- Email: [email protected]
wall experiment Magnum-PSI
Fusion Engineering and Design 86 (2011) 1745–1748
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
The target for the new plasma/wall experiment Magnum-PSI M.A. van den Berg a,∗ , S. Brons a , O.G. Kruijt a , J. Scholten a , R. Pasquet a , P.H.M. Smeets a , B. Schweer b , G. De Temmerman a a b
FOM-Institute for Plasma Physics Rijnhuizen, Association EURATOM-FOM, Trilateral Euregio Cluster, P.O. Box 1207, 3430 BE, Nieuwegein, The Netherlands Forschungszentrum Juelich GmbH, IEF-4, Euratom association, Trilateral Euregio Cluster, 52425 Juelich, Germany
a r t i c l e
i n f o
Article history: Available online 23 February 2011 Keywords: Plasma surface interaction Magnum-PSI Divertor target ANSYS Heat transfer
a b s t r a c t The construction of Magnum-PSI is in its final stage. The aim is to provide a controlled and highly accessible linear plasma device to perform the basic plasma-surface interaction research needed for the design of the plasma facing components of future fusion devices. This contribution will focus on the thermal challenges imposed by those extreme conditions on the design of the target holder of Magnum-PSI. The target holder is designed to allow the exposure of large size targets with variable inclination angles with respect to the magnetic field. A test set up was made to test different interlayers (grafoil® , soft metal sheets) and improve the thermal contact between the target and the heat sink. In addition, a modular target holder for sequential exposure of smaller size targets has been designed. Finite element modeling using the ANSYS code was used to optimize the cooling geometry and to predict the temperature profiles due to the heat load of the plasma. Experiments were done on the Pilot-PSI linear device to validate the thermal calculations. Calorimetry and infrared thermography were used to experimentally measure the temperature profile on the target and the heat deposition. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the fusion device ITER, the divertor will have to be operated in a semi-detached regime to maintain the heat flux density on the divertor within the engineering limits for steady-state power handling [1]. In this regime, ion fluxes of 1024 m−2 s−1 are expected at the strike-point, with a steady state heat flux of 10 MW/m2 with a relatively low electron temperature of 1–10 eV. Plasma-surface interaction studies under those conditions motivated the design of the Magnum-PSI device. Magnum-PSI is a linear plasma device with a steady state 3T magnetic field [2]. The plasma is produced by a cascaded arc source. The required ion and power fluxes were already achieved in the Pilot-PSI device [3]. In the target chamber, user defined targets can be exposed to the plasma. The different targets can be attached to a target manipulator, allowing for target rotation about the same axis as the plasma beam and tilting with respect to the magnetic field. A broad range of diagnostics will be used to monitor the plasma and target conditions. For example, the surface temperature will be monitored by a fast infrared camera and a multi wavelength pyrometer. After plasma exposure, the target can be retracted to the Target Exchange and Analysis Chamber (TEAC,
∗ Corresponding author. Tel.: +31 30 6096 793. E-mail address: [email protected] (M.A. van den Berg). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.01.112
see Fig. 1), without breaking the vacuum, for further analysis. The TEAC is designed to provide good diagnostic conditions and access after exposure and has more than 30 viewing ports. The TEAC can be insulated from the main vacuum system by two large gate valves and has its own vacuum pump system. This set up allows the target manipulator to be retracted from the exposure position in the target chamber to the TEAC in less than 30 s. After closing the valves, the TEAC can be pumped down to 5 × 10−5 Pa. Main surface diagnostics in the TEAC include laser induced desorption, laser induced ablation and laser induced breakdown spectroscopy.
2. Target manipulator for Magnum-PSI The targets are mounted onto the target manipulator, which mainly consists of a cooled base plate (target holder), a worm wheel for the target rotation, and a 5 m long bellow system, containing the cooling water tubes, sensor cables and electrical cables. The target manipulator is designed to accommodate different target sizes and geometry. For example, large (60 × 12 cm, 100 kg) targets can be rotated ±120◦ and tilted ±90◦ with respect to the magnetic field. In addition, a holder allowing 5 circular targets (30 mm diameter) to be mounted simultaneously and exposed sequentially, perpendicular to the magnetic field, is designed (see Section 3).
