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Universität Augsburg Max-Planck-Institut für Plasmaphysik Plasma Diagnostics of Ion Sources Ursel Fantz Ursel Fantz [email protected] Diagnostics – The Window to the Knowledge f Langmuir probes: φ pl , n e , T e f Absorption techniques: n s Cs, H¯ f Emission spectroscopy: n T e HH H¯ f Emission spectroscopy: n s , T s e, H, H 2 , H CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Monitoring and Quantification – Spatial and Temporal Resolution Preliminary considerations What do I want to know? f identify the quantity (and the reason for it) f define the required precision f temporal behaviour and required time resolution f necessity for spatial resolution f necessity for spatial resolution f Accessibility of the ion source? Adequate diagnostic technique? f diagnostic ports f test stand or continuous operation f extensive or simple setup f data acquisition and evaluation f risks and feasibility f reliability f f reliability f costs and time (manpower) needed f CAS on Ion Sources, 6 th June 2012 Ursel Fantz, p. 2 f f

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  • Universität AugsburgMax-Planck-Institut für Plasmaphysik

    Plasma Diagnostics of Ion SourcesUrsel FantzUrsel Fantz

    [email protected]

    Diagnostics – The Window to the Knowledge

    Langmuir probes: φpl, ne, Te

    Absorption techniques: ns → Cs, H¯

    Emission spectroscopy: n T → e H H H¯Emission spectroscopy: ns, Ts → e, H, H2, H

    CERN Accelerator School, Senec, Slovakia 29th May – 8th June 2012

    Monitoring and Quantification – Spatial and Temporal Resolution

    Preliminary considerations

    What do I want to know?

    identify the quantity (and the reason for it)define the required precisiontemporal behaviour and required time resolutionnecessity for spatial resolutionnecessity for spatial resolution …

    Accessibility of the ion source?Adequate diagnostic technique? y

    diagnostic portstest stand or continuous operation

    q g q

    extensive or simple setupdata acquisition and evaluation

    risks and feasibilityreliability

    reliabilitycosts and time (manpower) needed

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 2

    ……

  • General plasma diagnostics techniques

    invasive – non-invasive ; active – passive ; basic – specific parameters

    Method Standard Sophisticated ExtrasLangmuir probe single probe

    (cylindrical or planar)double or triple

    probespecial method:

    Boyd-Twiddyemissive probe

    emission spectroscopy

    optical wavelength range with fibre optics

    extended wavelength range

    sophisticated system spectral resolution,

    & survey spectrometer VUV, UV or IF type of detectorabsorption

    spectroscopywhite light absorption

    techniquetuneable laser

    absorptioncavity-ringdownspectroscopy

    Laser methods laser induced fluorescence

    TDLAS

    mass residual gas analyser energy resolved spectrometry mass spectrometry

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 3

    Introduction into techniques and applications for example in [1] – [7]

    Example case: IPP ion source for negative hydrogen ions

    ICP: f = 1 MHz, P = 70 kW, p = 0.3 Pa6s plasma (4s beam), every 3 min: BATMANcw, up to 1 hour, every 3 min: MANITU

    H⎯ formation and losses ...

    volume process

    magnet boxesmagnet boxes

    source bodyat 50°Coven at

    cesium followed byH2 + e- → H2(v) + e- + hν

    H ( ) H¯ H

    H2(B,C)p

    magnet boxes

    Cs inlet

    magnet boxes

    Cs inlet

    > 120°C H2(v) + e- → H¯ + H

    dissociative attachmentH2⎯

    f

    groundedgridx

    yx

    ygroundedgridx

    yx

    y

    grid at150°C

    surface processH, Hx+ + surface e- → H¯

    i llow work function

    plasmagrid

    extractiongrid

    gridxz xz

    plasmagrid

    extractiongrid

    gridxz xz caesium layer

    destruction processesH¯ + e- → H + 2e-

    t tiexpansion

    plasmageneration(driver)

    ∅ 24 t tiexpansion

    plasmageneration(driver)

    ∅ 24

    H + e → H + 2eH¯ + H+ → H + HH¯ + H → H + H + e-

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 4

    extractionsystem

    pchamber∅ = 24 cm

    ℓ = 15 cm 32×59×23 cm3extractionsystem

    pchamber∅ = 24 cm

    ℓ = 15 cm 32×59×23 cm3... determine source optimisation

  • Example case: sources for negative hydrogen ions

    Ion sources for negative hydrogen ions: ionising – recombining plasma

    Filter field≈ 10 mT

    H, Hx+ + surface e- → H¯Filter field

    SRF power100 kWp = 0.3 Pa Plasma generation

    ionising plasma

    , xCs evaporation → low work function

    gas feed

    eHx+HH¯

    ionising plasmaionisation: α ≈ 0.1dissociation: δ ≈ 0.3T 10 V 5 1018 3

    g H Te ≈ 10 eV, ne ≈ 5×1018 m-3

    H⎯ generationrecombining plasmaCsevaporation N

    Driver∅ 24cm

    Expansionregion

    Extractionregion

    g pTe ≈ 1 eV, ne ≈ 5×1017 m-3H¯/ne ≈ 0.1 – 5Cs+/n ≈ 0 01 – 0 1

    evaporation

    ∅ 24cm region region

    focus of diagnostics (and modelling)

