# One-Body Capillary Plasma Source for Plasma Accelerator Research at e-LABs
## 2 Development of the One-Body Capillary Plasma Source  
### Sapphire-Based, One-Body Capillary Gas Cell  
Laser machining is a common method for the manufacture of sapphire-based capillary gas cells due to the material's hardness and the small diameter of less than 1 mm [9; 21; 22]. Short laser pulses are employed to engrave sapphire plates, creating gas feedlines and longitudinal capillary holes. In the case of cylindrical capillaries, semi-circular carving is implemented on each plate along the capillary axis, and the capillary gas cell is assembled by combining the plates. However, the machining conditions must be precise to fulfill the quality requirements of capillary shape and straightness [22]. In addition, an incomplete assembly of capillary cells may result in gas leaks and suboptimal capillary lines, particularly in longer capillary gas cells.  
Diamond tool-based machining is an effective method for manufacturing harder materials such as sapphire [25; 26; 27]. In general, holes are produced by diamond drilling using an electroplated diamond tool, in which diamond grains are electrodeposited onto metal or alloy substrates. As illustrated in Figure 1, a sapphire capillary cell was manufactured using diamond machining. All processing to create the capillary gas cell was performed using diamond drill bits on a single block of sapphire by a local company (COMA Technology Co., Gumi, Republic of Korea). The capillary and gas lines of a 1 mm or larger hole diameter were formed using the diamond drilling. The hole depth depends on the tool design of the manufacturers and, in this paper, the longest capillary produced was 15 mm in length with a 1 mm diameter. This length was achieved by drilling from one side of a sapphire block; thus, it can be extended to 30 mm by drilling on both sides. Capillaries with smaller diameters can be manufactured using thin, standard diamond drill bits. Although the restricted length leads to a limitation of the hole depth, it is possible to extend the capillary length through the coupling of multiple sapphire blocks. The design of capillaries for the stated objective is currently in progress and will not be covered within the scope of this paper. In addition to hole processing, internal thread manufacturing for gas delivery and the capillary holder was adopted. One-touch fittings (EOCC03M3, KCC Co., Ltd., Seoul, Republic of Korea) were connected to the sapphire block for gas inlets. The simple design of this one-body gas cell has allowed for the realization of a compact capillary gas cell without the need for housing, which is typically required in sandwiched capillary designs. The one-body design of the gas cell allows for a compact and leak-free capillary gas cell that does not require housing, making it easily adaptable to various plasma accelerator research. Furthermore, for discharge plasma applications, oxygen-free electrolytic copper electrodes were installed at both ends of the gas cell, and the utilization of PEEK material for the bolts and holders close to the electrodes has prevented unexpected discharges. Discharge experiments using electrodes will be discussed in Section 4.