Do the splices limit us to 5TeV – plans for the 2010 run

M. Koratzinos, CERN, Geneva, Switzerland

Abstract

There are about 10000 main circuit splices in the LHC. Each one needs to conform to certain specifications regarding its resistance at cold and the integrity of its surrounding copper stabilizer. We review the results of the (warm) copper stabilizer measurements, the results of the splice measurements at cold and the allowed space for these variables as a function of LHC energy. It emerges that the limiting factor for operating safely at high energies is our knowledge of copper stabilizer resistances. The principal methods at our disposal for addressing this problem are then reviewed.

Splices

Types of splices

The main circuits of the LHC (RB, RQD, RQF per sector) have about 24000 splices. Out of these there are:

10170 interconnect splices and

13796 magnet splices

Magnet splices are protected by the magnet diodes (so that in case of problems they have to deal only with the energy stored in that specific magnet and not the whole string of magnets in the same circuit). Interconnect splices are not protected by diodes and have to deal with the full current of the circuit at all times. The nominal interconnect splice resistance is 300pΩ at cold (essentially the resistance of the solder material, SnAg), whereas at warm the resistance is 10μΩ (the resistance of the copper stabilizer). For the LHC to operate safely at a certain energy, there is a limit to how big a splice resistance can be, both at cold and at warm.

Individual splices are measurable only with invasive methods. Under non-invasive conditions, we are limited by the amount of voltage taps available. What we can measure between successive voltage taps is a busbar segment. A typical bus bar segment for the RB bus contains 2 or 3 splices, whereas a typical bus bar segment for the RQ bus contains 8 splices. Non-invasive methods at non-superconducting temperatures measure the resistance of the splices but also the resistance of the bus bar segment. Busbar excess resistance is the resistance of a busbar minus its nominal resistance.

Measurements at cold

Motivation for measuring splice resistances comes from the 19 September 2008 accident. The calorimetric analysis of test PLI3.a2 done on 15 September 2008 yields the following value for the excess resistance of a splice in the vicinity of magnet C24R3: R 23R3-16R3, excess = 234 ± 15 nΩ. This is three orders of magnitude higher than the nominal value. This prompted us to measure the resistance of as many splices as we could in a variety of ways:

Calorimetry: using the superfluid properties of helium, together with he very small heat capacity of the system at 1.9K, the cryogenic system is able to measure the resistive heat released by a high-resistance splice. Resistive power as low as 1Watt in one cryogenic subsector (16 magnets, 350t of cold mass, 250m long) can be detected using this method. The temperature rise is 1mK/hour/watt. This means that excess resistances of as low as 40nΩ can be detected using this method.

We have also performed (ad hoc) measurements using the old QPS system which covers 6 out of 7 magnet splices (RB) and 6 out of 10 splices (RQ). Using both methods, two significant excess resistances were found:

B16R1 (MB2334) (100nΩ)

B32R6 (MB2303) (50nΩ)

Much more accurate results are expected from the nQPS campaign of 2009/2010.

Using the nQPS system as a series of accurate voltmeters, we can measure the resistance of a bus bar segment to <1nΩ. Hardware commissioning test PLI2.s1 (the so-called “Maya pyramid” test) comprises a series of current plateaus with a top current plateau of 2000A (lasting 30+ minutes) resembling a Maya pyramid. The results are much more accurate than the previous measurements and have yielded no excess resistance above 4nΩ anywhere in the machine. The highest significant splice resistances found above 2nΩ are:

2.87±0.14 (RQ circuit, sector 23)

2.32±0.14 (RB circuit, sector 34)

2.05±0.52 (RQ circuit, sector 34)

It should be noted that an excess resistance at cold of 2nΩ poses no problems under normal operation; however it might suggest a structural problem or a problem with the soldering procedure which might be more serious (See [1]. Therefore, the time evolution of such a resistance would need to be followed closely. Also, the excess resistances found are natural candidates to be checked by the new X-ray tomograph when it becomes operational.

