Bangerter, R O

Barnard, J J

Caporaso, G J

Fong, C G

Friedman, A

Godlove, T F

Hewett, D W

Judd, D L

Kirbie, H

Neil, V K

Newton, M A

Paul, A C

Reginato, L L

Sharp, W M

Yu, S S

CERN Document Server

Particle Accelerators, 1992, Vols. 37-38, pp. 489-492Reprints available directly from the publisherPhotocopying permitted by license only© 1992 Gordon and Breach Science Publishers, S.A.Printed in the United KingdomPRELIMINARY DESIGN FOR A RECIRCULATINGINDUCTION ACCELERATOR FOR HEAVYION FUSIONtS. s. YU, J. J. BARNARD, G. J. CAPORASO, A. FRIEDMAN, D. W. HEWETT,H. KIRBIE, M. A. NEWTON, V. K. NEIL, A. C. PAUL, L. L. REGINATO andW. M. SHARPLawrence Livermore National Laboratory,P. O. Box 808, L626, Livermore, CA 94550, USA.T. F. GODLOVEFM. Technologies, Inc., 10529-B Braddock Road, Fairfax, VA 20032, USA.R. O. BANGERTER, C. G. FONG and D. L. JUDDLawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA.(Received 5 December 1990)Substantial savings in size and cost over a linear machine may be achieved in an induction acceleratorin which a heavy ion beam makes many ('" 50) passes through one or more circular accelerators. Weexamine a point design for such an accelerator, consisting of four rings. We discuss the consequences ofthis design on emittance growth, longitudinal instability growth, vacuum requirements, pulser require-ments, pulsed-magnet requirements, acceleration schedule, and cost.The concept of recirculating induction accelerato,r as a fusion driver has been knownever since the early days of heavy ion fusion. The potential cost advantages of sucha machine are obvious. Multiple passes through induction cores (N1ap times) lead toa reduction of core material by a factor of I/N1ap ' In contrast to a linear machinewhose length is limited by the maximum accelerating gradient ("" 1 MV/m), thecircumference of a circular machine is limited by the effective radius of curvature forbending. The savings in length is about a factor of 5 for a given charge state. Thecost for focusing magnets and structures could be expected to be reduced correspond-ingly. In addition, multiple use of induction cores eases the constraints on minimizingthe total volt-seconds. One can then generally consider pulses with longer duration.For a given charge per bunch, this implies new freedom in reduced current and/ornumber of beamlets. The transverse dimensions of circular machines could beexpected to be much smaller.t Work performed under the auspices of the U.S. Department of Energy by Lawrence LivermoreNational Laboratory under contract W-7405-ENG-48 and at LBL by the Director, Office of EnergyResearch, Advanced Energy Projects Division, U.S. DOE under contract DE-AC03-76F00098.489490 S. S. YU et a/.TABLE 14-ring Recirculator Design ExampleIR LER MER HERIon Energy 3-20 MeV 20-100 MeV 0.1-1 Gev 1-10 GevPulse Duration 115-32.5 32.5-3.5 3.5-0.55 0.55-0.10(Jls)Circumference (m) 335 283 921 2591Current/beam (A) 0.9-3.1 3.1-29 29-182 182-1000No. of Beams 4 4 4 4No. of Laps 50 50 50 50Pipe Radius (m) 0.084 0.075 0.068 0.068Lattice Half 0.80 0.87 1.26 3.03Period (m)Induction Modules:Inner radius (m) 0.295 0.263 0.236 0.237Outer radius (m) 0.565 0.583 0.565 0.598Length (m) 0.404 0.481 0.591 0.542No. of Cores 224 221 607 1580Bends (Magnetic Dipoles):Length (m) 0.23 0.31 0.54 1.6Max. Mag. Field 0.7 1.5 1.1 1.0(T)No. of Bends 1424 1100 2712 3200Superconducting Magnetic Quadrupoles:Length 0.52 (0.40) 0.49 (0.39) 0.59 (0.48) 1.08 (0.96)(Eff. Length) (m)Max. Mag. Field 1.3 2.8 3.2 2.1(T)No. of Quads 1560 1184 2800 3304Offsetting the potential cost advantages described above, recirculators were ex-pected to have a more complex pulse-power system to accommodate the variablelap-to-lap format in the induction cells, along with time-varying dipoles to matchthe lap-to-lap increase in beam energy. The cost reduction potential of recirculatorscould be realized provided that the pulsers are not prohibitively expensive. Inaddition, new issues associated with the dynamics of space-charge dominated beamsgoing around bends and with possible beam loss due to long resident times must beaddressed.These considerations have led to an intensive study by a team from LLNL andLBL over the last few months. Our goals were to ascertain whether the recirculatorconcept is indeed technically viable, and to construct a realistic cost estimate on thebasis of a concrete point design.The present study focuses on a 4 MJ system, consisting of ions of mass 200 andcharge state + 1, accelerated to final energy of 10 GeV. The driver design exampleconsists of 4 rings. Acceleration from 100 MeV to 10 GeV is accomplished in thehigh-energy ring (HER) and the medium-energy ring (MER), each ring incrementingthe beam energy by one decade. The low energy end consists of the injection ringRECIRCULATING INDUCTION LINAC 491(IR), which is fed by a 3 MeV injector, and accelerates ions to 20 MeV, and thelow-energy ring (LER), which provides transition from IR to MER. The keyparameters of the 4-ring driver are shown in Table 1.Conceptual designs of the dipole and induction core pulsers for the 4-ring driverhave been constructed. The dipole field must be ramped to its peak value of 1-2 Tin ~ 1 millisecond. Very efficient energy recovery is required since the energy storedin the dipole fields is about ten times the total beam energy. A sinusoidally ringedcircuit with appropriate switching for energy recovery! is expected to be ~ 95%efficient. Multiple pulsing of the induction cells is accomplished in the IR and LERby means of hard-vacuum tubes and in the MER and HER with pulse-forming-networks together with appropriate switching, using thyratrons and magneticdevices. 