Rüdiger Schmidt and Karl Hubert Mess The challenges of protecting the super-conducting magnets and powering system Rüdiger Schmidt and Karl Hubert Mess Academic Training May 2008

During tests the energy of 2 MJ from the SPS beam was directed into a metal target (LHC 360 MJ) V.Kain

During tests the energy of 7 MJ in one of 154 magnets was released into one spot in the coil (interturn short) P.Pugnat

Overview Energy in beams and magnets ….and their controlled discharge Beam and powering interlock systems Quenches in superconducting magnets LHC powering architecture Powering system: magnet types and electrical circuit types Protection for the electrical circuits Procedure for commissioning of (the protection systems) an electrical circuit and recent experience Interlocks systems during beam operation

Energy stored in magnets and beam E dipole = 0.5  L dipole  I 2dipole Energy stored in one dipole operating at 7 TeV with 11850 A is 7.4 MJoule For 154 dipoles in one sector: ~1.2 GJoule For all 1232 dipoles in the LHC: ~9 GJoule E beam = N p  N bunches  Energy Energy stored in one beam with 2808 bunches, each 1.15  1011 protons at 7 TeV is 360 MJoule

LHC cycle and stored beam energy 7000 6000 energy ramp 25 MJ  360 MJ circulating beam 2808 bunches 5000 Energy [GeV/c] coast 360 MJ circulating beam 4000 3000 beam dump discharge of energy into beam dump block injection phase 3 MJ  25 MJ beam transfer circulating beam 2000 1000 12 batches from the SPS (every 20 sec) one batch 216 / 288 bunches 3 MJ per batch -4000 -2000 2000 4000 time from start of injection (s)

LHC cycle and stored magnet energy (main dipoles) 12000 10000 current ramp 36 MJ  9 GJ for 1232 dipole magnets 8000 coast 9 GJ Current [kA] 6000 4000 in case of quench discharge of energy into resistors injection current 760 A 0.53 T 2000 1 -4000 -2000 2000 4000 time from start of injection (s)

Livingston type plot: Energy stored magnets and beam based on graph from R.Assmann

What does this mean? The energy stored in the LHC magnets corresponds approximately to 8 such trains running at 280 km/h Sufficient to heat up and melt more than 10 tons of Copper!! 90 kg of TNT The energy of an 200 m long fast train at 155 km/hour corresponds to the energy stored in one LHC beam 8 litres of gasoline 15 kg of chocolate It’s the time for the energy release (instantaneous power) that matters !!

Machine Protection during all phases of operation During commissioning of the powering system and operation: protection of magnets, busbars connecting magnets and High Temperature Superconductor current leads is mandatory The magnet protection and powering interlock systems become operational during this time, long before starting beam operation In case of failure, the energy of the superconducting magnets must be discharged into resistors During beam commissioning and operation: protection from the injection process, during the energy ramps and at 7 TeV is mandatory The only component that can stand a loss of the full beam is the beam dump block - all other components would be damaged The LHC beams must ALWAYS be extracted into the beam dump blocks, at the end of a fill and in case of failure In general, a failure in an electrical circuit leads to beam extraction

Beam dumping system in IR6 Septum magnet deflecting the extracted beam H-V kicker for painting the beam Q5L Beam Dump Block Q4L Fast kicker magnet about 700 m Q4R about 500 m Q5R Beam 2 AB - Beam Transfer Group

LHC Machine Protection Overview Beam Dumping System Beam Interlock System Injection Interlock

LHC Machine Protection Powering System >50% of all interlocks Beam Dumping System Beam Interlock System Injection Interlock Powering Interlocks sc magnets Powering Interlocks nc magnets Magnet Current Monitor Magnets Power Converters MPS (several 1000) Power Converters ~800 AUG UPS Cryo OK

LHC Machine Protection Access System Beam Dumping System Beam Interlock System Injection Interlock Powering Interlocks sc magnets Powering Interlocks nc magnets Magnet Current Monitor Access System Magnets Power Converters MPS (several 1000) Power Converters ~800 AUG UPS Cryo OK Doors EIS

LHC Machine Protection Vacuum System Beam Dumping System Beam Interlock System Injection Interlock Powering Interlocks sc magnets Powering Interlocks nc magnets Magnet Current Monitor Access System Vacuum System Magnets Power Converters MPS (several 1000) Power Converters ~800 AUG UPS Cryo OK Doors EIS Vacuum valves Access Safety Blocks RF Stoppers

