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Gas fueling effect on plasma profile in Heliotron J
Journal of Nuclear Materials 438 (2013) S453–S458
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Gas fueling effect on plasma profile in Heliotron J T. Mizuuchi a,⇑,1, H.Y. Lee b, K. Mukai c, K. Yamamoto b, S. Kobayashi a, H. Okada a, S. Yamamoto a, S. Ohshima a, L. Zang b, K. Nagasaki a, T. Minami a, T. Kagawa b, T.Y. Minami b, K. Mizuno b, Y. Wada b, S. Arai b, H. Yashiro b, H. Watada b, K. Hashimoto b, N. Kenmochi b, Y. Nagae b, M. Sha b, Y.I. Nakamura b, K. Kasajima b, N. Nishino d, K. Hosoi e, Y. Nakashima e, Y. Nakamura b, S. Konoshima a, F. Sano a a
Institute of Advanced Energy, Kyoto Univ., Gokasho, Uji 611-0011, Japan Graduate School of Energy Science, Kyoto Univ., Gokasho, Uji, Japan National Institute for Fusion Science, Toki, Japan d Graduate School of Engineering, Hiroshima Univ., Higashi-Hiroshima, Japan e Plasma Research Center, Univ. of Tsukuba, Tsukuba, Japan b c
a r t i c l e
i n f o
Article history: Available online 17 January 2013
a b s t r a c t The optimization of gas fueling scenarios has been studied to improve the plasma performance in Heliotron J. Gas fueling effects on the plasma profile are discussed by comparing two fueling methods, a short pulse H2-beam fueling with a supersonic molecular-beam injection (SMBI) technique and a high intensity gas-puff fueling (HIGP) with a conventional gas puff technique. The maximum plasma stored energy Wp after SMBI is about 20% higher than that after HIGP in this NBI-only sustained plasma experiment, where the line-averaged density is almost the same (3 1019 m3) for both cases. The electron and ion temperatures in the SMBI case are higher than those are in the HIGP case at the Wp-peak timing. A peaked density profile is observed after SMBI, while it is flat or slightly hollow for the case of HIGP. These observations point out the importance of SMBI fueling for plasma density control to obtain better plasma performance. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Fueling and recycling control is one of the key issues to obtain high density and high performance plasma in magnetic confinement devices from two aspects; (1) profile control of the plasma density and (2) reduction of neutrals in the peripheral region. Core fueling by ice pellets is well known as a technique to realize favorable fueling from these aspects. The pellet system is, however, complicated and it is not easy to make pellets small enough for density control in medium or small size devices. A supersonic molecular-beam injection (SMBI) technique [1] is, on the other hand, considered to be an effective fueling method for deeper penetration of neutral particles into the core plasma compared to conventional gas-puffing (GP). SMBI is applied not only for core density control but also as an effective edge modification technique in fusion devices [2–4]. The optimization of gas fueling scenarios has been studied to improve the plasma performance in Heliotron J, which is a helical-axis heliotron device with an L/M = 1/4 helical coil (hR0i = 1.2 m, ha p i = 0.12–0.17 m, hB 0 i 6 1.5 T) [5,6] based on the ⇑ Corresponding author. 1
E-mail address: [email protected] (T. Mizuuchi). Presenting author.
0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.01.092
helical-axis heliotron concept [7,8]. Here L and M are the pole number of the helical coil and its helical pitch number, respectively. A fueling method with SMBI technique has been successfully applied to Heliotron J plasma [9]. It is considered that local fueling with a short pulse intense molecular beam can increase the core plasma density, while avoiding confinement degradation due to the edge cooling caused by excess neutrals in the peripheral region, which is often observed in the case of conventional GP control. SMBI can also affect plasma fluctuations. Fast camera observation of filament structure in edge turbulence has revealed that SMBI can change its rotation direction and/or speed [10,11]. Similar change is observed at L–H transition in Heliotron J. In a previous PSI conference [9], we have reported the effects of SMBI fueling on the global parameters such as the plasma stored e , and the divertor particle energy Wp, the line-averaged density n flux, which is represented by the summation of the ion-saturation currents for all probe pins of a divertor-probe-array at section #3.5 (see Fig. 1). Recently, the response of edge plasma caused by different gas fueling methods is investigated, especially focused on the change of the plasma radial profile [12,13]. In this paper, we discuss the effects of SMBI fueling on the core plasma profile comparing SMBI fueling and high intensity short pulse GP fueling with a conventional gas-puff system. Although the latter fueling method e like the SMBI case, it cannot avoid can make a rapid increase of n
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Fig. 1. Experimental set-up. A layout of heating, fueling and diagnostic equipment is illustrated.
