# One-Body Capillary Plasma Source for Plasma Accelerator Research at e-LABs
## 1 Introduction  
Plasma sources using gas targets have been extensively studied for their potential applications in laser-plasma acceleration (LPA) [1; 2; 3; 4; 5; 6; 7]. Capillary gas cells, which possess a small diameter along the longitudinal direction, have been commonly employed as a gas target due to their stability and controllability of the gas flow [5; 8]. Early LPA experiments utilized gas cells as discharge plasma waveguides to enhance electron energy. The use of capillary discharge waveguides (CDW) featuring a parabolic density profile enables the transport of laser pulses over a prolonged distance beyond the Rayleigh length, resulting in an increased acceleration length and subsequently higher-energy electron beams [9; 10; 11]. In 2006, the group led by [PERSON] at LBNL demonstrated a significant electron energy gain in LPA by using hydrogen-filled capillary discharge plasma [3]. They subsequently succeeded in achieving 8 GeV electron beams in 2019 [12]. In addition to their use as acceleration columns, discharge plasma sources have also been employed as plasma lenses for focusing electron beams [13; 14; 15]. Such plasma lenses, characterized by their high magnetic field gradient and low chromatic dependence, play a crucial role in LPA by enabling the production of a relatively large energy bandwidth in the electron beam. Unlike permanent magnet quadrupoles, active plasma lenses can produce azimuthally symmetric magnetic fields whose strength increases radially, enabling the symmetric focusing of electron beams.  
The capillary cells were also demonstrated to be effective as steady-flow gas targets for LPA without the need for electrical discharge [5; 16; 17; 18; 19]. Upon irradiation with high-power laser pulses, gas breakdown occurs within the gas cell, resulting in the formation of plasma bubbles. Despite the absence of a preformed plasma channel, these cells have been shown to produce stable electron beams in terms of both reproducibility and beam quality due to the stationary nature of the gas distribution. In 2008, research conducted by [PERSON]'s group demonstrated the substantial advantage of utilizing capillary gas cells in generating reproducible, high-quality electron beams [5]. Subsequent studies verified the stable operation of plasma-based electron accelerators for prolonged periods of time, such as 24 h, utilizing the same concept [20].  
Capillaries for LPA applications are typically constructed by combining two sapphire plates that have been laser-micromachined on their surfaces [19; 21; 22]. The sapphire material is known for its robustness against high temperatures, allowing it to withstand high-voltage discharged plasmas or tightly focused laser beams for prolonged periods of time. This property is particularly advantageous for LPA plasma sources that operate at high repetition rates or with high-intensity laser systems. However, there is a potential risk of gas leakage due to incomplete coupling of the sapphire plates. Alternative designs, such as one-body capillary cells made of materials such as glass or plastic, have been proposed to address this issue. However, the lifetime of these alternatives has not been fully established in LPA experiments [23; 24]. Therefore, there is a need for the development of one-body capillary sources made from harder and more durable materials.  
In this paper, we present the development of a sapphire-based, one-body capillary plasma source manufactured using a micromachining method with a diamond tool. The design and characteristics of the constructed capillary gas cell are outlined in Section 2. The performance of the gas cell is evaluated through laser wakefield acceleration experiments in Section 3. In Section 4, the potential application of the capillary discharge plasma as an active plasma lens is examined. Finally, we provide a summary and outlook in Section 5.