For the high luminosity upgrade of the Large Hadron Collider (LHC), it is planned to replace the existing triplet quadrupole magnets with Nb₃Sn quadrupole magnets, which provide a comparable integrated field gradient with a significantly increased aperture. These magnets will be powered through a novel superconducting link based on MgB₂ cables. One option for the powering layout of this triplet circuit is the use of cryogenic bypass diodes, where the diodes are located inside an extension to the magnet cryostat and operated in superfluid helium. Hence, they are exposed to radiation. For this reason the radiation hardness of existing LHC type bypass diodes and more radiation tolerant prototype diodes needs to be tested up to the radiation doses expected at their planned position during their lifetime. A first irradiation test is planned in CERN's CHARM facility starting in spring 2018. Therefore, a cryo-cooler based cryostat to irradiate and test LHC type diodes in-situ has been designed and constructed. This paper will describe the properties of the sample diodes, the experimental roadmap and the setup installed in CHARM. Finally, the first measurement results will be discussed

Bernhard, A.

Denz, R.

D’Angelo, G.

Favre, M.

Hagedorn, D.

Kirby, G.

Koettig, T.

Monteuuis, A.

Mueller, A. -S.

Rodriguez-Mateos, F.

Siemko, A.

Stachon, K.

Valette, M.

Vammen Kistrup, L.

Verweij, A.

Will, A.

Wollmann, D.

English

Will, Andreas

Bernhard, Axel

D'Angelo, Giorgio

Denz, Reiner

Favre, Mathieu

Hagedorn, Dietrich

Kirby, Glyn

Kistrup, Lucas

Koettig, Torsten

Monteuuis, Arnaud

Mueller, Anke-Susanne

Rodriguez-Mateos, Felix

Siemko, Andrzej

Stachon, Krzysztof

Valette, Matthieu

Verweij, Arjan

Wollmann, Daniel

CERN Document Server

Experimental Setup to Characterize the Radiation Hardness of Cryogenic Bypass Diodes for the HL-LHC Inner Triplet Circuits

Mueller, A.-S.

