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The MAST neutral beam injection system
Fusion Engineering and Design 56 – 57 (2001) 529– 532 www.elsevier.com/locate/fusengdes
The MAST neutral beam injection system M.P.S. Nightingale a,*, G.W. Crawford a, S.J. Gee a, D.J. Hurford a, D. Martin a, M.R. Simmonds a, R.T.C. Smith a, C.C. Tsai b, S.E.V. Warder a a
EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK b Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Abstract Two neutral beam injectors will provide both plasma heating and current drive on the MAST tokamak using beamlines on loan from Oak Ridge National Laboratory. Progress is reported regarding the following engineering challenges in achieving beam energies up to 70 keV and pulse lengths up to 5 s: design of carbon fibre composite, hypervapotron and inertial beamline components capable of withstanding high heat fluxes; re-design of the ORNL accelerators to improve voltage capability and reliability; and the design and commissioning of indirectly-heated cathodes capable of 5 s operation. © 2001 EFDA-JET. Published by Elsevier Science B.V. All rights reserved. Keywords: Beam energies; MAST tokamak; Plasma; Injection system
1. Introduction Neutral beam injection is to be carried out on the Mega-Ampere Spherical Tokamak (MAST) [1,2] at Culham Science Centre using two injectors provided on loan from Oak Ridge National Laboratory (ORNL) upgraded to operate at 80 A/70 keV/2.5 MW per injector in deuterium for pulse lengths up to 0.5 s, and 65 A/61 keV/2.0 MW for 0.5 –5 s. These performance levels lie significantly above those achieved so far using these injectors by Gardner et al. at ORNL [3] and by Williams et al. at Princeton Plasma Physics Laboratory [4] and the beamlines, designed and built during the * Corresponding author. Tel.: + 44-1235-46-4693; fax: + 44-1235-46-4626. E-mail address: [email protected] (M.P.S. Nightingale).
1970’s, will require the modifications discussed below.
2. Upgrade issues
2.1. Shinethrough protection A major objective of the MAST programme will be the achievement of 0.5 MA of neutral beam-driven current drive, which requires tokamak operation at a peak plasma density of order 2× 1019 m − 3. Operation at such low densities requires the MAST vessel wall to be protected from sputtering and thermal damage caused by shinethrough, and so two beam stops of the design shown in Fig. 1 have been installed inside the MAST vessel. Each beam stop comprises 17 interlocking 300× 150× 25 mm3 2D carbon fibre com-
0920-3796/01/$ - see front matter © 2001 EFDA-JET. Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 3 4 3 - X
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M.P.S. Nightingale et al. / Fusion Engineering and Design 56–57 (2001) 529–532
posite (CFC) tiles, equipped with thermocouples to provide beam profile and alignment data. The tiles were supplied by Dunlop Limited (Aviation Division) of the UK, and have high thermal conductivity along the axes perpendicular to beam incidence in order to minimise mechanical stresses by transverse heat diffusion. Table 1 shows the measured properties of the tiles supplied by Dunlop. Modelling by Akers et al. using the LOCUST code [5] predicted a peak shinethrough power density of 1 MW m − 2 for 5 s current drive experiments using 2 MW 61 keV sources injecting into a plasma of peak electron density of 2×1019 m − 3. 3D transient modelling of the performance of the tiles, carried out using COSMOS [6], predicted that the surface temperature rise for the central tile will be 247 °C. In the event that the beam is incident onto the beam stop without plasma present, the COSMOS model predicted that the upper limit tile temperature of 1200 °C will be reached in 0.4 s for a full (2.5 MW), and an interlock has been fitted that switches the beam off in B 1 ms if the MAST plasma current does not lie within a pre-set window and the injector is set to operate into the MAST vessel.
