Undulator Gap Research Articles

Neural networks for ID gap orbit distortion compensation in PETRA III

Undulators are used in storage rings to produce extremely brilliant synchrotron radiation. In the ideal case, a perfectly tuned undulator always has a first and second field integrals equal to zero. But, in practice, field integral changes during gap movements can never be avoided for real-life devices. As they significantly impact the circulating electron beam, there is the need to routinely compensate such effects. Deep Neural Networks can be used to predict the distortion in the closed orbit induced by the undulator gap variations on the circulating electron beam. In this contribution several state-of-the-art deep learning algorithms were trained on measurements from PETRA III. The different architecture performances are then compared to identify the best model for the gap-induced distortion compensation. It is found that realistic data, with several gaps moving simultaneously to arbitrary gaps must be used to train the neural networks. The deep feed-forward neural network was found to be more effective than the recurrent and convolutional neural networks.

Cryogenic calibration for Hall probes

A superconducting undulator (SCU) prototype with a magnet length of 4m is being developed for the SHINE (Shanghai High Repetition Rate XFEL and Extreme Light Facility) project. The period length of the undulator is 16mm, the period number is 250, and the pole gap is 5mm. A point measurement system with three Hall probes will be used to measure the magnetic fields in the gap. The Hall probes fixed on a sledge will experience temperatures of about 4.2K to 50K when they move through the undulator gap. A temperature-dependent calibration system for these hall probes is necessary. The paper introduces the configuration of the cryogenic calibration system, and the result for the sensitivity and nonlinearity of one Hall probe from 2.8K to 300K over a magnetic field range of ±1.9T.

High-precision positioning of Hall probes for SCU magnetic field measurement

A planar superconducting undulator (SCU) prototype with the magnetic length of 4 m and the magnetic gap of 5 mm is being developed for Shanghai High Repetition rate XFEL and Extreme Light facility (SHINE) to generate vertical linear polarization FELs. The magnetic field measurement device based on Hall probes has been built for the SCU. Three Hall probes mounted on a sledge are used to measure the magnetic field in the undulator gap. The three-dimensional positioning system based on the Gaussian laser beam for the sledge and the Hall probes is presented in detail in this paper.

Photon Diagnostic for FLASH2020+: Analysis of Spontaneous Radiation for Undulator Alignment

Within the FLASH2020+ program, DESY is aiming to significantly improve the flexibility and availability of its pioneering FEL FLASH for users. Increasing the electron beam energy to 1.35GeV will make wavelengths available up to the oxygen K-edge on the fundamental harmonic. Exchanging the current fixed gap undulators in the FLASH1 branch for variable gap devices will allow for fully independent operation of both branches in the future. The 11 new Apple III type variable polarization undulator segments will have a period length of 33mm and a length of 2.5m each.This paper presents the development of a photon diagnostic station for FLASH2020+, an additional tool for alignment and commissioning of the undulators. The diagnostic station will be located about 50m downstream from the last undulator. It will allow for the measurement and correction of the electron beam orbit as well as for the measurement of the undulator K-values. In addition, the system can be used to investigate the phasing of the undulators in two adjacent cells by analysing their spontaneous radiation.

Development and magnetic field measurement of a 0.5-m-long superconducting undulator at IHEP.

Undulators are important devices for accelerator-based light sources whose magnetic field quality determines the photon beam performance. Superconducting-type undulators have been developing rapidly in recent years around the world. The insertion device group at the Institute of High Energy Physics (IHEP) in China started an R&D project to develop prototypes of superconducting undulators (SCUs). A half-meter-long planar SCU has been produced recently. The SCU was designed based on the simulation program OPERA-3D giving a period length of 15 mm and a gap of 7 mm. This prototype was manufactured and fabricated precisely, and was then tested by vertically submerging in liquid helium in a Dewar. After quench training several times, the maximum current in the main coils reached 480 A. The magnetic field was measured by Hall probes mounted on a sledge in the middle of the undulator gap. The peak magnetic field reached 1 T. The measurement results indicate that correction coils with suitable current can not only optimize the magnetic first and second integrals but also reduce the phase error, which is expected bydesign.