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thermocouples which are necessary to monitor the bulk target temperature. Calculations in ANSYS are done to determine the best channel cooling geometry, based on the beam properties. Fixed parameters are the following: • Cooling water temperature and pressure: 20 ◦ C and 1 MPa. • Gaussian heat flux profile with peak power >10 MW/m2 and radii from 10 to 100 mm. • Tunable parameters: • Cooling channel geometry: diameter of a channel, distance between channels and number of channels. • Surface contact improving material (see Section 3).
Fig. 1. The Target Exchange and Analyses Chamber (TEAC), without the top lid. Left the two gate valves can be seen. Inside the TEAC the large target holder in yellow is shown. The 5 m bellow system designed to retract the target from exposure to the TEAC is also shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2.1. Target holder for large targets In the design of the first target holder the main consideration was to create a large flexibility in mounting different shapes and sizes, and still have adequate cooling. In order to mimic the conditions expected in ITER, castellated targets (tungsten and carbon) with active and passive cooling have been designed, that can be mounted on the large target holder. The holder consists of a stainless steel plate of 320 mm in height, 120 mm in width and 10 mm in thickness, with 12 cooling channels milled into it. Brazed on top is a 3 mm copper lid which is milled after brazing, to 2.5 mm, to ensure a good flatness of the final surface. A copper lid was chosen to ensure good heat transfer to the cooling channels. As shown in Fig. 2, 52 holes are available for target mounting. The holes are 24 mm apart in length and width. The smaller holes can be used to insert
The water pressure available for the target is 1 × 106 Pa [4]. If the losses in the water supply pipes are about 2 × 105 Pa, ca. 7 × 105 to 8 × 105 Pa is left for the target base plate. The total flow reserved for the target is 50 l/min. Assuming 12 channels to cover the whole width of the target, the maximum flow per channel is 4 l/min. With a length of 0.3 m the smallest diameter for an acceptable pressure drop (7.5 × 105 Pa) is calculated to be 2 mm. The film coefficient in one channel then becomes 60 kW/m2 K. To determine the optimum position of the cooling channels, different geometries were modeled using ANSYS, with the common constraint that room for the bolts and thermocouples is needed. The calculations resulted in the following conclusions: • The temperature difference due to the distance between cooling channels of 5 or 7 mm in perfect conducting layers is negligible. • The temperature difference due to the distance between cooling channels of 5 or 7 mm with imperfectly conducting layers is also within the Ansys tolerances, and therefore negligible. • The temperature difference due to all channels or only the 4 in the middle, with perfect conducting layers, is about 110◦ (difference gets higher if conducting layers are not perfect) and can be adjusted by closing the cooling channels on the outside of the holder. • The temperature difference between all cooling channels and only 4 cooling channels gets bigger for lower layer conduction. 2.2. Mock up for the Magnum-PSI target
Fig. 2. The target manipulator and the holder for large targets. The copper lid is made transparent to show the water cooling channels, the mounting holes and the little holes for thermocouples. Behind the target holder, a part of the manipulator is shown.
To validate the ideal way of target mounting onto the holder, a test set up was made to determine and optimize the total heat conductivity in several configurations [5]. The test set up is half the size in the length of the Magnum-PSI target to compensate for the lower heat load during the tests, so that the water temperature rise remains measurable. Besides the length, the other major difference is the way the outgoing water channels are constructed (see Fig. 3). At the end of the plate, the eight water channels are soldered to eight cylindrical water tubes, so the possibility to measure the temperature of each channel is created for spatially resolved measurements. Other dimensions are similar to the Magnum-PSI target. Brass is chosen for the target material in this test set up to have a good thermal conductivity and mechanical strength to ensure good mounting capability. To improve the thermal contact between the target and the cooling plate, a thin layer of a suitable material is added. For this purpose several materials have been selected, based on both their compatibility with high temperatures and vacuum environment, and their mechanical properties. The tested materials include grafoil® (flexible graphite foil), silverfoil and Apiezon® H Grease. To make a good comparison between the materials, the measured thermal conductivity is normalized to the thickness of the layer to give the heat transfer coefficient between the target and the cooled copper plate. Since the materials are
M.A. van den Berg et al. / Fusion Engineering and Design 86 (2011) 1745–1748
Fig. 3. The mock up for the large Magnum-PSI target holder. The incoming water connection is at the backside of the target holder, the 8 outgoing water channels for spatially resolved temperature measurements are shown on the right side.
