- Email: [email protected]
Design aspects of high-power targets
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 521 (2004) 136–142
Design aspects of high-power targets W.L. Talbert*, D.M. Drake, H.-H. Hsu, M.T. Wilson TechSource Inc., PO Box 31057, Santa Fe, NM 87594-1057, USA
Abstract The design of high-power targets for production of intense beams of radioactive ions requires the following: a reliable determination of the power deposited, knowledge of the thermal properties of the materials incorporated in the design, and a comprehensive thermal analysis of designs to evaluate the conceptual approaches. From the thermal analysis, iterations in the design approaches can lead to effective target conceptual designs that can bypass a series of prototype tests. Emphasis will be made for conductive cooling approaches, for which comprehensive thermal conductivity data are required for all components. Additional thermal properties are needed if radiative cooling or shielding is incorporated in target concepts. The example is presented of a uranium/carbon target to produce fission-product activities, featuring sensitivities to power dissipation and thermal properties assumed in the analyses. r 2003 Elsevier B.V. All rights reserved. PACS: 29.25.Rm; 29.25. t Keywords: Radioactive beams; Two-step target; Numerical analysis
1. Introduction Recently, considerable attention has been given to the design of targets to produce intense beams of radioactive ion beams from high-power beams of light ions. The principal approach to obtain more intense beams compared to traditional ISOL systems relies on increasing the incident production beam intensity. A incident production beam intensity of 100 mA is considered to be a reasonable goal for design of new high-power targets. In the past, targets to produce radioactive ion beams have required external heating for operation at temperatures adequate for effective release of the *Corresponding author. Tel.: +1-505-995-0799; fax: +1505-998-7656. E-mail address: [email protected] (W.L. Talbert).
produced activities because the incident production beam intensities have been inadequate to provide sufficient internal heating through beam interactions with the target material. Application of external heating is relatively simple to control and has the added feature that it results in a nearly uniform temperature distribution within the target material. For intense production beams, the internally generated heating results in thermal conditions which require that the target be cooled. It should be recognized that any cooling process requires that temperature gradients exist within the target so that the heat generated can be removed—the extent of the gradients depends on the cooling approach and the thermal properties of the target material. In particular, when cooling is achieved using water as the coolant, large gradients within the target can be expected.
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.141
ARTICLE IN PRESS W.L. Talbert et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 136–142
In this work, issues associated with external conductive cooling are presented, both for direct cooling of a so-called ‘‘thick’’ target, and for cooling (or containment of heat) for a two-step target. This latter target concept employs a solid primary target irradiated by the production beam and a secondary target illuminated by secondary neutrons produced by the production beam interactions within the primary target. A thermal analysis of any target requires knowledge of the energy deposition distribution within the target. We have used the beam (and secondary particle) interaction results obtained from application of the MCNPX code [1]. This numerical approach provides tracking of the primary beam as well as all secondary particles that may induce energy losses and/or nuclear interactions resulting in the production of radioactivities of interest. The determination of the energy deposition distribution is followed by a thermal analysis using a finite element code such as ALGOR [2], CFDesign [3], or COSMOS [4]. In the thermal analysis, the energy deposition distribution is regarded as a heat source, and cooling (or containment) is predicted for a design concept using known thermal properties of the target and containment materials, such as thermal conductivity and radiative emittance (for cases where surface temperatures lead to important losses of heat by radiative transfer). The validity of this numerical approach for developing a high-intensity target concept has been demonstrated [5] in an experiment using a test target at the TRIUMF/ISAC facility [6] in December, 1999. Because this approach of using numerical analyses was demonstrated to be valid, with careful attention to details in the analysis and fabrication approach it is expected that a highintensity target concept can be evaluated numerically and realistic target designs developed with confidence, bypassing multiple (and resourceintensive) prototype tests.
