- Email: [email protected]
Requirements specification for the Neutral Beam Injector on FAST
Fusion Engineering and Design 86 (2011) 974–977
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Requirements specification for the Neutral Beam Injector on FAST M. Baruzzo a,∗ , T. Bolzonella a , G. Calabro b , F. Crisanti b , A. Cucchiaro b , D. Marcuzzi a , W. Rigato a , M. Schneider c , P. Sonato a , M. Valisa a , P. Zaccaria a , J.F. Artaud c , V. Basiuk c , A. Cardinali b , F. Imbeaux c , L. Lauro Taroni a , M. Marinucci b , P. Mantica d , F. Zonca b a
Consorzio RFX, EURATOM-ENEA Association, Corso Stati Uniti 4, 35127 Padova, Italy Associazione Euratom/ENEA sulla Fusione, CP 65-00044 Frascati, Rome, Italy c CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France d Istituto di Fisica del Plasma ‘P.Caldirola’, Associazione Euratom-ENEA-CNR, Milano, Italy b
a r t i c l e
i n f o
Article history: Available online 3 April 2011 Keywords: Fusion FAST NBI
a b s t r a c t This paper discusses the scientific and technical requirements for a Neutral Beam Injection system on the FAST tokamak and describes a preliminary conceptual design of a suitable injector. FAST is being proposed as a European experiment in support to the operations on ITER and to the design of DEMO. The specific mission of this device is an integrated approach to a number of outstanding burning plasmas physics and operational issues with an emphasis on the impact of fast particles on turbulent transport. Such scientific requirements set a series of technical challenges regarding the injector and the coupling of the injector to the FAST main chamber that are addressed in the paper. A preliminary conceptual design of the injector is proposed which attempts to meet the stated requirements. © 2011 Published by Elsevier B.V.
1. Introduction FAST is being proposed as a European experiment in support to the operations on ITER and to the design of DEMO [1,2]. The specific mission of this relatively compact (R = 1.82 m, a = 0.64 m) device is an integrated approach to a number of outstanding burning plasmas physics and operational issues with an emphasis on the impact of fast particles on turbulent transport. In this respect the inclusion of a Neutral Beam Injector to complement the ICRH, LH and ECRH systems is of particular relevance. The reference H-mode scenario in FAST features a single-null diverted, 6.5 MA of plasma current, 7.5 T of toroidal magnetic field and 30 MW of total input power [1,2], with a central electron density of about 2.5 × 1020 m−3 . The main physics requirements for a beam on FAST have been investigated, and several simulations varying beam parameters (species, energy, geometry, divergence) have been performed by means of the NEMO/SPOT codes [3] simulating the interaction of the beam with the standard FAST H-mode plasma. A full scenario simulation has also been performed by using NEMO/SPOT coupled to the CRONOS transport code [4] in order to assess the effect of the NBI in a self consistent way. The resulting requirements set a series of technical challenges regarding the injector and the coupling of the injector to the FAST main chamber. A preliminary conceptual
∗ Corresponding author. Tel.: +39 0498295074. E-mail address: [email protected] (M. Baruzzo). 0920-3796/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.fusengdes.2011.03.012
design of the injector is proposed which attempts to overcome these challenges.
