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Physical design of the neutralizer for CFETR negative ion based neutral beam injection prototype
Fusion Engineering and Design 148 (2019) 111316
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Physical design of the neutralizer for CFETR negative ion based neutral beam injection prototype
T
Zhaoyuan Zhanga, Guodong Wanga, , Changqi Chena, Jianglong Weib, , Yuanlai Xieb, Ling Taob, Lizhen Liangb, Chundong Hub ⁎
a b
⁎
School of Mechanical Engineering, Hefei University of Technology, Hefei, 230009, PR China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, PR China
ARTICLE INFO
ABSTRACT
Keywords: Neutral beam injection Neutralizer Particle tracing model Gas flow analyses
A research project of the CFETR Negative ion based Neutral Beam Injection (NNBI) prototype has been started in China. The objectives of the CFETR NNBI protype are to produce a negative hydrogen ion beam of > 20 A to 200–400 kV for 3600 s and to attain a neutralization efficiency of > 50%. Thus, a neutral beam power of > 2 MW is foreseen onto the calorimeter (i.e., beam dump). A gas neutralizer is applied inside the beamline of CFETR NNBI prototype, to achieve the neutralization of negative ion beam through the charge-exchange collisions with the gas target. The most critical issues for the gas neutralizer are to balance the contradictive requirements of high neutralization efficiency, low gas inflow, and low heat load. During the physical design of the neutralizer for CFETR NNBI prototype, two proposals have been considered and studied in detail. One has single beam channel and other one has two narrow channels separated by a panel. The narrow channel design is benefic to decrease gas conductance, and thus reduce the required gas inflow to attain the maximal neutralization efficiency. However, the inserted panel will restrict the extraction area of the beam source, where the aperture columns corresponding to the panel should be masked to avoid the direct beam interception. But the leading edge of the inserted panel will still suffer a high thermal load due to the stray particles from the beam source. A 3D model of the whole beamline of CFETR NNBI prototype has been developed to study the gas flow and the beam transport in the beamline. Based on this model, these two neutralizer designs have been evaluated in detail, in terms of the relation between neutralization efficiency and gas inflow, and the power load on the neutralizer components.
1. Introduction A new magnetic confinement fusion device, named China Fusion Engineering Test Reactor (CFETR), is under design in China. CFETR is a fully superconducting tokamak device which has a similar scale to the fusion experimental reactor ITER. It aims to complete the critical science and technology towards a demonstration fusion reactor DEMO. Hence, its primary missions are demonstrating full cycle of fusion energy production, self-sustain with a tritium breeding ratio (TBR) > 1.0, steady-state operation with a duty cycle of about 0.3∼0.5 [1]. The hydrogen isotopes (i.e., H, D and T) neutral beam injection (NBI) system is a promising plasma heating and current drive system, which will also be applied to CFETR. For large-scale fusion devices like ITER, CFETR and DEMO, the required injection beam energy is larger than 200 keV/amu to attain more power deposition in the core plasma. At such a high beam energy, a negative ion based neutral beam injection
⁎
(NNBI) system is inevitable [2]. Because the neutralization efficiency reduces sharply for positive ions above 50 keV/amu, but it is acceptable for negative ions. While most of NBI systems worldwide are based on positive ions, only two NNBI systems have been routinely in operation for the Large Helical Device (LHD) [3] and JT-60U tokamak [4] in Japan. According to their operation experience, the NNBI system is much more complex, large and elusive than the P-NBI system. Faced to the more challenging operational parameters required for ITER NNBI system, several related test facilities (i.e., ELISE [5], SPIDER [6], MITICA [7]) have been or are planned to be developed step by step, in order to verify and optimize the physical and engineering design of the beam source and the overall injector. As a critical step towards the CFETR NNBI system, a pilot project of the CFETR NNBI prototype is in progress to investigate the challenging science and technology issues. The missions of CFETR NNBI prototype include: (1) to develop a RF-driven negative hydrogen ion
Corresponding authors. E-mail addresses: [email protected] (G. Wang), [email protected] (J. Wei).
https://doi.org/10.1016/j.fusengdes.2019.111316 Received 26 June 2019; Received in revised form 29 August 2019; Accepted 29 August 2019 Available online 03 September 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 148 (2019) 111316
Z. Zhang, et al.
beam source which is required to extract and accelerate > 20 A negative ion beam to 200–400 kV for 3600 s; (2) to develop a beamline system which can neutralize and transport the negative ion beam with the neutralization efficiency of 50% for 3600 s; (3) to develop the technology of high voltage power supply, high voltage transmission and high voltage insulation. A gas neutralizer is used for the neutralization of negative ion beam inside the beamline system of CFETR NNBI prototype. The neutralization process is based on the beam-gas collisions in the neutralizer, but which will cause not only the neutralization of negative ions, but also the double-stripping of negative ions and the ionization of neutrals. Hence, the amount of gas molecules should be suitable to attain an optimal neutralization efficiency and the critical parameter is the gas line integrated density (i.e. ngas dl , also called gas target thickness). The variation of beam species with the gas line integrated density can be descried via a set of differential equations [8]:
dF 1 =1 d nl dF0 = d nl dF1 = d nl
(
10
10 F 1
+
10 F1
01 F0
11 F 1
+
01 F0
10 F1
+
11 ) F 1
Fig. 2. Calculation model for the beamline system of CFETR NNBI prototype.