2. Conductive cooling design approach Considerable experience has been obtained in the use of traditional, low-intensity targets that
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require external heating to enable efficient release of the radioactivities produced. Such heating results in a reasonably uniform temperature distribution within the target, and no cooling is required. However, when progressing to targets using high-intensity incident beams the internal heating by the beam interactions is significant and, perhaps, even catastrophic if the target is not cooled. In designing a target to be externally cooled, careful attention is required in addressing the thermal coupling between the heat source and the coolant system. Incomplete thermal contact between components of the target can lead to unpredictable temperature distributions. The conductive path to the cooling system can even be designed to provide containment of the heat generated by an intense beam, acting as a thermal barrier. Targets providing radioactive ion beams through interactions with an incident high-energy light ion beam need to retain sufficient heat to enable release of the radioactivities produced. Also, it must be recognized that the temperature gradients within a conductively cooled target must be sufficient to allow transfer of heat out of the target, resulting in a non-uniform temperature distribution. The simplest thermal barrier is that of contact thermal resistance, where the incomplete contact of adjoining components results in restrictive heat conduction across the junction. This approach has been used with considerable success in space applications (where it is desired to have a system thermally isolated, yet mechanically stable), but prediction of the degree of isolation has been developed through empirical means. In general, metallic targets have the best characteristics to provide a reliable cooling approach design. The metallic target foil must be thin enough to allow diffusion of the radioactivities out of the target material. Thermal properties of metals and even metallic compounds, including thermal conductivities, have been determined to temperatures near the melting points. Thus, a careful design of a conductively cooled target is possible for a refractory metallic target utilizing stacked foils. Amorphous, or granular, targets cannot be treated with the same level of confidence.
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An example of the design issues for metallic targets is provided in a test target that was highly instrumented and tested at the ISAC facility in 1999 [5]. This target, of stacked molybdenum foils and spacers, employed water coolant and was designed to operate with maximum temperatures in the vicinity of 1700 C. The target was fabricated from foils of molybdenum alternating in a stack with foils with the center portion cut out in order to simulate a realistic RIB target (the spacers allow an effusion path to an ion source for the radioactivities). The stack was diffusion bonded to provide a solid thermal coupling to the coolant system. The target is described in detail elsewhere [5] and consisted of a cylinder with stacked foils and spacers and incorporated two longitudinal fins that were shaped to provide a varying thermal barrier to cooling tubes that ran the length of the target on the outside edge of the fins. The shaping was required because the heat deposition profile in the target varied markedly from the entrance to exit ends of the target due to the beam diverging as it traverses the target, as shown in Fig. 1 for a 0.95-cm radius molybdenum target (with 10 radial segments), 15 cm long, and irradiated by a 100 mA proton beam of energy 500 MeV. The molybdenum target had a density of essentially half solid (about 5 g/cm3) which varied slightly from the front of the target to the rear.
The cooling fins were integral to the target, being extensions of the target foils and spacer foils, and extended into the diffusion bonded structure (of full-density molybdenum). Therefore, thermal contact between the target and cooling system was assured. The known temperature-dependent thermal conductivity of molybdenum was used in the thermal analysis. From the thermal analysis, the anticipated experimental temperature profiles at the junction of the target with the conductive fin for four of the thermocouples are shown in Fig. 2 as the smooth lines, with the observed temperatures indicated by the symbols. The predictions and experimental points are shown for currents ranging from 10 to 100 mA. For further details, the reader is referred to the more complete description contained in Ref. [5]. This test target was used to validate the numerical approach to develop a realistic target design. A total of 16 high-temperature thermocouples monitored the temperatures at selected locations along the surface of the target, and the observed temperatures corresponded closely to the values predicted over a range of incident beam intensities. This agreement provided the validation of the numerical design approach using coupled energy deposition and thermal analysis codes. The issues associated with conductively cooled targets can be summarized by noting that attention must be given to the inclusion of robust conductive
Fig. 1. Energy deposition density profile for the molybdenum target.
Fig. 2. Results of validation experiment for molybdenum target.
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pathways to the coolant system, and also that the thermal properties of the target components be well known.
3. Two-stage target design approach The two-stage target consists of two components, the primary target irradiated by an energetic light ion beam and the secondary target illuminated by neutrons produced from beam interactions in the primary target. The principal scientific benefit of this concept is that the mass separated radioactivities that result are principally neutronrich fission products with little interference from neutron-deficient high-energy spallation products. A technical benefit is that the secondary target can incorporate materials that are unable to tolerate intense production beams under direct irradiation, because the energy deposition in the secondary target is largely due to the neutron interactions, which are limited to nuclear reactions, and not from ionization processes, as would be the case for the production beam ions. 3.1. Primary target design The primary target considered in this work is a solid rhenium cylinder of 1.5-cm diameter and 6.5cm length. Rhenium is chosen because of its high density (therefore short effective length in neutron production) and high melting point (3180 C). The choice of rhenium, however, carries a challenge with respect to a relatively low thermal conductivity (about half that of tungsten at elevated temperatures). This makes cooling the intensely heated target a challenging design problem, especially when constrained to using water as the coolant (as is the case for ISAC). The target of very modest dimensions has a heat load of nearly 21 kW for an incident beam intensity of 100 mA. This substantial heating has demanded several concept iterations to achieve cooling adequate to render the maximum temperature (at the target axis, entrance end) below that of molten rhenium. The relatively low thermal conductivity of rhenium requires large thermal gradients for transfer of heat to the cooling system,
Fig. 3. Two-step target primary temperature distribution for an incident beam intensity of 100 mA.
