A novel approach for the improvement of the sensitivity of the flow visualization system on supersonic molecular beam of tokamak fuelling

Supersonic molecular beam injection is a robust alternative to control the particles in fusion plasma devices. It is widely used in many experimental studies on the plasma physics. Direct visualization of beam makes it possible for precise optimization of the beam characteristics. However, a normal visualization system does not fully meet the requirements of the measurement of the beam distribution in the low gas density vacuum environment. This paper reports a newly designed multi-pass visualization system without changing the schlieren mirrors. This multi-pass system is designed to make the light passing the testing area several times, where the deflection angle of the light would simultaneously increase. Thus, the sensitivity of the schlieren image is enhanced. The testing result proves the improvement of the sensitivity, which makes it possible to optimized the beam structure in low gas density working conditions.