Conceptual Design of Neutral Beam Injection System for EAST

Transcription

1 Conceptual Design of Neutral Beam Injection System for EAST HU Chundong (胡纯栋), NBI Team Institute of Plasma Physics, Chinese Academy of Sciences, Hefei , China Abstract Neutral beam injection (NBI) system with two neutral beam injections will be constructed on the Experimental Advanced Superconducting Tokamak (EAST) in two stages for high power auxiliary plasmas heating and non-inductive current drive. Each NBI can deliver 2 4 MW beam power with kev beam energy in s pulse length. Each elements of the NBI system are presented in this contribution. Keywords: neutral beam injection system, tokamak, EAST, auxiliary heating PACS: Cx, Ak, Gj, t DOI: / /14/6/30 1 Introduction The Experimental Advanced Superconducting Tokamak (EAST) is a non-circular advanced steady-state experimental device. The first plasma was successfully achieved in September, 2006 [1]. The scientific mission of EAST project is to develop a steady-state operated advanced tokamak of 1000 s to establish scientific and technological bases for an attractive fusion reactor as a future energy source [2]. In order to achieve this requirements, a NBI system of 4 8 MW with s needs to be constructed as a high power auxiliary heating and non-inductive current drive system [3 5]. The EAST NBI system comprises two neutral beam injectors, which will be constructed in two stages, respectively. According to the schedule, a test bed for the R&D of the neutral beam injectors will begin to work in From that time, the first injector will be finished in the following three years. The two injectors have the same structure, each of which is mainly made up of two bucket ion sources and one beamline. The vacuum vessel of the beamline is divided into three parts for convenient installation and maintenance. The designed power is 2 MW at 50 kev and 4 MW at 80 kev. EAST NBI will works with tangential injection positioned at 19.5 degree to the radial in co-injection direction [6]. The layout of neutral beam injector and several main parameters are shown in Fig. 1 and Table 1, respectively. The length of the beamline is about 6 m from the exit grid of accelerator to the center of the tokamak plasma. The estimated weight is 40 tons. The beam centerline is 6.18 m above the ground floor, which is of the same height as the EAST horizontal midplane. Fig. 2 shows the make-up of a complete EAST-NBI, and its key conceptual design is described in the following sections. supported (a) (b) Planform view of neutral beam injectors layout Elevation view of the neutral beam injector from S-S Fig.1 The layout of the neutral beam injector system for EAST (color online) by National Natural Science Foundation of China (No ), the Chinese Academy of Sciences Knowledge Innovation Project: the study of neutral beam steady-state operation of the key technical and physical problems

2 Table 1. Parameters of the EAST-neutral beam injector Parameter name Number of ion source Angle between two extraction direction Beam energy Beam current Pulse length Beam power Fig.2 line) kev A s 2 4 MW Sketch map of the neutral beam injector (color on- Ion source The ion source includes the plasma generator and accelerator, as shown in Fig. 3. The parameters of the ion source are presented in Table 2. It is a hot cathode arc source named bucket ion source, which has the features of large volume plasma production and large area ion extraction at high ion density [7 10]. In the arc chamber, the primary electrons produced by 32 hot cathode filaments would be used to collide with the operating gas to generate plasma. To guarantee the plasma production at high density and high uniformity, a multipole cusp magnetic field is formed by permanent magnets around the arc chamber. The engineering structure of the ion source is shown in Fig. 4. From experimental experiences on several large tokamaks [11 15], this kind of ion source can provide enough source plasma. The beam species is deuterium, and the atomic fraction is as high as 80%. Table 2. Parameters of the ion source Parameter name Source sort Bucket ion source Number of filaments 32 Beam cross section at source 120 mm 480 mm Number of acceleration 4 electrode Extraction sort Multi-sort Divergence angle X: 0.6, Y : Beam species D : D : D3+ =80%:14%:6% Arc power 80 kw Ion beam is extracted and accelerated by a fourelectrode acceleration system with multiple slit type apertures, as shown in Fig. 4. The extraction cross section is 120 mm 480 mm, and the beam divergence (1/e half angle) is expected to be 0.6 degree parallel to the slots and 1.2 degree perpendicular to the slots. It is a special feature of the slot grid accelerator that the parallel and perpendicular divergence angles are different. The transparency of accelerator grid is 0.6, and the designed maximum extracted ion density is 0.24 A/cm2. The power deposit on the grids is expected to be 1% to 2% of the total beam power and the power of each grid will less than 1 kw when the cooling water goes through the pipe inside the grids. Six Langmuir probes are installed in arc chamber in front of the extraction system to measure the properties of source plasma [16] and to feed signals back to control the arc power based on the measurement of plasma density to achieve stable arc discharge. Furthermore, active cooling water is used to remove the heating deposit on the components, such as filament plant, arc chamber and accelerator grids, which is very important for stable and long pulse operation of ion source. Fig.4 Structural engineering map of the ion source (a) Arc chamber, (b) Accelerator system (color online) 3 Fig Sketch map of the neutral beam injector Beamline The EAST-NBI vacuum vessel is cylindrical and divided into three parts. To obtain a vacuum excellent performance, rational layout of each inner element of the beamline is very important. The rear cryopanel,

