel to the total

    magnetic field.

  • C-Mod ICRF has Similar Characteristics as ITER

    Field aligned antenna

    Toroidally Aligned Antennas

    Antenna power density exceeds anticipated ITER power density.

    Strong single pass absorption (SPA).Metallic PFCs (Mo) that has similar sputtering 

    characteristics as tungsten.Scrape off layer is opaque to neutrals.

    I2.2 - Wukitch 20th Topical Conf. RF Power in Plasmas 7

  • Field Aligned Antenna has Achieved Electrical Performance Similar to Toroidally Aligned Antenna

    Electrically the Field Aligned antenna hasperformed well.• Antennas conditioned very quickly in plasmas

    (~15 discharges) to 2 MW (~6 MW/m2).• Operation over wide range of q95 (3-5.5) has

    seen little variation in performance.▪ Misalignment to magnetic field is

  • FEM Simulation Utilizes Detailed Antenna Geometry and Simplified Core Wave Propagation

    • C-Mod plasma and magnetic data are used in the model.

    Limitations of the model:• RF sheath boundary conditions are not implemented.• Plasma density and temperature do not evolve with application of RF power.• RF potentials are calculated in vacuum region ~1 cm in front of the Faraday

    screen.

    I2.2 - Wukitch 920th Topical Conf. RF Power in Plasmas

    Finite Element Method:• 3-D toroidal FEM• Antenna CAD model is

    utilized.• Cold plasma permittivity

    tensor rotated alongmagnetic field.

    • Use artificial collisionaldamping to modelminority absorption.

    M. Garrett et al., Fus. Eng. Design 9, 1570 (2012).

  • FEM Simulation show Reduced Integrated E|| for the Field Aligned Antenna

    For Field Aligned antenna, the integrated E|| fields are reduced for allantenna phases.• For dipole, estimated integrated E|| is reduced particularly at the peaks.• For monopole, integrated E|| is significantly reduced.

    I2.2 - Wukitch 1020th Topical Conf. RF Power in Plasmas

    1 2 3−0.3−0.2

    −0.1

    0

    0.1

    0.2

    0.3

    Potential [kV]

    Polo

    idal

    Coo

    rdin

    ate

    [m]

    (Pos

    ition

    alo

    ng A

    nten

    na)

    Toroidally AlignedField Aligned

    Dipole Phasing

    1 2 3 4 5−0.3

    −0.2

    −0.1

    0

    0.1

    0.2

    0.3

    Potential [kV]

    Polo

    idal

    Coo

    rdin

    ate

    [m]

    (Pos

    ition

    alo

    ng A

    nten

    na)

    Toroidally AlignedField Aligned

    Monopole Phasing

  • FEM Simulation show Reduced Integrated E|| for the Field Aligned Antenna

    For Field Aligned antenna, the integrated E|| fields are reduced for allantenna phases.• For dipole, estimated integrated E|| is reduced particularly at the peaks.• For monopole, integrated E|| is significantly reduced.

    ▪ Lower than dipole– a surprising prediction.

    I2.2 - Wukitch 1120th Topical Conf. RF Power in Plasmas

    1 2 3−0.3−0.2

    −0.1

    0

    0.1

    0.2

    0.3

    Potential [kV]

    Polo

    idal

    Coo

    rdin

    ate

    [m]

    (Pos

    ition

    alo

    ng A

    nten

    na)

    Toroidally AlignedField Aligned

    Dipole Phasing

    1 2 3 4 5−0.3

    −0.2

    −0.1

    0

    0.1

    0.2

    0.3

    Potential [kV]

    Polo

    idal

    Coo

    rdin

    ate

    [m]

    (Pos

    ition

    alo

    ng A

    nten

    na)

    Toroidally AlignedField AlignedField Aligned Dipole

    Monopole Phasing

  • High Z metallic materials are favored forplasma facing components (PFCs).• Impurity sources and contamination are

    more difficult to manage.• Plasma tolerance of high Z materials is

    much lower than low Z materials.