Control Scheme of Phase-Shifter for Photon Energy Scan

Variable gap undulator widely used in X-ray free-electron laser (XFEL) enables the photon energy scan by changing its gap. A phase-shifter should be incorporated to compensate for the phase mismatch between the electron bunches and X-ray pulses arising while those traverse the drift space between undulator segments. The uncertainties in both the undulator parameter and the drift space distance introduce an error in calculating the optimum gap distance of the phase-shifter for the different undulator K. The phase-shifter gap needs to be set where the error is within the tolerable range. The control scheme we propose can maintain full FEL intensity over the scanned photon energies.

Optimization of SASE operation for an X-ray free-electron laser Using multiple objective evolutionary algorithms

Performance of Self Amplifed Spontaneous Emission (SASE) operation for an X-ray Free-Electron Laser (FEL), is optimized by Multiple Objective Evolutionary Algorithms (MOEA). Different types of hybrid planar undulators are considered to achieve 0.5 Å ≤ λ FEL ≤ 1.5 Å wavelength range. This demand is regarded as a continuous optimization problem, and hence, an evolutionary algorithm is designed to find optimal FEL performance parameters. An encoding scheme comprising of undulator period (λ u ) and undulator gap (g) is adopted here to find optimal values for saturation power (P sat ) and λ FEL subject to several physical constraints on λ u and g such as the ratio of g/λ u and L sat . It is shown that MOEA gives optimal solutions for estimation of L sat with plausible P sat values.

A custom high-stability pulse generator for the test of a novel in-situ magnetic sensor developed to detect radiation damage in undulators

High-performance analog-to-digital converters (ADC) have been widely applied in many areas of science. For instance, magnetic field measurements based on the Faraday’s induction law require high-precision voltmeters to measure induced voltages. In the context of free-electron lasers technology, the Magnetic Measurements Team at SLAC National Accelerator Laboratory proposed a novel in-situ radiation damage detection system (RDDS) for detecting small field variations in undulators. The system measures the flux change in a flexible printed-circuit coil attached to the magnet array during the undulator gap movement. The gap movement changes the magnetic field, which induces a voltage signal in the coil that is measured and integrated with an ADC. Although the system is capable of detecting relative flux changes better than 100ppm, drift in the ADC’s gain or offset can cause apparent changes in the relative flux. This paper describes the first attempt to developed a high-precision verification circuit to perform ADC testing in the framework of the novel RDDS. The circuit generates a reference voltage pulse with a voltage–time integral relative precision better than 50ppm for a few hundreds of mVs — the typical order of magnitude measured with the RDDS. The circuit’s design combines a fast and precise switch with a low-noise voltage reference. Long-term measurements allowed statistical analysis and showed that averaging the voltage–time integral of ten pulses gives the required 50ppm stability. Moreover, reproducibility tests confirmed that the circuit’s output is invariable under small power supply instabilities and equipment shutdown. Instruments and applications designed to quantify the magnetic field by integrating voltage signals may use the pulse generator proposed in this paper for verification purposes.

Control of undulator radiation using a Laser Plasma Acceleration Source

Spontaneous undulator radiation emission, after the COXINEL line using a Laser Plasma acceleration (LPA) source, has been observed. The line enables to manipulate the electron beam phase space such as emittance, dispersion and energy spread along a 10 m long transport. The large divergence is handled at a very early stage to mitigate the chromatic emittance, using high gradient permanent magnet based quadrupoles mounted on translation tables. The operating energy is between 161-180 MeV focused in a 2-m long cryo-ready undulator with a period of 18 mm emitting light in the Ultra-Violet range. The spectral flux is characterized using a spectrometer. The wavelength is tuned by either changing the electron beam energy or by adjusting the undulator gap. The radiation pattern signature is illustrated alongside its dependence on the energy spread that is modified by introducing a slit in a magnetic chicane where a small relative bandwidth of 5% has been achieved.

Single-shot transverse coherence measurements with Young's double pinholes at FLASH2

We measured the transverse coherence at FLASH2, a variable gap undulator line at the FLASH free-electron laser user facility at DESY in Hamburg. We demonstrate, theoretically and experimentally, a revised version of Young’s double pinhole approach to perform single-shot, repeatable and non-invasive transverse coherence measurements. At beamline FL24 of FLASH2, the transverse coherence of pulses was systematically characterized at wavelengths of 8, 13.5 and 18 nm for different FEL source settings. We determine degrees of coherence of 57% to 87% in the vertical and horizontal direction, respectively. These measurements can facilitate the planning of novel, coherence-based experiments at the FLASH facility.

