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- ItemTemperature and pressure profiles of an ablation-controlled arc plasma in air(Bristol : IOP Publ., 2019) Becerra, Marley; Pettersson, Jonas; Franke, Steffen; Gortschakow, SergeyExperimental measurements of the spatial distribution of temperature and composition of ablation-controlled arc plasmas are a key to validate the predictions of metal evaporation and polymer ablation models. Thus, high-speed photography and space-resolved spectroscopic measurements have been performed to characterize a stable air arc plasma jet controlled by ablation of a polymer nozzle made of Polyoxymethylene copolymer (POM-C) or polyamide (PA6). The spectroscopic analysis is performed along a plane perpendicular to the arc jet axis for a current of 1.8 kA, corresponding to an estimated current density of ~65 A mm-2. Temperature and partial pressure profiles of the plasma for copper, hydrogen and carbon in the gas mixture are estimated as an inverse optimization problem by using measured side-on radiance spectra and radiative transfer spectral simulations. It is shown that the generated ablation-controlled arc has a complicated, non-uniform gas composition. Thus, the generated arc jet has a thin metallic core with a lower almost constant hydrogen pressure, surrounded by a thicker hydrogen and carbon mantle at partial pressures slightly lower than atmospheric pressure. The separation of hydrogen and carbon in the core is a consequence of demixing of the polymer vapour in the plasma. It is found that the overall shape of the temperature and pressure profiles obtained for the arc plasmas with the POM-C and PA6 nozzles are similar although differ in peak values and width. © 2019 IOP Publishing Ltd.
- ItemDouble-propagation mode in short-gap spark discharges driven by HV pulses with sub-ns rise time(Bristol : IOP Publ., 2020) Höft, H.; Becker, M.M.; Kolb, J.F.; Huiskamp, T.The object of this study is the investigation of spark discharges ignited by unipolar positive rectangular high voltage (HV) pulses with 200 ps rise time and (15 ± 2) kV amplitude with 3 ns duration full width at half maximum in synthetic air in a 1.2 mm pin-to-pin gap (tungsten electrodes) at atmospheric pressure. The discharge development was recorded by synchronised iCCD and streak camera measurements in single-shot operation, revealing a two-stage propagation mode. The discharge started with a fast initial breakdown across the entire gap (∼10 mm ns−1) during the HV slope, followed by a much slower (∼0.1 mm ns−1) propagation originating from both electrodes towards the gap centre. The combination of high-resolution diagnostics with numerical modelling indicated that the initial breakdown phase is caused by the rapid increase of electric field strength during the steep HV slope, which leads to the simultaneous fast propagation of a positive and a negative streamer.
- ItemElectric field determination in transient plasmas: in situ & non-invasive methods(Bristol : IOP Publ., 2022) Goldberg, Benjamin M.; Hoder, Tomáš; Brandenburg, RonnyOne of the primary basic plasma parameters within transient nonequilibrium plasmas is the reduced electric field strength, roughly understood as the ratio of the electrical energy given to the charged species between two collisions. While physical probes have historically been used for electric field measurements, recent advances in high intensity lasers and sensitive detection methods have allowed for non-invasive optical electric field determination in nearly any discharge configuration with time-resolution up to the sub-nanosecond range and sub-millimeter spatial resolution. This topical review serves to highlight several non-invasive methods for in situ electric field strength determination in transient plasmas ranging from high vacuum environments to atmospheric pressure and above. We will discuss the advantages and proper implementation of (i) laser induced fluorescence dip spectroscopy for measurements in low pressure RF discharges, (ii) optical emission spectroscopy based methods for nitrogen, helium or hydrogen containing discharges, (iii) electric field induced coherent Raman scattering, and (iv) electric field induced second harmonic generation. The physical mechanism for each method will be described as well as basic implementation and highlighting recent results.
- ItemSpatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges in helium with nitrogen admixtures(Bristol : IOP Publ., 2017-09-20) Bogaczyk, Marc; Tschiersch, Robert; Nemschokmichal, Sebastian; Meichsner, JürgenThis work reports on the spatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges driven by sine-wave feeding voltage at a frequency of 2 kHz in helium with small nitrogen admixtures. The discharge gap of 3 mm is bounded by glass plates on both plane electrodes. Priority is given to the lateral discharge inhomogeneities, underlying volume- and surface-memory effects, and the breakdown mechanism. Therefore, relevant processes in the discharge volume and on the dielectric surfaces were investigated by ICCD camera imaging and optical emission spectroscopy in combination with electrical measurements and surface charge diagnostics using the electro-optic Pockels effect of a bismuth silicon oxide crystal. The number of current pulses per half-cycle of the sine-wave voltage rises with increasing nitrogen admixture to helium due to the predominant role of the Penning ionization. Here, the transition from the first glow-like breakdown to the last Townsend-like breakdown is favored by residual species from the former breakdowns which enhance the secondary electron emission during the pre-phase of the later breakdowns. Moreover, the surface charge measurements reveal that the consecutive breakdowns occur alternately at central and peripheral regions on the electrode surface. These spatial inhomogeneities are conserved by the surface charge memory effect as pointed out by the recalculated spatio-temporal development of the gap voltage.
- ItemVerified modeling of a low pressure hydrogen plasma generated by electron cyclotron resonance(Bristol : IOP Publ., 2022) Sigeneger, F.; Ellis, J.; Harhausen, J.; Lang, N.; van Helden, J.H.A self-consistent fluid model has been successfully developed and employed to model an electron cyclotron resonance driven hydrogen plasma at low pressure. This model has enabled key insights to be made on the mutual interaction of microwave propagation, power density, plasma generation, and species transport at conditions where the critical plasma density is exceeded. The model has been verified by two experimental methods. Good agreement with the ion current density and floating potential—as measured by a retarding energy field analyzer—and excellent agreement with the atomic hydrogen density—as measured by two-photon absorption laser induced fluorescence—enables a high level of confidence in the validity of the simulation.
- ItemCombined toxicity of indirubins with cold physical plasma in skin cancer cells in vitro(Bristol : IOP Publ., 2022) Berner, Julia; Bekeschus, SanderCold physical plasma is a partially ionized gas that generates various components identified as potential anticancer compounds. Due to its topical application, cold plasmas are suitable, especially in dermatological applications. We, therefore, tested the cold plasma effects in skin cancer cells in vitro. An atmospheric pressure argon plasma jet was used as the plasma source. The plasma exposure alone reduced the metabolic activity and induced lethal effects in a treatment time-dependent fashion in both cell lines investigated. This was accompanied by executioner caspases 3 and 7, cleavage indicative of apoptosis and reduced cell migration and proliferation. Recent research also indicated roles of novel indirubin derivatives with potent anticancer effects. Three candidates were tested, and reduced metabolic activity and viability in a dose-dependent manner were found. Strikingly, one compound exerted notable synergistic toxicity when combined with plasma in skin cancer cells, which may be promising for future in vivo experiments.
- ItemSpectroscopic study of plasma nitrocarburizing processes with an industrial-scale carbon active screen(Bristol : IOP Publ., 2020) Puth, A.; Kusýn, L.; Pipa, A.V.; Burlacov, I.; Dalke, A.; Hamann, S.; van Helden, J.H.; Biermann, H.; Röpke, J.The active screen plasma nitrocarburizing technology is an improvement of conventional plasma nitrocarburizing by providing a homogeneous temperature distribution within the workload and reducing soot formation. In this study, an industrial-scale active screen (AS) made of carbon-fibre-reinforced carbon serves as the cathode as well as the carbon source for the plasma-chemical processes taking place. The pulsed dc discharge was maintained at a few mbar of pressure while simultaneously being fed with a mixed gas flow of hydrogen and nitrogen ranging from 10 to 100 slh. Using in situ infrared laser absorption spectroscopy with lead salt tuneable diode lasers and external-cavity quantum cascade lasers, the temperatures and concentrations of HCN, NH3, CH4, C2H2, and CO have been monitored as a function of pressure and total gas flow. To simulate industrial treatment conditions the temperature of the sample workload in the centre of the reactor volume was kept at 773 K by varying the plasma power at the AS between 6 and 8.5 kW. The resulting spectroscopically measured temperatures in the plasma agreed well with this value. Concentrations of the various species ranged from 6 × 1013 to 1 × 1016 cm−3 with HCN being the most abundant species.