and the chosen solution is to employ four cooling tubes running the length of the target, on the surface, or possibly imbedded into the target a short distance. The energy deposition density distribution for the rhenium target is similar to that in Fig. 1, except that the intensity of energy deposition is about a factor of 4 higher and the target length is less than half that for Fig. 1. The thermal analysis for a quadrant of the target (for analysis purposes, the model is symmetric around one of the cooling tubes) is shown in Fig. 3. In the figure, the maximum temperature is well within the maximum temperature ‘‘allowed’’ for the rhenium target, and the cooling tube is attached to the surface of the target. The water coolant flow conditions (about 5 l/min per tube, with a temperature rise of about 80 C) are not severe, and within standard engineering practice [7]. This configuration for the primary target provides relative ease in fabrication. It has been noted that the problem solution is affected by the finite element zoning mesh size near the cooled region. Fine mesh sizes are required in regions of the target where heat flow is large, and the cooling tube region channels large heat flow from the rest of the target. 3.2. Secondary target design The secondary target in the two-step target concept is an axially concentric annular cylinder of 1.25-cm inner radius and annular thickness 2.1 cm and consisting in this example of an amorphous mixture of uranium and carbon. The density of the secondary target material is assumed to be 2.5 g/cm3, and the mixture ratio of U/C is 1.0. This type of material has been used with great success
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to produce neutron-rich radioactivities as well as heavy spallation products in traditional radioactive beam targets through direct beam irradiation at low intensities [8], and its thermal conductivity has been measured recently at Argonne National Laboratory [9]. The thermal properties are not ideal, as the resulting values for thermal conductivity can be considered typical of thermal insulators (1–2% that of the rhenium metal). The energy deposition density rate in the secondary target is much less than for the primary, as seen in Fig. 4. The total energy deposition in the secondary target is approximately 1.2 kW for a beam incident on the primary target of 100 mA intensity, and is mainly due to fission processes. This small energy deposition rate (only about 6% of that in the primary) leads the thermal analysis to focus on containment and distribution of the heat in the secondary target rather than cooling. Note that the energy deposition rate persists toward the downstream end. This is due to direct deposition of energy from protons scattered into the secondary target from the primary target. The distribution shown in the figure indicates that the highest temperatures can be expected in the inner central part of the target. The energy distribution is principally from fission events and indicates the neutron flux distribution within the target that is responsible for the fission distribution except for
Fig. 4. Energy deposition density distribution in the secondary target for a rhenium primary target.
the downstream end. In fact, it is not useful for the secondary target to extend beyond the ends of the primary target. A thermal analysis using the thermal properties of 2.5 g/cm3 U/C and the energy deposition distribution results in the temperature distribution shown in Fig. 5 for a quadrant of the secondary target. The figure shows the bare U/C temperature distribution (the primary target has been removed from the graphic) for a secondary target with radiation shields on the outside surface, the inner surface and the ends (shown). The calculation is sensitive to the radiative view factors and properties (such as temperature-dependent emittances) contained in the concept. It is considered that the U/C material should operate above 1600 C to provide reasonable fission product release conditions, and below 2200 C to avoid carbonization of the secondary target material with the (rhenium) containment vessel. As seen in the figure, a very large thermal gradient exists within the target material (as expected from the low thermal conductivity values), and the desired temperature conditions are not achieved for any region of the target. The deposited energy is insufficient to raise the temperature profiles of the large volume into the desired operating realm. There is also apparently little prospect to make the secondary target of uniform temperature, possibly even if external heating is employed. Various possibilities exist for mitigating the large thermal gradients within the target material ranging from inserting into the target rhenium ‘‘fins’’ to distribute the heat better through the superior thermal conductivity of the rhenium, to
Fig. 5. Temperature distribution for secondary target.