2. Model descriptions
(1)
The beamline system of CFETR NNBI prototype is based on the design of the ITER heating neutral beam (HNB) injectors [9]. Because the science and technology issues related to the ITER HNB injectors have been systematically researched and developed for decades, and their designs have high reliability and availability. The beamline system of CFETR NNBI prototype mainly includes a beamline vessel, a gas neutralizer, a residual ion dump (RID) with electrostatic removing, a calorimeter and two sets of cryopump. The major difference is that the beam source is installed outside the beamline vessel for the CFETR NNBI prototype, instead of inside a beam source vessel for the ITER HNB. For maintenance accessible, there is a gate valve between the beam source and the beamline vessel of CFETR NNBI prototype. A basic design model of the beamline system is shown in Fig. 2. The beam source is not included in the model instead of the exit of the grids system and the beam source duct. The beam source duct is 0.4 m wide, 1.8 m high, and 0.5 m long. The inner space of the gate valve is simplified to a cylinder which is 2.2 m in diameter and 0.5 m in length. The gas neutralizer is of rectangular cross-section, 3 m long, 1.7 m wide and 0.32 m wide corresponding to the beam profile. The neutralizer entrance is located 1 m downstream of the front plate of the beamline vessel, and the exit is 0.5 m upstream of the RID. Hence, the gas exiting the neutralizer into these two gaps can be pumped away by the two lateral cryopumps. That is beneficial to suppress the stripping losses in the beam source and the re-ionization losses in the RID. The sophisticated cryopumps are simplified by two vertical cryopanels with the length of 8 m and height of 2.6 m. The front edge of each cryopanel and the neutralizer entrance are located in the same plane.
where, the F-1,0,1 represents the fraction of H−1, H0 and H+ beam species, the ij means the cross section of Hi changing to Hj. For either 200 keV or 400 keV negative hydrogen ion beam of CFETR NNBI prototype, the beam species evolutions are illustrated in Fig. 1. The largest neutralization efficiencies are both > 55% and the corresponding gas target thickness are 6.3 × 1019 m-2 and 11.2 × 1019 m-2. Note that, the growth of neutralization efficiency is slow near the largest point. To reduce the re-ionization loss, the optimal target thickness is a bit smaller (6.0 × 1019 m-2 for 200 keV and 10 × 1019 m-2 for 400 keV, respectively), where the fractions of H0 and H+ are equal. Furthermore, such a power equal on the positive and negative ion dump is a visualized reference during the operation. The neutralizer is the first component downstream the beam source inside the beamline system, so the stray particles from the beam source (e.g. beam halo, accelerated electrons) will deposit on the neutralizer walls inevitably. Hence, the amount and also the density of the power load is another critical issue for the neutralizer. The physical design of the neutralizer of the CFETR NNBI prototype is described in this paper. The basic design criterion is minimizing the gas inlet for the required neutralization efficiency and meanwhile reducing the beam power on the neutralizer as low as possible.
3. Gas flow in the neutralizer There are two gas sources in the CFETR NNBI prototype, one is from the beam source for plasma generation, the other is from the neutralizer for beam neutralization. The beam source contains four RF drivers which are arranged in a line on the expansion chamber. The non-ionized hydrogen gas is assumed to be 3 Pa·m3/s exiting from the beam duct. Although the residual gas is not ionized, it is heated to a high temperature (about 650 K according to experimental results [10]). The filling hydrogen gas for neutralizer is injected from two vertical injection tubes on the lateral walls. These two injection tubes are located in the midway along the neutralizer, that is 2 m downstream the neutralizer entrance. The gas injection rate can be adjusted and the gas temperature is 293.15 K (i.e., room temperature). Only the cryopumps
Fig. 1. Beam species evolution with different gas target thickness originated from 200 keV and 400 keV H− beam. 2
