HiLumi LHC FP7 High Luminosity Large Hadron Collider Design Study. Milestone Report. Cryogenic Scenarios for the Cold Powering System

Transcription

1 CERN-ACC HiLumi LHC FP7 High Luminosity Large Hadron Collider Design Study Milestone Report Cryogenic Scenarios for the Cold Powering System Ballarino, A (CERN) et al 27 May 2014 The HiLumi LHC Design Study is included in the High Luminosity LHC project and is partly funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement This work is part of HiLumi LHC Work Package 6: Cold powering. The electronic version of this HiLumi LHC Publication is available via the HiLumi LHC web site < or on the CERN Document Server at the following URL: < CERN-ACC

2 Grant Agreement No: HILUMI LHC FP7 High Luminosity Large Hadron Collider Design Study Seventh Framework Programme, Capacities Specific Programme, Research Infrastructures, Collaborative Project, Design Study MILESTONE REPORT CRYOGENIC SCENARIOS FOR THE COLD POWERING SYSTEM MILESTONE: MS57 Document identifier: Due date of deliverable: End of Month 30 (April 2014) Report release date: 10/06/2014 Work package: Lead beneficiary: Document status: WP6: Cold Powering CERN Final Abstract: This document reports the results of the preliminary studies made in order to optimize the cooling scheme of the Cold Powering Systems for the High Luminosity Upgrades. Following a review of the cooling possibilities that can be envisaged using the existing LHC infrastructure at P1, P5 and P7 and the expected new cryogenic infrastructure at P1 and P5, cryogenic cooling schemes satisfying the electrical requirements are proposed for each of the different cold powering applications. Grant Agreement PUBLIC 1 / 14

3 Copyright notice: Copyright HiLumi LHC Consortium, For more information on HiLumi LHC, its partners and contributors please see The HiLumi LHC Design Study is included in the High Luminosity LHC project and is partly funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement HiLumi LHC began in November 2011 and will run for 4 years. The information herein only reflects the views of its authors and not those of the European Commission and no warranty expressed or implied is made with regard to such information or its use. Delivery Slip Name Partner Date Authored by A. Ballarino, U. Wagner, Y. Yang CERN, SOTON 19/05/14 Edited by A. Ballarino CERN 25/05/14 Reviewed by L. Rossi [Project coordinator] CERN 26/05/14 Approved by Steering Committee 27/05/14 Grant Agreement PUBLIC 2 / 14

4 TABLE OF CONTENTS 1. INTRODUCTION COMMON BOUNDARY CONDITIONS COMMON COOLING SCHEME CONSIDERATIONS NAMING CONVENTION COOLING SCHEME LHC POINT DESCRIPTION OF THE COOLING SCHEME CRYOGENIC CONTROL COST OF COOLING COOLING SCHEME IN LHC POINTS P1 AND P COOLING SYSTEM FOR THE ARC CURRENT FEED BOX General description Control COOLING SYSTEM FOR THE CURRENT FEED BOXES OF HIGH LUMINOSITY MATCHING SECTIONS AND INNER TRIPLETS CONCLUSIONS REFERENCES ANNEX: GLOSSARY Grant Agreement PUBLIC 3 / 14

