Insertion Device Gap Research Articles

Hydraulic-assist driver for compact insertion devices

We have developed and tested a novel type of driver for insertion devices. In our approach, we compensate for the magnetic forces acting on the insertion device (ID) arrays by installing a number of compact hydraulic cylinders along the ID.These assisting cylinders are connected to a single hydraulic line and activated by the same pressure. A computer-controlled closed-loop feedback system maintains the target ID array position and an acceptable load on the mechanical driver. This is accomplished by simultaneously controlling both the hydraulic assist pressure and the mechanical driver position. The quantity and location of the compact hydraulic cylinders are optimized to keep the deformation of ID structural components in an acceptable range.The hydraulic cylinders compensate for over 90% of the strong forces generated by the magnetic field. This reduces the load applied to a conventional mechanical driver to less than 10% of the total load. The implementation of this type of driver can make insertion devices much more compact without compromising their performance.In the proof-of-concept experiments, our novel hydraulic-assist driver demonstrated a long-term stability and reproducibility of the ID gap within 2μm. Magnetic field stability and reproducibility was within 6e−5 of the normalized field strength or better.Here we will describe the concept, details of our test setup and the results of the proof-of-concept experiments. We will also discuss the advantages of the hydraulic-assist driver over conventional mechanical drivers, as well as possible applications and future developments.

Dynamic pressure monitoring in the front-end vacuum systems at the TPS facility

The Taiwan Photon Source Front End (TPS FE) vacuum systems are located between the storage ring and beam lines and include various high-heat-load (HHL) components. We investigate the change in dynamic pressure over time in the FE vacuum system, including measurements of HHL component temperatures, residual gas analyzer (RGA) data analysis, and the effect of changing the insertion device (ID) gap during beam current operation. Our results reveal that the exponential slopes of dynamic pressures in the FE versus accumulated beam dose is nearly −0.5 for all cases. With beam current at 400 mA and ID gap close to 7 mm, the dynamic pressure and HHL component temperature are 1.5 × 10−8 Pa and 45 °C, respectively. The RGA data show that the main dynamic pressure contributions are H2, H2O, CO and CO2. H2O, CO and CO2 come from the chamber surface, while H2 comes from the vacuum chamber materials. The dynamic pressure was ∼2.3 × 10−11 Pa/mA at 1000 Ah, and it continues to decrease with an increase in accumulated beam dose. These results indicate that the total dynamic pressure reaches to a value of 10−7 Pa at full operation current in the FE systems.

Interplay of Touschek scattering, intrabeam scattering, and rf cavities in ultralow-emittance storage rings

The latest generation of storage ring-based light sources employs multibend achromat lattices to achieve ultralow emittance. These lattices make use of a large number of weak bending magnets which considerably reduces the amount of power radiated in the dipoles in comparison to power radiated from insertion devices. Therefore, in such storage rings, parameters such as emittance, energy spread, and radiated power are—unlike 3rd generation storage rings—no longer constant during a typical user shift. Instead, they depend on several varying parameters such as insertion device gap settings, bunch charge, bunch length, etc. Since the charge per bunch is usually high, intrabeam scattering in medium-energy storage rings with ultralow emittance becomes very strong. This creates a dependence of emittance on stored current. Furthermore, since the bunch length is adjusted with rf cavities but is also varied as insertion device gaps change, the emittance blowup from intrabeam scattering is not constant either. Therefore, the emittance, bunch length, and hence the resulting Touschek lifetime have to be calculated in a self-consistent fashion with 6D tracking taking into account not only the bare lattice and rf cavity settings, but also momentary bunch charge and gap settings. Using the MAX IV 3 GeV storage ring as an example, this paper demonstrates the intricate interplay between transverse emittance (insertion devices, emittance coupling), longitudinal emittance (tuning of main cavities as well as harmonic cavities), and choice of stored current in an ultralow-emittance storage ring as well as some implications for brightness optimization. (Less)

Local correction schemes to counteract insertion device effects

Insertion devices (IDs) of various types provide light of high brilliance to experimenters at the Swiss Light Source (SLS) beamlines. However, changes in the photon energy and polarization by movement of the ID gap and phase shift cause orbit distortions that result in a displacement of the photon beam in both angle and position at the beamline. A feed-forward correction scheme has been developed to quantify and precisely correct these effects using designated correctors local to the photon source. The settings for these correctors are determined using an orbit configuration consisting of 73 digital beam position monitors (DBPMs) and associated correctors; recently commissioned X-ray beam position monitors (XBPMs) located at the beamline front-end are also included in the correction algorithm to further constrain the photon beam to its specified position. The feed-forward correction procedure is finally implemented at the local processor level and applied at a rate of 10Hz. A photon pointing stability at the sub-microradian level is achieved. The entire gap scan, feed-forward generation and subsequent verification can be completed from within a few minutes to several hours depending on the complexity of the ID. The methodology of the procedure is described and the results from several ID analyses are presented. The effect of the ID on the betatron tune is also discussed. Both feed-forward and feedback procedures have been implemented to maintain the horizontal and vertical components of the tune at the desired working points.

