Quench Protection Study of a 11 T Nb 3 Sn Model Dipole for the High Luminosity LHC

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1 3LPo2E-05 [L44] 1 Quench Protection Study of a 11 T Nb 3 Sn Model Dipole for the High Luminosity LHC Susana Izquierdo Bermudez, Franco Mangiarotti, Gerard Willering, Marta Bajko, Luca Bottura, Jose Ferradas Troitino, M. Guinchard, Frederic Savary, Giorgio Vallone. Abstract The planned upgrade of the LHC collimation system requires the installation of 11 T Nb3Sn dipole magnets in the dispersion suppressor areas. Due to the large stored energy density and the low copper stabilizer section, the quench protection of these magnets is particularly challenging. The baseline protection scheme after installation in the main dipole circuit is based on quench heaters and a bypass diode. The maximum allowable temperature during quench has a primary importance. In one of the latest short model magnets, full protection studies were performed up to a quench integral and hot spot temperatures well beyond the design value in order to understand the limits. Measurements are compared to electrotransient and thermo-mechanical models to evaluate quench propagation, temperature rise in the conductor and thermal stress due to temperature gradients in the coil and surrounding structure. Index Terms High Luminosity LHC, Quench Protection, High Field Nb 3Sn magnet. T I. INTRODUCTION HE High Luminosity upgrade of the Large Hadron Collider (HiLumi-LHC) envisages the replacement of two 8.3 T main bending (MB) dipoles with shorter 11 T Nb 3Sn magnets delivering the same integrated strength and making room for additional collimators [1]. The protection of these magnets is particularly challenging due to the high stored energy density (125 J/cm 3 ) and the high current density in the copper stabilizer (770 A/mm 2 ) [2],[3]. Upon quench detection, the stored magnetic energy has to be quickly dissipated in the thermal mass of the magnet itself, while the current decreases as the magnet resistance increases, in a freewheeling rampdown process through a by-pass diode. The time available for quench detection and active quench initiation through the quench heaters is a few tens of ms (30 to 40 ms). The protection system of the 11 T magnet was designed such that the maximum temperature after quench is below 350 K, which is considered as a safe limit for high field Nb 3Sn Manuscript receipt and acceptance dates will be inserted here. This work was supported by the High Luminosity LHC Project at CERN. (Corresponding author: S. Izquierdo Bermudez) Susana Izquierdo Bermudez, Franco Mangiarotti, Gerard Willering, Marta Bajko, Luca Bottura, Jose Ferradas Troitino, M. Guinchard, Frederic Savary, Giorgio Vallone are with CERN, 1211 Geneva, Switzerland. ( susana.izquierdo.bermudez@cern.ch) Color versions of one or more of the figures in this paper are available online at Digital Object Identifier will be inserted here upon acceptance. TABLE I CONDUCTOR AND COIL PROPERTIES Coil Strand lay out Cu/SC Coil Resistance (293 K), mω 116 RRP 150/ RRP 150/ RRR 293K/20K Fig. 1. Cross section of half coil, showing the magnetic field at nominal current, ka (left). Coil pack geometry, including the strain gauge location (right). magnets to avoid magnet degradation [4]. A set of high temperature tests were performed in one of the latest 11 T single aperture short model magnets, MBHSP106 [5], in order to understand the limits and operational margin. This paper presents an analysis of these tests. II. MAGNET PARAMETERS AND FEATURES 11 T coils are made with a Rutherford-type cable composed of 40 strands of 0.70 mm diameter. The cable insulation consists of a C-shaped mica foil folded around the cable, and braided S-2 11-Tex glass fibre. The total insulation thickness is 100 μm. Each coil consists of 56 turns, 22 in the inner layer and 34 in the outer layer, which are wound, reacted and impregnated together using CTD-101K epoxy resin. Most relevant quench protection parameters for the coils assembled in MBHSP106 are summarized in Table I. Coils are surrounded by ground insulation made of 5 layers of 125 μm thick polyimide foils, stainless steel protection shells and laminated collars. The collared coil assembly (see Fig. 1, right) is placed inside a vertical split yoke and two stainless steel half shells. The welding of the half shells compress the yoke around the collars providing a rigid support structure [6]. The quench protection system of the magnet relies on quench heaters attached to the outer coil surface, covering around 50 % of the coil turns [3]. They are composed of 25- μm stainless steel strips bonded to a 50-μm layer of polyimide in the so-called trace. The trace is positioned on outer surface Template version 8.0c, 7 August IEEE will put copyright information in this area See for more information.

