Suppression of Ag deposition by Ar gas in mtorr range and its implication to mitigation of impurity deposition on first mirrors

•A higher pressure difference between the tube and the chamber is obtained at a lower gas flow using a baffled duct.•Ar gas confined in the tube at >10 mtorr effectively suppresses the deposition of Ag (up to > 97%) onto QCM in the tube.•The mitigation efficiency is strongly dependent on the pressure in the tube.

We observe that Ar gas confined in a tube maintained at a pressure higher than 10 mtorr by a baffled duct is very effective in minimizing the deposition of incoming Ag particles onto the surface of quartz crystal microbalance (QCM) in the tube. Ar gas is introduced into the tube and the gas flow to the main chamber is minimized by a baffled duct. In this way, the tube pressure of 10 mTorr or higher is achieved. The pressure is found to be enough to suppress the deposition of nearly 97% Ag particles onto the QCM in the tube. The suppression of the Ag deposition is successfully explained by reduced mean-free path at the high pressure in the tube. We propose that the present approach can be very promising in the mitigation of impurity particles on first mirrors in thermonuclear reactors.