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Performance of fueling pellet injectors for the large helical device
Fusion Engineering and Design 81 (2006) 2655–2660
Performance of fueling pellet injectors for the large helical device M. Hoshino a,∗ , R. Sakamoto b , H. Yamada b,c , R. Kumazawa b , T. Watari b , the LHD Experimental Groupb a
Department of Energy Engineering and Science, Nagoya University, Nagoya 464-8603, Japan b National Institute for Fusion Science, Toki 509-5292, Japan c The Graduate University for Advanced Studies, Sokendai, Hayama 240-0193, Japan Available online 22 August 2006
Abstract Techniques of solid hydrogen pellet injection into plasmas towards steady state operation as well as further reliable hardware have been demonstrated for the large helical device (LHD). The conversion of hardware is performed to attain higher reliability and reproducibility of pellets. Major issues to improve the quality of pellets are the cooling temperature of pellet production by a cryo-cooler and the incident angle to the inner wall of the guide tube for pellets. The former is improved from 12.6 to 10.6 K by the reduction of the heat conduction and radiation to produce the robust pellets. The latter is optimized from 5.41◦ to 1.73◦ at the maximum by the low-angle pellet path to mitigate the destruction of pellets in the guide tube. It has been confirmed by these conversion that the quality of pellets is significantly improved, i.e., the probability of intact pellets is increased to ∼100%. Also, the controllability of the pellet size is desired as needs of pellet injection experiments. It has been demonstrated by the property of pellet production related to the cooling temperature and the pressure of supplied hydrogen gas. © 2006 Elsevier B.V. All rights reserved. Keywords: LHD; Fueling pellet injector; In-situ pipegun method; Cryo-cooler; Pellet production
1. Introduction Gas puffing and solid hydrogen pellet injection have been main fueling tools in many experimental devices and they both are planned for the international thermonuclear experimental reactor (ITER) [1]. Significant fueling capability to operate at high density for long duration is required for the next generation experi∗ Corresponding author. Tel.: +572 58 2222x1219; fax: +572 58 2618. E-mail address: [email protected] (M. Hoshino).
0920-3796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2006.07.025
ment like ITER. However, a larger device meets an issue that the fueling efficiency of gas puffing declines because most of fueled gas is ionized by a hot and thick scrape-off layer before it penetrates into the confinement region [2]. The increment of neutral gas causes problems of degradation of the confinement. With respect to the tritium inventory, the degradation of the fueling efficiency also becomes problematic. Therefore gas puffing in a fusion reactor may be not necessarily convincing and fulfilled the required performance on fueling. Injection of solid hydrogen pellets is superior to gas puffing in terms of deeper penetra-
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tion and higher fueling efficiency [3]. The understanding of physical mechanism of pellet ablation in hot plasmas is important to optimize the fueling scenario. Also, reliable engineering techniques of pellet injectors should be established to fulfill these physical requirements. From a viewpoint of hardware, a pellet injection technique has been made significant progress in the 1970s and many experiments have been performed with the aim of high-speed projection, high repetition and reliable operation [4]. The cryogenic property of hydrogen isotopes is a major key for the performance of a pellet injector since the gas of H2 , D2 and T2 is generally frozen in the production zone cooled in the ultracold temperature of less than ∼20 K. Fig. 1 shows the state diagram of hydrogen. With respect to hydrogen (H2 ), the gas cannot be solidified if the gas temperature is larger than 13.9 K. Also, this critical temperature decreases when the gas pressure becomes less than 7 kPa. The condition of these critical temperatures and pressures is referred to the triple point. The state of fuel hydrogen is determined by the relationship between the cooling temperature and the pressure of supplied hydrogen gas. Therefore the cryogenic zone of pellet production on a pellet injector must be carefully controlled to produce intact pellets. In particular, it is important for establishment of reliable pellet production to study the relation between the temperature and the quality of pellets. In this paper, principal prop-
Fig. 1. The state diagram of hydrogen at extremely low temperatures. The temperature and pressure of the triple point are about 13.9 K and 7 kPa, respectively.
erties for pellet production concentrated on cryogenics are reported.
