doe plasma science center - probe measurements of...
TRANSCRIPT
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
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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)
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More complex plasma: beyond the limitations of the Druyvesteyn method
Higher pressures (plasmas with near-probe collisions)
Magnetic fields
Anisotropy
Plasma electron spectroscopy (PLES)
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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
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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.
More complex plasma: beyond the limitations of the Druyvesteyn method
Higher pressures (plasmas with near-probe collisions)
Magnetic fields
Anisotropy
Plasma electron spectroscopy (PLES)
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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:
More complex plasma: beyond the limitations of the Druyvesteyn method
Higher pressures (plasmas with near-probe collisions)
Magnetic fields
Anisotropy
Plasma electron spectroscopy (PLES)
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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).
More complex plasma: beyond the limitations of the Druyvesteyn method
Higher pressures (plasmas with near-probe collisions)
Magnetic fields
Anisotropy
Plasma electron spectroscopy (PLES)
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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
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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
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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.