    Cs /ne ≈ 0.01 0.1

    Main issuesProduction and destruction of negative ions

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 5

    (and modelling) on plasma close to the grid

    gExtraction of negative ionsReduction of co-extracted electrons

    Langmuir probes: φpl, ne, Te, (EEDF)

    Main principle

    plasma chamber stick a wire into the plasma

    probe tip(tungsten wire)

    s c a e o e p as atungsten, ∅≈ 100 µm, l = 1 cm

    choose a reference electrode( g )insulator

    (ceramics)potential of plasma chamber

    apply a variable voltage

    voltage supply (-50 V to + 50 V)

    typ. from - 50 V to + 50 V

    resistor

    voltage measurementI – V

    characteristics

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 6

    Recommended text books [1], [2], [5], [6] & publications and programs by F.F. Chen [8]

  • Langmuir probes: φpl, ne, Te, (EEDF)

    Main principle

    plasma chamberφ

    16electron

    probe tip

    φfloating

    101214

    transition

    electronsaturation

    [mA

    ]

    ion saturation

    probe tipφprobe

    φplasma 68

    10

    e cu

    rrent

    nTe

    EEDF

    024

    prob

    e neni

    5 10 15 20 25

    -20

    b lt [V]probe voltage [V]

    φflφpl

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 7

    Langmuir probes: φpl, ne, Te, (EEDF)

    Floating potential

    116

    electronion saturation same fluxes for ions and electrons

    8101214

    transitionsaturation

    ent [

    mA

    ]

    ion saturation sa e u es o o s a d e ec o sΓions = Γelectronssame currents

    246

    8

    prob

    e cu

    rre no probe current

    5 10 15 20 25

    -202p

    Iprobe = 0 → φfl5 10 15 20 25

    probe voltage [V]

    φfl

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 8

  • Langmuir probes: φpl, ne, Te, (EEDF)

    Plasma potential

    16 electron

    ion saturationdetermined by ambipolar diffusion

    8101214

    transitionsaturation

    ent [

    mA

    ]

    ion saturation turning point of I-V characteristicszero-crossing of second derivative

    V2 ]2

    46

    8

    prob

    e cu

    rre

    d2 I / d φpr2 → φpl

    1

    2

    e [m

    A /

    V

    φ5 10 15 20 25-202p

    0

    deriv

    ativ

    e φpl5 10 15 20 25

    probe voltage [V]φpl

    f ith hi h i l l

    5 10 15 20 25

    -1se

    cond

    for curves with high noise levelcrossing of linear fits toelectrons saturation current and transition close to turning point

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 9

    5 10 15 20 25probe voltage [V]

    transition close to turning point

    Langmuir probes: φpl, ne, Te, (EEDF)

    Electron energy distribution function (EEDF) and electron temperature

    16 electron

    ion saturationdistinguish: EEDF = sqrt(E) × EEPF

    8101214

    transitionsaturation

    ent [

    mA

    ]

    ion saturation

    EEDF

    e.g.: Maxwell function probability function

    plot log(I ) versus E = φ φ

    for both: normalisation

    246

    8

    prob

    e cu

    rre TeEEDF plot log(Ipr) versus E = φpl − φpr

    d(ln I ) / dE) = e / ( kbTe )slope yields Te for Maxwell EEDF

    2

    mA

    / V2 ]

    100slope → Te5 10 15 20 25-202p b e

    0

    1

    eriv

    ativ

    e [m

    φpl10-2

    10-1

    [eV-

    3/2 ]

    5 10 15 20 25probe voltage [V]

    φpl

    -1

    0

    seco

    nd d

    e

    EEPF

    10-4

    10-3

    EEP

    F

    Te also from potential differenceφpl−φfl = kbTe × ln(sqrt(mi/(2πme)))

    ≈ 2-3 × T

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 10

    5 10 15 20 25probe voltage [V]

    0 1 2 3 4 5 6energy [eV]

    ≈ 2 3 × Te

  • Langmuir probes: φpl, ne, Te, (EEDF)

    The electron density

    electron saturation current16 electronion saturation e ec o sa u a o cu e

    8101214

    transitionsaturation

    ent [

    mA

    ]

    ion saturation

    effeesate AvenI 41

    , =

    246

    8

    prob

    e cu

    rre

    neproblem: effective probe areadue to increase of plasma sheath

    5 10 15 20 25

    -202p

    take current at plasma potential→ A = A5 10 15 20 25

    probe voltage [V]φpl

    → Aeff = Aprobe

    eple Tk

    ml

    In

    φ2

    )(=Probe geometry

    eBprpre Tkelr π2

    needs Te

    Probe geometry influences shape of electron saturation current

    cylindrical probe (standard case)

