# One-Body Capillary Plasma Source for Plasma Accelerator Research at e-LABs
## 3 Laser-Plasma Electron Acceleration with a Capillary Gas Cell  
To investigate the capabilities of the one-body capillary gas cell for electron acceleration in LPA, we conducted the electron acceleration experiments using a 150 TW laser system at the Institute for Basic Science (IBS), GIST [30]. A schematic of the experimental setup is illustrated in Figure 4. The laser used in the experiment had a center wavelength of 800 nm and a bandwidth of 76 nm (FWHM), and the measured laser pulse duration was 25 fs (FWHM). The laser beam reflected by a holed mirror was focused using a spherical mirror with a 1 m focal length, resulting in a laser spot size of 24 \(\upmu\)m (FWHM) at the focus. The normalized vector potential (\(a_{0}\)) of approximately 1.6 was determined by considering the encircled energy fraction in the spot size, the reflectivity of the deformable mirror, and the reflectivity of the holed mirror. A 7 mm long, one-body capillary gas cell was mounted on a 5 axis motorized stage inside a vacuum chamber (\(\sim\)\(4\times 10^{-5}\) Torr) and was used for the laser wakefield acceleration (LWFA) experiment. The cell was filled with pure helium gas at a pressure of 150 mbar. Considering the full ionization of helium gas (ionization intensity of \(8.8\times 10^{15}\) W/cm\({}^{2}\)) by the intense laser pulse used in this experiment (the peak laser pulse intensity of \(\sim\)\(10^{19}\) W/cm\({}^{2}\)), the estimated plasma density was \(7.2\times 10^{18}\) cm\({}^{-3}\). The laser was deflected by a tilted aluminum foil after passing through the capillary, and the electrons generated were diagnosed as they passed through it. The diagnostic devices were covered by black-anodized foils to prevent stray light from the laser-induced plasma and deflected laser pulse from reaching them. To analyze the energy spectrum of the electron beams, two phosphor imaging plates (LANEX 1, 2) were placed before and after a 20 cm long permanent dipole magnet with a field strength of 1 T. The trajectory deviation recorded in the images captured by LANEX 1 was used to calibrate the electron energy spectrum recorded at LANEX 2. The electron beam spectra obtained from six consecutive shots are shown in Figure 5a. The beam energy, which remained nearly constant at 200 \(\pm\) 25 MeV, highlighted the stability of the newly developed capillary gas cell as a plasma acceleration source. Figure 5b provides a comparison with the particle-in-cell (PIC) simulation results, which were performed using the cylindrical grid algorithm, Fourier-expanded in the azimuthal direction [31]. The simulation parameters were similar to the experimental conditions, with a normalized vector potential of the laser \(a_{0}\ \approx\ 1.6\), laser spot size of 25 \(\upmu\)m, pulse duration of 25 fs, and plasma density of \(n_{0}=7\times 10^{18}\) cm\({}^{-3}\). The simulation box dimensions were \(z\times r=60\ \upmu\)m\(\times 200\ \upmu\)m, \(\Delta z=\lambda_{laser}/25\), and \(\Delta r=10\Delta z\), with macroparticles of \(N_{z}\times N_{r}=2\times 6\) and azimuthal modes of 0, 1, and 2. The overall plasma profile was consistent with the CFD simulation (Figure 5c), and the laser focus was targeted at \(z=3\) mm, which was 1/3 of the way into the flattop region. Results showed that the maximum electron beams were generated at a plasma position of approximately \(z=3.5\) mm (Figure 5d). Figure 3: (**a**) CFD simulation result for the 7 mm long capillary gas cell. (**b**) Gas density temporal evolution in the capillary gas cell. The gas density was stabilized after about 50 ms of gas-filling time. After this point, the laser peak intensity began to decrease, accompanying weaken wakefields. As the laser propagated, the laser-driven wakefields disappeared and the injected electron beams started to create beam-driven wakefields, which then generated and accelerated the secondary electron beam, as shown in Figure 5e.