relatively soft, the thermal contact is expected to vary under pressure. For all samples except the silverfoil the difference in the heat transfer coefficient does not change significantly for a torque higher than 1.50 N m (see Fig. 4). With high torque (2.00 N m) the materials have about the same coefficient (4.5–5.5 kW/m2 K). Of the tested materials, 0.15 g of Apiezon® H Grease is the best (5.5 kW/m2 K ± 11%) at an applied torque of 2.00, but the heat transfer coefficient of the Apiezon® H Grease depends strongly on the amount of grease applied. Since the difference between the materials is not significant, grafoil® is advised as a contact improving material. Grafoil® is readily available, it does not leave any residues behind and can be applied easily. 3. Model verification of the small target holder The first plasma in Magnum-PSI will have a diameter of about 10 mm. When targets are to be exposed perpendicular to the plasma, the size of the target manipulator allows several targets to be mounted on the same holder. This holder is designed to accommodate up to 5 circular targets of 30 mm, only one target being
5.5
2
Fig. 5. The target holder for multiple small targets. The red tube is the water supply and also the rotating axis. It is made of aluminum bronze alloy. The red wheel is a worm wheel, also made of aluminum bronze alloy. Internally the water is transported to the back of the copper base plates (the yellow circles). Five targets can be mounted, a clamping ring being shown on the central target. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
exposed at a given time, the others being shadowed from direct plasma impact (see Fig. 5). The target holder rotates around an aluminum bronze alloy water tube by means of an aluminum bronze worm wheel. In the stainless steel body the water is transported to the copper plates on which the targets can be mounted. Inserted in the water channel is a thermocouple for calorimetry. To validate thermal modeling, experiments were done on PilotPSI using a small target holder (see Fig. 6), with a plasma beam similar to that in Magnum-PSI. The flow and temperature of the target cooling water are measured, allowing the total power to the target to be determined by calorimetry. A high speed IR camera (FLIR SC7500-MB) is used to measure the 2D target temperature profile during plasma exposure. Heat fluxes to the target are calculated with the THEODOR code [6], a 2D inverse heat transfer code. 2D slab geometry approximation is used to describe the target. The free parameters in THEODOR
0.01" grafoil 0.02" grafoil 0.004" silver 0.006" silver 0.06 g grease 0.15 g grease 0.23 g grease No layer
5.0
Heat transfer [kW/m K]
1747
4.5 4.0 3.5 3.0 2.5 2.0 0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Torque [Nm] Fig. 4. The measured heat transfer of different materials. The materials act as a contact improving layer between the actively cooled heat sink and the target material. The heat transfer improves slightly if more torque is applied. There is no significant difference in heat transfer between the best options for grafoil® , grease and silverfoil.
Fig. 6. . A simplified model of the Pilot-PSI target. The grey body is made of stainless steel, the yellow part is made of copper and is cooled by a water flow coming from the bottom, the green layer is a contact improvement layer (grafoil® ) and the light blue is the tungsten target. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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14
Gauss fit Heat flux THEODOR
12
2
Heat flux [MW/m ]
10 8 6 4 2 0 -15
-10
-5
0
5
10
15
Radius [mm] Fig. 7. The heat flux from THEODOR and the gaussian fit. The asymmetric profile needs to be clarified. Only the left side of the profile is used for the fit.