3 HU Chundong et al.: Conceptual Design of Neutral Beam Injection System for EAST neutralizer and ion dump are installed in the first part, the bending magnet is installed in the second part and the front cryopanel and calorimeter are installed in the third part, as shown in Fig. 5. There is other active cooling water system used to remove the power deposit on the beam inner elements, especially the ion dump and calorimeter. Fig Schematics of the beamline (color online) The first part of beamline and its inner elements The neutralizer is designed to achieve a neutralization efficiency of 55% [17]. It is a long region which uses the gas flow out of the ion source to provide a certain charge exchange gas target for the conversion of ions into neutrals. Provisions are made at selected points along its length to provide additional target gas. The neutralizer is made of copper plate with external water cooling. At the end of the neutralizer is a magnetic shield. Fig.6 Inner structure of the first part of beamline (color online) The ion dump is installed at the bottom of the vacuum vessel and formed a unitary part with neutralizer. It is a V shaped set of copper plates which also have external water cooling. And the function of the ion dump is to absorb the power of the residual ions which are reflected by the bending magnet, which can be up to 4 MW. Besides, a cylindrical cryopanel named rear cryopanel with pumping area up to 8 m2 is also installed in the first part of the vacuum vessel. 3.2 The second part of beamline and inner elements The non-neutralized ions in the beam are reflected into an ion dump by a 180 degree reflecting magnet [18]. The reflecting magnet made by hollow water-cooled electrical coils is installed in the second part of the vacuum vessel. This scheme has the advantage of keeping the gas load arising from ion reflection in the high pressure section of the beamline. The magnet has two 20.5 cm gaps for the two sources, the field intensity of each magnetic field can be up to 1700 Gs and the deflection radius of full energy D+ is 42 cm. During the bending process of the residual ions, few ions will be neutralized by collision with the background molecule in the gap of the magnet. Thus, the resulting energetic atom will move along the tangent direction of the bending track. So, the ion shields are needed to be installed to protect the magnet and forward cryopanel. The magnet shielding fingers are used to shield the ion drift between the dumps and the magnet. The main parameters of the reflecting magnet are listed in Table 3. And a collimator is used on the magnet entrance side to protect the exposed edges of the gap. Fig. 7 shows the structure of the second part of beamline. Table 3. Main parameters of the reflection magnet Parameter name Magnetic field strength Deflection radius Space of magnet field 1700 Gs 42 cm 470 mm 180 mm 1375 mm Distance between magnet poles 20.5 cm Fig.7 line) Structure of the second part of beamline (color on- 3.3 The third part of beamline and inner elements A circular disk shape cryopanel named front cryopanel and the calorimeter are installed in the third 569