    ICRF compatibility with high Z PFC hasextensively investigated at C-Mod.• ICRF heated H-mode performance with

    high Z PFCs is insufficient.

    Challenges for ICRF Utilization: Impurities

    0.04

    0.08

    0.12

    1

    2

    0.5

    1.5

    2.5

    0.6 0.7 0.8 0.9.

    1

    2

    3

    951205026

    PRAD [MW]

    PICRF [MW]

    WMHD [MJ]

    1.0

    H ITER89

    Time (s)

    bare high Z PFC

    I2.2 - Wukitch 20th Topical Conf. RF Power in Plasmas 12

    Adapted from Greenwald et al., Nucl. Fusion 37, 793 (1997).

  • High Z metallic materials are favored forplasma facing components (PFCs).• Impurity sources and contamination are

    more difficult to manage.• Plasma tolerance of high Z materials is

    much lower than low Z materials.

    ICRF compatibility with high Z PFC hasextensively investigated at C-Mod.• ICRF heated H-mode performance with

    high Z PFCs is insufficient.• ICRF impurity contamination has been

    can be controlled by boron coatings.

    • Low Z film lifetime is limited and doesnot scale to reactor

    • Seek to develop ICRF antennas withoutneeding to resort to boronization.

    Challenges for ICRF Utilization: Impurities

    0.04

    0.08

    0.12

    1

    2

    0.5

    1.5

    2.5

    0.6 0.7 0.8 0.9.

    1

    2

    3

    951205026&960116010

    PRAD [MW]

    PICRF [MW]

    WMHD [MJ]

    1.0

    H ITER89

    Time (s)

    bare high Z PFC

    boronized high Z PFC

    I2.2 - Wukitch 20th Topical Conf. RF Power in Plasmas 13

    Adapted from Greenwald et al., Nucl. Fusion 37, 793 (1997).

  • Compare Measurements of Core Impurity Contamination, Local Impurity Sources

    Compare antenna performancebetween toroidally and field alignedantenna in near axis minority Habsorption scenario with RF powerup to 3 MW.• Magnetic field 5.2-5.4 T with currents

    0.6-1.3 MA.

    Core impurity content is monitoredusing VUV spectroscopy MoXXXI.

    Local impurity (Mo I) sources aremonitored with visiblespectrometer:

    • Multiple views of each antennaand poloidal protection limiter.

    FA-antTA-ant

    I2.2 - Wukitch 20th Topical Conf. RF Power in Plasmas 14

  • In L-mode, Field Aligned Antenna has Lower Impurity Contamination

    Prior to boronization in L-modedischarges, plasma response is morefavorable for power from FieldAligned antenna.

    Impurity contamination is lower.

    • Radiated power is 25% lower forcomparable injected power.

    • Core molybdenum content issignificantly reduced.

    I2.2 - Wukitch 1520th Topical Conf. RF Power in Plasmas

    0.03

    0.05WMHD [MJ]

    1

    2Prad (MW)

    0.2

    0.4Mo XXXI (a.u.)

    0.7 0.8 0.9 1 1.1

    1

    2PRF (MW)

    Time (s)

    1120

    5310

    17, 0

    20

    Toroidally Aligned

    Field Aligned

  • Core Mo is significantly lower for FieldAligned antenna compared toToroidally Aligned antennas.

    • Rise time on the core Mo content issignificantly slower for the Field Alignedantenna than the Toroidally Aligned-antennas.

    Field Aligned antenna has lower radiatedpower.

    • Radiated power is ~20-30% lower thanfor the Toroidally Aligned antennas inEDA H-mode.

