Shape-control of surface-launched plasma bullets by designing spatial profiles of applied voltage

抄録

Atmospheric-pressure plasma jet (APPJ) using helium gas can be used for remote plasma processing in open air conditions. A large number of application examples using APPJ have been reported. The APPJ is known to be formed as a result of propagation of plasma bullets. Plasma bullets are generally launched from the nozzle of a narrow DBD tube with helium gas. This scheme has not been changed for approximately two decades.

Recently, we have discovered that plasma bullets can be launched vertically from a planar surface of a dielectric plate if we employ the pulse power with very high dV/dt [1]. We call them surface-launched plasma bullets (SLPBs).

We can propose various new possibilities by eliminating the restriction of “only launching from the nozzle”. For example, the SLPBs can hydrophilize the entire inner surfaces of 3D-printed PLA bone-regeneration scaffolds (continuous porous dielectrics) while conventional APPJs cannot [2]. Furthermore, it may be possible to design the shape of the bullets in contrast to the fact that the shape of conventional plasma bullets is limited to elongated balls or comets. In this study, we investigated the possibility of generating sheet-shaped plasma bullets.

The shape of the reactors examined in this study are shown in Fig. 1. The grounded electrode (GND) is on the top and powered electrode (HV) is on the bottom. Although it is impossible in principle to change the potential distribution of a metal electrode, we examined effect of spatial profile of the bottom-electrode voltage as a study that can be done only by simulation. The discharge gas was a mixture of 0.1% N2 and 99.9% He. The bottom-electrode potential (i.e. applied voltage) was ( 15 kV ){ 1 - exp( - t/T ) }, where T was 10 ns. The spatial profile was the Gaussian profile of exp( - r² / ( 2R² )), where R was infinity or 20 mm. Numerical simulation was performed using an axisymmetric fluid model coupled with Poisson equation and Boltzmann solver (BOLSIG+). Gas-phase reactions in this model were electron impact ionization of He, excitation of He to metastable state, and penning ionization of N₂ by metastable He. Photo ionization was not involved in the model.

Figure 1(a) shows the results for R = infinity, i.e. V(r) = constant. In this case, the bullet was not launched. Figure 1(b) shows the results for R = 20 mm. In this case, the bullet was launched. The shape of the bullet was dome shape. The results obtained in this study suggest that sheet-like plasma bullet, which corresponds to large volume atmospheric pressure plasma, may be possible if we can properly design the electrode-potential spatial profile.

Acknowledgments: This work has been supported by the MEXT/JSPS KAKENHI (19H01888, 19K03811, 20K20913, 23H01166), and the Joint Usage / Research Program of Center for Low-Temperature Plasma Science, Nagoya University.

[1] Shirafuji, T. and Oh, J.-S. 11th Int. Conf. Reactive Plasmas / 2022 Gaseous Electronics Conf., ER5.00006 (2022).

[2] Shirafuji, T. et al. 42nd Int. Symp. Dry Process, H-1 (2021).