## 5 Chicane Commissioning
### Procedures  
In the beam transport preparation, the goal is to achieve the beam that enters the chicane via the reference path. First, the beam straight must propagate through the center of the first chicane dipole magnet. This can be performed by centering the beam to the beam pipe at both screens HIGH2.SCR2 and HIGH2.SCR3 located upstream and downstream of the chicane (Figure 2), respectively. Note that the vertical beam transport and dispersion measurement have no interference from the earth magnetic field; thus, the beam is sent from the booster without the use of y-steeres. Moreover, with the bunch charge of 50 pC, the quadrupole magnets are not used in the commissioning phase. This will ensure the beam propagating straight through the center of the first chicane dipole without any steering force exerted by quadrupole magnets, when practically the beam travels inexactly through the center of all operating quadrupole magnets. In other words, the use of such a low bunch charge simplifies the beam transport and thereby the commissioning.  
Then, the momentum dispersion measurement starts by turning on all chicane dipole magnets. To control magnetic hysteresis properly, all dipole magnet currents are set to the magnitude of 3.5 A prior to setting (decreasing the magnitude) to any sets of the testing currents. The set with the constraints in Equation (1) is tuned such that the beam reappears at the center of the beam pipe at screen HIGH2.SCR3 (\(\Delta y=0\)) again. Then, all chicane dipole magnet currents \(D_{1}\), \(D_{2}\), \(D_{3}\), and \(D_{4}\) are recorded together as a sample set of dipole currents. Note that such a tuning condition with the constraints leads to only one degree of freedom where the dipole magnet current \(D_{1}\) becomes only one tuning knob to minimize dispersion.  
For the measurement of the momentum dispersion, beam vertical position \(\langle y\rangle\) is recorded at HIGH2.SCR3, where booster phase \(\phi_{2}\) in the range of (0, 10 deg) with a step of 1 deg with respect to the maximum mean momentum gain phase of the CDS booster (MMMG phase) is set to vary the beam energy or momentum. Then, this is repeated  
Figure 5: Contour line of \(y\)-position (or offset position \(\Delta y\)) and density plot of momentum dispersion (normalized by beam momentum) of 17-MeV/c electron beam at the screen HIGH2.SCR3 located downstream of the chicane as a function of its chicane dipole magnet currents with the constraints in Equation (1), where \(\Delta y\) represents a distance in the \(y\)-axis from the beam reference path. The simulation is performed without space charge effects.  
without the use of the chicane, as all chicane dipole magnets are degaussed in order to record the \(\langle y\rangle\) background of this measurement. Figure 6 shows a plot of the beam vertical position as a function of beam momentum for one sample set of the dipole magnet currents for a bunch charge of 50 pC. Therefore, the momentum dispersion \(\mathrm{d}y/\mathrm{d}p\) for the sample set of the dipole magnet currents is obtained by a linear regression fitting with the subtraction of the background. Note that the background is a result of imperfection in the booster beam-based alignment (BBA) and the use of steerer magnets, while it shows that the beamline without the chicane is also dispersive.