the saturation (or degradation) of Wp in higher density condition in contrast to the SMBI case [9]. 2. Experimental setup Fig. 1 schematically shows a layout of heating, fueling and some diagnostic equipment for this experiment. For density control, usually used is a conventional gas-puffing system with four piezoelectric valves. These valves are installed at the inboard-side ports (low field side injection) at 90° intervals around the torus (‘‘Gas’’ in Fig. 1). These ports are located in the low field side nevertheless they are in inboard side since the field strength is usually higher near the helical coil. The amount of H (or D) atoms from the GP system with the plenum pressure of 0.15 MPa is pre-programmed to control the line-averaged density. Since, due to the space limitation, the nozzles of these valves do not directly see the plasma, the induced gas from the valve diffuses to the discharge chamber after multiple reflections in an elbow-shaped pipe connecting the nozzles to the chamber. An SMBI system of hydrogen is equipped on two horizontal outboard-side ports (the port number: #3.5, #11.5). The system consists of a fast piezoelectric valve with a short conic (or Laval-type) nozzle with the orifice size of 0.2 mm/. The amount of H atoms injected with the SMBI is controlled by changing the pulse width of each SMBI valve under a fixed plenum pressure (Ppl 1–2 MPa). The initial hydrogen or deuterium plasma is produced by using second harmonic X-mode ECH (70 GHz, <0.30 MW). The hydrogen beam is injected for NBI experiments by using one or two tangential beam-lines (BL-1 and BL-2. The acceleration voltage is <30 kV and the beam power is <0.7 MW/beam-line). 3. Effects of SMBI fueling on core plasma profile The peculiarities of SMBI fueling are its directional motion of the injected gas, the small area of plasma–beam interaction and a massive gas injection in a short time. As discussed in [9], the absolute value of the beam speed itself is not so fast (1–2 km/s) in this experiment, comparable to that for the thermal speed of H2 introduced by normal GP. As for the plasma–beam interaction area, judging from visible-light images measured with a fast
camera [10,14], the core part of H2-beam radius is less than 10 cm near the LCFS. To investigate the effect of directional motion, a direct comparison between the directional and non-directional gas injection was performed by using a movable plate in front of the #3.5-SMBI e and Ha at #3.5 nozzle. Fig. 2 shows time traces of Wp and n (Ha#3.5) for two discharges under the same heating condition (simultaneous heating by ECH and NBI, ECH+NBI); the shutter is open in the case (a) and closed in the case (b). Also plotted are the information of GP condition, ‘‘GP’’ (the pre-programmed control voltage for the gas-puffs) and ‘‘gas’’ (a manometer signal measured at a gas-puff port). A large spike of Ha#3.5 indicates the SMBI timing (t 208.5 ms). When the shutter is closed, the SMBI beam hits the plate and then H2 molecules diffuse to the plasma, which is suggested by a low and slow response of Ha#3.5 shown in Fig. 2b. More directly, chord profiles of Ha-intensity measured with the Ha-array at #3.5 are compared in Fig. 2c and d for two timings. Just after SMBI (t 209 ms, Fig. 2c), very strong (>10) emission is observed for sight lines crossing the injected beam line inside the LCFS (R < 1.31 m). On the other hand, only 3 ms later timing (Fig. 2d), almost the same intensity and chord profile are observed for both cases. These observations indicate that the injected beam penetrates well into the plasma core e shows raregion in the open shutter case. As shown in Fig. 2a, n pid increase just after SMBI and then gradually increases up to 2.6 1019 m3 during 30 ms in the directly injected case (the open shutter case). The stored energy also shows rapid increase with a slight delay, keeps the elevated value for 10 ms and then e becomes saturated. In starts to decrease when the increase of n the case of closed shutter (Fig. 2b), on the other hand, the in e and Wp are slow and mild. The density already satcreases of n urates at 1.41019 m3 and starts to decrease at 15 ms after SMBI. Since the amount of injected gas is the same for these e after the SMBI inditwo cases, the difference of the attainable n cates the effectiveness of SMBI in the over-all fueling efficiency. The differences of edge density profile for these two discharges measured with a microwave reflectometer at the torus section of #15.5 are discussed in [12]. In the open shutter case, the edge density profile, which is rather flat before SMBI, rapidly changes to a very peaked one after SMBI, while the profile change is slow and mild in the closed shutter case.
