UNIQUE FEATURES IN MAGNET DESIGNS FOR R AND D ENERGY RECOVERY LINAC AT BNL.
In this paper we describe the unique features and analysis techniques used on the magnets for a R&D Energy Recovery Linac (ERL) [1] under construction at the Collider Accelerator Department at BNL. The R&D ERL serves as a test-bed for future BNL ERLs, such as an electron-cooler-ERL at RHIC [2] and a future 20 GeV ERL electron-hadron at eRHIC [3]. Here we present select designs of various dipole and quadruple magnets which are used in Z-bend merging systems [4] and the returning loop, 3-D simulations of the fields in aforementioned magnets, particle tracking analysis, and the magnet's influence on beam parameters. We discuss an unconventional method of setting requirements on the quality of magnetic field and transferring them into measurable parameters as well as into manufacturing tolerances. We compare selected simulation with results of magnetic measurements. A 20 MeV R&D ERL (Fig. 1) is in an advanced phase of construction at the Collider-Accelerator Department at BNL, with commissioning planned for early 2009. In the R&D ERL, an electron beam is generated in a 2 MeV superconducting RF photo-gun, next is accelerated to 20 MeV in a 5 cell SRF linac, subsequently passed through a return loop, then decelerated to 2 MeV in the SRF linac, and finally is sent to a beam dump. The lattice of the R&D ERL is designed with a large degree of flexibility to enable the covering of a vast operational parameter space: from non-achromatic lattices to achromatic with positive, zero and negative R56 parameter. It also allows for large range tunability of Rlz and lattice RS4 parameters (which are important for transverse beam-break-up instability). Further details of the R&D ERL can be found elsewhere in these proceedings [5]. The return loop magnets are of traditional design with the following exceptions: (a) The bending radius of the 60{sup o} dipole magnets is 20 cm, which is rather small. We use 15{sup o} edges on both sides of the dipoles to split very strong focusing evenly between the horizontal and vertical planes (so-called chevron-magnet). (b) The requirements on field quality of the loop's quadrupoles had been determined by the requirement to preserve a very low normalized transverse slice emittance of electron beam ({var_epsilon} {approx} 1 mm-mrad). We used direct tracking of a sample electron beam to verify a high degree of the emittance preservation. (c) Each quadrupole is equipped with a dipole trim coil, which can be also used to excite a sextupole component, if required, for emittance preservation of e-beam with a large energy spread. One of the unique features of all ERLs is the necessity for merging low and high energy electron beams. In the R&D ERL, 2 MeV from the SRF gun merges with the 20 MeV electron beam coming around the return loop into the same trajectory at a position within the SRF linac. In the linac, injected bunch is accelerated to 20 MeV, while the returned or ''used'' bunch is decelerated to 2 MeV. The challenge for a merger design is to provide conditions for emittance compensation [5] and also for achromatic conditions of a low energy, space-charge dominated-e-beam [4,6]. The scheme which satisfies these requirements (called 2-bend [4]) is used on the R&D ERL. The Z-bend is approximately 4-meter long. It bends the beam trajectory in the vertical plane. It is comprised of four dipole magnets designed to be equally focusing in both planes, with bending radius {approx} 60 cm, and bending angles of: +15{sup o}, -30{sup o}, +30{sup o} and -15{sup o}. The beam dynamics in the Z-bend results in a large-size (centimeters) near-laminar electron beam [7]. The large beam size and very low slice emittance of the e-beam dictates the tolerances on the magnetic field to be very tight. The integrated nonlinear kicks should not exceed {approx} 20 micro-radian per magnet at a typical radius {approx} 1 cm. The magnets in the Z-bend are rather short (15 cm effective length for the 15{sup o} magnet) and have a rather large aperture of 6 cm. Analysis predicts that the influence of various field components on the emittance growth are complicated by the fact that the beam trajectory bends significantly in the Einge fields. Hence, we decided to use direct tracking in the calculated fields extracted from Opera3d of test beam to evaluate and to minimize influence of magnetic field on the beam emittance. In addition, we used predictions of Opera3d and compared them with results of magnetic measurements for the return loop dipole and quadrupole. One of the features of the loop magnets is that they are fabricated with a very high geometric tolerance, allowing them to be an excellent test bed for bench-marking our predictions. Agreement with the prediction provides us with sufficient confidence that Z-bend magnets will preserve beam emittance.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States)
- Sponsoring Organization:
- Doe - Office Of Science
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 913082
- Report Number(s):
- BNL-79225-2007-CP; R&D Project: 18086; KB0202012; TRN: US0800642
- Resource Relation:
- Conference: 22ND PARTICLE ACCELERATOR CONFERENCE ; ALBUQUERQUE, NEW MEXICO; 20070625 through 20070629
- Country of Publication:
- United States
- Language:
- English
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