Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator...

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Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to the Knowledge Langmuir probes: φ pl , n e , T e Absorption techniques: n s Cs, Hˉ Emission spectroscopy: n s , T s e, H, H 2 , Hˉ Monitoring and Quantification – Spatial and Temporal Resolution Universität Augsburg Max-Planck-Institut für Plasmaphysik [email protected]

Transcript of Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator...

Page 1: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

Plasma Diagnostics of Ion Sources Ursel Fantz

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

Diagnostics – The Window to the Knowledge

Langmuir probes: φpl, ne, Te

Absorption techniques: ns → Cs, Hˉ

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

Monitoring and Quantification – Spatial and Temporal Resolution

Universität Augsburg Max-Planck-Institut für Plasmaphysik

[email protected]

Page 2: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Preliminary considerations

What do I want to know?

identify the quantity (and the reason for it) define the required precision temporal behaviour and required time resolution necessity for spatial resolution …

Accessibility of the ion source?

diagnostic ports test stand or continuous operation risks and feasibility reliability …

Adequate diagnostic technique?

extensive or simple setup data acquisition and evaluation reliability costs and time (manpower) needed …

Page 3: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

General plasma diagnostics techniques

Method Standard Sophisticated Extras Langmuir probe single probe

(cylindrical or planar) double or triple

probe emissive probe

special method: Boyd-Twiddy

emission spectroscopy

optical wavelength range with fibre optics & survey spectrometer

extended wavelength range

VUV, UV or IF

sophisticated system spectral resolution,

type of detector absorption

spectroscopy white light absorption

technique tuneable laser

absorption cavity-ringdown spectroscopy

Laser methods laser induced fluorescence

TDLAS

mass spectrometry

residual gas analyser energy resolved mass spectrometry

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

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

Page 4: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Example case: IPP ion source for negative hydrogen ions

ICP: f = 1 MHz, P = 70 kW, p = 0.3 Pa

∅ = 24 cmℓ = 15 cm 32×59×23 cm3

∅ = 24 cmℓ = 15 cm 32×59×23 cm3

source bodyat 50°C

grid at150°C

oven at> 120°C

6s plasma (4s beam), every 3 min: BATMAN cw, up to 1 hour, every 3 min: MANITU

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

H2(v) + e- → Hˉ + H

H2(B,C)

dissociative attachment H2‾

surface process H, Hx

+ + surface e- → Hˉ

caesium layer low work function

H‾ formation and losses ...

volume process

destruction processes Hˉ + e- → H + 2e- Hˉ + H+ → H + H Hˉ + H → H + H + e-

... determine source optimisation

Page 5: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Example case: sources for negative hydrogen ions

Ion sources for negative hydrogen ions:

gas feed

Filter field

N

S

Driver ∅ 24cm

Expansion region

Extraction region

e Hx

+

H Hˉ

RF power 100 kW

p = 0.3 Pa

≈ 10 mT

Plasma generation ionising plasma ionisation: α ≈ 0.1 dissociation: δ ≈ 0.3 Te ≈ 10 eV, ne ≈ 5×1018 m-3

H‾ generation recombining plasma Te ≈ 1 eV, ne ≈ 5×1017 m-3

Hˉ/ne ≈ 0.1 – 5 Cs+/ne ≈ 0.01 – 0.1

Cs evaporation

H, Hx+ + surface e- → Hˉ

Cs evaporation → low work function

focus of diagnostics (and modelling)

on plasma close to the grid

Main issues Production and destruction of negative ions Extraction of negative ions Reduction of co-extracted electrons

ionising – recombining plasma

Page 6: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Langmuir probes: φpl, ne, Te, (EEDF)

Main principle

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

probe tip (tungsten wire)

insulator (ceramics)

voltage supply (-50 V to + 50 V)

plasma chamber

resistor

voltage measurement

stick a wire into the plasma tungsten, ∅≈ 100 µm, l = 1 cm

choose a reference electrode potential of plasma chamber

apply a variable voltage typ. from - 50 V to + 50 V

I – V characteristics

Page 7: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Langmuir probes: φpl, ne, Te, (EEDF)

