Challenges for magnetic design of a compact booster fed by single power supply
Proceedings of the 2003 Bipolar/BiCMOS Circuits and Technology Meeting (IEEE Cat. No.03CH37440)
In design of full energy Booster injectors, commonly used in the modern accelerator facilities, there is a tendency of avoiding saturation of the magnetic elements in order to avoid losses associated with tune change during the energy ramp. Typical maximum field in the bending magnets of the modern Booster projects of 1.0-1.4 T results in the large circumference. For 0.27-1.2 GeV full energy Booster injector for the Duke FEL storage ring, recently under design and fabrication, there was an
... , there was an ultimate goal to fit it into existing storage ring room to avoid cost extensive building construction. Therefore, the Booster ring has to be compact, therefore the maximum field in the bending magnets was accepted 1.76 T. With a different level of saturation in the bending magnets, focusing and defocusing quadruples it was not possible to avoid a tune change with the energy rise. However, the ratio of saturation levels for the elements was optimized to avoid crossing of any significant resonance while ramping through the entire energy range. The lattice was simulated for different energies based on the results of 3D calculations of the magnetic elements with the use of MERMAID 3D code  . Another challenging part of the design was supplying all the dipoles and quadrupoles by single power supply. BOOSTER LATTICE A fast Booster-shynchrotron providing for a full energy top-off injection is commonly accepted in nowadays an integral part of any modern accelerator facility, specifically for SR facility. The one for the Duke FEL storage ring was proposed in the year 2000 as a part of the DOE proposal to improve drastically performance of the High Energy γ Source (HIγS) at Duke [2, 3] . The Booster will provide for top-off replacement of up 4 nC/sec of electron loss while producing intensive γ-rays beam of high energy. Extraction energy must be variable within 0.3-1.2 GeV range. Existing 270 MeV linac will be injector for the Booster. The RF frequencies of the Booster and the storage ring are identical. The odd ratio of the harmonic numbers of the ring and the Booster 64/19 provides for extraction of individual bunch from any bucket of the Booster into selected RF buckets of the storage ring. The lattice is optimized for the fast 11 nS pulse kicker providing for the single bunch extraction. The value of that kick has to be as low as possible. Thus, we have chosen vertical single kick symmetrical injection/extraction scheme with β y ≈25 m (Fig.2) at the location of the kickers and septum magnets. Q y ≈1/2 allows to install them in the opposite straight sections. Table 1: Main parameters of the Booster (at 1.2 GeV) Maximum beam energy [GeV] 1.2 Injection energy [GeV] 0.27 Average beam current [mA] 100 Circumference [m] 31.902 Bending radius [m] 2.273 RF frequency [MHz] 178.55 Number of bunches 8 -19 Shortest operation cycle [sec] 2.5 Energy rise time, min [sec] 0.5 -0.8 Beam emittance ε x , ε y [nm . rad] 350/ 15 Maximum β x / β y / η x [m] 25.4/9.4/1.4 Betatron tunes Q x /Q y 2.43/ 0.46 Momentum compaction factor 0.153 Natural chromaticity C x /C y -1.7/ -3.7 Damping times τ x,y / τ s [mS] 3.16 / 1.58 Energy loss per turn [KeV] 80.7 Energy spread σE/E 6.8⋅10 -4 Magnetic System: Dipoles (G=2.7 cm): ea./B max [T]/L eff [m] 12/ 1.76/ 1.19 Quadrupoles (D=5.0 cm): QF1: ea./ G max [T/m]/ L eff [m] 4/ 27.6/ 0.151 QF2: ea./ G max [T/m]/ L eff [m] 4/ 19.5/ 0.151 QD : ea./ G max [T/m]/ L eff [m] 8/ 8.4/ 0.131 Supplied by the same current I max [A] 700 Sextupoles (D=6.0 cm): SF: ea./ B″ max [T/m 2 ]/ L eff [m] 4/ 100/ 0.098 SD: ea./ B″ max [T/m 2 ]/ L eff [m] 4/ 70 / 0.098 We have originally planned a single turn injection , though stacking is also considered. The orbit is predistorted prior to the kick by strong vertical trim dipoles located in the injection/extraction straight section and providing for 10 mm local orbit bump at septum magnets. The pre-distortion of the orbit relieves vertical aperture constrains associated with 27 mm dipole gap (∼24 mm stay-clear). The kick value required for the injection/extraction is 0.675 mrad. The designed repetition rate for the extraction kicker is up to 25 Hz. Figure 1: Layout of the Booster synchrotron in the North-East corner of the Duke FEL storage ring building.