S Simon Hübner

Electron properties in an atmospheric helium plasma jet determined by Thomson scattering

In this work we present Thomson scattering measurements on a nanosecond pulsed high voltage dielectric barrier discharge (DBD)-like helium plasma jet, operated in ambient air. With the low detection limit offered by a triple grating spectrograph equipped with a high quantum efficiency intensified charge-coupled device (ICCD) camera, temporally and spatially resolved electron densities and mean energies have been mapped. 7 kV peak with 250 ns width pulses at 20 kHz are applied to the inner cylindrical shaped electrode of a DBD. This results in a peculiar hollow electron density profile in the vicinity of the jet nozzle with maximum values of ne = 5 × 1018 m−3 and mean energies of up to 2.5 eV. Further downstream, the profile collapses radially and contracts. A much higher electron density is found (2 × 1019 m−3) while the mean energy is lower (0.5 eV).

The radial contraction of argon microwave plasmas studied by Thomson scattering

Radial electron density ne(r) and temperature Te(r) profiles of a microwave argon plasma at intermediate pressure were investigated by Thomson scattering. This method allows one to get ne(r) and Te(r) spatially resolved without any a priori assumption on the shape of the profile. Data were acquired in the pressure range 5–88 mbar where a transition from wall-stabilized to a radially contracted plasma mode was observed.It was found that the fitting of the radial profile can be done with a Bessel function for which the boundary radius R defined by ne(R) = 0 is a free parameter. For pressures above 20 mbar the electron density profile undergoes radial contraction, so R goes down from 3 mm at 5 mbar (wall position) to 2.09 mm at 88 mbar. The electron temperature Te(r) on the other hand is flat in the centre and rises towards the wall. For low pressures, this rise is moderate but for pressures of 20 mbar and above the increase is more pronounced.

Experimental investigation of the electron energy distribution function (EEDF) by Thomson scattering and optical emission spectroscopy

The electron temperature of an argon surface wave discharge generated by a surfatron plasma at intermediate pressures is measured by optical emission spectroscopy (OES) and Thomson scattering (TS). The OES method, namely absolute line intensity (ALI) measurements gives an electron temperature which is found to be (more or less) constant along the plasma column. TS, on the other hand, shows a different behaviour; the electron temperature is not constant but rises in the direction of the wave propagation. In the pressure range of this study, it is theoretically known that deviations from Maxwell equilibrium are expected towards the end of the plasma column. In this paper, we propose a combination of methods to probe the electron energy distribution function (EEDF) in this relatively high-pressure regime. The ALI method combined with a collisional–radiative model allows one to measure the effective (Maxwellian) creation temperature of the plasma while TS measures the mean electron energy of the EEDF. The differences between the two temperature methods can be explained by the changes in the form of the EEDF along the plasma column. A strong correlation is found with decreasing ionization degree for different pressures. Numerical calculations of the EEDF with a Boltzmann solver are used to investigate the departure from a Maxwellian EEDF. The relatively higher electron temperature found by TS compared with the ALI measurements is finally quantitatively correlated with the departure from a Maxwellian EEDF with a depleted tail. (Some figures may appear in colour only in the online journal)

Electron densities and energies of a guided argon streamer in argon and air environments

In this study we report the temporally and spatially resolved electron densities and mean energies of a guided argon streamer in ambient argon and air obtained by Thomson laser scattering. The plasma is driven by a positive monopolar 3.5 kV pulse, with a pulse width of 500 ns and a frequency of 5 kHz which is synchronized with the high repetition rate laser system. This configuration enables us to use the spatial and temporal stability of the guided streamer to accumulate a multitude of laser/plasma shots by a triple grating spectrometer equipped with an ICCD camera and to determine the electron parameters. We found a strong initial ne-overshoot with a maximum of 7 × 1019 m−3 and a mean electron energy of 4.5 eV. This maximum is followed by a fast decay toward the streamer channel. Moreover, a 2D distribution of the electron density is obtained which exhibits a peculiar mushroom-like shape of the streamer head with a diameter significantly larger than that of the emission profile. A correlation of the width of the streamer head with the expected pre-ionization channel is found.

Density of atoms in Ar*(3p54s) states and gas temperatures in an argon surfatron plasma measured by tunable laser spectroscopy

This study presents the absolute argon 1 s (in Paschens’s notation) densities and the gas temperature, Tg, obtained in a surfatron plasma in the pressure range 0.65 10 mbar, for which the pressure broadening can no more be neglected. Tg is in the range of 480-750 K, increasing with pressure and decreasing with the distance from the microwave launcher. Taking into account the line of sight effects of the absorption measurements, a good agreement is found with our previous measurements by Rayleigh scattering of Tg at the tube center. In the studied pressure range, the Ar(4 s) atom densities are in the order of 1016−1018 m−3, inc...

