Combining laser interferometry and plasma spectroscopy for spatially resolved high-sensitivity plasma density measurements in discharge capillaries

Precise characterization and tailoring of the spatial and temporal evolution of plasma density within plasma sources are critical for realizing high-quality accelerated beams in plasma wakefield accelerators. The simultaneous use of two independent diagnostics allowed the temporally and spatially resolved detection of plasma density with unprecedented sensitivity and enabled the characterization of the plasma temperature in discharge capillaries for times later than 0.5 µs after the initiation of the discharge, at which point the plasma is at local thermodynamic equilibrium. A common-path two-color laser interferometer for obtaining the average plasma density with a sensitivity of 2 × 1015 cm−2 was developed together with a plasma emission spectrometer for analyzing spectral line broadening profiles with a resolution of 5 × 1015 cm−3. Both diagnostics show good agreement when applying the spectral line broadening analysis methodology of Gigosos and Cardenoso in the temperature range of 0.5 eV–5.0 eV. For plasma with densities of 0.5–2.5 × 1017 cm−3, temperatures of 1 eV–7 eV were indirectly measured by combining the diagnostic information. Measured longitudinally resolved plasma density profiles exhibit a clear temporal evolution from an initial flat-top to a Gaussian-like shape in the first microseconds as material is ejected out from the capillary. These measurements pave the way for highly detailed parameter tuning in plasma sources for particle accelerators and beam optics.