LHC Machine Protection Beam Loss Monitors Dumping System Beam Interlock System Injection Interlock Powering Interlocks sc magnets Powering Interlocks nc magnets Magnet Current Monitor Beam loss monitors BLM Access System Vacuum System Magnets Power Converters Monitors aperture limits (some 100) Monitors in arcs (several 1000) MPS (several 1000) Power Converters ~800 AUG UPS Cryo OK Doors EIS Vacuum valves Access Safety Blocks RF Stoppers

LHC Machine Protection Collimation System ~99% of all interlocks Collimator Positions Environmental parameters Collimation System Beam Dumping System Beam Interlock System Injection Interlock Powering Interlocks sc magnets Powering Interlocks nc magnets Magnet Current Monitor Beam loss monitors BLM Access System Vacuum System Magnets Power Converters Monitors aperture limits (some 100) Monitors in arcs (several 1000) MPS (several 1000) Power Converters ~800 AUG UPS Cryo OK Doors EIS Vacuum valves Access Safety Blocks RF Stoppers

LHC Machine Interlocks Devices Movable Detectors Beam Loss Monitors BCM Experimental Magnets Collimator Positions Environmental parameters Safe Beam Parameter Distribution LHC Devices LHC Devices Safe LHC Parameter Software Interlocks Sequencer Operator Buttons CCC LHC Experiments Transverse Feedback Beam Aperture Kickers Collimation System SpecialBLMs Beam Dumping System Beam Interlock System Safe Beam Flag Injection Interlock Powering Interlocks sc magnets Powering Interlocks nc magnets Magnet Current Monitor RF System Beam loss monitors BLM Beam LifetimeFBCM Access System Vacuum System Screens / Mirrors BTV Timing System (Post Mortem Trigger) Magnets Power Converters Monitors aperture limits (some 100) Monitors in arcs (several 1000) MPS (several 1000) Power Converters ~800 AUG UPS Cryo OK Doors EIS Vacuum valves Access Safety Blocks RF Stoppers

Operational margin of a superconducting magnet Applied Magnetic Field [T] Bc Bc critical field 8.3 T quench with fast local loss of ~5 · 106 protons QUENCH Tc critical temperature quench with fast local loss of ~5 · 109 protons 0.54 T 1.9 K 9 K Temperature [K]

Quench - transition from superconducting state to normalconducting state Quenches are initiated by an energy in the order of mJ Movement of the superconductor by several µm (friction and heat dissipation) Failure in cooling Beam losses To limit the temperature increase after a quench The quench has to be detected The magnet current has to be switched off within << 1 sec For main magnets: the energy stored in the quenching magnet is distributed inside the magnet by force-quenching the magnet coils using quench heaters For magnets powered in series: the quenching magnet is isolated from the other magnets using a power diode

Superconducting wire and cable Filament diameter 6 m Wire diameter 1 mm Typical value for operation at 8 T and 1.9 K: 800 A width 15 mm Rutherford cable current ~12000 A

Power into superconducting cable after a quench Specific heat of copper at 300 C:

LHC Powering in 8 Sectors 5 Powering Sector: 154 dipole magnets about 50 quadrupoles total length of 2.9 km 4 6 DC Power feed LHC Octant 3 DC Power 7 27 km Circumference Powering Subsectors: long arc cryostats triplet cryostats cryostats in matching section 2 8 Sector 1 P.Proudlock

Magnet in one arc cell of 110 m length 6 main dipole magnets (12 kA) 2 arc quadrupole magnets (12 kA) Lattice sextupole magnets in arcs (600 A) Multipole and other correctors in arcs Powered in series 752 arc orbit corrector magnets powered individually (60 A) Correctors to adjust beam parameters (trim quadrupoles, orbit correctors, etc., 80 – 600 A) Powered individually SSS sextupole corrector (MCS) quadrupole MQF orbit corrector quadrupole MQD orbit corrector quadrupole MQF orbit corrector main dipole MB main dipole MB main dipole MB main dipole MB main dipole MB main dipole MB special corrector (MQS) lattice sextupole (MS) decapole octupole corrector (MCDO) special corrector (MO) lattice sextupole (MS) special corrector (MO) lattice sextupole (MS) F0D0 cell 110 m

