To achieve the desired energy gain in an accelerator the RF fields of the accelerating TM010 mode need to be precisely controlled in amplitude and phase with respect to the beam’s arrival or injection time. Especially for SRF photoinjector cavities low time-jitter synchronization between the photocathode laser and the RF field has to be maintained. In modern linear electron accelerators the field has to be controlled within better than 0.01 deg. in phase and 1e-4 for relative amplitude error.
This is further of importance as any field deviations in the injector regime, where the beam is not fully relativistic yet and energy variations lead to additional time jitter of the beam. This might be further increased in dispersive sections, such as bunch compressors.
In continuous wave (CW) operation of superconducting cavities the main error source for field deviations are given by microphonics detuning of the thin-walled niobium structures. For an e.g. TESLA cavity the TM010 resonance detunes as 0.3 Hz/nm. Figure 1 shows an overview of all possible error sources contributing to the cavity detuning.
For many applications, like e.g. low current FELs or the main Linac of an ERL as BERLinPro, the cavities can be operated at low coupling, meaning high loaded quality factor. Thus the bandwidth of the resonance can be of the order of 10s of Hertz or even lower. While reducing the thermal burden on RF components as the fundamental power coupler, the fields become more susceptible to detuning due to external forces on the cavity walls.
To control the field a twofold approach is followed:
- The field is maintained by having an overhead of RF power available to compensate for amplitude and phase shifts. This is done by a low power loop (Low Level RF) which measures the field vector with respect to a reference – a low noise RF source- and reduces the residual error by modulating the forward signal supplied to the high power amplifier.
- The cavity is kept on resonance by mechanical tuning systems (see Figure 2) which comprise a stepper motor driven lever arm or similar mechanics to coarse tune the cavity on resonance. To compensate for detuning in the up to sub-millisecond regime these tuners are often equipped with piezo-based tuning systems.
At HZB in recent years for CW operation of SC cavities several tuning systems have been characterized and implemented for fast detuning control. Microphonics was in depth characterized and different compensation algorithms were developed and tested.