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The PROTO-SPHERA load assembly
Fusion Engineering and Design 74 (2005) 179–183
The PROTO-SPHERA load assembly S. Papastergiou ∗ , F. Alladio, A. Mancuso, P. Micozzi Associazione ENEA-EURATOM sulla Fusione, Centro Ricerche Frascati, C.P. 65, 00044 Frascati, Rome, Italy Available online 4 October 2005
Abstract PROTO-SPHERA is a proposed spherical torus where a hydrogen plasma arc, in a form of a screw pinch field fed by electrodes, replaces the central conductor. This simply connected magnetic configuration, if fusion relevant, might strongly simplify the design of a fusion reactor. The machine design philosophy, basic geometry and operating conditions together with the major components like the vacuum vessel, coils, electrodes, protection components, divertor, etc. are analyzed. The thermal and electromagnetic behavior as well as the predicted and permitted key stresses will be discussed in order to demonstrate that the design, construction and reliable operation of the machine are feasible. Reference is also made to the proposed Multi-Pinch experiment using the START vacuum vessel to demonstrate the feasibility and stability of the PROTO-SPHERA configuration. © 2005 Elsevier B.V. All rights reserved. Keywords: PROTO-SPHERA; Multi-Pinch experiment; Spherical torus
1. Introduction PROTO-SPHERA [1] is a relatively small device of 2 m in diameter, 3.5 m in height (including the service connections) and weighs approximately 120 kN. The basic machine principle (Fig. 1) is for a vacuum vessel (VV), which provides the ultra-high vacuum enclosure and supports the PF coils, the anode, the cathode and other basic protection elements. These components protect the coils from the hot electrodes while other elements, like a divertor and the internal support structure, complete the machine design.
∗ Corresponding author. Tel.: +39 06 94005619; fax: +39 06 94005393. E-mail address: [email protected] (S. Papastergiou).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.235
The machine is operating at room temperature and the VV can sustain 10−8 mbar, while the whole load assembly can also be baked to 80–90 ◦ C. 2. Machine components Fig. 1 shows the PROTO-SPHERA VV. It is a nonmagnetic stainless steel (AISI 304L) cylindrical vessel, 2 m in diameter, 2.5 m high and 18 mm thick. The flat top and bottom flanges facilitate the use of space and maintenance, but they require to be 30 mm thick to accommodate the vacuum forces. The PF coils of PROTO-SPHERA, also shown in Fig. 1, have to be located very close to the plasma, and therefore must be positioned inside the VV. The PF coils are water cooled to achieve a frequent pulse
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Fig. 1. Cross-section of the PROTO-SPHERA assembly.
rate, are arranged co-axially and are sustained by the internal support structure which withstands the electromagnetic forces and accommodates thermal expansion during normal operation, baking and disruptions. In addition, the coil supports incorporate insulation plates (stainless steel plasma sprayed with alumina) to maintain the appropriate potential in different parts of the machine. The anode and cathode, the two electrodes for producing the screw pinch plasma, are perhaps the most technologically demanding components. Their design is based on the experience of the PROTO-PINCH experiment (a simplified linear plasma arc experiment that demonstrated the principle of PROTO-SPHERA at ENEA, Frascati) [2]. Fig. 2 shows the main characteristics and a preliminary design of the anode. This cylindrical component is formed by six sectors, each with five modules. Each module is made from OFHC Cu with its surface exposed to the plasma arc protected by a Cu–W (5%) alloy to resist excessive transient temperatures of up
to 1000 ◦ C. Gas puff in each individual module, up to a total of 30 mbar l/s, is performed through 20, 10 mm diameter holes to spread the arc energy and avoid melting. Fig. 3 shows the main features and a preliminary design of the cathode. This cylindrical component is made from 378 tungsten (W) coils supported by a dispenser assembly, which also feeds the current to the
Fig. 2. The anode assembly.
S. Papastergiou et al. / Fusion Engineering and Design 74 (2005) 179–183
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no more than 100 MPa. However, the electromagnetic forces on the coils require to be sustained by the supports. Although these forces can be as much as 10 times the coil weight, they can be easily accommodated.