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Fig. 2. (a) Time traces of line-averaged density, stored energy for a series of ECH+NBI plasma for an open condition of the shutter in front of SMBI (#3.5-SMBI). (b) The similar time traces for a closed shutter case under the same heating condition. Chord profiles of Ha at the SMBI section (#3.5) for the open shutter case ( ) and the closed case ( ) at two different timings, (c) 209 ms and (d) 212 ms, respectively. Sight lines of Ha-array and magnetic surfaces are shown in (e) with a schematics of #3.5-SMBI.
The similar comparison for the effects of open/close shutter was performed for a different series of ECH+NBI plasma with the #11.5SMBI. Fig. 3a and b shows the time evolution of the chord intensities of SX emission at section #7.5 (the sightlines are shown in Fig. 3c) as contour plots for the open/closed shutter cases, respectively. The SX signals show clear responses to a gas injection in the open case, while the change of SX-intensity in the closed shutter case is not so clear for all chords. In the open shutter case, a rapid drop in intensity is observed even for the chords viewing the plasma core, with a little time delays from the edge to the core. Then a rapid rise in intensity is observed. The time response of this increasing is more rapid in the open shutter case compared to
the closed case. These observations might indicate the creation of a local cold spot by SMBI without shutter. SMBI effects on the core plasma profile were investigated for e and the stored NBI plasmas. Figs. 4a and 5a show time traces of n energy Wp with gas fueling and heating sequences for NBI sustained plasmas. Here a case of fueling by a short pulse intensive GP (Fig. 5) is compared to the case of SMBI (Fig. 4) to make a more e for both cases. Electron temperature similar time response of n Te(r/a) and density ne(r/a) from TVTS for the cases of SMBI and HIGP are shown in Figs. 4b and 5b, respectively. Figs. 4c and 5c show radial profiles of ion temperature Ti(r/a) from CXRS [15] for each discharge. Plasmas are initiated by using a short pulse of
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Fig. 3. Comparison of the chord profile of SX emission observed at #7.5-section between (a) open and (b) closed shutter conditions for #11.5-SMBI, respectively. Here, the plasma-heating scenario is similar to that in the case shown in Fig. 2. (c) Sightlines of SX-array and magnetic surfaces. (d) Schematics of #11.5-SMBI with sight lines of Haarray in this section.
Fig. 4. (a) Time trace of NBI sustained plasma with SMBI. (b) Radial profiles of electron temperature and density from TVTS at t = 235 ms. (c) Radial profiles of ion temperature from CVRS at t = 225, 235 ms.
second harmonic X-mode ECH (70 GHz, <0.40 MW) and then sustained by two-staged NBI heating (H0-beam, Ebeam < 30 keV, Pinj. 0.6 MW per a beam-line), where the first NBI (NBI#1) sustains the plasma and then the second NBI (NBI#2) is superimposed. Just e is increased by using SMBI or GP to before the start of NBI#2, n study a target plasma scenario for NBI. In the discharge shown in Fig. 4, a single pulse of SMBI from the outboard horizontal port
#3.5 is injected at t 216 ms, which is indicated in a large spike of Ha#3.5. In the discharge shown in Fig. 5, a short pulse (10 ms) of high intensity GP (HIGP) is used instead of SMBI, where other discharge conditions are almost the same. e steps up just after the fueling of As shown in Figs. 4a and 5a, n SMBI or HIGP. The stored energy drops a short time after this gas injection, but it rapidly increases in the second NBI phase. The
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Fig. 5. (a) Time trace of NBI sustained plasma with HIGP. (b) Radial profiles of electron temperature and density from TVTS at t = 237 ms. (c) Radial profiles of ion temperature from CVRS at t = 225, 235 ms.