Fusion Engineering and Design 148 (2019) 111316
Z. Zhang, et al.
are considered in the model, because the pumping speed of the turbo molecular pumps is too low. The two vertical cryopanels are assumed to be a mean temperature of 90 K and an effective capture factor of 0.3. The other surfaces in the vacuum vessel are set to be room temperature. The gas flow in the whole vacuum vessel is assumed to be under the free molecular flow, where the collisions between gas molecules can be ignored. Here the angular coefficient method is used to solve molecular flows that computes the total flux arriving at a point by integrating the flux arriving from all other surface elements visible from that point. The macroscopic variables in the vicinity of the surface can be derived from kinetic theory. Compared to the Monte Carlo method [10–12], the calculation consumption and the statistical noise of the angular coefficient method is smaller. A finite element program is used to implement the angular coefficient method based on the COMSOL Multiphysics environment. The mesh of the calculation model was built through free tetrahedral. The number of tetrahedral elements is 172,903. The maximum and minimum element size is 0.25 m and 0.0025 m, respectively. In this case the gas molecules are assumed to be adsorbed and subsequently diffusely emitted from the surface (this is often referred to as total accommodation). When emitting from the gas inlets or reflecting from the walls, the velocity components of the gas molecules follow the Boltzmann-Maxwell distribution and the angle follows the Knudsen’s cosine law. Basically, the gas neutralizer of CFETR NNBI prototype has two design options. The most common one for NBI systems is a single gas cell, the cross section of which has a similar shape and size as the ion beam profile. But for the ITER HNB injector, the neutralizer is divided into 4 adjacent vertical channels, to reduce the gas conductance of the neutralizer and consequently to reduce the gas inflow needed for the optimum neutralization of the beam. Specific to the CFETR NNBI prototype, the two designs are not difficult to interconvert. A single large channel (320 mm wide) can be transformed to two narrow channels (145 mm wide) by inserting a panel (30 mm wide) into the neutralizer. The distribution of gas density along the centerline of the neutralizer channel is shown in Fig. 3 with the gas injection quantity from the neutralizer of 0, 5, 10, 15, 20 Pa·m3/s. The display range includes the whole vacuum vessel. The gas concentrates inside the neutralizer and the contribution from the beam source is so small. The gas densities are higher in the design of two narrow channels as expected. Hence, the design of two narrow channels is better in the view of gas load limitation. As shown in Fig. 4, the gas target thickness increases linearly with the gas puff quantity. The slope of the line of two narrow channels is larger than that of single large channel by a factor of 1.5. To attain the optimal target thickness for 200 keV H− beam, the gas flow rates are 17.4 Pa·m3/s and 11.5 Pa·m3/s for a single large channel and for two narrow channels, respectively. For 400 keV H− beam, the single large
Fig. 4. Gas target thickness for two designs of neutralizer.
channel is not applicable anymore, because the required gas inlet rate is up to 30 Pa·m3/s, which will cause too much gas load to the cryopumps. But for two narrow channels, the required gas flow rate is more acceptable (19.1 Pa·m3/s). 4. Power loads on the neutralizer The problem of the divided channels design of the neutralizer is that the inserted panels may intercept a large beam power. Therefore, the negative ion beam should also be separated to several columns corresponding to the neutralizer channels. As a result, the extracted ion current will be reduced due to the masked apertures. However, the inserted panels will still suffer a great heat load by the stray particles, which are caused by the beam core with small divergence, the beam halo with large divergence, the beam deflection due to magnetic field, the misalignment due to the thermal deformation or the manufacturing and assembly tolerances of the planar grids [13]. Specific to the CFETR NNBI prototype, the extraction area of the beam source is 0.32 × 1.6 m2, which is about half of the ITER beam source. The aperture array of the grids system is displayed in Fig. 5. It is obvious that the aperture array is divided into two columns in the horizontal direction, to match the two channels of the neutralizer. In the vertical direction, the aperture array is divided into four blocks corresponding to the four segments of each grid. Hence, the total 768 beamlets are divided into 8 groups. A 1/4 model of beam transmission is considered due to the symmetry, in order to estimate the beam power deposited on the
Fig. 3. Gas density profiles along the centerline of neutralizer channel: (a) single large channel design, (b) double narrow channel design. 3
Fusion Engineering and Design 148 (2019) 111316
Z. Zhang, et al.
A typical power load on the neutralizer is shown in Fig. 6 for the 400 keV beam (11.2 MW) with ωc = 7 mrad and m = 3 mrad. The largest power load density (2.74 MW/m2) is located at the leading edge of middle panel, although the deposited power is only 100 kW. To avoid the accidental damage, the leading edge should be chamfered to decreasing the power load density and be enhanced the heat removal ability. Similarly, the entrance dump also suffers high power load density, which is needed to be carefully treated. The power load density on the channel walls is relatively low, especially on the lateral walls. Indeed, except the middle panel, Fig. 6 can directly reflect the power load situation for single channel design. Because of the beamlets divergence, the power load is higher on the downstream part of middle panel and lateral wall. Although the distance between outside apertures and lateral wall is larger than that between outside apertures and middle panel, the power load on the middle panel is much higher due to the misalignment. A little convergence of the beamlets groups appears on the lateral walls and middle panel, which is also caused by the misalignment. The influence of divergence and misalignment on the beam power load are indicated in Fig. 7. As expected, the total power load and the maximum power load density on the neutralizer are increased with the beamlet divergence and the misalignment. The total beam power load on the neutralizer is 5.9%∼10.9% of the initial beam power. The only exception is the cases of total power load between m = 0 and m = 1 mrad. It seems that the total power load of m = 1 mrad is not higher, but a little lower than that of m = 0 mrad. The reason can be revealed by the power load on each component of the neutralizer. Apparently, the major contributions of the power load on the neutralizer are from the middle panel and the lateral wall. Their variations with the misalignment are opposite. For the cases of m = 0 and m = 1 mrad, these two power loads are nearly the same, so the total power loads also have little difference. A similar opposite trend can be found between the leading edge and the entrance dump. The power load on each component has guiding significance for the thermo-mechanical design of the neutralizer for CFETR NNBI prototype. 5. Discussions and conclusions This paper is focused on the power load from the beam particles. While the energic electrons emitted from the beam source may be another important power load on the neutralizer. Such electrons are produced via the stripping of negative ions and the ionization of background gas within the accelerator and are further accelerated to high energy. When evaluating the gas flow and the power load in the beamline system, the distance from the beam ions emitting surface to the neutralizer is a key influence factor. By now, the only certainly is the distance between the neutralizer entrance and the front plate of the beamline vessel. But the designs of the beam source and the large-scale gate valve still need further improvement and confirmation. Based on the established design concepts, a 3D model of the beamline system of the CFETR NNBI prototype was developed, which includes all the key components. Firstly, the gas flow in the beamline system was simulated for two design concepts of the neutralizer via an angular coefficient method. The design of two narrow channels can increase the gas density in the neutralizer because of the reduction effect to the gas conductance. Consequently, the required gas flow rate of two narrow channels is ∼2/3 of the single large channel to attain an optimal neutralization efficiency. For 200 keV H− beam, the gas flow rates are 17.4 Pa·m3/s and 11.5 Pa·m3/s for single large channel and for two narrow channels, respectively. For 400 keV H− beam, the single large channel is not applicable anymore and the required gas flow rate is 19.1 Pa·m3/s for two narrow channels. To estimate the beam power load on the neutralizer, 5000 simulation particles for each beamlet (i.e., total 3,840,000 particles for all 768 beamlets) were produced via a Monte Carlo method based on the
Fig. 5. Aperture array of grids system for the beam source of CFETR NNBI prototype.
