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Chopping Simulations Results 

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

17/06/2010

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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.

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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)

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L4L.QD

A03010

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03030

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A03050

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03070

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L4L.QFB03090

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C03110

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E03150

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03190

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A03200

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A03220

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c-27Po

wer

[W]

Chopper Line

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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.

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Pow

er [

W/m

]

Lenght (m)

DTL

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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 Line ON – 700V

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Chopper OFF

Chopper ON

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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%

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Voltage [V]

Chopper efficiency % Input beam

stopped by the dump

Input beamto PSB

Extinction ratio

500 93.94 0.5 x 10-1

600 99.35 0.6 x 10-2

700 99.93 0.6 x 10-3

800 99.99 0.7 x 10-4

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

200 300 400 500 600 700 800

Exti

ncti

on R

atio

Chopper plates voltage [V]

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Some numbers

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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

Fe<Nf / Ne

100 µs

. . .

Nf Ne

. . .

Ring 3Ring 2Ring 1

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If chopper driver rise/fall time > 2ns Partially deflected bunches

Partially deflected bunches

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Voltage [V]

Chopper efficiency

% Input beam stopped by the

dump

Input beamto PSB

Extinction ratio

200 30.6 6.9 x 10-1

300 55.4 4.4 x 10-1

400 78.3 2.1 x 10-1

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).

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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.

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References

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[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