DOE Plasma Science Center - Probe Measurements of ...doeplasma.eecs.umich.edu/files/Web_Demidov_Vladimir_2011...Probe Measurements of Electron Energy Distributions in Gas Discharge

Probe Measurements of

Electron Energy

Distributions in Gas

Discharge Plasmas,

Part 2

Valery Godyak1 and Vladimir I. Demidov2

1RF Plasma Consulting, Brookline, MA 02446, USA2West Virginia University, Morgantown, WV, USA

1

Plasma Science Center

Predictive Control of Plasma Kinetics

Outline

I. Introductory remarks

II. MIB probe

III. Instrumental functions

IV. More complex plasma: beyond the limitations of the Druyvesteyn method:

A. Higher pressures (plasmas with near-probe collisions)

B. Magnetic fields

C. Anisotropy

D. Plasma electron spectroscopy (PLES)

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Introductory remarks:

Development of novel diagnostics is one of the important tasks of the LTP Center.

The electric probe is seen as a simple and attractive instrument used many authors.

Sophisticated probe constructions allow measurements in different types of plasmas.

These probe constructions have not been yet fully exploited.

Magnetically insulated baffled (MIB) probe is an example of probe diagnostics, which has been developed by the LTP Center.

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Magnetically insulated baffled probes (MIB)

A MIB probe offers the advantages of direct measurements of the plasma properties, while being non-emitting and electrically floating.

The MIB probes can be used in

◦ technologically important LTP plasmas

◦ basic plasma research, and

◦ fusion related plasmas.

V. I. Demidov, M. E. Koepke, and Y. Raitses, Rev. Sci. Instrum. 81, 10 E129, 2010

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Multi-baffled probe design

Instrumental functions in probe measurements

The result of measurements of the EEPF is a convolution of the real EEPF and the instrumental function A:

H. Amemiya, Japan J. Appl. Phys. 15, 1767, 1976

V. I. Demidov and N. B. Kolokolov, Sov. Phys. Tech. Phys. 26, 533, 1981

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Measurements of instrumental functions

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A simple circuit allows measuring

instrumental functions

IV trace of the system

V. I. Demidov and C. A . DeJoseph, Rev. Sci.

Instrument, 76, 086105, 2005

The measured instrumental function

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Measured EEDs in argon-rf-afterglow plasma without

(dots) and with (solid line) an additional artificial maximum

(indicated by arrow). The gas pressure is 30 mTorr, the

repetition frequency is 400 Hz, and the time after current

interruption is 0.7 ms.

The measured instrumental function of

the SMARTProbe (1). The same function

in

the presence of potential oscillations with

an amplitude of 2.5 V (2).

The measured instrumental functions in afterglow plasma

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Instrumental function A(ε) measured in a neon-afterglow

plasma (1). The calculated function for the “clean” probe (2).

The calculated function for a probe with electron reflection

with reflection coefficients of 1-0.016 V-1 (3) and 1-0.056 V-1 (4).

An instrumental function obtained

from a probe with a dirty surface

V. I. Demidov, N. B. Kolokolov, and O. G. Toronov, Sov. Phys. Tech. Phys. 29, 230, 1984

Ne*+Ne*→Ne++Ne+ef

More complex plasma: beyond the limitations of the Druyvesteyn method

Higher pressures (plasmas with near-probe collisions)

Magnetic fields

Anisotropy

Plasma electron spectroscopy (PLES)

9



Magnetic fields

Anisotropy


10

Higher pressures (plasma with some near-probe collisions)

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These equations can be used in

a weakly-collisional plasma

J. D. Swift, Proc. Phys. Soc. London 79, 697, 1962

A. I. Lukovnikov, M. Z. Novgorodov, Brief.

Communications on Physics, 1971, #1, 27

Higher pressures (plasma with many near-probe collisions)

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Thin probe sheaths (sufficiently

high electron density) or arbitrarily

thick sheaths and vDe = const (e.g.,

in argon plasma)

He afterglow, 40 Torr

Y. B. Golubovsky, V. M. Zakharova, V. I. Pasunkin, and L. D. Tsendin, Sov. J. Plasma Phys. 7, 340, 1981.

General case pressure

13

Thin probe sheaths (sufficiently

high electron density) or arbitrarily

thick sheaths and vDe = const (e.g.,

in argon plasma)

Calculated ln(I”e) (left) and ln(-I’eΨ/ε) (right) for a

Maxwellian EEPF (Ψ = 1 (1), Ψ = 5 (2), Ψ = 20 (3), Ψ = 0.3

(4), Ψ = 1 (5), Ψ = 2 (6)) and the model Maxwellian EEPF

(dashed line)

M. A. Malkov, High Temp., 29, 180, 1991.

R. R. Arslanbekov, N. A. Khromov, and

A. A. Kudryavtsev, PSST 3, 528, 1994.



Magnetic fields

Anisotropy


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Strong magnetic fields

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Parallel probe:

Perpendicular probe:

Y. B. Golubovsky, V. M. Zakharova, V. I. Pasunkin, and

L. D. Tsendin, Sov. J. Plasma Phys. 7, 340, 1981.

Magnetic fields

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Arbitrary magnetic field:

The EEDF obtained by a probe in

the CASTOR tokamak edge plasma.