2. Experimental set-up Two pellet injectors, an in situ pipegun pellet injector [5] and a repetitive pellet injector [6] with a screw extruder, are installed in LHD. They are routinely utilized in concert with experimental needs [7], and indeed studied to demonstrate pellet injectors with high performance for fueling on a reactor. On the in situ pipegun pellet injector, solid hydrogen pellets are simultaneously produced in 8 barrels which are cooled at about 10 K with a Gifford–McMahon (GM) cycle cryo-cooler [8] which has a refrigeration cycle of high efficiency, performance and reliability. The internal diameter of barrel is 3.0 mm and the length contacting the heat sink of Cu (∼10 K measured by thermocouples) cooled by a cryo-cooler is 3 mm; the nominal pellet mass is 1.0 × 1021 atoms. Pellets are injected into LHD plasmas by the pneumatic pipegun method using high pressure He gas. It has been confirmed that the pellet velocity varies from 1000 to 1200 m/s by the pressure of accelerating gas, and this fact agrees with the prediction of the ideal gun theory [9] in the difference of 20–30%. The pellet mass is measured by the microwave cavity of the TE103 mode [10] and is about 6.0–8.5 × 1020 atoms, which is consistent with the size of pellets. Fig. 2 shows the section view of the cryogenic vacuum chamber where pellets are formed, on the in situ pipegun pellet injector. A heat sink made of oxygen free copper is connected to the cold head on the second stage of a cryo-cooler (1 W at 4.2 K and 8 W at 10 K) and is surrounded with the thermal shield on the first stage (37 W at 40 K). Heat sinks of Cu are two blocks and cooled by each cryo-cooler in the cryogenic vacuum chamber. The structure of heat sinks before and after the conversion is shown in Fig. 3. The slits on the heat sinks prevents a heat load by propellant gas in the neighbor barrels. Barrels with the wall thickness of 0.5 mm are brazed by the heat sink 3 mm long along their paths, and are connected to H2 , He gas supply and vacuum pumping. The number of barrels is eight (four barrels for each heat sink). The funnel is used to package eight guide tubes before the injection to LHD and forces the angle of ∼4◦ at the maximum on the guide tube.
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Fig. 2. Schematic diagram of the cryogenic chamber.
3. Improvement of the quality of pellets The improvement of the quality of pellets due to the conversion regarding the pellet production zone and the pellet path is described in this section. The cooling temperature of the pellet production zone in a cryogenic chamber is measured by thermocouples to clarify the heat load with eight barrels. The cooling temperature using two compressors of and cold heads on two heat sinks A and B is 10.2 and 12.6 K when eight barrels are installed, respectively. The difference of cooling temperature on A and B may imply that the cooling capability on B is worse, even as an similar operational condition. Higher cooling temperature (12.6 K) on the heat sink B causes the degradation of the strength of
pellets produced by the compressor and the cold head on B since the mechanical strength of solid hydrogen pellets produced on the temperature of ∼10.2 K is twice as high as that of 12.6 K in Ref. [11]. As shown in Table 1, intact pellets of ∼65% and ∼0% are observed on A and B, respectively. Thus, the improvement of cooling temperature is the requisite condition of intact pellets on B. Also, the incident angle to the inner wall of the guide tube of ∼5.41◦ at the maximum by the funnel, which the scattering angle of 1◦ is considered when pellets leave the muzzle of the guide tube, would cause the damage of the pellet due to a collision in the funnel. Indeed, intact pellets of ∼65% are verified on the heat sink A of 10.2 K being colder than B, of which pellets projected by the low-angle path of the guide tube (inner
Table 1 Before and after the conversion of the heat sink, the cooling temperature on the heat sinks A and B with the barrels and the probability of intact pellets Compressor
The number of barrels
Cooling temperature (K)
Intact pellets (%)
Before conversion
A B
4 4
10.2 12.6
65 0
After conversion
A B
6 4
10.2 10.6
100 100
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Fig. 3. Conversion of the heat sink of Cu in unit of mm. The width of the heat sinks is reduced to enlarge the distance from the thermal shield and consequently to mitigate the heat radiation.