- ItemEnhancing surface production of negative ions using nitrogen doped diamond in a deuterium plasma(Bristol : IOP Publ., 2020) Smith, Gregory J.; Ellis, James; Moussaoui, Roba; Pardanaud, Cédric; Martin, Céline; Achard, Jocelyn; Issaoui, Riadh; Gans, Timo; Dedrick, James P.; Cartry, GillesThe production of negative ions is of significant interest for applications including mass spectrometry, particle acceleration, material surface processing, and neutral beam injection for magnetic confinement fusion. Methods to improve the efficiency of the surface production of negative ions, without the use of low work function metals, are of interest for mitigating the complex engineering challenges these materials introduce. In this study we investigate the production of negative ions by doping diamond with nitrogen. Negatively biased (-20 V or-130 V), nitrogen doped micro-crystalline diamond films are introduced to a low pressure deuterium plasma (helicon source operated in capacitive mode, 2 Pa, 26 W) and negative ion energy distribution functions are measured via mass spectrometry with respect to the surface temperature (30 °C to 750 °C) and dopant concentration. The results suggest that nitrogen doping has little influence on the yield when the sample is biased at-130 V, but when a relatively small bias voltage of-20 V is applied the yield is increased by a factor of 2 above that of un-doped diamond when its temperature reaches 550 °C. The doping of diamond with nitrogen is a new method for controlling the surface production of negative ions, which continues to be of significant interest for a wide variety of practical applications. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemThe kINPen—a review on physics and chemistry of the atmospheric pressure plasma jet and its applications(Bristol : IOP Publ., 2018-5-16) Reuter, Stephan; von Woedtke, Thomas; Weltmann, Klaus-DieterThe kINPen® plasma jet was developed from laboratory prototype to commercially available non-equilibrium cold plasma jet for various applications in materials research, surface treatment and medicine. It has proven to be a valuable plasma source for industry as well as research and commercial use in plasma medicine, leading to very successful therapeutic results and its certification as a medical device. This topical review presents the different kINPen plasma sources available. Diagnostic techniques applied to the kINPen are introduced. The review summarizes the extensive studies of the physics and plasma chemistry of the kINPen performed by research groups across the world, and closes with a brief overview of the main application fields.
- ItemFoundations of plasmas for medical applications(Bristol : IOP Publ., 2022) von Woedtke, T.; Laroussi, M.; Gherardi, M.Plasma medicine refers to the application of nonequilibrium plasmas at approximately body temperature, for therapeutic purposes. Nonequilibrium plasmas are weakly ionized gases which contain charged and neutral species and electric fields, and emit radiation, particularly in the visible and ultraviolet range. Medically-relevant cold atmospheric pressure plasma (CAP) sources and devices are usually dielectric barrier discharges and nonequilibrium atmospheric pressure plasma jets. Plasma diagnostic methods and modelling approaches are used to characterize the densities and fluxes of active plasma species and their interaction with surrounding matter. In addition to the direct application of plasma onto living tissue, the treatment of liquids like water or physiological saline by a CAP source is performed in order to study specific biological activities. A basic understanding of the interaction between plasma and liquids and bio-interfaces is essential to follow biological plasma effects. Charged species, metastable species, and other atomic and molecular reactive species first produced in the main plasma ignition are transported to the discharge afterglow to finally be exposed to the biological targets. Contact with these liquid-dominated bio-interfaces generates other secondary reactive oxygen and nitrogen species (ROS, RNS). Both ROS and RNS possess strong oxidative properties and can trigger redox-related signalling pathways in cells and tissue, leading to various impacts of therapeutic relevance. Dependent on the intensity of plasma exposure, redox balance in cells can be influenced in a way that oxidative eustress leads to stimulation of cellular processes or oxidative distress leads to cell death. Currently, clinical CAP application is realized mainly in wound healing. The use of plasma in cancer treatment (i.e. plasma oncology) is a currently emerging field of research. Future perspectives and challenges in plasma medicine are mainly directed towards the control and optimization of CAP devices, to broaden and establish its medical applications, and to open up new plasma-based therapies in medicine.