3.10 Fuelling System 3.10.1 Gas Fuelling System 3.10.1.1 Overview The gas fuelling system is the equipment to inject gas into the vacuum vessel. The equipment consists of injection, delivery, vacuum pumping and system controller parts. We designed the system configuration and systematic diagram. 3.10.1.2 Designed Systematic Diagram (1) Design condition The design condition of the equipment is shown in Table 3.10.1-1. The gas puffing conditions of normal discharge, TDC and glow discharge are described. The mass flow rate for the common operation mode is comparable with that used in the JET high density experiments. Therefore, the mass flow rate could be sufficient to increase the density up to Greenwald limit at the plasma current of 2-3 MA. At the plasma current of 5.5 MA, it is difficult to increase the density up to Greenwald limit by gas-puffing only as described in Section 2.9.4 at least with keeping high confinement condition. Thus, a combination of gas-puffing and pellet injection is required at the plasma current of 5.5 MA. In impurity seeding experiments on JT-60U for high radiation operation, the Ar mass flow rate is equivalent to hydrogen mass flow rate of several Pam 3 /s. The hydrogen mass flow rate can increase up to 60 Pam 3 /s for impurity gas, which can be considered to be enough to enhance the radiation loss. The mass flow rate similar to that in JT-60U is considered to be sufficient for massive gas injection in disruption mitigation experiments, because these experiments are planned to be performed with relatively low plasma current. When the experiments will be performed with high plasma current, the background pressure can be increased from 0.2 to 0.4 Pa. (2) Gas Injection Port Position The continuous injections are planed for TDC and Glow discharging at the port section N0.5 or nearly that is other side of the exhaust port. The positioning of Common operation mode is basically located at both main plasma region and divertor region. The detailed gas outlet position depends on the position of piezoelectric actuator valve. The operation characteristics of piezoelectric actuator valve are now being examined and the time response is being measured with a long tube. Based on these results, the detailed position of gas outlet will be determined. (3) Systematic configuration The injection unit consists of injection valves to control gas flow into the vacuum vessel, delivery tubes and these supports. The delivery unit consists of gas cylinder of stored vessel, pressure controller to control gas pressure to the injection valves and delivery tubes. The vacuum pumping unit consists of pumps to evacuate unnecessary gas from the injection unit and the delivery unit. The system controller unit controls the injection, the delivery and the vacuum pumping units, and communicates to the ZENKEI system for the linkage between the other systems and the sequence of plasma discharge. The system configuration diagram of JT-60SA gas fuelling system is shown in Fig. 3.10.1-1. The system line consists of four injection lines, four delivery lines and three vacuum pumping lines. There are manifolds on the vacuum vessel. It s used for normal operations and continuous injections for TDC and glow discharge. The reservoir tank of the delivery unit is used for gas storage from gas cylinder, and the gas pressure is controlled to keep predetermined 3. Plant Description Sec.3.10 Page 1
pressure by control valves. (4) Operation mode The operation modes corresponding to the operation conditions are shown in Table 3.10.1-1. a. Common operation mode It is possible to inject simultaneously four gases using four injection lines. The gas flow is controlled by several multilayer piezoelectric actuator valves. The method of the gas reaching the plasma is studied that both gas injecting through the manifold and the baffle plate for direct injection. b. TDC mode In He-TDC mode, the line-3 of the injection and delivery parts or the line-4 of the injection and delivery parts are used. Pressure of vacuum vessel is controlled by a multilayer piezoelectric actuator valve. c. Glow discharge mode In Glow discharge mode, the line-3 of the delivery part or the line-4 of the delivery part is used. Pressure of vacuum vessel is controlled by a mass flow controller. 3.10.1.3 Design of Instruments (1) Gas injection valve Type : Multilayer piezoelectric actuator valve Qty. : 16 (type-h 8, type-l 8) Gas flow (H2) : type-h 4.5~45Pam 3 /s : type-l 0.5~5.8Pam 3 /s OT : RT DP : ~0.4 MPa (2) Pressure controller Type : Control valves with reservoir tank or Pressure Controller (3) Gas cylinder stands Qty : 6 cylinders in total Line-1 1 cylinder Line-2 1 cylinder Line-3 2 cylinders Line-4 2 cylinders 3. Plant Description Sec.3.10 Page 2
