Low Mach number shock wave induced modulation of plasma conductivity

Summary form only given, as follows.We have previously shown the effects of acoustic shock waves with Mach numbers of 1.4 to 2.9 on low-pressure glow discharges in nitrogen by measuring the large local changes in the electric field in the immediate vicinity of the shock front as well as the global changes in discharge voltage and current. The electrical effects were shown to be discharge polarity dependent. Using floating probes, time resolved measurements showed a sharp drop in the local electric field propagating with the shock front, indicating a local region of enhanced conductivity at the shock front and a slow decay of this ionization. This observation, with the requirement for current continuity, suggested that a triple- or quadruple space charge layer was created at the shock front. The associated high electric fields can then lead to local excitation and ionization enhancement if the electron Debye length exceeds the shock layer thickness. We now have measured this excitation enhancement with a DC biased, floating double Langmuir probe. The probe current, proportional to electron density, increases by a factor of 3.5 to 4.3, depending on discharge polarity, when the shock wave passes the probe position. Spatially and temporally resolved measurements of the 2-0 vibrational transition of the B/sup 3//spl Pi//sub g/ -A/sup 3//spl Sigma//sub u//sup +/ first positive band of N/sub 2/ indicated five orders of magnitude enhancement of direct electron impact excitation of the B state compared to the steady state conditions at the shock position. This implies a jump of E/n from 40 Td (unperturbed discharge) to >200 Td in the shock front, correlating to six orders of magnitude local ionization rate enhancement. These localized ionization and electric field modifications will lead to a large enhancement of the local Joule heating near the multiple layers and provide a mechanism for shock wave modifications in a plasma reported by various authors.