2 Magnet current [ka] Iss (1.9 K) = ka 9 ms/14.8 MA 2 s 17 ms/15.5 MA 2 s Protection delay[ms]/ QI [MA 2 s] 35 ms/15.7 MA 2 s I/Iss = 91 % I/Iss = 89 % ms/13.7 MA 2 s 3 ms/14.1 MA 2 s 40 ms/15.3 MA 2 s ms/13.8 MA 2 s 27 ms/15.7 MA 2 s 6 ms/14.7 MA 2 s 22 ms/16.1 MA 2 s 12 ms/15.2 MA 2 s 40 ms/ 17.5 MA 2 s 20 ms/15.7 MA 2 s 30 ms/15.9 MA 2 s Quench number Training HT natural quench HT InL provoked QI study Fig. 2. Quench history of MBHSP106 for the second and third thermal cycle at 1.9 K. of the reacted coil, and it is covered by a layer of glass before coil impregnation. In order to increase the coil volume heated by the protection strips, inter-layer protection heaters were installed in the coils assembled in MBHSP106. The stainless steel strips are placed during winding, and insulated from the coil turns with one layer of mica and two layers of S2-glass per side. The total thickness of the quench heater assembly is 0.5 mm, which is identical to the nominal layer-to-layer insulation of the 11 T coils. Electrical tests after impregnation in a practice coil demonstrated good inter-layer heaters to coil insulation up to 8 kv. In spite of the good electrical performance of the heaters after coil fabrication, two out of the four interlayer heater circuits failed during cool down and powering [7]. III. QUENCH PERFORMANCE The short models of the 11 T magnet are typically trained at 1.9 K using a 60-mΩ dump resistor to decrease the energy deposited in the bath allowing a faster recovery. MBHSP106 is a particular case, since the first training campaign was done at 4.5 K due to test station availability reaching a maximum current of ka, which corresponds to 85 % of the short sample limit [5]. In the second and third thermal cycle, the magnet was re-trained at 1.9 K and a set of high temperature (HT) tests were done in order to explore the limits in terms of maximum allowable temperature during quench. Every high temperature test was followed by a standard training quench dumping part of the energy on the external resistor in order to assess the magnet performance reproducibility and possible detraining effects. Figure 2 shows the quench history for the second and third thermal cycles of MBHSP106 at 1.9 K. The plot includes for each HT test the protection delay and quench integral (defined as the integral over time of the square of the magnet current). After an initial training and a set of quench integral (QI) tests, HT natural quenches were done. During these tests, the magnet is ramped to quench, protected only with outer layer quench heaters. Every step, the firing of the heaters is delayed Fig. 3. Measured quench integral as a function of the magnet current for high temperature natural quenches. by 3 ms up to a maximum of 40 ms. As it can be seen in Fig. 2, the quench current progressively increased. In all the cases, the quench origin is in the block 3 of the inner coil layer (see Fig. 1). The quench current increased by 200 A after the quench with 22 ms delayed protection. A thermal cycle was done after this quench in order to assess the training memory. The first quench after the thermal cycle was 80 A higher than the maximum current previously reached. After two provoked quenches (QI study), HT test with ms delayed protection were done. Only the HT quench with 35 ms delayed protection caused a detraining of 3 % with respect to the maximum quench current. The detraining was recovered after one standard quench. Due to the fast quench propagation and resistive growth of the initial quench zone for the HT natural quenches, the current is dumped before the firing of the heaters. This translates in a marginal increase of the quench integral for an increase of the protection delay from 22 ms to 40 ms (see Fig. 3). The maximum quench load achieved with HT natural quenches was 16 MA 2 s. For the case of 40 ms delayed protection, 60 % of the quench load is dissipated before the firing of the heaters. In order to reach larger quench load, HT Inter Layer (IntL) provoked quenches were done at nominal current. For these tests, a quench is provoked at nominal current with one of the inter-layer quench heaters, using the minimum required power to start a quench. Upon quench detection, outer layer heaters are fired to protect the magnet. Every step, the firing of the heaters is delayed by 10 ms. Validation quenches showed 2 % of permanent degradation after the HT IntL provoked quench at nominal current with 40 ms delayed protection and no degradation up to a protection delay of 30 ms. A. Hot Spot Temperature IV. ANALYSIS Due to the lack of specific instrumentation, the hot spot temperature at the initial quench location is not available experimentally. Therefore, the temperature is derived from the measured current using the adiabatic approximation under the following assumptions:

3 3 Coil 116 (OL) Coil 116 (IL) Coil 117 (IL) Coil 117 (OL) Fig. 4. Adiabatic estimate of hot spot temperature as a function of the quench integral for the HT test. Continuous line shows the computed adiabatic cable temperature assuming a constant field of 12 T and RRR = 100. Fig. 5. Derived average temperature from voltage measurements for each instrumented coil segment and computed 2D temperature map (I mag = ka; Protection delay = 40 ms) The conductor specific heat is calculated as a weighted average of all conductor components (Nb 3Sn, Copper, Epoxy and G10). In order to account for the transposed cable geometry of the Rutherford cable, the material fraction are computed using the amount of each component in a differential volume of the cable. NIST database [8] has been used for all material properties. The average coil Residual Resistivity Ration (RRR) was measured during the magnet test (see Table I), but the specific RRR of the quenching segment is not available. Therefore, the analysis is performed using the average coil RRR. A ± 20 % variation on the RRR has an impact of ± 15 K on the hot spot temperature calculation. Fig. 6. Measured temperature of the pole turn as a function of the quench integral for the HT test. Continuous line shows the computed adiabatic cable temperature assuming a constant field of 12 T and RRR = 100. B is the magnetic field, RRR is the Residual Resistivity Ratio, l is the segment length, and A Cu is the cross sectional area of The peak field in the conductor where the quench starts copper in the cable. At the end of the decay, the magnetic field is considered, and it is scaled with the measured magnet current to account for the copper magneto- is zero and the temperature of the cable can be derived by inverting Eq. 2. Figure 5 shows an example of the measured average temperature based on resistive voltages for each instruresistivity. If instead of the peak, the average field is considered, the hot spot temperature is K lower. mented coil segment at the end of the current decay for the HT Figure 4 compares the adiabatic temperature of the cable assuming a constant field of 12 T to the inferred hot spot tem- IntL provoked quench at nominal current with a protection delayed by 40 ms. Voltage measurements give an average equivalent temperature based on the length between the voltage perature as a function of the quench integral. Under the considered assumptions, a hot spot temperature of 420 K did not taps, which is below the actual peak temperature. The 2D cause any performance degradation. A 2 % degradation on the computed temperature map using ROXIE quench model [9] is load line is experienced when the maximum temperature during quench reaches 480 K. also shown in the plot. The computed hot spot temperature is 480 K, consistent with the adiabatic estimate. In all high temperature test, the quench starts in block 3 (see B. Voltage Measurements Fig. 1). Since each conductor of block 3 is not individually monitored with voltage taps, a measurement of the local hot The coils are instrumented with a series of voltage taps spot temperature based on resistive voltages is not available along their winding. The voltage difference between consecutive pairs of voltage taps (u) is measured during quench, which experimentally. Only pole turn and mid-plane turns are individually monitored, with six voltage taps per turn. The quench is composed of an inductive (u i) and resistive (u r) voltage: load deposited in the pole turn is 2.5 to 4.5 MA 2 s lower since u = u i + u r = L di/dt + R I (1) the quench needs ms to propagate to the pole turn. Nevertheless, these measurements can be used to validate the assumptions outlined in Section A for the adiabatic estimation of where L is the inductance of the measured segment, I is the magnet current and R is the resistance, that depends on the the hot spot temperature. Figure 6 shows the temperature of temperature (T) through the resistivity of the copper stabilizer the pole turn for a 60 cm length segment derived from resistive voltage measurements for all high temperature tests as a η Cu. R=η Cu(T, B, RRR ) l/a Cu (2) function of the quench (2) load deposited in the specific segment.