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 11

    planar probe → Apr >> sheathspherical probe

    Langmuir probes: φpl, ne, Te, (EEDF)

    Ion density (positive ions)

    ion saturation current16

    electronion saturation ion saturation current

    basically three theories availableOML: Orbital-Motion-Limited

    8101214

    transitionsaturation

    ent [

    mA

    ]

    ion saturation

    ABR: Allen-Boyds-ReynoldsBRL: Bernstein-Rabinowitz-Langmuirll f th i lli i l

    ni246

    8

    prob

    e cu

    rre

    all of them assuming collision-less plasma sheath, i.e. λ(ions) > r(sheath)

    5 10 15 20 25

    -202p

    simplest case OML (r /r < 3)5 10 15 20 25probe voltage [V] simplest case: OML (rpr /rsheath < 3)

    i

    eBprii m

    TkeAnI = needs Techoose proper φprim

    needs mioften unclear, e.g. hydrogen

    guide line: φ = φpl − 10 × kTe

    check if ne = ni is fulfilled

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 12

    often unclear, e.g. hydrogen→ H+, H2+, H3+Isat, i at fixed φpr: useful as monitor signal

  • Langmuir probes: φpl, ne, Te, (EEDF)

    Specific features – to keep in mind

    invasive method→ probe size versus plasma volume

    level of noise → EEPF for typically three orders of magnitudeaverage, smoothing, filtering

    RF field → oscillating φplmeasured curve ≠ real curve

    urre

    nt

    real curve

    oscillation

    prob

    e cu measured curve

    RF compensationactive or passive

    magnetic field → gyro motionuse Ii instead of I 20 10 0 10 20 30

    active or passive

    use Iion instead of Ie

    negative ions → I− instead of Iefor same mass Isat ion = I sat symmetric curves

    probe voltage-20 -10 0 10 20 30

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 13

    sat,ion sat, y

    For monitoring or quantification with spatial and temporal resolution !

    Langmuir probes: φpl, ne, Te, (EEDF)

    Double probe

    probe 1 A1 two probe tips of same sizeprobe 1 A1

    e a

    two probe tips of same sizedistance > 2 Debye length

    voltage between the probes

    compatible with quartz or ceramicchamber

    vol

    tage

    plas

    ma voltage between the probes

    both probes are floatingno reference potential needed

    chamberand RF field

    1

    2 ion saturationprobe 1

    A]

    Iprobe 2 A2

    0

    1

    transition regionurre

    nt [m

    A Ii1

    symmetric curve: ion saturation currentI+ II

    -2

    -1 transition region

    Ii2

    cu

    ion saturationprobe 2

    eB

    satii

    mTkeA

    In ,=

    2,,

    ,−+ += isatisatsati

    III

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 14

    -40 -20 0 20 40voltage between the tips [V]

    imTe from transition region

  • Example: Langmuir probes in ion source & Boyd-Twiddy setup

    Comparison of EEDF10-1

    Boyd TwiddyH2, 80 kW, 0.36Pa voltage ramp is superimposed by

    Boyd-Twiddy method: direct measurement of EEDF

    2[eV

    -3/2

    ]

    Boyd-Twiddyd=5cm

    voltage ramp is superimposed byAC modulated signalmeasure frequency spectrum

    B. Crowley, S. Dietrich PSST 18 (2009) 014010

    10-2Langmuir probewithout RF compensation

    f (E

    ) [

    Max

    of probe current

    H2,80kW, 0.36Pa0 5 10 15 20 25 30

    10-3

    Energy [eV]

    Vpl= 21.5 V

    axwellTe =6.6eV

    10-1

    100

    -3/2

    ]Energy [eV]

    SophisticatedLangmuir probe system

    driver exit10-2

    10

    Te=1.2eVTe=1.4eV

    Te=2.9eVTe=6.6eV

    f(E) [

    eV-

    Te=14eV

    nce [

    cm]

    59

    1419McNeely et al.

    PSST 18 (2009) 014011

    for RF ion sources

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 15

    plasma grid10-30 5 10 15 20 25 30

    e

    Energy [eV]

    distan0

    5

    Maxwellian EEDF

    All about photons: emission and absorption

    Radiation of low temperature plasmas

    Colourful plasmas ! Neutrals atoms and molecules

    Hepink

    Ions single chargedElectrons ne

  • All about photons: emission and absorption

    Emission versus absorption spectroscopy

    photon energy E = hνl h (E E )/h

    Emission Absorptionwavelength λ = (Ep-Ek)/hc Einstein coefficients Apk, Bkp

    Emissionpassive

    Absorptionactive

    it d t tp

    it d t tp

    excited state

    λpk, Apk

    p

    impactit ti k

    excited state

    λpk, Bkp

    p

    k

    radiation field Lλλpq, Bqp

    q

    excitation k

    λpq, Apq q

    k

    g [ ]

    tens

    ity

    nten

    sity

    806 808 810 812 814 816Wavelength [nm]

    Int

    Wavelength [nm]806 808 810 812 814 816

    In

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 17

    VIS: simple equipmentinformation of upper level p

    expensive equipmentinformation of lower level q

    Absorption techniques: nspecies → Cs, Ar, He, ...