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incorporating in the target material whiskers of graphite fiber having enhanced thermal conductivity along their length, to introducing radiative pathways within the target material. The most appealing possibility for reducing the thermal gradients, however, is to incorporate U/C material having an enhanced intrinsic thermal conductivity. In recent measurements at Argonne, material samples of density up to 3.3 g/cm3 have been preliminarily measured to have an increased thermal conductivity compared to the original samples of density 2.5 g/cm3. While an increase in density results in an increase of the energy loss, it also allows a reduction in the annular thickness to achieve the same fission rate. This fact, combined with more attractive thermal properties, makes the desired temperature distributions in the secondary target easier to achieve. It is anticipated that, when reliable values of thermal properties for increased density secondary target material are available, thermal analyses of a secondary target configuration employing such increased density materials will result in more acceptable internal temperature distributions. To anticipate the effect of an increase in thermal conductivity, a thermal analysis was performed on the configuration of Fig. 5 with identical conditions except for a doubling of the temperaturedependent thermal conductivity (which is preliminarily indicated in recent measurements at Argonne National Laboratory for material of density 3.3 g/cm3 [9]). The result is shown in Fig. 6, where it is seen that the temperature distribution within the target has markedly smaller gradients, and the
Fig. 6. Temperature distribution for the secondary target, assuming twice the ‘‘normal’’ thermal conductivity for the target material.
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distribution of heat through the target has improved. However, the maximum temperature has also decreased. It is clear from this analysis that the secondary target will require external heating to achieve operating temperatures in the desired range, even at the maximum incident production beam current of 100 mA. One approach suggested is to electrically heat the radiation shield next to the outer surface. An engineered design for this revision to the original concept has yet to be performed. An additional consequence of increasing the target material density is that the volume of the target can be reduced to maintain the same fission rate. Fig. 7 shows the effect on total fission rate of change in density, where it is seen that higher densities can lead to smaller annular radii for the secondary target to maintain the fission rate of a 2.1-cm thick annulus of 2.5 g/cm3 density. If the volume (or radius) of the secondary target can be reduced while maintaining the total fission rate, it is reasonable to expect that the release of the fission products originating in the target volume will be enhanced. This assumes that the release properties of the more dense target material are not affected by the (modest) increase in density. For example, a secondary target of density 3.3 g/cm3 could have an annular thickness of approximately 1.4 cm, compared to the 2.1 cm in the original concept, and the volume is reduced by just under a factor of 2. The reliability of such considerations awaits verification by measurements
Fig. 7. Dependence of total fission rate on secondary target material density.
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on more dense target material, hopefully in progress. The design of a two-step target for a test at the ISAC facility (presented above) is a work in progress at the time of writing, with the primary target illustrated considered nearly ready for engineering design. While it is hoped that a satisfactory secondary target concept will be developed in the near future, unless the thermal properties of the U/C material can be modified to be more favorable this goal represents a serious challenge and a optimal design may result in compromised performance in providing radioactivities being released in timely and quantitative manner. The small energy deposited in the secondary target of the two-step target concept fulfills the desire that the target can be driven to higher production rates than for direct irradiation. The price to pay for this extension in production rate is that external heating of the secondary target appears to be required.
4. Conclusions The design and evaluation of concepts for production of intense beams of radioactive ions is possible using numerical techniques widely available. The issues consist mainly of energy deposition distributions and thermal analysis for cooling or heat containment. Conductive cooling with water as the coolant appears to be feasible for targets having high-energy deposition densities. A complete knowledge of the thermal properties of the materials is required for realistic development of target concepts operating under highpower conditions. Perhaps the most compelling data requirements are those of thermal conductivity, although at high operating temperatures radiative properties are also important.
For the two-step target concept, the opposing views of extraction of heat and conservation of heat are exhibited in the two target components of the concept. In the latter case, consideration of the distribution of heat within the target through conductive processes is a very important activity. Radiation shielding has an important role in concept development for containment of the deposited energy and requires good knowledge of hemispherical emittances as a function of temperature for the target components.
Acknowledgements This work is supported by the US Department of Energy under SBIR Grant DE-FG0301ER83314.
