2. The Compact Muon Beam Line CMBL 41
2.6. Accurate G4BL Simulation of the CMBL & simulation based optimization
In order to improve the transmission to the end of the CMBL a more rened G4BL model of the last part of the beam line was created. All elements were positioned according to alignment measurements in the area that were made at the end of the CMBL beam time 2014 /2015. The beam for the simulation was extracted from the phase space measure-ments that were carried out at the stage I position with the "nal focus" optics (shown in
gure 2.69). All drawings were checked again and vacuum chamber apertures were imple-mented accurately. The currents that were applied in the simulation were directly taken from the "nal focus" optics setting, that was determined in the CMBL setup. In order to simulate comparable beam proles at the end also the Mylar vacuum window and the air column between the window and the pill counter were implemented in the simulation. The results of this simulation are presented below. Figure 2.74 shows the graphical output of the G4BL simulation. The beam prole that is extracted at the position that corresponds
Figure 2.74.: The graphical output shows the muon beam (yellow trajectories) propagation through the CMBL to the nal focus position. The implementation of the Mylar window and the air column at the end leads to scattering of the muon beam.
to the position of the pill counter during the beam time is shown in gure 2.75. The beam prole shows a good agreement with gure 2.64. The beam prole parameters for both simulation and measurement are listed in table 2.4. The beam widths, correlations and the transmission show a good agreement. Deviations of the beam centroid can be explained by the fact, that the simulated beam starts on-axis parallel to the centreline, whereas the quadrupole tuning in gure 2.70 implied an oset. For further validation the QSM current was tuned in the simulation to the same values that were used for the phase space mea-surements at the nal focus. The comparison plot is shown in gure 2.76. Over a wide range of applied QSM currents a good agreement between the measured beam size and the simulation result is observed. This implies that not only the prole for the nal focus setting is correctly modelled but also the phase space is well described. The deviations in the horizontal direction can be partially attributed to contributions from the coupling associated with dispersion, since the beam spot is only regarded to be achromatic around the nal focus setting. Further sources of deviations are due to the rst-order beam imple-mentation and contributions e.g. from hysteresis. The beam envelopes are shown in gure 2.77. Discontinuities in the envelope plot can mainly be identied with particle losses as
Figure 2.75.: The muon beam prole was extracted from the G4BL simulation at the corresponding Pill3 position and shows a good agreement with the measured beam prole in gure 2.64.
Table 2.4.: Comparison between the measurements during the CMBL test beam 2014/2015 and the G4BL simulation results. The measured transmission is given as the range from the results obtained at the nal focus in 2015 and 2014.
Beam Parameter Measurement CMBL 2014/2015 G4BL simulation
¯
x (mm) 0.4 6.1
¯
y (mm) 3.8 -0.1
σx (mm) 10.4 11.1
σy (mm) 25.4 25.0
ρx,y 0.026 -0.015
Transmission from stage I to the end (%) 58.0 - 61.3 60.6
Figure 2.76.: The QSM in the G4BL simulation was tuned to the same current values as for the phase space measurements in the experimental area. The plot compares the respective widths for the measurements and the simulation.
Figure 2.77.: Shown are the beam envelopes for the G4BL simulation with nal focus settings applied to the magnets.
illustrated in gure 2.78. Finally the transmission eciency has to be multiplied by the survival probability of∼96.5 %, since the simulation was done with muon decay disabled.
The corrected transmission from the Pill1 to the Pill3 position amounts to 60.6 %. Figure
Figure 2.78.: The plot shows the beam losses that occur on the beam line apertures according to the G4BL simulation for nal focus magnet settings. The simulation was done with disabled muon decay. Therefore the transmission has to be scaled with the survival probability between the beginning and the end, yielding 60.6 %.
2.79 shows the prole at the stage I measurement position for both the simulation and the measurement.
The validation checks described in this section show that the simulation based on the recon-structed phase space is able to properly reproduce the transverse phase space, the central proles and the transmission. Therefore the simulation is used as a basis to identify regions of particle losses. Figure 2.78 illustrates that these regions are mainly at the dipole vacuum chambers. Therefore new eldmaps of the ASL and the ASK were generated in TOSCA that have a larger gap. Running the simulation with loose aperture constraints shows that after optimization of the beam line with the Optima G4BL code a total transmission of up to 87 % can be achieved by introducing larger vacuum chambers.
(a) Measured beam prole at the Pill1 position for
nal focus optics (b) Simulated beam prole at the Pill1 position
for nal focus optics
Figure 2.79.: The horizontally cut measured prole at the Pill1 position for the nal focus tune is shown on the left. The simulation based on the reconstructed phase space on the right shows a comparable cut.