Magnetic field mapping simulations of sub-mm period undulators for the next generation X-ray free electron lasers

There is a growing interest on the compact XFELs to be used in applications such as medical science (surgery, fat removal), material science and military. In order to build a compact XFEL, constructing an undulator with sub-millimeter period (λu) is mandatory. Many problems arise due to miniaturization of undulators; such as keeping the magnetic field high while still having a considerable gap to minimize beam scraping. In this work, RADIA program has been employed for the modeling of sub millimeter period undulators (20μm<λu<400μm) with three different magnet configuration; namely Up–Down​ (↑↓↑↓…), Halbach (Wiggler) (↑→↓←↑…) and Hybrid (→↑←↓→…). Effects of sub-millimeter undulator period and gap on the magnetic field pattern of the undulator have been discussed.

Design and development of a laser interferometer for magnet alignment and phase error analysis in an undulator

In this paper we discuss design and development of a laser interferometer for undulator measurement application. The vertical misalignment of the magnets in the undulator results in non uniformity in the undulator gap and leads to non uniform magnetic field along the length of the undulator. The non uniformity in the magnetic field gives rise to trajectory wander and phase error. The trajectory wander and the phase error are the source of reduction in spontaneous emission and free electron laser gain. The effect is more severe for free electron lasers operating at higher harmonics. It is demonstrated that the laser interferometer can be used effectively to measure and correct the magnet alignments of the undulator.

A Novel $In-Situ$ Radiation Damage Diagnostic System for Undulators

Radiation damage has been a known problem at many synchrotron radiations and free-electron lasers facilities, especially those that use hybrid or pure permanent magnet undulators to produce high brightness light. Field strength loss due to demagnetization makes it necessary to remove the insertion device from the tunnel for inspection and repair, which brings several inconveniences. We aimed to develop an in situ radiation damage detection system for undulators capable of diagnosing relative field changes of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> (100 ppm) or smaller. The system measures the flux change in a flexible printed circuit board coil attached to the undulator magnet array by periodically measuring the integral flux change during the opening and closing of the undulator gap. If the magnet block damage occurs, the signal from the coil will decrease proportionally to the damage. First tests performed on variable gap hybrid undulators for the Linac Coherent Light Source II (LCLS-II) at SLAC National Accelerator Laboratory showed that the system is capable of measuring flux changes with a relative precision of approximately 0.7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> over several days. Relative field changes smaller than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> were detected in short-term measurements. The novel system that we propose here opens the possibility of detecting radiation damage in undulators during operation.

2D and 3D simulations of the permanent magnet undulator for generation of MIR-FEL

This research focuses on computer modelling and magnetic field simulations of the permanent magnet undulator at the PBP-CMU Electron Linac Laboratory, Chiang Mai University. This undulator magnet will be used for generation of the mid infrared free electron laser (MIR-FEL). It has 40 periods with a period length of 40 mm. The magnetic field simulations were conducted by using computer programs PANDIRA and RADIA for 2D and 3D models, respectively. The simulation results obtained from both models are presented and discussed in this paper. The magnetic field can be varied by adjusting a gap of the undulator. A maximum magnetic field of 0.23 Tesla can be achieved for the undulator gap of 26 mm. The simulated magnetic field distributions along the undulator magnetic axis obtained from the 3D model were used to calculate the deflecting angle and trajectory of electron. The achievement of this study will be used as a significant information for further installation and operation of the undulator magnet at our facility.

Quantitative investigation of linear arbitrary polarization in an APPLE-II undulator.

Insertion devices are utilized at synchrotron radiation facilities around the world for their capability to provide a high-brilliance X-ray beam. APPLE-II type undulators are especially important for their capacity to switch between a variety of photon beam polarization states. A high-precision soft X-ray polarimeter has been used to investigate the polarization calibration of an APPLE-II undulator (period length λu = 64 mm) installed on beamline I06 at Diamond Light Source. Systematic measurement of the beam polarization state at a range of linear arbitrary angles has been compared with the expected result for a given set of undulator gap and row phase parameters calculated from theory. Determination of the corresponding Stokes-Poincaré parameters from the measured data reveals a discrepancy between the two. The limited number of energy/polarization combinations included in the undulator calibration tables necessitates the use of interpolated values for the missing points which is expected to contribute to the discrepancy. However, by modifying the orbit of the electron beam through the undulator by at least 160 µm it has been found that for certain linear polarizations the discrepancies can be corrected. Overall, it is suggested that complete correction of the Stokes-Poincaré parameters for all linear angles would require alteration of both these aspects.

