Walter Scott, Jr. College of Engineering

Graduate Exam Abstract

Kristian Dehne
M.S. Final
Feb 08, 2023, 3:30 pm - 5:00 pm
ERC Electronics Classrom A210
Demonstration of Filament-Guided Electrical Discharges from a High Average Power 1 kHz Picosecond Laser
Abstract: Atmospheric propagation of ultrashort, high energy laser pulses is of interest for applications including remote sensing, directed energy, and guiding lightning. In this work, the filamentation of high energy picosecond laser pulses at repetition rates up to 1 kHz is demonstrated and the guiding of electrical discharges in air at high repetition rates is studied. Diode-pumped solid state lasers in a chirped pulse amplification (CPA) layout have emerged as the modern choice for the generation of high pulse energies at high repetition rates. The design and performance of the diode-pumped Yb:YAG chirped pulse amplification (CPA) laser system utilized in this work is described in detail. This compact CPA system, that combines a room temperature regenerative amplifier and cryogenically cooled Yb:YAG amplifiers, is capable of producing compressed pulses of < 5 ps duration with up to 1.1 J of energy at 1 kHz repetition rate. This record Joule-level 1 kHz repetition rate picosecond laser (average power output of more than 1 kW) has enabled the results presented in this work.

The compressed output pulses from the Yb:YAG laser induce filamentation which results from the counterbalance between Kerr self-focusing and plasma refraction defocusing. The result is a narrowly plasma channel which is capable of propagating for hundreds of meters. Upon recombination of the plasma, a hydrodynamic response of the atmospheric medium leads to a density depression of similar geometry to the filament channel. The result is a preferential path which is capable of both triggering and guiding electrical discharges. The majority of previous laser-guided discharge studies have been conducted at repetition rates of 10 Hz where the medium completely recovers before the next laser pulse arrives. This work reports on the physics of laser filament-guided electric discharges in air initiated by high energy (up to 250 mJ), 1030 nm wavelength laser pulses of ~7 ps duration at repetition rates up to 1 kHz. A breakdown voltage reduction of up to 4.2 X was measured and determined to result primarily from the perturbation caused by a single laser pulse, with cumulative effects playing only a secondary role. A current proportional to the laser pulse energy is shown to appear as soon as the laser pulse arrives, initiating a high impedance phase of the discharge channel evolution. Full breakdown, characterized by impedance collapse and the onset of high current conduction, occurs 100s of nanoseconds to a few microseconds later. The gaps between the filamentary plasma channel and the electrodes are observed to play a role in the delay between arrival of the laser pulse and the onset of a discharge. Critically, the breakdown voltages measured for 100 Hz and 1 kHz repetition rates are shown to be nearly equivalent. This is consistent with the results of interferometry data which shows that the filament formed by a single laser shot causes a deep density depression up to 75%, compared with the 20% density depression measured 10 microseconds prior to the arrival of a laser pulse in a sustained 1 kHz sequence. The physical insight gained from this work on the formation of laser filament-guided discharges in air at 1 kHz repetition rate can be expected to contribute to their use in applications.
Adviser: Jorge Rocca
Co-Adviser: N/A
Non-ECE Member: Samuel Brewer, Physics
Member 3: Mario Marconi, ECE
Addional Members: N/A
Program of Study: