We concentrated on the south side trigger today. We found one of the thyratrons on the south side was triggered unstably. Inside that thyratron, the small arcing was observed during the period between two trigger signals. Mike suggested to replace that one immediately. When I lifted the thyratron board, I am surprised to find the conduct tape was so dirty.
This conduct tape connected the thyratron board to the ground, that's why the sparks on the plate were always obtained. So we removed the old dirty conduct tape and put a brand new copper tape.
Because we don't have new thyratron CX1622 at hand, I installed a used one which was probably still good.