    In H-mode, Field Aligned Antenna has Lower Impurity Contamination

    0.2

    0.4

    0.6

    P RA

    D [

    MW

    ]

    0.2

    0.4

    0.6

    Mo

    XX

    XI [

    AU

    ]

    Toroidally Aligned

    H-mode0.5 0.6 0.7 0.8 0.9 1.0

    Time (s)

    0.2

    0.6

    1.0

    1.4

    P IC

    RF

    [MW

    ]

    Field Aligned

    I2.2 - Wukitch 20th Topical Conf. RF Power in Plasmas 16

  • Toroidally Aligned Antenna Molybdenum Source Correlates with RF Power

    Compare the response of the local Mosource for each antenna view when theToroidally Aligned and Field Alignedantenna are powered separately.

    Strong Molybdenum source response atthe Toroidally Aligned antenna whenthe Torioidally Aligned antenna ispowered.• Mo source at the Toroidally Aligned

    antenna increases with each power step.

    I2.2 - Wukitch 1720th Topical Conf. RF Power in Plasmas

    1

    2

    4

    3

    TA Antenna View

    Mo

    I Brig

    htne

    ss [A

    .U. 3

    86 n

    m]

    0.6 0.7 0.8 0.9 1.0Time (s)

    TA Ant.

    .

    PICRF [MW]

    1

    2

  • Toroidally Aligned Antenna Molybdenum Source Correlates with RF Power

    Compare the response of the local Mosource for each antenna view when theToroidally Aligned and Field Alignedantenna are powered separately.

    Strong Molybdenum source response atthe Toroidally Aligned antenna whenthe Torioidally Aligned antenna ispowered.• Mo source at the Toroidally Aligned

    antenna increases with each power step.

    Weak Molybdenum source response atthe Toroidally Aligned antenna whenField Aligned antenna is powered.

    I2.2 - Wukitch 1820th Topical Conf. RF Power in Plasmas

    1

    2

    4

    3

    TA Antenna View

    Mo

    I Brig

    htne

    ss [A

    .U. 3

    86 n

    m]

    0.6 0.7 0.8 0.9 1.0Time (s)

    TA Ant.

    FA Ant.

    .

    PICRF [MW]

    1

    2

  • Field Aligned Antenna Molybdenum Source Correlates with Toroidally Aligned Antenna Power

    I2.2 - Wukitch 1920th Topical Conf. RF Power in Plasmas

    1

    2

    4

    3

    TA Antenna View

    Mo

    I Brig

    htne

    ss [A

    .U. 3

    86 n

    m]

    0.6 0.7 0.8 0.9 1.0Time (s)

    TA Ant.

    FA Ant.

    .

    PICRF [MW]

    1

    2

    Measurable Mo source response at Field Aligned antenna when ToroidallyAligned antenna is powered.

    1

    0.5

    Mo

    I Brig

    htne

    ss [A

    .U. 3

    86 n

    m] FA Antenna View

    0.6 0.7 0.8 0.9 1.0Time (s)

    .

    PICRF [MW]

    1

    2

    TA Ant.

  • Field Aligned Antenna Molybdenum Source has Weaker Response to Field Aligned Power

    I2.2 - Wukitch 2020th Topical Conf. RF Power in Plasmas

    1

    2

    4

    3

    TA Antenna View

    Mo

    I Brig

    htne

    ss [A

    .U. 3

    86 n

    m]

    0.6 0.7 0.8 0.9 1.0Time (s)

    TA Ant.

    FA Ant.

    .

    PICRF [MW]

    1

    2

    Measurable Mo source response at Field Aligned antenna when ToroidallyAligned antenna is powered.

    Mo source response is lower at Field Aligned antenna when Field Alignedantenna is active.

    1

    0.5

    Mo

    I Brig

    htne

    ss [A

    .U. 3

    86 n

    m] FA Antenna View

    0.6 0.7 0.8 0.9 1.0Time (s)

    .

    PICRF [MW]

    1

    2

    TA Ant.

    FA Ant.

  • Challenges for ICRF Utilization: RF Enhanced Heat Flux

    RF enhanced heat flux needs to be managed to prevent exceeding thepower handling capability of the antenna and other PFCs.• Steady state operation becomes challenging.

