Laser-initiated channels for ion transport: Breakdown and channel evolution

The electrical breakdown and discharge evolution in CO2 laser-heated molecular gases has been studied. With the laser tuned to a vibrational mode of NH3, C2H4, CH2CHCN, or CH3OH the breakdown potential decreases as much as 10-fold for laser pulse energies up to 35 J/cm2. The subsequent 50–142-cm discharges are straight, stable, and reproducible. Analogous tests in D2 and air yield only a small alteration of breakdown potential and do not cause a straight discharge. The expansion of the initial laser-heated gas has been modeled by the CHARTB hydrocode with the addition of the NH3 equation of state in tabular and analytic form to that code. The breakdown characteristics and initial expansion stage confirm the earlier calculation of laser heating to 1900–2100 °K. Experimental observations of the discharge evolution in NH3 have measured (1) the radial expansion velocity by streak-camera photography of the Hβ emission zone, (2) the plasma temperature by the Niv/Niii line-ratio method, and (3) the electron-density profile by holographic interferometry. The central zone of the channel is heated to 5.5 eV and expands with a radial velocity of 1.0–1.2 mm/μs for the case of a 27-kA discharge in 20 Torr of NH3. Preliminary hydrocode simulations of the discharge show qualitative agreement with observations.