For SwissFEL, two types of cavity BPMs are used. In the linac, injector and transfer lines, low-Q dual-resonator cavity BPMs with a loaded-Q factors (QL) of ~40 and 3.3GHz mode frequency allow easy separation of the two adjacent bunches with 28ns bunch spacing. For the undulators that receive only single bunches from a beam distribution kicker with 100Hz repetition rate, dualresonator BPM pickups with higher QL are used. The baseline version for the undulator BPMs is a stainless steel pickup with QL=200 and 3.3GHz frequency. In addition, an alternative version with copper resonators, QL=1000 and 4.8GHz frequency has been investigated. For all pickups, prototypes were built and tested. The status of pickup and electronics development as well as the latest prototype test results are reported. INTRODUCTION Due to different apertures in different parts of SwissFEL, three different BPM pickups are needed. While the injector and linac have BPM pickup apertures of 38mm (“BPM38”) and 16mm (“BPM16), the undulator BPMs have 8mm (“BPM8”). Injector and Linac BPMs The planned cavity BPM system for injector and linac is similar in architecture to that already developed by for E-XFEL and FLASH2 [1,2,3,4]. In order to independently measure position and charge of the two bunches with 28ns spacing (compared to 222ns for E-XFEL), the SwissFEL BPM38 and BPM16 pickups have a lower QL of ~40, compared to QL=70 for E-XFEL. The signal frequency is fo=3.284 GHz for BPM38 and BPM16, which is still safely below the cut-off frequency of the larger (38mm aperture) beam pipe. Moreover, this frequency is an integer multiple of the machine reference clock frequency (142.8MHz), where the bunch spacing is four reference clock periods. As a result, both bunches have (nearly) the same IQ phase, which simplifies the algorithms for BPM signal processing and local oscillator phase feedback (see ref [3]). Finally, the choice of this frequency also allows direct reuse of large parts of the EXFEL cavity BPM electronic system that also work at 3.3GHz (with a similar aperture of 40.5mm for the warm E-XFEL beam transfer lines), thus minimizing the development effort. For QL=40 and fo=3.284 GHz the resulting decay time τ=QL/(π·fo) of the BPM38 and BPM16 pickup signals is 3.9 ns. After 28ns, the cavity signal is decayed to <0.1% of the initial amplitude before the second bunch arrives, thus causing only negligible RF crosstalk between bunches. In order to be able to use the same 160MSPS ADCs like for E-XFEL, the RFFE shapes the output pulses for the two bunches such that enough samples for a sufficiently accurate computation of the beam position are available, using a simple algorithm in the BPM electronics FPGAs to eliminate any bunch-to-bunch crosstalk caused by the pulse shaper. Compared to pickups with high Q where overlapping pickup signals can have constructive or destructive interference, the lowQ RFFE output pulse shaper only causes small overlap of baseband amplitude signals, where a simple (scalar) subtraction of the overlapping signal pulses is sufficient to supress bunch-to-bunch crosstalk, without the necessity to use phase information. The RMS noise requirements of the SwissFEL linac BPMs is <5μm for the 16mm aperture cavity and <10μm for the 38mm cavity (see Table 1) at 10-200pC bunch charge. Tests with a slightly modified E-XFEL BPM electronics have already demonstated sub-micron resolution for a BPM16 prototype pickup installed at the SwissFEL Injector Test Facility SITF [1]. Undulator BPMs The BPMs in the SwissFEL undulator intersections have higher resolution and precision requirement than the linac and injector sections (see Table 1). From the experience gained with the E-XFEL and FLASH2 cavity BPM system we decided to use BPM pickups of the same basic structure, but with higher QL. The main reasons for this decision are: 1. In contrast to EXFEL, the SwissFEL undulators are operated in single bunch mode with 100Hz repetition rate, therefore we do not need a low QL to avoid bunch-to-bunch crosstalk of pickup output signals. 2. With higher QL, more data samples are available per bunch when using a similar ADC sampling rate. This reduces the impact of ADC noise on the BPM position noise and thus increases the ratio of measurement range to noise (for higher bunch charges), which is typically ~1000 for QL~40-70. 