Main principle

probe tip φprobe

plasma chamber

φfloating

φplasma

5 10 15 20 25-202468

10121416

transition

electronsaturation

prob

e cu

rrent

[mA]

probe voltage [V]

ion saturation

φfl

φpl

ne ni

Te

EEDF cylindrical probe

Page 8: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Langmuir probes: φpl, ne, Te, (EEDF)

Floating potential

5 10 15 20 25-202468

10121416

transition

electronsaturation

prob

e cu

rrent

[mA]

probe voltage [V]

ion saturation

φfl

same fluxes for ions and electrons Γions = Γelectrons

same currents

no probe current

Iprobe = 0 → φfl

Page 9: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

5 10 15 20 25

-1

0

1

2

seco

nd d

eriva

tive

[mA

/ V2 ]

probe voltage [V]

φpl

Langmuir probes: φpl, ne, Te, (EEDF)

Plasma potential

5 10 15 20 25-202468

10121416

transition

electronsaturation

prob

e cu

rrent

[mA]

probe voltage [V]

ion saturation

φpl

determined by ambipolar diffusion turning point of I-V characteristics zero-crossing of second derivative

d2 I / d φpr2 → φpl

for curves with high noise level crossing of linear fits to electrons saturation current and transition close to turning point

Page 10: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

5 10 15 20 25

-1

0

1

2

seco

nd d

eriva

tive

[mA

/ V2 ]

probe voltage [V]

EEPF

φpl

0 1 2 3 4 5 610-4

10-3

10-2

10-1

100

EEP

F [e

V-3/2

]

energy [eV]

slope → Te

Langmuir probes: φpl, ne, Te, (EEDF)

Electron energy distribution function (EEDF) and electron temperature

5 10 15 20 25-202468

10121416

transition

electronsaturation

prob

e cu

rrent

[mA]

probe voltage [V]

ion saturation

φpl

distinguish: EEDF = sqrt(E) × EEPF

Te

EEDF

e.g.: Maxwell function probability function

plot log(Ipr) versus E = φpl − φpr

Te also from potential difference φpl−φfl = kbTe × ln(sqrt(mi/(2πme))) ≈ 2-3 × Te

d(ln I ) / dE) = e / ( kbTe )

for both: normalisation

slope yields Te for Maxwell EEDF

Page 11: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Langmuir probes: φpl, ne, Te, (EEDF)

The electron density

electron saturation current

5 10 15 20 25-202468

10121416

transition

electronsaturation

prob

e cu

rrent

[mA]

probe voltage [V]

ion saturation

φpl

ne

effeesate AvenI41

, =

problem: effective probe area due to increase of plasma sheath

take current at plasma potential → Aeff = Aprobe

eB

e

prpr

ple Tk

melr

In

πφ

2)(

=

needs Te

Probe geometry influences shape of electron saturation current cylindrical probe (standard case) planar probe → Apr >> sheath spherical probe

Page 12: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Langmuir probes: φpl, ne, Te, (EEDF)

Ion density (positive ions)

ion saturation current

basically three theories available OML: Orbital-Motion-Limited ABR: Allen-Boyds-Reynolds BRL: Bernstein-Rabinowitz-Langmuir

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

ni

5 10 15 20 25-202468

10121416

transition

electronsaturation

prob

e cu

rrent

[mA]

probe voltage [V]

ion saturation

simplest case: OML (rpr /rsheath < 3)

i

eBprii m

TkeAnI = needs Te

needs mi often unclear, e.g. hydrogen → H+, H2

+, H3+

choose proper φpr

guide line: φ = φpl − 10 × kTe

check if ne = ni is fulfilled

Isat, i at fixed φpr: useful as monitor signal

Page 13: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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 magnitude average, smoothing, filtering

RF field → oscillating φpl

measured curve ≠ real curve

magnetic field → gyro motion use Iion instead of Ie

negative ions → Ineg.ion instead of Ie for same mass Isat,ion = I sat,neg. ion → symmetric curves

prob

e cu

rren

t

probe voltage -20 -10 0 10 20 30

measured curve

real curve

oscillation

RF compensation active or passive

For monitoring or quantification with spatial and temporal resolution !

