Note: Real-time monitoring via second-harmonic interferometry of a flow gas cell for laser wakefield acceleration

Abstract

The use of a gas cell as a target for laser wakefield acceleration (LWFA) offers the possibility to obtain stable and manageable laser-plasma interaction process, a mandatory condition for practical applications of this emerging technique, especially in multi-stage accelerators. In order to obtain full control of the gas particle number density in the interaction region, thus allowing for a long term stable and manageable LWFA, real-time monitoring is necessary. In fact, the ideal gas law cannot be used to estimate the particle density inside the flow cell based on the preset backing pressure and the room temperature because the gas flow depends on several factors like tubing, regulators, and valves in the gas supply system, as well as vacuum chamber volume and vacuum pump speed/throughput. Here, second-harmonic interferometry is applied to measure the particle number density inside a flow gas cell designed for LWFA. The results demonstrate that real-time monitoring is achieved and that using low backing pressure gas (<1 bar) and different cell orifice diameters (<2 mm) it is possible to finely tune the number density up to the 10(19) cm(-3) range well suited for LWFA.

Density characterization of discharged gas-filled capillaries through common-path two-color spectral-domain interferometry

“…[16][17][18][19][20][21][22][23]) concepts. The approach relies on a single femtosecond laser pulse traveling through a frequency-doubling crystal, thus transforming into two co-propagating pulses (the fundamental at frequency ω and the harmonic at 2ω).…”

“…Finally, a single-shot spectrum is recorded, delivering the broad-bandwidth modulated spectrum from which both the plasma phase velocity (phase of the spectral fringes) and group velocity (spacing of the spectral fringes) can be retrieved. The common-path TCI efforts to date, including measurements of gas and plasma densities [20,23], have been performed in the spatial domain (spatial interferometry) and have targeted the phase velocity through fringe tracking. The spectral-domain approach presented in this Letter avoids the necessity for temporal overlap of the pulses (the delay between the two pulses can be quite arbitrary), while the broad-bandwidth-enabled group velocity measurement removes the need for multi-shot phase tracking.…”

Optical diagnostics for density measurement in high-quality laser-plasma electron accelerators

“…In the case of non-cylindrically symmetric targets, multiple measurements along different lines of sight are necessary to implement a 3D tomographic reconstruction procedure [19, 20] . Finally, in the case of homogeneous samples, such as uniformly filled gas cells, the density is actually given by the average density measured by means of interferometry [21] .…”

“…To date, SHIs have mostly been developed and applied to monitor the free-electron density in magnetically confined large plasma machines [43, 44] . Recently, due to their versatility and robustness, SHIs have been successfully applied also for the characterization and monitoring of gas targets for LWFA (namely discharged gas-filled capillaries [45, 46] and gas cells [21, 47] ).…”

Novel gas target for laser wakefield accelerators

“…A long gas medium can be produced by using gas cells 14 , which are small chambers filled with gas having apertures for the entrance of laser and the exit of accelerated electrons. Some gas cells can have a variable length from 0 up to a few centimeters 15 . One of the main flaws of such gas cells design is its length which has to be limited when a low density uniform profile is required.…”