Superconducting Magnets for Accelerators

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Superconducting Magnets for Accelerators. J. G. Weisend II National Superconducting Cyclotron Lab Michigan State University. Introduction. Superconducting Magnets are highly engineering devices material wire cable magnet Want to show examples - PowerPoint PPT Presentation


<ul><li><p>Superconducting Magnets for AcceleratorsJ. G. Weisend IINational Superconducting Cyclotron LabMichigan State University</p></li><li><p>IntroductionSuperconducting Magnets are highly engineering devicesmaterial wire cable magnetWant to show examplesToo much material for a 45 min talkSee backup slides &amp; suggested readings for more informationWont really cover HTS or SCRF</p><p>*J. G. Weisend II</p><p>J. G. Weisend II</p></li><li><p>AcknowledgementThis talk includes many slides graciously provided by Luca Boturra of CERN</p><p>J. G. Weisend II</p></li><li><p> Why superconductivity anyhow ?Abolish Ohms law !no power consumption (although need refrigeration power)high current densityampere turns are cheap, so dont need iron (although often use it for shielding)Consequenceslower running cost new commercial possibilitiesenergy savingshigh current density smaller, lighter, cheaper magnets reduced capital costhigher magnetic fields economically feasible new research possibilitiesGraphics by courtesy of M.N. Wilson </p><p>J. G. Weisend II</p></li><li><p>Abolish Ohms law -Normal-conducting dipoleSuper-conducting dipolePotential for saving 7 MW of the 15 MW estimated total power consumption of an accelerator complex</p><p>Iron weight [tons]10Peak voltage [V]34Average AC loss power [W]1.3</p><p>Iron weight [tons]15Peak voltage [V]41Resistive power [W]27000</p><p>J. G. Weisend II</p></li><li><p>Superconductors Pre-history thus the mercury at 4.2 K has entered a new state, which, owing to its particular electrical properties, can be called the state of superconductivityH. Kamerlingh-Onnes (1911)</p><p>J. G. Weisend II</p></li><li><p>Cooper PairsSuperconductorpaired electrons are bosons, a quasi particle in condensed statezero resistanceNormal conductorscattering of e-finite resistance</p><p>J. G. Weisend II</p></li><li><p>Hey, what about field ?Cool downRaise fieldMeissner &amp; Ochsenfeld, 1933superconductingnormal-conducting</p><p>J. G. Weisend II</p></li><li><p>Lattice of quantum flux linesObservation on Pb-4at% In magnetised by a field of 3000 Oe and decorated by Co particles</p><p>Essmann &amp; Truble, 19670 = h/2e = 2.07 x 10-15 WbFlux quantumSupercurrentGraphics by courtesy of Superconductor Lab, Oslo</p><p>J. G. Weisend II</p></li><li><p>Critical temperature and field NOTE: of all the metallic superconductors, only NbTi is ductile. All other are brittle inter-metallic compoundsThe upper critical field BC2 and temperature TC of metallic superconductors are mutually related</p><p>Both BC2 and TC are determined by the chemistry of the materialHTSGraphics by courtesy of M.N. Wilson </p><p>J. G. Weisend II</p></li><li><p>Hey, what about current ?A current flowing in a magnetic field is subject to the Lorentz force that deviates the charge carriers:F = J x BThis translates into a motion of the fluxoids across the superconductor energy dissipation loss of superconductivityTo carry a significant current we need to lock the fluxoids so to resist the Lorentz force. For this we mess-up the material and create pinning centers that exert a pinning force FP</p><p>J. G. Weisend II</p></li><li><p>Pinning mechanismsMicrostructure of Nb-TiGraphics by courtesy of Applied Superconductivity Center at NHMFLPrecipitates in alloysGrain boundaries in inter-metallic compoundsMicrostructure of Nb3SngrainToday we are engineering the structure of superconductors at a microscopic scale via heat treating, alloying and cold work</p><p>J. G. Weisend II</p></li><li><p>Jc(B,T,)The maximum current