Page 14: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Langmuir probes: φpl, ne, Te, (EEDF)

Double probe

-40 -20 0 20 40

-2

-1

0

1

2

transition region

ion saturationprobe 1

Ii2

curre

nt [m

A]

voltage between the tips [V]

Ii1

ion saturationprobe 2

probe 1

probe 2

A1

A2

volta

ge

pla

sma

two probe tips of same size distance > 2 Debye length

voltage between the probes both probes are floating no reference potential needed

compatible with quartz or ceramic chamber and RF field

symmetric curve: ion saturation current

i

eB

satii

mTkeA

In ,=

2,,

,−+ +

= isatisatsati

III

Te from transition region

Page 15: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

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

Comparison of EEDF

0 5 10 15 20 25 3010-3

10-2

10-1

Langmuir probewithout RF compensation

f (E)

[eV-3

/2]

Energy [eV]

Boyd-Twiddy

Vpl= 21.5 V

MaxwellTe =6.6eV

d=5cmH2, 80 kW, 0.36Pa

driver exit

plasma grid 10-3

10-2

10-1

100

0 5 10 15 20 25 30

H2,80kW, 0.36Pa

Te=1.2eVTe=1.4eV

Te=2.9eVTe=6.6eV

f(E) [

eV-3

/2]

Energy [eV]

Te=14eV

distan

ce [c

m]

05

914

19

Maxwellian EEDF

voltage ramp is superimposed by AC modulated signal measure frequency spectrum of probe current

Boyd-Twiddy method: direct measurement of EEDF

McNeely et al. PSST 18 (2009) 014011

Sophisticated Langmuir probe system for RF ion sources

Page 16: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

CAS on Ion Sources, 6th June 2012 Ursel Fantz, p. 16

All about photons: emission and absorption

Radiation of low temperature plasmas

He pink

Ne red

N2, air

orange

H2, H

purple

Colourful plasmas ! Neutrals atoms and molecules Ions single charged Electrons ne << nn

a + ef → a* + es → a + hν + es

collisions and spontaneous emission

electronically excited state

radiation

level q

level p

e impact excitation

Emission of light in the UV-VIS-IR

Page 17: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

All about photons: emission and absorption

Emission versus absorption spectroscopy

photon energy E = hν wavelength λ = (Ep-Ek)/hc Einstein coefficients Apk, Bkp

Emission passive

Absorption active

excited state

λpk, Apk

q

p

impact excitation k

λpq, Apq

excited state

λpk, Bkp

q

p

k

radiation field Lλ λpq, Bqp

806 808 810 812 814 816

g [ ]

Wavelength [nm]

Inte

nsity

Wavelength [nm]

806 808 810 812 814 816

Inte

nsity

VIS: simple equipment information of upper level p

expensive equipment information of lower level q

Page 18: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Main principle of line absorption

light source detector optics

absorbing medium

white light tunable laser

vacuum or plasma spectrometer with CCD diode with filter

Non-invasive and line of sight integrated method !

