10 Years of Experiment Control at SLS Beam Lines: an Outlook to SwissFEL

The Swiss Light Source (SLS) at the Paul Scherrer Institute (PSI), the first 3rd generation synchrotron light source designed to operate in top-up regime, started user operation end of 2001 with four beam lines [1]. Today, after nearly 10 years of consolidated user operation with up to 18 beam lines, we are looking back to briefly describe the success story based on EPICS controls toolkit. From the experience gained at the SLS we outlook towards the X-ray free-electron laser SwissFEL, the next challenging PSI project, starting operation by 2016. PTD EXPERIMENTAL ANATOMY The EPICS-toolkit provided flexible and easy-to-scale distributed control system architecture, both for the beam line and experimental control setups. We briefly discuss the main elements of the distributed beamline control system rigorously based on "Positioner-Trigger-Detector" (PTD) anatomy. The bulk of the “Positioner” motion control is based on MAXv motor controllers hosted in VME systems which are attached to separate power amplifiers (motor box) giving 8-axis motion control per one MAXv and motor box. The motor device control is the EPICS motor record: the main building block on top of which the whole motorized functionality is build up mainly with SynApps [2]. To be more specific, transform, sseq sequential, table configurable optical table setups, combined with the motor-record wait-for-completion functionality, tremendously simplify the software integration of slits, monochromators and mirror units*. The Concept of EPICS Positioner The beamline “Positioner” is the most critical issue for reliable operation. Typically it is bundled multi-axis motion control functionality. For example slits, monochromators and mirror units are set of motors with precise physical meaning in engineering units such as slit width or pitch-tilt-yaw settings of an optical unit. A more complex example of multi-axis Positioner is the sample manipulator. The integration of a 6-axis CarvingTM system (Fig.1) developed at PSI, meets the requirements for safe translation and user-friendly positioning inside vacuum chambers. Shutter interlocks and dynamic motion volume control are essential to prevent in-vacuum accidental crashes. A noteworthy fact is that positioning requirements for all five CarvingTM systems at SLS are based on select, fanout, transform, calcout and sseq records with configurable behaviour instead of implementing dedicated solutions via programming or scripting. In the earlier days of beamline commissioning, the spectroscopy beam lines success story started with a simple transform record that bundled the monochromator and insertion device “set-energy” command by simply typing in the desired photon energy. The EnergyPositioner has mandatory readback value. The busy record – a smart wait-for-completion flag – aggregates monochromator plus insertion device energy-positioners into one logical block with implicit callback for point-topoint scans. In sscan setup this functionality minimizes the effort for setting up arbitrary experimental setups. Our experience is that an adequate PTD sscan setup understands most experimental needs used for beamline commissioning and even more advanced experiments with image data acquisition (Fig.1). Since the sscan P-T-D links are possible to change dynamically, it is also a guideline for implementing experiments by adapting existing experimental schemes instead of rewriting them. Figure 1: PTD experiment with areaDetector (AD). a) 1x1mm ROI (in-set) evaluating the vertical beam size by scanning the exit slit through the beamline focus (dashed line, courtesy M.Munthwiler). b) -scan with VGScienta electron-analyzer for mapping zero-energy electronic states in momentum space with Carving manipulator (courtesy of L.Patthey). VGScienta AD credit: J. O'Hea and F.Yuan, Diamond Light Source Ltd. The Concept of EPICS Trigger and Detector Strictly speaking, triggers are usually associated with some hardware TTL signal in order to measure voltage, CCD, pixel detectors or channeltron. In the latter case the COmplete PHotoEmission Experiment, a world class 3D ___________________________________________ * To illustrate the wealth of EPICS records we shall henceforth denote them in cursive writing Proceedings of ICALEPCS2013, San Francisco, CA, USA TUPPC066 Experiment Control ISBN 978-3-95450-139-7 729 C op yr ig ht c ○ 20 14 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s spin-polarimetry setup, is in-fact a slick sscan masterpiece example reliably working since 2001 without any modification. The Positioner is here the electron analyzer low/high voltage window; the Trigger is a scaler record acting also as a Detector by counting TTL pulses from Mott detectors. A more basic example is e.g. triggered voltage readout from current amplifier. Based on Hytec ADC VME cards with buffered sampling and averaging, they are soft-triggered (Fig.3). The integration time is adjustable by feeding the readout into compress records for adjusting the signal averaging. To make this sscan-aware, auxiliary calcout and busy records linked with the compress record provide the desired effect. To summarize, the rich variety of EPICS records are “nuts” and “bolts” for the PTD experimental approach. An adequate PTD design opens the doors for sscan solution for most experimental demand. Configuration and Installation EPICS has no object oriented capabilities but individual “nuts” and “bolts” are grouped according to their functionality. There is a way to mimic a class hierarchy between them at least on the configuration side with Project-Objects-Properties weakly related by naming convention, substitution and template files (Fig.2). For the beamline motion control, template files implement motion axes with well defined properties (e.g. mandatory initialization procedure to position the axis in a defined state). The substitution file groups axes into one logical block and naming convention ensures that an installation procedure deploys the whole project or a part of it into the target VME or Linux IOC system [3].