KITopen

EXPERIMENTAL SETUP TO CHARACTERIZE THE RADIATIONHARDNESS OF CRYOGENIC BYPASS DIODES FOR THE HL-LHCINNER TRIPLET CIRCUITS∗†A. Will‡1,2, A. Bernhard2, G. D’Angelo1, R. Denz1, M. Favre1, D. Hagedorn1, G. Kirby1, T. Koettig1,A. Monteuuis1, A.-S. Mueller2, F. Rodriguez-Mateos1, A. Siemko1, K. Stachon1,M. Valette1, L. Vammen Kistrup3, A. Verweij1, D. Wollmann11 CERN, 1211 Geneva 23, Switzerland2 Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany3 KEA Copenhagen School of Design and Technology, Copenhagen, DenmarkAbstractFor the high luminosity upgrade of the Large HadronCollider (LHC), it is planned to replace the existing tripletquadrupole magnets with Nb3Sn quadrupole magnets, whichprovide a comparable integrated field gradient with a signif-icantly increased aperture. These magnets will be poweredthrough a novel superconducting link based on MgB2 cables.One option for the powering layout of this triplet circuit isthe use of cryogenic bypass diodes, where the diodes are lo-cated inside an extension to the magnet cryostat and operatedin superfluid helium. Hence, they are exposed to radiation.For this reason the radiation hardness of existing LHC typebypass diodes and more radiation tolerant prototype diodesneeds to be tested up to the radiation doses expected at theirplanned position during their lifetime. A first irradiation testis planned in CERN’s CHARM facility starting in spring2018. Therefore, a cryo-cooler based cryostat to irradiateand test LHC type diodes in-situ has been designed andconstructed. This paper will describe the properties of thesample diodes, the experimental roadmap and the setup in-stalled in CHARM. Finally, the first measurement resultswill be discussed.INTRODUCTIONHL-LHC Inner Quadrupole TripletThe replacement of the inner triplet quadrupole magnetsin interaction point IP1 and IP5 of the LHC by Nb3Sn mag-nets in the High Luminosity Large Hadron Collider projectrequires a redesign of the powering scheme. In order to feedcurrents up to 18 kA to the triplet, MgB2 cable based super-conducting links will be installed, connecting the magnetsin the LHC tunnel with the power converters in the new radi-ation free underground cavern [1, 2]. The powering schemeincludes bypass diodes for magnet protection in case of aquench [3]. One option would use cryogenic bypass diodes,as shown in Fig. 1, located in the magnet cryostat, close tothe beam axis and immersed in liquid helium. As opposedto diodes located in the new cavern, this option would avoid∗ Work supported by the Wolfgang Gentner Programme of the GermanFederal Ministry of Education and Research.† Work supported by the HL-LHC project.‡ andreas.will@cern.chFigure 1: Picture and schematics (not to scale) of cryogenicbypass diode as used in LHC and potentially to be used inHL-LHC.large over-currents through the superconducting link in caseof failures.On the other hand they will be exposed to high radiationdoses, leading to a potential degradation of the diode charac-teristics, which has to be quantified. The integrated radiationdose at the location of the cold diodes is estimated to reachup to 30 kGy and 1015 n/cm2 over the HL-LHC lifetime [4].Previous Qualification CampaignsAs known from previous qualification campaigns, expo-sure of diodes to radiation leads to a rise of the forwardbias voltage, potentially up to an open circuit in the worstcase. During qualification campaigns for the LHC, diffu-sion type diodes supplied by Dynex1 with a base widthof around 10 µm had been tested up to 2 kGy respectively3 · 1013 n/cm2 [5, 6]. The same type of diode had later beenqualified to doses up to 1.2 kGy and 2.2 · 1014 n/cm2 to beused at FAIR [7], however not radiation exposed at 4 K, forlower maximum currents and with intermediate warm-ups.New Prototype DiodesLHC type diodes and two new prototype diodes, alsoproduced by Dynex, are currently being tested and qualifiedfor the potential use in the HL-LHC inner triplet. The twonew prototypes have base widths of 0 ± 5 µm and -10 ±5 µm. The radiation hardness is expected to increase withdecreasing base width, while on the other hand the reverseblocking voltage is decreasing.1 Dynex Semiconductor Ltd (Lincoln, GB)9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPMG006WEPMG0062620ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.07 Accelerator TechnologyT31 Subsystems, Technology and Components, OtherFigure 2: Picture of the cryostat setup demonstrating theassembly and placement of the diodes.CHARM Irradiation FacilityThe radiation tests will be performed at the CERN high-energy accelerator test facility (CHARM), where 24 GeV/cprotons, extracted from CERN’s Proton Synchrotron (PS),are impacting a metal target to create a mixed radiationenvironment [8]. A new irradiation position close to thefacility’s target and below the beam axis has been evaluatedfor this purpose. The