References [1] L.S. Waters (Ed.), MCNPX users manual: Monte Carlo N-particle transport code system for multi-particle and high-energy application, Los Alamos National Laboratory Report LA-UR-02-2607, v. 2.3.0, 2002. [2] The ALGOR code, v. 12, ALGOR Incorporated, Pittsburgh, PA, 2000. [3] CFDesign v. 3.1, Blue Ridge Numerics, Charlottesville, VA, 1999. [4] The COSMOS/M Code, Structural Research and Analysis Corporation, Los Angeles, CA, 1988. [5] W.L. Talbert, D.M. Drake, M.T. Wilson, J.W. Lenz, H.-H. Hsu, Nucl. Phys. A 701 (2002) 303c. [6] P.G. Bricault, M. Dombsky, P.W. Schmor, G. Stanford, Nucl. Instr. and Meth. B 126 (1997) 231. [7] G.P. Celata, M. Cumo, A. Mariani, Int. J. Heat and Mass Transfer 36 (1993) 1269. [8] A.H.M. Evensen, R. Catherall, P. Drumm, P. Van Duppen, O.C. Jonsson, E. Kugler, J. Lettry, O. Tengblad, V. Tikhonov, H.L. Ravn, Nucl. Instr. and Meth. B 126 (1997) 160. [9] J.P. Greene, Private communication, 2001, 2002.
Nuclear Instruments and Methods in Physics Research A 521 (2004) 136–142
Design aspects of high-power targets W.L. Talbert*, D.M. Drake, H.-H. Hsu, M.T. Wilson TechSource Inc., PO Box 31057, Santa Fe, NM 87594-1057, USA
Abstract The design of high-power targets for production of intense beams of radioactive ions requires the following: a reliable determination of the power deposited, knowledge of the thermal properties of the materials incorporated in the design, and a comprehensive thermal analysis of designs to evaluate the conceptual approaches. From the thermal analysis, iterations in the design approaches can lead to effective target conceptual designs that can bypass a series of prototype tests. Emphasis will be made for conductive cooling approaches, for which comprehensive thermal conductivity data are required for all components. Additional thermal properties are needed if radiative cooling or shielding is incorporated in target concepts. The example is presented of a uranium/carbon target to produce fission-product activities, featuring sensitivities to power dissipation and thermal properties assumed in the analyses. r 2003 Elsevier B.V. All rights reserved. PACS: 29.25.Rm; 29.25. t Keywords: Radioactive beams; Two-step target; Numerical analysis
1. Introduction Recently, considerable attention has been given to the design of targets to produce intense beams of radioactive ion beams from high-power beams of light ions. The principal approach to obtain more intense beams compared to traditional ISOL systems relies on increasing the incident production beam intensity. A incident production beam intensity of 100 mA is considered to be a reasonable goal for design of new high-power targets. In the past, targets to produce radioactive ion beams have required external heating for operation at temperatures adequate for effective release of the *Corresponding author. Tel.: +1-505-995-0799; fax: +1505-998-7656. E-mail address: [email protected] (W.L. Talbert).
produced activities because the incident production beam intensities have been inadequate to provide sufficient internal heating through beam interactions with the target material. Application of external heating is relatively simple to control and has the added feature that it results in a nearly uniform temperature distribution within the target material. For intense production beams, the internally generated heating results in thermal conditions which require that the target be cooled. It should be recognized that any cooling process requires that temperature gradients exist within the target so that the heat generated can be removed—the extent of the gradients depends on the cooling approach and the thermal properties of the target material. In particular, when cooling is achieved using water as the coolant, large gradients within the target can be expected.
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.141
ARTICLE IN PRESS W.L. Talbert et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 136–142
In this work, issues associated with external conductive cooling are presented, both for direct cooling of a so-called ‘‘thick’’ target, and for cooling (or containment of heat) for a two-step target. This latter target concept employs a solid primary target irradiated by the production beam and a secondary target illuminated by secondary neutrons produced by the production beam interactions within the primary target. A thermal analysis of any target requires knowledge of the energy deposition distribution within the target. We have used the beam (and secondary particle) interaction results obtained from application of the MCNPX code [1]. This numerical approach provides tracking of the primary beam as well as all secondary particles that may induce energy losses and/or nuclear interactions resulting in the production of radioactivities of interest. The determination of the energy deposition distribution is followed by a thermal analysis using a finite element code such as ALGOR [2], CFDesign [3], or COSMOS [4]. In the thermal analysis, the energy deposition distribution is regarded as a heat source, and cooling (or containment) is predicted for a design concept using known thermal properties of the target and containment materials, such as thermal conductivity and radiative emittance (for cases where surface temperatures lead to important losses of heat by radiative transfer). The validity of this numerical approach for developing a high-intensity target concept has been demonstrated [5] in an experiment using a test target at the TRIUMF/ISAC facility [6] in December, 1999. Because this approach of using numerical analyses was demonstrated to be valid, with careful attention to details in the analysis and fabrication approach it is expected that a highintensity target concept can be evaluated numerically and realistic target designs developed with confidence, bypassing multiple (and resourceintensive) prototype tests.
