S. Marks

Fabrication of a Short-Period

Lawrence Berkeley National Laboratory develops high-field Nb3Sn magnets for HEP applications. In the past few years, this experience has been extended to the design and fabrication of undulator magnets. Some undulator applications require devices that can operate in the presence of a heat load from a beam. The use of Nb3Sn permits operation of a device at both a marginally higher temperature (5-8 K) and a higher Jc, compared to NbTi devices, without requiring a larger magnetic gap. A half-undulator device consisting of 6 periods (12 coil packs) of 14.5 mm period was designed, wound, reacted, potted and tested. It reached the short sample current limit of 717 A in 4 quenches. The non-Cu Jc of the strand was over 7,600 A /mm2 and the Cu current density at quench was over 8,000 A/mm2 . Magnetic field models show that if a complete device was fabricated with the same parameters one could obtain beam fields of 1.1 T and 1.6 T for pole gaps of 8 mm and 6 mm, respectively.

{\rm Nb}_{3}{\rm Sn}

Current design of permanent magnet wiggler/ undulators use either pure charge sheet equivalent material (CSEM) or the CSEM-Steel hybrid configuration. Hybrid configurations offer higher field strength at small gaps, field distributions dominated by the pole surfaces and pole tuning. Nominal performance of the hybrid is generally predicted using a 2-D magnetic design code neglecting transverse geometry. Magnetic measurements are presented showing transverse configuration influence on performance, from a combination of models using CSEMs, REC (Hc = 9.2 kOe) and NdFe (Hc = 10.7 kOe), different pole widths and end configurations. Results show peak field improvement using NdFe in place of REC in identical models, gap peak field decrease with pole width decrease (all results less than computed 2-D fields), transverse gap field distributions, and importance of CSEM material overhanging the poles in the transverse direction for highest gap fields.

Superconducting Undulator

A new undulator beamline at the Advanced Light Source, Lawrence Berkeley National Laboratory is described. This new beamline has an Apple II type undulator which produces linearly and elliptically polarized X-rays. A high resolution monochromator directs the radiation to two branchlines. The first branchline is optimized for spectroscopy and accommodates multiple endstations simultaneously. The second branchline features a photoemission electron microscope. A novel feature of the beamline is the ability to produce linearly polarized radiation at arbitrary, user-selectable angles. Applications of the new beamline are also described.

CSEM-Steel Hybrid Wiggler/Undulator Magnetic Field Studies

Superconducting undulator (SCU) technology has the potential to significantly enhance the performance of synchrotron radiation sources for storage ring and FEL applications. Since 2002, our team at Lawrence Berkeley National Laboratory has been performing R&D on superconducting undulators, including the fabrication of three Nb3Sn prototypes. We have demonstrated experimentally the possibility to provide the prototype with trim coils that could be used for phase error correction. The research effort that we report here demonstrates the possibility to add degrees of freedom to the field correction provided by these coils in a cryogenic environment. By means of bridge of superconducting switches, we can modify the current direction through a trim coil. Here we describe the design of the experimental bridge we fabricated, the results we obtained and finally the generalized concept one could plan to apply to correct the phase errors with trim coils connected to a network of superconducting bridges.

A SOFT X-RAY UNDULATOR BEAMLINE AT THE ADVANCED LIGHT SOURCE WITH CIRCULAR AND VARIABLE LINEAR POLARIZATION FOR THE SPECTROSCOPY AND MICROSCOPY OF MAGNETIC MATERIALS

Multiple trim magnets (MTMs), also known as ‘‘magic fingers,’’ are an arrangement of magnets for reducing integrated magnetic‐field errors in insertion devices. The idea is to use transverse arrays of permanent magnets, hence the name ‘‘multiple trim magnets,’’ above and below the midplane, to correct both normal and skew longitudinal magnetic‐field integral errors in a device. MTMs are typically installed at the ends of an ID. Adjustments are made by changing either the size, position, or orientation of each trim magnet. Application of the MTMs to the ALS undulators reduced both the normal and skew longitudinal field integral errors, over the entire 20 mm×60 mm ‘‘good field region,’’ of the beam aperture by as much as an order of magnitude. The requirements included corrections of field and gradients outside the multipole convergence radius. Additionally, these trim magnet arrays provided correction of the linear component of the integrated field gradients for particles with trajectories not parallel to ...

Superconducting switch concept applied to superconducting undulator phase‐error correction

Procedures are described for obtaining precise magnetic field integral values for Hall probe scans of undulators. A technique is discussed for scaling the even portion of a nonlinear Hall probe calibration based upon sets of scans with the Hall probe installed in opposing orientations. An example is discussed where a 293 G-cm error is corrected by scaling which corresponds to an average adjustment of 0.7 G over a 3.0 T range. An analysis of measurement uncertainty is presented. A quantitative example is presented which shows that the expected level of uncertainty is of the order of 20 G-cm for undulator field integrals using the Advanced Light Source (ALS) magnet measurement facility. Sets of duplicate field integral measurements of ALS undulators demonstrate an accuracy and variation of results of better than 20 G-cm. A comparison of integrated Hall probe scans with integral coil measurements shows good agreement. >

Multiple trim magnets, or ‘‘magic fingers,’’ for insertion device field integral correction

The possibility exists of achieving significantly lower emittances in an electron storage ring by increasing the horizontal betatron tune. However, existing magnet locations and strengths in a given ring may be inadequate to implement such an operational mode. For example, the ALS storage ring[1] could lower its emittance to one third of the current value by increasing the horizontal tune from 14.25 to 16.25. Nevertheless, this would come with the cost of large chromaticities that could not be corrected with our existing sextupole magnets. We discuss such operational issues and possible solutions in this paper.

Precise integration of undulator Hall probe scans

The magnetic end design of pure-permanent magnet Apple-II elliptically polarizing undulators (EPU) is discussed. Constraints on end block dimensions and positions are presented that guarantee steering and displacement free systems in both transverse directions and at all gaps for mu=1 material. For block material with mu>1 some beam steering (i.e. integrated dipole) may occur due to the ends; in particular, the integrated dipole strength varies with EPU phase. An optimization process is presented that assumes small perturbations about the mu=1 solution and minimizes the variation in steering with EPU phase. We present numerical and experimental results that quantify the reduction in integrated dipole variation with phase

A low emittance Lattice for the advanced ligght source

This paper describes a design study with the objective of optimizing spectral performance of an elliptical wiggler to be installed at the Lawrence Berkeley Laboratory Advanced Light Source (ALS). This device is to produce circularly polarized radiation in the energy range of 50 eV–10 keV. A figure of merit, which is a function of flux density and degree of circular polarization, is introduced as the objective function for optimization. An optimum set point for a particular photon energy is characterized by values of peak vertical field, horizontal deflection parameter, and vertical aperture. Optimum performance is evaluated for the nominal ALS operating energy of 1.5 GeV.

Elliptically Polarizing Undulator End Designs

Insertion devices for the Advanced Light Source (ALS) incorporate up to 3000 magnet blocks each for pole energization. In order to minimize field errors, these magnets must be measured, sorted and assigned appropriate locations and orientations in the magnetic structures. Sorting must address multiple objectives, including pole excitation and minimization of integrated multipole fields from minor field components in the magnets. This is equivalent to a combinatorial minimization problem with a large configuration space. Multi-stage sorting algorithms use ordering and pairing schemes in conjunction with other combinatorial methods to solve the minimization problem. This paper discusses objective functions, solution algorithms and results of application to magnet block measurement data. >