Lattice Design of 2-loop Compact ERL. High Energy Accelerator Research Organization, KEK Miho Shimada and Yukinori Kobayashi

Transcription

1 Lattice Design of 2-loop Compact ERL High Energy Accelerator Research Organization, KEK Miho Shimada and Yukinori Kobayashi

2 Introduction Wepromote the construction of the compact Energy Recovery Linac(cERL) as the test facility of 5GeV-ERL. Main parameter Maximum energy Current Charge per bunch Repetition Normalized emittance MeV ma pc 1.3 GHz mm-mrad Energy spread < 3 x10-4 Bunch length low emittance bunch compression 1-3 ps 0.1 ps Why 2-loop ERL High energy beam is feasible at a reasonable low cost in a small space Challenging beam dynamics and operation

3 Lattice design of 2-loop compact ERL (cerl) Adjustment chicane Circumference m Merger section Dump for 5MeV Branch chicane 500V DC electron gun Sizes of magnets Main superconductor cavities For merger, extraction and adjustment chicanes For Linear lattice BEND 300mm x 300mm 700mm x 785.3mm QUAD 300mm x 100mm 600mm x 200mm Extract section Small magnets are the same size as one for 5MeV injector beam. They don t affect on the linear lattice for high energy beam. Large magnets can make a magnetic field at T ( inner loop) and 1.3T( outer loop ).

4 Layout of injector and merger section Injector beam merges with circulating beam at the angle of 16 degree. More detail information will be presented by Dr. T. Miyajima (tomorrowmorninginwg2 ) Merger section circulating Change in orbit ( emphasized) Branch Chicane Branch chicane injection Energy ratio of circulating to injection should be large because the circulating beam is also kicked and need to be bumped at the merger section. Minimum energy ratio is 1:7( 5 : 65 MeV) and then the bump height is 60 mm. Orbit of the outer loop changes if the energy ratio of energy at the outer loop to inner loop. Branch chicane compensates the change in the orbit.

5 How to design the linear lattice of 2-loop ERL Outer loop Inner loop Accelerator and decelerator Scheme of the lattice design using SAD code (Strategy Accelerator Design) STEP1 accelerator and decelerator linac Three kinds of energy (acceleration and deceleration) pass through the same quadrupole magnets four times. STEP2 inner loop Dispersion function is zero at the straight section (achromatcondition). Basically, the inner loop is operated with an isochronous condition ( R56 = 0 ) to maintain the bunch length. STEP3 outer loop Dispersion function is zero at the straight section (achromatcondition). In low emittance mode, the optics of the outer loop is also isochronous. A short bunch is achieved at the outer loop with non-zero R56 in bunch compression mode. ( Bunch the compression optics are not shown in this presentation. )

6 STEP1 : Design of accelerator linac Calculation The quadrupolemagnets for deceleration can not be controlled independently because the beam passes through the same magnet with the acceleration. Sometimes, the beam size will blow up after deceleration down to 5MeV. It should be flexible for an initial condition of the twissparameter because the merger section and the first triplet should be optimized by another simulation including the space charge effect at low energy. How to calculate Three triplets are optimized for two accelerator and decelerator sections at the same time using dummy inner and outer loop. Dummy loop consists of some qadrupole magnets. The simplest way to suppress the betatronfunction over all make a symmetric optics with respect to the linac for accelerating and decelerating. First acceleration Second acceleration First deceleration Second deceleration Electron Gun Dummy inner loop Dummy outer loop Dump Fig. All accelerator and decelerator linacof 2-loop ERL with dummy loops

7 Lattice design result 1 Dummy inner loop Dummy outer loop Temporal initial twissparameters ; bx13 m, ax -2, by 0.7 m, ay 0. Last triplet Betatronfunction is suppressed at the main cavity. Horizontal betatronfunctionincreases up to 90 m at the last triplet just before the extraction. It may cause the large beam sizein the bunch compression mode because the horizontal emittanceincreased by the CSR effect. Lattice design result 2 Last triplet Maximum horizontal betatron function at the last triplet is suppressed down to 20 m. On the other hand, the betatronfunction at the main cavity is larger than above result.

8 STEP2 : Design of Inner loop Two Triplets The triplets used for achromat and isochronous condition. Matching section The other quadrupolemagnets are used for matching of twiss parameter to the accelerator linac section. Fig. Example of the achromat and isochronous optics of inner loop

9 STEP3 : Design of outer loop Uncontrollablearea. Basically, the quadrupole is optimized for the inner loop. Two triplets for achromat and isochronous condition Matching section for matching the twiss parameter of linac Fig. Example of the achromat and isochronous optics of outer loop

10 Summary Scheme of design of 2-loop cerl 2-loop cerlis divided into three sections; accelerator linac, inner loop and outer loop. Accelerator and decelerator linacis designed at first, two loops are matched with the linac. Accelerator & decelerator linac Three kinds of energy pass through the same quadrupole magnets. Two accelerator and decelerator linacis optimized at the same time using dummy inner and outer loop. The maximum betatronfunction at the last triplet can be suppressed down to 20 m by a symmetric optics. Achromat and isochronous condition of inner and outer loop Triplets between the bending magnets are used for an achromatand isochronous optics. Other quadrupolemagnets are used for matching with the accelerator and decelerator linac.

11 Branch Chicane Change in orbit in outer loop ( emphasized) ~10 mm Energy of circulating beam depends on the acceleration of main cavity. case 1 : 65 MeV: 125 MeV ( 5MeV injection, 60 MeVacceleration per circle ) case 2 : 125 MeV: 245 MeV( 5MeV injection, 120 MeVacceleration per circle ) Branch chicane compensate the change in the orbit caused by the change in the energy ratio of outer to inner loop. Maximum of the horizontal orbital change is close to 10 mm.