Observation of Impurity Accumulation After Hydrogen Multi-Pellet Injection in Large Helical Device

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1 Plasma Science and Technology, Vol.15, No.3, Mar Observation of Impurity Accumulation After Hydrogen Multi-Pellet Injection in Large Helical Device DONG Chunfeng ( ) 1, Shigeru MORITA 1,2, Motoshi GOTO 1,2, WANG Erhui ( ) 2, Gen MOTOJIAMA 1, Izumi MURAKAMI 1, Ryuichi SAKAMOTO 1,2, Norimasa YAMAMOTO 3 1 National Institute for Fusion Science, Toki , Gifu, Japan 2 Department of Fusion Science, Graduate University for Advanced Studies, Toki , Gifu, Japan 3 Center of Applied Superconductivity & Sustainable Energy Research, Chubu University, Kasugai , Aichi, Japan Abstract Impurity accumulation is studied for neutral beam-heated discharges after hydrogen multi-pellet injection in Large Helical Device (LHD). Iron density profiles are derived from radial profiles of EUV line emissions of FeXV-XXIV with the help of the collisional-radiative model. A peaked density profile of Fe 23+ is simulated by using one-dimensional impurity transport code. The result indicates a large inward velocity of 6 m/s at the impurity accumulation phase. However, the discharge is not entirely affected by the impurity accumulation, since the concentration of iron impurity, estimated to be to the electron density, is considerably small. On the other hand, a flat profile is observed for the carbon density of C 6+, which is derived from the Z eff profile, indicating a small inward velocity of 1 m/s. These results suggest atomic number dependence in the impurity accumulation of LHD, which is similar to the tokamak result. Keywords: impurity accumulation, EUV spectrometer, inward velocity PACS: m, Hc, Kz DOI: / /15/3/08 1 Introduction Impurity study is one of the indispensable subjects in magnetic confinement fusion research, since the performance of fusion plasmas is strongly affected by the impurity behavior. Therefore, a considerable amount of attention has been paid to the study on impurity behaviors [1]. A phenomenon of impurity accumulation appeared in the plasma core, which is predicated by the neoclassical transport theory [2], has also been studied in many tokamaks, such as ISX-B [3], PBX [4], JET [5], and HL-2A [6]. In order to avoid plasma disruption and excessive radiation losses in the plasma cores, the physical mechanism of impurity accumulation has to be investigated. The typical results from tokamak experiments indicate that the impurity accumulation is much stronger for heavy impurities, compared with light impurities, such as carbon and oxygen [7,8]. Until now, many analyses have been done on the impurity accumulation for metallic impurities. As a result, a large inward convective velocity is found for the metallic impurities during the accumulation phase. In Large Helical Device (LHD), the increase in spectral line emissions from highly ionized ions of iron has been observed after hydrogen ice pellet injection during neutral beam-heated discharges. It is a good opportunity to study the impurity accumulation in helical devices and to compare the result with that of the tokamak. For this purpose, the radial distribution of iron ions is measured using a space-resolved extreme ultraviolet (EUV) spectrometer to obtain the iron ion density profile. The radial profile of carbon ions is derived from the Z eff profile, which is measured with the EUV bremsstrahlung profile. The density profiles of iron and carbon ions are analyzed with a one-dimensional impurity transport code to derive the diffusion coefficient (D) and inward convection velocity (V ). In this paper, the result of impurity accumulation in LHD is presented and an analysis on the impurity transport coefficients during impurity accumulation phase after hydrogen multi-pellet injection is discussed for heavy and light impurities of iron and carbon, respectively. 2 Experimental arrangements In the present study, the LHD is operated at magnetic axis of R ax =3.63 m and toroidal magnetic field of B t = T. It is noted that the sign of B t indicates the direction of the coil current, i.e., the negative value means the counterclockwise equivalent current showing co-injection dominant discharge, which indicates higher beam deposition power. The discharge is initiated by tangentially injected negative ion source NBI (N-NBI) support by LHD project (NIFS11ULPP010) and partly supported by the JSPS-NRF-NSFC A3 Foresight Program in the field of Plasma Physics