Measurements at warm – non-invasive

A copper stabilizer with no continuity coupled to a superconducting cable badly soldered to the stabilizer limits the maximum safe energy of the machine [2]. Such a bad joint manifests itself as a splice with higher than nominal resistance at warm. Measuring such a resistance is challenging, with only the measurement at 300K for the RB bus being relatively easy. For measuring the RB copper stabilizer resistances at 80K we need to control the temperature and the RRR of the busbar segment to a very good degree. Measuring the RQ at 300K is on the limit of accuracy, whereas measuring the RQ at 80K is very difficult. The table below shows the factors affecting the resistance measured at warm, together with the typical resistance of a segment and of a defect that we are trying to measure. Column 4 shows the effect of a temperature change in the segment by 1K. Column 5 shows the effect of an increase in the cross section of the copper stabiliser, whereas the last column shows the effect of the RRR of the busbar segment increasing from 100 to 150.

Table 1: Factors affecting the resistance at warm

Bus segm. (μΩ)	Typical resis. (μΩ)	Typical defect (μΩ)	Temp. change +1K	Xsection →+50um	RRR 100→ 150
RB@ 300K	2000	50 (2.5%)	7 (0.4%)	12 (0.6%)	7 (0.3%)
RQ@ 300K	11000	50 (0.5%)	40 (0.4%)	85 (0.8%)	40 (0.3%)
RB@ 80K	270	7 (2.5%)	8 (2.9%)	2 (0.6%)	7 (2.6%)
RQ@ 80K	1500	7 (0.5%)	44 (2.9%)	11 (0.8%)	40 (2.6%)

A substantial effort to map all busbar segment resistances was undertaken during the period April to July 2009. Measurements (referred to as Biddle measurements) were taken by hand using an accurate hand-held device in the tunnel in all sectors. The measurements had a 1% accuracy and, predictably, worked o.k. for the RB at 300K (where we were looking for a 2.5% defect), but worked less well for the RQ and at 80K. Five sectors were measured at room temperature and the worst splices were opened up and repaired. Three sectors were measured at 80K (where the precision was insufficient). The table below shows the situation in the five sectors measured at room temperature after the repairs. The last column shows the highest excess resistance in a sector at the 90% confidence level. This is higher than column 4, as the measurement accuracy is of the order of 15 μΩ.

Table 2: recap of Biddle measurements

Circuit/ Sector	Temperature spread (K)	Excess resistance spread	Highest remaining excess resistance	Excess resistance limit 90%CL
A12 RB	1.1	13	37	51
A34 RB	1.9	10	35	47
A45 RB	0.9	17	53	78
A56 RB	0.4	9	20	34
A67 RB	0.6	14	31	48

Measurements at warm – invasive

In addition to the Biddle measurements covering the whole machine, a number of interconnect splices were opened and measured invasively. These measurements are referred to as R16 measurements. Most of these were made following an opening of an interconnect after an indication of a high excess resistance by the Biddle measurements. The worse R16 resistance measured was 70μΩ, where the nominal resistance is 10μΩ. Therefore the highest R16 excess resistance is Rexcess,worst=60 μΩ.

The gamma-ray picture of that specific splice interconnect can be seen in the following picture.

Figure 1: the interconnect with the worst measured R16 resistance

Worst remaining splice

Since reliable information about the resistance of splices exists only for part of the machine, the worst possible splice expected would necessarily be the result of a calculation based on statistical extrapolation. The only reliable Biddle measurements are the RB measurements at 300K (5 sectors).

Worst measured excess resistance RB: 74±15μΩ(A45)

Worst remaining excess resistance RB: 53±15μΩ (A45)

The worst measured R16 measurement is 60±1 μΩ. However, the statistic of the ‘worse splice seen in a sample of a distribution’ is not a particularly robust one. Therefore we have used the following analysis: we have used the sample of the (accurate) R16 measurements to derive the cumulative distribution of excess resistances. This can be seen in the figure below. The two curves (using different assumptions) fit the data rather well. This extrapolation, together with the estimation of what fraction of an unbiased sample of the interconnect splice data the R16 sample represents, gave the following value of the maximum excess resistance for all main circuits at the 90% confidence level: Rexcess = 98μΩ. [3]. Since such a high value was considered unrealistic, the value used for subsequent discussions on the safe energy of the LHC was Rmax ≈ 90 μΩ [4].