A power pulsed low-pressure argon microwave plasma investigated by Thomson scattering: evidence for molecular assisted recombination

A squared-wave power pulsed low-pressure plasma is investigated by means of Thomson scattering. By this method the values of the electron density and temperature are obtained, directly. The plasma is created by a surfatron launcher in pure argon at gas pressures of 8–70 mbar. Features of the pulse rise and decay are studied with microsecond time resolution. During the pulse rise we observe initial high temperature values, while the density is still rising. At power switch-off we find decay times of the electron density that are smaller than what is expected on the basis of diffusion losses. This implies that the dominant decay mechanism in the studied pressure regime is provided by molecular assisted recombination.

Revision of the criterion to avoid electron heating during laser aided plasma diagnostics (LAPD)

A criterion is given for the laser fluency (in J/m2) such that, when satisfied, disturbance of the plasma by the laser is avoided. This criterion accounts for laser heating of the electron gas intermediated by electron-ion (ei) and electron-atom (ea) interactions. The first heating mechanism is well known and was extensively dealt with in the past. The second is often overlooked but of importance for plasmas of low degree of ionization. It is especially important for cold atmospheric plasmas, plasmas that nowadays stand in the focus of attention. The new criterion, based on the concerted action of both ei and ea interactions is validated by Thomson scattering experiments performed on four different plasmas.

Determination of electron-impact transfer rate coefficients between argon 1s2 and 1s3 states by laser pump???probe technique

In a microwave argon plasma, the electron-impact population transfers between the first four excited states of argon are studied by time-resolved laser pump?probe technique. Metastable atoms in the 1s5 state (in Paschens notation) are selectively pumped up to the 2p3 state, with a nanosecond pulsed dye laser tuned to the 706?nm argon transition and the temporal response of the densities in the 1s3, 1s4 and 1s5 states are monitored by time-resolved laser diode absorption. The electron density and temperature are also measured by Thomson scattering along the plasma column for different pressures. The rate coefficient measured for the 1s3 to 1s2 state transfer, for which only rough estimations exist in the literature is found to be 9???10?13?m3?s?1, almost five times larger than the value commonly assumed.

Mass spectrometry of atmospheric pressure plasmas

Atmospheric pressure non-equilibrium plasmas (APPs) are effective source of radicals, metastables and a variety of ions and photons, ranging into the vacuum UV spectral region. A detailed study of these species is important to understand and tune desired effects during the interaction of APPs with solid or liquid materials in industrial or medical applications. In this contribution, the opportunities and challenges of mass spectrometry for detection of neutrals and ions from APPs, fundamental physical phenomena related to the sampling process and their impact on the measured densities of neutrals and fluxes of ions, will be discussed. It is shown that the measurement of stable neutrals and radicals requires a proper experimental design to reduce the beam-to-background ratio, to have little beam distortion during expansion into vacuum and to carefully set the electron energy in the ionizer to avoid radical formation through dissociative ionization. The measured ion composition depends sensitively on the degree of impurities present in the feed gas as well as on the setting of the ion optics used for extraction of ions from the expanding neutral-ion mixture. The determination of the ion energy is presented as a method to show that the analyzed ions are originating from the atmospheric pressure plasma.

Spatio-temporal dynamics of a pulsed microwave argon plasma: ignition and afterglow

In this paper, a detailed investigation of the spatio-temporal dynamics of a pulsed microwave plasma is presented. The plasma is ignited inside a dielectric tube in a repetitively pulsed regime at pressures ranging from 1 up to 100 mbar with pulse repetition frequencies from 200 Hz up to 500 kHz. Various diagnostic techniques are employed to obtain the main plasma parameters both spatially and with high temporal resolution. Thomson scattering is used to obtain the electron density and mean electron energy at fixed positions in the dielectric tube. The temporal evolution of the two resonant and two metastable argon 4s states are measured by laser diode absorption spectroscopy. Nanosecond time-resolved imaging of the discharge allows us to follow the spatio-temporal evolution of the discharge with high temporal and spatial resolution. Finally, the temporal evolution of argon 4p and higher states is measured by optical emission spectroscopy. The combination of these various diagnostics techniques gives deeper insight on the plasma dynamics during pulsed microwave plasma operation from low to high pressure regimes. The effects of the pulse repetition frequency on the plasma ignition dynamics are discussed and the plasma-off time is found to be the relevant parameter for the observed ignition modes. Depending on the delay between two plasma pulses, the dynamics of the ionization front are found to be changing dramatically. This is also reflected in the dynamics of the electron density and temperature and argon line emission from the plasma. On the other hand, the (quasi) steady state properties of the plasma are found to depend only weakly on the pulse repetition frequency and the afterglow kinetics present an uniform spatio-temporal behavior. However, compared to continuous operation, the time-averaged metastable and resonant state 4s densities are found to be significantly larger around a few kHz pulsing frequency.