Impact of Gas Backing Pressure and Geometry of Conical Nozzle on the Formation of Methane Clusters in Supersonic Jets

We present an experimental investigation of the dependence of the production of large methane clusters on the cluster source conditions. The clusters were produced at room temperature through supersonic expansion of methane gas at the backing pressures P-0 ranging from 10 to 84 bar using five conical nozzles of different geometries. The Cluster size was characterized by Rayleigh scattering measurements and calibrated with Coulomb explosion of the clusters at P-0 = 44 bar subjected to an ultraintense laser pulse. A quantitative evaluation of the performance of the conical nozzles against the nozzle geometry and the backing pressure was made by introducing a parameter delta. Differ from the idealized case where the performance of the conical nozzle can be described by the equivalent sonic nozzle of diameter d(eq), in the present work, the "effective equivalent sonic-nozzle diameter" of the conical nozzle defined by d(eq)* = delta d(eq) is introduced. delta represents the deviation of the performance in cluster formation of the conical nozzles from that predicted on the basis of the concept of the equivalent diameter d(eq) = d/tan alpha, with d being the throat diameter, and alpha the half-opening angle of the conical nozzle, Experimental results show that the cluster growth process will be restricted when the gas backing pressure P-0 is higher and/or d/tan alpha of the conical nozzle becomes larger, resulting in smaller delta. From the experimental data, delta can be expressed by an empirical relation delta = A/[P-0(B)(d/tan alpha)(1.36)], where A = 8.4 and B = 0.26 for 24 bar <= P-0 <= 54 bar (2.8 mm < d/tan alpha < 4.5 mm), and A = 72 and B = 0.80 for 54 bar <= P-0 <= 84 bar (2 mm <= d/tan alpha <= 7 mm). For all the cases investigated in this work, delta was found to lie between about 0.2 and 1.0, and the average radii of the methane clusters were measured to be 1-7 nm, depending on the experimental conditions. For lack of the experimental data on methane cluster formation with sonic nozzles, the data from the "sonic-like" conical nozzles were applied. Consequently, the delta values provided in this work for the conical nozzles remain relative in nature.