3. Direct digital quadrature downconversion from a finite IF frequency is able to eliminate systematic measurement errors due to phase and amplitude imbalance. Position and charge readings are thus less sensitive to sampling phase or bunch arrival fluctuations, increasing robustness and overall accuracy. ___________________________________________ # markus.stadler@psi.ch Proceedings of IBIC2014, Monterey, CA, USA WEPD13 BPMs and Beam Stability Wednesday poster session ISBN 978-3-95450-141-0 675 Co py rig ht © 20 14 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs Table 1: SwissFEL BPM Pickup Specifications (EXFEL undulator pickup shown for comparison only) Type Aperture (mm) QL Freq. (GHz) Bunch spacing Resonator Sensitivity RMS Position Noise (μm) Location Reference (V/nC) Position (mV/nC/ μm) BPM8 8 200 3.284 1 bunch 100Hz 47.5 5.2 <1 Undulators BPM8 8 1000 4.855 1 bunch 100Hz 58 4.3 <1 Undulators BPM16 16 40 3.284 28ns (2 bunch mode) 135 7.1 <5 Injector/Linac BPM38 38 40 3.284 28ns (2 bunch mode) 66 5.7 <10 Injector/Linac EXFEL 10 70 3.3 222ns 45 2.8 <1 Undulators It should be noted that choosing a higher QL (for a given pickup geometry, by having weaker coupling) does not affect the BPM position noise at very low bunch charges, or at higher bunch charges and small measurement ranges where RFFE noise dominates over ADC noise. UNDULATOR PICK-UP DESIGN AN STATUS Recently we have successfully tested two BPM8 undulator pickup prototypes at the SwissFEL Injector Test Facility (SITF) with beam: One made from stainless steel, with QL~200 and f=3.284GHz, and one with copper resonators and an outer stainless steel hull with QL~1000 and f=4.855GHz (see Figure 1).After getting experience with the production and confirming that the copper version can be produced on time and on budget, we decided to use it in the SwissFEL undulators since it allows to reach better resolution than the 3.3GHz steel version both at high charge (due to the higher QL) and at low charge (due to the higher frequency that improves sensitivity for a given decay time). A detailed discussion of further design considerations may be found in [1]. The design of the SwissFEL cavity pickup was based on the experience gained during the E-XFEL cavity BPM project over the last few years as is true for the entire PSI SwissFEL BPM activities. The pickup bodies for BPM38, BPM16 and the BPM8 version with QL=200 are all made from stainless steel (316LN). Details of the design are given in [1,5,6,7]. QL=200 is a compromise between the goals of having a high QL and keeping the resistive losses reasonably low. In order to reach a much higher QL of ~1000, we designed a BPM8 pickup with copper resonators. Figure 1: Cross-section of BPM8 QL=1000 pickup. As shown in Figure 1 the pickup has an outer hull and vacuum flanges made from stainless steel, while the inner part with the resonators is made from copper. This combines the advantages of copper resonators – easy machining and low losses – with the advantages of stainless steel for the outer part of the pickup – easy welding of the RF feedthroughs, and solid vacuum flanges. Table 2: Status of SwissFEL cavity BPM pickups Cavity Quantity for at SwissFEL Status BPM16 111 Prototype installed at SwissFEL Test Injector Facility (SITF). Test OK, series production started BPM38 6 Prototype installed at SITF. Test OK. Production started BPM8 (QL=200) 27 Prototype installed at SITF. Preliminary Test OK or BPM8 (QL=1000) Prototype installed at SITF. Preliminary test OK. WEPD13 Proceedings of IBIC2014, Monterey, CA, USA ISBN 978-3-95450-141-0 676 Co py rig ht © 20 14 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs BPMs and Beam Stability Wednesday poster session During production, the copper block is first brazed into the steel hull. After machining the three body parts, they are brazed together. Then the glass ceramic RF feedthroughs are welded to the body, followed by a final vacuum test. The current status of the SwissFEL cavity pickups is summarized in Table 2. All SwissFEL undulator BPM8 pickups will be horizontally and vertically adjustable using high-precision motorized movers. This allows absolute calibration of the undulator BPM position during operation. The BPM16 and BPM38 pickups will have supports that can only be adjusted manually for cost reasons. Figure 2: SwissFEL undulator BPM8 pickup in an undulator intersection, mounted together with a quadrupole magnet on a motorized 2D mover stage. LOW-Q & HIGH-Q RF FRONT-END ELECTRONICS To process the BPM38 and BPM16 pickups signals, a slightly modified version of the already existing E-XFEL cavity BPM RF electronics will be used [3,4]. A block diagram of this electronics is shown in Figure 3. Figure 3: RFFE electronics use for low-Q BPM. The short RF pulse signal (compared to the ADC sampling interval) produced by a low-Q pickup makes it adequate to use analog I/Q downconversion. If high-Q cavities are used it becomes however more attractive to do the downconversion in the digital domain. A block schematics of the BPM8 electronics (with 3.3GHz and QL=200 or 4.8GHz and QL=1000) is shown in Figure 4. Figure 4: High-Q RFFE electronics. Instead of using an I/Q downconverter, a single mixer is sufficient to convert the pickup signals to an IF frequency of ~40MHz. The IF portion (downstream of the mixers) uses different filter characteristics, which represents only a slight change in circuit design compared to the low-Q RFFEs. For the 3.3GHz BPM8 pickup, large parts of the E-XFEL / FLASH2 CBPM electronics could have been reused due to the same signal frequency. For the 4.8GHz version that is now