1. HL-LHC Long term behaviour of magnets Test facility limitations
M. Bajko
23rd of May CERN

2. Table of contents Endurance test type What we check? How do we check
Thermal cycles
Mechanical cycles
What we check?
Electrical integrity
Protection
Performance stability – Magnet training memory
Mechanical stability vs conductor stability
How do we check
On models/prototypes/spares/string
Procedures , criteria, tolerances
Other non technical limitations
Summary
Endurance test – WP3– Marta Bajko

3. Thermal cycles Endurance test type
Between room temperature ( 300 K ) and 1.9 K with control on the thermal gradients
Endurance test type
Thermal cycles
Mechanical cycles
Nr of cycles representatives for the magnet life time: 10. (We plan to do 1 TC for the series)
Possible in all vertical and horizontal test stands: Siegtal, Long, HFM, Cluster D, Cluster A
The thermal gradient in checked with sensors attached or integrated in to the magnets.
Cooling and warming process is different between vertical and horizontal test stands but also between Long or Siegtal and HFM or D. The second once are using He gas for the process injected at the required temperature while the old vertical cryostats uses the evaporation of the LHe injected or present in the vessel. Horizontal benches used He gas injected into the cryostat.
Thermal gradients are influenced by the size of the magnet wrt the size of the cryostat and the use of a magnetic measurement shaft
Time consuming: typically is at least 36 hours depending on the cryostat, the size of the magnet and the occupancy of the rest of the test stands so the availability of cooling and warming capacity
Endurance test – WP3– Marta Bajko

4. Mechanical cycling or current cycling
Up to Imax ( max=nominal or ultimate) of 20 kA
Endurance test type
Thermal cycles
Mechanical cycles
Nr of cycles representatives for the magnet life time:
Possible in all vertical and horizontal test stands: Siegtal, Long, HFM, Cluster D, Clustre A
The current and the strain gauges can be measured and recorded for each cycle
Cycles can not be performed during night for safety reasons
Time consuming: typically if we respect the machine ramp rate , a cycle is about 1h.
Cycling can be speed up with high ramp rates up to 200 A/s and cycling the magnets with > I ultimate if this is acceptable
Protection of the magnet in case of voltage spikes, flux jumps can seriously limit the process as an adaptation of the threshold is needed
Endurance test – WP3– Marta Bajko

5. Checks of electrical integrity
HV withstand level compliant with the specifications
Instrumentation is allowing at least to produce the trigger signal for protection
Thermal cycles
Mechanical cycles
No particular reason to do electrical checks before and after mechanical cycles.
At room temperature and at LH, before and aftre the thermal cycle
Continuity test is done systematically before and after each cycle. No risk associated to it as is with low current and is necessary for the integrity and the safety of the magnet
High Voltage test is done systematically before and after each cycle, BUT AT DIFFERENT LEVEL OF VOLTAGE! This test can be destructive if the procedure is not respected and not adapted to the environment ( temperature, pressure and type of gas). Voltage levels should be reduced after the first cool down to LHe. The number of discharges at high voltage should be limited to the strict necessary. There are ongoing tests to explore possibility of performing HV test in He gas at 200 K 1 bar and some measurements have been already performed in cluster D on a MQXFS magnet. Limit can be the test stand typically the old cryostats designed for max 1000 V between the current leads and ground. The horizontal benches are designed as the HFM and cluster D current leads for 3000 V but depending on the test also connectors should be checked or adapted to the tests.
Endurance test – WP3– Marta Bajko

6. Check of magnet protection integrity
Electrical soundness of the outer layer QH
Electrical continuity and correct connection of the CLIQ bus bars
Thermal cycles
Mechanical cycles
No particular reason to do electrical checks before and after mechanical cycles.
Magnet protection check is done systematically at the beginning of a run
The endurance of their electrical integrity can be checked with a series of discharges in the quench heaters followed by high voltage test
The test can be destructive as typically a degraded quench heaters is only visible when is completely sectioned and can cause a major default both on the magnet and on the cryostat. Therefore systematic continuity test of the quench heater strips is mandatory before every single discharge.
One successful test, in my opinion is not well representative for a series but can qualify a design provided that the strip used is perfect.
CLIQ system should be tested offline for its own endurance. There is no evident mechanism that may lead to the degradation of the inter filament resistance on which the system good functioning is based but is relaying on the trigger signal
Endurance test – WP3– Marta Bajko