1600 1400
Temperature [ºC]
1200 1000 800
between the copper surface and the water bulk. Since the water temperature only rises 5◦ during the plasma exposure, this is deemed acceptable. A temperature profile is calculated with the heat flux parameters from THEODOR (Fig. 8). In this case, the temperature profile of ANSYS and the IR-camera are in good agreement. Next step is modeling the stack of materials as used in PilotPSI (see Fig. 6). As the surface and cooling parameters from the first model are verified, the tunable parameters are the two heat resistances between the two contact transitions: the tungsten target to the grafoil® , and the grafoil® to the water-cooled copper heat sink. These two heat resistances are varied so that the top temperature from thermography and ANSYS are again in agreement. Fig. 8 shows a comparison between the three temperature profiles. The THEODOR code is widely used to calculate surface heat fluxes in tokamaks like ASDEX [6], and MAST [8]. In both devices, THEODOR has been bench marked with other diagnostics. The temperature ANSYS calculates with the same input as THEODOR is reasonable good. Without any adjustment the top temperature and profile shape are in good agreement. The temperature from the model with the stack of materials is a bit broader. This suggests that the heat resistance between the two surfaces cannot be modeled with just one parameter, but might have a temperature or radial dependence. More investigation is needed to benchmark these calculations and quantify the role of the heat resistance on the discrepancies. 4. Outlook
600 400
IR Camera Temperature 1 mm tungsten Pilot target
200 0 -20
-15
-10
-5
0
5
10
15
20
Radius [mm] Fig. 8. . The temperature profile recorded by the IR camera, and the profiles calculated by ANSYS, with the heat flux from THEODOR as input.
are the rear and front boundary conditions. Surface effects can be accounted for by introducing a surface parameter, ˛ [7], the ratio of the heat conductivity, to the thickness of the layer in W m−2 K−1 . In the present case, the surface effect is expected to be negligible because of the absence of erosion/deposition effects (low Te ). The rear surface is modeled by a heat transmission boundary condition which depends on the difference between the surface temperature and that of the surrounding material. Since the plasma exposure duration is long compared to the heat penetration time through 1 mm of tungsten, the heat transmission at the back has a strong influence on the results. The rear boundary condition is adjusted to get a match between the calorimetry and the IR data. The heat flux determined by thermography is the input for the ANSYS model. A Gaussian function is chosen to fit the heat flux. From the fit the peak heat flux and the FWHM (full width at half maximum) are derived (see Fig. 7). THEODOR models only one material, in this case tungsten of 1 mm. An ANSYS model is made with the same dimensions and material properties, surface and water cooling parameters. The water is simulated by a bulk temperature and a film coefficient
The comparison between the THEODOR code and the ANSYS model of the Pilot-PSI target will continue, to reduce the difference in the temperature profiles. Furthermore experiments with Pilot-PSI with a single material target, instead of the stack of materials, are foreseen to estimate the influence of different materials. After the commissioning period, ITER relevant materials and geometries will be tested in Magnum-PSI. To reduce thermal stress castellated targets and mono blocks of tungsten are foreseen, and will also be modeled in Ansys. Acknowledgment Albrecht Herrmann is greatly acknowledged for his help with the THEODOR code. References [1] R.A. Pitts, A. Kukushkin, A. Loarte, A. Martin, M. Merola, C.E. Kessel, et al., Phys. Scr. 014001 (2009), doi:10.1088/0031-8949/2009/T138/014001. [2] J. Rapp, W.R. Koppers, H.J.N. van Eck, G.J. van Rooij, W.J. Goedheer, B. de Groot, et al., fusengdes, doi:10.1016.2010.04.009. [3] G.J. van Rooij, V.P. Veremiyenko, W.J. Goedheer, B. de Groot, A.W. Kleyn, P.H.M. Smeets, et al., Appl. Phys. Lett. 90 (121501) (2007). [4] O.G. Kruijt, J. Scholten, P.H.M. Smeets, S. Brons, H.J.N., van Eck, R.S. Al, M.A. van den Berg, et al., These proceedings SOFT 2010, Thermal effects and component cooling in Magnum-PSI. [5] P.M.J. Koelman, Optimizing the heat conductivity in the Magnum target, Bachelor Thesis, Fontys University, 2009. [6] A. Herrmann, W. Junker, K. Gunther, S. Bosch, M. Kaufmann, J. Neuhauser, et al., Plasma Phys. Control Fusion 37 (1995) 17. [7] A. Hermann, Limitations for divertor heat flux calculations of fast events in tokamaks, in: Contribution to the 27th Conference on Plasma Physics and Controlled Fusion, Budapest, Hungary, 2001. [8] G. De Temmerman, E. Delchambre, J. Dowling, A. Kirk, S. Lisgo, P. Tamain, Plasma Phys. Control Fusion 52 (2010) 095005.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