maximum Wp in the SMBI (Fig. 4) and the HIGP (Fig. 5) cases are e (3 1019 m3) is almost 4 kJ and 3.4 kJ, respectively, where n the same for both cases. As shown in Figs. 4b and c and 5b and c, Te and Ti in the SMBI case are higher than those in the HIGP case at the Wp-peak timing (235 ms and 237 ms, respectively). More clear difference is observed in the density profile. A peaked density profile is observed after SMBI (235 ms), while it is flat or a little bit hollow (237 ms) for the case of HIGP. This is qualitatively consistent with the edge density profile reconstructed from AM microwave reflectometer data. The reflectometer data indicates the dynamics of edge density profile during the increasing phase of Wp after SMBI or HIGP. After a density increase just after the short pulse fueling, temporal reduction of the edge density is observed, suggesting the core density peaking, and then the edge density and its radial gradient increases for both the cases [12]. Such behavior is not observed after the second NBI when neither SMBI nor HIGP is used.
turbulence has revealed that SMBI can change its rotation direction and/or speed. These effects on the edge turbulence could improve the plasma performance after SMBI. In addition, recent density fluctuation measurements at different r/a with a BES system [16] suggests that SMBI can affect at least some kind of MHD activities inside the last-closed flux surface, which will be discussed more detailed in the near future. In a condition of NBI plasma, density fluctuations observed in the BES signal, which may be caused by some MHD mode relating to high energy ions, disappeared during 10 ms after SMBI. The Mirnov signal also decreased at this time. This observation suggests some change of excitation condition of the MHD mode as well as the more preferable change of NBI deposition profile toward core heating due to ne(r) modification and expected low edge neutral density caused by SMBI. In summary, comparing gas fueling effects on plasma profile between SMBI and GP, the importance of SMBI fueling for plasma density control to obtain better plasma performance is pointed out. Acknowledgements
4. Discussions and summary Effectiveness of SMBI fueling has been studied based on the profile data. This experiment revealed that SMBI fueling could make a more peaked density profile compared to the conventional gas puff fueling. In a short pulse intensive gas puff fueling, the amount of e is about 30– gas necessary to obtain the same increment of n 40% higher compared to the case of SMBI fueling. This should be attributed to the beam-like motion of SMBI neutrals with a high flux density in a small area at the last closed flux surface. In such a condition, the beam can make a local cold spot and the beam neutrals can deeper penetrate before they are ionized. In the dise charges shown in Figs. 4 and 5, almost the same increment of n was obtained. Therefore, the amount of the gas in the case of HIGP should be large compared to that in the SMBI case, resulting higher neutral density in the plasma edge in the case of HIGP. This expected difference in the neutral density outside the plasma after SMBI or HIGP might contribute to make the observed different density-profile at 20 ms after the fueling. SMBI can also affect plasma fluctuations. As discussed in [10,11], fast camera observation for filament structure in edge
The authors are grateful to the Heliotron J supporting group for their excellent arrangement of the experiments. This work is performed with the support and under the auspices of the Collaboration Program of the Laboratory for Complex Energy Processes, IAE, Kyoto University, the NIFS Collaborative Research Program (NIFS10KUHL030, etc.), the JSPS-CAS Core University Program, and the NIFS/NINS project of Formation of International Network for Scientific Collaborations, as well as the Grant-in-Aid for Sci. Research. References [1] L. Yao, in: New Developments in Nuclear Fusion Research, Nova Sci. Publ., 2006, pp. 61–87. [2] G. Giruzzi et al., Nucl. Fusion 49 (2009) 104010. [3] H. Takenaga et al., Nucl. Fusion 49 (2009) 075012. [4] A. Murakami et al., Plasma Fusion Res. 6 (2011) 1402135. [5] F. Sano et al., J. Plasma Fusion Res. 3 (2000) 26. [6] T. Obiki et al., Nucl. Fusion 41 (2001) 833. [7] M. Wakatani et al., Nucl. Fusion 40 (2000) 569. [8] M. Yokoyama et al., Nucl. Fusion 40 (2000) 261. [9] T. Mizuuchi et al., J. Nucl. Mater. 415 (2011) S443–S446.
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[10] N. Nishino et al., J. Nucl. Mater. 415 (2011) S447–S450. [11] L. Zang, et al., in: The 18th International Stellarator/Heliotron Workshop, Canberra, 2012. [12] K. Mukai et al., Plasma Fusion Res. 6 (2011) 1402111. [13] T. Mizuuchi, et al., FEC2010, EXC/P8-11.