barrels) account ∼71% though the ratio of intact pellets of the high-angle path (outer barrels) is ∼41%. This difference would be derived from the incident angle at the maximum because the angle of the outer barrel is larger than that of inner barrel in the funnel. These results indicate that the degradation of the quality of pellets is caused by a collision in the guide tube. Major issues to improve the quality of pellets are the cooling temperature producing pellets and the incident angle in the guide tube by considering the experimental condition on A and B. The conversion of the in situ pipegun pellet injector was performed. The width of the heat sinks was reduced (i.e., the heat sinks was cut) to enlarge the distance from the thermal shield and consequently to mitigate the heat radiation as shown in Fig. 3. The distance between the heat sink and the thermal shield before and after the conversion of the heat sink was broadened from 11 to 16 mm for a lengthwise direction and from 7 to 17 mm for a crosswise direction. The optimization of a pellet path (the funnel is
not used and the angle of a guide tube is 1.73◦ at the maximum) was performed and the wall thickness of barrels was reduced from 0.5 to 0.3 mm for the mitigation of the heat conduction. The number of the barrel was increased from 4 to 6 on A because of experimental needs and was remained in 4 on B, totally 10 barrels were installed. After the conversion, the cooling temperature on A and B is 10.2 and 10.6 K with the barrels, respectively (see Table 1). This result shows the improvement of cooling capability since the temperature increase is controlled in spite of an increase of the number of the barrels on A and the temperature on the heat sink B is obviously improved. Consequently, the quality and the reproducibility of pellets have been significantly improved by the conversion of cryogenic pellet production and the optimization of a pellet path. An intact pellet has been confirmed by the picture by a shadowgraph method in Fig. 4(b). The probability of an intact pellet is improved from ∼65% on A and 0% on B to both ∼100%. Also, the increase of the line-averaged electron density and the pellet penetration depth in pelletfueled experiments of LHD are observed between a cracked pellet and an intact pellet of 3.0 mm cylinder. These same pellet are injected into plasmas on the same magnetic configuration (the measured pellet mass, the pellet velocity, the central electron temperature, and the total heating power are about 7.0 × 1020 atoms, 1200 m/s, 2.0 keV, and 7.0 MW, respectively). The increase of central electron density is 1.1 and 3.7 × 1019 m−3 and the pellet penetration depth normalized by the minor radius is 0.7 and 1.4, for a cracked pellet and an intact pellet. An intact pellet is superior to a cracked pellet in terms of the pellet penetration depth as well as the increase of the central electron density. However, the improvement due to the conversion of heat sinks and wall thickness cannot be quantitatively estimated. It is important for the quantitative estimation of the improvement to demonstrate each effects in further investigation.
4. Properties of pellet production The property of pellet production is examined on the basis of the state diagram of hydrogen at cryogenic temperature on an in situ pipegun pellet injector. Fig. 5 shows the schematic diagram for pellet production. The
M. Hoshino et al. / Fusion Engineering and Design 81 (2006) 2655–2660
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Fig. 4. Pictures of (a) a cracked pellet of 3.0 mm cylinder before the conversion and (b) an intact pellet of 3.8 mm cylinder after the conversion by a shadowgraph method.
cooling temperature measured by thermocouples on the heat sink of Cu is ∼10.2 K, and there is temperature distribution between the pellet production zone in the pellet production chamber and outside the chamber (∼300 K). This difference arises the temperature gradient along the barrel. The temperature gradient along the barrel (1/4 in. SUS 316) in the chamber is calculated to demonstrate the temperature increase by the heat load in Fig. 5. Using the initial condition (the cooling temperature and the interface temperature of the vacuum are 10.2 and 300 K), the equation of heat conduction is solved with neglecting the thermal radiation. By controlling the pressure of supplied hydrogen
Fig. 5. Schematic diagram for pellet production with the cryogenic property of hydrogen and the calculated temperature gradient along the barrel.
gas, the length of the pellet can be controlled since the region of solidification along the barrel is widened outwardly. Therefore pellets can be longer than the length of the part cooled by the heat sink (3 mm). For example, when the pressure increase to 7 kPa the increment of the pellet length is ∼1 mm in Fig. 5. Using the mass detector and the shadowgraph method after pellet projection, this controllability of the pellet size (i.e., mass) is confirmed in Fig. 6. The pellet mass increases until the pressure of the triple point, and H2 pellets of 3 mm cylinder and length are controlled from 100% to 130%. The mass increase of 30% measured by the mass detec-
Fig. 6. Relation between the pressure of supplied hydrogen gas and the pellet mass for 3 mm cylinder pellets.
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tor agrees that of the increment of the pellet length ∼1 mm. When the pressure is larger than the pressure at the triple point, the mass is almost returned to the level of the initial size in Fig. 6. This implies that a pellet is melted by the heat propagation of a liquid layer because a liquid layer outside a solid is formed when the pressure is larger than the pressure of ∼7 kPa. The improvement of capability of the present pellet injector of LHD is expected when D2 is used instead of H2 . Since the temperature of the triple point on D2 (∼18.7 K) is higher than that of H2 (∼13.9 K), the flexibility of pellet size can be improved in comparison with H2 . The mass of 3 mm size pellet is envisaged to increase to about 183% for D2 in contrast with about 130% for H2 because of further expanding of the freezing region.
