Measuring Extremely Low Densities of Weakly Ionized Plasma with Picosecond Avalanche Breakdown

Daniel Woodbury, University of Maryland, College Park

Photo of Daniel Woodbury

Avalanche ionization breakdown of air with picosecond mid-infrared laser pulses is an exceptionally sensitive remote probe of the elevated air ionization levels that surround radioactive sources and thus of the sources themselves. By exponentially increasing the electron density in the vicinity of a single-seed source to detectable levels, electron avalanche is the analogue of single-photon detection in photomultiplier tubes. While we previously demonstrated the ability to detect radioactive material based on avalanche from ionization byproducts near the source [Sci. Adv. 5, eaav6804 (2019)], we have subsequently measured the low density (down to 103 cm-3) of electron seeds directly by imaging the location of spatially isolated breakdown sites. This unique capability comes from our picosecond, mid-IR laser driver, which both avoids the competing effect of multi-photon ionization and limits electron diffusion and hydrodynamic motion during the pulse, maintaining the spatial separation of initial avalanche seed sites. We use these direct-density measurements to benchmark diagnostics that could be used to remotely detect low-level ionization near a radioactive source in the field. We also will present ongoing work on measuring other low-density plasma in ultrafast laser-air interactions.

Abstract Author(s): D. Woodbury, R.M. Schwartz, E. Rockafellow, H.M. Milchberg