Electromagnetic Undulator Research Articles

Comments on the Physics of Microwave-Undulators

The properties of electromagnetic undulators, produced by the field of a high-power microwave, are discussed. We analyze the conditions to be satisfied that allow the treatment of the emission process by relativistic charges moving through it in full analogy with the case of their static counterparts. We critically review the often claimed possibility of exploiting them in programs aimed at reducing the sizes and costs of an actual free electron laser (FEL) device. We analyze several possible configurations for FELs based on powerful microwave undulators and conclude that the present level of technology, even though not yet mature, should be improved to allow a breakthrough either for low- or high-gain devices.

Gyrotrons as High-Frequency Drivers for Undulators and High-Gradient Accelerators

Gyrotrons are used as high-power sources of coherent radiation operating in pulsed and CW regimes in many scientific and technological fields. In this paper, we discuss two of their numerous applications. The first one is in gyrotron-powered electromagnetic wigglers and undulators. The second one is for driving high-gradient accelerating structures in compact particle accelerators. The comparison, between the requirements imposed by these two concepts on the radiation sources on one hand and the output parameters of the currently available high-performance gyrotrons on the other hand, show that they match each other to a high degree. We consider this as a manifestation of the feasibility and potential of these concepts. It is believed that after the first successful proof-of-principle experiments they will find more wide usage in the advanced FEL and particle accelerators.

Optical klystron undulator radiation with electromagnetic wave undulator

In the present study we proposed optical klystron in an electromagnet wave undulator. We consider the circularly polarized electromagnetic waves are travelling collinearly in opposite direction formed a beat wave. Spectral properties of the beat wave optical klystron are analysed and investigated using Lienard-Wiechert potential. We observed spikes as well as sidebands on the either side of the fundamental frequency.

Transverse confinement of electron beams in a 2D optical lattice for compact coherent x-ray sources

Compact coherent x-ray sources have been the focus of extensive research efforts over the past decades. As a result, several novel schemes like optical and nano-undulators for generating x-ray emissions in ‘table-top’ setups are proposed, developed, and assessed. Despite the extensive efforts in the past decades, there exists no operational FEL based on optical or electromagnetic undulators. By combining the particle confinement capability of optical cavities with wiggling motion inside an optical undulator, this paper proposes a new concept for making a compact coherent x-ray source. The full-wave solution of first-principle equations based on finite-difference time-domain and particle-in-cell is performed to simulate inverse-Compton scattering (ICS) off both free and confined electrons. It is shown that the strong space-charge effect in a low-energy electron beam (5 MeV) is the main obstacle in acquiring coherent gain through the ICS mechanism with a 10 micrometer laser. Subsequently, it is shown that by confining the electron beam at the field nodes of an optical cavity, the space-charge effect is compensated, and additionally, the ultrahigh charge density enables high FEL-gain at confinement spots. The full-wave numerical simulations predict enhancement of about three orders of magnitude in the radiation efficiency when ICS is carried out with confined electrons compared to free electrons. These theoretical results show promising potential as a new scheme for implementing a compact coherent x-ray source.

Recirculated Wave Undulators for Compact FELs

Particular schemes of Free Electron Lasers (FELs) are designed to exploit wave undulators. We consider a system employing a recirculated electromagnetic undulator provided by a high-power laser in a resonator cavity. The aim is to establish from calculations a set of realizable parameters for such a device. Indeed, novel generation electron accelerators push forward the limits on the accelerating fields, reducing to the sub-meter scale the length over which the electrons can gain enough energy for lasing in the VUV/X-ray region of the electromagnetic spectrum. On the other hand, these innovative technologies do not solve yet the problem associated with the saturation length and therefore of the undulator length, which can be as long as several tens of meters. The option of a FEL based on a wave undulator might provide a valid solution in this respect.

