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Optimization of Chopper Line Design for Enhanced Beam Dynamics in LINAC Systems

This study presents the modifications made to the chopper line, strategically positioned between the RFQ and DTL, to optimize pulse timing and reduce energy losses in the high-energy particle acceleration process. Utilizing simulations with PathManager, we assessed the impact of various voltage levels on chopping efficiency and particle transmission rates. Key findings indicate an exponential decrease in particle transmission efficiency with increased voltage and shed light on the importance of fine-tuning the pulse duration for specific commissioning scenarios requiring low-intensity beams.

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Optimization of Chopper Line Design for Enhanced Beam Dynamics in LINAC Systems

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  1. Chopping Simulations Results M. Garcia Tudela, JB. Lallement, PA. Posocco, A. Lombardi, G.Bellodi, M. Eshraqi, E. Sargsyan, L. Hein

  2. Chopper Line - Introduction • Placed between the RFQ and DTL. Figure 1.- MEBT Scheme. • Aim: Modify the time structure of the pulse, avoiding losses at high energy. • Removes the bunches that would fall outside the bucket of the PSB at injection. (133/355) • Removes the bunches during the rising time of the distributor (1µs gap) in the TL. • Generate low intensity beams. • Allow the matching to the DTL. • Beam Dynamics chopper ON, fate of the particles  Simulations with code PathManager[3] • Field maps from the electromagnetic simulations [5] to have a realistic approach.

  3. Chopper Line ON – 700V • Nominal case. Losses in the MEBT. Figure 2.- Beam power loss map [watts per element] in the MEBT. • Input beam: 106 macro particles • 0.04 % Duty cycle  PSB rep. rate 1Hz, 400 µs pulse. • ~ 0.065 % Beam current after the MEBT. ( From 63.5 mA to 41µA)

  4. Chopper Line ON – 700V • Tracking the beam up to the PSB Figure 3.- Beam power loss map [watts per meter] in the DTL. • No beam loss in the CCDTL or PIMS. • Worst case in the transfer line is 0.25 W/m. • 77% of the partially chopped beam (at the output of the dump) is transmitted along the LINAC up to the end of the transfer line.

  5. Chopper Line ON – 700V • Figure 5.- TL beam energy. Chopper OFF. • Figure 4.- TL output beam (Nominal beam and deflected beam superimposed) . • Transmission, chopper ON: 0.06 % • Figure 6.- TL beam energy. Chopper ON. Chopper ON Chopper OFF

  6. Chopper Line ON – Other approaches • Chopping efficiency applying different voltages to the plates for the same input beam. • Chopper OFF : • Transmission after the MEBT 96.3% • Transmission to PSB 89.4%

  7. Some numbers • Beam pulse 0.4 ms  Beam pulse per ring 0.1 ms • Repetition rate 1Hz • Bunch frequency 352.2 MHz • Number of bunches per pulse (0.4 ms): 3.5 x 104 • Number of bunches per pulse per ring (0.1 ms) : 0.875 x 104 • Number of particles per pulse to PSB: 1x1014 • Number of particles per bunch 1.14 x10 9 • Nf : Number of bunches filled in a pulse ( chopper OFF ) • Ne: Number of bunches empty in a pulse( chopper ON ) Extinction ratio criterion: • Other commissioning scenarios: • pulse 10 ns per ring ~ 3 bunches  Fe < 0.34 x10 -3 • 700 v : Fe= 0.6 x 10-3 Number of particles chopper on is comparable to the number of particles during the pulse. • pulse 30 ns per ring ~ 10 bunches  Fe < 0.87 x10 -3 Ring 1 Ring 2 Ring 3 100 µs . . . . . . Nf Ne Fe<Nf/ Ne

  8. Partially deflected bunches • If chopper driver rise/fall time > 2ns  Partially deflected bunches 200 V 300 V 400 V 5 * RMS Emittance, superimposed • The level of the losses along the linac is in the order of mW. • Figure 7.- TL output deflected beams for every voltage (X-Y).

  9. Conclusions • The percentage of particles at the end of the TL decreases exponentially with the voltage. • The larger the pulse required in a ring , the less strict the extinguish factor required. • For the nominal case: 700 v • 0.6 %o particles of the input beam transmitted to the PSB. • For some commissioning scenarios requiring very low intensity beam this value could be not enough.

  10. References • [1] F. Gerigk, M. Vretenar editors, “LINAC4 Technical Design Report”, CERN-AB-2006-084 ABP/RF • [2] N.V Mokhov and W.Chou editors, “Beam Halo and scraping”, Proc. 7th ICFA mini-workshop on high intensity and high brightness hadron beams, Interlaken resort, Wisconsin, United States, 1999 • [3] A. Perrin and J.F Amand, Travel v4.07, users manual,CERN (2003). • [4] R.Duperrier, N. Pichoff, D. Uriot, “CEA Saclay codes review”, ICCS Conference 2002, Amsterdam • [5] T. Kroyer, F. Caspers, E. Mahner, “The CERN SPL Chopper Structure: A Status Report”, CERN-AB-2007-004, CARE-Report-06-033-HIPPI • [6] M. Garcia Tudela, JB. Lallement, A. Lombardi, “Chopper Line Studies”, CERN-sLHC-Project-Note-0012

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