2 DONG Chunfeng et al.: Observation of Impurity Accumulation After Hydrogen Multi-Pellet Injection with high-energy of 180 kev, and maintained by two N-NBIs and perpendicularly injected two positive ion source NBIs (P-NBI) with low energy of 40 kev. Hydrogen multi-pellet injection is utilized to produce highdensity plasmas, in which the line-average electron density is one order higher than that in normal gas-puffed LHD discharges. In the present study, two EUV spectrometers are used as described below. The vertical profile of iron line emissions is measured in the wavelength range of Å by the space-resolved EUV spectrometer [9]. The intensity of line emissions is absolutely calibrated with high accuracy by using a newly developed method based on visible and EUV bremsstrahlung profile measurements [10]. Moreover, the space-resolved EUV spectrometer also provides the radial profile of effective ion charge, Z eff, derived from the bremsstrahlung continuum. Another EUV spectrometer working as an impurity monitor is used to record the time behavior of impurity line emissions [11]. The profiles of electron density, n e, and temperature, T e, are measured by Thomson scattering system with YAG lasers along the major radius at a horizontally elongated plasma cross section [12]. The Thomson scattering system has been optimized to measure the temperature range from 50 ev to 10 kev [13]. 3 Experimental results and discussions The discharge waveform with multi-pellet injection is plotted in Fig. 1. The time evolution of CVI intensity normalized to electron density, plotted in Fig. 1(e), is taken from another shot with plasma parameters similar to Figs. 1(a) (d). Eight hydrogen ice pellets are injected into the plasma at discharge times of 3.70 s, 3.75 s, 3.80 s, 3.85 s, 3.90 s, 3.95 s, 4.00 s, and 4.05 s, respectively. During the pellet injection, the line-averaged electron density is rapidly increased up to cm 3, and the density is gradually decreased with a certain decay time. The value is one order higher compared to that before the pellet injection. Simultaneously, the central electron temperature drops from 2.5 kev to 0.27 kev, while it gradually increases from 4.2 s. The iron line emissions from highly ionized charge stages of FeXXIII and FeXXIV start to increase from 4.2 s, whereas the intensity of FeXV almost keeps constant and the density decreases. The CVI intensity also shows a slight increase after the hydrogen pellet injection. The time behaviors of FeXXIII and FeXXIV in the plasma core and FeXV in the plasma edge suggest impurity accumulation of iron ions at the plasma core. On the other hand, the total radiation power does not increase after the pellet injection, indicating a conflict with the experimental result seen in tokamaks. In order to investigate the experimental result, the density profile of iron ions is analyzed at three time slices denoted with the shadowed area in Fig. 1(a). Fig.1 Time evolutions of: (a) Line-averaged electron density, (b) Central electron temperature, (c) Total radiation power, (d) Intensities of FeXV, FeXXIII and FeXXIV normalized to electron density in discharge #102086; and (e) CVI intensity normalized to electron density in discharge # Discharge # has similar plasma parameters to discharge # (color online) The electron density and temperature profiles measured at the three time slices are shown in Fig. 2. The density profile peaks at the center with the hydrogen multi-pellet injection. The density peaking factor, defined by the ratio of central density to density at ρ=0.8, after the pellet injection gradually drops from 2.01 at t=4.137 s to 1.75 at t=4.937 s. On the contrary, the T e profile becomes flat indicating that the product of n e and T e is constant. The iron density profiles of Fe 14+ and Fe 23+ are analyzed from absolute emissivity profiles of spectral lines of FeXV ( Å, 3s 21 S 0 3s3p 1 P 1 ) and FeXXIV ( Å, 2s 2 S 1/2 2p 2 P 3/2 ), respectively. The emission coefficients necessary for the analysis are calculated with a collisional-radiative (CR) model [14]. Although the density profiles of Fe 14+ shown in Fig. 3(a) are nearly the same between t=4.137 s and s, a small increment of iron influx is observed at t=4.937 s. It is probably explained by the enhanced recycling on the divertor plates after hydrogen multi-pellet injection because a small amount of iron is already deposited on the divertor plates. The FeXXIV cannot be observed until t=4.3 s due to the low T e during the pellet injection, as seen in Fig. 1(d). It begins to appear from t=4.4 s again and monotonically increases until t=5 s with increasing T e, as seen in Fig. 1(d). The density profile of Fe 23+ is then shown in Fig. 3(b) at two time slices. It is clear that the density profile of Fe 23+ is centrally 231