[14] N. Nishino, et al., This conference, P1–68. [15] H.Y. Lee et al., Plasma Fusion Res. 7 (2012) 1402019. [16] S. Kobayashi, et al., in: 19th Topical Conf. High-Temp. Plasma Diag, Monterey, 2012, P1–12.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Gas fueling effect on plasma profile in Heliotron J T. Mizuuchi a,⇑,1, H.Y. Lee b, K. Mukai c, K. Yamamoto b, S. Kobayashi a, H. Okada a, S. Yamamoto a, S. Ohshima a, L. Zang b, K. Nagasaki a, T. Minami a, T. Kagawa b, T.Y. Minami b, K. Mizuno b, Y. Wada b, S. Arai b, H. Yashiro b, H. Watada b, K. Hashimoto b, N. Kenmochi b, Y. Nagae b, M. Sha b, Y.I. Nakamura b, K. Kasajima b, N. Nishino d, K. Hosoi e, Y. Nakashima e, Y. Nakamura b, S. Konoshima a, F. Sano a a
Institute of Advanced Energy, Kyoto Univ., Gokasho, Uji 611-0011, Japan Graduate School of Energy Science, Kyoto Univ., Gokasho, Uji, Japan National Institute for Fusion Science, Toki, Japan d Graduate School of Engineering, Hiroshima Univ., Higashi-Hiroshima, Japan e Plasma Research Center, Univ. of Tsukuba, Tsukuba, Japan b c
a r t i c l e
i n f o
Article history: Available online 17 January 2013
a b s t r a c t The optimization of gas fueling scenarios has been studied to improve the plasma performance in Heliotron J. Gas fueling effects on the plasma profile are discussed by comparing two fueling methods, a short pulse H2-beam fueling with a supersonic molecular-beam injection (SMBI) technique and a high intensity gas-puff fueling (HIGP) with a conventional gas puff technique. The maximum plasma stored energy Wp after SMBI is about 20% higher than that after HIGP in this NBI-only sustained plasma experiment, where the line-averaged density is almost the same (3 1019 m3) for both cases. The electron and ion temperatures in the SMBI case are higher than those are in the HIGP case at the Wp-peak timing. A peaked density profile is observed after SMBI, while it is flat or slightly hollow for the case of HIGP. These observations point out the importance of SMBI fueling for plasma density control to obtain better plasma performance. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Fueling and recycling control is one of the key issues to obtain high density and high performance plasma in magnetic confinement devices from two aspects; (1) profile control of the plasma density and (2) reduction of neutrals in the peripheral region. Core fueling by ice pellets is well known as a technique to realize favorable fueling from these aspects. The pellet system is, however, complicated and it is not easy to make pellets small enough for density control in medium or small size devices. A supersonic molecular-beam injection (SMBI) technique [1] is, on the other hand, considered to be an effective fueling method for deeper penetration of neutral particles into the core plasma compared to conventional gas-puffing (GP). SMBI is applied not only for core density control but also as an effective edge modification technique in fusion devices [2–4]. The optimization of gas fueling scenarios has been studied to improve the plasma performance in Heliotron J, which is a helical-axis heliotron device with an L/M = 1/4 helical coil (hR0i = 1.2 m, ha p i = 0.12–0.17 m, hB 0 i 6 1.5 T) [5,6] based on the ⇑ Corresponding author. 1
E-mail address: [email protected] (T. Mizuuchi). Presenting author.
0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.01.092
helical-axis heliotron concept [7,8]. Here L and M are the pole number of the helical coil and its helical pitch number, respectively. A fueling method with SMBI technique has been successfully applied to Heliotron J plasma [9]. It is considered that local fueling with a short pulse intense molecular beam can increase the core plasma density, while avoiding confinement degradation due to the edge cooling caused by excess neutrals in the peripheral region, which is often observed in the case of conventional GP control. SMBI can also affect plasma fluctuations. Fast camera observation of filament structure in edge turbulence has revealed that SMBI can change its rotation direction and/or speed [10,11]. Similar change is observed at L–H transition in Heliotron J. In a previous PSI conference [9], we have reported the effects of SMBI fueling on the global parameters such as the plasma stored e , and the divertor particle energy Wp, the line-averaged density n flux, which is represented by the summation of the ion-saturation currents for all probe pins of a divertor-probe-array at section #3.5 (see Fig. 1). Recently, the response of edge plasma caused by different gas fueling methods is investigated, especially focused on the change of the plasma radial profile [12,13]. In this paper, we discuss the effects of SMBI fueling on the core plasma profile comparing SMBI fueling and high intensity short pulse GP fueling with a conventional gas-puff system. Although the latter fueling method e like the SMBI case, it cannot avoid can make a rapid increase of n
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Fig. 1. Experimental set-up. A layout of heating, fueling and diagnostic equipment is illustrated.
