A. Takayama

A.M. Ito

H. Nakamura

S. Saito

National Institute for Fusion Science (NIFS-Repository)

     The aim of this collaborative work is that the development of the integrated simulation method on the control of the core and peripheral plasma for DEMO. For this aim, we carry out modeling on the critical physical issues on the control of the core and peripheral plasma for DEMO. In 2010, we focused on the following topics: (1) proposal and its critical issue on the control method for the core and peripheral plasma for the commissioning period for DEMO, (2) 1-D plasma modeling on the SOL-Divertor plasma to understand the stability and control of detachment in the divertor plasma, (3) improvement of modeling and code of the integrated core-peripheral plasma. Those results were presented in the domestic and international conferences shown in references. As for the first topic, we firstly surveyed what kind of control method is required in a power plant based on the thermal and nuclear power plant. The inevitable control methods are controls of the rating operation, start-up operation, stop-down operation, and commissioning operation. Accordingly, we investigated the control method of plasma start-up operation including the commissioning operation for a tokamak fusion power plant. To propose the integrated control method for the core and peripheral plasma of the tokamak power plant, the analysis on MHD stability, current drive operation and divertor transport was carried out. Here, we proposed the ratio of tritium in the fuel density to control the fusion power to keep the high density preferable for the divertor detachment1). Another candidate for the plasma start-up scenario is also collaboratively discussed, and the ratio of helium ash in the plasma ion density is concluded to be also pursued. The other plasma control method on the ELM was also discussed. The integrated transport code with the pellet injection was also developed, and the modeling on the ELM triggered by the pellet injection was carried out2). That integrated transport code could reproduce the phenomena of ELM generation by the rapid growth of plasma pressure by the pellet fueling. This integrated transport code will be applied to the development of the control method of ELM consistent with the fueling control for the plasma operation of ITER and DEMO. As for the second topic, we have developed a one dimensional SOL-divertor code to investigate the physics of detachment phenomena and to control it in the divertor for the plasma operation in ITER and DEMO3). This code can handle the radial transport effect in the SOL and divertor region. When the detachment is appeared in the divertor region, the energy flux comes into the detachment flux tube from the outer attachment flux tube. Our divertor code can analyze the stability on the detachment front including the effect of such radial transport. We analyzed the effect of such radial transport on the stability of the detachment front. The result of this code qualitatively showed that the radial particle and energy transport in the divertor region is effective to stable the detachment front. To analyze it more precisely, the neutral transport modeling is a critical issue to discuss the stability limit quantitatively. As for the third topic, we investigated the precise modeling on the peripheral plasma in order to integrate the core plasma modeling. First of all, the simulation study on the particle wall interaction was carried out, and the effect of impurity species on the erosion was estimated in the LHD configuration. This result made the modeling basis for the effect of the impurity generated in the divertor region on the core plasma performance.  The detachment physics is also investigated. When the detachment is occurred in the tokamak configuration, the reduction of the ion particle onto the divertor plate is observed. However, this phenomenon has not been reproduced in the framework of the fluid transport modeling in the SOL-divertor region. We investigated whether the particle diffusion to the dome region and the super-sonic flow have an important effect on the ion flux or not. The result did not show the clear relationship between those phenomena and the reduction of the ion flux. We will also investigate the effect of the drift and orbit induced flow in the next year. Finally, we also discussed the standardization of the integrated transport code, and the way to apply it to the DEMO design activity. The following considerations were pointed out. The system code for the design activity is based on the 0 dimensional modeling. However, recent improvement of PC performance probably enables the system analysis based on the multi-dimensional modeling such as the equilibrium and MHD stability code. Furthermore, how collaborative alliance among the experimental study, the integrated modeling study, and the DEMO design study should be made was also discussed. Those important issues will be discussed in the following year.   1)  R.Hiwatari, et al.,Plasma commissioning scenario and initial tritium inventory for Demo-CREST FTP/P6-17, 23rd IAEA Fusion Energy Conference, Korea, Oct. 2010. 