3 Plasma Science and Technology, Vol.15, No.3, Mar peaked at t=4.937 s, suggesting impurity accumulation in LHD discharges. to our previous study on LHD impurity transport [15], Dq is assumed to be spatially constant. The value of V is given by a linear function: Vq (r) = Vq (a) (r/a), Fig.2 Profiles of (a) electron density and (b) electron temperature at three time slices measured by Thomson scattering system (color online) (2) where Vq (a) denotes the inward velocity at plasma edge boundary: r = a. A detailed description of the transport code is given in Refs. [16] and [17]. The diffusion coefficient and inward convective velocity can be evaluated using this code in addition to the density profile of iron ions. The simulated results are shown in Fig. 4. In Fig. 4(a), the Fe23+ profile is simulated for different D values of 0.05 m2 /s, 0.2 m2 /s and 1.0 m2 /s. The inward convective velocity of V = 1 m/s is used as the common value in LHD plasmas, which is obtained from the former impurity transport study [15]. It is clear that the peaked Fe23+ profile cannot be reproduced, although a variety of diffusion coefficients are considered. Fig. 4(b) shows the simulated result for different V values of 1 m/s, 3 m/s, 6 m/s and 10 m/s. Here, D is fixed to be 0.2 m2 /s, which is a common value in LHD plasmas [13]. One can understand that the peaked Fe23+ profile can be well reproduced by the relatively large inward velocity of V = 6 m/s. Compared to the common V value of 1 m/s usually used in LHD [15], the number of 6 m/s is definitely large. Fig.3 Time evolutions of the density profiles of (a) Fe14+ and (b) Fe23+ ions (color online) In order to study the formation of peaked profile of Fe23+ observed at t=4.937 s, a one-dimensional impurity transport code is adopted to simulate the Fe23+ density profile. In this code, cylindrical plasma is assumed with a diffusive/convective model. The particle flux is expressed by Γq = Dq (r) nq + Vq (r) nq, r (1) where Γq is the particle flux, nq the ion density, Dq the diffusion coefficient and Vq the inward convective velocity of impurity ions in the qth charge state. According 232 Fig.4 Simulated density profiles of Fe23+ at t=4.937 s (a) with different D and fixed V and (b) with different V and fix D. The solid line denotes the experimental result (color online) The total number of iron ions is calculated by integrating all the charge states and the whole plasma volume as NFe = Ne, where NFe and Ne are the total number of iron ions and electrons, respectively. It indicates that the iron density is still extremely low, even if impurity accumulation occurs. The total radiation power from iron impurity in the range from ρ=0

4 DONG Chunfeng et al.: Observation of Impurity Accumulation After Hydrogen Multi-Pellet Injection to 0.5 is calculated to be 30 kw, which is considerably small compared with the total radiation power of 2 MW measured by the bolometer system. The result also gives a clear answer to the question why the total radiation power keeps constant (see Fig. 1). The low level of iron impurity concentration seems to originate from the enhanced impurity screening in the ergodic layer of LHD [18]. The effective ion charge, Z eff, is a key parameter for evaluating the light impurity contamination in fusion plasmas. The Z eff profile is calculated at t=4.137 s and s based on the EUV bremsstrahlung profile measurement. The result is shown in Fig. 5. The Z eff shows a flat profile distribution at both time slices. It also reconfirms that the iron impurity content in LHD plasma is really low. Calculation shows that the contribution of iron to Z eff is at t=4.937 s. The Z eff values change from 1 to 4 between the two discharge times. Since the dominant impurity in LHD is uniquely carbon [19], the increase in Z eff is attributed to carbon density only. The density profile of C 6+ can be then calculated from the Z eff profile, as shown in Fig. 6. Compared with Fe 23+ density profile, the C 6+ profile reveals an entirely flat profile. The concentration of carbon increases up to 10% of n e during the impurity accumulation phase. Fig.6 Density profiles of the C 6+ ion calculated from the Z eff profiles shown in Fig. 5 (color online) Fig.7 Simulated density profile of C 6+ (a) with different D and fixed V and (b) with different