Spectrometer Solenoid Background Info

1. Spectrometer Solenoid Background Info Steve Virostek Lawrence Berkeley National Lab MICE Spectrometer Solenoid Review: Phone Meeting October 5, 2009

2. Summary • Description of the present situation with the spectrometer solenoid magnets • “Magnet 1” is the first magnet assembled and tested (in ’08) • Due to various problems, Magnet 1 was disassembledto make appropriate repairs • To save time, the modifications were implemented first on Magnet 2 which had been wound but not fully assembled • Until recently, Magnet 2 was expected to be commissioned first and sent to RAL (after magnetic measurements at FNAL) as the upstream spectrometer magnet in the MICE channel • Current plans are to improve Magnet 2 quickly and complete re-assembly of Magnet 1 with additional improvements

3. Magnet 1 Issues • Several mechanical errors were discovered after magnet assembly • The internal alignment of the cold-mass with respect to the end flanges was estimated to be incorrect by about 10 mm • The support stand placement was also off and would have interfered one side with the iron disk mounting points • The latter problem was addressed externally, but internal misalignment required that the magnet be disassembled • Before proceeding, we decided to cool and train the magnet • Had the magnet passed its acceptance test, we likely would have accommodated the misalignment some other way, but this was not the case

4. Magnet 1 Issues (cont’d) • After cooling the cold mass down with liquid cryogens, an attempt was made to train the magnet • The training only reached 196 A as compared to the 270 A needed to achieve a specified field of 4 T in the central coil • From measurements and observations, the cryocoolers were not maintaining the LHe level and the thermal shield temperature was about 120 K rather than the specified 80 K • These two issues were due to the thermal siphon line being plugged by frozen nitrogen & inadequate thermal connection between the three cooler first stages and the shield • Also, the pressure rise observed during quench was too high • This was due to the fact that the single vent line was partially blocked by the instrumentation wires within it

5. Magnet 1 Issues (cont’d) • Based on this result, several changes were made to Magnet 2 • The siphon tubes were replaced by a direct connection through the top of the cold mass (less efficient thermally, but eliminates the risk of a plugged siphon tube.) • The shield/cooler connections were replaced w/thicker Al straps in place of the original thin Cu straps, an LN reservoir was added to speed shield cool-down and protect the HTS leads in a power failure, and a 2nd vent line was added • Work proceeded to complete Magnet 2 assembly while starting Magnet 1 disassembly • A portable CMM from LBNL was used to ensure cold mass alignment for Magnet 2 • Appropriate fiducials were added to the vacuum vessel OD

6. Magnet 2 Issues • After completing the described modifications earlier this year, an attempt was made to cool Magnet 2 with cryogens • A blockage developed in the internal fill line • The path of the fill line made it difficult to clear the blockage, and the vendor moved the stinger to a vent line • Continuing the fill process led to a leak in a bellows flange in the 2nd vent line (the one not being used for filling), venting the vacuum space to helium and aborting the cool-down • In response to this problem, the routing for the fill line of Magnet 1 has been changed to avoid sharp bends and thus improve the ability to clear a blockage

7. Magnet 2 Issues (cont’d) • After warming up and venting the magnet, the vent lines were modified by removing the bellows flanges • Since the initial blockage was likely a cool-down procedural issue, a safer and more robust technique for filling the magnet was devised (Bross/FNAL) and which worked well • The next cool-down was completed in ~3 days w/o incident • However, the shield temperature fell slowly to only about 110-115 K at the ends of the cylinder, resulting in added heat flow into the cold mass via the cold-mass supports • The improved thermal connections to the cooler 1st stages & the LN reservoir did not solve the previous shield problems • The coolers are expected to maintain the LHe level after filling; we were losing ~1% of the LHe overnight (unpowered)

8. Magnet 2 Issues (cont’d) • At this point, training began and appeared to be going well • At 238 A (w/all coils in series), one of the HTS leads burned out due to a higher than allowable temp. (at upper end) • The upper lead temperature without current was ~75 K, increasing to ~85 K at >200 A, resulting in failure of the lead farthest from the coolers • The lead problem was a surprise, as it was not noticed in the earlier Magnet 1 tests (no firm explanation for this yet) • It may simply be a monitoring error in the earlier measurements; the feedthroughs themselves are the same ones used before • We are currently thermally testing the feedthroughs and leads in an off line test to see what can be learned

9. Magnet 2 Modification • A sol’n was devised that won’t require magnet disassembly • Our plan to add a single-stage Cryomech AL330 cooler (to arrive 11/4/09) provides 170 W at 55 K and requires modifications only in the “turret” area • Should lower lead temp. sufficiently to train to full current • This mod could remove enough heat to permit the other 3 coolers to maintain LHe level (i.e. closed system); however, the new cooler won’t substantially affect the shield temp. • There was also a problem with the power supply controller where it would ramp up in current but could not ramp down. Modifications to the power supply circuit external to the magnet have been proposed and briefly tested - we believe these will solve the problem

10. Magnet 1 Modification • Due to the Magnet 2 uncertainty, Magnet 1 is being modified to permit the addition of a 4th two-stage cooler • The modification also allows a single-stage cooler to be added for the leads, resulting in up to 5 coolers on Magnet 1 • This configuration will be tried on Magnet 1 if Magnet 2 (with 3 two-stage coolers and a single-stage cooler) cannot maintain the LHe level without external top-up • In addition, the thermal connections to the Magnet 1 shield will be improved through flexible copper-sheet connections between the stage 1 cold heads and the shield body • Also, the thermal conductivity of the shield will be increased locally by the addition of some 1100-series Al cladding to the areas where the cold-mass support intercepts are attached

11. Magnet 2 Fallback • If the addition of a single-stage cooler does not turn Magnet 2 into a closed system, there are two options remaining: • i) Live with the thermal properties as they are and top up the magnet with external cryogens as needed • ii) Disassemble the magnet and modify it to improve the shield thermal connections as well as to accommodate a fourth two-stage cooler (along with the option of keeping the single-stage cooler) • The 2nd option will lead to 3–6 months of additional delay in delivering the 2nd magnet to RAL and will create additional risks of damage or assembly errors in the repair process • For these reasons, the first option is preferred