The first task for beam set-up is to set the magnet for the desired pion rate while minimizing the muon and positron contaminations.
Figure: Beam line set-up for optimization during the test run of 1992: secondary beam (central trajectory); quadrupole magnets; concrete shielding for proton channel; vertical steering magnet; mylar windows, lead collimator(opening: 7 cm vertical, 2 cm horizontal, thickness cm; horizontal steering magnet; quadrupole triplet; multi-wire proportional chamber(MWPC); scintillator , area.
At a given pion momentum, the nominal magnet settings are used as the initial inputs of an optimization algorithm. With these initial settings, the optimization algorithm known as OPTIMA calculates the particle rate in a scintillator counter placed at a waist.
Figure: Electronics diagram for beam optimization and Time Of Flight analysis.
Then, for each unit in the beam channel except the momentum defining magnet, OPTIMA varies the corresponding setting within a given range with a given step size around the initial value for a maximum rate in the scintillator.
Figure: Time separation observed on the oscilloscope between , and at 116 MeV/c.
The observed counting rate in the scintillator is normalized to the proton rate during the counting period to account for fluctuations in the primary proton current.
Figure: Time of flight in the secondary beam line of , and at various momenta. For the pion beta experiment, the central beam momentum is set at .
When all the magnets are set at their optimum values, OPTIMA calculates again the rate in the scintillator and returns the ratio of the final rate to the initial rate as the gain. An iterative process is then performed until the gain is close to unity.
Figure: Beam line arrangement for phase space studies during the 1993 test run.
The beam line set-up and the electronics diagram for optimization purposes
during the test run of 1992 is shown in figure and
figure respectively. The MWPC monitors
the horizontal and vertical beam profiles at the waist.
A time of flight (TOF) analysis is conducted before and after the optimization
in order to determine the fractions of pions and the contaminating
particles (muons and positrons) in the beam.
The optimization scintillator provides the start signal for
the Time to Analog Converter while the stop signal is the suitably delayed RF signal,
which marks a particular phase of the accelerator frequency
(see table ).
The optimization branch of the electronics involves the selection of the
particle type --- in this case --- for which the magnets are to be
optimized using the coincidence, , in
figure . Triggering an oscilloscope with the scintillator and
looking at the RF, one observes at 116 MeV/c, a clean separation between
particle types as shown in figure .
This observation agrees well with the
expected separation displayed in figure . Once the selection is made,
the coincidence output is sent to a scaler whose counts are used by the optimization algorithm.