Permanent Undulator Research Articles

Helical undulators of magnetized ring sectors

A periodic helical array of magnetized ring rare-earth sectors (quasi-helices) creates a helical field on its axis, close in structure and magnitude to the field of a solid helix. In some cases, such permanent undulators, made of a relatively small number of readily available magnets, are easier to manufacture and assemble than a system containing magnetized helices made of one piece. The field magnitude of the quasi-helices depends only weakly on the number of sectors per undulator period. According to calculations and measurements, the prototype Halbach-type helical undulator assembled from longitudinally and “radially” magnetized quasi-helices consisting of NdFeB ring sectors with a period of 2 cm and a relatively large inner diameter of 8 mm, provides an on-axis field of about 0.6T. By reducing the inner diameter to 5 mm, one can achieve twice as strong a field. When assembling such an undulator, to weaken the mutual effect of strongly interacting magnetized elements, it turned out to be structurally more convenient, while maintaining the position of all ring sectors, to use the division of the undulator not into quasi-helices, but into cylindrical sectors shifted along the axis and rotated relative to each other. Permanent undulators made from ring sectors can provide high oscillation velocity of electrons and seem promising for increasing the efficiency of Free Electron Lasers in various frequency ranges.

Experimental Verification of a Permanent Undulator for a Strong Helical Field

Experimental Verification of a Permanent Undulator for a Strong Helical Field

Status of the development of Delhi Light Source (DLS) at IUAC

A project to construct a compact pre-bunched Free Electron Laser by using a normal conducting photocathode electron gun has been undertaken at Inter University Accelerator Centre (IUAC), New Delhi, India. In this facility, the short laser pulses from a high power laser system will be split into many pulses (2–16) commonly known as ‘Comb beam’ and will strike the photocathode material (metal and semiconductor) to produce electron beam bunches. The electrons will be accelerated up to an energy of ∼8MeV by a copper cavity operated at a frequency of 2860MHz and the beam will be injected into a compact, planar permanent undulator magnet to produce THz radiation. The radiation frequency designed to be tuned in the range of 0.15–3THz by varying the magnetic field of the undulator and/or changing the energy of the electron. The separation of the laser micro-pulses will be varied by adjusting the path length difference to alter the separation of the electron micro-bunches and to maximise the radiation intensity.

FEL polarization control studies on Dalian coherent light source

The polarization switch of a free-electron laser (FEL) is of great importance to the user scientific community. In this paper, we investigate the generation of controllable polarization FEL from two well-known approaches for Dalian coherent light source, i.e., crossed planar undulator and elliptical permanent undulator. In order to perform a fair comparative study, a one-dimensional time-dependent FEL code has been developed, in which the imperfection effects of an elliptical permanent undulator are taken into account. Comprehensive simulation results indicate that the residual beam energy chirp and the intrinsic FEL gain may contribute to the degradation of the polarization performance for the crossed planar undulator. The elliptical permanent undulator is not very sensitive to the undulator errors and beam imperfections. Meanwhile, with proper configurations of the main planar undulators and additional elliptical permanent undulator section, circular polarized FEL with pulse energy exceeding 100 μJ could be achieved at Dalian coherent light source.

Development of cryogenic permanent undulators operating around liquid nitrogen temperature

The cryogenic permanent magnet undulator (CPMU) is an insertion device in which permanent magnets (PMs) are cooled to a cryogenic temperature to improve magnetic performances in terms of remanence and coercivity. Operation of CPMUs is expected to be much easier than that of existing superconductive undulators (SCUs) with low-temperature superconducting coils, since the operating temperature can be much higher than that of liquid helium. In addition to pure PMs, high-permeability pole pieces and/or high-temperature superconductors (HTSCs) can be exploited to enhance the magnetic field of the CPMU. Towards realization of this concept, a number of R&Ds are in progress at SPring-8: field measurement under a cryogenic environment and reinforcement of HTSC samples. Encouraging results have been obtained from preliminary studies.

THE BROOKHAVEN FREE ELECTRON LASER EXPERIMENT

The status of the free-electron laser experiment at Brookhaven is reported, with the description of the permanent undulator already built and ready to be installed on the VUV storage ring.