that can be carried by the superconductor is the current at which:|J x B| = FPThe above expression defines a critical surface:JC(B,T,) = FP / BCritical surface of a LHC NbTi wireJc (5 T, 4.2 K) 3000 A/mm2</p><p>J. G. Weisend II</p></li><li><p>Practical Superconductor FactsSuperconductor s are relatively poor conductors when normalSuperconductors do have resistive losses when the current is variedSuperconductors cant generally be used as a bulk material. They are divided into filaments (tens of mm in DIA) housed in a good conductor matrix. This:Prevents flux jumping and heatingIncreases stability</p><p>J. G. Weisend II</p></li><li><p>Engineering current density All wires, tapes and cables contain additional components:Left-overs from the precursors of the SC formationBarriers, texturing and buffering layersLow resistance matricesThe SC material fraction is hence always &lt; 1: = ASC / AtotalTo compare materials on the same basis, we use an engineering current density:JE = JC x </p><p>J. G. Weisend II</p></li><li><p>Nb-Ti manufacturing routeNbTi is a ductile alloy that can sustain large deformationsIC(5 T, 4.2 K) 1 kANbTi billetGraphics by courtesy of Applied Superconductivity Center at NHMFLLHC wire</p><p>J. G. Weisend II</p></li><li><p>Nb3Sn manufacturing routes Nb3Sn is brittle and cannot be drawn in final form. The precursors are drawn and only later the wire is heat-treated to 650 C, to form the Nb3Sn phaseGraphics by courtesy of Applied Superconductivity Center at NHMFLIC(12 T, 4.2 K) 1.5 kA</p><p>J. G. Weisend II</p></li><li><p>Practical conductors: high JEMultifilamentary wires have current carrying capability of 100 1000 AInsulated with varnish or glass-braids they can be used to make all kind of small size magnetsLarge size magnets (e.g. LHC dipoles) require invariably large operating currents (10 to 100 kA) to:Decrease inductance,Lower the operating voltage,Ease the magnet protectionRutherford cables are ideally suited for this taskJE 500 A/mm2LHC cable prototype</p><p>J. G. Weisend II</p></li><li><p>Practical conductors: low JESuper-stabilized conductor, a superconducting cable (e.g. Rutherford) backed by a large amount of good normal conductor (e.g. Al)JE 50 A/mm2Internally-cooled conductor, e.g. Cable-In-Conduit Conductor (CICC), a rope of wires inserted in a robust conduit that provides a channel for cooling</p><p>J. G. Weisend II</p></li><li><p>we expect the magnet to go resistive i.e. to 'quench', where the peak field load line crosses the critical current line86422468101214Field T1234567Current density kA/mm210 Temperature KPeak field5 T Bore fieldCritical line and magnet load linesNbTi critical current IC(B)quench !NbTi critical surfacee.g. a 5 T magnet design</p><p>J. G. Weisend II</p></li><li><p>Temperature margin Temperature rise may be caused bysudden mechanical energy releaseAC lossesResistive heat at joints Beams, neutrons, etc.We should allow temperature headroom for all foreseeable and unforeseeable events, i.e. a temperature margin:</p><p>T = TCS-TopT1.5 KIop5 T6 TTopTCSNbTi critical current Ic(T)</p><p>J. G. Weisend II</p></li><li><p>Current quench IQField quench BQLoadline quench ImaxTemperature quench TCSOperating marginsPractical operation always requires margins:Critical current margin: Iop/IQ 50 %Critical field margin: Bop/BQ 75 %Margin along the loadline: Iop/Imax 85 %Temperature margin: TCS - Top 12 KThe margin needed depends on the design and operating conditions (see later)</p><p>J. G. Weisend II</p></li><li><p>Cooling of SuperconductorsVarious Possibilities ExistIndirect (adiabatic magnets)contact to a heat sink through conduction (cryoplant, cryocooler)in practice no cooling on the time scale of interest for stability and quenchbath cooling (pool boiling magnets) pool of liquid cryogen at atmospheric pressure and saturation temperature (e.g. helium at 4.2 K)boiling heat transferforce-flow