7. Check of performance indicators
Max 3 training quenches to Inominal
Attaining the Iultimate
Capability to stay 8 h at Inominal
Stability of strain data measured
Min ( < 3) nr of training quenches to Inominal, x= ? Training quenches to Iultimate.
x h at Inominal
Stability of strain measurements…. but
….are we sure that we are not limited to see only a part of the performance stability?
Image credits
Creator: undefined
Credit: Copyright:solerf/123RF
Information extracted from IPTC Photo Metadata
11 T program gave some new informations (dedicated talk from G. Willering)
A small but visible degradation of performance in 109 short model after successive thermal cycles
A relevant degradation of performance in second prototype after one thermal cycle (so-called hybrid)
A visible degradation after ~350 K hotspot
extract from Ezio’s and Gerard ‘s presentation
We can perform precision measurements (VI), on any of the test stand provided that there is an adequate instrumentation
We can perform RR studies up to 400 A/s depending on the test stand and magnet L.
We can perform training between K but is difficult to know the exact temperature of the coils and most probably they have a thermal gradient
We can perform measurements at 4.2 K where the margins are smaller and degradation if any is easier to see
Endurance test – WP3– Marta Bajko

8. Mechanical stability vs conductor stability
See the slides of Gerard on this subject. Example of diagnostics that is distinguish between mechanical instability or conductor instability, local or more global effects.
Endurance test – WP3– Marta Bajko

9. How do we check endurance?
Do we plan to make endurance test on series? I guess NO
Should we plan endurance test on prototype (length is important?)
We can use well performing model magnets for most of the endurance tests
10 thermal cycles
current cycles to Inominal(or equivalent charge with less cycles and higher current)
HV test ( should we do in gas? which margin to operation, which pressure, which criteria)
HiMiits ( for >350 K is affecting most of the time only locally and the less robust part of a magnet)
Test at 4.2 K ( to K and measure VI before and after an endurance in temperature or current)
RR study ( at 1.9 and 4.2 K but not always easy if the magnet is only checked to Iultimate and training is not finished)
We need procedures before tests to allow us getting the max info out of a tests! ( references, measurements at strategic moments) and acceptance criterias + tolerances.
We should plan for test which maybe not successful or showing limits for which we need diagnostic tools .
Endurance test – WP3– Marta Bajko

10. Other non technical limits of the test stands
SM18 HOSTS test stands for :
Magnets, Links, RF cavities for HL-LHC,
Spares for LHC (magnets, leads, diodes) and
R&D magnets and cavities
HL-LHC IT STRING
SM18 is sharing:
Cryogenics (primary water)
Powering and
Handling capacity
The availability of cryogenics ( LHe or pumping ) and of the powering ( 20 kA PC is covering several cryostats in the vertical benches) maybe a limit of an endurance test
The assumptions for test plan from which the test planning is derived
Every single magnet will be tested at cold and up to Iultimate= 108% Inom in nominal cooling conditions (1. 9K)
Systematic Powering at Inom for 8 h
High Voltage test will be performed according to individual magnet electrical insulation and design values up to 3400 V for 120 s, <20 mA following for each magnet the specified test conditions and values
Magnet protection will be done following the individual design CLIQ, QH, EE
Magnet protection is based on quench detection at mV, verification time of 10 ms and protection simultaneously with all systems, variable threshold detection is foreseen for Nb3Sn magnets
Splice resistances measured individually and < 2 nOHM
Quench localisation with Vtaps and quench antenna
Magnetic measurement with shaft ( need of anticryostat)
A typical test plan is of 7 weeks TC included and max 20 quenches / magnet
Endurance test – WP3– Marta Bajko

11. Planning SM18
Endurance test – WP3– Marta Bajko

12. Summary
DIAGNOSTICS
is the key element that allows us to say something on the endurance test with combined analysis of all different measurements and effects.
INSTRUMENTATION is mandatory unless we are blind or on top of the iceberg .
TIME is working against us when we come to the endurance test of any type.
A WELL DEFINED TEST PLAN for which we agree on the methodology for each type of magnet in function of its specificity and the criterias that we wish to use to qualify a test would be a basic ingredient of an endurance test.
SM18 MAIN LIMITS ARE IN THE NON TECHNICAL ASPECTS: planning , LHe availability. We have developed methods and improved or built test stand allowing to make endurance test
Endurance test – WP3– Marta Bajko