M. A. Malkov, High Temp., 29, 180, 1991.

R. R. Arslanbekov, N. A. Khromov, and

A. A. Kudryavtsev, PSST 3, 528, 1994.

V. I. Demidov, S. V. Ratynskaia, K. Rypdal, and

R. J. Armstrong, Phys. Plasmas, 6, 350, 1999.

Restriction for fast-sweeping probe:



Magnetic fields

Anisotropy


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Anisotropy (spherical probe)

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The Driuvesteyn formula

is valid and provide EEDF.

Information about angular

Distribution of ions is lost.

The EEPF in a low-pressure (0.1 Torr) hydrogen

constricted arc plasma at the discharge axis.

Y. M. Kagan, B. P. Lavrov, and R. I. Lyaguschenko, Sov. Phys. Tech. Phys. 22, 349, 1977.

Anisotropy (cylindrical probe)

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Ip’’ with respect to the potential V measured

at the discharge axis at a distance Z from

the cathode by probes in two mutually

perpendicular orientations. At Z > 2 mm,

I’’ is the same for both probes. The helium

pressure is 2.3 Torr, the discharge current

is 0.5 A.

V I Demidov, N B Kolokolov, A P Mezentsev, A S Mustafaev, Sov. J. Plasma Phys., 12, 866, 1986.

A P Mezentsev, and A Smustafaev, Sov. Phys. Tech. Phys., 30, 1319, 1985.

Anisotropy (general case)

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Coefficients fj in a helium low-pressure (0.5 Torr)

positive column: f0 (1), f1 (2), f2 (3), f3 (4) and f4 (5)

V. L. Fedorov, Sov. Phys. Tech. Phys. 30, 584, 1985

Modeling anisotropic EDF

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The polar diagram f(v) for electrons calculated for different

numbers of probe orientations K: K = 3 (1); K = 5 (2);

K = 7 (3); K = 9 (4); model function (5).



Magnetic fields

Anisotropy


22

Plasma Electron Spectroscopy

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Atomic and molecular processes in plasmas can change

and shape form of electron energy distribution functions (EEDF).

Due to this, measurements of the EEDF allow in principle

analyzing those processes and measuring densities of

participating particles. This principle

can be used for development of gas

analytical detectors.

Afterglow may be convenient for this

purpose: low electron temperature.

Plasma electron spectroscopy in afterglow

24

A method for analyzing the fine structures

of the energetic portion of the EEDF in

an afterglow plasma is known as plasma

electron spectroscopy (PLES) in afterglow (V. Demidov et al., Sov. Phys. J., 1987; RSI, 2002)

N. Kolokolov and A. Blagoev,

Physics-Uspekhi, 1993.

Measurements in

He/N2 mixture

Probe measurements of the EEDF in negative glow

25

A. N. Soldatov et al., Sov. Phys. J., 1974 C. A. DeJoseph, Jr. et al., Europhysics News, 2007

Electrons from plasma-chemical

processes are observable, but

poorly resolved.

Measurements in He plasma.

He*+He*→He++He+ef (14.4 eV)

He*+e→He+ef (19.8 eV)

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1. Technically simpler. Does not require temporal resolution.

As a result, the sensitivity is much higher.

2. Reduced influence of the ion current on the measurements.

As a result, the energy resolution is higher.

3. Much greater area.

As a result, the sensitivity is much higher.

4. Simpler to create small size plasmas

As a result, it is simpler to make micro-gas-detectors.

Benefits of the new approach

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The experimental device

Cathode (C)

Negative glow (NG)

Cylindrical Wall (W)

Faraday dark space (FDS)

Anode (A)

Demidov V.I., Adams S. F., Blessington J., Koepke M. E., and

Williamson J. M., Contributions to Plasma Physics, 50, 808, 2010.

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Ne*+e→Ne+ef (16.6 eV)

Ar*+e→Ar+ef (11.5 eV)

O+O-→O2+ef (3.6 eV)

Experiments in Ne, Ar and O2/Ar

Gas pressure: Ne (3 Torr),

Ar (0.5 Torr), and

Ar/O2 (0.5 Torr, 5% of Ar)

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Finally…

The goal of this review is to increase awareness of the problems

pertaining to the relationship between the actual plasma

parameters

and the probe experiment design. Main sources of error in

EEDF measurements, remedies to avoid EEDF distortions and

examples

of positive resolutions of the problems were presented here for

different types of gas-discharge plasmas. We also introduce the

reader to unconventional methods of electron-distribution

diagnostics in collisional, magnetized and anisotropic plasmas

that are still under development and remain a challenge for

budding scientists.

DOE Plasma Science Center - Probe Measurements of ...doeplasma.eecs.umich.edu/files/Web_Demidov_Vladimir_2011...Probe Measurements of Electron Energy Distributions in Gas Discharge

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