Numerical Study on the Ablation Effects of Tungsten Irradiated by High-intensity Pulsed Ion Beam

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1 Physics Procedia (011) International Conference on Physics Science and Technology (ICPST 011) Numerical Study on the Ablation Effects of Tungsten Irradiated by High-intensity Pulsed Ion Beam Di Wu a *, Mingkai Lei b, Xiaopeng Zhu b, Ye Gong b a College of Physical Science and Technolog Dalian Universit Dalian 1166, China b Surface Engineering Laborator Dalian University of Technolog Dalian 11604, China Abstract Based on the ion current density model of TEMP type ion beam accelerator, and utilizing the deposited energy of the high-intensity pulsed ion beam (HIPIB) in tungsten by Monte Carlo method as source term, a two-dimensional thermal conduction model controlling the irradiation process was built. The evolution of tungsten temperature during the irradiation period was obtained and analyzed. The results show that the melted thickness of tungsten surface is about 0.3 m at shooting center of target irradiated by HIPIB at the end of a pulse under the ion peak current density of 100 A/cm, the materials of tungsten surface in thickness of about 0. m reach boiling point; and among the ion peak current density of A/cm, the melting ablation depth is less than 1 m according to the calculation. While the ion peak current density increased to about 330 A/cm, the vaporizing ablation phenomenon appeared on the shooting center of tungsten surface. 011 Published by Elsevier B.V. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of Garry Lee. Keywords: High-intensity pulsed ion beam; tungsten; temperature fields; numerical method 1. Introductions High-energy ions escaped from constrained plasma are mainly deposited their energy on some parts of the first wall such as the hole column and diverter in magnetic confinement fusion reactor. In addition, while plasma disrupts, the energy will also deposit in these area of the first wall in the future international tokamak experimental reactor (ITER) in milliseconds [1]. The components must utilize high heat flux * Corresponding author. Tel.: ; fax: address: wudi@dlu.edu.cn Published by Elsevier B.V. Selection and/or peer-review under responsibility of Garry Lee. Open access under CC BY-NC-ND license. doi: /j.phpro

2 Di Wu et al. / Physics Procedia (011) materials. Tungsten, having the highest melting point in metal, low vapor pressure, good thermal conductivity and doing not form a hydride etc, is one of the most probable materials for diverter armor of high vaporization temperature and heat conductivity. This motivates the investigation of tungsten under severe heat loads at disruptions, edge localized modes (ELMs) events etc. Recent years, AUG [] and JET [3] or plasma guns such as MK-00UG, QSPA-Kh50 [4] were often used to simulate the erosion process of high heat loading problems. High-intensity pulsed ion beam (HIPIB) is a new technique developed in recent ten years [5, 6], the energy of a single particle in the beam may reach several tens of kev to a few of MeV. High energy of the beam is efficiently deposited into target materials in the range m in a short pulsed width usually less than 1 s under the power density up to 10 9 W/cm to rapidly melt and strongly vaporize the near-surface layer. Tungsten surface irradiated by HIPIB can be used to simulate the radiation effects produced on the diverter armor while plasma disrupts. In this paper equipment TEMP 6 was used to research the temperature effects in tungsten target irradiated by HIPIB under high heat flux; it lays the foundation for the study of thermal stress in tungsten target irradiated by HIPIB also, so as to study the connection problems of the armor with other structure better. y. Physical models HIPIB is mainly determined by the sort of ions, the voltage of magnetically insulated ion diode (MID), the ion current density and the width of pulsed time. Different kinds of HIPIB accelerators may produce different kinds of ions and their ratio. The interaction between HIPIB and tungsten target is the process of ions interacting with target. During the process, ions deposit their energy inside tungsten to make the surface materials melting or vaporizing. The Schematic diagram of theoretical model of the metal tungsten plate irradiated by HIPIB is shown in Fig 1. The origin of coordinate is located at the beam shooting centre; positive x direction points to the depth direction, and positive y takes arbitrary direction along surface due to the symmetric distribution of the ion beam, the model of the ion beam built as [7]: ( t t ) 0U U ( t ) A exp[ ] 1 ( y y 0 ) ( J ( y, t ) B exp[ ] exp[ t t 0 J ) ] () O Thermal Influence Region Ion Range Fig.1 Schematic diagram of theoretical model of the metal W plate irradiated by HIPIB x (1)