4. Predicted and permitted stresses
Fig. 3. The cathode assembly.
coils. The dispensers are made from Mo to resist high temperatures, which in the coils can reach 2750 ◦ C. The cathode is composed by six sectors, each powered by a six-phased power supply and 24 dispensers form each sector, each carrying three coils of null field type. The six-phased AC power supply gives 8 MJ to the cathode. The heating time to the working temperature of 2600 ◦ C of the coil wires is 15 s. As soon as the screw pinch plasma breaks down, the coil temperature increases to 2750 ◦ C. The hot electrodes and particularly the cathode radiate at high temperatures and could damage the coil insulation if the coils were left unprotected. Thus, in order to protect the coils from mainly the cathode and also to respect horizontal machine symmetry, Cu protection components, sprayed black, were designed. Divertor plates are also used to accommodate the diverted thermal power from the plasma. The thermal flux impinging in the steady state phase of the discharge on the divertor plates is evaluated assuming that the spherical torus can be sustained for 1 s. Based on the calculated equilibrium configurations, the position of the divertor plates have been chosen as indicated in Fig. 1.
3. Electromagnetic forces and stresses The electromagnetic analysis of forces and stresses in the different machine components was made assuming a change in the magnetic field of 50 T/s, a permanent field of 500 G and a time constant of 1 ms. Cut outs were introduced in the machine components so that the resultant eddy current stresses were
The allowable stress for the stainless steel components at the relatively low operating temperature is approximately 150 MPa. This value can accommodate both the maximum eddy current (bending) stresses of 100 MPa and the maximum thermal stresses in the divertor of 320 MPa. According to ASME III–NB 3221 both criteria (of 100 MPa less than 1.5 × 150 MPa and 100 + 320 MPa less than 3 × 150 MPa) are satisfied. Note that the 1.5 factor in the ASME criteria is required because the eddy current stresses are of bending (and not of uniform across the cross-section) nature. For the Cu protection components (not the electrodes), it is proposed to use half-hardened Cu so that the electromagnetic stresses of 100 MPa can be withstood. The minimum yield and ultimate strengths have to be 115 and 230 MPa, respectively, at room temperature to satisfy the previously mentioned ASME criteria. These values should be defined in the material specification and the manufacturer could achieve them with the appropriate heat treatment. Note that during operation, annealing of these Cu components under significant eddy current stresses is not expected, because the temperature there will be less than 100 ◦ C.
5. The Multi-Pinch experiment Fig. 4 gives the cross-section of an interim experiment, called Multi-Pinch [3], to verify the stability of the PROTO-SPHERA configuration. This experiment was suggested by an international panel of experts that judged positively PROTO-SPHERA in March 2002. In order to minimize the investment, this experiment uses the START VV elongated by two cylinders to achieve the required height of 3.5 m. A reduced number of PF coils (without water cooling), simplified electrodes and a suitably adjusted support structure are employed in order to perform this experiment. No coil protections
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Fig. 4. The Multi-Pinch load assembly.
are required because the energy used in the electrodes is relatively small. 6. Conclusions The design and the engineering analysis performed, together with the proposed Multi-Pinch experiment, should provide a high degree of confidence that the con-
struction and reliable operation of PROTO-SPHERA are feasible. References [1] F. Alladio, A. Mancuso, P. Micozzi, et al., PROTO-SPHERA, ENEA, Serie Energia, Associazione Euratom-ENEA sulla Fusione, RT/ERG/FUS/2001/14, 2001, ISSN 1124/7932.
S. Papastergiou et al. / Fusion Engineering and Design 74 (2005) 179–183 [2] F. Alladio, L. A. Grosso, A. Mancuso, et al., Results of ProtoPinch Testbench for the Proto-Sphera experiment, in: Proceedings of the 27th EPS Conference on Controlled Fusion and Plasma Physics, Budapest, 12–16 June 2000, ECA vol. 24B, 2000, p. 161.
183
[3] F. Alladio, A. Mancuso, P. Micozzi, S. Papastergiou, F. Rogier, The Multi-Pinch experiment, in: Proceedings of the Innovative Confinement Concepts Workshop (ICC2004), 25–28 May 2004, Madison, Wisconsin, USA, plasma.physics.wisc.edu/ icc2004/.