=

),()0,(ln1)(

lII

lk

λλλ

[ ]lIlI )(exp)0,(),( λκλλ −=Absorption in a medium with path length l

)(λκabsorption coefficient statistical weights gi, gk

with Ladenburg relation π

λλλκ8

)(4

0 ik

k

ik

lineki

Acg

gnd =∫

=

lineiki

kk d

lII

lAggcn λ

λλ

λπ

),()0,(ln18

40

Density of lower state ground state, metastables

Page 19: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

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

852.105 852.110 852.125 852.130 852.135

FF

34

44

54

2333

T = 1000 K

Inte

nsity

Wavelength [nm]

43

sum of Doppler-broadenedhyperfine lines

take line broadening into account

six hyperfine lines overlap to two peaks: ∆λ ≈ 21 pm

mTk

cB

FWHMD2ln8

,λλ =∆

Doppler profile

852.080 852.100 852.120 852.140

0.0

0.5

1.0

1.5

0.00

0.05

0.10

0.15

Sign

al w

hite

light

abs

orpt

ion

Sign

al L

aser

abs

orpt

ion

Wavelength [nm]

Laserabsorption

white lightabsorption

appearance depends on technique

apparatus profile (spectrometer) Doppler profile

white light absorption ↔ laser absorption

Page 20: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Example: Cs line at 852.1 nm white light absorption versus laser absorption

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

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

being perfectly in the range required for the ion sources

851.6 851.8 852.0 852.2-0.02

0.00

0.02

0.04

nCs = 1.4 x 1015 m-3

Abso

rptio

n [a

. u.]

Wavelength [nm]

white light absorption

0.0 0.1 0.2 0.3 0.4

0.000

0.005

0.010

0.015

0.020

Abso

rptio

n [a

. u.]

Rel. wavelength [a. u.]

laser absorption

nCs = 3.5 x 1013 m-3

Page 21: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Straightforward analysis

=

lineiki

kk d

lII

lAggcn λ

λλ

λπ

),()0,(ln18

40

... but ....

Line saturation

strong absorption: nk × l correction factors by profile fitting

... and .... 0

2

4

measured signal

Gaussian fit

heavy absorption

mediumabsorption

Abso

rptio

n [a

. u.]

Wavelength

lowabsorption

0

1

(b)

Rel.

inte

nsity

(a)

Depopulation effect strong intensity attenuation of laser to ≈1% → trade-off with temporal resolution

in vacuum with plasma → subtract emission

Page 22: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Laser absorption for Cs in ion sources

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

ND-filter 2.0 interference filter 852 nm

lens f = 25.4 mm

beamdiameter11.5 mm

diodelaser

fiber105 µm

NA = 0.22

photodiode

fiber200 µm

NA = 0.22

lens f = 25.4 mm

ND-filter 2.0 interference filter 852 nm

lens f = 25.4 mm

beamdiameter11.5 mm

diodelaser

fiber105 µm

NA = 0.22

photodiode

fiber200 µm

NA = 0.22

lens f = 25.4 mm

Simple and robust setup for application to ion sources !

single mode laser, 150 mW λ = 851-853 nm with ∆λFWHM = 0.01 pm

Page 23: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

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

5450 5455 5460 54650

1

2

3

4

0

1

2

3

4

5

RF off

Cs e

miss

ion

[a. u

.]

Cs emission

Cs-d

ensit

y [1

015 m

-3]

Time [s]

RF on

Cs densitylaser absorption

BATMAN #68562

3 s before pulse: vacuum 6 s plasma with 4 s beam: plasma 90% of Cs is ionized

30 s after pulse: Cs peak & decay time

Page 24: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Cavity – Ringdown – Absorption – Spectroscopy → CRDS

high reflectivity mirrors R>99.9%

dcavity detector

transmitted signal

plasma with length l

laser

pumping of an optical cavity by a (tunable) laser source measurement of signal decay after laser source is switched off

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

0t

0 e(t) τ−⋅= II

)1(0 Rcd−

=τ; )0

1'

1(ld

cσ1n

ττ−⋅⋅

⋅=

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

Page 25: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Cavity – Ringdown – Absorption – Spectroscopy → CRDS

0 5 10

0.0

0.2

0.4

0.6

0.8

1.0

limited by time resolution

increasedensity

τ0 (reflectivity of mirrors)

norm

alize

d in

tens

ity

Time [µs]

empty cavity

)0

1'

1(ld

cσ1n

ττ−⋅⋅

⋅=

Hˉ density: line of sight averaged detection limit ≈ 1015 m-3

Hˉ = 5×1017 m-3 → t = 8 µs 0 50 100 150 200

0.2

0.4

0.6

0.8

1.0

plasma

τ = 47 µs

τ0 = 62 µssig

nal [

a.u.