    Main principle of line absorption

    light source detectorabsorbing mediumlight source detectoroptics

    absorbing medium

    white light tunable laser

    vacuum or plasma spectrometer with CCDdiode with filter

    Non-invasive and line of sight integrated method !

    [ ]lIlI )(exp)0,(),( λκλλ −=Absorption in a mediumwith path length l)(λκabsorption coefficient

    statistical weights g g[ ]lIlI )(exp)0,(),( λκλλwith path length l

    ⎟⎟⎠

    ⎞⎜⎜⎝

    ⎛=

    )()0,(ln1)(

    lII

    lk

    λλλ with Ladenburg relation

    λλλκ8

    )(4

    0 ikikki

    Agnd =∫

    statistical weights gi, gk

    ⎟⎠

    ⎜⎝ ),(

    )(lIl λ π8kline cg

    ∫ ⎟⎟⎞

    ⎜⎜⎛

    = k dlIgcn λλπ ),(ln18Density of lower state

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 18

    ∫ ⎟⎟⎠

    ⎜⎜⎝

    =lineiki

    k dIlAgn λ

    λλ )0,(ln4

    0Density of lower state

    ground state, metastables

  • Absorption techniques: nspecies → Cs, Ar, He, ...

    Example: Cs line at 852.1 nmresonance line 6 2S1/2 – 6 2P3/2 with hyperfine structure

    take line broadening into accountDoppler profile

    1 5 0 15 onwhite light

    appearance depends on technique

    mTk

    cB

    FWHMD2ln8

    ,λλ =Δ

    1.0

    1.5

    0.10

    0.15

    ht a

    bsor

    ptio

    abso

    rptio

    n absorption

    5 4

    T = 1000 K

    ity

    sum of Doppler-broadenedhyperfine lines

    0.5 0.05

    nal w

    hite

    lig

    gnal

    Las

    er a

    Laserabsorption

    F F4 42 3

    3 3

    Inte

    nsi

    4 3852.080 852.100 852.120 852.140

    0.0 0.00 Sig

    n

    Sig

    Wavelength [nm]

    absorption

    852.105 852.110 852.125 852.130 852.135

    3 4

    Wavelength [nm] apparatus profile (spectrometer) Doppler profile

    white light absorption ↔ laser absorption

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 19

    Wavelength [nm]

    six hyperfine lines overlap to two peaks: Δλ ≈ 21 pm

    apparatus profile (spectrometer) Doppler profile

    Absorption techniques: nspecies → Cs, Ar, He, ...U. Fantz, C. Wimmer

    Example: Cs line at 852.1 nmwhite light absorption versus laser absorption

    U. Fantz, C. WimmerJ. Phys. D 44 (2011) 335202

    0 04 white light absorption0.020

    laser absorption

    0.02

    0.04

    nCs = 1.4 x 1015 m-3

    on [a

    . u.]

    0.010

    0.015

    n [a

    . u.] nCs = 3.5 x 10

    13 m-3

    0.00

    Abso

    rptio

    0.005

    Abso

    rptio

    n

    851.6 851.8 852.0 852.2-0.02

    Wavelength [nm]0.0 0.1 0.2 0.3 0.4

    0.000

    A

    Rel. wavelength [a. u.]

    Improved detection limit for laser absorption: factor > 40 !sensitivity range: 3×1013 m-3 1017 m-3 (path length = 15 cm)

    g [ ]

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 20

    sensitivity range: 3×1013 m 3 ̵ 1017 m 3 (path length = 15 cm)being perfectly in the range required for the ion sources

  • Absorption techniques: nspecies → Cs, Ar, He, ...

    Straightforward analysis

    1in vacuum

    ∫ ⎟⎟⎠

    ⎞⎜⎜⎝

    ⎛= kk dI

    lIlA

    gcn λλλ

    λπ

    )0(),(ln18 4

    lowabsorption

    1

    ensi

    ty

    (a)with plasma → subtract emission

    ∫ ⎟⎠

    ⎜⎝lineiki

    k IlAg λλ )0,(40

    ... but ....h b ti

    mediumabsorption

    0

    Rel

    . int

    e

    Line saturation

    strong absorption: nk × l4

    Gaussian fit

    heavy absorption

    a. u

    .]

    0

    (b)g p kcorrection factors by profile fitting

    and

    2 measured signalbs

    orpt

    ion

    [a... and ....