4 4 Fig. 7. Measured bulk temperature of the coil as a function of the magnetic energy density, compared to the enthalpy of the composite coil. Fig. 9. Stress evolution on the collar nose during powering. Fig. 8. Measured and computed stress in the collar nose for the different assembly steps in MBHSP106, compared to the coil stress. The continuous line corresponds to the computed adiabatic temperature assuming a constant field of 12 T. A good agreement is found, with a difference between experimental values and adiabatic estimates of K. Figure 7 compares the temperature of the coils inferred from voltage measurements as a function of the energy density with the enthalpy of the composite coil (including conductor insulation). The agreement is excellent showing that the whole magnetic energy stored in the magnet is converted into heat, and causes a corresponding increase in the enthalpy of the coils through the current dump, in a more or less adiabatic process with negligible heat exchange to the bath and surrounding structure. V. STRAIN MEASUREMENTS The coil stress is indirectly monitored measuring the strain on the collar nose (see Fig. 1). A total 12 collars are instrumented with half-bridge strain gauges measuring the bending and compression stress [10], [11]. Figure 8 shows the evolution of the measured collar stress at the different assembly steps and after cool down for MBHSP106. The error bars represent the standard deviation of the measurements across the different locations. Measurements are compared to the Finite Element (FE) model and to the expected coil stress. At the maximum collaring force, when the keys are inserted, the stress on the collar nose is close to 150 MPa, which corresponds to a coil stress of 80 MPa. After releasing the collaring press, the stress in the coil is 50 MPa. Due to the differential thermal contraction, the stress in the coil decreases to 30 MPa Fig. 10. Measured collar compression stress during powering as a function of the square of the normalized current for one of the instrumented collars. Fig. 11. Measured change on the stress level at the beginning of the powering cycle as a function of the quench integral of the previous quench. after cool down. In order to match the measured change of stress during cool down, the integrated coil contraction in the FE model had to be increased from 3.6 mm/m [10] to 4.2 mm/m. During powering, as the magnet current increases, the electromagnetic forces pull the coil from the pole, with a decrease of the stress measured on the strain gauge location. With the level of pre-stress applied in MBHSP106, the contact pressure between coil and pole is expected to be close to zero for a magnet current of 6-8 ka. After this level, the stress does not vary linearly with the electromagnetic forces anymore. During a quench, the current decays within few milliseconds, with a sudden decrease in collar nose stress (ΔQuench in Fig. 9). The evolution of the collar stress during subsequent ramps was analyzed for the different types of quenches performed in MBHSP106. As shown in Fig. 10, there is not a significant change on the initial stress level or the stress evolution as

5 5 TABLE II HOT SPOT TEMPERATURE AT INOM FOR SHORT MODEL MAGNETS BASED ON THE QUENCH INTEGRAL TEST. Magnet Cu/SC RRR 293K/20K QI* [MA 2 s] T Adiab [K] DP SP SP SP *Assumes 15 ms for quench detection and validation and 4 ms of heating firing delay Fig. 12. Measured on the change of compression stress on the collar nose as a function of the coil average temperature at the end of the quench. function of the electromagnetic forces for the different types of quenches. Figure 11 shows the lack of correlation between the energy deposited in the coil and the initial stress level for the following quench. The reduction of azimuthal preload in the quenching coil during high temperature quenches observed in TQS01c [12] is not visible in the 11 T measurements. During a quench, the current quickly decays and the conductor