]

µs

empty cavity (vacuum)

Example H‾ cross section: photodetachment

0.0 0.5 1.0 1.50

1

2

3

4

cros

sec

tion

σ [1

0-21 m

2 ]

wavelength [µm]

H- + hν → H + e

Nd-YAG laser

Page 26: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

The main principle

spectrometer detector optics

plasma

Non-invasive and line of sight integrated method !

UV and VUV: vacuum system IF: detector

πλε

4/)( hcApn pkpk =

Measures density of excited state ...

... which depends on plasma parameters !

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

Page 27: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

What information can be obtained from the line emission ?

499 500 5010.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

[a. u

.]

Wavelength [nm]

Imax n(p) Apk Intensity: plasma parameters

density and temperature of neutrals, ions, electrons

insight in plasma processes

∆λ Tgas Doppler broadening: particle temperature

Line profile: broadening mechanism

λ0 Element

Wavelength: species

Wavelength shift: particle velocity

⇒=∫ 1λλ dPline

λλ εε Ppk=

Page 28: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

selection rules

Radiation

. . .

.

n(k)

Ion

n(p)

. . .

.

Energy

. . .

pkA

1,0 ±=∆L

0=∆S

Atoms

1,0 ±=∆J

Appearance depends on spectral resolution !

587 588 589 590 591 5920.00.20.40.60.81.01.2

3 2P1/2 - 3 2S1/2

Inte

nsity

[a.u

.]

Wavelength [nm]

Na D-line

3 2P3/2- 3 2S1/2

366 368 370 372 374 376 378 380 3820.0

0.2

0.4

0.6

0.8

1.0

3-52-4

1-3

Inte

nsity

[a.u

.]

Wavelength [nm]

Nitrogen N2C 3Πu - B

3Πgv'-v''

0-2

SLS Ln +

+12 . . .

.

. . . .

.

.

.

.

n(p), v’, J’

n(k), v’’, J’’

Ion

Radiation '''''' JJvvpkA

Molecules

0=∆Σgu ↔

1,0''' ±=∆=− JJJP, Q, R branch

Σ+Λ+ Λ12S −+,

,ug

Page 29: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Spectroscopic system

Optics Spectrometer Detector

photomultiplier λ scan ∆λ, ∆t diode array λ range CCD, ICCD pixel size - ∆λ intensity

focus length spectral resolution ∆λ grating spectral resolution Blaze - intensity slits entrance slit ∆λ exit slit - detector

fibre very flexible VIS: glass, quartz, UV enhanced lens and aperture Imaging optics solid angle

Page 30: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

The spectroscopic system is determined by the purpose !

survey spectrometer pocket size ∆λ ≈ 1-2 nm 1m spectrometer good optics ∆λ ≈ 20 pm Echelle spectrometer high resolution ∆λ ≈ 1-2 pm

time resolution detector spatial resolution detector, lines of sight intensity detector, spec., optics spectral resolution detector, spec., optics

line profile line shift

line monitoring very simple ∆t, poor ∆λ

less information

relative intensities common technique poor ∆t, ∆λ, ∆x, flexible

moderate information

absolute intensities expensive technique poor ∆t, ∆λ, ∆x, flexible

powerful tool

Inte

nsity

Wavelength]

∆λImax

Inte

nsity

TimeLOS

1

4 y

Page 31: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Calibrated spectrum spectral radiance [W/m2/sr/nm]

Measurement intensity [counts/s]