4 part of the vacuum vessel. The pumping area of the front cryopanel is up to 6 m2. Together with the rear cryopanel, the two crypanels provide a required differential vacuum pressure distribution condition in the beamline to avoid the re-ionization of the neutral beam [19]. The calorimeter is used to measure the beam power and profile, the design of which is similar with that of ion dump. To improve the reliability of measurement, calorimeter adopts two kinds of measuring methods: (1) sampling method [20]. First, the temperature increments at certain selected sampling spots are measured by thermocouples, and then the relevant power densities of these spots can be calculated; finally, fitting the power density of all points at the calorimeter to obtain the beam profile; (2) cooling water method. Measuring the heat taken away by the cooling water of calorimeter can gives the total beam energy the calorimeter suffered, and the energy divided by the pulse duration is the total beam power. Cable drivers are used to retract the calorimeters in several minutes. Fig. 8 shows the structure of the third part of beamline. 4.1 Filament is used to supply primary electrons to generate plasma. In order to gain the required source plasma, the filament should supply a large filament current to produce sufficient primary electrons. To increase the lifetime of the filaments, the filament voltage loading mode is designed as slow slope ascent and the unloading mode is designed as slow decent mode. The rise time is selected between 3 s and 7 s to reduce the overshoot current, and the descent time is about 1 s or 2 s to avoid induced electric field damage to the filaments. The source filament power supply specifications are given in Table 4. Table 4. 4 Structure of the third part of beamline (color on- 570 Unit Voltage V 20 Voltage regulation % ±3 Current ka Pulse width 5.5 steady-state Arc power supply The arc power supply is used to supply power to produce the plasma with ample temperature, density and spatio-temporal distribution. The optics of the extracted ion beam depends strongly on the density of the plasma, np, in the arc chamber. Consequently, np must 2/3 be changed with Va, where Va is acceleration voltage. Meanwhile, the ion beam current is proportional to np, and np is approximately proportional to the arc current. So a stable and high-energy power supply is needed to generate steady arc current for steady ion beam extraction. The arc power supply specifications are given in Table 5. Table 5. Power supply Separate power supply sub-systems are being proposed for each of the two ion sources in order to ensure operational flexibility and ease of construction, maintenance and future expansion. Each sub-system will provide appropriate filament, arc, acceleration grid, gradient grid, and suppressor grid power to its associated ion source. Each sub-system will perform the electrical functions of primary line power conditioning, transformation and rectification, regulation, overload protection, monitoring, and control necessary for conditioning and operating its associated ion source. The filament and arc power supplies are referenced to the acceleration voltage. The power suppliers of acceleration grid, gradient grid and suppressor grid are referenced to the ground potential. The following sections summarize the requirements of each of the two power supply sub-systems. Filament power supply specifications Parameter 4.2 Fig.8 line) Filament power supply Arc power supply specifications Parameter Unit Voltage V 200 Voltage regulation % ±1 Current ka 3 Pulse width 4.3 steady-state Acceleration grid power supply The acceleration grid power supply is connected to the acceleration grid; its specifications are given in Table 6. The power supply will be designed to permit independent interruption of current to any source and crowbar within tens µs in the event of a spark, while not disturbing the normal pulsing of other adjacent sources. The sketch of acceleration power supply is shown in Fig. 9.

5 HU Chundong et al.: Conceptual Design of Neutral Beam Injection System for EAST Table 6. Acceleration power supply specifications Parameter Unit Voltage, nominal kv 100 Voltage regulation % ±1 Current A 100 Pulse length s 100 Repetition period min 10 Voltage rise-time µs 20 Fig.9 Sketch map of the acceleration power supply 4.4 Gradient grid power supply When the voltage of acceleration power supply is up to 100 kv, secondary acceleration is useful for good ion optics. A resistive divider housed within the modulator is connected with the acceleration power supply and supplies power to the gradient grid. Usually, it is 75% 90% of the acceleration power supply. 4.5 Suppressor grid power supply Table 7 lists the specifications of the suppressor grid power supply. The high energy ions may impact on the grids and produce electrons. The purpose of this grid is to prevent backstreaming electrons flowing back into the ion source. So the voltage of the suppressor grid is negative to the ground potential. Table 7. Suppressor grid power supply specifications Parameter Unit Voltage, nominal kv 4 Voltage regulation % ±1 Current A 20 Pulse length s 100 Repetition period min 10 Voltage rise-time µs 5 5 Control system The control system of EAST-NBI (NBICS) is needed to monitor completely the whole operational process of the experiment. The key issue for the control system is to keep high reliability, expandability, and continuity. The following should be taken into consideration in the conceptual design stage: a. The control system should ensure secure operation of the EAST-NBI. b. The control system should provide a stable repeatability during the operation. c. The control system should adopt structural/modular design to guarantee high reliability. d. The control system should offer the user diagnostic methods to check the system and to restore the system components. e. The control system should be composed of commercially marketed components, which are well supported by manufacturing company in order to easily obtain replacement parts. f. The control system should provide enough application interfaces among operator, physical analyst and device. Due to these requirements, an opening distributed system is the best choice for NBICS. Considering current demands, a control system for EAST-NBI test bed is being constructed, as shown in Fig. 10. According to different functioning, this complicated system can be divided into three constituent parts: Computer Data Processing System, Measurement and Control System, Alarm and Interlock Protection System. 5.1 Computer data processing system (CDPS) CDPS is the key component to realize good flexibility with extension and high speed data acquisition, storage and transfer of NBICS. It can implement four remote control functions in the centre control room or through the Internet: (1) remote operating the EAST-NBI experiment, (2) storing and processing the experiment data, (3) remote alarm and protection of the EAST- NBI system, and (4) remote monitoring. 5.2 Measurement and control system (MCS) MCS is the critical system to achieve the stable operation of the high-powered NBI system. It should be simplified as much as possible to ensure high reliability, and any invalidation is not permission. Specifically, the MCS should be furnished with the following functions: a. Providing kinds of control methods, including remote control in the control room and field manual/automatic control; b. Real-time acquiring and storing the operational data; c. A well-designed human-machine interface; d. Credible interlock and protection among each system; e. Flexible alarm and operational parameters setting; f. Real-time feedback control based on the Langmuir probe; g. Synchronous and cooperative operation among each system; h. Operation monitoring and control; i. Carrying out different operating modes of ion source or beamline; j. Patrol checking on the key equipments before the operation. 571