Types of electrical circuits There are more than 50 different types of magnets Magnets can be powered in series or individually As an example, powering and protection of an individual orbit corrector magnet (60 A, 9 kJ) and a circuit including 154 main dipole magnets (12 kA, 1.2 GJ) is very different risks, magnet protection, interlocks, commissioning procedures, etc. 1618 electrical circuits grouped into nine “Electrical Circuit Types” (eight for circuits with superconducting magnets, one for circuits with normal conducting magnets) The attribution of an electrical circuit to a type depends on the energy stored in the magnets of the electrical circuit, and the way of protecting magnets, busbars and current leads Commissioning procedures are essentially identical for electrical circuit types

Sector 7-8 and magnet cryostats IR7 Cleaning Matching section Arc cryostat (3 km) Matching section Inner Triplet IR8 LHCb DFBMH DFBAN DFBAO DFBMC DFBMA DFBX IR Quadrupoles Correctors 154 Arc dipole magnets and correctors Short straight sections with quadrupoles and correctors IR Quadrupoles Correctors Insertion dipoles IR Quadrupoles Correctors Each cryostat has its own set of PC. Only the beam, the access, the vaccuum, and the cryogenics couple Each cryostat one PPC (arc needs two, because fed from both sides) PPC close to the PC Power Converters (60A) for 94 orbit corrector magnets

Sector 7-8 and power converters IR7 Cleaning Matching section Arc cryostat (3 km) Matching section Inner Triplet IR8 LHCb DFBMH DFBAN DFBAO DFBMC DFBMA DFBX Energy extraction Each cryostat has its own set of PC. Only the beam, the access, the vaccuum, and the cryogenics couple Each cryostat one PPC (arc needs two, because fed from both sides) PPC close to the PC UJ76 RR77 UA83 Power Converters for 34 electrical circuits and other equipment Power Converters for 72 electrical circuits and other equipment

Conditions for powering Cryogenics: correct conditions 1.9K, 4.5K, other conditions Safety systems ready (AUG – arret urgence general, UPS – uninterruptible power supplies, …) Power converter ready Magnet protection system ready Power converters Energy extraction Operator / Controls: must give permission to start powering Powering Interlock Controller (PIC) Beam Interlocks Quench in a magnet inside the electrical circuit Warming up of the magnet due to quench in an adjacent magnet Warming up of the magnet due to failure in the cryogenic system AUG or UPS fault Power converter failure

Main dipoles in arc cryostat Time for the energy ramp is about 20-30 min (Energy from the grid) Time for regular discharge is about the same (Energy back to the grid) DFB DFB Magnet 2 Magnet 4 Magnet 152 Magnet 154 Magnet 1 Magnet 3 Magnet 5 Magnet 153 Energy Extraction: switch closed Energy Extraction: switch closed Power Converter

Main dipoles: quench of a magnet Quench in one magnet: Resistance and voltage drop across quenched zone Quench is detected: Voltage across magnet exceeds 100 mV for >10 ms DFB DFB Magnet 2 Magnet 4 Magnet 152 Magnet 154 Magnet 1 Magnet 3 Magnet 5 Magnet 153 Energy Extraction: switch closed Energy Extraction: switch closed Quench Detector Power Converter

Main dipoles: magnet protection Quench heaters warm up the entire magnet coil: energy stored in magnet dissipated inside the magnet (time constant of 200 ms) Diode in parallel becomes conducting: current of other magnets through diode Resistance is switched into the circuit: energy of 153 magnets is dissipated into the resistance (time constant of 100 s for main dipole magnets) DFB DFB Magnet 2 Magnet 4 Magnet 152 Magnet 154 Magnet 1 Magnet 3 Magnet 5 Magnet 153 Energy Extraction: switch open Energy Extraction: switch open Quench Detector Quench Heater PS Power Converter

Magnet and busbar quench detection To detect a quench: U = R  I (about 1 V need to be detected) But one needs to substract from U1: a) U2 = Rwarm  I b) During energy ramp: U = dI/dt  L  154 = 77  (U3 + U4) DFB DFB Magnet 2 Magnet 4 Magnet 152 Magnet 154 Magnet 1 Magnet 3 Magnet 5 Magnet 153 U2 U3 U4 U1 Power Converter World FIP - deterministic Fieldbus

Simpler way to discharge the energy? assume one magnet quenches assume the magnets in the string have to be discharged in, say, 200 ms the inductance is about 15 H, the current about 12 kA with U = l  dI/dt Discharge with about 1 MV: not possible