The PROTO-SPHERA load assembly S. Papastergiou ∗ , F. Alladio, A. Mancuso, P. Micozzi Associazione ENEA-EURATOM sulla Fusione, Centro Ricerche Frascati, C.P. 65, 00044 Frascati, Rome, Italy Available online 4 October 2005
Abstract PROTO-SPHERA is a proposed spherical torus where a hydrogen plasma arc, in a form of a screw pinch field fed by electrodes, replaces the central conductor. This simply connected magnetic configuration, if fusion relevant, might strongly simplify the design of a fusion reactor. The machine design philosophy, basic geometry and operating conditions together with the major components like the vacuum vessel, coils, electrodes, protection components, divertor, etc. are analyzed. The thermal and electromagnetic behavior as well as the predicted and permitted key stresses will be discussed in order to demonstrate that the design, construction and reliable operation of the machine are feasible. Reference is also made to the proposed Multi-Pinch experiment using the START vacuum vessel to demonstrate the feasibility and stability of the PROTO-SPHERA configuration. © 2005 Elsevier B.V. All rights reserved. Keywords: PROTO-SPHERA; Multi-Pinch experiment; Spherical torus
1. Introduction PROTO-SPHERA [1] is a relatively small device of 2 m in diameter, 3.5 m in height (including the service connections) and weighs approximately 120 kN. The basic machine principle (Fig. 1) is for a vacuum vessel (VV), which provides the ultra-high vacuum enclosure and supports the PF coils, the anode, the cathode and other basic protection elements. These components protect the coils from the hot electrodes while other elements, like a divertor and the internal support structure, complete the machine design.
∗ Corresponding author. Tel.: +39 06 94005619; fax: +39 06 94005393. E-mail address: [email protected] (S. Papastergiou).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.235
The machine is operating at room temperature and the VV can sustain 10−8 mbar, while the whole load assembly can also be baked to 80–90 ◦ C. 2. Machine components Fig. 1 shows the PROTO-SPHERA VV. It is a nonmagnetic stainless steel (AISI 304L) cylindrical vessel, 2 m in diameter, 2.5 m high and 18 mm thick. The flat top and bottom flanges facilitate the use of space and maintenance, but they require to be 30 mm thick to accommodate the vacuum forces. The PF coils of PROTO-SPHERA, also shown in Fig. 1, have to be located very close to the plasma, and therefore must be positioned inside the VV. The PF coils are water cooled to achieve a frequent pulse
180
S. Papastergiou et al. / Fusion Engineering and Design 74 (2005) 179–183
Fig. 1. Cross-section of the PROTO-SPHERA assembly.
rate, are arranged co-axially and are sustained by the internal support structure which withstands the electromagnetic forces and accommodates thermal expansion during normal operation, baking and disruptions. In addition, the coil supports incorporate insulation plates (stainless steel plasma sprayed with alumina) to maintain the appropriate potential in different parts of the machine. The anode and cathode, the two electrodes for producing the screw pinch plasma, are perhaps the most technologically demanding components. Their design is based on the experience of the PROTO-PINCH experiment (a simplified linear plasma arc experiment that demonstrated the principle of PROTO-SPHERA at ENEA, Frascati) [2]. Fig. 2 shows the main characteristics and a preliminary design of the anode. This cylindrical component is formed by six sectors, each with five modules. Each module is made from OFHC Cu with its surface exposed to the plasma arc protected by a Cu–W (5%) alloy to resist excessive transient temperatures of up
to 1000 ◦ C. Gas puff in each individual module, up to a total of 30 mbar l/s, is performed through 20, 10 mm diameter holes to spread the arc energy and avoid melting. Fig. 3 shows the main features and a preliminary design of the cathode. This cylindrical component is made from 378 tungsten (W) coils supported by a dispenser assembly, which also feeds the current to the
Fig. 2. The anode assembly.
S. Papastergiou et al. / Fusion Engineering and Design 74 (2005) 179–183
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no more than 100 MPa. However, the electromagnetic forces on the coils require to be sustained by the supports. Although these forces can be as much as 10 times the coil weight, they can be easily accommodated.
