# One-Body Capillary Plasma Source for Plasma Accelerator Research at e-LABs
## 5 Discussion  
We successfully developed and investigated a sapphire-based, one-body capillary gas cell for plasma accelerator applications. The sapphire capillary gas cell was manufactured by diamond machining. Due to its simple, one-body structure, there was no need to be concerned about gas leakage and assembly. The operational properties of the gas cell were investigated by using the longitudinal interferometry and CFD simulations. In gas density measurements, the result showed that the maximum gas density was reached after 50 ms of gas injection. To confirm the stability of the capillary gas cell, a laser-plasma electron acceleration experiment with the capillary cell was performed using a TW laser system. From the LWFA experiment, a stable and reproducible electron beam of 200 MeV was generated. The experimental results were analyzed using the PIC simulation, and it was demonstrated that both results were in good agreement. In addition, to investigate its potential application as an active plasma lens, a pulsed discharge system was applied to the gas cell to generate a capillary discharge plasma. An analysis of the discharge current of 140 A predicted the capability to achieve a focusing gradient of 97 T/m. Although it is necessary to find more optimal conditions according to the property of electron beams generated at e-LABs, the possibility of its use as an active plasma lens was demonstrated. The characteristic results of the capillary plasma source shown in this paper provided useful information for its application in LPA, with external injection planned to be carried out in the e-LABs facility (e-LABs is an acronym standing for \"electron linear accelerator for basic science\").  
Conceptualization, I.N., H.S. and M.K.; software, S.L. and M.-H.C.; formal analysis, S.-h.K. and D.J.; investigation, S.L., S.-h.K., I.N., M.-H.C. and M.K.; writing--original draft preparation, S.L. and M.K.; writing--review and editing, M.-H.C., D.J., I.N., H.S. and M.K. All authors have read and agreed to the published version of the manuscript.  
Figure 7: **(a)** Normalized current density and the azimuthal magnetic field in a 1 mm diameter capillary plasma. An electron beam with a rms size of 179 \(\upmu\)m was used to investigate the focusing property by the developed discharge capillary. **(b)** Different focal position in terms of the discharge current. The spot size was maintained at 13 \(\upmu\)m regardless of the current.  
**Funding:** This research was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT) [No. 2020R1C1C1011840, 2020R1C1C1010477, 2021R1C1C1003255, 2021R1F1A1062911, 2022R1A2C2009768, 2022R1A2C3013359].  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in the study are available on request from the corresponding author.  
**Acknowledgments:** The authors acknowledge all e-LABs and CoReLS members who participated in operations.  
**Conflicts of Interest:** The authors declare no conflict of interest.