Challenges for quench protection Detection of quenches for all main dipole and quadrupole magnets (1600 magnets powered in 24 electrical circuits) Voltage across the dipole magnet chain up to 180 V during ramping - 1 V needs to be detected in presence of noise etc. During discharge the quench detectors are connected to equipment at high voltage (1000 V) Detection of quenches in about 800 other circuits Global quench detection for circuits operating at 600 A Inductance of magnets change as a function of current Detection of quenches across all HTS current leads (2000) with very low voltage threshold ~ 3 mV for 1 sec across HTS part Systems must be very safe in order not to damage equipment (any quench must be detected) Systems must be very reliable in order not to disturb operation (the system should not trigger in case of noise etc.)

Energy extraction switch house 13 kA Energy extraction resistors MB Energy extraction switch 13 kA Diode for 13 kA

Objective of powering tests Why the lengthy commissioning? Most components in the electical circuit have been tested before (e.g. all magnets) Short circuit tests of the power converters have been done Validate the entire electrical circuits for the first time Busbars ok? 70000 superconducting connections ok? Magnets still ok (after storage)? All other systems ok? Protection systems work? Switch-off for different failure cases ok? Discharge of 1 GJoule ok? All interlock systems ok? Test protection in case of quench, of water cooling problems, of a failure in the UPS system, of an AUG activation Test protection in case of a problem in the cryogenic system

One of ~1700 interconnections: busbars and tubes

Steps in the commissioning procedure Verification of the correct functioning of the interlock systems at low current in case of quench: fire quench heaters and fast switch off power converter in case of power converter failure: extraction of energy stored in the circuit in case of UPS failure: switch off power converter in case of CRYO failure: slow abort of power converter Verification of magnet protection by firing quench heaters at low current Verification of magnet protection by firing quench heaters at high current For circuits with energy extraction systems Verification of the energy extraction functionality by switching the resistor into the circuit at different current levels

Commissioning procedure for quadrupoles (example) Tests of interlock system at very low current Tests at intermediate current Tests at nominal current

RD4: Fast Power Abort from 200A (circuit quench via magnet protection system) time [s] 41

Fast Power Abort from 350A (Fast Abort request via powering interlock system) time [s] 42

Fast Power Abort from 5500A (PNO.C2) time [s] 43

Quench from 5500A time [s] 44

Quench - detail time [s] 45

Quench detection and beam dump trigger detector threshold reached Quench trigger Quench Diode opens Current bypasses magnet Quench Heaters fire 15 – 130 ms 3 - 200 ms 10 ms time 1.5 ms 7 - 9 ms Energy extraction Current decay starts time 3-4 ms 0.2 ms < 0.4 ms Powering Interlock triggered Beam Interlock triggered Beam extracted

Response to Quenches Quench detected (+ 0 ms) Current decay starts (+ 9 ms) 3.4 ms 73 ms Interlocktriggered (+ 5 ms) Beam would be dumped (+ 5.6 ms) 0.01% of beam would lost (+91 ms) 11/11/2018 andres.gomez.alonso@cern.ch 47

Conclusions Protection has high priority, a failure can lead to substantial equipment damage Several systems are involved in protection: magnet protection system, powering interlock system, power converters, cryogenics, UPS, AUG … The commissioning of the machine protection systems is mandatory for safe operation The complexity of the LHC powering system is unprecedented and requires the application of strict procedures for commissioning The procedures have been automised, and therefore commissioning could become very efficient In case of a failure (quench, failure of a power converter, failure of a supply system such as water and electricity) the beam will be dumped. During hardware commissioning more than 50% of all interlock channels are being tested. Commissioning experience: worked in general very well, but the lengthy work is fully justified (a number of non-conformities were detected)

Acknowledgement Many colleagues have been involved in the work that is presented here, as CERN staff, as project associates and as industrial support. It is not possible to list all the names, but I very much appreciate the enthusiastic work of all them. Teams responsible for protection systems: Magnet Protection, Powering Interlocks, Beam Interlocks, Beam Dumping System, Beam Loss Monitors, Collimation System Teams responsible for systems involved in powering: Power Converters, Vacuum, Cryogenics, (cold) Electrical Engineering Teams responsible for service systems: Water cooling, Ventilation, Access System, AC and DC distribution Other teams: Controls and Networking, Operation, Magnet “Owners” Collaborators from abroad (US, Japan), Project Engineers