2. Determination of physics requirements The main physics requirements for a beam on FAST are that in order to be relevant to burning plasmas the beam generated fast particles have to be super-Alfvenic, and the beam energy must be high enough to assure deep penetration in the plasma and also drive preferentially electron heating to emulate alpha heating in a reactor. Given the FAST toroidal field, injection energies have to be larger than 0.7 MeV for hydrogen injection and 1 MeV for deuterium injection to create super-Alfvenic ions. Fast ion pressure has to be of the order of a few per cent of the magnetic pressure, and the injected power has to significantly contribute to the total required input power, leading to an overall design injected power of 10 MW. Momentum input and current drive capabilities are important ingredients as well, for this reason tangential injection is mandatory. Beam steering is also desirable to extend the study to different profiles of fast ions densities and power deposition, along with the possibility to inject for the whole flat top phase, of the order of tens of seconds, to study NBI effect in stationary regimes. NBI fast ion birth profiles were calculated using NEMO, which takes into account the cross sections of all of the ion generation processes, plasma shape, plasma density, ion and electron temperature profiles. It also takes into account the NBI parameters as the
M. Baruzzo et al. / Fusion Engineering and Design 86 (2011) 974–977
Fig. 1. Deposited power on electrons (blue) and ions (red) as a function of normalized radius. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
3D configuration of the injector, the injected species and the ion energy. The plasma shape, density and temperature profiles used were taken from a CRONOS simulation [4] of FAST H-mode standard scenario with 30 MW of injected ICRH power, where heat diffusion equations were solved using Bohm/gyro-Bohm diffusivities with prescribed electron and ion densities. The ion birth profile is then used in SPOT [5], a Monte Carlo code which generates test particles and follows their path along magnetic field lines until they thermalize. The outputs of this step are important NBI related quantities, such as NBI driven current, deposited power and injected torque. The output deposited power densities are plotted in Fig. 1. Using this code an extensive parametric scan has been performed varying ion energy, ion species, beam divergence and injection angle, to evaluate the most effective configuration to fulfill the NBI scientific requirements for FAST. Explored parameters are shown in Table 1. The results of the scan are plotted in Fig. 2 for the deuterium injection case [6]. First of all a large difference in fast ion density profiles can be noticed between on-axis injection (blue red) and off-axis injection (green, cyan), in fact in the first case they are steep and peaked in the center, while in the second case profiles are centred at = 0.4, much broader towards the edge and almost zero in the center [6]. The effect of a larger divergence (dashed lines) is of blurring the beam and making the deposition profile less localized. This can be important in the center, because it lowers the central fast ion density by 20% in the on-axis case, or makes it different from zero in the off-axis case.
975
Fig. 2. Fast ion density as a function of normalized radius for the parametric scan in the deuterium injection case.
The reduction of injection energy from 1 MeV to 700 keV has the effect of enhancing the ion generation towards the edge-middle zone, slightly lowering the central fast ion density in the on-axis case, while it has little effect in the off-axis case. The results with hydrogen injection are qualitatively similar to the deuterium ones, with similar ion density and driven current profiles. The reference choice for NBI in FAST is 1 MeV D injection, to have superalfvénic injection and good neutron yield, and it is also the design value used for beam line components sizing. The injection of 0.7 MeV H is the option that maximizes the effect on fast ion population with smaller engineering efforts (power supply and insulation design, beam shine through and reionization in the duct), even though reducing the neutron yield. The use of the smaller divergence value is recommended, to obtain better deposition localization and larger fast ion  in the plasma core. Both on-axis and off-axis injection seem feasible, and the beam steering is recommended for the flexibility in plasma scenario design. A full scenario simulation has been also performed using NEMO coupled to CRONOS, to simulate FAST H-mode scenario with a plasma current of 6.5 MA, 30 MW ICRH input power and 10 MW NBI power. In this simulation heat diffusion equations were solved using Bohm/gyro-Bohm diffusivities with prescribed electron and ion densities, as in the initial one. In Fig. 3 the NBI impact on temperature profiles can be seen for a case with the same NBI parameters as SCAN1. In blue the temper-
Table 1 Performed parametric scan for FAST NBI.
SCAN1 SCAN2 SCAN3 SCAN4 SCAN5 SCAN6 SCAN7 SCAN8 SCAN9 SCAN10 SCAN11 SCAN12 SCAN13 SCAN14 SCAN15 SCAN16
Isotope
E (MeV)
Divergence
Direction
D D D D H H H H D D D D H H H H
1.00 1.00 0.70 0.70 1.00 1.00 0.70 0.70 1.00 1.00 0.70 0.70 1.00 1.00 0.70 0.70
5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦
On axis Off axis On axis Off axis On axis Off axis On axis Off axis On axis Off axis On axis Off axis On axis Off axis On axis Off axis
Fig. 3. Electron (dash) and ion temperature (fill) as a function of normalized radius without (blue) and with NBI (green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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M. Baruzzo et al. / Fusion Engineering and Design 86 (2011) 974–977
Fig. 4. FAST Neutral Beam Injector.