Gain and efficiency of table-top terahertz free-electron lasers driven by a microtron accelerator

There are difficulties in developing high-power and small-size terahertz (THz) sources that can be used for field applications. In an attempt to design small-size THz free-electron laser (FEL) devices capable of producing higher output powers in the THz spectral region of 1–2 THz, we have developed a microtron accelerator that can accelerate electron beams to 3–6 meV, with a macropulse current of more than 40 mA. The new THz FELs use hybrid electromagnetic (EM) undulators that are two to four times shorter in length than the previous undulator and waveguide resonators with mode cross-sectional areas that are more than two times smaller than the parallel-plate waveguide in the existing FEL. We confirm that the gains and losses of the compact FELs are sufficient for lasing, and we estimate that average output power of approximately 1 W is possible with an efficiency approximately 10 times greater than the existing FEL. The minimum size of the THz FEL system, including a high-voltage pulse modulator, is estimated to be approximately 1.5 m × 2.0 m.

Design concept of an electromagnetic undulator to produce arbitrarily polarized light

An undulator magnet design is described consisting of four electromagnet arrays to produce arbitrarily polarized photon beams including a linear inclined mode. Polarization changes are effected by a current switch rather than mechanical longitudinal translations. The capability of choosing a variety of current excitations in each double coil of the structure can produce any desired polarization mode. This concept exhibits an advantage over mechanical designs especially for superconducting design versions of this undulator. In this paper, we will discuss the magnet structure concept, the current distribution in each double coil and magnetic field features for various polarizing modes. In addition, the concept of current switching will be discussed, which allows to reduce the number of individual current sources from eight to one.

Flux jumps in ring-shaped and assembled bulk superconductors during pulsed field magnetization

Bulk (RE)BCO, where RE is a rare-earth element or yttrium, superconductors fabricated in the form of rings are potentially useful for a variety of solenoidal-type applications, such as small, high field nuclear magnetic resonance and electromagnetic undulators. It is anticipated that the practical exploitation of these technologically important materials will involve pulse field magnetization (PFM) and, consequently, it is important to understand the behavior of ring-shaped samples subjected to the PFM process. Macroscopic flux jumps were observed in PFM experiments on ring-shaped bulk samples when the peak applied field reaches a threshold magnitude, similar to behavior reported previously in cylindrical samples. Magnetic flux jumps inward when the thermal instability is triggered, however it subsequently flows outwards from the sample, resulting in a relatively low trapped field. This behavior is attributed to a variety of effects, including the inhomogeneity of the material, which may lead to the formation of localized hot spots during the PFM process. In order to further elucidate this phenomena, the properties of a structure consisting of a bulk superconducting ring with a cylindrical superconductor core were studied. We observe that, although a flux jump occurs consistently in the ring, a critical state is established at the boundary of the ring-shaped sample and the core. We provide a detailed account of these experimental observations and provide an explanation in terms of the current understanding of the PFM process.

Simulation and measurement of electromagnetic undulator prototype for production of THz radiation

This research focused on magnetic field simulation and measurement of an electromagnetic undulator prototype for generation of terahertz (THz) radiation from electron beam with energy of about 10 MeV at the PBP-CMU Electron Linac Laboratory of the Plasma and Beam Physics Research Facility. This undulator prototype has 6.5 periods with a period length of 64 mm. Its magnetic field was measured by using a Hall probe with a measuring system, which its position movement can be precisely controlled with automatic computer interface. The magnetic field of end-poles was compensated by adjusting the end-pole currents. It was found that at the main-pole current of 0.76 A, the optimal end-pole currents at the entrance and exit o f the undulator were 0.27 A and 0.19 A, respectively. At these currents the undulator has the transverse good field region of 19 mm. For magnetic field simulation, the computer program RADIA was used to model the undulator prototype for the ideal design. This was done for comparing the magnetic fields from the simulation and the measurement. The magnetic field of the two cases were not the same because the constructed prototype has different gaps for different lo ngitudinal positions. However, they could give the radiation in the THz regime, which were 97.6 μm for the simulation and 96.7 ± 0.1 μm for the measurement.