2)  N. Hayashi,et al., Integrated simulation of ELM triggered by pellet through energy absorption and transport enhancement, THS/P3-02, 23rd IAEA Fusion Energy Conference, Korea, Oct. 2010 3)  M. Nakamura et al., "Multi-layer" one-dimensional model for stability analysis on partially detached divertor plasmas, to be published in J. Nucl. Mater. (2011) We have investigated interactions between hydrogenatoms and carbon materials such as graphite by molecu-lar dynamics (MD) simulations in order to contribute un-derstanding the divertor physics and designing an appro-priate conﬁguration. MD simulation is time-consuming,and thus, is limited to about nanometer order and theenergy range to about keV order.In order to investigate plasma-wall interactions(PWI) by numerical simulation, we have to overcomethe limitations imposed by MD simulation. A comple-mentary model based on binary collision approximation(BCA) can be established. There have been a numberof PWI-related works on BCA-based simulations. TheBCA simpliﬁes interactions between material elementsand reduces them to a sequence of binary collisions. Abeneﬁt of the model is that it is rather simple and re-quires less computing resources than an MD model. It,however, can only be applied in a high energy rangewhere multi-body interactions can be neglected.We employ the BCA-based simulation codeACAT1), which was developed to simulate atomiccollisions in an amorphous target within the frameworkof the BCA. Projectile particles are traced through bi-nary collisions. Target particles, with which projectilescollide, are randomly distributed in each unit cell whosesize is R0 = N−1/3, where N is the number density of thetarget material. In terms of randomly distributed targetparticles, this code employs the Monte Carlo method andfocuses on atomic collisions in an amorphous target.In each interaction between the projectile and thecollision pair of the target material, we employ theMoliere approximation to the Thomas-Fermi potential asthe interatomic potential V (r). The scattering angle isanalytically given as a function of the impact parameterand the kinetic energy of the projectile. The trajecto-ries of particles are approximated as their asymptotes.Thus, they consist of linked straight-line segments. Thedetailed expressions for particle trajectories are given inreferences1, 2).Three parameters are deﬁned: bulk binding energyEB, displacement energy Ed, and minimum energy Ec.Let us consider a collision from which the original pro-jectile emerges with kinetic energy E1 after transferringkinetic energy T to the target (collision pair) atom. Thetarget atom is displaced when its kinetic energy exceedsthe displacement energy Ed. If T > Ed is satisﬁed, thetarget atom is added to the collision cascade with kineticenergy E2 = T − EB. Each projectile is traced while itskinetic energy exceeds a given minimum energy Ec.We have extended the ACAT code described aboveas follows: While a target atom is produced randomlywithin a cubic collision cell in the original code, our ex-tended version employs rectangular parallelepiped cells,and the positions of target atoms are given as the initialcondition. The positions of projectile and target atomsare stored in computer memory, which enables us to treatstructured target materials as well as amorphous ones. Inthe present version, it is assumed that each cell containszero or one target atom. Thus, we can obtain the posi-tion of the target (collision pair) atom from the storeddata when the collision cell is speciﬁed. We call the ex-tended version “AC∀T” code2), which stands for atomiccollisions in any structured target. The notation ∀ im-plies that the code can handle any structure includingcrystalline and amorphous structures.If the kinetic energy of each projectile T becomesless than a predetermined minimum energy Ec, tracingof the projectile is stopped and its location and velocityare stored, which can be used for the initial condition ofa subsequent MD simulation. Here, Ec is determined bythe validity condition of the BCA.We simulate the injection of a hydrogen atom withenergy 1.0 keV into a target graphite as an example prob-lem. The incident angle is ﬁxed to 0◦, that is, normalto the target surface, and the incident point is variedfor each trial. We ﬁnd the region where the trajectoryis almost straight. This may be caused by a type ofchanneling. This type of trajectory is not realized in theoriginal ACAT code because it assumes an amorphoustarget. We observe that the projectile trajectory imme-diately before stopping is horizontal with ﬂuctuations.In this region, the projectile travels between graphenelayers. This type of trajectory is also newly reproducedby our extension of the ACAT code. Thus we expectthat the AC∀T code will be useful for the evaluation ofchanneling eﬀects. Channeling is a phenomenon wherea projectile ion injected parallel to the crystallographicaxis or the face of a target material undergoes small-angle scattering without collision and penetrates deeplyinto the material. Channeling generally increases a rangeof a projectile.We are currently developing a hybrid code basedon our MD simulation code and our BCA-based code“AC∀T”. As the BCA-based code has a relatively smallcomputational load, the resultant hybrid simulation codeis expected to simulate carbon materials of submicron or-der. The AC∀T code itself exhibits new features and ispromising for the evaluation of channeling eﬀects. Eval-uations of channeling eﬀects and the eﬀects of target ma-terial crystallite states on the sputtering yield remain asfuture works.1) Yamamura,Y., Mizuno,Y.: IPPJ-AM-40, Inst.Plasma Phys., Nagoya University (1985).2) Takayama,A., Saito,S., Ito,A.M., Kenmotsu,T.,Nakamura,H.: Jpn. J. Appl. Phys. 50 (2011) 01AB03.365§26. Development of  Binary-Collision-Approximation-Based Simulation Code Applicable to Any Structured Target MaterialTakayama, A., Ito, A.M., Nakamura, H.,  Saito, S. (Nagoya Univ.)

Development of Binary-Collision-Approximation-Based Simulation Code Applicable to Any Structured Target Material

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Development of Binary-Collision-Approximation-Based Simulation Code Applicable to Any Structured Target Material

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