BEAM OPTICS STUDY FOR THE COMPACT ERL IN JAPAN

T. Shiraga, N. Nakamura, H. Takaki, ISSP, University of Tokyo, Kashiwa 277-8581, Japan R. Hajima, JAEA, Tokai 319-1195, Japan

K. Harada, M. Shimada, T. Miyajima, Y. Kobayashi, S. Sakanaka, KEK, Tsukuba 305-0801, Japan

Abstract Beam optics of the compact ERL was studied with the

simulation code “elegant” including CSR(Coherent Synchrotron Radiation) effects and optimized for each of three operation modes. In the optimized optics of HC(High Current) and LE(Low Emittance) modes, the normalized emittance was well preserved and excellent energy recovery was achieved. In the optimized optics of BC(Bunch Compression) mode, an ultra-short bunch of several tens picoseconds was achieved and the horizontal and vertical beam sizes were well suppressed by tuning the betatron functions and the decelerating RF phase to transport the beam to the beam dump without serious beam loss. In this paper, we present the results of the design study on the compact ERL optics.

OVERVIEW OF THE COMPACT ERL A compact ERL(energy recovery linac) is planned to be

constructed in Japan in order to demonstrate excellent ERL performances and to test key components such as low-emittance photocathode gun and superconducting RF cavity[1]. The layout of the compact ERL is shown in Fig. 1. The compact ERL consists of two TBA(triple bend achromat) arc sections, two long straight sections and injector and dump sections. The injector section generates the bunched beam and accelerates the beam to 5 MeV. Two main superconducting(SC) cavity modules in one long straight section accelerate the beam to 85-165 MeV for the acceleration field of 10-20 MV/m and also decelerate the accelerated beam down to the injection energy level for energy recovery. The other long straight section has a chicane system generating an orbit bump for tuning the orbit path length and hence the decelerating RF phases of the main SC cavity modules. It can also be used for insertion devices and SR user experiments. The two TBA arc sections can control the bunch length using their quadrupole and sextupole magnets. The compact ERL has three operation modes: HC(High Current), LE(Low Emittance) and BC(Bunch Compression) modes, which can realize a high current of 100 mA, a low normalized emittance of 0.1 mm·mrad and a ultra-short bunch of less than 100 fs, respectively. Table 1 lists the parameters of the compact ERL used for our beam optics study.

Figure 1: Layout of the compact ERL.

Table 1: Parameters of the compact ERL for three modes Parameter HC LE BC RF frequency[GHz] 1.3 Beam energy[MeV] 85-165 Injection energy[MeV] 5 (10)* Injection rate [GHz] 1.3 1.3 ≤ 0.001 Beam current[mA] 100 10 - Bunch charge[pC] 77 7.7 ≥ 77 Emittance**[mm·mrad] 1 0.1 - Bunch length[ps] 1-3 1-3 < 0.1

*Increased up to 10 MeV, **Normalized emittance

DESIGN CONDITION AND STRATEGY In the beam optics study of the compact ERL with

elegant[2], the injector section including three bending magnets for injection was not considered and an initial beam with six-dimensional Gaussian distribution was given at the exit of the injector section. The initial horizontal and vertical betatron functions and their derivatives were assumed to be 20 m and zero. The bending magnets of the chicane system were normally turned off and effects of the bending magnets in the chicane and extraction systems on the optics were neglected.

In the compact ERL, R56 of the 1st arc section was utilized for the bunch compression. This parameter is expressed by

R56 = ηρC∫ ds , (1)

where η and ρ are the dispersion function and the bending radius in a section C. The deviation of the path length Δz or the time delay Δt is given by

Δz = cβΔt = R56

Δpp

+ T566Δpp

⎛

⎝ ⎜

⎞

⎠ ⎟

2

+ ... . (2)

Here Δp is the momentum deviation from the reference momentum p and T566 is the 2nd order coefficient of the transfer matrix elements. In BC mode, the optimum R56 value of the 1st arc section for bunch compression is approximately expressed with the accelerating RF phase φRF and the RF wave number kRF as follows:

R56 ≅ 1kRF sin φRF

. (3)

The detail of the R56 setting is shown in ref. [3]. R56 of the 2nd arc section is normally set to the same value with the opposite sign to recover the bunch length.

The design procedure for the compact ERL optics is as follows:

• R 56’s of the 1st and 2nd TBA arc sections are set to the optimum values for each operation mode. The

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Proceedings of PAC09, Vancouver, BC, Canada TU5RFP084

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S. Sakanaka etM. Borland, PhT. Shiraga, MaH. Sakai et al.,

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REFEREt al., in these phys. Rev. ST-aster Thesis (i, ERL07, Dare

bunch (a) at iny modules andexpressed in u

ertical beam de.

h length at exitngth, (b) initiam spread, (d) de.

NCES proceedings. -AB 4, 070701in Japanese), 2esbury, 2007.

nitial position,d (c) at exit ofunit of mec.

sizes along

t of the 1st arcal normalizedbeam energy

1 (2001). 2009.

, f

g

c d y

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