At ESRF, the European Synchrotron Radiation Facility in Grenoble, electrons are accelerated, via a 200 MeV Linac and a 6 GeV synchrotron booster, and injected into the storage ring at 10 Hz rate. Two thin septum blade magnets and an eddy current sheet type septum magnet provide the final deflection of the injected beam. The operational requirements of the e- injection scheme and the resulting demanding hardware specifications are recalled. The pulsed septum magnets are briefly described. The design, circuit layout and construction of the power converters are related with emphasis on innovative aspects of general interest. Results of tests during commissioning are reported

Perrine, J P

Thivent, M

Völker, F V

Perrine, J-P

Thivent , Μ

Völker, F

CERN Document Server

The pulsed power converter and septum magnet system for injection into the electron storage ring at ESRF

THE PULSED POWER CONVERTER AND SEPTUM MAGNET SYSTEMFOR INJECTION INTO THE ELECTRON STORAGE RING AT ESRFJ.P.Perrine, ESRF.  M.Thivent and F.Völker, CERNAbstractAt ESRF, the European Synchrotron Radiation Facilityin Grenoble, electrons are accelerated, via a 200 MeVLinac and a 6 GeV synchrotron booster, and injectedinto the storage ring at 10 Hz rate. Two thin septumblade magnets and an eddy current sheet type septummagnet provide the final deflection of the injectedbeam. The operational requirements of the e− injectionscheme and the resulting demanding hardwarespecifications are recalled. The pulsed septum magnetsare briefly described. The design, circuit layout andconstruction of the power converters are related withemphasis on innovative aspects of general interest.Results of tests during commissioning are reported.1. INTRODUCTIONThe ESRF Storage Ring (SR) [1] injectors comprise a200 MeV Linac and a fast cycling Booster Synchrotron(BS). A repetition rate of 10 Hz keeps the beamcharging times short compared with the beam lifetime.The actual parameters of the SR are summarized inTable 1.Table 1: Parameters of the ESRF Storage RingNominal beam energy (GeV)  6Beam current (mA)                                  200Number of bunches ≤ 992H/V beam emittance (nm⋅rad) 4/0.04Beam lifetime at 100 mA (h)  70Number of straight sections  32Radiation sources:  Bendings, Undulators and Wigglers2. OPERATIONAL REQUIREMENTSThe ESRF complex and the SR injection layout areschematically shown in Fig.1. Fig.1: ESRF complex and SR injection layout.After the Linac the electrons are one turn (1 µs) injectedinto the BS. Here the particle energy is increased, with a50 ms oscillating ramp-up of magnet currents. At 6 GeVthe beam is extracted from the BS and enters the transferline (TL). Injection into the SR takes place in thehorizontal plane by a combination of three thin and thickseptum magnets (S1-S2 and S3) to keep the apertureneeds small. These magnets produce the final deflectionto align the particle trajectory to be parallel to, butdisplaced by about 20 mm from the circulating beam.Four fast bump magnets, arranged symmetrically aroundthe midpoint of the SR high beta injection straightsection, provide a local orbit deviation to move thestored beam close to the S3 septum. The injected particlepulse is then captured within the acceptance of the SR.3. THE PULSED SEPTUM MAGNETSTable 2 summarizes the characteristics of the monoturnseptum magnets S1-S2 and S3, shown in Fig.2.Table 2: Characteristics of magnets: S1-S2             S3Deflection angle (mrad)                            44 20Physical/magnetic length (mm)        978/950 580/500Yoke cross section (mm2)                120x120 80x80Gap aperture HxV (mm)                      48x13 25x10Nominal field (T)                                    0.90 0.87Nominal excitation current (kA)               9.6 8Impedance L/R (µH/mΩ)                      5/0.7 1.9/2Integrated field (T⋅m)                             0.88 0.44Total septum blade thickness (*) (mm)         7 2 to 4Lamination steel TRANSIL 280 (3% Si)(*) including the magnetic screenS1 and S2 are two current sheet septum magnets [2],electrically connected in series. The 1 mm thick stainlesssteel vacuum chamber in the gap is water cooled. For avacuum of < 10-8 mbar, the chamber is to be baked at∼300 °C. In doing so the septum must be disassembledand the magnet displaced horizontally. The H/V beamdimensions in the line are 18/10 mm and the sagitta is 22mm. The yoke is composed of glued 0.35 mm sheets.The end plates and the lamination are bolted together.The magnetic screen is a 1.5 