neutralizer. A Monte Carlo method was used to produce simulated beam particles based on the biGaussian function [14,15]. The single beamlet divergence consists of beam core and beam halo. The fraction of power carried by the halo f is 15% and the divergence ωh is 30 mrad, which are assumed to be standard in all cases. The misalignment considered in this paper is simply a result of thermal deformation. According to the experimental and simulated results, the heat loads on the grid segments mainly located on the top surface and inside the aperture, which causes the grid segments are convex to upstream direction. As a result, the normal direction of each beamlet emission surface may have some deviation. Here, the misalignment angle of each beamlet is supposed to be the same (m = 0, 1, 2, 3 mrad), but their deviation directions are different. All deviations are assumed to point to the center of the grid segments. There are 5000 simulation particles for each beamlet. For calculating the power load density, the mesh of the neutralizer walls was built via free quad. The quad element size is 50 × 50 mm2 on the lateral surface and 50 × 10 mm2 on the leading edge. In addition, for better diagnosing and studying the beamlet properties, there is no requirement of beam focusing for the CFETR NNBI prototype. According to the design values of extracted ion current density (300 A/m2) and accelerator losses (∼20%), the maximum H- beam current emitted from beam source is 28 A. Thus, the maximum beam power is expected to be 11.2 MW at the beam energy of 400 keV. 4
Fusion Engineering and Design 148 (2019) 111316
Z. Zhang, et al.
Fig. 6. Typical power load on the neutralizer for the 400 keV beam with ωc = 7 mrad and m = 3 mrad.
Fig. 7. Beam power load on the neutralizer at different divergence and misalignment: (a) total power load and power losses of initial beam power, (b) maximum power density, (c) power load on the leading edge and entrance dump, (d) power load on the middle panel and lateral wall.
5
Fusion Engineering and Design 148 (2019) 111316
Z. Zhang, et al.
biGaussian function. The divergence of beam core, the fraction and divergence of beam halo and the misalignment of beamlet group are considered in the estimation. All these factors will increase the power load on the neutralizer, the maximum power load is up to 10.9% of the initial beam power. The leading edge of the inserted panel for two channels design need an extra protection, because the power load density (up to 2.7 MW/m2) is extremely higher than the other parts of the neutralizer.
[2] Y. Takeiri, Negative ion source development for fusion application (invited), Rev. Sci. Instrum. 81 (2010) 02B114. [3] Y. Takeiri, O. Kaneko, K. Tsumori, et al., High performance of neutral beam injectors for extension of LHD operational regime, Fusion Sci. Technol. 58 (2010) 482–488. [4] M. Hanada, A. Kojima, Y. Tanaka, et al., Progress in development and design of the neutral beam injector for JT-60SA, Fusion Eng. Des. 86 (2011) 835–838. [5] B. Heinemann, U. Fantz, W. Kraus, et al., Towards large and powerful radio frequency driven negative ion sources for fusion, New J. Phys. 19 (2017) 015001. [6] V. Toigo, S. Dal Bello, E. Gaio, et al., The ITER neutral beam test facility towards SPIDER operation, Nucl. Fusion 57 (2017) 086027. [7] V. Toigo, R. Piovan, S. Dal Bello, et al., The PRIMA test facility: SPIDER and MITICA test-beds for ITER neutral beam injectors, New J. Phys. 19 (2017) 085004. [8] J. Kim, H.H. Haselton, Analysis of particle species evolution in neutral-beam injection lines, J. Appl. Phys. 50 (1979) 3802–3807. [9] R.S. Hemsworth, D. Boilson, P. Blatchford, et al., Overview of the design of the ITER heating neutral beam injectors, New J. Phys. 19 (2017) 025005. [10] A. Krylov, R.S. Hemsworth, Gas flow and related beam losses in the ITER neutral beam injector, Fusion Eng. Des. 81 (2006) 2239–2248. [11] X. Luo, C. Day, 3D Monte Carlo vacuum modeling of the neutral beam injection system of ITER, Fusion Eng. Des. 85 (2010) 1446–1450. [12] J.L. Wei, C.D. Hu, L.Z. Liang, et al., Modeling the gas flow in the neutralizer of ITER neutral beam injector using Direct Simulation Monte Carlo approach, Fusion Eng. Des. 88 (2013) 46–50. [13] P. Agostinetti, G. Chitarin, G. Gambetta, et al., Two key improvements to enhance the thermo-mechanic performances of accelerator grids for neutral beam injectors, Fusion Eng. Des. 109 (2016) 890–894. [14] M. Dalla Palma, E. Sartori, M. Zaupa, et al., Simulation of the beamline thermal measurements to derive particle beam parameters in the ITER neutral beam test facility, Rev. Sci. Instrum. 89 (2018) 10J111. [15] P. Veltri, P. Agostinetti, M. Dalla Palma, et al., Evaluation of power loads on MITICA beamline components due to direct beam interception and electron backscattering, Fusion Eng. Des. 88 (2013) 1011–1014.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are very grateful to the other members of ASIPP NBI team for their continuous support and excellent work. This work was partly supported by National Key R&D Program of China (Contract No. 2017YFE300101, 2017YFE300103) and National Natural Science Foundation of China(Contract No. 11575240, 11975264). References [1] Y. Wan, J. Li, Y. Liu, et al., Overview of the present progress and activities on the CFETR, Nucl. Fusion 57 (2017) 102009.
