Frequency Sweeping

First measurements of the resonant depolarization frequency were done by hand, i.e. the signal generator fed a sine wave signal to the amplifier of a kicker magnet whereby the parameters (sweep range, sweep velocity, gain, etc.) of the signal generator had to be set at the machine manually and the crucial time values (determining the current sweep frequency) had to be noted as well.

In order to get around this tedious and not very accurate method a program has been introduced which allows to control the experiment from the control room with much higher accuracy. The program queries the necessary parameters, writes them to EPICS channels (which can be logged) and, upon start, sends a trigger signal to the signal generator which, on receiving this signal, begins the sweeping of a previously defined range. While sweeping the program writes the actual sweep frequency to an EPICS channel which is written to the same data output files as the measured values (beam current, lifetime, etc.) and therefore enables precise linking between measured values and current experimental parameters.

An example of this procedure is the determination of the resonant depolarizing frequency. When a dip in lifetime is observed, the data is queried for the sweep frequency set on the signal generator during depolarization. This allows precise identification of resonant frequencies as well as measurement of the resonance width. Figure 9 shows an example for the correlation of sweep frequency and depolarization of the beam. When the dip in lifetime occurs (second curve from top) or the rise of losses starts (bottom curve) the current sweep frequency is identified to be the depolarizing frequency. Because the width of this dip (for more detail see chapter 5) is much wider than the precision of the sweep frequency, the width of the depolarization dip is the dominating contribution to the resonance uncertainty.

Figure 9: The experiment as seen from the SLS control system. Shortly before 6:05 the resonant depolarization frequency is reached. The top curve shows the current excitation frequency during the sweep allowing the begin of the dip to be localized at 581,25 kHz.

Figure 10 shows the expert panel (provided by the SLS controls group) for setting frequency generator values within the SLS control system.

Figure 10: The panel to set sweep frequency parameters from the SLS control room.