ature profiles without NBI are plotted, while profiles with NBI are plotted in green. 3. Injector requirements A Neutral Beam Injector (NBI) is a complex system able to generate, accelerate and neutralize ions to produce a beam of neutral particles with the goal of transfer power and momentum to the plasma. The main subsystems present in a NBI are the following and can be seen in Fig. 4: • Beam Source (BS) and Accelerator: to produce and accelerate ions. • Neutralizer: to neutralize the accelerated ions. • Residual Ion Dump (RID): to remove the ions still present after the neutralization. • Calorimeter: to measure NBI power and profile. • High Voltage Bushing (HVB) and Transmission Line (TL): to supply electrical power at the right voltage to produce and accelerate ions. • Vacuum pumping system: to maintain the right vacuum level. A dedicated NBI system design shall be carried out to be adapted and to fulfill all the requirements of the device. The compactness of the FAST tokamak, associated with the relatively high input power (10 MW) in charge of the NBI system, and the geometrical constraints for the neutral beam direction axis along with the beam foot-print and shape inside the plasma, demand a huge effort to meet all the design requirements for a NBI system [7]. Given the similar requested ion energy and input power, the high level design takes advantage of the general solutions employed in ITER NBI [8], even though most of the components had to be resized or modified to comply with the space limits in FAST vacuum vessel and equatorial port. These components are characterized by an aperture relatively high and rather narrow. Therefore the space available for the neutral beam is quite large in the vertical direction but very limited in the horizontal plane. In addition the optimization of the neutral beam axis position and direction produces a further reduction of the horizontal space available, as it can be seen in Figs. 5 and 6. The space limits have driven the geometrical features of the neutral beam considering also that the footprint of the beam, at the entrance of the main plasma, shall be contained in a rectangle 400 mm high and 200 mm wide. The porthole height (1.46 m) leaves
Fig. 5. Critical points between FAST equatorial port and Neutral Beam (horizontal section).
vertical space enough for beam steering and off-axis injection as well. Among all the requirements, the most stringent one is the respect of the horizontal space and the distances inside the equatorial port assigned to the NBI. Therefore the minimum waist of the beam has been positioned inside the port itself. In order to comply with such geometrical constraints the neutral beam shall be composed by two beamlet groups in the horizontal direction and five in the vertical one. Each beamlet group will consist of 16 beamlets in the vertical direction and 5 beamlets in the horizontal one (Fig. 7). This configuration has been chosen to assure the best compromise between the narrow access and the requirement of a tangential injection of 10 MW input power using high energy ions. The requirements and the conceptual design of a dedicated NBI system composed by a Beam Source (BS) complete with 1 MV extraction and acceleration grids (Accelerator), a Neutralizer, a RID, a Calorimeter, a Vacuum vessel, a vacuum pumping system and a High Voltage Bushing and Transmission Line have been fixed allowing an estimation of the components dimensions and main features. 3.1. NBI system: components description The Neutral Beam Injector for FAST will consist of a Beam Source, including the 1 MV accelerator, and the High Voltage Bushing, that supplies electric power, gas inlet and cooling at the different electrical voltages. The BS shall foresee a dedicated positioning and adjustment system allowing two working configurations with dif-
Fig. 6. FAST equatorial port and Neutral Beam (vertical section).
M. Baruzzo et al. / Fusion Engineering and Design 86 (2011) 974–977
Fig. 7. Section of the BS and accelerator grids beamlet group detail.