estimated dose on the diodes at thisposition is 0.5 kGy and 1013 n/cm2 per week [9]. Within thisyear’s irradiation period, it is aimed to achieve a total doseof 16 kGy and >3.2 · 1014 n/cm2.CRYOSTAT DESIGNThe diodes should be tested in-situ during the irradiationperiod without intermediate warm up. The temperatures ofmain interest to perform the measurements are 4.2 K and77 K. This allows a comparison to previous measurements,performed in liquid helium and liquid nitrogen. However,the characteristics at other temperatures is of interest as well.Furthermore, as access to the facility is very limited, the sys-tem has to be remotely controlled, with an effective distanceof ≈ 40 m from the control room to the cryostat inside theirradiation area, following a concrete cable chicane.Therefore, a two stage pulse tube cryocooler-based cryo-stat has been designed, which can be operated with the firststage at 77 K, and the second stage at 4.2 K simultaneously.Furthermore, both stages allow a wider temperature rangeto be set using heaters on both stages.Figure 2 shows the opened setup without the vacuumvessel and thermal shield. The two stacks of four diodeseach are attached to the two stages. The stacks are equippedwith two diodes with -10 µm base width on the positionclosest to beam (D3+D4 and D5+D6), one diode with 0 µmbase width below (D7, D2) and one LHC type referencediode at the bottom (D1, D8), with the largest distance tothe beam.Each diode is equipped with two voltage and two currentleads, allowing individual four-point measurements. Thediodes on the stack attached to the 77 K stage are connectedin series; two current leads allow to apply current pulses offew ms duration with amplitudes up to 18 kA. The diodes onthe stack attached at the 4 K stage are insulated from eachother.There are copper spacers between the diodes, acting asheat sinks during the measurements. The temperatures arecontinuously monitored at each of the cryo-cooler’s stageplates as well as on three heat sinks within each stack. Thetemperature of each diode is, therefore, known with a preci-sion of ±0.2 K.Commissioning of the CryostatDuring the commissioning of the cryostat system, an equi-librium temperature of the first stage of 33 K was reached.The diode stack attached to this stage converges to a tem-perature range of 43.6 K for D4 to 44.3 K for D1, due to theadditional heat load from the high current leads, which aredirectly connected to the stack. The second stage convergedto a temperature of 2.9 K, the diode stack attached to thisstage achieved a temperature range of 3.5 K for D8 and 3.8 Kfor D5. Both temperatures give a sufficient margin in coolingpower and allow the diode temperatures to be set accurately,using the stage heaters with feedback from the temperaturesensors on the stacks.In-Situ Measurements during IrradiationFour quantities will be measured to evaluate the degrada-tion of the diode characteristics as a function of absorbeddose, three of them can be recorded on both stacks at allavailable temperatures:• The turn-on voltage Vto, by applying a triangular volt-age pulse with a defined voltage ramp rate and simul-taneously measuring the current through and voltagedrop over the diode. The voltage ramp rate is matchedto the ramp rates observed in the LHC main dipole mag-nets during a quench, which is in the order of 50 V/s.Voltage ramp rates up to 10 kV/s are possible.• The reverse bias voltage Vrev up to a reverse bias currentof 1 mA.• The capacitance using an LCR meter and an excitationvoltage amplitude below Vto.On the 77 K stack, it is possible to additionally test thefull forward characteristics of the diodes V f (I) as a functionof the applied current up to 18 kA. This is done step-wise9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPMG00607 Accelerator TechnologyT31 Subsystems, Technology and Components, OtherWEPMG0062621ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Figure 3: Turn-on voltage measured on D5 (N-base width-10 µm) for different voltage ramp rates at 4.5 K.by applying half-sinusoidal current pulses of varying ampli-tudes with a pulse width of few milliseconds and measuringthe voltage drop across each diode in the stack.FIRST MEASUREMENTSAfter the successful commissioning of the cryostat in thefacility, before the irradiation had started, a first full charac-terization of the diodes in the setup has been performed.Turn-On VoltageAt 77 K, the temperature rise during a measurement dueto ohmic losses is expected to be negligible. However theheat capacity of one diode at 4.2 K, consisting mainly ofcopper, is in the order of 0.5 J/K. The deposited energy dueto the measurement can lead to a measurable temperaturerise of the diode and the adjacent heat sinks, even for smalland short DC currents. The results of the turn-on voltagemeasurements at 4.5 K for various voltage ramp