2.2. Accelerator de6elopment The present ORNL ion accelerators are equipped with epoxy insulators. The accelerators have only been operated at beam energies up to 55 keV and are known to suffer from accelerator de-conditioning which is presumed to be due to outgassing from the epoxy causing contamination of the accelerator grids, leading to high voltage breakdowns. In order to obtain 70 keV operation with maximum reliability, the epoxy insulators are to be replaced with porcelain insulators of the design [7] proven on the JET, ASDEX-U and TEXTOR NBI systems. In order to use the existing triode accelerator grids, including their alignment systems, the re-designed re-entrant accelerator geometry shown in Fig. 2 is being assembled, as shown in Fig. 3. The grids are mounted on stainless steel grid holders, separated using post insulators. The resulting design allows the entire stack to be assembled on the
Fig. 1. One assembled carbon fibre composite beam stop prior to installation within the MAST vessel.
bench, including the setting of final grid alignment and spacing, before insertion into the insulator. Inner shields have been added to the DECEL (3rd) grid holder and the ground support tube to prevent plasma particles from the beam impinging on the section of porcelain insulator across which the high voltage stress will occur.
2.3. Long pulse cathode de6elopment The present ORNL duopigatron ion source is powered by oxide-coated filaments, which cannot meet the requirement for pulse length due to heating of the oxide filament coating by back ion Table 1 MAST NBI CFC tile properties measured by DUNLOP limited Parameter
Parallel to tile front face
Perpendicular to tile front face
Thermal 310 910 at 65 9 5 at 300 °C conductivity 300 °C, 133 at 29 at 1000 °C (Wm−1 K−1) 1000 °C Thermal expansion B1.0 13.0 (10−6K−1) Density (kgm−3) 1750 9400 Porosity (%) 7.1 90.3 Edge flexural 190 9 13 at strength 300 °C and (MNm−2) 139 937 at 1000 °C
M.P.S. Nightingale et al. / Fusion Engineering and Design 56–57 (2001) 529–532
531
Fig. 2. Schematic of the upgraded accelerator structure.
bombardment. These filaments are to be replaced with indirectly-heated cathodes presently under development at ORNL. One thousand two hundred amperes of arc current is to be produced using 200 cm2 La2O3-doped molybdenum emitters, of the type developed previously by Schechter and Tsai [8], heated to 1800 °C using graphite heaters. The assembly is then mounted inside the actively-cooled cathode chamber shown in Fig. 4, with the source intermediate electrode geometry modified to provide the required thermal management. To date, the prototype cathode has delivered 500 A for 5 s and 600 A for 0.5 s during testing at reduced power.
Fig. 3. Installation of the re-designed accelerator assembly into the porcelain insulator.
Fig. 4. Indirectly-heated cathode components during assembly at ORNL: the cathode (containing its graphite heater) is on the left; the interface flange is the centre; and the cathode liner of (including the tantalum heat shield) is on the right. The inner bore of the cathode liner is 8.75 cm diameter.
2.4. High heat flux components Power densities of up to 114 and 38 MW m − 2 (normal incidence) are estimated to be incident upon the beamline calorimeter and residual ion dump (RID), respectively. These are to be handled using inclined Cu–Cr –Zr hypervapotron panels based upon the design used on JET as described by Falter and Thompson [9], but modified to handle incident power densities up to 15 MW m − 2 for 5 s, by narrowing the water channel depth from 10 to 3.4 mm in order to increase the water flow rate to 6.2 and 8.2 ms − 1 for the calorimeter and RID, respectively, and reducing the front face thickness from 6 to 5 mm. With these two changes, extrapolation of data published by Falter and Thompson suggests that the hypervapotron front surface will reach a temperature of 4699 20 °C after 1.5 s at 15 MW m − 2, which lies below the 500 °C at which coherent precipitation is likely to become a problem. The 500 mm long 112 mm wide 30 mm deep panels are being supplied by Zanon (Italy), and are inclined at angles of 7.1 and 22° for the calorimeter and RID respectively, leading to power density reductions of 8.1 and 2.7. A schematic of one MAST NBI panel is shown in Fig. 5.
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M.P.S. Nightingale et al. / Fusion Engineering and Design 56-57 (2001) 529–532
Fig. 5. Schematic of a 50 cm long MAST NBI hypervapotron panel, with a cutaway showing the cooling channel design.
2.5. Neutraliser input scraper
Acknowledgements
An additional re-design has been required to the input of the neutraliser. Modelling has predicted that the power density incident at normal incidence onto the input face of the existing neutraliser might result in power densities of up to 30 MW m − 2, which is unacceptable for 5 s pulse length. The entire neutraliser input section has therefore been removed, and replaced with a conical copper scraper angled at 9.5° to the beam axis, which reduces the incident power density to acceptable levels.