    0

    Ab

    WavelengthDepopulation effect

    strong intensity

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 21

    strong intensityattenuation of laser to ≈1% → trade-off with temporal resolution

    Laser absorption for Cs in ion sources

    Tuneable diode laser – Fibre optics – Photo diode with interference filter

    diodelaserdiodelaser

    single mode laser, 150 mWλ = 851-853 nm with ΔλFWHM = 0.01 pm

    lens f = 25.4 mm

    fiber105 µm

    lens f = 25.4 mmlens f = 25.4 mm

    fiber105 µm

    lens f = 25.4 mm

    ND-filter 2.0 interference filter 852 nmbeamdiameter11.5 mm

    µNA = 0.22

    ND-filter 2.0 interference filter 852 nmbeamdiameter11.5 mm

    µNA = 0.22

    photodiode

    fiber200 µm

    NA = 0.22photodiode

    fiber200 µm

    NA = 0.22

    Simple and robust setup for application to ion sources !

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 22

  • Example: laser absorption for on-line monitoring of Cs dynamics

    On-line monitoring: vacuum – plasma (6s with 4s beam) - vacuum

    30 s after pulse:Cs peak & decay time

    3

    45

    3 ]Cs densitylaser absorption

    BATMAN #68562

    Cs peak & decay time

    2

    3

    3

    4

    RF off ion

    [a. u

    .]

    y [1

    015

    m-3

    RF on

    laser absorption

    1

    2

    1

    2

    RF off

    Cs

    emis

    s

    Cs-

    dens

    ity

    RF on

    5450 5455 5460 54650 0

    1 C

    Cs emission

    C

    5450 5455 5460 5465Time [s]

    3 s before pulse: vacuum

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 23

    3 s before pulse: vacuum 6 s plasma with 4 s beam: plasma90% of Cs is ionized

    Absorption techniques: nspecies → H⎯, Cs, Ar, H3+...

    Cavity – Ringdown – Absorption – Spectroscopy → CRDSpumping of an optical cavity by a (tunable) laser source

    measurement of signal decay after laser source is switched off

    dcavity detectorlaser

    measurement of signal decay after laser source is switched off

    high reflectivity mirrorsR>99.9%

    transmitted signal

    plasmawith length l

    Measurement of laser light attenuation trapped in a high-finesse optical cavitytransfers absorption signal from wavelength into time dependence

    0t

    0 e(t)τ−⋅= II

    )1(0 Rd

    =τ; )1'

    1(ld1n −⋅⋅=

    additional absorption ττ ′→0empty cavity with decay time τ0

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 24

    0 )1(0 Rc −)

    0'(

    lcσ ττ⋅

  • Absorption techniques: nspecies → H⎯, Cs, Ar, H3+...

    Cavity – Ringdown – Absorption – Spectroscopy → CRDSExample H⎯

    1.0

    )11(d1n −⋅⋅= cross section: photodetachment

    0 6

    0.8τ0 (reflectivity of mirrors)nt

    ensi

    ty

    empty cavity

    )0'

    (lcσ

    nττ⋅

    = cross section: photodetachment

    3

    4

    0-21

    m2 ] H- + hν → H + e

    0 2

    0.4

    0.6

    density

    0

    mal

    ized

    in

    1

    2

    3

    sect

    ion

    σ [1

    0.0

    0.2

    limited by time resolution

    increasedensity

    norm

    0 8

    1.0

    .]

    0.0 0.5 1.0 1.50

    1

    cros

    s

    wavelength [μm]

    Nd-YAG laser

    0 5 10Time [μs]

    0.6

    0.8

    τ0 = 62 μs

    sign

    al [a

    .u.

    empty cavity (vacuum)

    H¯ density: line of sight averageddetection limit ≈ 1015 m-3 0.2

    0.4plasma

    τ = 47 μss

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 25

    H¯ = 5×1017 m-3 → t = 8 µs0 50 100 150 200 μs

    Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    The main principle

    spectrometer detectorplasma spectrometer detectoroptics

    plasma

    UV and VUV: vacuum system IF: detector

    Non-invasive and line of sight integrated method !

    λε /)( hcApn=

    Measures density of excited state ...

    πε

    4)( Apn pkpk =

    ... which depends on plasma parameters !

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 26

    Recommended text books [1], [4], [7], [9], [10]

  • Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    What information can be obtained from the line emission ?

    I n(p) A Intensity: plasma parameters

    1.0

    Imax n(p) Apkdensity and temperature ofneutrals, ions, electrons

    0 6

    0.8

    u.]

    insight in plasma processes

    Line profile: broadening mechanism

    ⇒=∫ 1λλ dPline λλ εε Ppk=

    0.4

    0.6

    ensi

    ty [a

    . u

    Δλ Tgas Doppler broadening: particle temperature

    Line profile: broadening mechanism

    0 0

    0.2Int

    e

    Wavelength: species

    Wavelength shift: particle velocity

    499 500 501

    0.0

    Wavelength [nm]

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 27

    λ0 Element

    Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    ... Ion.