5. Conclusion On the in situ pipegun pellet injector in LHD, the conversion of hardware is performed to improve the quality of pellets. The production of intact pellets is significantly concerned with the cooling temperature of pellet production and the destruction of pellets in the pellet guide tube should be mitigated after pellet projection. The cooling temperature is improved from 12.6 to 10.6 K by the reduction of the heat conduction and radiation in the cryogenic zone of pellet production. The incident angle to the inner wall of the guide tube for pellets is optimized from 5.41◦ to 1.73◦ at the maximum. It has been verified by these conversion that the probability of intact pellets is improved from 65% on the heat sink A (10.2 K) and 0% on B (12.6 K) to both ∼100%. The controllability of the pellet size is also confirmed that the pellet mass of 3 mm cylinder and length is increased by 30% by controlling the cooling temperature and the pressure of supplied hydrogen gas, and the increment is consistent with prediction by
the calculated temperature gradient. After the conversion of hardware, pellet production with high reliability and reproducibility is demonstrated in pellet injection experiments on LHD. Acknowledgments The authors would like to thank the LHD technical staff for their encouragement and support. This work is supported by NIFS under contract nos. NIFS05ULPP521 and ULPP522. References [1] ITER Physics Basis Editors, et al., Chapter1: overview and summary; chapter 9: opportunities for reactor scale experimental physics, Nucl. Fusion 39 (1999) 2137–2638. [2] L.R. Baylor, et al., Pellet fueling technology development leading to efficient fueling of ITER burning plasmas, Phys. Plasmas 12 (2005) 056103. [3] S.L. Milora, W.A. Houlberg, et al., Pellet fuelling, Nucl. Fusion 35 (1995) 657–754. [4] S.K. Combs, Pellet injection technology, Rev. Sci. Instrum. 64 (1993) 1679–1698. [5] H. Yamada, et al., Development of pellet injector system for large helical device, Fusion Eng. Des. 49/50 (2000) 915– 920. [6] H. Yamada, et al., Repetitive fueling pellet injection in large helical device, Fusion Eng. Des. 69 (2003) 11–14. [7] R. Sakamoto, et al., Impact of pellet injection on extension of the operational region in LHD, Nucl. Fusion 41 (2001) 381– 386. [8] R.C. Riedy, Low temperature, high performance G–M refrigerator, Cryogenics 33 (1993) 653–658. [9] L.D. Landau, E.M. Lifshitz, Fluid Mechanics, Pergamon Press, London, 1987, pp. 361–373. [10] M.J. Gouge, et al., A combined microwave cavity and photographic diagnostic for high-speed projectiles, Rev. Sci. Instrum. 61 (1990) 2102–2105. [11] L.A. Alekseeva, et al., Temperature dependence of the yield stress of solid hydrogen, Sov. J. Low Temp. Phys. 8 (1982) 158–159.
Performance of fueling pellet injectors for the large helical device M. Hoshino a,∗ , R. Sakamoto b , H. Yamada b,c , R. Kumazawa b , T. Watari b , the LHD Experimental Groupb a
Department of Energy Engineering and Science, Nagoya University, Nagoya 464-8603, Japan b National Institute for Fusion Science, Toki 509-5292, Japan c The Graduate University for Advanced Studies, Sokendai, Hayama 240-0193, Japan Available online 22 August 2006
Abstract Techniques of solid hydrogen pellet injection into plasmas towards steady state operation as well as further reliable hardware have been demonstrated for the large helical device (LHD). The conversion of hardware is performed to attain higher reliability and reproducibility of pellets. Major issues to improve the quality of pellets are the cooling temperature of pellet production by a cryo-cooler and the incident angle to the inner wall of the guide tube for pellets. The former is improved from 12.6 to 10.6 K by the reduction of the heat conduction and radiation to produce the robust pellets. The latter is optimized from 5.41◦ to 1.73◦ at the maximum by the low-angle pellet path to mitigate the destruction of pellets in the guide tube. It has been confirmed by these conversion that the quality of pellets is significantly improved, i.e., the probability of intact pellets is increased to ∼100%. Also, the controllability of the pellet size is desired as needs of pellet injection experiments. It has been demonstrated by the property of pellet production related to the cooling temperature and the pressure of supplied hydrogen gas. © 2006 Elsevier B.V. All rights reserved. Keywords: LHD; Fueling pellet injector; In-situ pipegun method; Cryo-cooler; Pellet production
1. Introduction Gas puffing and solid hydrogen pellet injection have been main fueling tools in many experimental devices and they both are planned for the international thermonuclear experimental reactor (ITER) [1]. Significant fueling capability to operate at high density for long duration is required for the next generation experi∗ Corresponding author. Tel.: +572 58 2222x1219; fax: +572 58 2618. E-mail address: [email protected] (M. Hoshino).