This work was funded by the UK Department of Trade and Industry and EURATOM, using equipment on loan from Oak Ridge National Laboratory and the US Department of Energy.
3. Conclusions Several engineering challenges need to be overcome to achieve the required performance from MAST NBI. The ion source accelerator structure and cathodes are being replaced; and high heat fluxes are being handled for pulse lengths up to 5 s using a combination of hypervapotrons, CFC beam stops and inertial scrapers. At present: the accelerator structures are being assembled; the prototype cathodes are being tested; some hypervapotron panels have arrived, and assembly of the first residual ion dump is underway; and the CFC beam stops and inertial scrapers have been installed.
References [1] M.P.S. Nightingale et al., Neutral beam injection on MAST, Proc. 20th Symp. Fusion Tech., 1998, pp. 387 – 390. [2] M. Cox, et al., The Mega Amp spherical tokamak, Fusion Eng. and Design 46 (1999) 397 – 404. [3] W.L. Gardner, et al., Properties of an intense 50-kV neutral-beam injection system, Rev. Sci. Insts. 53 (1982) 424 – 431. [4] M.D. Williams et al., Installation and start-up of the PDX neutral beam injection system, Proc. 9th Int. Symp. Fusion Eng., 1981, pp. 760 – 762. [5] R.J. Akers et al., Neutral beam heating in the START spherical tokamak, submitted to Nucl. Fusion, 2000. [6] COSMOS/M Version 1.71, Supplied by Structural Research and Analysis Corporation, June 1994. [7] G. Duesing, et al., Neutral beam injection system, Fusion Technology 11 (1987) 163 – 202. [8] D.E. Schechter, C.C. Tsai, Indirectly heated cathodes and duoplasmatron-type electron feeds for positive ion sources, Proc. 9th Int. Symp. Fus. Eng., 1981, pp. 1515 – 1518. [9] H-D. Falter, E. Thompson, Performance of hypervapotron beam-stopping elements at JET, Fusion Tech. 29 (1996) 584 – 595.
The MAST neutral beam injection system M.P.S. Nightingale a,*, G.W. Crawford a, S.J. Gee a, D.J. Hurford a, D. Martin a, M.R. Simmonds a, R.T.C. Smith a, C.C. Tsai b, S.E.V. Warder a a
EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK b Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Abstract Two neutral beam injectors will provide both plasma heating and current drive on the MAST tokamak using beamlines on loan from Oak Ridge National Laboratory. Progress is reported regarding the following engineering challenges in achieving beam energies up to 70 keV and pulse lengths up to 5 s: design of carbon fibre composite, hypervapotron and inertial beamline components capable of withstanding high heat fluxes; re-design of the ORNL accelerators to improve voltage capability and reliability; and the design and commissioning of indirectly-heated cathodes capable of 5 s operation. © 2001 EFDA-JET. Published by Elsevier Science B.V. All rights reserved. Keywords: Beam energies; MAST tokamak; Plasma; Injection system
1. Introduction Neutral beam injection is to be carried out on the Mega-Ampere Spherical Tokamak (MAST) [1,2] at Culham Science Centre using two injectors provided on loan from Oak Ridge National Laboratory (ORNL) upgraded to operate at 80 A/70 keV/2.5 MW per injector in deuterium for pulse lengths up to 0.5 s, and 65 A/61 keV/2.0 MW for 0.5 –5 s. These performance levels lie significantly above those achieved so far using these injectors by Gardner et al. at ORNL [3] and by Williams et al. at Princeton Plasma Physics Laboratory [4] and the beamlines, designed and built during the * Corresponding author. Tel.: + 44-1235-46-4693; fax: + 44-1235-46-4626. E-mail address: [email protected] (M.P.S. Nightingale).
1970’s, will require the modifications discussed below.
