WAVEFRONT-BASED PLASMA CHARACTERIZATION

STRAIGHTFORWARD AND HIGHLY SENSITIVE

Key properties of laser-produced plasma are electron density, shape and propagation. A sensitive characterization of these properties over time helps understanding plasma creation and expansion, and monitoring its quality. Controlling the electron density as well as the neutral gas density also helps optimizing the plasma generation process: nozzle design, laser pulse illumination, gas pressure and homogeneity…. Phasics high resolution wavefront sensor is a straightforward and highly sensitive solution for plasma and neutral gas density diagnosis. It is based on a patented technology: the quadriwave lateral shearing interferometry1. The solution overcomes the classical Mach-Zender interferometer (MZI) limitations by offering compactness, very low measurement noise and compatibility with ultrashort probes . Measuring the phase shift in a direct set-up without a reference arm, it is insensitive to the vibrations that often occur in high energy laboratories and is easy to integrate. This document presents the measurement principle and shows the outcomes of a comparison between Phasics solution and a MZI.

“DIRECT SET-UP WITH NO REFERENCE ARM”

Like standard MZI, Phasics solution relies on measuring the phase shift induced by the plasma to a probe beam propagating through the plasma. Reverse Abel transform is then used to retrieve the density maps. Distinctively Phasics wavefront sensor uses no reference arm. It directly measures the phase shift induced by the plasma (or gas jet) refractive index change linked to its electron density. Consequently the sensor acts as a camera that images the plasma or gas jet. It integrates into a set-up of small footprint (figure 2) and is very easy to align . Furthermore Phasics technology is achromatic and works with a simple halogen source or a LED . The sensor is also compatible with femtosecond sources for fs-flash measurements to follow the plasma expansion at the very early stage of its creation.

Figure 2: Set-up based on Phasics wavefront sensor. Any spatially coherent source can be used. An imaging system is used to adjust the field of view to the sensor dimensions

Figure 1: Electron density of a laser induced Helium plasma. Courtesy of LOA 2

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波面センシングテクノロジーによる プラズマ/ガスジェット 観測

“8.4 TIMES MORE SENSITIVE THAN MZI”

Since Phasics sensor is a self-aligned interferometer, it is very little sensitive to vibrations. Consequently the sensor shows a substantially lower noise level than MZI which is highly sensitive to vibrations due to the use of two separated beams. This was experimentally proven by measurements made at LOASIS3 in Berkeley on a Helium gas jet measured 1 mm above the nozzle. For each technique, the RMS deviation phase map was calculated from the acquisition of 188 phase maps (figure 3). Experimental conditions and post treatment were exactly the same. The resulting average of the maps is 11.4 mrad for Phasics solution when it is of 95.7 mrad for the MZI. Phasics method is therefore 8.4 times more sensitive and the noise is more homogeneously distributed 3.

“ACCURATE, EVEN AT LOW GAS PRESSURE”

At LOA2, both techniques were integrated on the same set-up to perform simultaneous acquisitions and rigorously compare them. Measurements were made on an axisymmetric thin Argon gas jet at various gas pressures. The same post treatment (Abel inversion routine and filter) was applied. As shown in figure 4, Phasics and MZI techniques provide similar filtered profiles at 20 bars while Phasics’ shows less noise. At 2 bars, Phasics technique manages to get a profile while it is not possible with the MZI due to its consequent noise level. This profile agrees with numerical fluid dynamics simulation.

20 bars 2 bars

Figure 4: Neutral gas density profiles at 200 µm above the nozzle exit with a filter median applied. Courtesy of LOA 2

“REPEATABLE SHOT-TO-SHOT MEASUREMENT”

Phasics wavefront sensor low noise level ensures repeatable shot-to-shot measurements, even at low pressure. Its high spatial resolution (300 x 400 measurement points) and high sensitivity (<2nm RMS phase shift) ensure precise measurement and reliable calculations . Thus it provides an accurate characterization to efficiently follow and compare gas jet or plasma. With the Plasma module, Phasics SID4 HR wavefront sensor is now a complete and flexible tool for high power laser facilities. The single instrument flexibly covers laser beam characterization, adaptive optics and plasma diagnosis . The software solutions dedicated to each application provide insightful analysis tools and results.

REFERENCES 1 J. Primot, N. Guérineau, “Extended Hartmann test based on the pseudoguiding property of a Hartmann mask completed by a phase chessboard”, Appl. Opt. 39, p. 5715-5720 (2000). 2 LOA: Laboratoire d’Optique appliqué, SourceLAB Project, ENSTA

3 G. R. Plateau, N. H. Matlis,& al, « Wavefront-sensor-based electron density measurements for laser-plasma Accelerators », LOASIS Program, Lawrence Berkeley National Laboratory (LBNL), REVIEW OF SCIENTIFIC INSTRUMENTS 81, 033108, 2010, doi: 10.1063/1.3360889.

Figure 3: RMS deviation maps averaged on 188 phase maps 3 for MZI and Phasics techniques