    Energy Atoms

    SLS Ln +

    +12l ...Ion

    Molecules

    Σ+Λ+ Λ12S −+,ug

    selection rules...

    n(p)

    ...

    .

    1,0 ±=ΔL

    SL+ .......

    n(p), v’, J’0=ΔΣ

    Σ+Λ ,ug

    Radiation

    n(p)

    pkA

    ,

    0=ΔS1,0 ±=ΔJ

    .

    .

    n(k) v’’ J’’

    Radiation'''''' JJvvpkAgu ↔

    1,0''' ±=Δ=− JJJP Q R branch

    n(k)

    1.2] Na D-line1.0

    ] Nitrogen N2

    ≈n(k), v’’, J’’P, Q, R branch

    0 40.60.81.0 3 2P1/2 - 3

    2S1/2

    ensi

    ty [a

    .u.]

    3 2P3/2- 3 2S1/2

    0.4

    0.6

    0.8

    2-4

    1-3

    ensi

    ty [a

    .u.] g 2

    C 3Πu - B 3Πg

    v'-v''0-2

    ...587 588 589 590 591 5920.00.20.4

    Inte

    W l th [ ]366 368 370 372 374 376 378 380 382

    0.0

    0.2 3-52-4

    Inte

    W l th [ ]

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 28

    ..

    Appearance depends on spectral resolution !Wavelength [nm] Wavelength [nm]

  • Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    Spectroscopic system

    OpticsSpectrometerDetector

    h t lti lifocus lengthspectral resolution Δλ fibphotomultiplier

    λ scanΔλ, Δtdiode array

    spectral resolution Δλgratingspectral resolutionBlaze - intensity

    fibrevery flexibleVIS: glass, quartz, UV enhanceddiode array

    λ rangeCCD, ICCDpixel size - Δλ

    yslitsentrance slit Δλexit slit - detector

    UV enhancedlens and apertureImaging opticssolid angle

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 29

    pixel size - Δλintensity

    g

    Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    The spectroscopic system is determined by the purpose !

    time resolution detector Δλ

    Imax

    nsity

    time resolution detectorspatial resolution detector, lines of sightintensity detector, spec., optics In

    tens

    ity

    Δλ

    Inte

    n

    TimeLOS

    1 spectral resolution detector, spec., opticsWavelength]

    1

    4y

    survey spectrometer pocket size Δλ ≈ 1-2 nm1m spectrometer good optics Δλ ≈ 20 pm

    Echelle spectrometer high resolution Δλ ≈ 1-2 pm line profilepline shift

    line monitoringvery simpleΔt, poor Δλ

    relative intensitiescommon techniquepoor Δt, Δλ, Δx, flexible

    absolute intensitiesexpensive techniquepoor Δt, Δλ, Δx, flexible

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 30

    Δt, poor Δλ

    less informationpoor Δt, Δλ, Δx, flexible

    moderate informationpoor Δt, Δλ, Δx, flexible

    powerful tool

  • Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    Calibration of the spectroscopic system

    Wavelength: pixel → nmt l l l λ t bl

    Radiance - Intensityt W/ 2/ h/ 2/

    Calibrated spectrum810

    Ulbricht sphere

    spectral lamps, plasma, λ tables counts → W/m2/sr, ph/m2/s

    Calibrated spectrumspectral radiance [W/m2/sr/nm]

    0246

    Measurement i t it [ t / ]

    0.20.30.4 x106

    0

    intensity [counts/s]

    Conversion factor100x10-6

    0.00.1

    spectral sensitivity

    ⎥⎦

    ⎤⎢⎣

    ⎡=×⎥

    ⎤⎢⎣

    ⎡( t / )

    photons4( t / )W

    22 hλπ

    1

    10

    100

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 31

    exposure time

    ⎥⎦

    ⎢⎣

    ⎥⎦

    ⎢⎣ (counts/s)nmsm(counts/s)nmsrm

    22 hc400 500 600 700 8001

    Wavelength [nm]

    Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    From intensity to plasma parameterpkpk ApnI )(=

    Population modelsPopulation modelsBoltzmann distribution (TE, LTE)

    Collisional radiative modelhigh ne relevance

    of photonsCorona equilibriumlow ne

    p

    Rate equations for excitation and de-excitation processesEnergyαβS

    ...

    . Ion....

    αβSelectron impact excitation and de-excitationabsorption and emission, heavy particle collisions, ....

    n(p)

    AX

    0)()(d

    )(d11 =−= ∑k pkepe ApnTXnnt

    pnCorona model

    rate coefficientemission rate coefficient

    01 nn ≅

    n(k)≈

    X

    ∑== k pkpkeppkepkepk AATXXwithTXnnI /)()( 10emission rate coefficient

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 32

    n(1)

    ≈CR model ,...),(0 ee

    effpkepk nTXnnI =

  • Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    Electron temperature from absolute line emission

    ,...),(0 eeeffpkepk nTXnnI =

    n0, ne known e

    pkee

    effpk nn

    InTX

    0,...),( =

    0 eepkepk

    e0

    s-1 ]

    Find suitable gases and diagnostic lines

    admixture of small amount

    17

    10-16

    ficie

    nt [m

    3 of diagnostic gas

    prominent example: Ar

    10-18

    10-17

    + 1 eV- 0.5 eVra

    te c

    oeffVery sensitive for low Te !