ferent beam axis angles in the vertical plane and a fine adjustment of the BS position for the alignment after the installation and maintenance operations. A set of Beam Line Components (BLCs) necessary for the neutralization, removal of residual ions and beam dumping, and a set of cryopanels are foreseen (Fig. 4). All the components shall be housed inside a dedicated vacuum vessel composed by two different vessels, the Beam Line Vessel (BLV) and Beam Source Vessel (BSV), jointed together. The Beam Line Vessel shall be connected to the assigned equatorial port with the interposition of a valve to allow the separation of FAST and NBI vacuum vessels. The overall dimensions of the vessel are 14 m and 9.5 m of length and height respectively. The Beam Source will be composed of five identical drivers disposed in a vertical column connected to a single expansion chamber. The source will supply −1 MV negative ions to the accelerator, which is composed by seven grids plus the bias plate, forming a column of five horizontal couples of beamlet groups. One common frame structure will support the weight of BS and Accelerator. The 1 MV grids will consist of plasma grid, extraction grid, bias plate (at the electrical potential of about −1 MV) and a set of five accelerating grids from −800 kV up to the grounded grid by steps of 200 kV. Each pair of beamlet group is right inclined in the vertical plane to focus the neutral beam inside the FAST vacuum chamber. In the horizontal plane, the focalization is done by an optimized distance between the beamlet groups and the inclination of the Neutralizer and RID panels. The Neutralizer, the RID and the Calorimeter are positioned along the beam with different goals. The Neutralizer and the RID are passed through by the beam while the Calorimeter (when closed) acts as a dump stopping the beam. The Neutralizer positioned along the beam and immediately downstream the BS will consist of a set of three actively cooled metallic plates having the function to limit the space where gas is injected for the neutralization of the negative accelerated ions and to minimize the gas conductance. The RID, positioned downstream the Neutralizer, will consist of two actively cooled plates at different electrical potentials to cap-
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ture the ions still present inside the beam after the neutralization. The electrical potential difference between the RID plates shall be optimized. The Calorimeter will consist of a single panel actively cooled. The calorimeter panel foresees two working positions, open and closed. The closed position allows intercepting the neutral beam; the open one allows the beam to enter inside the FAST vacuum vessel. A vacuum compatible actuator shall be foreseen to vary the inclination of the Calorimeter panel. A dedicated vacuum pumping system will consist of a cryopanel housed inside the BLV on the left side of the BLCs. Additional smaller cryopanels will be foreseen inside the BSV and inside the equatorial port. Dedicated forevacuum pumping access shall be foreseen. The first proposal for the components installation and maintenance handling foresees one lateral big access in the BLV. The cryopanel will be fixed by means of proper rail allowing thermal expansion on the lateral lid. The removal of both the lid and cryopanel will be simultaneous. This solution allows to minimize the number of apertures, to simplify maintenance and installation operations, to avoid removable cryogenic feedthroughs and to minimize the number of handling tools. The BSV foresees one rear lid necessary for installation and maintenance handling of the Beam Source. 3.2. Further issues The main issues to be investigated concern the little room inside the equatorial port, and the high power deposition linked to it. An actively cooled duct and a cryopanel seem to be necessary, along with a beam scraper before the entrance of the tokamak vessel. The thermal loads due to the available space and layout shall be analyzed carefully in order to avoid excessive heating of the components. Moreover the regeneration of the currently foreseen cryopanel inside the equatorial port implies a back flow of gas to the tokamak chamber, for this reason it might need a dedicated valve and gas extraction. The heat removal in the RID, which is composed by only two plates, is an issue that has to be faced as well in a more advanced design. 4. Conclusions In this paper the main physical and technical requirements for a Neutral Beam Injector in FAST device have been outlined. Given these requirements a NBI conceptual design for FAST has been carried out, and in parallel the effects of such an NBI on FAST H-mode tokamak scenario have been simulated using NEMO and CRONOS codes. The whole project is ultimately characterized by challenging technical issues, but can provide important improvements to FAST operative scenarios. References [1] [2] [3] [4] [5] [6] [7] [8]