coolingsupercritical or two-phase flow, warm or cold circulationsuperfluid coolingstagnant bath, heat removal through counter-flow heat exchange</p><p>J. G. Weisend II</p></li><li><p>Helium as a coolant</p><p>J. G. Weisend II</p></li><li><p>Cryogenic RefrigerationCooling for superconducting magnets is well within the state of the artMany commercial options exist (either off the shelf or custom)The cryogenic system is best designed along with the magnet system as opposed to added on later</p><p>J. G. Weisend II</p></li><li><p>Cryogenic RefrigerationExamplesCryocooler: 0.1 W @ 4 KHelium Refrigerator Coldbox (CTI400)1200 W at 4.5 K</p><p>J. G. Weisend II</p></li><li><p>Stability of Superconducting MagnetsWhen a s/c magnet undergoes a temperature rise, there are 2 possibilities:It can cool back down and remain superconductingIt can warm up above Tc and quench (become normally conducting)Which occurs depends on the amount of heat generation and cooling</p><p>J. G. Weisend II</p></li><li>A prototype temperature transientheat pulseeffect of heat conduction and coolinggeneration&gt;coolingunstablegeneration</li><li><p>Perturbation spectrummechanical eventswire motion under Lorentz force, micro-slipswinding deformationsfailures (at insulation bonding, material yeld)electromagnetic eventsflux-jumps (important for large filaments, old story !)AC loss (most magnet types)current sharing in cables through distribution/redistributionthermal eventscurrent leads, instrumentation wiresheat leaks through thermal insulation, degraded coolingnuclear eventsparticle showers in particle accelerator magnetsneutron flux in fusion experiments</p><p>J. G. Weisend II</p></li><li><p>Current sharingTIopTopICstabilizersuperconductorstabilizersuperconductor</p><p>J. G. Weisend II</p></li><li><p>Stability of Superconducting MagnetsFully cryostable: magnet will recover regardless of size of normal zone ( disturbance) May be true of large detector magnets e.g BaBar detector or MRI magnetsPartially cryostable: magnet will recover if normal is not too big more typical of accelerator magnets</p><p>J. G. Weisend II</p></li><li><p>Criteria for Fully Cryostable MagnetsStekly Criteria most conservative, doesnt account for end cooling&lt; magnet is stableEqual area theoremTakes into account the cooling of the conductor via conduction at the endsCan be expressed as a graphical solution comparing the areas under the cooling and heating curves ( see references )</p><p>J. G. Weisend II</p></li><li><p>Quench Detection &amp; ProtectionSuperconducting magnets store large amounts of energy either individually (20 MJ for the Babar detector magnet) or connected in series (10s of GJ)If all the energy is deposited in a small volume, bad things happen!</p><p>J. G. Weisend II</p></li><li><p>Quench Detection &amp; ProtectionThe goal is to rapidly and accurately detect the quench and safely dispose of the energyEvenly throughout the magnetIn an external dump resistorIn a coupled secondaryIn magnet strings, bypass the energy of the other s/c magnets away from the quenching oneRemember its the stored energy in the magnet(s) not the power supply thats problem</p><p>J. G. Weisend II</p></li><li><p>Detecting QuenchesCant just measure voltage directly as magnet ramping causes voltage and give a false signalThe general approach is to subdivide the magnet with voltage taps and build a bridge circuit that cancels out voltage due to rampingRedundant QD systems are necessaryOther measurements such as temperature, helium level or vacuum level might be used to look for precursors to trouble but take care not to over interlock the magnet</p><p>J. G. Weisend II</p></li><li><p>Strategy: energy dumpthe magnetic energy is extracted from the magnet and dissipated in an external resistor:</p><p>the integral of the current:</p><p>can be made small by:fast detectionfast dump (large Rdump)B.J. Maddock, G.B. James, Proc. Inst. Electr. Eng., 