0920-3796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2006.07.025
ment like ITER. However, a larger device meets an issue that the fueling efficiency of gas puffing declines because most of fueled gas is ionized by a hot and thick scrape-off layer before it penetrates into the confinement region [2]. The increment of neutral gas causes problems of degradation of the confinement. With respect to the tritium inventory, the degradation of the fueling efficiency also becomes problematic. Therefore gas puffing in a fusion reactor may be not necessarily convincing and fulfilled the required performance on fueling. Injection of solid hydrogen pellets is superior to gas puffing in terms of deeper penetra-
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M. Hoshino et al. / Fusion Engineering and Design 81 (2006) 2655–2660
tion and higher fueling efficiency [3]. The understanding of physical mechanism of pellet ablation in hot plasmas is important to optimize the fueling scenario. Also, reliable engineering techniques of pellet injectors should be established to fulfill these physical requirements. From a viewpoint of hardware, a pellet injection technique has been made significant progress in the 1970s and many experiments have been performed with the aim of high-speed projection, high repetition and reliable operation [4]. The cryogenic property of hydrogen isotopes is a major key for the performance of a pellet injector since the gas of H2 , D2 and T2 is generally frozen in the production zone cooled in the ultracold temperature of less than ∼20 K. Fig. 1 shows the state diagram of hydrogen. With respect to hydrogen (H2 ), the gas cannot be solidified if the gas temperature is larger than 13.9 K. Also, this critical temperature decreases when the gas pressure becomes less than 7 kPa. The condition of these critical temperatures and pressures is referred to the triple point. The state of fuel hydrogen is determined by the relationship between the cooling temperature and the pressure of supplied hydrogen gas. Therefore the cryogenic zone of pellet production on a pellet injector must be carefully controlled to produce intact pellets. In particular, it is important for establishment of reliable pellet production to study the relation between the temperature and the quality of pellets. In this paper, principal prop-
Fig. 1. The state diagram of hydrogen at extremely low temperatures. The temperature and pressure of the triple point are about 13.9 K and 7 kPa, respectively.
erties for pellet production concentrated on cryogenics are reported.
2. Experimental set-up Two pellet injectors, an in situ pipegun pellet injector [5] and a repetitive pellet injector [6] with a screw extruder, are installed in LHD. They are routinely utilized in concert with experimental needs [7], and indeed studied to demonstrate pellet injectors with high performance for fueling on a reactor. On the in situ pipegun pellet injector, solid hydrogen pellets are simultaneously produced in 8 barrels which are cooled at about 10 K with a Gifford–McMahon (GM) cycle cryo-cooler [8] which has a refrigeration cycle of high efficiency, performance and reliability. The internal diameter of barrel is 3.0 mm and the length contacting the heat sink of Cu (∼10 K measured by thermocouples) cooled by a cryo-cooler is 3 mm; the nominal pellet mass is 1.0 × 1021 atoms. Pellets are injected into LHD plasmas by the pneumatic pipegun method using high pressure He gas. It has been confirmed that the pellet velocity varies from 1000 to 1200 m/s by the pressure of accelerating gas, and this fact agrees with the prediction of the ideal gun theory [9] in the difference of 20–30%. The pellet mass is measured by the microwave cavity of the TE103 mode [10] and is about 6.0–8.5 × 1020 atoms, which is consistent with the size of pellets. Fig. 2 shows the section view of the cryogenic vacuum chamber where pellets are formed, on the in situ pipegun pellet injector. A heat sink made of oxygen free copper is connected to the cold head on the second stage of a cryo-cooler (1 W at 4.2 K and 8 W at 10 K) and is surrounded with the thermal shield on the first stage (37 W at 40 K). Heat sinks of Cu are two blocks and cooled by each cryo-cooler in the cryogenic vacuum chamber. The structure of heat sinks before and after the conversion is shown in Fig. 3. The slits on the heat sinks prevents a heat load by propellant gas in the neighbor barrels. Barrels with the wall thickness of 0.5 mm are brazed by the heat sink 3 mm long along their paths, and are connected to H2 , He gas supply and vacuum pumping. The number of barrels is eight (four barrels for each heat sink). The funnel is used to package eight guide tubes before the injection to LHD and forces the angle of ∼4◦ at the maximum on the guide tube.
