Effect of ion-neutral collisions in presheath on plasma flow and potential measurements

초록

Collision effects on plasma can not be discarded in deducing plasma parameters by applying electric probes. However, there have been few researches on the collision al effects in plasma flow and plasma potential measurements which are measured by the Mach and capacitive probes, respectively. To reslove the collisonal effect in Mach probe analysis, effects of ion-neutral collision in the presheath on deducing plasma flow are investigated. To analyze experimental results of collosionless, weakly collisional and highly collisional plasmas flow measurments, a new inmagnetized collisional Mach probe theory is suggested. The unmagetized collisional Mach theory includes ionization, charge and momentum transfer of ion in a presheath region of a Mach probe. In the collisionless Mach probe experiment at Hanbit plasma, unmagnetized rotational flow velocity showed similar results by collisional and collisionless model. In the weakly collisional Mach probe experiment at University of Califonia at Irvin(UCI), results by laser induced fluorescence(LIF) showed a good agreement with collisional fluid model rather than collisionless model. The UCI plasma consisted of a background rf plasma and an ion beam. The background plasma flow was measured as 1 79+-17 m/s by using the LIF, while Mach probe data was deduced as 222+-67, 230 +-70, and 161+-49 m/s by using collisionless particle-in-cell(PIC), collisionless kinetic, and collisional fluid model, respectively. Measurementof Mach probe in ion beam with background plasma experiments showed small difference with average velocity ion beam and background plasma. For the highly collisional Mach probe experiments - Torch experiment, collisional Mach probe theory produces about 5 times smaller value than the collisoinless Mach probe theory. The collisioless models give about 5 km/s of flow velocity while collisional one gives less than 1 km/s. The reported numerical simulation of flow pressure torch (50 Torr), although there are few related papers, is close to that of the collisional fluid model. For the collisional effect on potential measurement, a new capacitive coupling method is introduced to measure plasma potential in atmospheric dielectric barrier dischage(DBD) plasma. The capacitive coupling method suggested to measure potential of atmospheric dielectric barrier discharge plasmas, measured a electric field of about 500 V/m at the outer region of electrode. The electric field is supposed to sustain the plasma flame at the outer region of electrodes overcoming high collisions at atmosopheric pressure. The collisional Mach probe model and new capacitive probe can be applied in collisional plasma, such as atmospheric DBD and plasma torch.