G. Calabrò, et al., Nucl. Fusion 49 (2009) 055002. A. Pizzuto, et al., Nucl. Fusion 50 (2010) 095005. M. Schneider et al. to be submitted to Nucl. Fusion. J.F. Artaud, et al., Nucl. Fusion 50 (2010) 043001. M. Schneider, et al., Plasma Phys. Control. Fusion 47 (2005) 2087–2106. M. Baruzzo et al., 37th Conf. on Plasma Physics, Dublin, 2010, P5.142. A. Kojima, et al., Rev. Sci. Instrum. 81 (2010) 02B112. J. Jaquinot, et al., Fus. Eng. Des. 84 (2009) 125–130.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Requirements specification for the Neutral Beam Injector on FAST M. Baruzzo a,∗ , T. Bolzonella a , G. Calabro b , F. Crisanti b , A. Cucchiaro b , D. Marcuzzi a , W. Rigato a , M. Schneider c , P. Sonato a , M. Valisa a , P. Zaccaria a , J.F. Artaud c , V. Basiuk c , A. Cardinali b , F. Imbeaux c , L. Lauro Taroni a , M. Marinucci b , P. Mantica d , F. Zonca b a
Consorzio RFX, EURATOM-ENEA Association, Corso Stati Uniti 4, 35127 Padova, Italy Associazione Euratom/ENEA sulla Fusione, CP 65-00044 Frascati, Rome, Italy c CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France d Istituto di Fisica del Plasma ‘P.Caldirola’, Associazione Euratom-ENEA-CNR, Milano, Italy b
a r t i c l e
i n f o
Article history: Available online 3 April 2011 Keywords: Fusion FAST NBI
a b s t r a c t This paper discusses the scientific and technical requirements for a Neutral Beam Injection system on the FAST tokamak and describes a preliminary conceptual design of a suitable injector. FAST is being proposed as a European experiment in support to the operations on ITER and to the design of DEMO. The specific mission of this device is an integrated approach to a number of outstanding burning plasmas physics and operational issues with an emphasis on the impact of fast particles on turbulent transport. Such scientific requirements set a series of technical challenges regarding the injector and the coupling of the injector to the FAST main chamber that are addressed in the paper. A preliminary conceptual design of the injector is proposed which attempts to meet the stated requirements. © 2011 Published by Elsevier B.V.
1. Introduction FAST is being proposed as a European experiment in support to the operations on ITER and to the design of DEMO [1,2]. The specific mission of this relatively compact (R = 1.82 m, a = 0.64 m) device is an integrated approach to a number of outstanding burning plasmas physics and operational issues with an emphasis on the impact of fast particles on turbulent transport. In this respect the inclusion of a Neutral Beam Injector to complement the ICRH, LH and ECRH systems is of particular relevance. The reference H-mode scenario in FAST features a single-null diverted, 6.5 MA of plasma current, 7.5 T of toroidal magnetic field and 30 MW of total input power [1,2], with a central electron density of about 2.5 × 1020 m−3 . The main physics requirements for a beam on FAST have been investigated, and several simulations varying beam parameters (species, energy, geometry, divergence) have been performed by means of the NEMO/SPOT codes [3] simulating the interaction of the beam with the standard FAST H-mode plasma. A full scenario simulation has also been performed by using NEMO/SPOT coupled to the CRONOS transport code [4] in order to assess the effect of the NBI in a self consistent way. The resulting requirements set a series of technical challenges regarding the injector and the coupling of the injector to the FAST main chamber. A preliminary conceptual
∗ Corresponding author. Tel.: +39 0498295074. E-mail address: [email protected] (M. Baruzzo). 0920-3796/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.fusengdes.2011.03.012
design of the injector is proposed which attempts to overcome these challenges.
