WG208

Proceedings of ERL09, Ithaca, New York, USA

LATTICE DESIGN OF 2-LOOP COMPACT ENERGY RECOVERY LINAC
Miho Shimada 1, Yukinori Kobayashi
High Energy Accelerator Research Organization, KEK
1-1 Oho, Tsukuba, Ibaraki, 305-0801
Abstract
Compact Energy Recovery Linac (Compact ERL) is planned to construct in the KEK site as a test facility of 5GeVERL project. For achieving the high energy with a limited refrigeration power, the electron beam is accelerated twice by
the same super-conducting cavity in a 2-loop design. At the branch of the two loops, a chicane is installed for flexibilityof the ratio of the lower energy in the inner loop to higher energy in the outer loop. The angle of merger of the injection
is 16 degree. The linear optics of the linear accelerator section is optimized for the two accelerator and two decelerator
beams using the “dummy loop”, which is used for determination of the twiss parameters of the entrance/exit of the twoloops. Both inner and outer loop are designed to be an achromat and isochronous.

1. Introduction
2.
5GeV energy recovery linac project has beenpromoted as the next generation light source, which isplanned to be sited in KEK. For saving the building cost,site area and the refrigerating power, 2-loop scheme is astrong candidate for KEK project as well as others
projects, such as Cornell Laboratory [1]. On the otherside, the higher order mode (HOM), which accumulated
in the superconducting cavity at lower current and thecomplicated beam dynamics are critical issues.
Compact ERL, which is a test facility of 5GeV-ERL,
is using 2-loop scheme for achieving the higher electronenergy with a limited refrigerating power. For
optimization of the optical functions, the main linac
section is should be careful because two accelerating andtwo decelerating beams pass though the same magnets.
In this paper, we proposed the method of optimisation of
optical functions and the simulation results for theCompact ERL.

Lattice layout of 2-loop Compact ERL

2.1 Main parameters and magnet layout
Figure 1 shows the schematic drawing of the layout ofthe Compact ERL. 500kV electron beam from DC
photo cathode electron gun [2] is accelerated up to 5MeV
at the injection section and lead to the main linac section
at the merger section, which is composed of three bending
magnets [3]. The electron beam accelerated up to 65 MeV
passes though the inner loop and then is accelerated again
up to 125 MeV. 125 MeV electron beam passing though
the outer loop is decelerated twice at the main linac
section down to 5 MeV. 5 MeV electron beam is lead tothe dump with the extraction section. Total electron path
length and the site area of the circulating section are 291.9
m and 47 m × 9.3 m, respectively. The lengths of the twomain superconducting cavities are 8m and 10m, whichcontain four 9 cell-cavities in one cryostat and in twocryostats, respectively [4]. 60MeV acceleration per a turnwill be upgrade to 120MeV in the future, in which themaximum electron energy is 245 MeV.

Fig. 1 : Schematic of 2-loop of compact ERL
Optics & Beam Dynamics
E-mail: miho.shimada@kek.jp

150Proceedings of ERL09, Ithaca, New York, USAWG208

Fig.2 : Layout of merger section
Fig.3 : Schematic drawing of energy branch chicane
and the change in orbit.
2.2 Merger section
The schematic drawing of the merger section is shownin Fig.2. 5 MeV injection electron beam are merged at thechicane with three bending magnets at the angle of 16
degree. A chicane is installed for the circulating beam
because to compensate the change in the orbit kicked by
the merging bending magnet. The minimum electronenergy of the circulating beam is 35 MeV and then thedisplacement of the orbit at the center of the chicane isabout 60 mm.
2.3 Energy branch chicane
The energy ratio between the inner and outer loop
depends on the injection and acceleration by the main
linac, e.g., 65:125 and 125:245. The change in the ratio is4%. For leading the full energy electron bean to the center
orbit of the outer loop for both energy ratios, we installed
the branch chicane at the entrance of the inner/outer loop.
The schematic drawing of the branch chicane is shown in
Fig.3, where the change in the orbit is enhanced. Thechange in the energy ratio, 4%, induces the orbital shift 10mm at the center of the branch chicane.

Optics & Beam Dynamics

3．Optical function of main linac
In the main linac, two cryostats of the superconducting
cavity are between the three triplets, and then the twoaccelerator and two decelerator beams pass though thesame triplets. The optical function should be designed not
to be large at the last triplet just before the extraction
section because the electron energy is low and thetransverse emittance becomes larger at the loop sections.
The focus strength of the triplets is optimized for thelowest energy beam. For the higher energy, the optical
function is controlled by the twiss parameter at theboundary to the loop section because the focus strength is
too small. The optical function is optimized using theSAD (Strategy Accelerator Design) and the two dummy
loops, which consists of four quadrupole magnets, areinserted for calculation of the twiss parameters at theboundaries. The twiss parameters at the exit of theinjector section is (βx, αx, βy, αy) = (13 m, -2, 0.7 m, 0).
Two results of the optical functions are shown in Fig.4.
Figure 4 (a) is minimized the optical function at the mainsuperconducting cavities. The optical function at theFig.4 : Optical function of the main linac
51

WG208Proceedings of ERL09, Ithaca, New York, USAFig.5 : Optical function of the inner loop

Fig.6 : Optical function of the outer loopす）linac and thedecelerator linac is larger than accelerator
betatron function at the last triplet is over 90 m. In such aoptics, the transverse beam size can be over the chamber
size at the deceleration linac and extraction section. On
the other side, in the symmetric optical function shown inFig.4 (b), the betatron function is suppressed down to 20
m at the last triplet.

4.

Optical function of inner and outer loops

The arc section of both inner and outer loops aredesigned to be an achromat and isochronous optics (R 56=0) for minimization of the transverse beam size andmaintaining the longitudinal bunch length. The two arc
sections are TBA like lattice consisting of four bending
magnets with the bending angle of 45 degree in order tochange R 56 for bunch compression. The outer loop
follows the branch chicane for electron beam with several
energy ratios. The triplets between the bending magnets
are used for the achromat and isochronous optics and thematching sections consist of several doublet and are usedfor matching the twiss parameter between the main linac
section and arc section. For simplicity, the edges of thebending magnets are rectangle at the branch chicane andsector at the arc sections.
Examples of the optical functions of the inner and outer
loop are shown in Fig.5 and Fig.6, respectively. The twiss
parameters at the boundary are the same with the Fig.4 (b).
Theoptical function has a flexibility over all and we canOptics & Beam Dynamics
find several optical function.
525.

Summary

We report the strategy of the liner optical function andthe first optical function of the 2-loop Compact ERL. At
first the optical function of the main linac is optimized byusing the dummy loops. At the loops, the triplets arecalculated for an achromat and isochronous optics, andfinally the matching sections are used for matching
between them. In ERL optics, it should be caureful to pay
attention that all electron beams with different energy passthough the same magnets with a reasonable opticalfunction. In this paper, we showed the example of thesymmetric optical function with low betatron function at
the last triplet.

References[1] G. H. Hoffstaetter et al. “Cornell ERL program”, inProceedings of ERL09, 2009
[2]
N.Nishimori, et al. “ JAEA/KEK gun status” inProceedings of ERL09, 2009[3] T.Miyajima, et al. “Tolerances for errors in ERL linacs” inProceedings of ERL09, 2009[4] H.Sakai, et al. “KEK ERL cryomodule development” inProceedings of ERL09, 2009