    Enhanced RF sheaths are thought to be the underlying physics• Sheaths increase the energy per ion• Radial profile results in convective cells that increase the particle flux.

    In ITER, the ICRF antenna design assumes an RF enhanced heat flux of 6MW/m2.• ~125 kW out of 20 MW or 0.625%• JET has measured between 2-10% is found on the septum and antenna limiters• Tore Supra has found ~3.5% for their classical antennas.

    I2.2 - Wukitch 2120th Topical Conf. RF Power in Plasmas

  • Field Aligned Antenna is Instrumented with Thermocouples

    In 5 out of 9 protection tiles, the TZMtiles are instrumented withthermocouples.• Thermojunction is pressed in direct

    contact with the TZM tile.• Thermocouple response time is

    insufficient to calculate the surfacetemperature or heat flux.

    • Use before and after shottemperatures with the tile mass andsurface area to calculate the energyflux for each instrumented tile.

    Side protection tiles are expected toreceive the majority of the heatflux.• FS rods, top and bottom tiles are

    shadowed by 5 mm and 3 mm,respectively.

    1

    9

    456

    1

    9

    456

    I2.2 - Wukitch 20th Topical Conf. RF Power in Plasmas 22

  • Total Energy Deposited is Lower with FA Antenna Powered

    Analyze the thermocouple data over athree month operational period.• Includes L-mode, H-mode• Current scan Ip=0.6-1.3 MA• Various outer gaps, 0.5-2 cm.• Primarily dipole phase with few monopole

    discharges.

    Isolated discharges where the FAantenna power (63 discharges) isgreater than 0.7 on injected RF joules(red squares).

    Compare with discharges where TAantenna power > 0.9 of the injectedpower (111 discharges).

    I2.2 - Wukitch 2320th Topical Conf. RF Power in Plasmas

    4

    3

    2

    1

    1 21.50.5Injected RF Energy [MJ]

    Ener

    gy to

    RF

    Lim

    iter [

    kJ]

    Toroidally Aligned AntennaField Aligned Antenna

    Monopole

    Discharges heated with the FA antenna have lower total energydeposited.• Monopole phasing has elevated energy deposited – about 3x that of dipole.

  • Spatial Distribution of Deposited Energy is Similar

    Profile is similar regardless of whichantenna is powered.• Top (#1) and bottom (#9) tiles have

    majority of deposited energy.

    When field aligned antenna is powered,tile #9 has less the 0.8 kJ depositedenergy.• Data above 0.8 kJ are discharges with

    lower hybrid power, locked modedischarges, or increasing power from TAantenna.

    Monopole phasing causes increaseddeposited energy on top tile.

    I2.2 - Wukitch 2420th Topical Conf. RF Power in Plasmas

    321Energy to RF Tile [kJ]

    4

    5

    6

    1

    9

    Tile

    Num

    ber

    2

    3

    7

    8

    Monopole

    Toroidally Aligned Antenna Field Aligned Antenna

    Estimate the total power deposited as a fraction of the coupled RF energy• linearly interpolate the energy deposited on tiles #2, #3, #7, and #8 ~ roughly

    doubles the measured deposited energy.• Assuming an equal amount on the right set of side protection tiles• the total deposited energy is ~6 kJ for 1.5 MJ injected or 0.4%.

  • Challenges for ICRF Utilization: Load Tolerance

    88 89 90 91 92

    Rmajor [cm]

    1018

    1019

    1020

    1021

    Cut-off density

    LCFS Main Plasma Limiter

    Ante

    nna

    Lim

    iter

    Scrape-off Layer

    Limiter

    Shadow

    Near

    SOL

    Far

    SOL

    ne

    [m-3

    ]

    I2.2 - Wukitch

    To maintain coupled power to the plasma, an ICRF antenna needs to be load tolerant • either intrinsically • or through external matching.

    Edge plasma density profile determines the antenna resistive loading.• Sets the distance to propagation

    and • Determines the transmission

    impedance.