5 Executive summary This document summarizes the results of the preliminary studies made to provide adapted cooling schemes for the Cold Powering Systems of the High Luminosity Upgrades. Different cryogenic scenarios satisfying the electrical requirements of the different superconducting circuits are elaborated, discussed and proposed. 1. INTRODUCTION Preliminary studies on cooling options for the Cold Powering Systems being developed for integration at LHC P1, P5 and P7 have indicated supercritical helium at 5 K, available in the existing and future LHC cryogenic systems, as the best choice for cooling the Superconducting (SC) Links. This fluid enables operation of the superconducting cables in the link within the specified temperature range both in the case of MgB 2 (T max 25 K) and of High Temperature Superconductors (HTS, YBCO or Bi-2223, T max 35 K). In this report, the results of the studies made for elaborating a cooling scheme using supercritical helium from the LHC refrigerators are presented and discussed. This report considers only MgB 2 as superconductor in the link between the LHC tunnel and the relocated current feed boxes. As this material has the lowest operating temperature, it is more demanding from the point of view of cooling if compared to high temperatures superconductors. The proposed cooling scheme could be extended to superconducting links incorporating HTS, which would require a lower helium mass flow rate. 2. COMMON BOUNDARY CONDITIONS The boundary conditions described below are valid for all systems and have an important influence on the definition of the individual cooling scheme. a) The superconducting link between the LHC tunnel and the current feed boxes contains copper stabilised MgB 2 cables. These cables are connected in the tunnel to the Nb-Ti magnet bus-bar and at the surface (P1 and P5) or in tunnel (P7) to the bottom end of the current leads. b) The connection between the Nb-Ti bus and the MgB 2 cables is done in a bath of liquid helium. c) The maximum temperature of the MgB 2 cables shall not exceed 20 K in nominal operating conditions. The cables are designed for operation up to 25 K at maximum current in order in order to have a safety margin of 5 K. The helium gas cooling the MgB 2 cables has a nominal operating temperature of 17 K, allowing for a 3 K margin on control variations in the cooling system. d) The cryostat of the link, i.e. the cryogenic transfer line that contains the MgB 2 cables, is of the semi-flexible design with two cooling circuits, one for the cold part housing the superconducting cables and one for the thermal shield. The heat load for this type of transfer line will be specified to be below 0.2 W/m on the inner circuit and about 2.5 W/m on the shield. e) The design of the current leads is based on that of the LHC leads. The cooling requirements of these current leads are derived from [1], where minimum mass flows and optimized geometries of current leads cooled by helium gas entering at different Grant Agreement PUBLIC 4 / 14

6 temperatures are reported in Figure 1 and Figure 2. For the present study, the case of helium gas entering the leads at about 20 K has to be considered. 3. COMMON COOLING SCHEME CONSIDERATIONS The combination of 0.2 W/m heat load on the inner circuit, 17 K maximum temperature and the length of the different link cryostats defines the mass flow necessary for the cooling of the superconducting link. The mass-flow for cooling the current leads in any of the distribution feed boxes is defined by the consumption of the current leads, i.e. by the total current transitioning through the feed box. This leads to two general cases for the cooling. Case 1 is characterized by the combination of a long link cryostat and a comparatively low total current in the feed box. In this case the static heat load of the link cryostat defines the necessary mass flow of cold helium. The current lead cooling will only consume part of the flow necessary for cooling the cryostat, and the flow in excess needs to be warmed to ambient temperature by a dedicated heater. This is the case of the Cold Powering System at LHC P7. Case 2 is characterized by a high total current in the feed box and a shorter link cryostat. In this case, the cooling of the current leads defines the helium mass flow requirements. The cooling flow of the thermal shield in the link can contribute to the cooling of the current leads. This is the case of the Cold Powering Systems at LHC P1 and P5. 4. NAMING CONVENTION The following naming conventions were adopted for the HL-LHC main sub-systems (see Figure 1) [2]: DFH is the cryostat containing the current leads, DSH is the semi-flexible cryostat with the MgB 2 cables (DFHA = arc, DFHM = matching sections, DFHX = inner triplets), and DF is the cryostat in the tunnel containing the electrical splices between the MgB 2 cables and the Nb-Ti bus (DFA = arc, DFM = matching sections, DFX = triplets). Figure 1: Naming conventions of the sub-systems of the Cold Powering System 5. COOLING SCHEME LHC POINT DESCRIPTION OF THE COOLING SCHEME For LHC P7 the existing DFBA and DFBM shall be replaced by a common feed box relocated in the underground gallery TZ76 at about 500 m distance from the tunnel. The existing 13 ka leads in the DFBA, used for energy extraction, will not be removed from the LHC tunnel and Grant Agreement PUBLIC 5 / 14