mm ARMCO sheet. Thewinding is water cooled (Q = 1.6 l/min., ∆p = 7 bar).The r.m.s. current density in the septum is 16 A/mm2.S3 is an “eddy current” septum magnet [3,4,5]. So thelamination is kept out of the high quality (10-10 mbar)vacuum of the SR and the electrical impedance seen by abunch of stored particles is minimized. The septum bladeis part of a 10 mm thick copper shielding box whichsurrounds the yoke of the magnet completely andextends beyond its ends. To make the shield workeffectively a short current pulse is needed. Acompromise with the current supply has led to a half-sinusoidal pulse of 60 µs duration. Thereby thepenetration depth in the copper is 0.7 mm. A 0.38 mmPERMALLOY magnetic screen, oven-brazed on theseptum blade, reduces the leakage field at the circulatingbeam to 10-3 of the gap field. The lamination iscomposed of 0.2 mm sheets insulated by oxide film. Thewinding conductor in the gap (10x7 mm2, hole 4 mm ∅),is water cooled. The external return conductor (4 x 30mm2) does not need forced cooling. The r.m.s. current inthe winding is 140 A. The septum blade is cooled (Q = 2l/min) by a separate copper pipe embedded in the thickeredges. The maximum total power dissipation is 90 Wand the temperature rise is < 65 °C. The magnet tank canbe moved by 6 mm to adapt the radial septum position tothe SR beam emittance.    Fig.2: Cross-section of S1-S2 and S3 magnets4. THE POWER CONVERTERSTable 3 shows the features of this equipment, developedand built in industry to a CERN specification [6].Table 3: Data on power converters: S1-S2             S3Regular pulse repetition rate (Hz)                9.5 to 10.5Max. charge voltage (kV)                                   2Max. charge current (A)                           10 5Energy storage capacitor (µF)                640 120Peak/r.m.s. load current (kA)             10/1.2 8/0.14Pulse duration (ms)                                 2.5 0.060Current flat-top (µs)                                 60 2Long term current flat-top stability (%)            0.1Load L/R (µH/mΩ)                                 5/2 3/54.1 Mode of operationAn electrical diagram is shown in Fig.3. The timingpulse sequence controls the capacitor charge and itsdischarge into the magnet: FOREWARNING pulse (FW)starts the 80 ms charge; WARNING (W) ends the 5 msvoltage stabilisation; START (ST) initiates thedischarge; MEASURE (MEA) monitors the current.Time windows after each pulse lock the completesequence. REMOTE or LOCAL actuation mode, currentsetting and timing is possible as well as pulsing at slowerrate or in single-shot. The actuations are OFF, STAND-BY, ON and RESET. The computer control interface, isbased on an RS-232 serial link. Isolation of in/out signalsto the electronics and the use of optic fibers to drive thepower switches provide for higher noise immunity.          Fig.3: Electrical diagram of power converters4.2 Layout of power converters.A power converter consists of two 19″ cubicles, housingthe charge circuit and the energy storage and switchingsection, located at ∼15 m distance from the magnets.4.2.1 Charge circuitThis includes an a.c. soft start and filtering section, a 6-pulse diode rectifier and a d.c. filter (Lo=5 mH, Co=2.25mF, fo=47 Hz). The d.c. link (Uo=540 V) powers adouble resonant d.c.-d.c. converter with four IGBTs(Insulated Gate Bipolar Transistors - 200L120 - FUJI/J)commutated at a frequency of fp to fs /2. The resonantcircuits are for S1-S2: Cs / Cp (µF) = 120 / 0.5, Ls / Lp(µH) = 9.3 / 200 and fs / fp (kHz) = 75 / 16; and for S3: Cs/ Cp (µF) = 120 / 0.066, Ls / Lp (µH) = 46 / 900 and fs / fp(kHz) = 91 / 20. Six ferrite-transformers (N1/N2=14/10)are connected to fast diode rectifiers with 2.1 µFcapacitors on the output. The transformer primaries arein parallel and the rectifiers in series. A choke (L2 = 10mH) on the HV side filters the current and protects thediodes when the capacitor voltage reverses polarity.4.2.2 Discharge circuit of converter S1-S2The capacitor is discharged by a thyristor TH1(N320CH40 - WESTCODE/GB) into the magnet via twocables in parallel and a pulse matching transformer(TRASFOR/CH) located under the magnet. Thetransformer characteristics are collected in Table 4.A saturating reactor L5 (40/5.5 µH) limits the initial di/dtof the pulse. The top of the sinusoidal current is flattenedby a third harmonic parallel resonant circuit L3-C3 (170µH-480 µF). Control of the magnet current during the 60µs flat-top is performed by a dynamic filter connectedacross an