## 1 Introduction  
Recently, plasma devices generating the streamer type of breakdown in a mixture of noble gases and air have been widely used in medicine (see, for example [1, 2]). The most typical plasma devices operate at 10-50 kHz frequency with the voltage of 2.5 kV-20 kV, applied to the electrode embedded inside of the dielectric tube. The streamer appears over the positive cycle of the applied voltage and propagates inside and outside of the dielectric tube [34]. Usually a noble gas is pumping through the dielectric tube since the critical voltage of the breakdown in the noble gases is essentially lower that in the atmospheric air. The streamers propagate over a laminar jet of a noble gas and induces multiple chemical reactions in the mixture of nitrogen, oxygen, water vapor and noble gas some distance apart from the dielectric tube inlet [5, 6]. The bio-target treated with the plasma jet is exposed to a chemical cocktail of different radicals, ions and the large electric field delivered by the streamer head. The efficiency of treatment depends on numerous parameters, such as the discharge voltage and frequency, plasma device geometry, type of working gas, velocity of flow gas, distance between a discharge tube nozzle and tissue, time of exposing to the plasma etc. The effect of different types of targets placed in free plasma jets on the plasma characteristics and OH-production was studied previously. In numerical calculations in Ref. [7], it was shown that an increase of permittivity of targets ranged from plastics to metals enhances the speed of ionization wave and the electron density in the plasma column. In Ref. [8], the images of plasma interaction with different targets showed an increase of intensity of the optical emission for the grounded electrode beneath. The electric field profile between the nozzle and target over the He plasma jet generated with 30 kHz discharge with 2 kV voltage amplitude was measured in Ref. [9], using optical emission spectroscopy on a forbidden line of Helium (\(2^{1}\)P\(4^{1}\)F). The electric field \(E\) delivered by the streamer to the metal grounded electrode is shown to be higher than \(E\) for the glass target case. A considerable rise in OH density values was found in Ref. [10] with the presence of the metal target compared to the free jet results at the high AC voltage amplitude of 10 or 14 kV. This enlarged value of the OH density was provided with the counter-propagating streamer after impinging the target. In Ref. [11], an original method to increase the OH generation was developed. Using multiple ring electrodes, more OH-radicals were generated. Compared to the case with only one ring, the device with 12 ring electrodes can generate 3-5 times more OH. It was shown that the multiple electrodes enhance the plasma and OH-radical production only inside the tube rather than in the plasma plume in the surrounding air. A higher discharge current and active species densities were achieved in Ref. [12] with the installation of an additional floating inner electrode in the plasma device compared to the device only with two outer electrodes. In Ref. [13], an external biased ring electrode installed between the plasma device and target was used to intensify the streamer characteristics near the target surface. It was shown that the surface electric field and ionization rate were much higher on the dielectric surface than on the conductive one, due to an accumulation of the surface charge, especially with the presence of negatively biased external electrode. In this work, in the experiment and 2D fluid model simulations, we study the influence of the presence of the grounded substrate beneath the plate with the media and cells on plasma characteristics and the efficiency of cancer cell treatment. Based on the theoretical and experimental results the optimal conditions of CAP jet are formulated and implemented in cancer cell-based experiments.