Conversion factor spectral sensitivity

exposure time

(counts/s)nmsm

photons4(counts/s)nmsrmW

22 hcλπ

400 500 600 700 8001

10

100 x10-6

Wavelength [nm]

0.00.10.20.30.4

x10602468

10

Ulbricht sphere

Calibration of the spectroscopic system

Wavelength: pixel → nm spectral lamps, plasma, λ tables

Radiance - Intensity counts → W/m2/sr, ph/m2/s

Page 32: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

From intensity to plasma parameter pkpk ApnI )(=

Population models Boltzmann distribution (TE, LTE)

Collisional radiative model Corona equilibrium

high ne

low ne

relevance of photons

n(1)

. . .

.

n(k)

Ion

n(p)

. . .

.

Energy α β S

A X

Rate equations for excitation and de-excitation processes electron impact excitation and de-excitation absorption and emission, heavy particle collisions, ....

CR model ,...),(0 eeeffpkepk nTXnnI =

0)()(d

)(d11 =−= ∑k pkepe ApnTXnn

tpn

∑== k pkpkeppkepkepk AATXXwithTXnnI /)()( 10

Corona model rate coefficient

emission rate coefficient 01 nn ≅

Page 33: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Electron temperature from absolute line emission

n0, ne known e

pkee

effpk nn

InTX

0,...),( =

,...),(0 eeeffpkepk nTXnnI =

1 2 3 4 5 6 7 810-19

10-18

10-17

10-16

+ 1 eV- 0.5 eV

+- 0.1 eVArI 750 nm

Emiss

ion

rate

coe

fficie

nt [m

3 s-1

]

Electron temperature [eV]

Find suitable gases and diagnostic lines

admixture of small amount of diagnostic gas

prominent example: Ar

Very sensitive for low Te !

Page 34: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Electron temperature from line ratio (relative calibration)

)()(

22

11

2

1

epke

epke

pk

pk

TXnnTXnn

II

= → ratio of rate coefficients for known densities

1 2 3 4 5 6 7 8 9 1010

100

1000

Ar750/He728

Ratio

of r

ate

coef

ficie

nts

Electron temperature [eV]

Find suitable gases and diagnostic lines

n1, n2 inert gases or n1 = n2

Ιpk undisturbed lines

ground state excitation

Xpk ratio depends on Te

Page 35: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Actinometry: density ratio from line ratio (relative calibration)

)()(

22

11

2

1

epke

epke

pk

pk

TXnnTXnn

II

= → ratio of densities for known rate coefficients

density ratio (nH/nH2)

density ratio (nAr/nH2)

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

)()(

2

2

2

434434

eHFuleH

eH

eHHFul

H

TXnnTXnn

II

= independent on ne

)()(

2

2

2

750750

eHFuleH

eAr

eArHFul

Ar

TXnnTXnn

II

=

1 2 3 4 5 6 78910 20 300.01

0.1

1

10Ar750 / H2Ful

Hγ / H2Ful

Hγ / Ar750

Ratio

of r

ate

coef

ficie

nts

Electron temperature [eV]

Te dependence negligible

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

Page 36: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Particle density from absolute line emission

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

eeeffpke

pk

nTXnI

n =

,...),(0 eeeffpkepk nTXnnI =

Knowledge of dominant excitation mechanism is essential !

Example: Cs and Cs+ lines

)(852852 eCs

eCsCs TXnnI =

needs ne, almost independent of Te

)(460460 eCs

eCsCs TXnnI

+

+

+

=

needs ne, strong dependence on Te

Cs:

Cs+:

1 2 3 4 5 6 7 8 9 1010-15

10-14

10-13

10-12

Cs+: 460 nmBorn approximation

Cs: 852 nm

Rate

coe

fficie

nt [m

3 /s]

Electron temperature [eV]

×1000

Page 37: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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Example: emission spectroscopy at the ion source

