Stable HiPIMS discharge in a high-aspect-ratio slender tube enabled by a coaxial magnetically confined hollow cathode

Achieving stable high-power impulse magnetron sputtering (HiPIMS) discharge in high-aspect-ratio slender tubes remains highly challenging because the confined geometry intensifies electron loss, restricts plasma sustainment, and aggravates target thermal instability. In this work, a coaxial magnetically confined hollow cathode was developed to realize stable HiPIMS discharge inside a high-aspect-ratio slender quartz tube. The proposed configuration integrates an ultrafine tubular hollow cathode, magnetic confinement, and internal water cooling, thereby enhancing electron confinement and plasma generation in the restricted space. Transient voltage-current characteristics show that increasing the discharge voltage from −540 to −600 V significantly raises the discharge current and the transferred charge per pulse. In-situ optical emission spectroscopy further reveals enhanced emission of Cu Ⅰ, Cu Ⅱ and Ar Ⅰ species with increasing voltage, indicating strengthened plasma activity and promoted metal ion generation. The reproducible operation of the confined pulsed discharge is supported primarily by the measured voltage-current waveforms. OES is used only as auxiliary, time-integrated optical-emission information. As a proof-of-concept demonstration of the proposed discharge configuration, Cu deposition was successfully achieved on the inner wall of a 6 mm inner diameter quartz tube. The deposited film exhibits continuous coverage, a dense microstructure, and a pronounced Cu (111) preferred orientation. These results demonstrate that the proposed coaxial magnetically confined hollow cathode provides an effective route for plasma ignition, discharge sustainment, and inner-wall deposition in highly confined slender tubes.