2The recirculating induction accelerator, like the linear version, requires multiplebeam operation to accommodate space-charge effects. To minimize the complexitiesof beam manipulation around the rings, we have chosen a design which consists offour parallel beams from injector to final extraction. No beam merging is required.The 4-beam configuration at the low energy end is made possible by operating atvery low currents, and therefore long pulses. At the 3 MeV injection into the IR, thebeamlet current is 0.87 A and the pulse duration is 115 /lS. With this low current, thebeamline consists of half lattice periods (L) which are ten ti~es longer than the piperadius (R). This aspect ratio allows adequate room for focusing, bending andextraction/injection without resorting to extremely short magnets. The beam isaccelerated at constant beam size to 20 MeV" at which point the current is 3.1 A,and the pulse duration is 32.5' /lS. In our design, the acceleration in the IR (as wellas in each of the other rings) is achieved in 50 laps. The core material required istherefore quite reasonable even though the pulses are long.The LER is designed to accommodate a relatively large pulse compression factor.The pulse duration at exit is 3.5 /lS. The hard-tube switches in the LER provide theflexibility required for the somewhat more delicate beam manipulations neededduring compression. Other constraints on focusing and bending are also eased bydesigning for only a fivefold increase in beam energy in the LER.The acceleration schedules in the MER and HER are both tailored for the switchingavailable. Since the pulse-forming-network can produce only one voltage waveformwith a fixed duration (although it can be repetition-rated for variable intervals) thebeam pulse duration is fixed during acceleration. After acceleration is complete ineach of the rings, the pulses are compressed to the desired pulse length in thelast couple of laps. The velocity tilt required for compression is induced by judiciouslydelaying some of the pulsers.The design of the MER and HER is by no means unique. We strive to maintaina balance between the dual goals of low cost and high efficiency. As an example, ourcells are designed with four times more Metglas than is required by the maximal fluxswing. This measure reduces the core losses drastically. The dipole fields employedreach a maximum of only 1 T. This design increases the number of dipoles (ascompared to a higher field design) but reduces the field energies required. Costreductions were achieved by employing low-field superconducting magnets (2 to 3 T),P.A.-S.492 s. S. YU et al.and embedding them inside the induction cores. On the basis of our first bottoms-upestimate, the efficiency of this driver is calculated to be 24%, and substantial savingsin cost over a linear machine appear to be borne out.The longitudinal instability for our recirculator point design is much milder thanits linear counterpart, primarily because of a much reduced impedance per unit lengthdue to the lower accelerating gradient. The low-frequency gain is calculated to be 5or 6 e-folds even before capacitative effects are taken into account. The transversebeam breakup instability can be kept quite small as long as care is taken tominimize the quality factor Q of the induction cells. Although detailed calculationsof the resonance crossing instability have not yet been performed, it is expected tobe insignificant because of the large changes in tune (~v > 1) from revolution torevolution.Our point design is based on 50 passes per ring. The long residence time can leadpotentially to beam loss due to charge exchange and stripping by background' gas.Charge exchange losses turn out to be exceedingly sensitive to beam radius (cxa- 4 ).By a judicious choice of focusing strengths, beam radii are kept to no less than 3 or4 cm in all rings, yielding ~ 1% loss due to charge exchange over the millisecondresidence. Vacuum requirements imposed by beam stripping are somewhat moredifficult to determine. The primary unknowns are related to the generation ofbackground gas as beam and background ions impinge upon the accelerator walis.Based on our best estimate of the ionization cross sections and desorption coefficients,beam loss could be kept sma1l 3 for background pressure of ~ 10- 10 Torr.Emittance growth of space charge dominated beams around bends is an area ofactive studies. Analytic work by Ed Lee and V. Kelvin Neil and numerical simulationsby A. Friedman have not to this point discovered mechanisms for major brightnessdegradations for matched beams, but further theoretical and experimental work isrequired to understand this central issue.REFERENCES1. L. Reginato, M. Newton, and S. Yu, "Concept for powering the dipole magnet in a recirculatinginduction accelerator for HIF," these Proceedings.2. M. Newton, L. Reginato, J. Barnard, and S. Yu, "High repetition rate pulser concepts for a recirculatingLINAC HIF driver," these Proceedings.3. J. Barnard, S. Yu, and A. Faltens, "Vacuum requirements for heavy ion recirculating induction linacs,"these Proceedings.

Preliminary design for a recirculating induction accelerator for heavy ion fusion

Preliminary design for a recirculating induction accelerator for heavy ion fusion

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