Experimental Study of Laser-Initiated Radiofrequency-Sustained High-Pressure Plasmas

Experiments are performed using 193-nm ultraviolet laser preionization of a seed gas in atmospheric pressure range argon and nitrogen to initiate a discharge that is sustained by 13.56-MHz radiofrequency (RF) power using efficient inductive wave coupling. High-density (4.5times10
¹²
/cm
³
line average density) large-volume (~500 cm
³
) 760-torr argon plasma is initiated and maintained for more than 400 ms with 2.2 kW of net RF power coupled to the plasma. Using the same technique, a 50-torr nitrogen plasma with line average electron density of 3.5times10
¹¹
/cm
³
is obtained. The nitrogen plasma volume of 1500 cm
³
is initiated by the laser and maintained by a net RF power of 3.5 kW for 350 ms. Measurements of the time-varying plasma impedance and optimization of the RF matching for the transition from laser-initiated to RF-sustained plasma are carried out. Both laser-initiated plasmas provide much larger plasma volumes at lower RF power densities than can be obtained by RF alone. Millimeter wave interferometry is used to determine the electron density and the total electron-neutral collision frequency. A new diagnostic technique based on interferometry is developed to evaluate the electron temperature in high-pressure plasmas with inclusion of the neutral heating. Broadband plasma emission spectroscopy is used to illustrate the changes in the ionized species character immediately after the laser pulse and later during the RF pulse