3 48 Di Wu et al. / Physics Procedia (011) where U ( represents the voltage of MID, J ( represents the HIPIB current densit it is a function of time t and the irradiation position y 0 represents the y axis value at the shooting centre on surface. A and B represent the maximal value of MID voltage and HIPIB current density respectivel t 0U and t 0J correspond to the time when voltage and ion current density reach the maximal values, and 1,, are the standard deviation for voltage and ion beam density respectively. Based on the equations, the energy and the amount of ions interacted with target in a short period of time can be obtained, and then the energy deposition of ions in target during the pulsed time can be calculated by TRIM code [8]. The energy transferred from ions to target will increase as time evolves during pulse time, it will change the temperature of target, especially in near surface layer. Taking the varying deposited energy being function of time in tungsten as thermal source term, a thermo-dynamic model has been constructed to calculate the temporal and spatial evolution profile of the ablation process in target; the twodimensional model controlling the irradiation process can be written in the Cartesian coordinate system as [9]: T T T C( T) ( ( T) ) ( ( T) ) Etot( x, (3) t x x y y E ( x, E ( x, E (4) tot s ph E ph Ll ( T ( x, Tm ) Lv ( T ( x, Tv ) (5) where, C(T) and (T) are mass densit specific heat and thermal conductivity respectivel E S (x, denotes the term of deposited energ E ph is the term which relates to latent heat, L l and L v denote latent heat of fusion and latent heat of evaporation respectively. Set function equal to one if temperature reaches melting or boiling point, other cases to zero. The initial condition is T (x, 0) =T 0, adiabatic conditions have been taken as boundary conditions, and the thermal radiation can be ignored [10]. The initial target temperature T 0 is 0 C. Finite differential method has been used to solve these equations. 3. Results and discussion The irradiation process of tungsten target by HIPIB has been simulated. The temperature dependent thermo-physical parameters of metal tungsten used in our calculation are taken from reference [11]. The width of the irradiation surface is 50 mm, the thickness of target is 50 m. In the TEMP-type ion source, the main ion species of the beam for the anode of magnetically insulated ion diode (MID) coated by polyethylene are approximately 70% H + and 30% C +, and the total pulse duration is 10 ns. According to the simulation results, the temperature contour of tungsten irradiated by HIPIB under the ion peak current density of 100 A/cm is drawn in Fig at time 80ns from the beginning of a pulse, the temperature distribution in the bulk can be seen clearly. For the short range of ions in tungsten, the thermal transportation depth is less than 1. m at the time. The melting ablation evolution profile of temperature on surface and inside the target is shown in Fig 3. It represents the case in the irradiation centre of tungsten.

4 Di Wu et al. / Physics Procedia (011) Fig. The temperature contour in tungsten irradiated by HIPIB at 80ns under ion peak current density of 100A/cm Fig.3 The slice profile of temperature in tungsten at shooting centre along the depth direction after the irradiation by HIPIB The temporal-spatial energy deposition in tungsten irradiating by HIPIB under the ion peak current density of 100 A/cm at the shooting centre during a pulse is shown in Fig 4. And the temporal-spatial temperature evolution of tungsten irradiating by HIPIB under the ion peak current density of 100 A/cm at the shooting centre during a pulse is shown in Fig 5. The surface materials on the irradiation centre already reached melting point near 60 ns from the beginning of a pulse. When the pulse finished, about 0.3 m top layer material was melted ablation. Fig.4 The temporal-spatial evolution of energy deposition in tungsten irradiating by HIPIB under the ion peak current density of 100 A/cm Fig.5 The temporal-spatial evolution of temperature of tungsten surface irradiating by HIPIB under the same condition as figure 4. Fig 6 gives the ablation distribution profile on the surface and inside the body of tungsten at the end of a pulse. The region in tungsten represented by "O" reached boiling point, by " " reached melting point at the end of a pulse. The most melted depth was in the irradiation centre.

