Noon Seminar Dr. Erik Power LLE: Flow-Cell concept

Demonstrating the Flow-Cell concept: an (actively) cool solution to the average power problem

Scaling CPA lasers to multi-kilowatt average powers with diffraction-limited performance requires management of thermal loads in the laser gain media, beam handling optics, and diffraction gratings used for pulse compression. Joule-class lasers operating at kilowatt average powers would require large beam sizes (~300 cm 2 or ~20 cm diam) with current gold gratings given their ~10 W/cm 2 average-power (~10 mJ/cm 2 at 1 kHz) handling capacity. I will present an experimental demonstration of the “flow-cell” actively cooled substrate, designed specifically to increase average power handling capacity so high average power (HAP) gratings can be operated close to their single-pulse fluence limit, ~100 mJ/cm 2 , with no thermal deformation penalty. The flow-cell design assumed an advanced broadband metallic grating design with 2.5% absorption to calculate a design target for an absorbed power density of 2.5 W/cm 2 with <λ/10 peak-to-valley deformation. Sub-aperture testing showed power handling up to 3.88 W/cm 2 with ~λ/15 deformation and a mechanical stabilization time < 30 s—behavior that is excellently matched by COMSOL models. Extending the models to full aperture illumination, the flow-cell is predicted to perform with <λ/10 deformation up to 2.6 W/cm 2 absorbed power density. I applied the same validated modeling techniques to investigate passive, back, and edge-cooling schemes, using the same constant-temperature boundary conditions used by Alessi, et al. to describe edge-cooled substrates. Model results predict the flow-cell can handle >10× the maximum average power of edge-cooled ULE (ultralow expansion glass), and >160× the maximum of passively cooled fused silica. Initial polishing trials for cordierite, the ceramic used to fabricate the prototype flow-cell, as well as LIDT and diffraction efficiency measurements for gold gratings on cordierite will be discussed.

Pizza will be served!