the saturation (or degradation) of Wp in higher density condition in contrast to the SMBI case [9]. 2. Experimental setup Fig. 1 schematically shows a layout of heating, fueling and some diagnostic equipment for this experiment. For density control, usually used is a conventional gas-puffing system with four piezoelectric valves. These valves are installed at the inboard-side ports (low field side injection) at 90° intervals around the torus (‘‘Gas’’ in Fig. 1). These ports are located in the low field side nevertheless they are in inboard side since the field strength is usually higher near the helical coil. The amount of H (or D) atoms from the GP system with the plenum pressure of 0.15 MPa is pre-programmed to control the line-averaged density. Since, due to the space limitation, the nozzles of these valves do not directly see the plasma, the induced gas from the valve diffuses to the discharge chamber after multiple reflections in an elbow-shaped pipe connecting the nozzles to the chamber. An SMBI system of hydrogen is equipped on two horizontal outboard-side ports (the port number: #3.5, #11.5). The system consists of a fast piezoelectric valve with a short conic (or Laval-type) nozzle with the orifice size of 0.2 mm/. The amount of H atoms injected with the SMBI is controlled by changing the pulse width of each SMBI valve under a fixed plenum pressure (Ppl 1–2 MPa). The initial hydrogen or deuterium plasma is produced by using second harmonic X-mode ECH (70 GHz, <0.30 MW). The hydrogen beam is injected for NBI experiments by using one or two tangential beam-lines (BL-1 and BL-2. The acceleration voltage is <30 kV and the beam power is <0.7 MW/beam-line). 3. Effects of SMBI fueling on core plasma profile The peculiarities of SMBI fueling are its directional motion of the injected gas, the small area of plasma–beam interaction and a massive gas injection in a short time. As discussed in [9], the absolute value of the beam speed itself is not so fast (1–2 km/s) in this experiment, comparable to that for the thermal speed of H2 introduced by normal GP. As for the plasma–beam interaction area, judging from visible-light images measured with a fast
camera [10,14], the core part of H2-beam radius is less than 10 cm near the LCFS. To investigate the effect of directional motion, a direct comparison between the directional and non-directional gas injection was performed by using a movable plate in front of the #3.5-SMBI e and Ha at #3.5 nozzle. Fig. 2 shows time traces of Wp and n (Ha#3.5) for two discharges under the same heating condition (simultaneous heating by ECH and NBI, ECH+NBI); the shutter is open in the case (a) and closed in the case (b). Also plotted are the information of GP condition, ‘‘GP’’ (the pre-programmed control voltage for the gas-puffs) and ‘‘gas’’ (a manometer signal measured at a gas-puff port). A large spike of Ha#3.5 indicates the SMBI timing (t 208.5 ms). When the shutter is closed, the SMBI beam hits the plate and then H2 molecules diffuse to the plasma, which is suggested by a low and slow response of Ha#3.5 shown in Fig. 2b. More directly, chord profiles of Ha-intensity measured with the Ha-array at #3.5 are compared in Fig. 2c and d for two timings. Just after SMBI (t 209 ms, Fig. 2c), very strong (>10) emission is observed for sight lines crossing the injected beam line inside the LCFS (R < 1.31 m). On the other hand, only 3 ms later timing (Fig. 2d), almost the same intensity and chord profile are observed for both cases. These observations indicate that the injected beam penetrates well into the plasma core e shows raregion in the open shutter case. As shown in Fig. 2a, n pid increase just after SMBI and then gradually increases up to 2.6 1019 m3 during 30 ms in the directly injected case (the open shutter case). The stored energy also shows rapid increase with a slight delay, keeps the elevated value for 10 ms and then e becomes saturated. In starts to decrease when the increase of n the case of closed shutter (Fig. 2b), on the other hand, the in e and Wp are slow and mild. The density already satcreases of n urates at 1.41019 m3 and starts to decrease at 15 ms after SMBI. Since the amount of injected gas is the same for these e after the SMBI inditwo cases, the difference of the attainable n cates the effectiveness of SMBI in the over-all fueling efficiency. The differences of edge density profile for these two discharges measured with a microwave reflectometer at the torus section of #15.5 are discussed in [12]. In the open shutter case, the edge density profile, which is rather flat before SMBI, rapidly changes to a very peaked one after SMBI, while the profile change is slow and mild in the closed shutter case.
T. Mizuuchi et al. / Journal of Nuclear Materials 438 (2013) S453–S458
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Fig. 2. (a) Time traces of line-averaged density, stored energy for a series of ECH+NBI plasma for an open condition of the shutter in front of SMBI (#3.5-SMBI). (b) The similar time traces for a closed shutter case under the same heating condition. Chord profiles of Ha at the SMBI section (#3.5) for the open shutter case ( ) and the closed case ( ) at two different timings, (c) 209 ms and (d) 212 ms, respectively. Sight lines of Ha-array and magnetic surfaces are shown in (e) with a schematics of #3.5-SMBI.
The similar comparison for the effects of open/close shutter was performed for a different series of ECH+NBI plasma with the #11.5SMBI. Fig. 3a and b shows the time evolution of the chord intensities of SX emission at section #7.5 (the sightlines are shown in Fig. 3c) as contour plots for the open/closed shutter cases, respectively. The SX signals show clear responses to a gas injection in the open case, while the change of SX-intensity in the closed shutter case is not so clear for all chords. In the open shutter case, a rapid drop in intensity is observed even for the chords viewing the plasma core, with a little time delays from the edge to the core. Then a rapid rise in intensity is observed. The time response of this increasing is more rapid in the open shutter case compared to
the closed case. These observations might indicate the creation of a local cold spot by SMBI without shutter. SMBI effects on the core plasma profile were investigated for e and the stored NBI plasmas. Figs. 4a and 5a show time traces of n energy Wp with gas fueling and heating sequences for NBI sustained plasmas. Here a case of fueling by a short pulse intensive GP (Fig. 5) is compared to the case of SMBI (Fig. 4) to make a more e for both cases. Electron temperature similar time response of n Te(r/a) and density ne(r/a) from TVTS for the cases of SMBI and HIGP are shown in Figs. 4b and 5b, respectively. Figs. 4c and 5c show radial profiles of ion temperature Ti(r/a) from CXRS [15] for each discharge. Plasmas are initiated by using a short pulse of
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Fig. 3. Comparison of the chord profile of SX emission observed at #7.5-section between (a) open and (b) closed shutter conditions for #11.5-SMBI, respectively. Here, the plasma-heating scenario is similar to that in the case shown in Fig. 2. (c) Sightlines of SX-array and magnetic surfaces. (d) Schematics of #11.5-SMBI with sight lines of Haarray in this section.
Fig. 4. (a) Time trace of NBI sustained plasma with SMBI. (b) Radial profiles of electron temperature and density from TVTS at t = 235 ms. (c) Radial profiles of ion temperature from CVRS at t = 225, 235 ms.
second harmonic X-mode ECH (70 GHz, <0.40 MW) and then sustained by two-staged NBI heating (H0-beam, Ebeam < 30 keV, Pinj. 0.6 MW per a beam-line), where the first NBI (NBI#1) sustains the plasma and then the second NBI (NBI#2) is superimposed. Just e is increased by using SMBI or GP to before the start of NBI#2, n study a target plasma scenario for NBI. In the discharge shown in Fig. 4, a single pulse of SMBI from the outboard horizontal port
#3.5 is injected at t 216 ms, which is indicated in a large spike of Ha#3.5. In the discharge shown in Fig. 5, a short pulse (10 ms) of high intensity GP (HIGP) is used instead of SMBI, where other discharge conditions are almost the same. e steps up just after the fueling of As shown in Figs. 4a and 5a, n SMBI or HIGP. The stored energy drops a short time after this gas injection, but it rapidly increases in the second NBI phase. The
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Fig. 5. (a) Time trace of NBI sustained plasma with HIGP. (b) Radial profiles of electron temperature and density from TVTS at t = 237 ms. (c) Radial profiles of ion temperature from CVRS at t = 225, 235 ms.