An inverse free electron laser acceleration-driven Compton scattering X-ray source

The generation of X-rays and γ-rays based on synchrotron radiation from free electrons, emitted in magnet arrays such as undulators, forms the basis of much of modern X-ray science. This approach has the drawback of requiring very high energy, up to the multi-GeV-scale, electron beams, to obtain the required photon energy. Due to the limit in accelerating gradients in conventional particle accelerators, reaching high energy typically demands use of instruments exceeding 100’s of meters in length. Compact, less costly, monochromatic X-ray sources based on very high field acceleration and very short period undulators, however, may enable diverse, paradigm-changing X-ray applications ranging from novel X-ray therapy techniques to active interrogation of sensitive materials, by making them accessible in energy reach, cost and size. Such compactness and enhanced energy reach may be obtained by an all-optical approach, which employs a laser-driven high gradient accelerator based on inverse free electron laser (IFEL), followed by a collision point for inverse Compton scattering (ICS), a scheme where a laser is used to provide undulator fields. We present an experimental proof-of-principle of this approach, where a TW-class CO2 laser pulse is split in two, with half used to accelerate a high quality electron beam up to 84 MeV through the IFEL interaction, and the other half acts as an electromagnetic undulator to generate up to 13 keV X-rays via ICS. These results demonstrate the feasibility of this scheme, which can be joined with other techniques such as laser recirculation to yield very compact photon sources, with both high peak and average brilliance, and with energies extending from the keV to MeV scale. Further, use of the IFEL acceleration with the ICS interaction produces a train of high intensity X-ray pulses, thus enabling a unique tool synchronized with a laser pulse for ultra-fast strobe, pump-probe experimental scenarios.

Development of a compact electromagnetic undulator for linac-based coherent THz radiation source in Thailand

A linac-based accelerator system at the Plasma and Beam Physics Research Facility of Chiang Mai University in Thailand can produce relativistic femtosecond electron bunches for generating coherent THz radiation. Originally, this accelerator system was built to produce high peak power THz/FIR radiation via transition radiation technique. To extend the system’s capability in production of higher average power and tunable coherent radiation, a project to add an undulator magnet as a source of coherent THz radiation was started. Construction of a compact 40-period electromagnetic undulator with short period length is in progress. Low electricity consumption, inexpensive materials and uncomplicated fabrication process are aimed. An undulator prototype was designed and built to investigate the magnetic field properties and to estimate the possibility on manufacturing of a future undulator magnet. A computer program RADIA was used to model a three-dimensional undulator prototype based on the fabricated magnet material and dimensions. This prototype has 4.5 periods consisting of 8 main-poles and 2 end-poles for magnetic field correction. The simulated peak magnetic field was 0.1858 T or equivalents to an undulator parameter of 0.98. This led to a calculated undulator radiation wavelength at the first harmonic of 109 μm for electron beam with energy of 10 MeV. This radiation is in the forward direction. After manufacturing, the magnetic field of the undulator prototype was measured and analyzed. The measurement result at an optimal excitation current of 5 A showed that an average period length of 54.8 mm and an average peak magnetic field of 0.1855 T were achieved. The measured undulator parameter was 0.97, which resulted in the undulator radiation wavelength of about 107 μm. A future electromagnetic undulator with 40 periods was designed and will be built based on the study results of this undulator prototype. The estimated angular flux density of the radiation emitted from 40 period undulator magnet is dominated at the first harmonic radiation wavelength of 109 μm, which is equivalent to the radiation frequency of 2.8 THz.

The prototype of new variable period undulator for Novosibirsk free electron laser

To improve the parameters of the second stage Novosibirsk free electron laser we plan to replace the existing electromagnetic undulator by permanent-magnet variable-period undulator (VPU). The VPUs have several advantages compared to conventional undulators, which include wider radiation wavelength tuning range and an option to increase the number of poles. Both these advantages will be realized in the new undulator under development in Budker INP. There are some technical problems, which have to be solved before this idea can be implemented in practice. To check the solution of these problems we designed and manufactured a small undulator prototype, which has just several periods. In this paper, the results of mechanical and magnetic measurements of this undulator prototype are presented and compared with simulations.