4. Predicted and permitted stresses
Fig. 3. The cathode assembly.
coils. The dispensers are made from Mo to resist high temperatures, which in the coils can reach 2750 ◦ C. The cathode is composed by six sectors, each powered by a six-phased power supply and 24 dispensers form each sector, each carrying three coils of null field type. The six-phased AC power supply gives 8 MJ to the cathode. The heating time to the working temperature of 2600 ◦ C of the coil wires is 15 s. As soon as the screw pinch plasma breaks down, the coil temperature increases to 2750 ◦ C. The hot electrodes and particularly the cathode radiate at high temperatures and could damage the coil insulation if the coils were left unprotected. Thus, in order to protect the coils from mainly the cathode and also to respect horizontal machine symmetry, Cu protection components, sprayed black, were designed. Divertor plates are also used to accommodate the diverted thermal power from the plasma. The thermal flux impinging in the steady state phase of the discharge on the divertor plates is evaluated assuming that the spherical torus can be sustained for 1 s. Based on the calculated equilibrium configurations, the position of the divertor plates have been chosen as indicated in Fig. 1.
3. Electromagnetic forces and stresses The electromagnetic analysis of forces and stresses in the different machine components was made assuming a change in the magnetic field of 50 T/s, a permanent field of 500 G and a time constant of 1 ms. Cut outs were introduced in the machine components so that the resultant eddy current stresses were
The allowable stress for the stainless steel components at the relatively low operating temperature is approximately 150 MPa. This value can accommodate both the maximum eddy current (bending) stresses of 100 MPa and the maximum thermal stresses in the divertor of 320 MPa. According to ASME III–NB 3221 both criteria (of 100 MPa less than 1.5 × 150 MPa and 100 + 320 MPa less than 3 × 150 MPa) are satisfied. Note that the 1.5 factor in the ASME criteria is required because the eddy current stresses are of bending (and not of uniform across the cross-section) nature. For the Cu protection components (not the electrodes), it is proposed to use half-hardened Cu so that the electromagnetic stresses of 100 MPa can be withstood. The minimum yield and ultimate strengths have to be 115 and 230 MPa, respectively, at room temperature to satisfy the previously mentioned ASME criteria. These values should be defined in the material specification and the manufacturer could achieve them with the appropriate heat treatment. Note that during operation, annealing of these Cu components under significant eddy current stresses is not expected, because the temperature there will be less than 100 ◦ C.
5. The Multi-Pinch experiment Fig. 4 gives the cross-section of an interim experiment, called Multi-Pinch [3], to verify the stability of the PROTO-SPHERA configuration. This experiment was suggested by an international panel of experts that judged positively PROTO-SPHERA in March 2002. In order to minimize the investment, this experiment uses the START VV elongated by two cylinders to achieve the required height of 3.5 m. A reduced number of PF coils (without water cooling), simplified electrodes and a suitably adjusted support structure are employed in order to perform this experiment. No coil protections
182
S. Papastergiou et al. / Fusion Engineering and Design 74 (2005) 179–183
Fig. 4. The Multi-Pinch load assembly.
are required because the energy used in the electrodes is relatively small. 6. Conclusions The design and the engineering analysis performed, together with the proposed Multi-Pinch experiment, should provide a high degree of confidence that the con-
struction and reliable operation of PROTO-SPHERA are feasible. References [1] F. Alladio, A. Mancuso, P. Micozzi, et al., PROTO-SPHERA, ENEA, Serie Energia, Associazione Euratom-ENEA sulla Fusione, RT/ERG/FUS/2001/14, 2001, ISSN 1124/7932.
S. Papastergiou et al. / Fusion Engineering and Design 74 (2005) 179–183 [2] F. Alladio, L. A. Grosso, A. Mancuso, et al., Results of ProtoPinch Testbench for the Proto-Sphera experiment, in: Proceedings of the 27th EPS Conference on Controlled Fusion and Plasma Physics, Budapest, 12–16 June 2000, ECA vol. 24B, 2000, p. 161.
183
[3] F. Alladio, A. Mancuso, P. Micozzi, S. Papastergiou, F. Rogier, The Multi-Pinch experiment, in: Proceedings of the Innovative Confinement Concepts Workshop (ICC2004), 25–28 May 2004, Madison, Wisconsin, USA, plasma.physics.wisc.edu/ icc2004/.