2. Upgrade issues
2.1. Shinethrough protection A major objective of the MAST programme will be the achievement of 0.5 MA of neutral beam-driven current drive, which requires tokamak operation at a peak plasma density of order 2× 1019 m − 3. Operation at such low densities requires the MAST vessel wall to be protected from sputtering and thermal damage caused by shinethrough, and so two beam stops of the design shown in Fig. 1 have been installed inside the MAST vessel. Each beam stop comprises 17 interlocking 300× 150× 25 mm3 2D carbon fibre com-
0920-3796/01/$ - see front matter © 2001 EFDA-JET. Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 3 4 3 - X
530
M.P.S. Nightingale et al. / Fusion Engineering and Design 56–57 (2001) 529–532
posite (CFC) tiles, equipped with thermocouples to provide beam profile and alignment data. The tiles were supplied by Dunlop Limited (Aviation Division) of the UK, and have high thermal conductivity along the axes perpendicular to beam incidence in order to minimise mechanical stresses by transverse heat diffusion. Table 1 shows the measured properties of the tiles supplied by Dunlop. Modelling by Akers et al. using the LOCUST code [5] predicted a peak shinethrough power density of 1 MW m − 2 for 5 s current drive experiments using 2 MW 61 keV sources injecting into a plasma of peak electron density of 2×1019 m − 3. 3D transient modelling of the performance of the tiles, carried out using COSMOS [6], predicted that the surface temperature rise for the central tile will be 247 °C. In the event that the beam is incident onto the beam stop without plasma present, the COSMOS model predicted that the upper limit tile temperature of 1200 °C will be reached in 0.4 s for a full (2.5 MW), and an interlock has been fitted that switches the beam off in B 1 ms if the MAST plasma current does not lie within a pre-set window and the injector is set to operate into the MAST vessel.
2.2. Accelerator de6elopment The present ORNL ion accelerators are equipped with epoxy insulators. The accelerators have only been operated at beam energies up to 55 keV and are known to suffer from accelerator de-conditioning which is presumed to be due to outgassing from the epoxy causing contamination of the accelerator grids, leading to high voltage breakdowns. In order to obtain 70 keV operation with maximum reliability, the epoxy insulators are to be replaced with porcelain insulators of the design [7] proven on the JET, ASDEX-U and TEXTOR NBI systems. In order to use the existing triode accelerator grids, including their alignment systems, the re-designed re-entrant accelerator geometry shown in Fig. 2 is being assembled, as shown in Fig. 3. The grids are mounted on stainless steel grid holders, separated using post insulators. The resulting design allows the entire stack to be assembled on the
Fig. 1. One assembled carbon fibre composite beam stop prior to installation within the MAST vessel.
bench, including the setting of final grid alignment and spacing, before insertion into the insulator. Inner shields have been added to the DECEL (3rd) grid holder and the ground support tube to prevent plasma particles from the beam impinging on the section of porcelain insulator across which the high voltage stress will occur.
2.3. Long pulse cathode de6elopment The present ORNL duopigatron ion source is powered by oxide-coated filaments, which cannot meet the requirement for pulse length due to heating of the oxide filament coating by back ion Table 1 MAST NBI CFC tile properties measured by DUNLOP limited Parameter
Parallel to tile front face
Perpendicular to tile front face
Thermal 310 910 at 65 9 5 at 300 °C conductivity 300 °C, 133 at 29 at 1000 °C (Wm−1 K−1) 1000 °C Thermal expansion B1.0 13.0 (10−6K−1) Density (kgm−3) 1750 9400 Porosity (%) 7.1 90.3 Edge flexural 190 9 13 at strength 300 °C and (MNm−2) 139 937 at 1000 °C
M.P.S. Nightingale et al. / Fusion Engineering and Design 56–57 (2001) 529–532
531
Fig. 2. Schematic of the upgraded accelerator structure.
bombardment. These filaments are to be replaced with indirectly-heated cathodes presently under development at ORNL. One thousand two hundred amperes of arc current is to be produced using 200 cm2 La2O3-doped molybdenum emitters, of the type developed previously by Schechter and Tsai [8], heated to 1800 °C using graphite heaters. The assembly is then mounted inside the actively-cooled cathode chamber shown in Fig. 4, with the source intermediate electrode geometry modified to provide the required thermal management. To date, the prototype cathode has delivered 500 A for 5 s and 600 A for 0.5 s during testing at reduced power.