Design of a Compact Electromagnetic Undulator for THz Radiation Production

The goal of this research is to design a compact electromagnetic undulator. This insertion device will be installed at the PBP–CMU–LINAC system of Chiang Mai University (CMU), Thailand, to produce THz radiation. The undulator magnet is designed by using 2D POISSON and 3D RADIA computer code to optimize the magnet dimensions. The width of iron pole (w) should be 12 – 14mm. The height of iron pole (L) should be about 85mm long. The thickness of return yoke should be more than 10mm and the lateral width (T) should be at least 50mm. With the undulator period length of 56mm, a 10 MeV electron beam is expected to produce THz radiation varying from 110μm at K = 1 to 76μm at K = 0.3.

Waveguide-Mode Terahertz Free Electron Lasers Driven by Magnetron-Based Microtrons

We have developed small-sized terahertz free-electron lasers by using low-cost and compact microtrons combining with magnetrons as high-power RF sources. We could stabilize the bunch repetition rate by optimizing a modulator for the magnetron and by coupling the magnetron with an accelerating cavity in the microtron. By developing high-performance undulators and low-loss waveguide-mode resonators having small cross-sectional areas, we could strengthen the interaction between the electron beam and the THz wave inside the FEL resonators to achieve lasing even with low-current electron beams from the microtron. We used a parallel-plate waveguide in a planar electromagnet undulator for our first THz FEL. We try to reduce the size of the FEL resonator by combining a dielectric-coated circular waveguide and a variable-period helical undulator to realize a table-top THz FEL for applying it to the security inspection on airports.

Analysis of spectral properties of harmonic undulator radiation of an electromagnet undulator

In this paper we analyze the spectral properties of undulator radiation with an electromagnet undulator for electron injected off the undulator axis. The electromagnet undulator is self bi-harmonic for locations near to the electromagnet. The electrons execute additional betatron oscillation when it injected off the undulator axis. It is observed that the electromagnet undulator can be used for harmonic lasing free electron laser operates at the cost of betatron oscillations for electron injected off the axis.

Design and optimization of an electromagnet undulator

In this paper we discuss the design and optimization of an electromagnet undulator. A short period electromagnet undulator has been reported and used for free electron laser experiment under the approximation of infinite permeability of the lamination core. Here we remove this approximation and present analytical expressions for an alternate design concept on the electromagnet undulator.

Analysis of undulator radiation with an electromagnet undulator

In this paper we discuss the theory of undulator radiation in an electromagnet undulator. We discuss the spectral properties of undulator radiation when electrons are injected off the undulator axis. This paper highlights the distinctive features of the radiation spectrum from electromagnet undulators, as compared to PPM undulators.

Inclusion of Finite Permeability on Design and Optimization of an Electromagnetic Undulator

We derive an alternate analytical expression for the optimum dimensions of an electromagnetic undulator with a finite permeability lamination core. The analytical expressions are compared with earlier results under an infinite permeability approximation of the lamination core of the electromagnetic undulator design.

Surface-micromachined Electromagnets for 100μm-scale Undulators and Focusing Optics

Abstract Leveraging recent advances in microelectromechanical system (MEMS) fabrication technologies, 100 μm gap 400 μm period electromagnetic undulators and 200 μm gap quadrupole optics have been successfully fabricated. Measurement of the electromagnet impedance closely matches simulations, corresponding to 0.135 T peak field on-axis in the undulator and 1400 T/m field gradients in the quadrupole for the initially tested devices. These microfabricated undulators and quadrupoles are envisioned as insertion devices and focusing lattices for future compact light sources.

Operation experience of the UE44 fixed gap APPLE II at SLS

All soft x-ray beamlines at the Swiss Light Source (SLS) are served with variable polarization from APPLE II [1] type and electromagnetic undulators. Three APPLE II type undulators are used: a twin and a single standard APPLE II (UE56 and UE54) and a fixed gap APPLE II (UE44) which follows the adjustable-phase undulator approach by R. Carr [2], [3]. The demand to rotate the linear polarization vector from 0 – 180° required all four magnet arrays to be shiftable. This opened the possibility to also vary the energy by a suitable shift of the magnet arrays with a simplified support structure lacking in any gap drive system [4], [5]. The current photon beam quality in linear and circular mode and the pros and cons of the operation of the UE44 will be discussed, namely the underestimated influence of gradients in the complex field distribution. As a consequence the spectra are degraded, but can be recovered by use of distributed coils or by a simple change in the operation mode.