The target for the new plasma/wall experiment Magnum-PSI M.A. van den Berg a,∗ , S. Brons a , O.G. Kruijt a , J. Scholten a , R. Pasquet a , P.H.M. Smeets a , B. Schweer b , G. De Temmerman a a b
FOM-Institute for Plasma Physics Rijnhuizen, Association EURATOM-FOM, Trilateral Euregio Cluster, P.O. Box 1207, 3430 BE, Nieuwegein, The Netherlands Forschungszentrum Juelich GmbH, IEF-4, Euratom association, Trilateral Euregio Cluster, 52425 Juelich, Germany
a r t i c l e
i n f o
Article history: Available online 23 February 2011 Keywords: Plasma surface interaction Magnum-PSI Divertor target ANSYS Heat transfer
a b s t r a c t The construction of Magnum-PSI is in its final stage. The aim is to provide a controlled and highly accessible linear plasma device to perform the basic plasma-surface interaction research needed for the design of the plasma facing components of future fusion devices. This contribution will focus on the thermal challenges imposed by those extreme conditions on the design of the target holder of Magnum-PSI. The target holder is designed to allow the exposure of large size targets with variable inclination angles with respect to the magnetic field. A test set up was made to test different interlayers (grafoil® , soft metal sheets) and improve the thermal contact between the target and the heat sink. In addition, a modular target holder for sequential exposure of smaller size targets has been designed. Finite element modeling using the ANSYS code was used to optimize the cooling geometry and to predict the temperature profiles due to the heat load of the plasma. Experiments were done on the Pilot-PSI linear device to validate the thermal calculations. Calorimetry and infrared thermography were used to experimentally measure the temperature profile on the target and the heat deposition. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the fusion device ITER, the divertor will have to be operated in a semi-detached regime to maintain the heat flux density on the divertor within the engineering limits for steady-state power handling [1]. In this regime, ion fluxes of 1024 m−2 s−1 are expected at the strike-point, with a steady state heat flux of 10 MW/m2 with a relatively low electron temperature of 1–10 eV. Plasma-surface interaction studies under those conditions motivated the design of the Magnum-PSI device. Magnum-PSI is a linear plasma device with a steady state 3T magnetic field [2]. The plasma is produced by a cascaded arc source. The required ion and power fluxes were already achieved in the Pilot-PSI device [3]. In the target chamber, user defined targets can be exposed to the plasma. The different targets can be attached to a target manipulator, allowing for target rotation about the same axis as the plasma beam and tilting with respect to the magnetic field. A broad range of diagnostics will be used to monitor the plasma and target conditions. For example, the surface temperature will be monitored by a fast infrared camera and a multi wavelength pyrometer. After plasma exposure, the target can be retracted to the Target Exchange and Analysis Chamber (TEAC,
∗ Corresponding author. Tel.: +31 30 6096 793. E-mail address: [email protected] (M.A. van den Berg). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.01.112
see Fig. 1), without breaking the vacuum, for further analysis. The TEAC is designed to provide good diagnostic conditions and access after exposure and has more than 30 viewing ports. The TEAC can be insulated from the main vacuum system by two large gate valves and has its own vacuum pump system. This set up allows the target manipulator to be retracted from the exposure position in the target chamber to the TEAC in less than 30 s. After closing the valves, the TEAC can be pumped down to 5 × 10−5 Pa. Main surface diagnostics in the TEAC include laser induced desorption, laser induced ablation and laser induced breakdown spectroscopy.
2. Target manipulator for Magnum-PSI The targets are mounted onto the target manipulator, which mainly consists of a cooled base plate (target holder), a worm wheel for the target rotation, and a 5 m long bellow system, containing the cooling water tubes, sensor cables and electrical cables. The target manipulator is designed to accommodate different target sizes and geometry. For example, large (60 × 12 cm, 100 kg) targets can be rotated ±120◦ and tilted ±90◦ with respect to the magnetic field. In addition, a holder allowing 5 circular targets (30 mm diameter) to be mounted simultaneously and exposed sequentially, perpendicular to the magnetic field, is designed (see Section 3).