    20th Topical Conf. RF Power in Plasmas 25

    Antenna geometry determines antenna reactance.• Modified by plasma which breaks symmetry of the off diagonal terms in the

    impedance matrix.

    Plasma load variations are encountered during confinement transitions and edge localized mode (ELMs) activity.

  • Field Alignment Improves Antenna Load Tolerance

    I2.2 - Wukitch 20th Topical Conf. RF Power in Plasmas 26

    Field aligned antenna has improved loadtolerance.• Reflection coefficient from Field Aligned

    antenna occupies less area thanToroidally Aligned antenna.

    • Impedance variation is reduced.

    • Q of the antenna, ratio of reactance toresistance, is approximately constant.

    • Impedance variation depends primarily onthe real part of the antenna load.

  • ELMs Loading Perturbation is Largely Resistive for Field Aligned Antenna

    I2.2 - Wukitch 20th Topical Conf. RF Power in Plasmas 27

    Field aligned antenna impedance changeis due resistive change.• Toroidally aligned antenna Q and

    impedance both vary.

    Speculation: antenna impedance matrixbecomes symmetric due to fieldalignment.

    Generic antenna impedance matrix:• a11 and a22 are the plasma resistive load• L11 and L22 are the strap self inductance• M12 and M21 are mutual coupling• b12 and b21 represent the coupling through

    the plasma – particularly E||

    Coupling variation through plasma results in large changes in reflectioncoefficient.

  • ICRF interaction with SOL: What is E|| Role?

    Oscillating E|| leads to DC sheathrectification.

    • I-V curve is highly non-linear andleads to excess electroncollection in presence of RF.

    I2.2 - Wukitch 2820th Topical Conf. RF Power in Plasmas

    I

    V

    e- current

    i+ current

    RF voltage

  • ICRF interaction with SOL: What is E|| Role?

    Oscillating E|| leads to DC sheathrectification.• I-V curve is highly non-linear and

    leads to excess electroncollection in presence of RF.

    • DC bias voltage is established onconducting surfaces to maintainambipolarity.

    Radial profile of RF enhancedsheaths leads to convection.

    E|| interacts with SOL results invariation in off diagonal terms inthe antenna impedance matrix.

    Does a field aligned antennareduce RF enhanced sheaths?

    I2.2 - Wukitch 2920th Topical Conf. RF Power in Plasmas

    I

    V

    e- current

    i+ current

    RF voltageDC offset

  • GPI

    Gas Puff Imaging Monitor Plasma Potential

    GPI diagnostic measures the poloidal velocity of the SOL turbulence.• Monitors the radial region between ~1 cm behind the antenna tile radius to the

    last closed flux surface.• Maps to the corners of both the Field Aligned and Toroidally Aligned antennas

    where the integrated E|| is expected to peak.• Vertical resolution of ~4 cm and radial resolution of ~0.4 cm.

    I2.2 - Wukitch 3020th Topical Conf. RF Power in Plasmas

    Field AlignedAntenna

    K PlasmaLimiter

    A‐B  Plasma Limiter

    Toroidally AlignedAntenna

    LH Coupler

  • SOL Potential Profile Estimated from GPI Measurements

    In the far SOL, turbulence is convected at the local ExB velocity.• Poloidal velocity and corresponding Er are small in ohmic discharge.

    I2.2 - Wukitch 3120th Topical Conf. RF Power in Plasmas

    0 1 2Distance into SOL (cm)

    -4

    -2

    0

    v(k

    m/s

    3

    2

    4

    6

    -620

    10

    0

    -10

    -20

    E r (k

    V/m

    )

    no RF

  • SOL Potential Profile Estimated from GPI Measurements

    In the far SOL, turbulence is convected at the local ExB velocity.• Poloidal velocity and corresponding Er are small in ohmic discharge.• Dramatic change in Er with application of RF.