2. Conductive cooling design approach Considerable experience has been obtained in the use of traditional, low-intensity targets that
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require external heating to enable efficient release of the radioactivities produced. Such heating results in a reasonably uniform temperature distribution within the target, and no cooling is required. However, when progressing to targets using high-intensity incident beams the internal heating by the beam interactions is significant and, perhaps, even catastrophic if the target is not cooled. In designing a target to be externally cooled, careful attention is required in addressing the thermal coupling between the heat source and the coolant system. Incomplete thermal contact between components of the target can lead to unpredictable temperature distributions. The conductive path to the cooling system can even be designed to provide containment of the heat generated by an intense beam, acting as a thermal barrier. Targets providing radioactive ion beams through interactions with an incident high-energy light ion beam need to retain sufficient heat to enable release of the radioactivities produced. Also, it must be recognized that the temperature gradients within a conductively cooled target must be sufficient to allow transfer of heat out of the target, resulting in a non-uniform temperature distribution. The simplest thermal barrier is that of contact thermal resistance, where the incomplete contact of adjoining components results in restrictive heat conduction across the junction. This approach has been used with considerable success in space applications (where it is desired to have a system thermally isolated, yet mechanically stable), but prediction of the degree of isolation has been developed through empirical means. In general, metallic targets have the best characteristics to provide a reliable cooling approach design. The metallic target foil must be thin enough to allow diffusion of the radioactivities out of the target material. Thermal properties of metals and even metallic compounds, including thermal conductivities, have been determined to temperatures near the melting points. Thus, a careful design of a conductively cooled target is possible for a refractory metallic target utilizing stacked foils. Amorphous, or granular, targets cannot be treated with the same level of confidence.
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An example of the design issues for metallic targets is provided in a test target that was highly instrumented and tested at the ISAC facility in 1999 [5]. This target, of stacked molybdenum foils and spacers, employed water coolant and was designed to operate with maximum temperatures in the vicinity of 1700 C. The target was fabricated from foils of molybdenum alternating in a stack with foils with the center portion cut out in order to simulate a realistic RIB target (the spacers allow an effusion path to an ion source for the radioactivities). The stack was diffusion bonded to provide a solid thermal coupling to the coolant system. The target is described in detail elsewhere [5] and consisted of a cylinder with stacked foils and spacers and incorporated two longitudinal fins that were shaped to provide a varying thermal barrier to cooling tubes that ran the length of the target on the outside edge of the fins. The shaping was required because the heat deposition profile in the target varied markedly from the entrance to exit ends of the target due to the beam diverging as it traverses the target, as shown in Fig. 1 for a 0.95-cm radius molybdenum target (with 10 radial segments), 15 cm long, and irradiated by a 100 mA proton beam of energy 500 MeV. The molybdenum target had a density of essentially half solid (about 5 g/cm3) which varied slightly from the front of the target to the rear.
The cooling fins were integral to the target, being extensions of the target foils and spacer foils, and extended into the diffusion bonded structure (of full-density molybdenum). Therefore, thermal contact between the target and cooling system was assured. The known temperature-dependent thermal conductivity of molybdenum was used in the thermal analysis. From the thermal analysis, the anticipated experimental temperature profiles at the junction of the target with the conductive fin for four of the thermocouples are shown in Fig. 2 as the smooth lines, with the observed temperatures indicated by the symbols. The predictions and experimental points are shown for currents ranging from 10 to 100 mA. For further details, the reader is referred to the more complete description contained in Ref. [5]. This test target was used to validate the numerical approach to develop a realistic target design. A total of 16 high-temperature thermocouples monitored the temperatures at selected locations along the surface of the target, and the observed temperatures corresponded closely to the values predicted over a range of incident beam intensities. This agreement provided the validation of the numerical design approach using coupled energy deposition and thermal analysis codes. The issues associated with conductively cooled targets can be summarized by noting that attention must be given to the inclusion of robust conductive
Fig. 1. Energy deposition density profile for the molybdenum target.
Fig. 2. Results of validation experiment for molybdenum target.
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pathways to the coolant system, and also that the thermal properties of the target components be well known.
