By Alex Rao, Pulsed Laser Effects (PLE) group, LLE
Introduction
Laser-induced breakdown spectroscopy (LIBS) uses a laser to generate plasma from a sample, producing light that reveals its composition through distinct spectral lines. This project employs LIBS to investigate the plasma generated by the laser-material interaction. Paired with an IsoPlane SCT 320 spectrometer for precise spectral analysis, the results highlight the diagnostic potential of LIBS, which could be extended to analyze key plasma properties such as temperature, density, and ionization state. This offers valuable insights for applications in propulsion, fusion, and industrial manufacturing.
Princeton IsoPlane SCT 320 Spectrograph
- Adjustable slit width for managing light level.
- Grating turret with three gratings to easily use each grating on the turret.
- Micrometer for adjusting focus onto the camera.
- Open back allows for various cameras to be used, both mounted and unmounted.
Experimental Setup
Below are the two lasers used for this experiment, and an outline of the general experimental setup:
The data is calibrated using a Hg and NeAr lamp by identifying known peaks, mapping pixels to wavelength, and applying the conversion. No efficiency measurement was performed on the detector. The spectrum is inverted relative to the image.
The data collected by the camera is initially in the form of an image. A line-out is taken of the image to create a spectrum in pixels vs intensity. Using the calibration process converts the pixel axis into wavelength. Once calibrated, the spectra at different wavelengths are stitched together to create one large spectrum.
Gratings
The key instrument in this experiment is the spectrometer, a Princeton IsoPlane SCT 320 spectrograph. This spectrometer uses the principle of grating dispersion to separate light into its component wavelengths.
Results
Measuring the air-breakdown spectra is important because it can then be compared to the experimental data for Al and graphite to help discern which peaks are air-breakdown peaks and which peaks are from the target material.
- Peaks inside the shaded boxes we are confident are from the target material.
- Peaks outside the shaded boxes are most likely from air-breakdown.
Conclusions
The data in the results section shows that air-breakdown is the most prominent spectrum when the materials were shot in air, while in vacuum the Qsmart laser was struggled to produce enough light to form clear spectra. In contrast, the MTW laser produced plenty of light for analyzing the material. When shot in air, air-breakdown dominated both the graphite and Al spectra. This means that, especially for PLE, where light levels could be problematic, many material peaks were obscured by air-breakdown. This is clearly shown with the Al spectra, where there are Al peaks in the MTW spectrum at the same wavelength as air-breakdown peaks in the PLE spectrum. Al produced more peaks than graphite and was able to produce peaks at lower pressure (20 torr compared to 432 torr) in the PLE setup. This means that more information is available for Al than for graphite.
Future Work
- Analyzing ways to improve the material signal with the lower energy laser.
- Shooting other materials of interest.
- Modelling plasma conditions with simulations
Acknowledgments
I would like to thank the PLE group at the LLE for all of their help and support throughout this process. Without their support, none of this would have been possible. This material is based upon work supported by the Department of Energy [National Nuclear Security Administration] University of Rochester “National Inertial Confinement Fusion Program” under Award Number(s) DE-NA0004144.