M. Hoshino et al. / Fusion Engineering and Design 81 (2006) 2655–2660
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Fig. 2. Schematic diagram of the cryogenic chamber.
3. Improvement of the quality of pellets The improvement of the quality of pellets due to the conversion regarding the pellet production zone and the pellet path is described in this section. The cooling temperature of the pellet production zone in a cryogenic chamber is measured by thermocouples to clarify the heat load with eight barrels. The cooling temperature using two compressors of and cold heads on two heat sinks A and B is 10.2 and 12.6 K when eight barrels are installed, respectively. The difference of cooling temperature on A and B may imply that the cooling capability on B is worse, even as an similar operational condition. Higher cooling temperature (12.6 K) on the heat sink B causes the degradation of the strength of
pellets produced by the compressor and the cold head on B since the mechanical strength of solid hydrogen pellets produced on the temperature of ∼10.2 K is twice as high as that of 12.6 K in Ref. [11]. As shown in Table 1, intact pellets of ∼65% and ∼0% are observed on A and B, respectively. Thus, the improvement of cooling temperature is the requisite condition of intact pellets on B. Also, the incident angle to the inner wall of the guide tube of ∼5.41◦ at the maximum by the funnel, which the scattering angle of 1◦ is considered when pellets leave the muzzle of the guide tube, would cause the damage of the pellet due to a collision in the funnel. Indeed, intact pellets of ∼65% are verified on the heat sink A of 10.2 K being colder than B, of which pellets projected by the low-angle path of the guide tube (inner
Table 1 Before and after the conversion of the heat sink, the cooling temperature on the heat sinks A and B with the barrels and the probability of intact pellets Compressor
The number of barrels
Cooling temperature (K)
Intact pellets (%)
Before conversion
A B
4 4
10.2 12.6
65 0
After conversion
A B
6 4
10.2 10.6
100 100
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Fig. 3. Conversion of the heat sink of Cu in unit of mm. The width of the heat sinks is reduced to enlarge the distance from the thermal shield and consequently to mitigate the heat radiation.
barrels) account ∼71% though the ratio of intact pellets of the high-angle path (outer barrels) is ∼41%. This difference would be derived from the incident angle at the maximum because the angle of the outer barrel is larger than that of inner barrel in the funnel. These results indicate that the degradation of the quality of pellets is caused by a collision in the guide tube. Major issues to improve the quality of pellets are the cooling temperature producing pellets and the incident angle in the guide tube by considering the experimental condition on A and B. The conversion of the in situ pipegun pellet injector was performed. The width of the heat sinks was reduced (i.e., the heat sinks was cut) to enlarge the distance from the thermal shield and consequently to mitigate the heat radiation as shown in Fig. 3. The distance between the heat sink and the thermal shield before and after the conversion of the heat sink was broadened from 11 to 16 mm for a lengthwise direction and from 7 to 17 mm for a crosswise direction. The optimization of a pellet path (the funnel is
not used and the angle of a guide tube is 1.73◦ at the maximum) was performed and the wall thickness of barrels was reduced from 0.5 to 0.3 mm for the mitigation of the heat conduction. The number of the barrel was increased from 4 to 6 on A because of experimental needs and was remained in 4 on B, totally 10 barrels were installed. After the conversion, the cooling temperature on A and B is 10.2 and 10.6 K with the barrels, respectively (see Table 1). This result shows the improvement of cooling capability since the temperature increase is controlled in spite of an increase of the number of the barrels on A and the temperature on the heat sink B is obviously improved. Consequently, the quality and the reproducibility of pellets have been significantly improved by the conversion of cryogenic pellet production and the optimization of a pellet path. An intact pellet has been confirmed by the picture by a shadowgraph method in Fig. 4(b). The probability of an intact pellet is improved from ∼65% on A and 0% on B to both ∼100%. Also, the increase of the line-averaged electron density and the pellet penetration depth in pelletfueled experiments of LHD are observed between a cracked pellet and an intact pellet of 3.0 mm cylinder. These same pellet are injected into plasmas on the same magnetic configuration (the measured pellet mass, the pellet velocity, the central electron temperature, and the total heating power are about 7.0 × 1020 atoms, 1200 m/s, 2.0 keV, and 7.0 MW, respectively). The increase of central electron density is 1.1 and 3.7 × 1019 m−3 and the pellet penetration depth normalized by the minor radius is 0.7 and 1.4, for a cracked pellet and an intact pellet. An intact pellet is superior to a cracked pellet in terms of the pellet penetration depth as well as the increase of the central electron density. However, the improvement due to the conversion of heat sinks and wall thickness cannot be quantitatively estimated. It is important for the quantitative estimation of the improvement to demonstrate each effects in further investigation.