Constraints

A. Verweij [2] has updated his calculations of what is the worst splice we can tolerate as a function of energy: For 5.0TeV (energy extraction time constant for the RB 75sec and for the RQ 15sec, RRR=100):

RB: 43 μΩ

RQ: 41 μΩ

For 3.5TeV (energy extraction time constant for the RB 50sec and for the RQ 10sec, RRR=100):

RB: 76 μΩ

RQ: 80 μΩ

If the RRR is 200, we can add 10 μΩ to the above values for both RB and RQ. Conversely, if the RRR in increased from 100 to 160, we gain 0.3TeV per beam.

Options

Regarding our options on increasing our knowledge of the main circuit splices that would allow safe running at a higher energy, I have tried to concentrate on options applicable this year, that is, relatively simple methods that would not need a huge infrastructure effort. I will also assume that the time available for major repairs in the machine this year will be minimal. Therefore I will focus on how we can increase our knowledge without running a major repair campaign. There are three broad categories of options during 2010:

Warm up and measure (with selective repairs) at 300K

Measure splices using low currents (RRR measurements)

Measure splices using high currents (Thermal Amplifier)

Option 1

This is conceptually the simplest method: warm up the three sectors we have not measured at warm, measure the busbar resistances, repair, and cool down again. (At the same time and in the shadow we can repair the 4-5 worse splices in the rest of the machine). The advantages are that this is something we know how to do. On the other hand, this is the option with the longest intervention and heavy re-commissioning will be needed. Another disadvantage is that our knowledge of the RQ bus in the sectors already measured (where we know it is not good enough) will not improve.

The time needed for such an option would be roughly three and a half months.

The probable gain would be that the highest resistance for the RB bus will go down from 90μΩ to 60μΩ.

Option 2

This option is motivated by the fact that currently the RRR measurements of the copper stabilizer of the machine are not very accurate and plagued by systematic errors. This has led us to take the conservative approach to assume a RRR value of 100 for the whole machine. A method has been proposed to measure the RRR with a precision of a few percent using the nQPS system by injecting a low current (20-30A) to the three main circuits of a sector. A type test has been reformed at the end of January 2010.

The advantages of such a method are that it uses low current so it is safe, and the moderate increase of temperature needed (we only need to go to 15K for a measurement) makes the test rather fast.

The disadvantage of this method is that it cannot measure excess resistances. The time needed for such a test would be of the order of 6 days. The possible gain after performing this measurement on all sectors would be to get confidence that the lowest RRR of the machine is higher than the currently assumed 100, allowing either a higher safe energy or more margin at 3.5TeV.

Option 3

This is the most involved method discussed here. It originates from an idea of H. Pfeffer and has been refined by A. Verweij, who also kindly provided the results of his calculations. The idea behind the technique is the following: A high current pulse warms up selectively the bad joints in a circuit “Thermal Amplifier”, in a highly non-linear way. Low current is used for reading out voltages using the nQPS system and hence identify areas where the temperature (a tell-tale sign of high resistance) has increased the resistance of the copper. The method is safe as the high current pulse will be applied in steps (to about 3000A and with a maximum duration of about 10 seconds). The beauty of the method is that it is a before-after measurement, meaning that various parameters that affect the resistance (for example RRR, geometry, etc.) cancel out. Also, the method is sensitive to the highest resistance in a segment, not to the sum of all splice resistances as in the other methods. In the following example, a ‘typical’ defect of 52μΩ has been introduced in the simulation. 3000A is passed from the circuit for 3 seconds. The circuit chosen for this simulation is the RB, and the temperature of the test has been chosen to be 80K. Very little energy is stored in the circuit as the power supply should ignite the diodes, therefore bypassing the high inductance magnets. With such a defect, the temperature of the joint after 3 seconds has already started running away and reaches 320K. After the three seconds, the current pulse of 3000A is switched off, leaving only 40A for the rest of the measurement. In the following figure, the solid line is the voltage of the bus bar segment in the case of no defect. There is a small difference before and after the pulse due to the fact that the bus bar segment warms up a little. However, the rise in voltage is much more pronounced in case of a defect (dashed curve). Actually, the voltage in case of a 52μΩ defect rises sharply and decays (with a time constant of about 5 seconds) to a new level higher than before (this will eventually decay with a much longer time constant). This (medium term) offset is what can be measured using the nQPS system.