    I2.2 - Wukitch 3220th Topical Conf. RF Power in Plasmas

    0 1 2Distance into SOL (cm)

    -4

    -2

    0

    v(k

    m/s

    3

    2

    4

    6

    -620

    10

    0

    -10

    -20

    E r (k

    V/m

    )

    no RF

    RF

  • SOL Plasma Potential Profile Estimated from GPI Measurements

    In the far SOL, turbulence is convected at the local ExB velocity.• Poloidal velocity and corresponding Er are small in ohmic discharge.• Dramatic change in Er with application of RF.

    Integrate Er profile to deduce potential profile with profile referenced to3Te/e.

    • Plasma potential is conserved on a field line.

    I2.2 - Wukitch 3320th Topical Conf. RF Power in Plasmas

    0 1 2Distance into SOL (cm)

    -4

    -2

    0

    v(k

    m/s

    3

    2

    4

    6

    -620

    10

    0

    -10

    -20

    E r (k

    V/m

    )

    no RF

    RF

    0

    100

    200

    Φp profile with ICRFreferenced to 3Te /e

    0.5 1.0 1.5 2.0 2.5

    (V)

    Distance into SOL (cm)

    Φ~3Te /e

  • Measured Potential for Field Aligned and Toroidally Aligned Antennas are Similar

    Impurity contamination differs between the antennas with the Field Alignedhaving lower sources and contamination.• Challenge to hypothesis that lower integrated E|| will lower RF enhanced

    sheaths.• Furthermore, difficult to reconcile lower impurity sources and contamination with

    E|| unchanged.I2.2 - Wukitch 3420th Topical Conf. RF Power in Plasmas

    1120

    9040

    13

    Field Aligned

    0.5 1.0 1.5PICRF (MW)

    100

    200

    300

    400

    φ max

    (V)

    Toroidally Aligned

  • Monopole Phasing does not Perform as well as Dipole

    Recall field aligned antenna inmonopole phasing was predictedto have the lowest integrated E||fields.

    Heating effectiveness with monopoleis significantly lower than dipolephasing.

    Measured plasma potential is higherfor monopole operation than thatmeasured during dipole operation.

    0.03

    0.05

    1

    2

    WMHD [MJ]

    Te0 (keV)

    Dipole phasing Monopole phasing

    0.6 0.7 0.8 0.9 1

    12

    1120613007&009

    PRF (MW)

    Time (s)

    I2.2 - Wukitch 20th Topical Conf. RF Power in Plasmas 35

    1120

    9180

    07 &

    112

    0918

    005

    Dipole

    0.0 0.5 1.0 1.5P ICRF (MW)

    0

    100

    200

    300

    400

    2.0

    Monopole

    500

    600

    700

    φ max

    (V)

  • What is Underlying Physics of Poor Performance with Monopole Phasing

    Wave spectrum is peaked at longwavelength if one only considers theantenna straps.• Good for coupling at long distances.

    • Dipole has peak at shorter wavelength –n~13.

    I2.2 - Wukitch 3620th Topical Conf. RF Power in Plasmas

    Monopole

    Dipole

    Pow

    er S

    pect

    rum

    (A.U

    .)

    Monopole

    Dipole

  • Hypothesis is that Monopole Spectrum Peaks at Short Wavelength

    FEM antenna-plasma model indicatesmonopole wave spectrum is peaked atvery short wavelengh.• Wavelength is about half dipole resulting in

    high n.

    • Image currents on antenna box includingthe septa significantly modify the launchedwave spectrum.

    • Dipole phasing image currents on septacancel – little modification of the analyticwave spectrum.

    I2.2 - Wukitch 3720th Topical Conf. RF Power in Plasmas

    Re(E+) (kV/m)

    Monopole

    Dipole

    MonopoleCurrents Add

    DipoleCurrents Cancel

  • Test Hypothesis with Mode Conversion Absorption Scenario

    Mode conversion heating scenarioenables measurement of corewaves with phase contrastimaging (PCI).• Monopole and dipole should have

    similar single pass absorption.

    Dipole plasma heating effectivenessis higher than monopole.

