Experimental results are presented on visualization of two-phase flow in capillary heat pipe by using proton radiography. Proton radiography is non-destructive testing technique making use of the difference in attenuation characteristics of proton beam in materials. 12MeV Proton beam is generated from Tandem accelerator. Beam diameter is 20mm. Proton beam passing through the test section are converted to visible light by fluorescent screen. Heat Pipe is thin-plate type (190mm X 50mm, 1.3mm in thikness), and 0.6×0.7mm channel is snaking in it. Normal speed video camera (recording rate is 60 frame per second) and high speed camera (1000 frame per second) are used. Small bubbles whose diameter are less than 0.6mm can be visualized clearly and recorded with the rate of 1000 frame per second.