2. Determination of physics requirements The main physics requirements for a beam on FAST are that in order to be relevant to burning plasmas the beam generated fast particles have to be super-Alfvenic, and the beam energy must be high enough to assure deep penetration in the plasma and also drive preferentially electron heating to emulate alpha heating in a reactor. Given the FAST toroidal field, injection energies have to be larger than 0.7 MeV for hydrogen injection and 1 MeV for deuterium injection to create super-Alfvenic ions. Fast ion pressure has to be of the order of a few per cent of the magnetic pressure, and the injected power has to significantly contribute to the total required input power, leading to an overall design injected power of 10 MW. Momentum input and current drive capabilities are important ingredients as well, for this reason tangential injection is mandatory. Beam steering is also desirable to extend the study to different profiles of fast ions densities and power deposition, along with the possibility to inject for the whole flat top phase, of the order of tens of seconds, to study NBI effect in stationary regimes. NBI fast ion birth profiles were calculated using NEMO, which takes into account the cross sections of all of the ion generation processes, plasma shape, plasma density, ion and electron temperature profiles. It also takes into account the NBI parameters as the
M. Baruzzo et al. / Fusion Engineering and Design 86 (2011) 974–977
Fig. 1. Deposited power on electrons (blue) and ions (red) as a function of normalized radius. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
3D configuration of the injector, the injected species and the ion energy. The plasma shape, density and temperature profiles used were taken from a CRONOS simulation [4] of FAST H-mode standard scenario with 30 MW of injected ICRH power, where heat diffusion equations were solved using Bohm/gyro-Bohm diffusivities with prescribed electron and ion densities. The ion birth profile is then used in SPOT [5], a Monte Carlo code which generates test particles and follows their path along magnetic field lines until they thermalize. The outputs of this step are important NBI related quantities, such as NBI driven current, deposited power and injected torque. The output deposited power densities are plotted in Fig. 1. Using this code an extensive parametric scan has been performed varying ion energy, ion species, beam divergence and injection angle, to evaluate the most effective configuration to fulfill the NBI scientific requirements for FAST. Explored parameters are shown in Table 1. The results of the scan are plotted in Fig. 2 for the deuterium injection case [6]. First of all a large difference in fast ion density profiles can be noticed between on-axis injection (blue red) and off-axis injection (green, cyan), in fact in the first case they are steep and peaked in the center, while in the second case profiles are centred at = 0.4, much broader towards the edge and almost zero in the center [6]. The effect of a larger divergence (dashed lines) is of blurring the beam and making the deposition profile less localized. This can be important in the center, because it lowers the central fast ion density by 20% in the on-axis case, or makes it different from zero in the off-axis case.
975
Fig. 2. Fast ion density as a function of normalized radius for the parametric scan in the deuterium injection case.
The reduction of injection energy from 1 MeV to 700 keV has the effect of enhancing the ion generation towards the edge-middle zone, slightly lowering the central fast ion density in the on-axis case, while it has little effect in the off-axis case. The results with hydrogen injection are qualitatively similar to the deuterium ones, with similar ion density and driven current profiles. The reference choice for NBI in FAST is 1 MeV D injection, to have superalfvénic injection and good neutron yield, and it is also the design value used for beam line components sizing. The injection of 0.7 MeV H is the option that maximizes the effect on fast ion population with smaller engineering efforts (power supply and insulation design, beam shine through and reionization in the duct), even though reducing the neutron yield. The use of the smaller divergence value is recommended, to obtain better deposition localization and larger fast ion  in the plasma core. Both on-axis and off-axis injection seem feasible, and the beam steering is recommended for the flexibility in plasma scenario design. A full scenario simulation has been also performed using NEMO coupled to CRONOS, to simulate FAST H-mode scenario with a plasma current of 6.5 MA, 30 MW ICRH input power and 10 MW NBI power. In this simulation heat diffusion equations were solved using Bohm/gyro-Bohm diffusivities with prescribed electron and ion densities, as in the initial one. In Fig. 3 the NBI impact on temperature profiles can be seen for a case with the same NBI parameters as SCAN1. In blue the temper-
Table 1 Performed parametric scan for FAST NBI.
SCAN1 SCAN2 SCAN3 SCAN4 SCAN5 SCAN6 SCAN7 SCAN8 SCAN9 SCAN10 SCAN11 SCAN12 SCAN13 SCAN14 SCAN15 SCAN16
Isotope
E (MeV)
Divergence
Direction
D D D D H H H H D D D D H H H H
1.00 1.00 0.70 0.70 1.00 1.00 0.70 0.70 1.00 1.00 0.70 0.70 1.00 1.00 0.70 0.70
5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 5.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦ 7.00E-03◦
On axis Off axis On axis Off axis On axis Off axis On axis Off axis On axis Off axis On axis Off axis On axis Off axis On axis Off axis
Fig. 3. Electron (dash) and ion temperature (fill) as a function of normalized radius without (blue) and with NBI (green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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M. Baruzzo et al. / Fusion Engineering and Design 86 (2011) 974–977
Fig. 4. FAST Neutral Beam Injector.