Widely tunable narrow-band coherent Terahertz radiation from an undulator at THU

There is anxious demand for intense widely tunable narrow-band Terahertz (THz) radiation in scientific research, which is regarded as a powerful tool for the coherent control of matter. We report the generation of widely tunable THz radiation from a planar permanent magnet undulator at Tsinghua University (THU). A relativistic electron beam is compressed by a magnetic chicane into sub-ps bunch length to excite THz radiation in the undulator coherently. The THz frequency can be tuned from 0.4 THz to 10 THz continuously with narrow-band spectrums when the undulator gap ranges from 23 mm to 75 mm. The measured pulse THz radiation energy from 220 pC bunch is 3.5 μJ at 1 THz and tens of μJ pulse energy (corresponding peak power of 10 MW) can be obtained when excited by 1 nC beam extrapolated from the property of coherent radiation. The experimental results agree well with theoretical predictions, which demonstrates a suitable THz source for the many applications that require intense and widely tunable THz sources.

Measurement of pre-bunched beam’s longitudinal form factor based on radiation from a tunable-gap undulator

The form factor, representing the statistical characteristics of a bunch's longitudinal distribution, is one of the most essential properties of a pre-bunched electron beam and is used for many types of frontier accelerator applications. We demonstrated the measurement of a pre-bunched beam's longitudinal form factor component based on coherent radiation from a widely tunable-gap undulator. The radiation energy from bunches with different longitudinal properties was measured as a function of undulator gap. The root-mean-square length of a 60 pC ultrashort quasi-Gaussian bunch generated by linac and chicane compression ranged from 75 fs to 240 fs, as obtained by fitting the radiation energy curve. Furthermore, the form factor component of the bunch train based on nonlinear longitudinal space charge oscillation was measured, and a higher-order harmonic component was observed with the proposed method than with the widely used coherent transition radiation method. The proposed method may satisfy the requirements of sub-fs bunch length measurement with proper undulator design.

The FLASH Facility: Advanced Options for FLASH2 and Future Perspectives

Since 2016, the two free-electron laser (FEL) lines FLASH1 and FLASH2 have been run simultaneously for users at DESY in Hamburg. With the installation of variable gap undulators in the new FLASH2 FEL line, many new possibilities have opened up in terms of photon parameters for experiments. What has been tested so far is post-saturation tapering, reverse tapering, harmonic lasing, harmonic lasing self-seeding and two-color lasing. At the moment, we are working on concepts to enhance the capabilities of the FLASH facility even further. A major part of the upgrade plans, known as FLASH2020, will involve the exchange of the fixed gap undulators in FLASH1 and the implementation of a new flexible undulator scheme aimed at providing coherent radiation for multi-color experiments over a broad wavelength range. The recent achievements in FLASH2 and the current status of plans for the further development of the facility are presented.

Magnetic design and modelling of a 14 mm-period prototype superconducting undulator.

The magnetic design of a ten-period (each period 14 mm) prototype superconducting undulator is reported using RADIA. The results of modelling the magnetic flux density are presented in an analytical formula. The dependence of the field integrals and phase error on the current density and undulator gap has been calculated, and temperature curves are determined for the models and are compared with earlier reported Moser-Rossmanith fits.

Phase matching strategy for the undulator system in the European X-ray Free Electron Laser

The undulator system in the European X-ray Free Electron Laser is mainly comprised of 5-m long undulator segments and 1.1 m long intersections in between. The longitudinal component of the electrons' velocity is reduced when traveling inside an undulator due to the wiggle motion. Therefore the optical phase is detuned. The detune effect is also from the undulator fringe field where electron longitudinal speed also deviates from the oscillation condition. The total detune effect is compensated by a magnetic device called phase shifter, which is correspondingly set for a specific undulator gap. In this paper we investigate the homogeneity of the fringe field from different undulators. Different phase matching criteria are studied. The field fitting technique for the phase matching in high accuracy is demonstrated in detail. The impact by air coil is also studied. Eventually the matching test by spontaneous radiation simulation is made. A test method for high sensitivity to matching error is proposed.