7 the related cables and current leads are therefore not included in the superconducting link and the new distribution feed box. This leads to a new hybrid feed box for the arc magnets, called DFA and to a connecting box close to Q6, called DFM. The new current feed box in TZ76 is called DFHA. The simplified flow scheme for the cooling of the Cold Powering System at LHC P7 is shown in Figure 2. The DFA includes the two 13 ka leads for the energy extraction. They are connected to the Nb-Ti bus in a similar way as in the existing DFBA. In a separate volume filled with liquid helium, the Nb-Ti bus of the 600 A circuits for the arc and Q6 are connected to the superconducting link. The helium for this liquid volume is taken from the QRL at the level of the DFM close to Q6. In the DFM this helium, which will be at about 5.3 K and 350 kpa at this location of the arc, is subcooled to about 4.6 K and used for cooling the Nb-Ti cable between Q6 and the DFA. At the level of the DFA the helium is expanded into the two phase volume creating the liquid necessary to cool the Nb-Ti superconductor and the electrical connections between the Nb-Ti and the MgB 2 conductor of the link. The superconducting link is cooled by the vapour evaporated from the two phase volume by means of an electrical heater. Along the superconducting link this helium warms up to 17 K and then enters the current leads. As mentioned in chapter 3 under case 1, the helium mass flow necessary for the cooling of the link will exceed the mass flow needed for the cooling of the current leads. The excess mass flow must therefore be either at least partially used for cooling the DFH box or directly warmed up via a separate heater before being returned at ambient temperature. The thermal shield for both the Nb-Ti link between DFM and DFA as well as the MgB 2 link between the DFA and DFHA is taken at about 20 K from line D in the tunnel at the level of the DFM. This helium is then passed through the two thermal shields of the cryostats and warmed to ambient temperature via an electrical heater in the DFHA after which it is returned at ambient temperature. Grant Agreement PUBLIC 6 / 14

8 Figure 2: Simplified flow scheme for the cooling of the Cold Powering System at LHC P CRYOGENIC CONTROL The control loops envisaged for the cooling of the Cold Powering System at LHC P7 are sketched in Figure 2. The level of the phase separator for the subcooling of the helium at 5.3 K and 350 kpa is controlled by the valve supplying the helium from line C. The level of the two-phase helium volume for the electrical connections between Nb-Ti and MgB 2 cables is controlled by the valve discharging the helium used for the cooling of the Nb- Ti link between Q6 and the DFA. The pressure of the liquid helium volume for the electrical connections between Nb-Ti and MgB 2 cables is controlled by an electrical heater in the liquid phase. This heater provides the mass flow of helium vapour necessary for the cooling of the superconducting link between the DFA and the DFHA. The temperature at the outlet of the link between DFA and DFHA is controlled by a control valve in the by-pass around the current lead. This valve such adapts the flow in the DSH such that the maximum temperature of 17 K is not exceeded. The flow for each current lead is controlled by a valve on the lead outlet, regulating the temperature at the cold end of the resistive part at about 30 K 40 K. Grant Agreement PUBLIC 7 / 14