6 Fig Schematic view of the control system for EAST-NBI test bed Alarm and interlock protection system (AIPS) The AIPS can provide real-time protection for the ion source, beamline, power supply and other elements of EAST-NBI system. The AIPS should present local and remote alarm information with presettable optical, electrical, acoustic or drawing & text signals. If necessary, the AIPS will directly execute protection and interlock in some high demanding system through hardware circuits. 6 Conclusion A high power and long pulse NBI system will be conducted on EAST and its conceptual design has been completed in EAST-NBI system comprises two neutral beam injectors, and the main design requirement of each injector is to deliver a deuterium neutral beam of 2 MW beam power with 50 kev beam energy or 4 MW with 80 kev beam energy, the pulse length is s. The construction of EAST-NBI system and the test bed are in process on schedule References Wan Y X. 2006, Overview Progress and Future Plan of EAST Projec. Presented at the 21st IAEA Fusion Energy Conference (Chengdu, China, October 16-21, 2006). Zhou D, St. John H, Hu Y M, et al. 2009, Plasma Sci. Technol., 11: 417 Sakuraba J, Akiba M, Arakawa Y. 1981, Rev. Sci. Instrum., 52: 689 Akiba M, Araki M, Horiike H, et al. 1982, Rev. Sci. Instrum., 53: 1864 Hu C D, Xie Y H, Xie Y L. 2009, Development of Long Pulse Neutral Beam Injector System for the EAST Tokamak. Presented at Chinese Nuclear Society Fall Meeting (Beijing, China, November 17-20, 2009). Wang J F, Wu B, Hu C D. 2010, Plasma Sci. Technol., 12: 289. Vella M C, Cooper W S, Pincosy P A, et al. 1988, Rev. Sci. Instrum., 59: 2357 Holmes A T J, Green T S, Newman A F. 1987, Rev. Sci. Instrum., 58: 1369 Akiba M, Araki M, Horiike H, et al. 1982, Rev. Sci. Instrum., 53: 1864 Goebel D M. 1982, Phys, Fluids, 25: 1093 Redi M H, Zarnstorff M C, White R B, et al. 1995, Nucl. Fusion, 35: 1191 Duong H H, Heidbrink W W. 1993, Nucl. Fusion, 33: 211 Putvinskij S V, Tubbing B J D, Eriksson L G, et al. 1994, Nucl. Fusion, 34: 495 Tobita K, Tani K, Neyatani Y, et al. 1992, Phys. Rev. Lett., 23: 3060 Tsai C C, Menon M M, Ryan P M, et al. 1982, Rev. Sci. Instrum., 53: 417 Xie Y H, Hu C D, Liu S, et al. 2010, Fusion Eng. Des., 85: 64 Hu C D, Wei J L, Liang L Z. 2010, The Simulation on Beam Interaction with Background Particles. Presented at the 19th International Conference on Cyclotrons and their Applications (Lanzhou, China, September 06-10, 2010) Liang L Z, Hu C D. 2010, Simulation and Design of a Reflection Magnet for EAST Neutral Beam System. Presented at the 2nd International Symposium on Negative Ions, Beams and Source (Takayama, Japan, November 16-19, 2010) Liang L Z, Hu C D, Xie Y L, et al. 2010, Chinese Phys. C, 34: 972 Xu Y J, Hu C D, Xie Y L, et al. 2010, J. Fusion Energ., 29: 395 (Manuscript received 17 May 2011) (Manuscript accepted 5 December 2011) address of HU Chundong: cdhu@ipp.ac.cn