115, 543, 1968</p><p>J. G. Weisend II</p></li><li><p>Strategy: heatersthe quench is spread actively by firing heaters embedded in the winding pack, in close vicinity to the conductor</p><p>heaters are mandatory in:high performance, aggressive, cost-effective and highly optimized magnet designswhen you are really desperate</p><p>advantages:homogeneous spread of the magnetic energy within the winding packdisadvantages:activehigh voltages at the heaterDoesnt work well with highly stable magnetswindingheater</p><p>J. G. Weisend II</p></li><li><p>Magnet stringsmagnet strings (e.g. accelerator magnets, fusion magnetic systems) have exceedingly large stored energy (10s of GJ):energy dump takes very long time (10100 s)the magnet string is subdivided and each magnet is by-passed by a diode (or thyristor)the diode acts as a shunt during the discharge</p><p>J. G. Weisend II</p></li><li><p>LHC dipoleBnominal8.3 (T)current11850 (A)stored energy 1 (MJ)cold mass 35 (tonnes)</p><p>J. G. Weisend II</p></li><li><p>Superconducting dipole magnet coilIdeal current distribution that generates a perfect dipolePractical approximation of the ideal distribution using Rutherford cables+JJ+JJ</p><p>J. G. Weisend II</p></li><li><p>Twin coil principleCombine two magnets in oneSave volume, material, cost</p><p>J. G. Weisend II</p></li><li><p>LHC dipole coilsBB</p><p>J. G. Weisend II</p></li><li><p>Coil windingBBCable insulationCoil winding machineStored coils10 m precision !</p><p>J. G. Weisend II</p></li><li><p>EndsInner layerLayer jumpEnds, transitions, and any deviation from the regular structure are the most delicate part of the magnet</p><p>J. G. Weisend II</p></li><li><p>Collaring and yoking175 tons/m85 tons/mFyokingcollaring</p><p>J. G. Weisend II</p></li><li><p>Cold mass</p><p>J. G. Weisend II</p></li><li><p>Cryostat</p><p>J. G. Weisend II</p></li><li><p>Finally, in the tunnel !</p><p>J. G. Weisend II</p></li><li><p>Example 2BaBar Detector S/C SolenoidProvided background field for particle identification for the BaBar detector at SLACPhysics requirements dictated a relatively thin solenoid</p><p>*J. G. Weisend II</p><p>J. G. Weisend II</p></li><li><p>Properties of BaBar SolenoidField: 1.5 TeslaStored Energy: 27 MJOperating Current: 4596ATc= 8.3KOperating Temp: 4.5KTotal Heat Load at 4.5K: 225liquid-liters/hrCryogenics: indirectly cooled using the force flow technique where the liquid He is circulated in cooling pipes welded to the outside diameter of the support tubeUses NbTi highly stabilized in a pure Al conductor</p><p>J. G. Weisend II</p></li><li><p>BaBar Detector Under Construction</p><p>J. G. Weisend II</p></li><li><p>BaBar Detector</p><p>J. G. Weisend II</p></li><li><p>Cryogenic Flow Schematic for BaBar Solenoid</p><p>J. G. Weisend II</p></li><li><p>BaBar SolenoidOperated almost continuously for ~ 10 yearsWas very stable only discharged due to loss of power, controls or coolingAvailability was &gt; 96% from the start and better than 98% during final 3 yearsImprovement due mainly to removing unnecessary interlocks and adding additional utility backupsMay still be used as part of the proposed Super B project in Italy</p><p>J. G. Weisend II</p></li><li><p>ConclusionsSuperconducting magnets make possible modern HEP acceleratorsThey are also key for MRI systems and heavy ion machines such as FRIB, NSCL and FAIRSuperconducting magnet design involves detailed engineering on a scale from the microscopic (flux pinning) to the immense ( multi ton, GJ magnets)Superconducting magnet design involves a wide range of disciplines: materials science, electrical engineering, mechanical design, cryogenics etc.HEP superconducting magnet requirements have driven and enabled many advances in s/c materials, wire and ancillary systems</p><p>*************************************</p></li></ul>


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