rates rangingfrom 100 V/s to 10 kV/s are shown exemplarily for DiodeD5 in Fig. 3. The heating for the slower voltage ramp ratesleads to a lower measured forward voltage. To determine theturn-on voltage it is therefore essential to choose the fastestavailable ramp rate to minimize this effect.Forward Bias CharacteristicsDue to the inductance of the long power cables from thepulse generator to the cryostat, the current pulse is signifi-cantly broadened, leading to a pulse-width of up to 3 ms. Theforward characteristics at 77 K and 298 K were measuredand are shown in Figure 4. All the diodes behave similarly.Temperature Rise Due to RadiationnDuring the irradiation of the diodes, i.e. when beam is im-pacting the CHARM target, a temperature rise of the secondstage stack up to a peak temperature of 5.9 K was observed.Thermal simulations of the setup in COMSOL2 showed this2 COMSOL Multiphysics® 5.3Figure 4: Characteristics of the diodes D1-4 on the 77 Kstack for different measured temperatures.to be in agreement with the heat load expected by the ra-diation. As the stack’s connection plate to the interface ismade from stainless steel, a rather bad thermal conductor atthese low temperatures, the heat load to the stack of few tensof milliwatt due to radiation is sufficient to lift the stack’stemperature to a higher equilibrium level. However, thediode annealing temperature is far above the observed value,therefore this will not affect experimental goals. The mea-surements will be performed during the weekly shutdown,where heating due to irradiation is not an issue, allowing thecharacterization of the diodes at the desired temperature of4.2 K.CONCLUSIONThe commissioning of a cryostat system to be used forin-situ qualification of cryogenic by-pass diodes under theinfluence of radiation has been performed successfully. Intotal, eight diodes of three different types were installed tobe qualified during the 2018 irradiation period. The charac-teristics for currents up to 18 kA at room temperature and77 K, as well as the characteristics up to 1 A at 4.5 K couldbe measured before the start of the irradiation. The setup iscurrently waiting to be installed permanently into CHARM,in the coming weeks.ACKNOWLEDGEMENTSWe would like to thank Dynex Semiconductor Ltd. forsupplying the new prototype diodes, as well as ScientificMagnetics for the mechanical design and construction of thecryostat. Furthermore we would like to thank S. Danzecaand J. Lendaro for the support in CHARM as well as A.Infantino and M. Krawina for the FLUKA simulations.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPMG006WEPMG0062622ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.07 Accelerator TechnologyT31 Subsystems, Technology and Components, OtherREFERENCES[1] A. Ballarino, “Development of superconducting links for theLHC machine”, Superconductor Science and Technology, vol.27, 044024 (2014), DOI: 10.1088/0953-2048/27/4/044024[2] “High-Luminosity Large Hadron Collider (HL-LHC): Tech-nical Design Report V. 0.1”, edited by G. Apollinari, I. Be-jar Alonso, O. Bruning, P. Fessia, M. Lamont, L. Rossi, andL. Tavian, CERN, Geneva, Switzerland, CERN-2017-007-M,CERN Yellow Reports: Monographs, 2017.[3] S. Yammine and H. Thiesen, “HL-LHC inner triplet poweringand control strategy”, in Proc. IPAC’17, Copenhagen, Den-mark, May 2017, paper WEPVA116.[4] R. Garcia Alia, “Updated FLUKA results for cold diode inHL-LHC inner triplet”, CERN, Geneva, Switzerland, Internalnote, Feb. 2018.[5] D. Brown et al., “Cryogenic testing of high current by-passdiode stacks for the protection of the superconducting mag-nets in the LHC”, CERN, Geneva, Switzerland, CERN-LHC-Project-Report 686, Jan. 2004.[6] R. Denz, A. Gharib, and D. Hagedorn, “Radiation resistanceand life time estimates at cryogenic temperatures of series pro-duced by-pass diodes for the LHC magnet protection”, CERN,Geneva, Switzerland, CERN-LHC-Project-Report 688, Jan.2004.[7] E. Floch et al., “Irradiation of bypass diodes up to 2.2E14Neutron/cm2 and 1.3 kGy for the FAIR project”, IEEE Trans-actions on applied superconductivity, vol. 17, no. 2, Jun. 2007.[8] A. Thornton, “CHARM facility test area radiation field de-scription”, CERN, Geneva, Switzerland, Rep. CERN-ACC-NOTE-2016-0041, Apr. 2016.[9] A. Infantino, R. Froeschl, and M. Krawina, “R2E & RPFLUKA simulations for the CHARM cryo-cooler test”, CERN,Geneva, Switzerland, CERN-EDMS-1907770 internal report,Feb. 2018.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPMG00607 Accelerator TechnologyT31 Subsystems, Technology and Components, OtherWEPMG0062623ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.

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Experimental Setup to Characterize the Radiation Hardness of Cryogenic Bypass Diodes for the HL-LHC Inner Triplet Circuits

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