» Plasma Diagnostics
From the experimental point of view, the characteristics of the discharge can be determined with the implementation of suitable diagnostic tools.
- Mass/Energy Analyser: an online mass spectrometer samples ions and neutral radicals from the discharge gas phase. The spectrometer can measure the composition and the energy distribution of charged species in the plasma as well as the composition of the neutral gas phase. Through a movable sampling orifice it allows to measure the spatial distribution, as well as the temporal evolution of the gas-phase composition directly in the region where material processing takes place.
Langmuir probe diagnostics: through the measurement of the voltage-current characteristics of carefully designed electrostatic probes it is possible to calculate plasma parameters such as the electron and ion density, the electron temperature and the plasma potential. Multi-pin probes allow to measure particle flux and the electric field inside the discharge region. An innovative Fast Sweep Langmuir probe has been developed by the research team, allowing the measurements of plasma parameters at frequencies up to 200 kHz. Such diagnostics works fine in low pressure plasmas, although operation in radio-frequency discharges requires suitable design for RF filtering [1]
OES (Optical Emission Spectroscopy): discharge spectra displays the emission lines of many of the reactive species produced in the plasma state. While identification is straightforward, a quantitative measure of concentration requires a detailed modelling of the excitation dynamics. A fast response (300 ps) photodiode is used for high frequency micro-discharges detection [2].- Electric probes: high voltages and high frequency electric fields require suitable electrical probes. Measurement of the discharge voltage and current can be performed both in DC and in radio-frequency discharges. The discharge voltage can be measured with commercially available HV probes, whereas Rogowski coils constitutes non-intrusive current probes useful to investigate plasmas. In particular, in atmospheric pressure sources the discharge regime often consists of intermittent channels where current flows for a few tens of nanoseconds (“plasma microdischarges”). At PlasmaPrometeo Rogowski coils suitable for the detection of these high-frequency current signals are adopted [3]. Source impedance and transmission line adaptation can be measured too [4].
- Photomultiplier tube: Fast Photomultiplier Tubes (PMTs) allow studying the temporal evolution of the light intensity emitted by plasmas. This is particularly useful when dealing with atmospheric pressure plasmas, where the light signal often appears as a series of pulses lasting some nanoseconds, because of the dynamics of the fast and intermittent “plasma microdischarges” that distinguish these discharge regimes [5].
» Further information is available in papers published in scientific and popular reviews:
[1] “A 400 kHz, fast-sweep Langmuir probe for measuring plasma fluctuations”
G.Chiodini, C.Riccardi and M.Fontanesi,
Review of Scientific Instruments 70 (6), 2681-2688 (1999).
[2] “Wettability and dyebility modulation of poly(ethylene terephthalate) fibres through cold SF6 plasma treatment”,
R.Barni, C.Riccardi, E.Selli, M.R.Massafra, B.Marcandalli, F.Orsini, G.Poletti and L.Meda,
Plasma Processes and Polymers 2, 64-72 (2005).
[3] “On the use of Rogowski coils as current probes for atmospheric pressure dielectric barrier discharges.”
Biganzoli,; Barni,; Riccardi,
Review of scientific instruments, 84, pp. 016101.1 – 016101.3, 2013, ISSN: 0034-6748.
[4] “Experimental study and simulations of electronegative discharges at low pressure”
C.Riccardi, R.Barni and M.Fontanesi,
Journal of Applied Physics 90 (8), 3735-3742 (2001).
[5] “Temporal evolution of a surface dielectric barrier discharge for different groups of plasma microdischarges.”
Biganzoli,; Barni,; Riccardi,
Journal of physics. D, Applied physics, 46 (2), pp. 025201 – 025211, 2013, ISSN: 0022-3727.