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thermocouples which are necessary to monitor the bulk target temperature. Calculations in ANSYS are done to determine the best channel cooling geometry, based on the beam properties. Fixed parameters are the following: • Cooling water temperature and pressure: 20 ◦ C and 1 MPa. • Gaussian heat flux profile with peak power >10 MW/m2 and radii from 10 to 100 mm. • Tunable parameters: • Cooling channel geometry: diameter of a channel, distance between channels and number of channels. • Surface contact improving material (see Section 3).
Fig. 1. The Target Exchange and Analyses Chamber (TEAC), without the top lid. Left the two gate valves can be seen. Inside the TEAC the large target holder in yellow is shown. The 5 m bellow system designed to retract the target from exposure to the TEAC is also shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2.1. Target holder for large targets In the design of the first target holder the main consideration was to create a large flexibility in mounting different shapes and sizes, and still have adequate cooling. In order to mimic the conditions expected in ITER, castellated targets (tungsten and carbon) with active and passive cooling have been designed, that can be mounted on the large target holder. The holder consists of a stainless steel plate of 320 mm in height, 120 mm in width and 10 mm in thickness, with 12 cooling channels milled into it. Brazed on top is a 3 mm copper lid which is milled after brazing, to 2.5 mm, to ensure a good flatness of the final surface. A copper lid was chosen to ensure good heat transfer to the cooling channels. As shown in Fig. 2, 52 holes are available for target mounting. The holes are 24 mm apart in length and width. The smaller holes can be used to insert
The water pressure available for the target is 1 × 106 Pa [4]. If the losses in the water supply pipes are about 2 × 105 Pa, ca. 7 × 105 to 8 × 105 Pa is left for the target base plate. The total flow reserved for the target is 50 l/min. Assuming 12 channels to cover the whole width of the target, the maximum flow per channel is 4 l/min. With a length of 0.3 m the smallest diameter for an acceptable pressure drop (7.5 × 105 Pa) is calculated to be 2 mm. The film coefficient in one channel then becomes 60 kW/m2 K. To determine the optimum position of the cooling channels, different geometries were modeled using ANSYS, with the common constraint that room for the bolts and thermocouples is needed. The calculations resulted in the following conclusions: • The temperature difference due to the distance between cooling channels of 5 or 7 mm in perfect conducting layers is negligible. • The temperature difference due to the distance between cooling channels of 5 or 7 mm with imperfectly conducting layers is also within the Ansys tolerances, and therefore negligible. • The temperature difference due to all channels or only the 4 in the middle, with perfect conducting layers, is about 110◦ (difference gets higher if conducting layers are not perfect) and can be adjusted by closing the cooling channels on the outside of the holder. • The temperature difference between all cooling channels and only 4 cooling channels gets bigger for lower layer conduction. 2.2. Mock up for the Magnum-PSI target
Fig. 2. The target manipulator and the holder for large targets. The copper lid is made transparent to show the water cooling channels, the mounting holes and the little holes for thermocouples. Behind the target holder, a part of the manipulator is shown.
To validate the ideal way of target mounting onto the holder, a test set up was made to determine and optimize the total heat conductivity in several configurations [5]. The test set up is half the size in the length of the Magnum-PSI target to compensate for the lower heat load during the tests, so that the water temperature rise remains measurable. Besides the length, the other major difference is the way the outgoing water channels are constructed (see Fig. 3). At the end of the plate, the eight water channels are soldered to eight cylindrical water tubes, so the possibility to measure the temperature of each channel is created for spatially resolved measurements. Other dimensions are similar to the Magnum-PSI target. Brass is chosen for the target material in this test set up to have a good thermal conductivity and mechanical strength to ensure good mounting capability. To improve the thermal contact between the target and the cooling plate, a thin layer of a suitable material is added. For this purpose several materials have been selected, based on both their compatibility with high temperatures and vacuum environment, and their mechanical properties. The tested materials include grafoil® (flexible graphite foil), silverfoil and Apiezon® H Grease. To make a good comparison between the materials, the measured thermal conductivity is normalized to the thickness of the layer to give the heat transfer coefficient between the target and the cooled copper plate. Since the materials are
M.A. van den Berg et al. / Fusion Engineering and Design 86 (2011) 1745–1748
Fig. 3. The mock up for the large Magnum-PSI target holder. The incoming water connection is at the backside of the target holder, the 8 outgoing water channels for spatially resolved temperature measurements are shown on the right side.