    10-19

    10 18 0.5 eV+- 0.1 eV

    ArI 750 nm

    Em

    issi

    on

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 33

    1 2 3 4 5 6 7 810Electron temperature [eV]

    Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    Electron temperature from line ratio (relative calibration)

    )(111

    epkepk TXnnI = → ratio of rate coefficients)(22

    2epkepk TXnnI

    = → ratio of rate coefficientsfor known densities

    Find suitable gases and diagnostic lines

    1000

    nts

    Find suitable gases and diagnostic lines

    n1, n2 inert gases or n1 = n2

    Ι undisturbed lines Ar750/He728

    coef

    ficie

    nΙpk undisturbed lines

    ground state excitation

    X ti d d T 100

    o of

    rate

    cXpk ratio depends on Te

    1 2 3 4 5 6 7 8 9 1010

    Rat

    io

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 34

    1 2 3 4 5 6 7 8 9 10Electron temperature [eV]

  • Emission spectroscopy: ns, Ts→ e, H, H2, H⎯ U Fantz et al Nucl Fusion 49 (2009) 125007

    Actinometry: density ratio from line ratio (relative calibration)

    )(111

    epkepk TXnnI = → ratio of densities

    U. Fantz et al., Nucl. Fusion 49 (2009) 125007

    )(222

    epkepk TXnnI=

    density ratio (n /n )

    → ratio of densitiesfor known rate coefficients

    T d d li ibl

    10

    nts

    density ratio (nH/nH2)

    )()(

    22

    434434H

    eH

    eHH

    H

    TXnnTXnn

    II

    = independenton ne

    Te dependence negligible

    1

    10Ar750 / H2Ful

    H / H2coe

    ffici

    en

    density ratio (nAr/nH2)

    )(2 eFuleHFul TXnnI e

    ArAr

    0 1

    1 Hγ / H2Ful

    H / Aro o

    f rat

    e )()(

    2

    2

    2

    750750

    eHFuleH

    eAr

    eArHFul

    Ar

    TXnnTXnn

    II

    =

    0.01

    0.1 Hγ / Ar750

    Rat

    io

    density ratio (nHe/nH2)dependence on Te (factor of 10)

    needs iteration

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 35

    1 2 3 4 5 6 7 8910 20 30Electron temperature [eV]

    →needs iteration

    Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    Particle density from absolute line emission

    ,...),(0 eeeffpkepk nTXnnI =

    ne, Te known ,...),(0 eeeffpke

    pk

    nTXnI

    n =

    0 eepkepk

    , ),( eepkeKnowledge of dominant excitation mechanism is essential !

    C 852

    Example: Cs and Cs+ lines

    )(852852 eCs

    eCsCs TXnnI =Cs:

    13

    10-12Cs: 852 nm

    [m3 /

    s]

    needs ne, almost independent of Te

    )(460460 eCs

    eCsCs TXnnI

    +

    +

    +

    =Cs+: 10-14

    10-13

    Cs+: 460 nmBorn approximationoe

    ffici

    ent

    ×1000

    )(460460 eeCsneeds ne, strong dependence on Te

    Cs

    10-15

    10 pp

    Rat

    e co

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 36

    1 2 3 4 5 6 7 8 9 10Electron temperature [eV]

  • Example: emission spectroscopy at the ion source

    Survey spectrometer and on-line monitoring

    H H i iti

    2000

    3000Hβ Hα

    Cs[cou

    nts] impurities

    Cs, Cs+

    H from Hγ

    0

    1000

    2000

    H2Hγ

    Inte

    nsity

    Cu, OH, N2 OCs+

    γ

    ne from Hβ/Hγne, Te diagnostic gas ArH¯ from H /H

    300 400 500 600 700 800 9000I

    Wavelength [nm]

    H from Hα/Hβ

    3000#19800Hδ

    50

    ]

    2000 #19776#19788

    #19800δ

    [cou

    nts]

    30

    40

    50

    4

    6Cs852

    y [c

    nm

    /ms] #64623

    nal [

    V]

    Cs diodeIntensity Transmission

    Cs filter

    diode & filter

    0

    1000

    #19776

    Cs 852#19800#19788In

    tens

    ity

    10

    202

    4

    off

    ne in

    tens

    ityUex on D

    iode

    sig

    n

    Cs852

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 37

    0 1 2 3 4 5 60 #19776

    Time [s]0 2 4 6 8 10 12

    0 0

    Time [s]