9 An electrical heater controls the warm end temperature of each current lead in order to maintain room temperature both in steady-state operation and for operations at currents lower than the nominal one COST OF COOLING In order to identify the cost of cooling for a cryogenic process one can best refer to an exergy balance [3]. The cooling solution for the cold powering at LHC P7, as described in the sections above, is about 5.9 times higher than the one for the current solution installed in the LHC machine where no superconducting links are used and therefore no specific cooling for this equipment is required. This relatively high increase in cooling is dominated by the following factors: The static heat load on the link cryostat. It should be noted that for the purpose of this analysis the static load was conservatively assumed to be 0.3 W/m. For commercially available cryostats, a static load below 0.25 W/m is guaranteed by the company when the shielding is at 77 K. For the LHC superconducting link cryostats, shielding with gas entering at about 20 K will enable a reduction of the load, and an average value along the line lower than 0.2 W/m will be specified. Because of the low-current rating of the LHC current leads at LHC P7, the cooling of the current leads requires less gas than the link cryostat itself. Only gas at ambient temperature is returned to the refrigerator system. An optimized design of the link cryostat and the use of the cold gas at the exit of the link for cooling the DFH box can reduce the cost of cooling. A study has shown that the use of a custom designed line with low heat load would have the potential to decrease the exergetic cost factor from 5.9 to 3.4. Considering an even more complicated line with three hydraulic circuits allowing the use of gas from the thermal screen cooling of the LHC machine for the thermal screen of the line can further reduce the factor to 2.7. The final design of the superconducting link cryostat will be a compromise between the cost of cooling, the cost of the line, and the requirements for the installation in LHC underground areas. A second option to decrease the cost of the cooling would be the use of HTS conductors in the link, which would allow for temperatures of the helium gas up to about 60 K. This would, for the same link cryostat configuration, lower the cost factor from 5.9 to about 3.6. Though the factor of increase seems high, the total cost of cooling compared to the base load of the refrigerators around P7 is low. Even the version with high exergetic cost can be well covered by the capacity margin of the existing LHC cryogenic installation. 6. COOLING SCHEME IN LHC POINTS P1 AND P5 At LHC P1 and P5 the Cold Powering Systems based on the use of superconducting links are required both for existing (DFBA and DFBL) and new (DFX) equipment. The block diagram for these points is sketched in Figure 3. Grant Agreement PUBLIC 8 / 14

10 Figure 3: Block diagram of the existing and new equipment in P1 and P5 As it is to be avoided that helium flow is mixed between different refrigerators, the relocated feed boxed for the arc current leads, DFHA, will be hydraulically connected to the existing refrigerators. For the two DFHM and DFHX, it is not envisaged to run current and helium lines through the experiment caverns in P1 and P5. Therefore two possibilities exist for the current feed boxes: a) combining all current leads in one single feed box or, as shown in Figure 3, b) have one feed box for the inner triplets and one for the matching sections. In view of the large currents to be transferred and of potentially different times of installation in the tunnel, solution b) is the present baseline and it is therefore retained as reference for the cooling of the Cold Powering Systems COOLING SYSTEM FOR THE ARC CURRENT FEED BOX General description The total current in the DFHA on both sides of each point is such that the flow necessary for the current leads cooling exceeds the flow necessary for the cooling of the cold part of the link. The cooling scheme for the superconducting link and current leads is sketched in Figure 4. A simplified flow scheme of the Cold Powering System at LHC P1 and P5 is shown in Figure 5. The DFA includes the volume filled with liquid helium where the Nb-Ti cables from the arc are connected to the superconducting link cables. The helium for this liquid volume is taken from the QRL at the level of the DFA. The superconducting link is cooled by the vapour evaporated from the two phase volume by means of an electrical heater. The helium warms up along the superconducting link up to about 17 K and is then mixed to the helium from the thermal shield cooling of the DSH. The thermal shield for the link between DFA and DFHA is taken at about 20 K from line D in the tunnel at the level of the DFM. This helium is then passed through the two thermal shields of the cryostats and warmed to about 80 K along the DFH line. The mixing temperature of the two flows will be at about 29 K. This combined flow is finally used to cool the current leads. An electrical heater in the DFHA is foreseen for transient or cool-down operations. Grant Agreement PUBLIC 9 / 14

11 Figure 4: Sketch of the cooling for the Cold Powering System of the arc current feed boxes at LHC P1 and P Control The control loops envisaged for the cooling of the Cold Powering System at LHC P1 and P5 are sketched in Figure 5. The level of the liquid helium volume for the connections between the Nb-Ti and the MgB 2 cables is controlled by the valve supplying the helium from line C. The temperature at the outlet of the link between DFA and DFHA is controlled to about 7 K by an electrical heater in the liquid phase. As the heat load on the cold line of the link can be considered as static, the heater is essentially set to a fixed value. The temperature at the outlet of the thermal screen of the link is controlled to about 80 K by a control valve taking the gas from the QRL. The exact set point for this temperature is given by the temperature of the mixed gas from cold line and shield which shall be at about 29 K. The flow for each current lead is controlled by a valve on the lead outlet. An electrical heater controls the warm end temperature of each current lead in order to maintain room temperature both in steady-state operation and for operations at currents lower than the nominal one. Grant Agreement PUBLIC 10 / 14

