## 1 Introduction  
Laser beam welding (LBW) employs a high-energy-density power source, and it can achieve relatively deep penetration depths with less heat input than other welding processes [1]. During LBW, deeper penetration depth is enabled by the high energy-per-unit length and/or the high energy density of the laser beam. However, a high energy-per-unit length can lead to keyhole instability and weld defects such as pores, because the high recoil pressure resulting from the high energy can cause melt ejections and frequent keyhole collapses [2]. On the other hand, the small laser beam size required for high energy density can reduce the ability to bridge the welding joint gap, which causes underfill and sagging in the weld cross-section [3]. Many trials have been conducted to achieve both deep penetration and weld quality, and the vacuum laser beam welding (VLBW) method has been proposed as one of the promising methods. VLBW enables deep penetration depth and welding stability under relatively low laser power, which is hard to achieve in atmospheric pressure laser welding [4]. These characteristics are attributed to the extraordinarily low ambient pressure. As the ambient pressure decreases, the material and welding characteristics that differ from atmospheric pressure are as follows. Under low ambient pressure, the evaporation temperature of the base material decreases. For steel, for example, the ambient pressure drop from atmospheric pressure to 10 Pa reduces the evaporation temperature from 3100 K to approximately 1850 K. The decrease in vaporization temperature can lower the energy required to vaporize the base metal and conduct keyhole mode welding [5]. In addition, the range between evaporation and melting temperatures (i.e., the temperature range of the liquid phase) decreases because the melting temperature of the metal is almost independent of the ambient pressure. Thin, molten keyhole walls resulting from the narrow temperature range of the liquid phase can enhance keyhole stability by reducing the number of keyhole collapses [6]. Additionally, the size of the plasma plume generated during laser welding decreases, so the energy absorbed into the plume can be reduced. Consequently, keyhole mode welding can be implemented even at low energy input, and a penetration depth more than twice that possible with atmospheric pressure welding can be achieved with VLBW [7; 8; 9]. In previous studies, the laser beam was typically delivered through a coupling glass that transmitted the laser beam and sealed the vacuum chamber. However, since VLBW is performed inside a vacuum chamber, the fumes generated during welding adhere to the inner chamber walls and the coupling glass. The contamination of coupling glass caused by the deposition of high-temperature welding fume particles gradually accumulates as the welding trial continues. Since laser energy is absorbed by the contaminated region of the coupling glass, laser transmittance decreases. Furthermore, the contamination and heat generation lead to thermal deformation and changes in the refractive index of the coupling glass, which changes the laser beam profile and the laser focal point [10]. Consequently, penetration depth or weld quality is inconsistent with increasing process time. Therefore, a protective system must be designed and applied to prevent contamination of the coupling glass. Previous researchers have suggested several protective systems to prevent contamination of the coupling glass in the vacuum chamber. [PERSON] et al. devised two separate vacuum chambers connected vertically, and a single hole on the bottom of the upper chamber acted as an aerodynamic window to prevent contamination of the coupling glass [4]. [PERSON] et al. applied a cylindrical protective system between the coupling glass and the vacuum chamber. N\({}_{2}\) shielding gas was supplied into the cylindrical protective system to prevent contamination of the coupling glass [7].