ature profiles without NBI are plotted, while profiles with NBI are plotted in green. 3. Injector requirements A Neutral Beam Injector (NBI) is a complex system able to generate, accelerate and neutralize ions to produce a beam of neutral particles with the goal of transfer power and momentum to the plasma. The main subsystems present in a NBI are the following and can be seen in Fig. 4: • Beam Source (BS) and Accelerator: to produce and accelerate ions. • Neutralizer: to neutralize the accelerated ions. • Residual Ion Dump (RID): to remove the ions still present after the neutralization. • Calorimeter: to measure NBI power and profile. • High Voltage Bushing (HVB) and Transmission Line (TL): to supply electrical power at the right voltage to produce and accelerate ions. • Vacuum pumping system: to maintain the right vacuum level. A dedicated NBI system design shall be carried out to be adapted and to fulfill all the requirements of the device. The compactness of the FAST tokamak, associated with the relatively high input power (10 MW) in charge of the NBI system, and the geometrical constraints for the neutral beam direction axis along with the beam foot-print and shape inside the plasma, demand a huge effort to meet all the design requirements for a NBI system [7]. Given the similar requested ion energy and input power, the high level design takes advantage of the general solutions employed in ITER NBI [8], even though most of the components had to be resized or modified to comply with the space limits in FAST vacuum vessel and equatorial port. These components are characterized by an aperture relatively high and rather narrow. Therefore the space available for the neutral beam is quite large in the vertical direction but very limited in the horizontal plane. In addition the optimization of the neutral beam axis position and direction produces a further reduction of the horizontal space available, as it can be seen in Figs. 5 and 6. The space limits have driven the geometrical features of the neutral beam considering also that the footprint of the beam, at the entrance of the main plasma, shall be contained in a rectangle 400 mm high and 200 mm wide. The porthole height (1.46 m) leaves
Fig. 5. Critical points between FAST equatorial port and Neutral Beam (horizontal section).
vertical space enough for beam steering and off-axis injection as well. Among all the requirements, the most stringent one is the respect of the horizontal space and the distances inside the equatorial port assigned to the NBI. Therefore the minimum waist of the beam has been positioned inside the port itself. In order to comply with such geometrical constraints the neutral beam shall be composed by two beamlet groups in the horizontal direction and five in the vertical one. Each beamlet group will consist of 16 beamlets in the vertical direction and 5 beamlets in the horizontal one (Fig. 7). This configuration has been chosen to assure the best compromise between the narrow access and the requirement of a tangential injection of 10 MW input power using high energy ions. The requirements and the conceptual design of a dedicated NBI system composed by a Beam Source (BS) complete with 1 MV extraction and acceleration grids (Accelerator), a Neutralizer, a RID, a Calorimeter, a Vacuum vessel, a vacuum pumping system and a High Voltage Bushing and Transmission Line have been fixed allowing an estimation of the components dimensions and main features. 3.1. NBI system: components description The Neutral Beam Injector for FAST will consist of a Beam Source, including the 1 MV accelerator, and the High Voltage Bushing, that supplies electric power, gas inlet and cooling at the different electrical voltages. The BS shall foresee a dedicated positioning and adjustment system allowing two working configurations with dif-
Fig. 6. FAST equatorial port and Neutral Beam (vertical section).
M. Baruzzo et al. / Fusion Engineering and Design 86 (2011) 974–977
Fig. 7. Section of the BS and accelerator grids beamlet group detail.