6
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Physical design of the neutralizer for CFETR negative ion based neutral beam injection prototype
T
Zhaoyuan Zhanga, Guodong Wanga, , Changqi Chena, Jianglong Weib, , Yuanlai Xieb, Ling Taob, Lizhen Liangb, Chundong Hub ⁎
a b
⁎
School of Mechanical Engineering, Hefei University of Technology, Hefei, 230009, PR China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, PR China
ARTICLE INFO
ABSTRACT
Keywords: Neutral beam injection Neutralizer Particle tracing model Gas flow analyses
A research project of the CFETR Negative ion based Neutral Beam Injection (NNBI) prototype has been started in China. The objectives of the CFETR NNBI protype are to produce a negative hydrogen ion beam of > 20 A to 200–400 kV for 3600 s and to attain a neutralization efficiency of > 50%. Thus, a neutral beam power of > 2 MW is foreseen onto the calorimeter (i.e., beam dump). A gas neutralizer is applied inside the beamline of CFETR NNBI prototype, to achieve the neutralization of negative ion beam through the charge-exchange collisions with the gas target. The most critical issues for the gas neutralizer are to balance the contradictive requirements of high neutralization efficiency, low gas inflow, and low heat load. During the physical design of the neutralizer for CFETR NNBI prototype, two proposals have been considered and studied in detail. One has single beam channel and other one has two narrow channels separated by a panel. The narrow channel design is benefic to decrease gas conductance, and thus reduce the required gas inflow to attain the maximal neutralization efficiency. However, the inserted panel will restrict the extraction area of the beam source, where the aperture columns corresponding to the panel should be masked to avoid the direct beam interception. But the leading edge of the inserted panel will still suffer a high thermal load due to the stray particles from the beam source. A 3D model of the whole beamline of CFETR NNBI prototype has been developed to study the gas flow and the beam transport in the beamline. Based on this model, these two neutralizer designs have been evaluated in detail, in terms of the relation between neutralization efficiency and gas inflow, and the power load on the neutralizer components.
1. Introduction A new magnetic confinement fusion device, named China Fusion Engineering Test Reactor (CFETR), is under design in China. CFETR is a fully superconducting tokamak device which has a similar scale to the fusion experimental reactor ITER. It aims to complete the critical science and technology towards a demonstration fusion reactor DEMO. Hence, its primary missions are demonstrating full cycle of fusion energy production, self-sustain with a tritium breeding ratio (TBR) > 1.0, steady-state operation with a duty cycle of about 0.3∼0.5 [1]. The hydrogen isotopes (i.e., H, D and T) neutral beam injection (NBI) system is a promising plasma heating and current drive system, which will also be applied to CFETR. For large-scale fusion devices like ITER, CFETR and DEMO, the required injection beam energy is larger than 200 keV/amu to attain more power deposition in the core plasma. At such a high beam energy, a negative ion based neutral beam injection
⁎
(NNBI) system is inevitable [2]. Because the neutralization efficiency reduces sharply for positive ions above 50 keV/amu, but it is acceptable for negative ions. While most of NBI systems worldwide are based on positive ions, only two NNBI systems have been routinely in operation for the Large Helical Device (LHD) [3] and JT-60U tokamak [4] in Japan. According to their operation experience, the NNBI system is much more complex, large and elusive than the P-NBI system. Faced to the more challenging operational parameters required for ITER NNBI system, several related test facilities (i.e., ELISE [5], SPIDER [6], MITICA [7]) have been or are planned to be developed step by step, in order to verify and optimize the physical and engineering design of the beam source and the overall injector. As a critical step towards the CFETR NNBI system, a pilot project of the CFETR NNBI prototype is in progress to investigate the challenging science and technology issues. The missions of CFETR NNBI prototype include: (1) to develop a RF-driven negative hydrogen ion
Corresponding authors. E-mail addresses: [email protected] (G. Wang), [email protected] (J. Wei).