Table 3.10.1 The design condition of JT-60SA Gas fuelling System. Operation mode Glow discharging (H 2, He) Injection valve Mass Flow Controller Qty. 1 Initial pressure (Pa) 10-5 Gas pressure(mpa) ~2x10-1 Mass flow rate (Pam 3 /s) ~8.5 TDC (He) Injection valve Multilayer piezoelectric actuator valve Qty. 1 Initial pressure (Pa) 10-5 Gas pressure(mpa) ~2x10-1 Mass flow rate (Pam 3 /s) 0.5-5.8 Common operation (D 2, H 2, Ar, CD 4 etc ) Injection valve Multilayer piezoelectric actuator valve Qty. 16 Initial pressure (Pa) 10-5 Gas pressure(mpa) ~2x10-1 Mass flow rate (Pam 3 /s) 90 3.10.1.4 Subject of Future Investigation Concerning the method of the gas reaching the plasma, we need to decide the structure and Fig. 3.10.1-1 Gas fuelling system configuration diagram. 3. Plant Description Sec.3.10 Page 3
quantity in consideration of the interference between the other equipments around the vacuum vessel. Concerning the gas puffing into baffle plate, we need to study flow control for the length and diameter of tube. Concerning the pressure control, we need to research other techniques because there is a limit to control gas pressure in the method using reservoir tank. 3. Plant Description Sec.3.10 Page 4
3.10.2 Pellet injection system 3.10.2.1 Introduction Pellet injection from the high-field-side (HFS) is expected as an effective particle fuelling method to the core plasma in JT-60SA. Recently, the pellet injection is also used for mitigation of the transient ELM heat load to the divertor plates. The pellet injection is considered to be a useful plasma control tool. As described in Section 2.9.4, the fuelling rate of more than 1x10 22 s -1 is required for sufficient density control. The pellet injection system in JT-60SA is designed based on the system, which is being developed in JT-60U with a centrifugal type accelerator [3.10.2-1]. In this section, the specifications of the pellet injection system in JT-60SA are described based on the present design. However, we keep options open, such as use of a gun type accelerator depending on the results for the development of the pellet injector in JT-60U. Since required injection speed is relatively low for the HFS injection (several hundred m/s), a gun type accelerator is also available. 3.10.2.2 Basic specifications The basic specifications of the pellet injection system are shown in the following. (1) Pellet acceleration : centrifugal type (2) Pellet size : (~2.5)x(~2.5)x(~2.5) (mm) (3) Injection speed : 100-1000 m/s (4) Injection frequency : 10 (Hz) (5) Injection duration : the maximum 100 (sec/shot) (6) Pellet species : H and D The particle fuelling rate is estimated to be ~8x10 21 /s with 10 Hz, which is smaller than the requirement. Therefore, we plan to install three pellet injectors to satisfy the requirements. In addition, we try to increase the injection frequency up to 20 Hz to increase the fuelling rate. An adequate pellet size is considered to be 2.1-2.5 mm, because the large size pellet injection significantly affects the plasma performance. In JT-60U, an ITB structure is transiently destroyed due to cold pulse propagation induced by 2.1 mm pellet absorption outside the ITB [3.10.2-2]. In JET, since an ITB structure vanishes when 4 mm pellet penetrates into the ITB region [3.10.2-3], the pellet size of 4 mm is considered to be too large. The pellet injector is developed to increase the injection frequency rather than the pellet size. For the ELM control, the smaller pellet size might be preferable. An adequate pellet size for the ELM control will be estimated and one pellet injector might be optimized for the ELM control. 3. Plant Description Sec.3.10 Page 5