    Lin ex

    820 840 860 880

    Example: emission spectroscopy: ns→ H⎯ U Fantz D Wünderlich

    A novel diagnostic technique for H¯ volume densityU. Fantz, D. Wünderlich

    NJP 8 (2006) 301

    Population mechanisms for H

    H(n)

    H+ H2+recombinationdissociativerecombinationH2

    +/H < 10-3Te > 1 eV

    Line ratios depend on ne, Te and H¯/H( )

    H H2H-

    dissociativeexcitation

    effectiveexcitation

    H/H2 > 0.1

    100 5x10171017

    Te = 3 eV

    p e, e

    H+ + H¯ → H(n=3) + H

    mutual neutralisation 100

    ne [m-3]

    5x101810185x1017

    Hα/Hβ

    ratio

    Collisional radiative modelHα 10 5x1018

    10185x10171017

    e [ ]

    H /H

    Line

    10-5 10-4 10-3 10-2 10-1 11

    Hβ/Hγ

    D it ti H-/H

    Measurement of Balmer line ratiosH /Hβ depends on H¯/H

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 38

    Density ratio H-/HHα/Hβ depends on H /HHβ/Hγ reflects ne and Te

  • Example: emission spectroscopy: ns→ H⎯ U Fantz D Wünderlich

    A novel diagnostic technique for H¯ volume densityU. Fantz, D. Wünderlich

    NJP 8 (2006) 301

    Population mechanisms for H143 /

    s]

    H /H

    H(n)

    H+ H2+recombinationdissociativerecombinationH2

    +/H < 10-3Te > 1 eV 12

    14

    019

    ph/m

    3 Hα/Hβ

    ( )

    H H2H-

    dissociativeexcitation

    effectiveexcitation

    H/H2 > 0.18

    10

    ensi

    ty [

    10 extraction

    H+ + H¯ → H(n=3) + H

    mutual neutralisation

    4

    6H-

    H /Ho, l

    ine

    inte

    Hβ/Hγ

    Collisional radiative modelHα

    0

    2Hα/Hβ Hγ

    Lin

    e ra

    tio

    H2, 80kW, 0.65Pa

    Measurement of Balmer line ratiosH /Hβ depends on H¯/H high H /Hβ ratio: H¯ = 1×1017 m-3

    -1 0 1 2 3 4 50

    Time [s]

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 39

    Hα/Hβ depends on H /HHβ/Hγ reflects ne and Te

    high Hα/Hβ ratio: H 1×10 mstable Hβ/Hγ ratio, i.e. stable ne and Te

    Emission spectroscopy: ns, Ts→ e, H, H2, H⎯

    Powerful diagnostic tool identification of speciesparticle densities

    ?? simple equipmentparticle densities particle temperatureson-line monitoring??insight in plasma processesspatial resolution by several lines of sight

    in-situ, non-invasive

    several lines of sight

    ,

    line of sight averaged resultsAnalysis

    based on atomicand molecular physics

    simple quite complext d b

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 40

    supported bycollisional radiative models

  • Summary

    The three W’s

    Plasma Diagnostics of Ion Sources

    e t ee sWhat do I want to know ? → and why?What is the adequate technique ? → effort versus gain!Wh t i th ibilit f th ? f ibilit !What is the accessibility of the source ? → feasibility !

    The three examplesL i b φ T (EEDF)Langmuir probes → φpl, ne, Te, (EEDF) Absorption techniques → nspecies → Cs, H⎯ Emission spectroscopy→ ns, Ts→ e, H, H2, H⎯ p py s s 2

    The three “keep-in-mind’s”Monitoring versus quantification → trends or full informationg qSpatial resolution → averaged or x-resolved (step width!))Temporal resolution → averaged or t-resolved (time scale!)

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 41

    Diagnostics – The Window to the Knowledge !

    References

    [1] F. F. Chen, J.P Chang, Lecture Notes on Principles of Plasma Processing(Kluwer/Plenum, 2003)

    [2] M. Lieberman, A. Lichtenberg, Principles of Plasma Discharges and Materials Processing(Wiley,1994)

    [3] B. Chapman, Glow Discharge Processes (Wiley, 1986)

    [4] R. Hippler, S. Pfau, Low Temperature Plasma Physics (Wiley, 2001)

    [5] D. Flamm, O. Auciello: Plasma Diagnostics, Volume 1 (Academic Press, Inc., 1989)

    [6] I.H. Hutchinson: Principles of Plasma Diagnostics (Cambridge University Press, 1987)

    [7] A. Thorne, Spectrophysics: Principles and Applications (Springer 1999)

    [8] http://www ee ucla edu/~ffchen/[8] http://www.ee.ucla.edu/~ffchen/

    [9] U. Fantz: Basics of plasma spectroscopyPlasma Sources Sci. Technol. 15 (2006), 137

    [10] U. Fantz: Emission Spectroscopy of Molecular Low Pressure PlasmasContrib. Plasma Phys. 44 (2004) 508

    CAS on Ion Sources, 6th June 2012Ursel Fantz, p. 42