relatively soft, the thermal contact is expected to vary under pressure. For all samples except the silverfoil the difference in the heat transfer coefficient does not change significantly for a torque higher than 1.50 N m (see Fig. 4). With high torque (2.00 N m) the materials have about the same coefficient (4.5–5.5 kW/m2 K). Of the tested materials, 0.15 g of Apiezon® H Grease is the best (5.5 kW/m2 K ± 11%) at an applied torque of 2.00, but the heat transfer coefficient of the Apiezon® H Grease depends strongly on the amount of grease applied. Since the difference between the materials is not significant, grafoil® is advised as a contact improving material. Grafoil® is readily available, it does not leave any residues behind and can be applied easily. 3. Model verification of the small target holder The first plasma in Magnum-PSI will have a diameter of about 10 mm. When targets are to be exposed perpendicular to the plasma, the size of the target manipulator allows several targets to be mounted on the same holder. This holder is designed to accommodate up to 5 circular targets of 30 mm, only one target being
5.5
2
Fig. 5. The target holder for multiple small targets. The red tube is the water supply and also the rotating axis. It is made of aluminum bronze alloy. The red wheel is a worm wheel, also made of aluminum bronze alloy. Internally the water is transported to the back of the copper base plates (the yellow circles). Five targets can be mounted, a clamping ring being shown on the central target. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
exposed at a given time, the others being shadowed from direct plasma impact (see Fig. 5). The target holder rotates around an aluminum bronze alloy water tube by means of an aluminum bronze worm wheel. In the stainless steel body the water is transported to the copper plates on which the targets can be mounted. Inserted in the water channel is a thermocouple for calorimetry. To validate thermal modeling, experiments were done on PilotPSI using a small target holder (see Fig. 6), with a plasma beam similar to that in Magnum-PSI. The flow and temperature of the target cooling water are measured, allowing the total power to the target to be determined by calorimetry. A high speed IR camera (FLIR SC7500-MB) is used to measure the 2D target temperature profile during plasma exposure. Heat fluxes to the target are calculated with the THEODOR code [6], a 2D inverse heat transfer code. 2D slab geometry approximation is used to describe the target. The free parameters in THEODOR
0.01" grafoil 0.02" grafoil 0.004" silver 0.006" silver 0.06 g grease 0.15 g grease 0.23 g grease No layer
5.0
Heat transfer [kW/m K]
1747
4.5 4.0 3.5 3.0 2.5 2.0 0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Torque [Nm] Fig. 4. The measured heat transfer of different materials. The materials act as a contact improving layer between the actively cooled heat sink and the target material. The heat transfer improves slightly if more torque is applied. There is no significant difference in heat transfer between the best options for grafoil® , grease and silverfoil.
Fig. 6. . A simplified model of the Pilot-PSI target. The grey body is made of stainless steel, the yellow part is made of copper and is cooled by a water flow coming from the bottom, the green layer is a contact improvement layer (grafoil® ) and the light blue is the tungsten target. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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14
Gauss fit Heat flux THEODOR
12
2
Heat flux [MW/m ]
10 8 6 4 2 0 -15
-10
-5
0
5
10
15
Radius [mm] Fig. 7. The heat flux from THEODOR and the gaussian fit. The asymmetric profile needs to be clarified. Only the left side of the profile is used for the fit.