3.10.2.3 Structure of pellet injection system The structure of the pellet injection system is shown in Fig. 3.10.2.3-1. The pellet is accelerated by a centrifugal type pellet accelerator. The centrifugal type can make the pellet injector compact compared with a gun-type. A pellet speed, an injection frequency and total number of injected pellets are easily changed in the centrifugal type accelerator. The pellet is formed by a screw type pellet extruder [3.10.2-4], which can extrude the pellet for a long duration. Fig. 3.10.2.3-1 Structure of pellet injection system 3.10.2.4 Pellet formation Fuel gas is introduced in the cylinder of the screw type pellet extruder to form the ice of hydrogen isotope which built in a screw, and cooled down in the cylinder, and the process of the pellet formation makes a solid hydrogen isotope. Thereafter, solid hydrogen isotope is molded by rotating a screw, and a bar-shaped pellet (~2.5) mm x (~2.5) mm is extruded. It is sent to the accelerator after bar-shaped pellet is cut in the length of ~2.5 mm. Then, pellet is accelerated with centrifugal force of the rotor, and injected into the plasma. 3.10.2.5 Main composition machine The basic arrangement plan of the pellet injection system is shown in Fig. 3.10.2.5-1~3 with injection angles. The main machine composition of the pellet injection system is also shown in Table 3.10.2.5-1. We plan to install the first set with the centrifugal type accelerator before the first plasma operation, which is used for JT-60U. However, the fabrication and installation of the 3. Plant Description Sec.3.10 Page 6
second and the third sets have not been planned yet because these are not squeezed into the national (NA) budget at present. These fabrication and installation are to be determined in future. And the second and the third sets will be chosen from the centrifugal type or a gun type in consideration of the pellet application. Three sets of the pellet injector system will be finally installed in JT-60SA in order to satisfy the requirement for particle fuelling in future. Pellet will be introduced from each horizontal port of P-5, P-12 and P-14, and 3 sets of the pellet flight tubes are installed inside the JT-60SA vacuum vessel for the HFS injection at a speed of 100-300 m/s. We plan to use the pellet injection with the centrifugal type accelerator at high speed up to 1000 m/s for the only low-field-side (LFS) injection without flight tube. In this case, a straight vacuum tube is directly connected to a port of JT-60SA. Pellet injection with adequate pellet size and speed from the LFS can be selected for ELM control. The vacuum in the pellet injection system can be separated from the JT-60SA vacuum vessel by using a torus isolation valve for the differential pumping. Moreover, the differential pumping system and a fuel-feeding system are necessary as well as utilities of the control power supply system. Pellet (HFS) Pellet (LFS) Fig. 3.10.2.5-1 Basic arrangement plan of Pellet injection system (P-5 section) Pellet (HFS) Pellet (LFS) Fig. 3.10.2.5-2 Basic arrangement plan of Pellet injection system (P-12 section) 3. Plant Description Sec.3.10 Page 7
Pellet (HFS) Pellet (LFS) Fig. 3.10.2.5-3 Basic arrangement plan of Pellet injection system (P-14 section) Table. 3.10.2.5-1. Whole main machine composition A main machine name Quantity Pellet injection system 3 Pellet flight tubes 3 Torus isolation vessel 3 Vacuum pumping system 1 Fuel-feeding system 1 Utilities 3 Torus isolation valve 3 References [3.10.2-1] K. Kizu et al., Fusion Engineering and Design 58-59 (2001) 331. [3.10.2-2] H. Takenaga et al., Proceedings of 33rd European Physical Society Plasma Physics Conference, 30I (2006) P-4.112. [3.10.2-3] A.A. Tuccillo et al., Nucl. Fusion 46 (2006) 214. [3.10.2-4] I. Viniar et al., Fusion Engineering and Design 58-59 (2001) 295. [3.10.2-5] N. Toyoshima, et al., The Design Study of the JT-60SU Device (No.10), JAERI-Research 97-008 (1997). 3. Plant Description Sec.3.10 Page 8