heats with negligible heat exchange to the magnet structure. In order to compute the thermal stress, the 2D conductor temperature map computed in ROXIE is imported to ANSYS to evaluate the stress. The maximum stress occurs at the end of the current decay, and it is mainly dependent on the bulk temperature of the coil. Figure 12 compares the expected change of the stress on the collar nose during quench (ΔQuench in Fig. 10) as a function of the average temperature of the coil at the end of the decay with the measured value. In the case of the MBHSP106, the maximum stress during quench is lower than the peak stress during collaring. VI. DISCUSSION Quench integral tests at different current levels are systematically done in all 11 T magnets in order to study the current decay, quench propagation and resistance build up in the magnets. During these tests, the magnet current is ramped to a given level and a quench is provoked by triggering all the outer layer quench heaters with nominal parameters [3]. The measured current decay can be used to estimate the maximum temperature in case of a quench in the inner layer pole turn assuming a certain delay for detection and validation. Table II summarizes the expected hot spot temperature assuming 15 ms for quench detection and validation and 4 ms of heater firing delay. The maximum inferred temperature in MBHSP107, the only magnet built with the final conductor (RRP 108/127) and heater lay-out (heaters impregnated) is 320 K, 160 K lower than maximum temperature in MBHSP106, responsible of 2 % degradation on the load line. The temperature is 10 K higher for MBHSP106 and MBHSP105 due to the lower copper content. Due to the large variability of RRR in the coils assembled in MBHDP101, the hot spot temperature will range from 280 K to 330 K depending on the initial quench location. VII. CONCLUSIONS High temperature tests in MBHSP106 showed 2 % of permanent degradation after a quench at nominal current with 40 ms delayed protection. The computed conductor temperature for this quench is 480 K, which is above the glass transition temperature of CTD-101K epoxy (386 K). Due to the lack of specific instrumentation, temperature measurements on the actual hot spot location are not available experimentally and have been derived based on the adiabatic approximation. The local voltage measurements of the pole turn have been used to validate the adiabatic assumptions. The hot spot temperature expected during quench is 320 K at nominal current and 340 K at ultimate. High temperature tests on MBHSP106 showed no performance degradation up to a peak temperature of 420 K. The strain gauges in the collars do not show a reduction of azimuthal preload following a high temperature test. No signs of degradation of the magnet insulation scheme were observed and visual inspection on the coils will be performed after magnet disassembly to identify signs of epoxy softening and redistribution at the quench start location. VIII. ACKNOWLEDGEMENTS The authors thank the technical staff for contributions to magnet design, construction and test. They are also grateful to Ezio Todesco and Paolo Ferracin for the useful discussions. IX. REFERENCES [1] G. Apollinari, I. Bejar Alonso, O. Brüning, M. Lamont, and L. Rossi, High-Luminosity Large Hadron Collider (HL-LHC): Preliminary Design Report, CERN Yellow Reports Monographs, CERN, Geneva, [2] S. Izquierdo Bermudez, et al., Quench Protection Studies of the 11T Nb 3Sn Dipole for the LHC Upgrade, IEEE Transactions on Applied Superconductivity, vol. 26, no. 4, , [3] S. Izquierdo Bermudez, et al., Quench Protection of the 11 T Nb 3Sn Dipole for the High Luminosity LHC, IEEE Transactions on Applied Superconductivity, vol. 28, no. 3, , [4] G. Ambrosio, Maximum allowable temperature during quench in Nb3Sn accelerator magnets, in Proc. Workshop Accelerator Magnet, Supercond., Des. Opt., CERN Yellow Report CERN , pp.43-46, DOI , arxiv: [5] G. 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