Using IMPACT T to perform an optimization of a DC gun system Including merger

Transcription

1 Using IMPACT T to perform an optimization of a DC gun system Including merger Xiaowei Dong and Michael Borland Argonne National Laboratory Presented at ERL09 workshop June 10th, 2009

2 Introduction An energy recovery linac (ERL) is a potential candidate for an Advanced Photon Source (APS) upgrade at Argonne National Laboratory. The injector is a key element of the ERL@APS upgrade since the beam properties are largely determined by the injector system. The APS ERL project has two investigated working modes, namely, high flux (HF) and high coherent (HC) modes1. High coherence mode: High flux mode: Bunch charge: 19 pc Bunch charge: 77 pc Bunch length: 2 3 ps Bunch length: 2 3 ps Normalized transverse emittance Normalized transverse emittance < 0.1 μm < 0.3 μm Energy spread < 140 kev Energy spread < 140 kev Beam energy: 10 MeV Beam energy: 10 MeV 1 G. Hoffstaetter, Status of Cornell ERL Project, fls2006.desy.de. 2

3 Design1 We keep the standard features of both the JLab2 and Cornell3 injectors: DC gun + solenoid + buncher + solenoid + booster configuration For now, a TESLA type 9 cell accelerating superconducting cavity4 is used as the booster. A zigzag type merger5 (BNL) is used to merge the beam from DC injector and high energy beam. Achromatic lattice: 10 bend, 40 cm drift, 20 bend, 81.6 cm drift, 20 bend, 40 cm drift, 10 bend X. Dong et al., Proc. PAC09, MO6RFP044. T. Siggins et al., NIM A 475 (2001) I. Bazarov et al., PRSTAB 8, (2005). 4 B. Aune et al., PRSTAB 3, (2000). 5 V. Litvinenko et al., NIM A 557 (2006)

4 Beam dynamics software IMPACT T1 is used to track particles through user defined external electromagnetic fields. It is unique in use of three dimensional space charge solvers based on an integrated Green function and a shifted Green function to treat image charge effects of a cathode. It has a comprehensive set of beamline elements. Bending magnets with space charge and CSR supported: essential for merger simulation. Includes short range longitudinal and transverse wakes. ASTRA2 (A Space Charge Tracking Algorithm) includes cylindrical space charge field solver. A side by side comparison with the simulation results from ASTRA demonstrated that IMPACT T is suitable for these simulations. GPT3 (General Particle Tracer), a commercial full 3D particle tracking code Crosscheck the modeling of the Z merger, showing good agreement on the emittance evolution in the bending plane. J. Qiang et al., J. Comp. Phys. 163 (2000) 434. K. Floettmann, 3 Pulsar Physics 1 2 4

5 Laser egg1 The shaping of the photo cathode drive laser pulse is critical for achieving sub micron beam emittances. The ellipsoidal laser pulse has been demonstrated to have the potential to deliver lower final transverse emittance than a uniform cylindrical (beer can) or pancake drive laser distributions. An experimentally feasible quasi ellipsoidal bunch is used in simulation: 28,000 macro particles for good statistics Laser pulse duration: 31.6 ps, not scalable Laser spot diameter: 2 mm 1 Y. Li and J. Lewellen, PRL 100, (2008) 5

6 Optimization of DC gun injector with Z merger The multi objective optimization technique was used successfully for the Cornell design1, because the large number of variables and the complexity of the physics in the injector makes analytical optimization impossible. We use a parallel geneticoptimizer which was developed by M. Borland and H. Shang Based on the nondominated sorting genetic algorithm II Can work with essentially any program or group of programs Optimizes user supplied penalty function depending on any calculated quantity Both single and multi objective optimization are supported Ten decision variables used in the optimization Laser pulse transverse size DC gun voltage 1st solenoid strength 2nd solenoid strength and position Buncher cavity voltage and phase Booster cavity voltage, phase and position 1 I. Bazarov et al., PRSTAB 8, (2005). 6

7 Examples of Rank 1 trial solutions HC Mode (19 pc/bunch) HF Mode (77 pc/bunch) Simulation Requirement Simulation Requirement Energy (MeV) Energy spread (kev) RMS bunch length (mm) / / / / Normalized εx/εy (μm) Normalized εz (μm) We got the 1st rank 1 solution for HC mode after 19,000 iterations. Starting from the optimal result for HC mode, optimization for HF mode was performed. Solutions satisfying all requirements emerged after 4,000 iterations. 7

8 Preliminary result: 19 pc/bunch 8

9 Preliminary result (2) 9

10 Optimum configurations HC HF Laser spot size XYrms (mm) DC gun voltage (kv) st solenoid strength (T) nd solenoid strength (T) nd solenoid position (m) Buncher cavity gradient (MV/m) Buncher cavity phase (degree) Booster cavity gradient (MV/m) Booster cavity phase (degree) Booster cavity position (m) Even though low DC gun voltage is sought by the optimization, the very high DC gun voltage appears to be required for ultra low final transverse emittances. 10

11 Effect of thermal emittance The effective thermal energy of the photocathode is assumed to be 24.5meV, corresponding to a normalized intrinsic emittance of μm. Starting from HC optimization, simulations were performed with variable thermal energy up to 50 mev to gauge importance of drive laser and photocathode material choices. 11

12 Summary We have optimized a DC injector design for ERL@APS with a zigzag merger using geneticoptimizer and IMPACTT. Optimization gives configurations meeting requirements of HC mode and HF mode. Ultra low emittance requires very high DC gun voltage of ~720kV (~750 kv) for HC (HF) mode, while the experimentally achievable gun voltage is ~350kV. Zigzag merger is well suited to preserving very low emittance. Essentially, no emittance growth is seen from the merger for even 77 pc/bunch. Our next step will be to use IMPACT T and elegant to explore a start to end model of a full ERL. This will permit more reliable evaluation the performance of the ERL, including collective effects (e.g., CSR) and jitter. 12