Survey spectrometer and on-line monitoring

300 400 500 600 700 800 9000

1000

2000

3000

H2

Hβ Hα

Cs

Inte

nsity

[cou

nts]

Wavelength [nm]

Cu, OH, N2 O Cs+

impurities Cs, Cs+ H from Hγ ne from Hβ/Hγ

ne, Te diagnostic gas Ar Hˉ from Hα/Hβ

0 1 2 3 4 5 60

1000

2000

3000

#19776

#19776#19788

#19800

Cs 852

#19800

#19788Inte

nsity

[cou

nts]

Time [s]0 2 4 6 8 10 12

0

10

20

30

40

50

0

2

4

6

off

Time [s]

Cs852

HγLi

ne in

tens

ity [c

nm

/ms] #64623

Uex on Dio

de s

igna

l [V]

Cs diode

820 840 860 880

Intensity

Cs852

Transmission Cs filter

diode & filter

Page 38: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Example: emission spectroscopy: ns→ H‾

A novel diagnostic technique for Hˉ volume density U. Fantz, D. Wünderlich

NJP 8 (2006) 301

H+ + Hˉ → H(n=3) + H

mutual neutralisation

H(n)

H

H+ H2+

H2 H-

recombination dissociative recombination

dissociative excitation

effective excitation

H2+/H < 10-3

H/H2 > 0.1

Te > 1 eV

Collisional radiative model Hα

Population mechanisms for H

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

10

100

5x10181018

5x10171017

ne [m-3]

5x101810185x10171017

Te = 3 eV

Hβ/Hγ

Hα/Hβ

Line

ratio

Density ratio H-/H

Line ratios depend on ne, Te and Hˉ/H

Measurement of Balmer line ratios Hα/Hβ depends on Hˉ/H Hβ/Hγ reflects ne and Te

Page 39: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Example: emission spectroscopy: ns→ H‾

A novel diagnostic technique for Hˉ volume density U. Fantz, D. Wünderlich

NJP 8 (2006) 301

H+ + Hˉ → H(n=3) + H

mutual neutralisation

H(n)

H

H+ H2+

H2 H-

recombination dissociative recombination

dissociative excitation

effective excitation

H2+/H < 10-3

H/H2 > 0.1

Te > 1 eV

Collisional radiative model Hα

Population mechanisms for H

high Hα/Hβ ratio: Hˉ = 1×1017 m-3 stable Hβ/Hγ ratio, i.e. stable ne and Te

-1 0 1 2 3 4 50

2

4

6

8

10

12

14

H-

Hα/Hβ Hγ

Lin

e ra

tio, l

ine

inte

nsity

[10

19 p

h/m

3 /s]

Hβ/Hγ

Hα/Hβ

Time [s]

extraction

H2, 80kW, 0.65Pa

Measurement of Balmer line ratios Hα/Hβ depends on Hˉ/H Hβ/Hγ reflects ne and Te

Page 40: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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

Powerful diagnostic tool

?

in-situ, non-invasive

simple equipment

line of sight averaged results Analysis

simple quite complex supported by

collisional radiative models

based on atomic and molecular physics

identification of species particle densities particle temperatures on-line monitoring insight in plasma processes spatial resolution by several lines of sight

Page 41: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

Summary

The three W’s What do I want to know ? → and why? What is the adequate technique ? → effort versus gain! What is the accessibility of the source ? → feasibility !

Plasma Diagnostics of Ion Sources

Diagnostics – The Window to the Knowledge !

The three examples Langmuir probes → φpl, ne, Te, (EEDF) Absorption techniques → nspecies → Cs, H‾ Emission spectroscopy→ ns, Ts→ e, H, H2, H‾

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

Page 42: Plasma Diagnostics of Ion Sources...Plasma Diagnostics of Ion Sources Ursel Fantz CERN Accelerator School, Senec, Slovakia 29 th May – 8 th June 2012 Diagnostics – The Window to

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

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/

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

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