5 50 Di Wu et al. / Physics Procedia (011) Fig 7 shows the spatial evolution of melting ablation process of tungsten during a pulse. It shows that melting ablation of specimen began at time about 5 ns from the beginning of a pulse; by time 80ns the maximum ablation depth reached 00 nm in target; by the end of a pulse, the value was 300 nm. At the same time, the melting ablation diameter on surface was about 15 mm. The melting ablation depth and Fig.6 The ablation case in tungsten after a pulse under the ion current density of 100A/cm Fig.7 The melting ablation process of tungsten during a pulse under the ion current density of 100A/cm velocity during a pulse are shown in Fig 8. It shows that the melting ablation process begins from about 50 ns, and the melted depth increases with the increasing of time. It rises quickly at beginning of a pulse, but over 100 ns, it increases slowly. The melting ablation velocity decreases with the increasing of time. This can be seen clearly from the velocity curve in the figure. The melting profiles in tungsten at the shooting center with several different ion current densities of HIPIB are shown in Fig 9 by the end of a pulse. It is shown that the melted depth increases with the increasing of ion current density of HIPIB. While the ion peak current density increased to about 330 A/cm, the vaporization appeared on the center of tungsten target surface. Fig. 8 Melting ablation process at shooting centre of tungsten irradiated by HIPIB during a pulse under the ion peak current density of 100A/cm Fig.9 The temperature distribution in tungsten irradiated by HIPIB with several different ion current densities at the end of pulse

6 Di Wu et al. / Physics Procedia (011) Conclusions While the ion peak current density is 100 A/cm, the most melting ablation depth is about 0.3 m, and it increases with the increasing of time during a pulse. Among the ion peak current density of A/cm, the melting ablation depth is less than 1 m. While the ion peak current density is 100A/cm, the melting ablation process starts at about 50 ns from the beginning of a pulse, and the melting ablation velocity is about 10m/s at the initial stage, it decreases with the increasing of time. While the ion peak current density increased to about 330 A/cm, the vaporizing ablation phenomenon appeared on the shooting center of tungsten surface. Acknowledgements The simulation works in this paper are supported by the National Natural Science Foundation of China ( ). References [1] Hao J K. Fusion reactors materials [M]. Beijing: Chemical industrial press 007, 17(in Chinese) [] Roth J and Janescchitz G. Impurity production and transport in the divertor tokamak ASDEX [J]. Nucl. Fusion, 1989, 9: [3] Coad J P, Andrew P, Hole D E, et al. Erosion/deposition in JET during the period [J]. J. Nucl. Mater., 003, : [4] Landman I S, Bazylev B N, Garkusha I E, et al. Simulation of tokamak armour erosion and plasma contamination at intense transient heat fluxes in ITER[J]. J. Nucl. Mater, 005, : [5] Remnev G E, Isakov I F, Opekounov, et al. High intensity pulsed ion beam sources and their industrial application [J]. Surf. & Coat. Technol., 1999,114: 06-1 [6] Petrov A V, Ryabchikov A I, Stepanov I B, et al. High current and high intensity pulsed ion-beam sources for combined treatment of materials[j]. Rev. Sci. Instrum., 000,71: [7] Wu D, Liu C, Zhu X P, et al. Research on ZrO Thermal Barrier Coatings Modified by High-Intensity Pulsed Ion Beam [J]. Chin.Phys.Lett., 008,5: [8] [9] Wu D, Gong Y, Liu J Y, et al. Two-Dimension Numerical Research on The Ablation Effects of Target Irradiated by Intense pulsed ion beam [J]. Acta. Phys. Sin., 006,55: [10] Rej D J, Davis H A, Remnev G E et al. Materials processing with intense pulsed ion beams[j]. J.Vac.Sci.Technol.A, 1997, 15(3): [11] Dai Z J, Mutoh Y, Sujatanond S. Numerical and experimental study on the thermal shock strength of Tungsten by laser irradiation [J]. Mater. Sci. Engineer. A, 008, 47: 6-34.