ferent beam axis angles in the vertical plane and a fine adjustment of the BS position for the alignment after the installation and maintenance operations. A set of Beam Line Components (BLCs) necessary for the neutralization, removal of residual ions and beam dumping, and a set of cryopanels are foreseen (Fig. 4). All the components shall be housed inside a dedicated vacuum vessel composed by two different vessels, the Beam Line Vessel (BLV) and Beam Source Vessel (BSV), jointed together. The Beam Line Vessel shall be connected to the assigned equatorial port with the interposition of a valve to allow the separation of FAST and NBI vacuum vessels. The overall dimensions of the vessel are 14 m and 9.5 m of length and height respectively. The Beam Source will be composed of five identical drivers disposed in a vertical column connected to a single expansion chamber. The source will supply −1 MV negative ions to the accelerator, which is composed by seven grids plus the bias plate, forming a column of five horizontal couples of beamlet groups. One common frame structure will support the weight of BS and Accelerator. The 1 MV grids will consist of plasma grid, extraction grid, bias plate (at the electrical potential of about −1 MV) and a set of five accelerating grids from −800 kV up to the grounded grid by steps of 200 kV. Each pair of beamlet group is right inclined in the vertical plane to focus the neutral beam inside the FAST vacuum chamber. In the horizontal plane, the focalization is done by an optimized distance between the beamlet groups and the inclination of the Neutralizer and RID panels. The Neutralizer, the RID and the Calorimeter are positioned along the beam with different goals. The Neutralizer and the RID are passed through by the beam while the Calorimeter (when closed) acts as a dump stopping the beam. The Neutralizer positioned along the beam and immediately downstream the BS will consist of a set of three actively cooled metallic plates having the function to limit the space where gas is injected for the neutralization of the negative accelerated ions and to minimize the gas conductance. The RID, positioned downstream the Neutralizer, will consist of two actively cooled plates at different electrical potentials to cap-
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ture the ions still present inside the beam after the neutralization. The electrical potential difference between the RID plates shall be optimized. The Calorimeter will consist of a single panel actively cooled. The calorimeter panel foresees two working positions, open and closed. The closed position allows intercepting the neutral beam; the open one allows the beam to enter inside the FAST vacuum vessel. A vacuum compatible actuator shall be foreseen to vary the inclination of the Calorimeter panel. A dedicated vacuum pumping system will consist of a cryopanel housed inside the BLV on the left side of the BLCs. Additional smaller cryopanels will be foreseen inside the BSV and inside the equatorial port. Dedicated forevacuum pumping access shall be foreseen. The first proposal for the components installation and maintenance handling foresees one lateral big access in the BLV. The cryopanel will be fixed by means of proper rail allowing thermal expansion on the lateral lid. The removal of both the lid and cryopanel will be simultaneous. This solution allows to minimize the number of apertures, to simplify maintenance and installation operations, to avoid removable cryogenic feedthroughs and to minimize the number of handling tools. The BSV foresees one rear lid necessary for installation and maintenance handling of the Beam Source. 3.2. Further issues The main issues to be investigated concern the little room inside the equatorial port, and the high power deposition linked to it. An actively cooled duct and a cryopanel seem to be necessary, along with a beam scraper before the entrance of the tokamak vessel. The thermal loads due to the available space and layout shall be analyzed carefully in order to avoid excessive heating of the components. Moreover the regeneration of the currently foreseen cryopanel inside the equatorial port implies a back flow of gas to the tokamak chamber, for this reason it might need a dedicated valve and gas extraction. The heat removal in the RID, which is composed by only two plates, is an issue that has to be faced as well in a more advanced design. 4. Conclusions In this paper the main physical and technical requirements for a Neutral Beam Injector in FAST device have been outlined. Given these requirements a NBI conceptual design for FAST has been carried out, and in parallel the effects of such an NBI on FAST H-mode tokamak scenario have been simulated using NEMO and CRONOS codes. The whole project is ultimately characterized by challenging technical issues, but can provide important improvements to FAST operative scenarios. References [1] [2] [3] [4] [5] [6] [7] [8]
G. Calabrò, et al., Nucl. Fusion 49 (2009) 055002. A. Pizzuto, et al., Nucl. Fusion 50 (2010) 095005. M. Schneider et al. to be submitted to Nucl. Fusion. J.F. Artaud, et al., Nucl. Fusion 50 (2010) 043001. M. Schneider, et al., Plasma Phys. Control. Fusion 47 (2005) 2087–2106. M. Baruzzo et al., 37th Conf. on Plasma Physics, Dublin, 2010, P5.142. A. Kojima, et al., Rev. Sci. Instrum. 81 (2010) 02B112. J. Jaquinot, et al., Fus. Eng. Des. 84 (2009) 125–130.