    Radiated power and core impuritycontamination are similar.• Suggests difference between

    monopole and dipole is notdominated by impuritycontamination.

    Carbon II responds strongly tomonopole phasing.• Indicates RF power is interacting

    with the SOL.I2.2 - Wukitch 3820th Topical Conf. RF Power in Plasmas

    0.03

    0.05

    .4

    6

    0.2

    0.4

    0.02

    0.06

    0.1

    0.060.1

    0.14

    1120905021

    0.8 0.9 1.1 1.2.

    0.51

    1.52

    PRAD [MW]

    Mo XXXI [AU]

    PICRF [MW]

    Te0 [keV]

    WMHD [MJ]

    C II [AU]

    Time (s)1.0

    DipoleMonopole

  • Monopole Spectrum is Inaccessible

    PCI measures no waves in theplasma core during monopolephasing.

    Full wave modeling shows thatthe monopole spectrumincluding the image currentslaunches waves that remainin the plasma periphery.

    I2.2 - Wukitch 3920th Topical Conf. RF Power in Plasmas

    -10

    -5

    0

    5

    10

    0

    2

    4

    6

    8

    10

    12

    0.6 0.8 1.0 1.2 1.4Time (s)

    1

    2

    k R (c

    m-1

    )

    PRF (MW)

    DipoleMonopole

    Inte

    nsity

    (101

    3 m-2

    /cm

    -1)2

    −30 −20 −10 0 10 20−40

    −20

    0

    20

    40

    V/m/MWabs

    −3.7

    −1.2

    −0.37

    −0.12

    0.13

    0.42

    1.3

    X = R − R0 (cm)

    Z (c

    m)

    Re(E_)V/m/MWabs

    nφ=3

    Monopole -(excluding image currents)

    −30 −20 −10 0 10 20−40

    −20

    0

    20

    40

    V/m/MWabs

    −3.3−1.1−0.33−0.110.00960.30.96

    X = R − R0 (cm)

    Z (c

    m)

    Re(E_)V/m/MWabs

    nφ=13

    Dipole

    −30 −20 −10 0 10 20X = R − R0 (cm)

    −40

    −20

    0

    20

    40

    Z (c

    m)

    V/m/MWabs

    −1.4

    −0.44

    −0.14

    −0.044

    1.6

    5

    16

    Re(E_)V/m/MWabs

    nφ=26

    Monopole -(including image currents)

  • Future Directions

    Add plasma response to antenna – plasma model.• Sheath boundary conditions are next physics to be included.• Reconstruct antenna impedance matrix from antenna-plasma model to

    investigate role of magnetic field and off diagonal terms.

    Investigate relationship between ICRF antenna and plasma potential.• Characterize plasma potential with additional emissive probes - same mapping

    as GPI and over wider set of plasma conditions.• Increase poloidal coverage to characterize poloidal profile of plasma potentials.• Does the tile geometry and orientation to the magnetic field play a role?• Can the antenna structure be biased to reduce RF enhanced plasma potential?

    Establish relationship between SOL plasma potential and impuritycontamination and sputtering.• Identify impurity source locations.• Is SOL impurity transport modified with RF?

    I2.2 - Wukitch 4020th Topical Conf. RF Power in Plasmas

  • Field Aligned ICRF Antenna Results are Promising

    Hypothesis: does a field aligned antenna improve ICRF antenna performance?

    • reduced impurity contamination and impurity sources,

    • has low RF enhanced heat flux, and

    • is more resilient to load variations than toroidally aligned antennas.

    However, our physics understanding of Field Aligned antenna is incomplete.

    • Plasma potentials associated with field aligned antenna operation are similar totoroidally aligned antenna operation.

    • Clarification of the underlying physics that influences the SOL plasma potential in thepresence of ICRF is required.

    Monopole antenna phasing has poor heating effectiveness due to poor wavepenetration in the plasma core.

    I2.2 - Wukitch 20th Topical Conf. RF Power in Plasmas 41