Scheme for precise correction of orbit variation caused by dipole error field of insertion device

We developed a scheme for precisely correcting the orbit variation caused by a dipole error field of an insertion device (ID) in a storage ring and investigated its performance. The key point for achieving the precise correction is to extract the variation of the beam orbit caused by the change of the ID error field from the observed variation. We periodically change parameters such as the gap and phase of the specified ID with a mirror-symmetric pattern over the measurement period to modulate the variation. The orbit variation is measured using conventional wide-frequency-band detectors and then the induced variation is extracted precisely through averaging and filtering procedures. Furthermore, the mirror-symmetric pattern enables us to independently extract the orbit variations caused by a static error field and by a dynamic one, e.g., an error field induced by the dynamical change of the ID gap or phase parameter. We built a time synchronization measurement system with a sampling rate of 100Hz and applied the scheme to the correction of the orbit variation caused by the error field of an APPLE-2-type undulator installed in the SPring-8 storage ring. The result shows that the developed scheme markedly improves the correction performance and suppresses the orbit variation caused by the ID error field down to the order of submicron. This scheme is applicable not only to the correction of the orbit variation caused by a special ID, the gap or phase of which is periodically changed during an experiment, but also to the correction of the orbit variation caused by a conventional ID which is used with a fixed gap and phase.

An overview of the information distribution system at the Advanced Photon Source

The Advanced Photon Source (APS) has been in operational mode for more than 5 yr. Currently there are over 40 beamlines in various phases of operation. The control system of choice at the APS is the Experimental Physics and Industrial Control System (EPICS). We have provided various interfaces to the beamlines from the APS control system. An overview of the various systems will be discussed. The General Control System Information (GCSI) uses dedicated computers as EPICS process variable gateways to provide data from the APS control system to each beamline. The GCSI architecture makes the APS control system secure, yet has the flexibility of providing access control to any data available on the APS control system. In addition, the gateway reduces the load on the APS control system equipment by making only one connection for each process variable accessed by multiple users. Each sector, consisting of a bending magnet and insertion device beamlines, is provided its own gateway, which resides on the local sector network. This scheme has the advantage of providing network security and more reliable operation. To provide real-time accelerator data to beamlines, each sector has been provided a chassis to display the storage ring current and other relevant information. These data are transmitted via a direct fiber link from the APS control system hardware to the beamlines. The beamlines are also provided VME-based hardware and associated EPICS software to retrieve key information provided via this fiber link. Some of the information on this link is beam current, lifetime, injection status, and sector specific information such as shutter status, insertion device gap, and energy, and storage ring and front-end beam position monitor signals. The data rates on this link are typically 10 Hz but can be as high as 272 kHz. This scheme allows the beamlines using EPICS-based software to seamlessly use the data from the APS control system without excessively impacting the system. For high-precision x-ray timing experiments, a new scheme has been designed to distribute the radio-frequency timing signals to the beamlines. Using VME-based hardware and EPICS software, the bunch clock signal is generated at the beamline for use in timing experiments. Data are provided at the radio frequency of 352 MHz. Beamlines have also been provided with hardware and software to generate the bunch clock signal for timing experiments.

Design of an undulator white beam profiler and test results on the Advanced Photon Source beamline (abstract)

At the Advanced Photon Source (APS), each insertion device (ID) beamline front end has two x-ray beam position monitors (XBPMs) to monitor the x-ray beam position for both the vertical and horizontal directions. The XBPMs measure photoelectrons generated by the chemical vapor deposited-diamond-based sensory blades and deduce the beam position by comparison of the relative signals from the blades [D. Shu, B. Rodricks, J. Barraza, T. Sanchez, and T. M. Kuzay, Nucl. Instrum. Methods. Phys. Res. A 319, 56 (1992)]. Performance challenges for an undulator XBPM during operation are contamination of the signal from the neighboring bending-magnet sources and the sensitivity of the XBPM to the ID gap variations. Problems are exacerbated because users change the ID gap during their operations, and hence the percentage level of the contamination in the front-end XBPM signals varies. The smart XBPM system partially solved these problems [D. Shu, H. Ding, J. Barraza, T. M. Kuzay, and M. Ramanathan, J. Synchrotron Radia. 5, 632 (1998)], but it is still very difficult to eliminate the contamination of the signal from the storage ring orbit-corrector magnets. A method was proposed by G. Decker and O. Singh [Phys. Rev. ST Accel. Beams 2, 112801 (1999)] that provides a solution to the long-standing problem of stray radiation-induced signals on photoemission-based XBPMs located on the ID beamline front end. The method involves the introduction of a chicane into the accelerator lattice that directs unwanted x rays away from the photosensitive XBPM blades. This technique has been implemented at the APS. In this paper, we present the design of an undulator white beam profiler that provides experimental confirmation of this technique.