maximum Wp in the SMBI (Fig. 4) and the HIGP (Fig. 5) cases are e (3 1019 m3) is almost 4 kJ and 3.4 kJ, respectively, where n the same for both cases. As shown in Figs. 4b and c and 5b and c, Te and Ti in the SMBI case are higher than those in the HIGP case at the Wp-peak timing (235 ms and 237 ms, respectively). More clear difference is observed in the density profile. A peaked density profile is observed after SMBI (235 ms), while it is flat or a little bit hollow (237 ms) for the case of HIGP. This is qualitatively consistent with the edge density profile reconstructed from AM microwave reflectometer data. The reflectometer data indicates the dynamics of edge density profile during the increasing phase of Wp after SMBI or HIGP. After a density increase just after the short pulse fueling, temporal reduction of the edge density is observed, suggesting the core density peaking, and then the edge density and its radial gradient increases for both the cases [12]. Such behavior is not observed after the second NBI when neither SMBI nor HIGP is used.
turbulence has revealed that SMBI can change its rotation direction and/or speed. These effects on the edge turbulence could improve the plasma performance after SMBI. In addition, recent density fluctuation measurements at different r/a with a BES system [16] suggests that SMBI can affect at least some kind of MHD activities inside the last-closed flux surface, which will be discussed more detailed in the near future. In a condition of NBI plasma, density fluctuations observed in the BES signal, which may be caused by some MHD mode relating to high energy ions, disappeared during 10 ms after SMBI. The Mirnov signal also decreased at this time. This observation suggests some change of excitation condition of the MHD mode as well as the more preferable change of NBI deposition profile toward core heating due to ne(r) modification and expected low edge neutral density caused by SMBI. In summary, comparing gas fueling effects on plasma profile between SMBI and GP, the importance of SMBI fueling for plasma density control to obtain better plasma performance is pointed out. Acknowledgements
4. Discussions and summary Effectiveness of SMBI fueling has been studied based on the profile data. This experiment revealed that SMBI fueling could make a more peaked density profile compared to the conventional gas puff fueling. In a short pulse intensive gas puff fueling, the amount of e is about 30– gas necessary to obtain the same increment of n 40% higher compared to the case of SMBI fueling. This should be attributed to the beam-like motion of SMBI neutrals with a high flux density in a small area at the last closed flux surface. In such a condition, the beam can make a local cold spot and the beam neutrals can deeper penetrate before they are ionized. In the dise charges shown in Figs. 4 and 5, almost the same increment of n was obtained. Therefore, the amount of the gas in the case of HIGP should be large compared to that in the SMBI case, resulting higher neutral density in the plasma edge in the case of HIGP. This expected difference in the neutral density outside the plasma after SMBI or HIGP might contribute to make the observed different density-profile at 20 ms after the fueling. SMBI can also affect plasma fluctuations. As discussed in [10,11], fast camera observation for filament structure in edge
The authors are grateful to the Heliotron J supporting group for their excellent arrangement of the experiments. This work is performed with the support and under the auspices of the Collaboration Program of the Laboratory for Complex Energy Processes, IAE, Kyoto University, the NIFS Collaborative Research Program (NIFS10KUHL030, etc.), the JSPS-CAS Core University Program, and the NIFS/NINS project of Formation of International Network for Scientific Collaborations, as well as the Grant-in-Aid for Sci. Research. References [1] L. Yao, in: New Developments in Nuclear Fusion Research, Nova Sci. Publ., 2006, pp. 61–87. [2] G. Giruzzi et al., Nucl. Fusion 49 (2009) 104010. [3] H. Takenaga et al., Nucl. Fusion 49 (2009) 075012. [4] A. Murakami et al., Plasma Fusion Res. 6 (2011) 1402135. [5] F. Sano et al., J. Plasma Fusion Res. 3 (2000) 26. [6] T. Obiki et al., Nucl. Fusion 41 (2001) 833. [7] M. Wakatani et al., Nucl. Fusion 40 (2000) 569. [8] M. Yokoyama et al., Nucl. Fusion 40 (2000) 261. [9] T. Mizuuchi et al., J. Nucl. Mater. 415 (2011) S443–S446.
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