This work presents the results of Thomson scattering measurements, optical emission spectroscopy and laser absorption spectroscopy applied to a high pressure nanosecond pulsed helium microdischarge. All data are recorded with high temporal resolution, giving an insight into the processes determining the discharge dynamics. From Thomson scattering measurements, the electron velocity distribution function is determined. Photoionization of helium Rydberg molecules presents a complication for the direct measurement of the electron density by Thomson scattering. Laser pulse energy variation measurements however allow to obtain absolute Rydberg state densities to be obtained. For the first time, the electron velocity distribution function and total Rydberg molecules density for a highpressure pure helium discharge are reported in this paper. These measurements provide new insights into high pressure pure helium discharge chemical pathways.

In this paper, we discuss the experimental results presented in Schregel et al (2016 Plasma Sources Sci. Technol. 25 054003) on a high pressure micro-discharge operated in helium and driven by nanosecond voltage pulses. A simple global plasma chemistry model is developed to describe the ions, excited atomic and molecular species dynamics in the ignition and early afterglow regimes. The existing experimental data on high pressure helium kinetics is reviewed and critically discussed. It is highlighted that several inconsistencies in the branching ratio of neutral assisted associative and dissociative processes currently exist in the literature and need further clarification.The model allows to pinpoint the mechanisms responsible for the large amounts of Rydberg molecules produced in the discharge and for the helium triplet metastable state in the afterglow. The main losses of electrons are also identified. The fast quenching of excited He (n > 3) states appears to be a significant source of Rydberg molecules which has been previously neglected. The plasma model finally draws a simplified, but still accurate description of high pressure helium discharges based on available experimental data for ion and neutral helium species.