https://doi.org/10.1016/j.fusengdes.2019.111316 Received 26 June 2019; Received in revised form 29 August 2019; Accepted 29 August 2019 Available online 03 September 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 148 (2019) 111316
Z. Zhang, et al.
beam source which is required to extract and accelerate > 20 A negative ion beam to 200–400 kV for 3600 s; (2) to develop a beamline system which can neutralize and transport the negative ion beam with the neutralization efficiency of 50% for 3600 s; (3) to develop the technology of high voltage power supply, high voltage transmission and high voltage insulation. A gas neutralizer is used for the neutralization of negative ion beam inside the beamline system of CFETR NNBI prototype. The neutralization process is based on the beam-gas collisions in the neutralizer, but which will cause not only the neutralization of negative ions, but also the double-stripping of negative ions and the ionization of neutrals. Hence, the amount of gas molecules should be suitable to attain an optimal neutralization efficiency and the critical parameter is the gas line integrated density (i.e. ngas dl , also called gas target thickness). The variation of beam species with the gas line integrated density can be descried via a set of differential equations [8]:
dF 1 =1 d nl dF0 = d nl dF1 = d nl
(
10
10 F 1
+
10 F1
01 F0
11 F 1
+
01 F0
10 F1
+
11 ) F 1
Fig. 2. Calculation model for the beamline system of CFETR NNBI prototype.
2. Model descriptions
(1)
The beamline system of CFETR NNBI prototype is based on the design of the ITER heating neutral beam (HNB) injectors [9]. Because the science and technology issues related to the ITER HNB injectors have been systematically researched and developed for decades, and their designs have high reliability and availability. The beamline system of CFETR NNBI prototype mainly includes a beamline vessel, a gas neutralizer, a residual ion dump (RID) with electrostatic removing, a calorimeter and two sets of cryopump. The major difference is that the beam source is installed outside the beamline vessel for the CFETR NNBI prototype, instead of inside a beam source vessel for the ITER HNB. For maintenance accessible, there is a gate valve between the beam source and the beamline vessel of CFETR NNBI prototype. A basic design model of the beamline system is shown in Fig. 2. The beam source is not included in the model instead of the exit of the grids system and the beam source duct. The beam source duct is 0.4 m wide, 1.8 m high, and 0.5 m long. The inner space of the gate valve is simplified to a cylinder which is 2.2 m in diameter and 0.5 m in length. The gas neutralizer is of rectangular cross-section, 3 m long, 1.7 m wide and 0.32 m wide corresponding to the beam profile. The neutralizer entrance is located 1 m downstream of the front plate of the beamline vessel, and the exit is 0.5 m upstream of the RID. Hence, the gas exiting the neutralizer into these two gaps can be pumped away by the two lateral cryopumps. That is beneficial to suppress the stripping losses in the beam source and the re-ionization losses in the RID. The sophisticated cryopumps are simplified by two vertical cryopanels with the length of 8 m and height of 2.6 m. The front edge of each cryopanel and the neutralizer entrance are located in the same plane.
where, the F-1,0,1 represents the fraction of H−1, H0 and H+ beam species, the ij means the cross section of Hi changing to Hj. For either 200 keV or 400 keV negative hydrogen ion beam of CFETR NNBI prototype, the beam species evolutions are illustrated in Fig. 1. The largest neutralization efficiencies are both > 55% and the corresponding gas target thickness are 6.3 × 1019 m-2 and 11.2 × 1019 m-2. Note that, the growth of neutralization efficiency is slow near the largest point. To reduce the re-ionization loss, the optimal target thickness is a bit smaller (6.0 × 1019 m-2 for 200 keV and 10 × 1019 m-2 for 400 keV, respectively), where the fractions of H0 and H+ are equal. Furthermore, such a power equal on the positive and negative ion dump is a visualized reference during the operation. The neutralizer is the first component downstream the beam source inside the beamline system, so the stray particles from the beam source (e.g. beam halo, accelerated electrons) will deposit on the neutralizer walls inevitably. Hence, the amount and also the density of the power load is another critical issue for the neutralizer. The physical design of the neutralizer of the CFETR NNBI prototype is described in this paper. The basic design criterion is minimizing the gas inlet for the required neutralization efficiency and meanwhile reducing the beam power on the neutralizer as low as possible.