Figure 2: Thermal Amplifier method: voltage rise after a current pulse is case of a perfect joint and of a joint with at 52 μΩ defect.

The simulation of the method was run with different values of initial temperatures, plateau durations, and defect sizes. In the case of a 52 μΩ defect and an initial temperature of 80K, the voltage difference (before and after the pulse) is 120uVolt in case of no defect, and 400uVolts in case of a 52μΩ defect. The difference of 280uVolts makes this very easy to detect, as the nQPS system used for reading out the voltages has a noise level of 5-10μV for 10 seconds. If we attempt the same measurement at lower initial temperatures we can go as low as 50K provided we now extend the pulse for 10seconds instead of 3. There our discriminating power (voltage difference of no defect vs voltage difference of 52μΩ defect) is as high as 430uVolts. The method fails to work at 40K, whereas for the RQ bus the effect is a factor of 4 smaller.

The method has a few engineering challenges. An important ingredient is the power supplies needed for such a measurement whose voltage requirements are as follows:

RB at 50K: a power supply able to deliver at least 190 Volts will be needed. This is just above the capabilities of the current RB power supply of 180V/13000A

RQ at 50K: total voltage needed here is 110Volts.

The requirements for the RB are just beyond the voltage available, whereas for the RQ circuit, the RB power supply will be sufficient. A small ‘igniter’ power supply will be needed to open the diodes in the RB case. With a bit of fine tuning the existing RB power supplies might suffice for this option.

The advantages of option 3 are as follows: It is sensitive to the worst splice in the segment. This is particularly interesting for the RQ bus where there are 8 splices in a bus bar segment and all other methods are sensitive to the sum of all excess resistances. It is also quick to perform once the infrastructure is there.

It is not without its disadvantages, however: Interlocking issues due to the high current level would need to be addressed, and there are engineering and integration issues. We estimate that the time needed to perform this method in a sector (a number of sectors can be done in parallel) is three weeks.

The possible gain is that using this method we can find all splices larger than 50μΩ and if none are above, say, 60μΩ this would enable us to run safely between 4-4.5TeV . A type test before more can be said about this method is imperative.

Conclusions

Splices at cold (in the superconducting state) have been measured with excellent accuracy and do not pose a problem.

Splices at warm (an indication of copper stabilizer continuity) have been measured in part of the machine and extrapolated to the whole machine using statistical methods.

worse splice measured: 60±1μΩ

worse splice known to exist in the machine: 53±15μΩ

worse splice extrapolated: 90μΩ

The current knowledge of the interconnect splices excludes 5TeV operation without major repairs after a warm up. Even the operation at 3.5TeV leaves no margin.

Two methods have been proposed to increase our knowledge of the interconnect splices

A low current method that can measure the RRR of the busbars

A high current method (the Thermal Amplifier) that is sensitive to the worst splices in all bus bar segments

Using any of the above methods would allow us to either run at a slightly higher energy (around 4TeV) or get a bigger margin at 3.5TeV.

References

[1]	P. Fessia, status of splices in 13kA circuits, these proceedings.

[2]	A. Verweij, minimum requirements for 13kA splices, these proceedings.

[3]	M. Koratzinos et al, Measurements of the resistance of main circuit busbar segments at the LHC, TE report to be published.

[4]	A. Siemko, Splice resistance measurements on Dipole and Quadrupole circuits and worst case estimates, LMC 5/8/2009.