Study of the damage produced by high velocity pellets on graphite first wall elements

In the RFX experiment the first wall is completely covered by graphite tiles and a multishot pellet injector for hydrogen (H) and deuterium (D) pellets with masses of 1.5~5·1020 atoms at velocity of 500~1500 m/s has been installed. Some concern existed about the possibility of seriously damaging the graphite with non-ablated hydrogen pellets. The paper presents a study performed by launching plastic and metal pellets at various velocities to evaluate the damage induced on graphite samples. The use of non-hydrogen pellet avoided the necessity of working in a vacuum environment and allowed to explore a wider parameter range than it would be possible with a single hydrogen pellet injector. The results obtained show that the amount of graphite dug out from the sample depends linearly on the kinetic energy only of the incoming pellet, with a threshold value of ?0.1~0.2 J. Tests performed with hydrogen pellets confirmed that, at low and medium velocity, little or no damage is done to the graphite and indicated that the threshold value for hydrogen is ?0.7 J. Hence in RFX, while H pellets fired at low velocity have an energy below threshold, the largest size pellets fired at high velocity, are expected to produce significant damage, i.e. removal of graphite masses comparable to the pellet size. Tests performed on Inconel elements of the vacuum vessel show that even the largest RFX pellet fired at 1500 m/s is not able to punch through a 1 mm thick Inconel sheet.