3. Gas flow in the neutralizer There are two gas sources in the CFETR NNBI prototype, one is from the beam source for plasma generation, the other is from the neutralizer for beam neutralization. The beam source contains four RF drivers which are arranged in a line on the expansion chamber. The non-ionized hydrogen gas is assumed to be 3 Pa·m3/s exiting from the beam duct. Although the residual gas is not ionized, it is heated to a high temperature (about 650 K according to experimental results [10]). The filling hydrogen gas for neutralizer is injected from two vertical injection tubes on the lateral walls. These two injection tubes are located in the midway along the neutralizer, that is 2 m downstream the neutralizer entrance. The gas injection rate can be adjusted and the gas temperature is 293.15 K (i.e., room temperature). Only the cryopumps
Fig. 1. Beam species evolution with different gas target thickness originated from 200 keV and 400 keV H− beam. 2
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are considered in the model, because the pumping speed of the turbo molecular pumps is too low. The two vertical cryopanels are assumed to be a mean temperature of 90 K and an effective capture factor of 0.3. The other surfaces in the vacuum vessel are set to be room temperature. The gas flow in the whole vacuum vessel is assumed to be under the free molecular flow, where the collisions between gas molecules can be ignored. Here the angular coefficient method is used to solve molecular flows that computes the total flux arriving at a point by integrating the flux arriving from all other surface elements visible from that point. The macroscopic variables in the vicinity of the surface can be derived from kinetic theory. Compared to the Monte Carlo method [10–12], the calculation consumption and the statistical noise of the angular coefficient method is smaller. A finite element program is used to implement the angular coefficient method based on the COMSOL Multiphysics environment. The mesh of the calculation model was built through free tetrahedral. The number of tetrahedral elements is 172,903. The maximum and minimum element size is 0.25 m and 0.0025 m, respectively. In this case the gas molecules are assumed to be adsorbed and subsequently diffusely emitted from the surface (this is often referred to as total accommodation). When emitting from the gas inlets or reflecting from the walls, the velocity components of the gas molecules follow the Boltzmann-Maxwell distribution and the angle follows the Knudsen’s cosine law. Basically, the gas neutralizer of CFETR NNBI prototype has two design options. The most common one for NBI systems is a single gas cell, the cross section of which has a similar shape and size as the ion beam profile. But for the ITER HNB injector, the neutralizer is divided into 4 adjacent vertical channels, to reduce the gas conductance of the neutralizer and consequently to reduce the gas inflow needed for the optimum neutralization of the beam. Specific to the CFETR NNBI prototype, the two designs are not difficult to interconvert. A single large channel (320 mm wide) can be transformed to two narrow channels (145 mm wide) by inserting a panel (30 mm wide) into the neutralizer. The distribution of gas density along the centerline of the neutralizer channel is shown in Fig. 3 with the gas injection quantity from the neutralizer of 0, 5, 10, 15, 20 Pa·m3/s. The display range includes the whole vacuum vessel. The gas concentrates inside the neutralizer and the contribution from the beam source is so small. The gas densities are higher in the design of two narrow channels as expected. Hence, the design of two narrow channels is better in the view of gas load limitation. As shown in Fig. 4, the gas target thickness increases linearly with the gas puff quantity. The slope of the line of two narrow channels is larger than that of single large channel by a factor of 1.5. To attain the optimal target thickness for 200 keV H− beam, the gas flow rates are 17.4 Pa·m3/s and 11.5 Pa·m3/s for a single large channel and for two narrow channels, respectively. For 400 keV H− beam, the single large
Fig. 4. Gas target thickness for two designs of neutralizer.
channel is not applicable anymore, because the required gas inlet rate is up to 30 Pa·m3/s, which will cause too much gas load to the cryopumps. But for two narrow channels, the required gas flow rate is more acceptable (19.1 Pa·m3/s). 4. Power loads on the neutralizer The problem of the divided channels design of the neutralizer is that the inserted panels may intercept a large beam power. Therefore, the negative ion beam should also be separated to several columns corresponding to the neutralizer channels. As a result, the extracted ion current will be reduced due to the masked apertures. However, the inserted panels will still suffer a great heat load by the stray particles, which are caused by the beam core with small divergence, the beam halo with large divergence, the beam deflection due to magnetic field, the misalignment due to the thermal deformation or the manufacturing and assembly tolerances of the planar grids [13]. Specific to the CFETR NNBI prototype, the extraction area of the beam source is 0.32 × 1.6 m2, which is about half of the ITER beam source. The aperture array of the grids system is displayed in Fig. 5. It is obvious that the aperture array is divided into two columns in the horizontal direction, to match the two channels of the neutralizer. In the vertical direction, the aperture array is divided into four blocks corresponding to the four segments of each grid. Hence, the total 768 beamlets are divided into 8 groups. A 1/4 model of beam transmission is considered due to the symmetry, in order to estimate the beam power deposited on the
Fig. 3. Gas density profiles along the centerline of neutralizer channel: (a) single large channel design, (b) double narrow channel design. 3