3. Two-stage target design approach The two-stage target consists of two components, the primary target irradiated by an energetic light ion beam and the secondary target illuminated by neutrons produced from beam interactions in the primary target. The principal scientific benefit of this concept is that the mass separated radioactivities that result are principally neutronrich fission products with little interference from neutron-deficient high-energy spallation products. A technical benefit is that the secondary target can incorporate materials that are unable to tolerate intense production beams under direct irradiation, because the energy deposition in the secondary target is largely due to the neutron interactions, which are limited to nuclear reactions, and not from ionization processes, as would be the case for the production beam ions. 3.1. Primary target design The primary target considered in this work is a solid rhenium cylinder of 1.5-cm diameter and 6.5cm length. Rhenium is chosen because of its high density (therefore short effective length in neutron production) and high melting point (3180 C). The choice of rhenium, however, carries a challenge with respect to a relatively low thermal conductivity (about half that of tungsten at elevated temperatures). This makes cooling the intensely heated target a challenging design problem, especially when constrained to using water as the coolant (as is the case for ISAC). The target of very modest dimensions has a heat load of nearly 21 kW for an incident beam intensity of 100 mA. This substantial heating has demanded several concept iterations to achieve cooling adequate to render the maximum temperature (at the target axis, entrance end) below that of molten rhenium. The relatively low thermal conductivity of rhenium requires large thermal gradients for transfer of heat to the cooling system,
Fig. 3. Two-step target primary temperature distribution for an incident beam intensity of 100 mA.
and the chosen solution is to employ four cooling tubes running the length of the target, on the surface, or possibly imbedded into the target a short distance. The energy deposition density distribution for the rhenium target is similar to that in Fig. 1, except that the intensity of energy deposition is about a factor of 4 higher and the target length is less than half that for Fig. 1. The thermal analysis for a quadrant of the target (for analysis purposes, the model is symmetric around one of the cooling tubes) is shown in Fig. 3. In the figure, the maximum temperature is well within the maximum temperature ‘‘allowed’’ for the rhenium target, and the cooling tube is attached to the surface of the target. The water coolant flow conditions (about 5 l/min per tube, with a temperature rise of about 80 C) are not severe, and within standard engineering practice [7]. This configuration for the primary target provides relative ease in fabrication. It has been noted that the problem solution is affected by the finite element zoning mesh size near the cooled region. Fine mesh sizes are required in regions of the target where heat flow is large, and the cooling tube region channels large heat flow from the rest of the target. 3.2. Secondary target design The secondary target in the two-step target concept is an axially concentric annular cylinder of 1.25-cm inner radius and annular thickness 2.1 cm and consisting in this example of an amorphous mixture of uranium and carbon. The density of the secondary target material is assumed to be 2.5 g/cm3, and the mixture ratio of U/C is 1.0. This type of material has been used with great success
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to produce neutron-rich radioactivities as well as heavy spallation products in traditional radioactive beam targets through direct beam irradiation at low intensities [8], and its thermal conductivity has been measured recently at Argonne National Laboratory [9]. The thermal properties are not ideal, as the resulting values for thermal conductivity can be considered typical of thermal insulators (1–2% that of the rhenium metal). The energy deposition density rate in the secondary target is much less than for the primary, as seen in Fig. 4. The total energy deposition in the secondary target is approximately 1.2 kW for a beam incident on the primary target of 100 mA intensity, and is mainly due to fission processes. This small energy deposition rate (only about 6% of that in the primary) leads the thermal analysis to focus on containment and distribution of the heat in the secondary target rather than cooling. Note that the energy deposition rate persists toward the downstream end. This is due to direct deposition of energy from protons scattered into the secondary target from the primary target. The distribution shown in the figure indicates that the highest temperatures can be expected in the inner central part of the target. The energy distribution is principally from fission events and indicates the neutron flux distribution within the target that is responsible for the fission distribution except for
Fig. 4. Energy deposition density distribution in the secondary target for a rhenium primary target.
the downstream end. In fact, it is not useful for the secondary target to extend beyond the ends of the primary target. A thermal analysis using the thermal properties of 2.5 g/cm3 U/C and the energy deposition distribution results in the temperature distribution shown in Fig. 5 for a quadrant of the secondary target. The figure shows the bare U/C temperature distribution (the primary target has been removed from the graphic) for a secondary target with radiation shields on the outside surface, the inner surface and the ends (shown). The calculation is sensitive to the radiative view factors and properties (such as temperature-dependent emittances) contained in the concept. It is considered that the U/C material should operate above 1600 C to provide reasonable fission product release conditions, and below 2200 C to avoid carbonization of the secondary target material with the (rhenium) containment vessel. As seen in the figure, a very large thermal gradient exists within the target material (as expected from the low thermal conductivity values), and the desired temperature conditions are not achieved for any region of the target. The deposited energy is insufficient to raise the temperature profiles of the large volume into the desired operating realm. There is also apparently little prospect to make the secondary target of uniform temperature, possibly even if external heating is employed. Various possibilities exist for mitigating the large thermal gradients within the target material ranging from inserting into the target rhenium ‘‘fins’’ to distribute the heat better through the superior thermal conductivity of the rhenium, to
Fig. 5. Temperature distribution for secondary target.