Fig. 3. Installation of the re-designed accelerator assembly into the porcelain insulator.
Fig. 4. Indirectly-heated cathode components during assembly at ORNL: the cathode (containing its graphite heater) is on the left; the interface flange is the centre; and the cathode liner of (including the tantalum heat shield) is on the right. The inner bore of the cathode liner is 8.75 cm diameter.
2.4. High heat flux components Power densities of up to 114 and 38 MW m − 2 (normal incidence) are estimated to be incident upon the beamline calorimeter and residual ion dump (RID), respectively. These are to be handled using inclined Cu–Cr –Zr hypervapotron panels based upon the design used on JET as described by Falter and Thompson [9], but modified to handle incident power densities up to 15 MW m − 2 for 5 s, by narrowing the water channel depth from 10 to 3.4 mm in order to increase the water flow rate to 6.2 and 8.2 ms − 1 for the calorimeter and RID, respectively, and reducing the front face thickness from 6 to 5 mm. With these two changes, extrapolation of data published by Falter and Thompson suggests that the hypervapotron front surface will reach a temperature of 4699 20 °C after 1.5 s at 15 MW m − 2, which lies below the 500 °C at which coherent precipitation is likely to become a problem. The 500 mm long 112 mm wide 30 mm deep panels are being supplied by Zanon (Italy), and are inclined at angles of 7.1 and 22° for the calorimeter and RID respectively, leading to power density reductions of 8.1 and 2.7. A schematic of one MAST NBI panel is shown in Fig. 5.
532
M.P.S. Nightingale et al. / Fusion Engineering and Design 56-57 (2001) 529–532
Fig. 5. Schematic of a 50 cm long MAST NBI hypervapotron panel, with a cutaway showing the cooling channel design.
2.5. Neutraliser input scraper
Acknowledgements
An additional re-design has been required to the input of the neutraliser. Modelling has predicted that the power density incident at normal incidence onto the input face of the existing neutraliser might result in power densities of up to 30 MW m − 2, which is unacceptable for 5 s pulse length. The entire neutraliser input section has therefore been removed, and replaced with a conical copper scraper angled at 9.5° to the beam axis, which reduces the incident power density to acceptable levels.
This work was funded by the UK Department of Trade and Industry and EURATOM, using equipment on loan from Oak Ridge National Laboratory and the US Department of Energy.
3. Conclusions Several engineering challenges need to be overcome to achieve the required performance from MAST NBI. The ion source accelerator structure and cathodes are being replaced; and high heat fluxes are being handled for pulse lengths up to 5 s using a combination of hypervapotrons, CFC beam stops and inertial scrapers. At present: the accelerator structures are being assembled; the prototype cathodes are being tested; some hypervapotron panels have arrived, and assembly of the first residual ion dump is underway; and the CFC beam stops and inertial scrapers have been installed.
References [1] M.P.S. Nightingale et al., Neutral beam injection on MAST, Proc. 20th Symp. Fusion Tech., 1998, pp. 387 – 390. [2] M. Cox, et al., The Mega Amp spherical tokamak, Fusion Eng. and Design 46 (1999) 397 – 404. [3] W.L. Gardner, et al., Properties of an intense 50-kV neutral-beam injection system, Rev. Sci. Insts. 53 (1982) 424 – 431. [4] M.D. Williams et al., Installation and start-up of the PDX neutral beam injection system, Proc. 9th Int. Symp. Fusion Eng., 1981, pp. 760 – 762. [5] R.J. Akers et al., Neutral beam heating in the START spherical tokamak, submitted to Nucl. Fusion, 2000. [6] COSMOS/M Version 1.71, Supplied by Structural Research and Analysis Corporation, June 1994. [7] G. Duesing, et al., Neutral beam injection system, Fusion Technology 11 (1987) 163 – 202. [8] D.E. Schechter, C.C. Tsai, Indirectly heated cathodes and duoplasmatron-type electron feeds for positive ion sources, Proc. 9th Int. Symp. Fus. Eng., 1981, pp. 1515 – 1518. [9] H-D. Falter, E. Thompson, Performance of hypervapotron beam-stopping elements at JET, Fusion Tech. 29 (1996) 584 – 595.