An apparatus for cooling ablation catheters via a coolant pump and liquid cooling medium controlled by a controller based on a characteristic state for an activity of a catheter and in addition a system that uses the apparatus for ablation of tissue in a human or animal body. The system includes an elongated ablation catheter, an ablation generator electrically connected to the catheter to generate a high-frequency energy pulse or a high-frequency energy field, the coolant pump connected to the catheter and the controller and a control line connected to the pump. The control line comprises an electric circuit designed to detect a high-frequency pulse or a high-frequency field.