Development of a density-tapered capillary gas cell for laser wakefield acceleration

Abstract

A capillary gas cell for laser wakefield acceleration was developed with the aid of three-dimensional computational fluid dynamics simulations. The gas cell was specially designed to provide upward density tapering in the longitudinal direction, which is expected to suppress the dephasing problem in laser wakefield acceleration by keeping the accelerated electrons in the acceleration phase of the wake wave. The density-tapered capillary gas cell was fabricated by sapphire plates, and its performance characteristics were tested. The capillary gas cell was filled with a few hundred millibars of hydrogen gas, and a Ti:sapphire laser pulse with a peak power of 3.8 TW and a pulse duration of 40 fs (full width at half maximum) was sent through the capillary hole, which has a length of 7 mm and a square cross section of 350 × 350 µm2. The laser-produced hydrogen plasma in the capillary hole was then diagnosed two-dimensionally by using a transverse Mach–Zehnder interferometer. The capillary gas cell was found to provide an upward plasma density tapering in the range of 1018 cm−3–1019 cm−3, which has a potential to enhance the electron beam energy in laser wakefield acceleration experiments.

Enhancement of electron energies in the laser wakefield acceleration using a density up-ramp capillary gas-cell

“…One can find that when lowering the pressure P 1 , the pressure in the entrance region of the capillary also decreases, which produces a long density up-ramp from the entrance to the second inlet (inlet 2). In [18], a high power laser pulse was injected into the capillary hole and the long plasma (not gas) density up-ramp was measured by the Mach-Zehnder interferometer. When a probe laser pulse in the interferometer passes through a plasma, it experiences a phase shift as the index of refraction is a function of the plasma electron density, leading to an intefrogram.…”

“…Figure 3 shows the representative LANEX images for different plasma densities n e1 = 2, 2.3, 2.5 × 10 19 cm −3 at the inlet 1 with the fixed plasma density n e2 = 2.5 × 10 19 cm −3 at the inlet 2. It can be deduced that the density up-ramp distances for cases (I), (II), and (III) are 1 mm, 4 mm, and 9 mm, respectively, based on the CFD simulations and experimental results in [18] which used a similar capillary gas-cell. As shown in figure 3, the flat density case (I) of n e1 = n e2 = 2.5 × 10 19 cm −3 generated a peak (brightest in the Lanex film) beam energy of 98 MeV with a cutoff energy of about 150 MeV (level of 20% of the peak brightness).…”

“…It was also shown that the laser pulse can be more self-focused as it propagates in the higher density region, leading to a longer distance propagation in the plasma and higher electron energies [13,14]. So far various methods have been developed to control the longitudinal plasma density profile [15][16][17][18][19][20]. For example, a double (discontinuous) discharge capillary structure was devised [15] and a discharge capillary with a linearly changing diameter was also developed [16].…”

“…For example, a double (discontinuous) discharge capillary structure was devised [15] and a discharge capillary with a linearly changing diameter was also developed [16]. The density up-ramp profile was also realized in a discharge capillary and a capillary gas-cell with gas feedlines [17,18]. In addition, the density up-ramp was demonstrated in a gas-jet by tilting it [19,20].…”

Measurements of plasma density profile evolutions with a channel-guided laser

“…Longitudinal interferometry experiments with a Mach-Zehnder interferometer require the capillary length to be less than a few millimeters, resulting in measurement error associated with end effects [69,71,72] . Transverse interferometry measurements require capillaries that are square, necessitating strong assumptions about the symmetry of the profile [73][74][75][76][77][78] and flat optical side windows to provide the optical path. The well-known spectroscopic method using the Stark broadening effect loses the ability to spatially resolve [79][80][81] .…”