APS Insertion Devices: Recent Developments and Results.

The Advanced Photon Source (APS) now has a total of 23 insertion devices (IDs). Over two-thirds of them are installed on the storage ring. The installed devices include 18, 27 and 55 mm-period undulators; an 85 mm-period wiggler; a 16 cm-period elliptical multipole wiggler; and many 33 mm-period undulators. Most of the IDs occupy storage-ring straight sections equipped with 8 mm vertical-aperture vacuum chambers. All of the IDs were measured magnetically at the APS and, in most cases, underwent a final magnetic tuning in order to minimize variation in the various integrals of the field through the ID over the full gap range. Special shimming techniques to correct magnetic field parameters in appropriate gap-dependent ways were developed and applied. Measurements of the closed-orbit distortion as a function of the ID gap variation have been completed, and results are in a good agreement with magnetic measurements. Spectral diagnostics of the ID radiation, including measurements of the absolute spectral flux, brilliance and polarization, show excellent agreement between calculated and measured results. Studies of the sensitivity of IDs to radiation exposure and measurements of the dose rate received by the IDs are in progress.

Smart X-ray beam position monitor system using artificial-intelligence methods for the Advanced Photon Source insertion-device beamlines.

At the Advanced Photon Source (APS), each insertion-device (ID) beamline front end has two X-ray beam position monitors (XBPMs) to monitor the X-ray beam position for both vertical and horizontal directions. Performance challenges for a conventional photoemission-type XBPM during operations are contamination of the signal from the neighbouring bending-magnet sources and the sensitivity of the XBPM to the insertion-device gap variations. Problems are exacerbated because users change the ID gap during their operations, and hence the percentage level of the contamination in the front-end XBPM signals varies. A smart XBPM system with a high-speed digital signal processor has been built at the Advanced Photon Source for the ID beamline front ends. The new version of the software, which uses an artificial-intelligence method, provides a self-learning and self-calibration capability to the smart XBPM system. The structure of and recent test results with the system are presented in this paper.

Field design studies of a planar hybrid/permanent magnet undulator with strong planar permanent magnet focusing

Undulators for Free Electron Laser (FEL) amplifiers driven by multi-GeV electron beams generally require superimposed focusing substantially stronger than that provided by their natural focusing fields. In recent years a wide variety of focusing schemes and configurations have been proposed for undulators of this class, ranging from conventional current-driven quadrupoles external to the undulator magnets to permanent magnet (PM) lattices inserted into, or adjacent to, the insertion device gap. In this paper we present design studies of the field structure of a flexible hybrid/PM undulator with superimposed planar PM focusing proposed for a 1.5 Å Linac Coherent Light Source (LCLS) undulator driven by an electron beam with a 1 mm mrad normalized emittance. Attainable field parameters, tuning modes, and trajectory performance of the proposed structure are discussed.

Performance of a liquid-nitrogen-cooled, thin silicon crystal monochromator on a high-power, focused wiggler synchrotron beam

An experiment was performed on beamline BL 3 at the European Synchrotron Radiation Facility to test the diffraction performance of a novel internally liquid-nitrogen-cooled, thin silicon crystal monochromator exposed to high heat loads. The beam parameters were chosen to closely match the conditions expected, in terms of absorbed power and beam profile, at the Advanced Photon Source (APS) for the closed gap undulator at 7 GeV and 100 mA. The cooled crystal was oriented at 11.4° in the symmetric Bragg geometry to diffract 30 keV x-rays from the Si(333) planes. The source was a 44-pole wiggler with the insertion device gap set at 25.0 mm corresponding to a deflection parameter, K, of 4.2. A tunable toroidal mirror was used to focus the wiggler beam onto the crystal. Double-crystal rocking curves were measured at several power values using different attenuators. The maximum total power absorbed by the 0.6-mm-thick crystal was 154 W at a storage ring current of 136 mA. The peak power density at normal incidence was about 420 W/mm2 corresponding to an absorbed peak power density on the crystal face of 83 W/mm2. No thermal-induced broadening of the rocking curve was observed above the average measured mounting/fabrication strain of 2 arcsec. Rocking curves were also measured as a function of coolant flow rate and pressure. No systematic broadening occurred due to flow-induced vibrations up to 6 ℓ/min. It has been demonstrated that thin silicon crystals directly cooled with liquid nitrogen can handle high power density synchrotron beams comparable to what is expected for the APS undulators with no appreciable thermal deformation.