1600 1400
Temperature [ºC]
1200 1000 800
between the copper surface and the water bulk. Since the water temperature only rises 5◦ during the plasma exposure, this is deemed acceptable. A temperature profile is calculated with the heat flux parameters from THEODOR (Fig. 8). In this case, the temperature profile of ANSYS and the IR-camera are in good agreement. Next step is modeling the stack of materials as used in PilotPSI (see Fig. 6). As the surface and cooling parameters from the first model are verified, the tunable parameters are the two heat resistances between the two contact transitions: the tungsten target to the grafoil® , and the grafoil® to the water-cooled copper heat sink. These two heat resistances are varied so that the top temperature from thermography and ANSYS are again in agreement. Fig. 8 shows a comparison between the three temperature profiles. The THEODOR code is widely used to calculate surface heat fluxes in tokamaks like ASDEX [6], and MAST [8]. In both devices, THEODOR has been bench marked with other diagnostics. The temperature ANSYS calculates with the same input as THEODOR is reasonable good. Without any adjustment the top temperature and profile shape are in good agreement. The temperature from the model with the stack of materials is a bit broader. This suggests that the heat resistance between the two surfaces cannot be modeled with just one parameter, but might have a temperature or radial dependence. More investigation is needed to benchmark these calculations and quantify the role of the heat resistance on the discrepancies. 4. Outlook
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IR Camera Temperature 1 mm tungsten Pilot target
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Radius [mm] Fig. 8. . The temperature profile recorded by the IR camera, and the profiles calculated by ANSYS, with the heat flux from THEODOR as input.
are the rear and front boundary conditions. Surface effects can be accounted for by introducing a surface parameter, ˛ [7], the ratio of the heat conductivity, to the thickness of the layer in W m−2 K−1 . In the present case, the surface effect is expected to be negligible because of the absence of erosion/deposition effects (low Te ). The rear surface is modeled by a heat transmission boundary condition which depends on the difference between the surface temperature and that of the surrounding material. Since the plasma exposure duration is long compared to the heat penetration time through 1 mm of tungsten, the heat transmission at the back has a strong influence on the results. The rear boundary condition is adjusted to get a match between the calorimetry and the IR data. The heat flux determined by thermography is the input for the ANSYS model. A Gaussian function is chosen to fit the heat flux. From the fit the peak heat flux and the FWHM (full width at half maximum) are derived (see Fig. 7). THEODOR models only one material, in this case tungsten of 1 mm. An ANSYS model is made with the same dimensions and material properties, surface and water cooling parameters. The water is simulated by a bulk temperature and a film coefficient
The comparison between the THEODOR code and the ANSYS model of the Pilot-PSI target will continue, to reduce the difference in the temperature profiles. Furthermore experiments with Pilot-PSI with a single material target, instead of the stack of materials, are foreseen to estimate the influence of different materials. After the commissioning period, ITER relevant materials and geometries will be tested in Magnum-PSI. To reduce thermal stress castellated targets and mono blocks of tungsten are foreseen, and will also be modeled in Ansys. Acknowledgment Albrecht Herrmann is greatly acknowledged for his help with the THEODOR code. References [1] R.A. Pitts, A. Kukushkin, A. Loarte, A. Martin, M. Merola, C.E. Kessel, et al., Phys. Scr. 014001 (2009), doi:10.1088/0031-8949/2009/T138/014001. [2] J. Rapp, W.R. Koppers, H.J.N. van Eck, G.J. van Rooij, W.J. Goedheer, B. de Groot, et al., fusengdes, doi:10.1016.2010.04.009. [3] G.J. van Rooij, V.P. Veremiyenko, W.J. Goedheer, B. de Groot, A.W. Kleyn, P.H.M. Smeets, et al., Appl. Phys. Lett. 90 (121501) (2007). [4] O.G. Kruijt, J. Scholten, P.H.M. Smeets, S. Brons, H.J.N., van Eck, R.S. Al, M.A. van den Berg, et al., These proceedings SOFT 2010, Thermal effects and component cooling in Magnum-PSI. [5] P.M.J. Koelman, Optimizing the heat conductivity in the Magnum target, Bachelor Thesis, Fontys University, 2009. [6] A. Herrmann, W. Junker, K. Gunther, S. Bosch, M. Kaufmann, J. Neuhauser, et al., Plasma Phys. Control Fusion 37 (1995) 17. [7] A. Hermann, Limitations for divertor heat flux calculations of fast events in tokamaks, in: Contribution to the 27th Conference on Plasma Physics and Controlled Fusion, Budapest, Hungary, 2001. [8] G. De Temmerman, E. Delchambre, J. Dowling, A. Kirk, S. Lisgo, P. Tamain, Plasma Phys. Control Fusion 52 (2010) 095005.