4. Properties of pellet production The property of pellet production is examined on the basis of the state diagram of hydrogen at cryogenic temperature on an in situ pipegun pellet injector. Fig. 5 shows the schematic diagram for pellet production. The
M. Hoshino et al. / Fusion Engineering and Design 81 (2006) 2655–2660
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Fig. 4. Pictures of (a) a cracked pellet of 3.0 mm cylinder before the conversion and (b) an intact pellet of 3.8 mm cylinder after the conversion by a shadowgraph method.
cooling temperature measured by thermocouples on the heat sink of Cu is ∼10.2 K, and there is temperature distribution between the pellet production zone in the pellet production chamber and outside the chamber (∼300 K). This difference arises the temperature gradient along the barrel. The temperature gradient along the barrel (1/4 in. SUS 316) in the chamber is calculated to demonstrate the temperature increase by the heat load in Fig. 5. Using the initial condition (the cooling temperature and the interface temperature of the vacuum are 10.2 and 300 K), the equation of heat conduction is solved with neglecting the thermal radiation. By controlling the pressure of supplied hydrogen
Fig. 5. Schematic diagram for pellet production with the cryogenic property of hydrogen and the calculated temperature gradient along the barrel.
gas, the length of the pellet can be controlled since the region of solidification along the barrel is widened outwardly. Therefore pellets can be longer than the length of the part cooled by the heat sink (3 mm). For example, when the pressure increase to 7 kPa the increment of the pellet length is ∼1 mm in Fig. 5. Using the mass detector and the shadowgraph method after pellet projection, this controllability of the pellet size (i.e., mass) is confirmed in Fig. 6. The pellet mass increases until the pressure of the triple point, and H2 pellets of 3 mm cylinder and length are controlled from 100% to 130%. The mass increase of 30% measured by the mass detec-
Fig. 6. Relation between the pressure of supplied hydrogen gas and the pellet mass for 3 mm cylinder pellets.
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tor agrees that of the increment of the pellet length ∼1 mm. When the pressure is larger than the pressure at the triple point, the mass is almost returned to the level of the initial size in Fig. 6. This implies that a pellet is melted by the heat propagation of a liquid layer because a liquid layer outside a solid is formed when the pressure is larger than the pressure of ∼7 kPa. The improvement of capability of the present pellet injector of LHD is expected when D2 is used instead of H2 . Since the temperature of the triple point on D2 (∼18.7 K) is higher than that of H2 (∼13.9 K), the flexibility of pellet size can be improved in comparison with H2 . The mass of 3 mm size pellet is envisaged to increase to about 183% for D2 in contrast with about 130% for H2 because of further expanding of the freezing region.
5. Conclusion On the in situ pipegun pellet injector in LHD, the conversion of hardware is performed to improve the quality of pellets. The production of intact pellets is significantly concerned with the cooling temperature of pellet production and the destruction of pellets in the pellet guide tube should be mitigated after pellet projection. The cooling temperature is improved from 12.6 to 10.6 K by the reduction of the heat conduction and radiation in the cryogenic zone of pellet production. The incident angle to the inner wall of the guide tube for pellets is optimized from 5.41◦ to 1.73◦ at the maximum. It has been verified by these conversion that the probability of intact pellets is improved from 65% on the heat sink A (10.2 K) and 0% on B (12.6 K) to both ∼100%. The controllability of the pellet size is also confirmed that the pellet mass of 3 mm cylinder and length is increased by 30% by controlling the cooling temperature and the pressure of supplied hydrogen gas, and the increment is consistent with prediction by
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