MICE Spectrometer Solenoid Review A summary of P.Fabbricatore on behalf the review board Amalia Ballarino, Elwyn Baynham, Tom Bradshaw, Mike Courthold, and Chris White. MICE ex officio members Alain Blondel and Andy Nichols 1.Review Charge 2.Actual review process 3.Findings 4.Conclusions and recommendations

1) Review and comment on the construction, cooling and testing of the MICE Spectrometer Solenoids under construction at Wang NMR. In particular, comment on: Original thermal model for magnet including analysis and assumptions. Interpretation of the existing results on the cooling of the magnet using cryocoolers and powering of the magnet. Updated thermal analysis based on the recent testing 2) Review the proposed modifications aiming at assuring successful and reliable powering of the magnet to full current. Comment on Updated thermal model, operating margin and ability to maintain the LHe with cryocoolers. New cryo-cooler implementation and mechanical modifications to the assembly Thermal and vacuum instrumentation and diagnostics 3) Review the power supplies and the operational abilities of the magnet in relation to the expected operating modes. 4) Review the presented schedule for implementing changes and the overall magnet successful delivery schedule. Evaluate possible risks and sources of delays. Review Charge

Starting from October 5 th the committee analyzed many documents (drawings, photos, log data) related to the design, construction and tests of the two MICE spectrometer solenoids. Three phone meetings were held with the LBNL MICE team responsible for the spectrometer magnets, in a very good atmosphere of openness and friendly collaboration. Finally an in-person meeting was held in Berkeley on Nov. 18 th and 19 th. A preliminary report was written and it is available at the review web site (reporting all the review phases). The committee acknowledges the strong efforts Mike Zisman, Mike Green and Steve Virostek in providing the required information and their availability in answering the many questions asked by the committee. Actual review process

The central point identified by the committee is related to the overall thermal balance of the magnet in the turret area : at the root of the problems found with the two magnets there are three causes:

1.The current leads cause a concentrated heat dissipation, which the copper plate is not able to efficiently drain away. 2.The thermal analysis indicates that cooling of the shield could be limited due to a high thermal impedance between copper plate and shield. 3.The temperature of the Al shield is quite high (105 K measured); in contrast the calculated heat load on the shield is quite low (15 W). In fact for the magnet 2 a LN2 reservoir was implemented, directly connected to both the thermal shield and to the copper plate hosting the first stages of the cryo- coolers

For a deeper understanding of the point 3, the committee analyzed: 1.The thermal flow through the connection between copper plate and shield (resulting in 60 W from shield to copper plate) 2.The heat balance at the copper plate (resulting in W power input of unknown nature) 3.The temperature distribution on the shield as resulting from a thermal analysis done by. F Trillaud (resulting in a maximum credible thermal to the shield input of W).

Al6061-Ti Finite Element Analysis done by F.Trillaud for the shield and comparison with measured temperatures

It is not easy to reconcile the above three points unless the thermal connection between the copper plate and the shield is imputed to be highly unsuitable to transfer even a limited heat flow and, at the same time, a different ‘agent’ is causing a 50–60 W heat load. It is still unclear whether there really exists this additional 60W heat load to the copper plate or rather a lack of cooling ( bad thermal contact or poor performance of one or more coolers)

Review and comment on the construction, cooling and testing of the MICE Spectrometer Solenoids under construction at Wang NMR. In particular, comment on: The system appear to be quite complex. From design point of view a more accurate analysis would have sufficed for a better design of the thermal connection between copper plate and shield and for a better sizing of the copper plate. The additional loss of 60 W at 65K (or bad performances of cryo-coolers giving 60 W less cooling power ) is mainly a construction problem. The committee recommendation It is very important to improve Frederic Trillaud’s thermal model to take into account both the LN 2 reservoir and the additional cryocooler and apply it to predict the results of cooling down and powering up of magnet 2.

Review the proposed modifications aiming at assuring successful and reliable powering of the magnet to full current Fixes to magnet 2. Besides replacing the damaged current lead, the modification to this magnet consists in adding a 4 th single stage cryo-refrigerator with high cooling power at 50 K (172 W). The committee feels that this approach should solve the problems of magnet 2. At the moment the operating margin is unknown (Cooling power Heat 4.2K). It should be evaluated with the thermal model. It is essential that the operating margin be measured at the occasion of the magnet 2 cool down and operation, taking advantage of the cold mass heater. From a general point of view this solution is technically acceptable only because the MICE schedule demands this magnet to be operative as soon as possible

Single-stage cooler Thermal link HTS leads

According FT model … This model is not yet completely realistic but the results are confirmed by simpler computations

Fixes to magnet 1. The foreseen fixes to magnet 1 appear to be more substantial because they are aimed to directly solve what appear to be one of the main problems (i.e. the poor thermal conductance between copper plate and shield). The fix envisaged also involves the addition of a single stage cryocooler, much in the same way as for magnet 2, with the possibility to add a 4 th two-stage cryocooler if needed All these fixes seem reasonable and moving towards the rigth direction but the committee recommends to use Frederic Trillaud’s finite element thermal model for a clear understanding of the expected temperature distribution and of the heat 4.2 K

Review the presented schedule for implementing changes and the overall magnet successful delivery schedule. Evaluate possible risks and sources of delays The committee accepts the decision of the MICE LBNL team to proceed with implementation of a single stage cryocooler in magnet 2, as there is significant evidence that this should allow the magnet to operate, and the procurement of a second single stage cryocooler to be installed in magnet 1. The improved instrumentation and additional fixes to magnet 1 aimed at improving the heat transfers are welcome. The committee recommends that the test of magnet 2 be carried out to the point where the magnet operation is demonstrated, the margin of operation determined and the possible boil-off rate measured. Cool down of the magnet should be continued until the temperatures at the HTS leads fall below the operating point of 64K, and until the liquid N2 reservoir is frozen or thermal equilibrium is reached. An important milestone should be reached in January 2010 to decide whether the magnet 2 meets the specifications or not. If not, then a decision will need to be made by the MICE management to rebuild magnet 2, and proceed with magnet 1. In order to help the decisions needed in this negative case it will be very important that the tests of magnet 2 cool down and power up be carried out in such a way as to reveal as much information as possible.