Time resolved observations of plasma evolution in the hollow cathode region of a transient hollow discharge

It is known that in a transient hollow cathode discharge (THCD), the formation of a moving virtual anode in the A-K space is causally linked to the emission of high energy electron beams originated in the hollow cathode region (HCR). In this work we report on time and space resolved observations of the formation and evolution of plasma inside the HCR which are time correlated with ionization processes inside the A-K gap. The experiments have been performed in Hydrogen, at pressures between 50 to 500 mTorr, at 30 kV applied voltage. Capacitive probes are used to monitor ionization growth in the A-K gap, in conjunction with time and space resolved spectroscopic observations in the HCR. High energy electron beams are also measured. Stark broadening observations of the Hβ line indicate that the electron density inside the HCR increases up to a few times 10¹⁵ cm⁻³ some time before breakdown in the A-K gap. This time is also related with the electron beam activity.