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incorporating in the target material whiskers of graphite fiber having enhanced thermal conductivity along their length, to introducing radiative pathways within the target material. The most appealing possibility for reducing the thermal gradients, however, is to incorporate U/C material having an enhanced intrinsic thermal conductivity. In recent measurements at Argonne, material samples of density up to 3.3 g/cm3 have been preliminarily measured to have an increased thermal conductivity compared to the original samples of density 2.5 g/cm3. While an increase in density results in an increase of the energy loss, it also allows a reduction in the annular thickness to achieve the same fission rate. This fact, combined with more attractive thermal properties, makes the desired temperature distributions in the secondary target easier to achieve. It is anticipated that, when reliable values of thermal properties for increased density secondary target material are available, thermal analyses of a secondary target configuration employing such increased density materials will result in more acceptable internal temperature distributions. To anticipate the effect of an increase in thermal conductivity, a thermal analysis was performed on the configuration of Fig. 5 with identical conditions except for a doubling of the temperaturedependent thermal conductivity (which is preliminarily indicated in recent measurements at Argonne National Laboratory for material of density 3.3 g/cm3 [9]). The result is shown in Fig. 6, where it is seen that the temperature distribution within the target has markedly smaller gradients, and the
Fig. 6. Temperature distribution for the secondary target, assuming twice the ‘‘normal’’ thermal conductivity for the target material.
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distribution of heat through the target has improved. However, the maximum temperature has also decreased. It is clear from this analysis that the secondary target will require external heating to achieve operating temperatures in the desired range, even at the maximum incident production beam current of 100 mA. One approach suggested is to electrically heat the radiation shield next to the outer surface. An engineered design for this revision to the original concept has yet to be performed. An additional consequence of increasing the target material density is that the volume of the target can be reduced to maintain the same fission rate. Fig. 7 shows the effect on total fission rate of change in density, where it is seen that higher densities can lead to smaller annular radii for the secondary target to maintain the fission rate of a 2.1-cm thick annulus of 2.5 g/cm3 density. If the volume (or radius) of the secondary target can be reduced while maintaining the total fission rate, it is reasonable to expect that the release of the fission products originating in the target volume will be enhanced. This assumes that the release properties of the more dense target material are not affected by the (modest) increase in density. For example, a secondary target of density 3.3 g/cm3 could have an annular thickness of approximately 1.4 cm, compared to the 2.1 cm in the original concept, and the volume is reduced by just under a factor of 2. The reliability of such considerations awaits verification by measurements
Fig. 7. Dependence of total fission rate on secondary target material density.
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on more dense target material, hopefully in progress. The design of a two-step target for a test at the ISAC facility (presented above) is a work in progress at the time of writing, with the primary target illustrated considered nearly ready for engineering design. While it is hoped that a satisfactory secondary target concept will be developed in the near future, unless the thermal properties of the U/C material can be modified to be more favorable this goal represents a serious challenge and a optimal design may result in compromised performance in providing radioactivities being released in timely and quantitative manner. The small energy deposited in the secondary target of the two-step target concept fulfills the desire that the target can be driven to higher production rates than for direct irradiation. The price to pay for this extension in production rate is that external heating of the secondary target appears to be required.
4. Conclusions The design and evaluation of concepts for production of intense beams of radioactive ions is possible using numerical techniques widely available. The issues consist mainly of energy deposition distributions and thermal analysis for cooling or heat containment. Conductive cooling with water as the coolant appears to be feasible for targets having high-energy deposition densities. A complete knowledge of the thermal properties of the materials is required for realistic development of target concepts operating under highpower conditions. Perhaps the most compelling data requirements are those of thermal conductivity, although at high operating temperatures radiative properties are also important.
For the two-step target concept, the opposing views of extraction of heat and conservation of heat are exhibited in the two target components of the concept. In the latter case, consideration of the distribution of heat within the target through conductive processes is a very important activity. Radiation shielding has an important role in concept development for containment of the deposited energy and requires good knowledge of hemispherical emittances as a function of temperature for the target components.
Acknowledgements This work is supported by the US Department of Energy under SBIR Grant DE-FG0301ER83314.
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