V and fixed D. The solid line denotes the experimental result (color online) Fig.5 Z eff profiles obtained from EUV bremsstrahlung profile measurement (color online) The carbon density profile at t=4.937 s is also simulated using the one-dimensional impurity transport code. Firstly, the carbon density profile is simulated by assigning D the value of 0.05 m 2 /s, 0.2 m 2 /s and 1.0 m 2 /s, respectively, where the value of V is fixed at 1 m/s. The result is shown in Fig. 7(a). Although any good fitting to the measured density profile cannot be reconstructed due to the assumed spatially constant D and the relatively large uncertainty in measured Z eff profile, the results seem to suggest a small D value near D=0.2 m 2 /s. With this D value, the carbon profile is simulated by changing V from 1 m/s to 10 m/s. The result is shown in Fig. 7(b). Only small V can reproduce the flat carbon density profile. A possible result is finally obtained at D=0.2 m 2 /s and V = 1 m/s, of which the values indicate the same ones as commonly used in the LHD [15]. This result reveals that the accumulation of light impurities (such as carbon) does not occur in the LHD too, similar to the tokamak result [20]. Finally, the uncertainties of the impurity density and Z eff values in the present analysis are estimated. The errors mainly originate from the Thomson scattering measurement, the atomic model, the EUV spectrometer system, the Abel inversion process [21], and the absolute intensity calibration [10]. The error is estimated to be 30% at most in the present study. 4 Summary Iron impurity accumulation at the plasma core after hydrogen multi-pellet injection is observed in LHD discharges. The density profiles of Fe 14+ and Fe 23+ are calculated by combining the absolute spectral line intensities of FeXV ( Å, 3s 21 S 0 3s3p 1 P 1 ) and FeXXIV ( Å, 2s 2 S 1/2 2p 2 P 3/2) with a CR model. One-dimensional impurity transport code is adopted to evaluate D and V. The simulated result of the Fe 23+ density profile suggests a large inward convective velocity of V = 6 m/s and D=0.2 m 2 /s. Since the total iron concentration during the impurity accumulation is estimated to be , the contribution of iron of to Z eff is therefore negligible. 233

5 Plasma Science and Technology, Vol.15, No.3, Mar The carbon density is obtained from the Z eff profile measurement, and the carbon concentration during the impurity accumulation is estimated to be 10% of the electron density. The carbon density profile can be reconstructed with a small inward convective velocity of V = 1 m/s and D=0.2 m 2 /s, indicating that no accumulation occurs for carbon. The result obtained in the present study is similar to the tokamak result [21], suggesting the importance of the density gradient in the impurity accumulation mechanism. Acknowledgements The authors acknowledge all the members of the LHD team for their technical support and LHD operation. References 1 Isler R C. 1984, Nucl. Fusion, 24: Hirshman S P and Sigmar D J. 1981, Nucl. Fusion, 21: Isler R C, Murray L E, Crume E C, et al. 1983, Nucl. Fusion, 23: Sesnic S S, Fonck R J, Ida K, et al. 1987, J. Nucl. Mater : Lawson K D, Alper B, Coffey I H, et al. 1998, Proc. 25th EPS Conf. on Controlled Fusion and Plasma Physics, Prada, 22C: Cui Z Y, Zhou Y, Li W, et al. 2008, Proc. 35th EPS Conf. on Controlled Fusion and Plasma Physics, Hersonissons, 32D: P Hawryluk R J and Suckewer S. 1979, Nucl. Fusion, 19: Guirlet R, Ciroud C, Parisot T, et al. 2006, Plasma Phys. Control. Fusion, 48: B63 9 Dong C F, Morita S, Goto M, et al. 2010, Rev. Sci. Instrum., 81: Dong C F, Morita S, Goto M, et al. 2011, Rev. Sci. Instrum., 82: Chowdhuri M B, Morita S, Goto M, et al. 2007, Rev. Sci. Instrum., 78: Narihara K, Yamada I, Hayashi H, et al. 2001, Rev. Sci. Instrum., 72: Yamada I, Narihara K, Funaba H, et al. 2010, Rev. Sci. Instrum., 81: 10D Dong C F, Morita S, Goto M, et al. 2012, Jpn. J. Appl. Phys., 51: Nozato H, Morita S, Goto M, et al. 2004, Phys. Plasmas, 11: Amano T, Mizuno J and Kako J. Simulation of impurity transport in tokamak. Int. Rep., IPPJ Morita S, Goto M, Muto S, et al. 2006, Plasma Sci. Tech., 8: Chowdhuri M B, Morita S, Kobayashi K, et al. 2009, Phys. Plasmas, 16: Zhou H Y, Morita S, Goto M, et al. 2010, Jpn. J. Appl. Phys., 19: Kaufmann M, Buchl K, Fussmann G, et al. 1988, Nucl. Fusion, 28: Zhou H Y, Morita S, Goto M, et al. 2010, J. Appl. Phys., 107: (Manuscript received 10 January 2012) (Manuscript accepted 14 August 2012) address of DONG Chunfeng: dong.chunfeng@nifs.ac.jp 234