Chopping Simulations Results

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