INSTRUMENTATION ACTIVITIES AT THE SwissFEL INJECTOR TEST FACILITY

The SwissFEL Injector Test Facility has been equipped with numerous prototype diagnostics (BPMs, screen monitors, wire scanners, optical synchrotron radiation monitor, compression (THz) monitor, bunch arrival time monitor, EO spectral decoding monitor, charge and loss monitor) specifically designed for the low charge SwissFEL operation modes. The design of the diagnostics systems and recent measurement results will be presented. THE SWISSFEL INJECTOR TEST FACILITY The SwissFEL Injector Test Facility (SITF) [1] is a 230 MeV electron linear accelerator that has been built to test concepts and components for SwissFEL, the upcoming Xray free electron laser at Paul Scherrer Institut [2]. Instrumentation installed at this test facility serves two purposes: • Provide a stable baseline diagnostics for beam development and test of acceleration and beam compression concepts • Test diagnostics designed for SwissFEL To fulfill the first requirement, the Diagnostics Section has implemented a series of standard position, profile and charge monitors. These monitors do not meet the specifications for SwissFEL, but they provide a stable basis to set up the linear accelerator and to test more advanced concepts. Figure 1 shows an overview of the monitors installed in the SwissFEL Injector Test Facility. In the following sections, the instrumentation is described in detail, starting with transverse position and profile measurements, longitudinal measurements and test of advanced concepts. There are several requirements for SwissFEL instrumentation: • Low-charge operation. The design charge for SwissFEL is between 10 and 200 pC, depending on the operating mode. It is foreseen that all diagnostics can be operated within this range. • Low emittance. SwissFEL is being designed with a normalized core slice emittance of 180 to 430 nm. This results in beam sizes down to 10 μm rms and requires diagnostics with corresponding resolution. • Pulse length. The design electron bunch length varies between 1 to 3 ps rms at the cathode to 6 to 30 fs rms at the undulators. Longitudinal diagnostics for such bunches have to be developed. • Stability requirements. The transverse and longitudinal parameters of the machine must be stabilized. • Two-bunch operation. The diagnostics for SwissFEL should be capable of distinguishing two bunches, separated by 28 ns. This is foreseen for a future upgrade of SwissFEL to a second beamline for soft X-rays. BEAM POSITION MONITORS The beam orbit in the SwissFEL Injector Test Facility (SITF) is measured by 19 resonant stripline beam position monitors (BPMs)[3]. The monitor pickups have four strips parallel to the beam, where each strip has an open and a shorted end, thus acting as lambda/4 resonator. The beam excites 500 MHz decaying sine signals on each of the strips. The signals are coupled out via antennas close to the shorted end of the strips. After filtering and amplification in an RF front-end (RFFE) electronics, the signals are digitized by a 5 GSample/s waveform digitizer. The digitized waveform is then processed by an FPGA carrier board to calculate beam position and amplitude. The single bunch resolution of this electronics that was developed by PSI is 7 μm rms for the beam position for charges between 5 and 1000 pC (dominated by digitizer noise) and still 35 μm rms at 1 pC (dominated by thermal RFFE noise). The charge resolution is 0.3% of the charge at 1-1000 pC and typ. 34 fC rms for charges 1 pC. Due to their robustness and low noise especially at low charge, the stripline BPMs are used for most charge measurements at SITF. However, dedicated charge monitors like ICTs or wall current monitors (see below) are still required for absolute charge calibration of the BPMs. Figure 2 shows the comparison of three adjacent beam position monitors. While the stripline BPMs satisfy the requirements of SITF, SwissFEL will use only cavity BPMs, thus providing a homogeneous BPM system along the whole accelerator [4]. A BPM test section at SITF with five cavity BPMs is being used for tests of cavity BPM prototypes for SwissFEL and E-XFEL, where sub-micron resolution of the cavity BPM electronics developed by PSI has already been demonstrated [5]. MOBL1 Proceedings of IBIC2013, Oxford, UK ISBN 978-3-95450-127-4 C op yr ig ht c ○ 20 13 by JA C oW — cc C re at iv e C om m on sA tt ri bu tio n 3. 0 (C C -B Y3. 0) 12 Overview and Commissioning