12 Figure 5: Simplified flow scheme of the cooling for the Cold Powering Systems at LHC P1 and P5 The pressure of the two-phase helium volume where the electrical connections between Nb-Ti and MgB 2 cables are made is controlled by a control valve by-passing part of the mixed flow around the current leads. This valve will be closed in normal operation and it will only operate for cool-down or in case the current lead control valve closes. The gas needed to cool the link is then passed around the current leads. An electrical heater in the by-pass warms the gas up to ambient temperature COOLING SYSTEM FOR THE CURRENT FEED BOXES OF HI- LUMINOSITY MATCHING SECTIONS AND INNER TRIPLETS The cooling system for the current feed boxes of the inner triplets and the matching sections at LHC P1 and P5 is still under study. At this stage, the general cooling principle retained is the same as for the arc current feed boxes in these points. For the feed boxes, the option exists to use a dedicated transfer line on surface level between the new refrigerator and the DFHX to cool the current leads instead of using the link for transferring gas from the tunnel as done for the cold powering system at LHC P7. Grant Agreement PUBLIC 11 / 14

13 A final cooling system will be elaborated in parallel with the work to be made for the definition of the new refrigerators and for the design of the DFA and DFH cryostats. 7. CONCLUSIONS A proposal for the cooling of the Cold Powering System connected to the existing systems at LHC P7 is now well defined, and preliminary studies for the cooling of the Cold Powering Systems at LHC P1 and P5 are elaborated. Small uncertainties, which will be cleared in a more advanced design phase, exist mainly on the heat loads to be expected in the system. In particular, the foreseen test of a horizontal demonstrator to be installed and operated in the SM18 will help to finally quantify the cryogenic performance. The design activity on the DFA cryostat for LHC P7 has started [4]. For the cooling of the Cold Powering Systems feeding the High Luminosity Inner Triples, a solution similar to the one proposed for the existing equipment arc and matching sections- at LHC P1 and P5 is proposed. Grant Agreement PUBLIC 12 / 14

14 8. REFERENCES [1] Ballarino, A. (2007) HTS Current Leads: performance overview in different operating modes, IEEE Trans. Appl. Supercond., Vol. 17, No. 2, June [2] Chemli, S. (2013), Naming of new components (DBF, SC Link and Interconnection box), Meeting of WP6, [3] Lebrun, Ph. (2009) Exergy Analysis of the Cryogenic Helium Distribution System for the Large Hadron Collider (LHC), AIP Conf. Proc (2010) ; Cryogenic Engineering Conference and International Cryogenic Materials Conference, Tucson, AR, USA, 28 Jun - 2 Jul 2009, pp [4] Ramos, D. (2014) Progress on conceptual design of DFA cryostat at LHC P7, Meeting of WP6, Grant Agreement PUBLIC 13 / 14

15 ANNEX: GLOSSARY Acronym Definition DFA Current connection box for the arc on LHC tunnel level DFM Current connection box for the matching section on LHC tunnel level DFX Current connection box for the inner triplet on LHC tunnel level DFHA Relocated current feed box for the arc on surface level or in remote underground cavern DFHM Relocated current feed box for the matching section on surface level DFHX Relocated current feed box for the inner triplet on surface level DSH Superconducting link containing MgB 2 or HTS cable, operating at temperatures of up to 25 K or 35 K DSL Existing LHC superconducting link containing Nb-Ti cables, operating at maximum 5.6 K HTS High Temperature Superconductor P7 LHC Point 7 P1 LHC Point 1 P5 LHC Point 5 Grant Agreement PUBLIC 14 / 14