Plasma interferometry and how the bound-electron contribution can bend fringes in unexpected ways

Area of Science:

• Plasma Physics
• Atomic Physics
• X-ray Optics

Background:

• Traditional plasma diagnostics assume refractive index depends solely on free electrons, leading to values less than 1.
• This approximation is insufficient for understanding plasma behavior under various conditions, especially with advanced light sources.
• Recent experiments with aluminum plasmas showed unexpected fringe bending, indicating a deviation from the standard model.

Purpose of the Study:

• To calculate the plasma refractive index across a range of elements and photon energies.
• To investigate the conditions under which bound-electron contributions become significant.
• To provide a more accurate understanding of plasma refractive index for future research.

Main Methods:

• Development and application of a novel average atom code.
• Calculation of refractive index for carbon, aluminum, titanium, and palladium plasmas.
• Exploration of photon energies from optical to 100 eV (soft x-rays).

Main Results:

• Demonstrated that bound-electron contributions frequently dominate over free electrons in determining plasma refractive index.
• Identified numerous conditions where the refractive index exceeds 1 due to bound electrons.
• Validated findings against recent experimental observations of anomalous fringe bending in aluminum plasmas.

Conclusions:

• The free-electron approximation for plasma refractive index is inadequate in many scenarios.
• Bound-electron effects are critical for accurate plasma characterization, particularly at higher photon energies and densities.
• Accurate refractive index calculations are essential for interpreting data from next-generation x-ray free-electron lasers and other advanced plasma sources.