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A typical power load on the neutralizer is shown in Fig. 6 for the 400 keV beam (11.2 MW) with ωc = 7 mrad and m = 3 mrad. The largest power load density (2.74 MW/m2) is located at the leading edge of middle panel, although the deposited power is only 100 kW. To avoid the accidental damage, the leading edge should be chamfered to decreasing the power load density and be enhanced the heat removal ability. Similarly, the entrance dump also suffers high power load density, which is needed to be carefully treated. The power load density on the channel walls is relatively low, especially on the lateral walls. Indeed, except the middle panel, Fig. 6 can directly reflect the power load situation for single channel design. Because of the beamlets divergence, the power load is higher on the downstream part of middle panel and lateral wall. Although the distance between outside apertures and lateral wall is larger than that between outside apertures and middle panel, the power load on the middle panel is much higher due to the misalignment. A little convergence of the beamlets groups appears on the lateral walls and middle panel, which is also caused by the misalignment. The influence of divergence and misalignment on the beam power load are indicated in Fig. 7. As expected, the total power load and the maximum power load density on the neutralizer are increased with the beamlet divergence and the misalignment. The total beam power load on the neutralizer is 5.9%∼10.9% of the initial beam power. The only exception is the cases of total power load between m = 0 and m = 1 mrad. It seems that the total power load of m = 1 mrad is not higher, but a little lower than that of m = 0 mrad. The reason can be revealed by the power load on each component of the neutralizer. Apparently, the major contributions of the power load on the neutralizer are from the middle panel and the lateral wall. Their variations with the misalignment are opposite. For the cases of m = 0 and m = 1 mrad, these two power loads are nearly the same, so the total power loads also have little difference. A similar opposite trend can be found between the leading edge and the entrance dump. The power load on each component has guiding significance for the thermo-mechanical design of the neutralizer for CFETR NNBI prototype. 5. Discussions and conclusions This paper is focused on the power load from the beam particles. While the energic electrons emitted from the beam source may be another important power load on the neutralizer. Such electrons are produced via the stripping of negative ions and the ionization of background gas within the accelerator and are further accelerated to high energy. When evaluating the gas flow and the power load in the beamline system, the distance from the beam ions emitting surface to the neutralizer is a key influence factor. By now, the only certainly is the distance between the neutralizer entrance and the front plate of the beamline vessel. But the designs of the beam source and the large-scale gate valve still need further improvement and confirmation. Based on the established design concepts, a 3D model of the beamline system of the CFETR NNBI prototype was developed, which includes all the key components. Firstly, the gas flow in the beamline system was simulated for two design concepts of the neutralizer via an angular coefficient method. The design of two narrow channels can increase the gas density in the neutralizer because of the reduction effect to the gas conductance. Consequently, the required gas flow rate of two narrow channels is ∼2/3 of the single large channel to attain an optimal neutralization efficiency. For 200 keV H− beam, the gas flow rates are 17.4 Pa·m3/s and 11.5 Pa·m3/s for single large channel and for two narrow channels, respectively. For 400 keV H− beam, the single large channel is not applicable anymore and the required gas flow rate is 19.1 Pa·m3/s for two narrow channels. To estimate the beam power load on the neutralizer, 5000 simulation particles for each beamlet (i.e., total 3,840,000 particles for all 768 beamlets) were produced via a Monte Carlo method based on the
Fig. 5. Aperture array of grids system for the beam source of CFETR NNBI prototype.
neutralizer. A Monte Carlo method was used to produce simulated beam particles based on the biGaussian function [14,15]. The single beamlet divergence consists of beam core and beam halo. The fraction of power carried by the halo f is 15% and the divergence ωh is 30 mrad, which are assumed to be standard in all cases. The misalignment considered in this paper is simply a result of thermal deformation. According to the experimental and simulated results, the heat loads on the grid segments mainly located on the top surface and inside the aperture, which causes the grid segments are convex to upstream direction. As a result, the normal direction of each beamlet emission surface may have some deviation. Here, the misalignment angle of each beamlet is supposed to be the same (m = 0, 1, 2, 3 mrad), but their deviation directions are different. All deviations are assumed to point to the center of the grid segments. There are 5000 simulation particles for each beamlet. For calculating the power load density, the mesh of the neutralizer walls was built via free quad. The quad element size is 50 × 50 mm2 on the lateral surface and 50 × 10 mm2 on the leading edge. In addition, for better diagnosing and studying the beamlet properties, there is no requirement of beam focusing for the CFETR NNBI prototype. According to the design values of extracted ion current density (300 A/m2) and accelerator losses (∼20%), the maximum H- beam current emitted from beam source is 28 A. Thus, the maximum beam power is expected to be 11.2 MW at the beam energy of 400 keV. 4
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Fig. 6. Typical power load on the neutralizer for the 400 keV beam with ωc = 7 mrad and m = 3 mrad.
Fig. 7. Beam power load on the neutralizer at different divergence and misalignment: (a) total power load and power losses of initial beam power, (b) maximum power density, (c) power load on the leading edge and entrance dump, (d) power load on the middle panel and lateral wall.
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biGaussian function. The divergence of beam core, the fraction and divergence of beam halo and the misalignment of beamlet group are considered in the estimation. All these factors will increase the power load on the neutralizer, the maximum power load is up to 10.9% of the initial beam power. The leading edge of the inserted panel for two channels design need an extra protection, because the power load density (up to 2.7 MW/m2) is extremely higher than the other parts of the neutralizer.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are very grateful to the other members of ASIPP NBI team for their continuous support and excellent work. This work was partly supported by National Key R&D Program of China (Contract No. 2017YFE300101, 2017YFE300103) and National Natural Science Foundation of China(Contract No. 11575240, 11975264). References [1] Y. Wan, J. Li, Y. Liu, et al., Overview of the present progress and activities on the CFETR, Nucl. Fusion 57 (2017) 102009.
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