Plasmas in general
It is quite remarkable that the fourth state of matter contributes more than 99 % to the visible matter in the universe. Whether plasmas are large or small, they all have one key feature in common: The content of sufficient energy for the dense coexistence of liberated electrons and positive ions within a defined volume. The resulting collective behavior of the charge carrying species leads to unique plasma properties such as the specific emission of light, the response to electrical and magnetic fields with distinct current/voltage characteristics and/or the formation of chemically reactive species. All these typical plasma properties impart unique features to many practical everyday life applications (e.g., plasma monitors, fluorescent lamps) and to vast industrial processes such as surface treatment or coating techniques in the semiconductor manufacturing sector, just to name a few. However, tailoring a plasma to a specific application requires careful assessment of the role of key parameters and their control, in order to maintain the desired state.
Plasma under investigation
Derived from an in-house developed photoionization source for mass spectrometric applications [1, 2] our group started to fundamentally characterize plasmas generated by a spark discharge. The principal setup, as shown in fig 1 left, is made of a hollow needle electrode assembly with a spark gap < 1 mm. A discharge gas is supplied through the cathode with a flow rate between 100 and 300 cm³/min and actively pumped at the anode to maintain a discharge pressure, typically between 2000 and 200 mbar. The triggered high voltage supply with amplitudes of up to 1.5 kV and repetition rates of 1.5 kHz is provided by two capacitors (2 nF) and a high-repetitive capacity charger on a hand sized circuit board (cf. fig 1 right) (DD20_10 C-Lader, Hartlauer Präzisionselektronik GmbH, Grassau, Germany). The duration of a single spark event is in the µs regime.
figure 1 (left) The spark discharge setup used as a photoionization source in a mass spectrometer (MS). In this example, the electrode assembly is implemented in the first pressure reduction stage of a common MS. (right) HV power supply of the spark source.
According to the correlation between the calculated  and experimentally determined helium Paschen curve (cf. fig 2) this setup can be fairly approximated by a simple, plane electrode assembly.
figure 2 Experimental and calculated Paschen curves.
In different collaborations, we have done measurements on other plasma types, such as DBDs, micro-plasma jets  and laser induced plasmas , as well. Thus, feel free to contact us (hkersten[at]uni-wuppertal.de), if you are interested to measure, e.g., the VUV emission characteristics of your plasma application.
What we are looking for
1. Light emission between 40 and 1200 nm (time resolved as well)
We would like to correlate the VUV emission characteristics with UV/VIS/NIR spectra, which is of mechanistic, as well as of valuable practical interest . Currently, we are setting up a free available spectral database of both regimes recorded under the same conditions. It can be used to predict VUV emissions by simply recording the much easier accessible UV/VIS/NIR range. We also record the temporal evolution of the emitted radiation of certain species during a spark event. Comparison with kinetic simulations will allow us to get deeper insight into the prevailing plasma chemistry.
2. Species generated in the plasma (time resolved as well)
Another way of getting a glimpse of the prevailing plasma chemistry is by means of mass spectrometry. We are particularly interested in mass spectrometric detection of electronically excited, metastable atoms and generated molecular species .
3. Time resolved current/voltage profiles
The current and voltage profiles of a spark event provide quantitative statements on, e.g., the electron density and the mean electron energy. In particular we are looking for: a) The breakdown voltage, b) the minimum voltage, c) the maximum current, d) the area underneath the current profile, which directly correlates to the total electron number and e) the fwhm of the current, as well as of the voltage profile.
Our analytical equipment so far (cf. fig 3)
1a VUV emission spectroscopy
(40 - 200 nm)
ARC VM-502 VUV spectrometer (Acton Research Corporation, Acton, MA, USA)
Resolution: 0.8 nm fwhm
The spark discharge is mounted inside the VUV spectrometer, directly in front of the entrance slit. Accordingly, the spectrometer is modified for operation with helium at elevated pressures
- vacuum system: 2 E-6 mbar
- gate valve to rapidly disconnect from the vacuum system and fill with helium to the desired discharge pressure
- scintillator-coated lens with Na-salicylate (custom made via piezo-nebulizer)
- Photomultiplier tube, R928, Hamamatsu Photonics, K.K., Hamamatsu City, Japan
- A/D converter, R232-ADC16/24, taskit GmbH, Berlin, Germany
- custom software (VB 2010 Express)
1b UV/VIS/NIR emission spectroscopy
(200 – 1200 nm)
Compact CCD spectrometer (AvaSpec-3848, Avantes BV, Eerbeek, The Netherlands)
Resolution: 0.7 nm fwhm
A fibre optic is mounted on the flange of the VUV spectromter, which covers the discharge region.
2 mass spectrometer
Quadrupole mass spectrometer HPR-60 (Hiden Analytical Ltd, Warrington, UK)
- direct sampling from atmospheric pressure plasmas (with up to 100% He)
- EI source with adjustable electron-energy (0.4 - 150 eV)
- operation in ± RGA and ± ion SIMS mode
- data acquisition with 0.1 µs resolution
- raw count accumulation
- adjustable scan dwell time
RTE 1054 oscilloscope from Rhode and Schwarz (R&S, Cologne, Germany)
A high precision 1 Ohm resistor in series with the grounded anode and a voltage divider (1 : 1E-5) allow us to quantitatively follow the current/voltage profiles.
figure 3 Analytical equipment.
 Kersten, H.; Wissdorf, W.; Brockmann, K. J.; Benter, T.; O'Brien, R.: VUV Photoionization within Transfer Capillaries of Atmospheric Pressure Ion sources. Proceedings of the 58th ASMS Conference on Mass Spectrometry and Allied Topics; Salt Lake City, UT, USA (2010)
 Kersten, H. Lightning meets Mass Spectrometry - Development of a windowless spark discharge and laser ionization source operating at atmospheric pressure; Südwestdeutscher Verlag für Hochschulschriften GmbH & Co. KG, Saarbrücken, ISBN: 978-3-8381-2799-6 (2011)
 Burm, K. T. A. L.: Calculation of the Townsend Discharge Coefficients and the Paschen Curve Coeficients Contributions to Plasma Physics. 47, 177-182 (2007)
 Benedikt, J.; Ellerweg, D.; Schneider, S.; Rügner, K.; Reuter, R.; Kersten, H.; Benter, T.: Mass spectrometry of positive ions and neutral species in the effluent of an atmospheric pressure plasma with hexamethyldisiloxane and oxygen. Journal of Physics D Applied Physics. 46 (46), 464017 (2013)
 Kersten, H.; Winkelmann, S.; Klopotowski, S.; Benter, T.: Spectroscopy of a miniature spark discharge in the range of 40 - 1200 nm. 22nd International Symposium on Plasma Chemistry (ISPC); Antwerpen, Belgium (2015)
 Barnes, I.; Klopotowski, S.; Brockmann, K.J.; Kersten, H.; Benter, T.: Spark discharge VUV lamps for Atmospheric Pressure Ionization – Mass spectrometric investigations of the plasma chemistry. Proceedings of the 61st ASMS Conference on Mass Spectrometry and Allied Topics; Minneapolis, MN, USA (2013)
 Bierstedt, A. Kersten, H.; Glaus, R.; Gornushkin, I.; Riedel, J.: Laser induced Plasma ion source for ambent mass spectrometry. 49th DGMS; Hamburg, Germany (2016)
Financial support from the German Research Foundation within project KE 1816/1-1 is gratefully acknowledged. Many thanks to Jun.-Prof. Dr. Jan Benedikt for providing literature.
For questions, suggestions and ideas please feel free to contact us hkersten[at]uni-wuppertal.de