## 2 Trigger Mechanism Structural Design  
Due to the need to reserve space for the extraction channel of the core, the available design space in the coring chamber is very limited. Therefore, a mechanical trigger mechanism using a combination of spring force and a contact pin was designed to assist the pressure-holding controller in completing the initial sealing. The structure can conserve the internal space of the pressure-maintaining core-removing device. As shown in Figure 2a, in the initial state of the core remover, the trigger mechanism was installed at the bottom of the core pressure-holding tank, and it sits on the valve seat of the pressure-holding controller to fix the valve cover of the pressure-holding controller at the position where it fits with the inner wall of the outer cylinder of the pressure-holding chamber (initial position). The trigger mechanism consists of the following parts: a pressure spring, a stabilizing sleeve, three contact pins spaced 120 degrees apart, the outer tube of the core, a core barrel, and its tube shoe. The pressure-holding controller and the trigger mechanism work together to ensure that the deep in situ core core pressure-holding chamber maintains deep in situ pressure; the valve cover of the pressure-holding controller is installed in the interlayer space between core core outer tube and core barrel, and valve. A tension spring is arranged between seat and valve cover to provide power to close the valve cover. When the core completes the drilling action, the core tube is filled with the in situ core obtained from drilling. At that time, the pressure-holding core starts the pressure-holding operation process, as shown in Figure 2b. This process can be divided into the four following stages: (1) Drilling: the core follows the outer drill to complete the drilling, the core barrel is filled with the captured in situ core, and the core barrel is pulled upwards. (2) Triggering: when the tube shoe of the core barrel is in contact with the contact pin of the trigger mechanism, it starts to drive the contact pin and the sleeve to lift up and compress the pressure spring.  
Figure 1: Physical diagram of pressure-holding controller based on principle of geometric intersection of cylinder and cone.  
(3) Locking: the contact pin reaches the groove position on the inner wall of the outer cylinder of the pressure-holding chamber, and the contact pin rotates along its axis to insert into the groove and lock the sleeve at that vertical height. Since the pressure-holding controller is not fixed by the sleeve, it is closed by its own weight and spring force. (4) Unlocking: the tube shoe of the core barrel is further raised to the target position, the contact pin self-unlocks, and the sleeve springs back.  
The three contact pins were evenly installed on the stable sleeve at a circumferential interval of 120 degrees. During the entire action process of the pressure-maintaining coring operation, the trigger mechanism can easily reach a state of force balance in the horizontal direction, preventing the damage caused by the core tube and the core. The mechanism is stuck due to the eccentric moment of the stabilizer sleeve.