Ionization induced plasma grating and its applications in strong-field ionization measurements

Abstract

An ionization induced plasma grating can be formed by spatially selective ionization of gases by the interference of two intersecting ultra-short laser pulses. The density modulation of a plasma grating can approach unity since the plasma is produced only where the two pulses constructively interfere and ionization does not occur in destructive interference regions. Such a large density modulation leads to efficient Thomson scattering of a second ultra-short probe pulse once the Bragg condition is satisfied. By measuring the scattering efficiency, it is possible to determine the absolute electron density in the plasma grating and thereby deduce the ionization degree for a given neutral gas density. In this paper, we demonstrate the usefulness of this concept by showing two applications: ionization degree measurement of strong-field ionization of atoms and molecules and characterization of extremely low-density gas jets. The former application is of particular interest for ionization physics studies in dense gases where the collision of the ionized electron with neighboring neutrals may become important-sometimes referred to as many-body ionization; and the latter is useful for plasma-based acceleration that requires extremely low-density plasmas.

Generation of ultrahigh-brightness pre-bunched beams from a plasma cathode for X-ray free-electron lasers

“…A premodulated plasma (both ions and electrons) is considered above. A technique for generating such ion density gratings in plasmas has recently been demonstrated 39 . In the ab initio simulations presented here, we use an alternative method to produce the plasma-density modulation that utilizes the ponderomotive force of two counter-propagating lasers (see Methods section) 40 .…”

Generation of meter-scale hydrogen plasmas and efficient, pump-depletion-limited wakefield excitation using 10 GeV electron bunches

“…Molecular hydrogen has an IP of 15.4 eV, larger than the 13.6 eV of hydrogen atoms. Therefore in these simulations, we employed atomic hydrogen but with an IP of 15.4 eV to accurately mimic the ionization pathway of molecular hydrogen by an ultrashort beam [21,40]. In the simulation, we utilized a moving window with dimensions of z = 10 cω −1 p (beam direction) and r = 7 cω −1 p (transverse direction), divided into 1000 and 400 cells along each direction, respectively, where cω −1 p represents the plasma skin depth for the normalized density of n e = 6.48 × 10 16 cm −3 .…”

“…These difficulties could potentially be overcome if a hydrogen plasma could be used instead of lithium plasma. The main difficulty is that the higher ionization potential (IP) of the hydrogen molecule (15.4 eV) requires either a high intensity (>5 × 10 14 W cm −2 ), multi-TW laser beam [21] or an electron beam with a peak current >30 kA for beaminduced ionization. We note that at the present time we have not considered even lower IP noble gases such as Ar, Kr, Xe because of possible beam-induced multiple ionization that is known to inject a substantial dark current into the wake [22,23] thereby increasing the complexity of a future multi-stage collider.…”

Plasma electron acceleration driven by a long-wave-infrared laser

“…The raw spectrometer data in Fig. 2b, taken at Δt = 0, shows both Stokes/anti-Stokes and difference-/sum-frequency generation (DFG/SFG) signals for seven different values of plasma density in the range 0.57≤n e ≤1.69 × 10 18 cm −3 , calibrated by independent optical measurements of the density profile n e (z) of the ionized gas jet with ± 10% accuracy using an ionization-induced plasma grating technique 34 . The magnitude ω p ∼ n 1=2 e of the Stokes/anti-Stokes shifts increased as expected with n e , whereas the DFG/SFG peaks remained at n e -independent frequencies and helped to calibrate the spectrometer's frequency scale.…”