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This work presents a computational study of the atmospheric pressure plasma of the COST jet source in He + 0.1% N2 buffer gas, incorporating the treatment of the VUV resonant radiation from He atoms at a wavelength of 58.4 nm. The plasma is driven by single- and multi-frequency waveforms at various peak-to-peak voltages between 400 V and 685 V. The simulations, which include the photoemission of electrons from the electrodes as well as the photoionization of the N2 molecules indicate that these processes, induced by the VUV radiation, have relatively little influence on the discharge characteristics due to other more efficient charged particle production mechanisms, e.g., Penning ionization. While the plasma characteristics are computed along one spatial dimension perpendicular to the electrodes, the tracing of the VUV photons is executed in the real 3D geometry of the plasma source to allow the computation of the VUV photon flux leaving the device through the orifice. Photon fluxes up to ≈ 5 Ă— 1 0 15 cmâˆ'2 sâˆ'1, corresponding to an energy flux of ≈18 mW cmâˆ'2 are found at multi-frequency operation. [ABSTRACT FROM AUTHOR]

Low-pressure (a few to hundreds of millitorrs) inductively coupled plasmas (ICPs), as typically used in microelectronics fabrication, often produce vacuum-ultraviolet (VUV) photon fluxes onto surfaces comparable to or exceeding the magnitude of ion fluxes. These VUV photon fluxes are desirable in applications such as sterilization of medical equipment but are unwanted in many materials fabrication processes due to damage to the devices by the high-energy photons. Under specific conditions, VUV fluxes may stimulate etching or synergistically combine with ion fluxes to modify polymeric materials. In this regard, it is desirable to control the magnitude of VUV fluxes or the ratio of VUV fluxes to those of other reactive species, such as ions, or to discretely control the VUV spectrum. In this paper, we discuss results from a computational investigation of VUV fluxes from low-pressure ICPs sustained in rare gas mixtures. The control of VUV fluxes through the use of pressure, pulsed power, and gas mixture is discussed. We found that the ratio, β, of VUV photon to ion fluxes onto surfaces generally increases with increasing pressure. When using pulsed plasmas, the instantaneous value of β can vary by a factor of 4 or more during the pulse cycle due to the VUV flux more closely following the pulsed power. [ABSTRACT FROM AUTHOR]

Microplasma sources typically take advantage of pd (pressure  ×  size) scaling by increasing pressure to operate at dimensions as small as tens of microns. In many applications, low pressure operation is desirable, which makes miniaturization difficult. In this paper, the characteristics of low pressure microplasma sources excited by microwave power are discussed based on results from experimental and computational studies. The intended application is production of VUV radiation for chemical analysis, and so emphasis in this study is on the production of resonant excited states of rare gases and radiation transport. The systems of interest operate at a few to 10 Torr in Ar and He/Ar mixtures with cavity dimensions of hundreds of microns to 1 mm. Power deposition is a few watts which produces fractional ionization of about 0.1%. We found that production of VUV radiation from argon microplasmas at 104.8 nm and 106.7 nm saturates as a function of power deposition due to a quasi-equilibrium that is established between the electron temperature (that is not terribly sensitive to power deposition) and the population of the Ar(4s) manifold. [ABSTRACT FROM AUTHOR]

Experimental observation of photoionization capable extreme ultraviolet and vacuum ultraviolet emission from nanosecond timescale, developing low temperature plasmas (i.e. streamer discharges) in atmospheric air is presented. Applying short high voltage pulses enabled the observation of the onset of plasma formation exclusively by removing the external excitation before spark development was achieved. Contrary to the common assumption that radiative transitions from the b (Birge–Hopfield I) and b (Birge–Hopfield II) singlet states of N2 are the primary contributors to photoionization events, these results indicate that radiative transitions from the c (Carroll–Yoshino) singlet state of N2 are dominant in developing low temperature plasmas in air. In addition to c transitions, photoionization capable transitions from atomic and singly ionized atomic oxygen were also observed. The inclusion of c transitions into a statistical photoionization model coupled with a fluid model enabled streamer growth in the simulation of positive streamers. [ABSTRACT FROM AUTHOR]

Optical emission spectroscopy in vacuum ultraviolet and UV spectral ranges is applied to study densities, and vibrational and rotational temperatures of the N2 molecule in a nitrogen-argon (0-95% Ar) plasma sustained at a pressure of 400 Pa by a helical cavity supplied with a power of 28 W and an excitation frequency of 27 MHz. The spatial investigation of all emission systems from UV to NIR shows a minimum situated in the middle of the helical structure and two maxima located at the positions where the RF power is transmitted to the gas and at the end of the helix. The minimum was deepest for emission of the first positive (1+) nitrogen system. This hollow shaped density profile due to the presence of a non-linear phenomenon in the discharge is constant whatever the gas composition. The emissions related to Lyman-Birge-Hopfield and the second positive (2+) systems of molecular nitrogen, and N(2P) atoms, are analyzed versus the Ar percentage. Additionally, the NO(A 2S+ - X 2P) and OH(A 2S+ - X 2P) emission systems coming from impurities are investigated. All the densities of the considered species increase with Ar amount. The rotational and vibrational temperatures of the emitter species are determined through the comparison between experimental and simulated spectra. In the case of a N2 discharge, all the rotational temperatures deduced through the nitrogen emission systems are equal and can be assimilated to the gas temperature. With the increase in the Ar amount, only the rotational temperature obtained from the 1+ system is close to the gas temperature. The rotational and vibrational temperatures related to the NO(A 2S+) species are constant whatever the gas composition. The vibrational distribution function of N2(a 1Pg) state presents a Boltzmann law with a vibrational temperature in the range 5600-8000 K (+-1000 K) for the N2-x% Ar mixture with x < 75%. When the Ar percentage increases above this limit, we observe strong deviations from the Boltzmann law and no temperature can be deduced. Some kinetic considerations, where the nitrogen and argon metastables play an important role, are discussed to explain the strong dependence of the temperatures and density species toward the Ar amount in the gas mixture. [ABSTRACT FROM AUTHOR]