inductance L4 (100 µH) [7]. Its power amplifierderives a fraction of the pulse current, using MOSFETtransistors (SKM 151 - SEMIKRON/D) under 200 Vdc.To recover the capacitor energy without affecting thepulse shape a thyristor TH2, fired ∼ 3 ms after TH1, andan inductance L6 (5 mH) are added. The duration of the∼500 A current pulse through TH2 is 6 ms. The dynamicfilter and the magnet currents are monitored bytransformers type 1025 and 1423 (PEARSON/US). Table 4: Characteristics of pulse transformerNumber of columns 3Iron cross-section (cm2) 776Weight (kp) 890Dimensions HxDxW (mm)  650x600x580Air gap (mm) 2.4Number of turns N1:N2 16:2Design ∫ u dt (V s), without d.c. core bias 1.5Winding resistance referred to primary (mΩ) 4.7Lσ/Lµ ref. to primary (µH/mH) 10/20Core lamination M6T35Winding insulation class H4.2.3 Discharge circuit of converter S3S3 has no matching transformer and TH1 must handlethe full current (8 kA, 60 µs). An SCR switch wouldsuffer from excessive current density in the vicinity of itsauxiliary turn-on thyristor before the plasma spreads out.Hence a GTO (Gate Turn Off) thyristor is used in stead(DG758 AX - GEC/GB). Due to the fine inter-digitationof the gate-cathode electrodes, all the silicon surfacecomes on at once and allows much higher initial di/dt.The GTO assembly includes an antiparallel and a seriesprotection diode. A third harmonic pulse shaping circuitL3-C3 (0.7 µH-100 µF) is present without dynamic filter.The saturating reactor L5 (5/0.1 µH) consists of a simpleC-core. The energy recovery path includes a diode D2and the inductance L6 (1 mH), selected >> Lload to keepthe pulse duration unchanged. The pulse transmissionline is made of three parallel coaxial cables.4.2.4 Electronics of S1-S2The charge voltage set point is proportional to themagnet current reference. After squaring, this signal isintegrated during the pulse to obtain the energydeposited into the magnet. To model its thermalbehaviour cooling between pulses is simulated by asuitable discharge time constant of the integrator. Theresulting signal tunes the capacitor voltage when themagnet heats up. The charge current I and voltage V areregulated by three parallel loops so as to limit the currentbelow a preset value, to maintain constant power (V⋅I)during charge and to perform the fine voltagestabilisation. The power reference is derived from thecapacitor energy at FW and W assuming constant chargetime Tc: (V⋅I)reference = ( Vreference2-Vrecovery2 )⋅C / (2⋅Tc ).5. TEST RESULTSAt the factory, after HV insulation tests, the chargecircuits have been run at 2 kV, 4 A d.c. on a resistiveload. Some wave-shapes, recorded during pulsedoperation, are shown in Fig 4.    Fig 4: Wave shapes recorded during tests: S1-S2 at 9 kA:a) Magnet current (5 kA/d-0.5 ms/d); b) Transformerprimary voltage (0.8 kV/d). S3 at 8 kA: c) Magnetcurrent (2 kA/d-20 µs/d); d) Capacitor voltage (0.5 kV/d)6. CONCLUSIONSThe requirements of the SR injection system have beensatisfied by two types of pulsed septum magnetsdesigned and assembled at CERN. The powerconverters, operating at 10 Hz, perform charge atconstant power and pulse current flat-top regulation via aparallel dynamic filter. A GTO switch for S3 and aparticularly reliable timing system characterize theequipment. The double-resonant soft-switched-modecharge circuit permits the fast pulse repetition rate andavoids mains reactive power and current harmonicstypical of a.c. thyristor controllers. Its characteristics ofcurrent source make the charger short-circuit proof andtherefore very robust and reliable. So far operationalrecords of this equipment at ESRF have been excellent.7. REFERENCES[1] ESRF Foundation Phase Report, 2/1987.[2] Etude des septa d’injection S1 et S2 de l’Anneau deStockage ESRF, PS/PA-S Note 89-22. R.Valbuena.[3] Design considerations for the septum magnets ofSincrorone Trieste, ST/M-89/12. G.Stange.[4] Etude du septum d’injection S3 de l’Anneau deStockage ESRF. PS/PA-S Note 90-14. R.Valbuena.[5] Magnetic calculations on the eddy current septummagnet for the ESRF, Note/AT/MA/90-08. F.Rohner.[6] Technical specification for the pulsed power suppliesof the septum magnets S1-S2, Feb. 1990. F.Völker.[7] The capacitor discharge power converters for thepulsed septum magnets in the PS straight sections 16and 58, CERN/PS/88-26 (PO). F.Völker et al.

The Pulsed Power Converter and Septum Magnet System for Injection into the Electron Storage Ring at ESRF

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