R.A. Bosch

Infrared radiation from bending magnet edges in an electron storage ring

The infrared radiation emitted by electrons entering or exiting a storage ring bending magnet, which is termed ‘‘edge’’ radiation, is computed. The numerical results are in agreement with experimental observations at a wavelength of 1 μm. A comparison is made with the infrared synchrotron radiation emitted from the central region of a bending magnet for wavelengths of 1 μm–1000 μm. The flux of the edge radiation is lower than that of the synchrotron radiation for the shorter wavelengths studied, and greater for the longer wavelengths. However, the brightness of the edge radiation is higher for all of the wavelengths studied. This suggests that edge radiation may be a promising infrared radiation source.

Infrared edge radiation beamline at Aladdin

An infrared beamline has been constructed at the 800 MeV electron storage ring, Aladdin. The beamline is located downstream of a short straight section, where it collects edge radiation produced by electrons exiting and entering the bending magnets at the ends of the straight section. Measurements at wavelengths of 1-5 /spl mu/m suggest that the edge radiation is brighter than standard synchrotron radiation. When used for Fourier transform infrared (FTIR) microspectroscopy, the signal-to-noise ratio is 20 times that measured with a glower source. Several applications of the beamline are described.

Extraction of edge radiation within a straight section of Aladdin

Edge radiation may be extracted by a slotted mirror located within a straight section of the Aladdin electron storage ring. For wavelengths exceeding 1 mm, this method is expected to provide a greater flux than a large aperture located downstream of a straight section or bending magnet source. The edge radiation within an Aladdin straight section may find application as a broadband source for wavelengths of 1 μm–60 mm, or as a diagnostic of the bunch length through coherent emission.

Throughput and noise of the Aladdin infrared beamline

The infrared beamline at the Aladdin electron storage ring utilizes edge radiation for spectromicroscopy. For wavelengths of 0.8-16 /spl mu/m, computations indicate that /spl sim/20% of the collected radiation is transported through the microscope. Transverse oscillations of the electron beam cause oscillations in microscope throughput that account for about one-third of the beamlines zero-burst noise.

Commissioning low emittance beam at Aladdin

The Aladdin storage ring is now routinely run in a low emittance configuration at 800 MeV. Vertical beam sizes and lifetime are comparable to the original lattice, while the horizontal beam size is reduced by a factor of three. Tools used to commission the new lattice include model based correction to obtain the design machine functions, and model independent correction to set the desired transverse coupling. Newly installed optical profile and position monitors, shunts to trim individual magnets, as well as implementation of a new control system scripting language, were important in achieving the desired results. Special attention was given to operation of the fourth harmonic bunch lengthening cavity used to improve the beam lifetime, and noise reduction in the RF system to improve photon beam quality on the infrared beamlines. In addition, compensation of undulators allows their strengths to be varied with minimum perturbation to the beam outside the regions of the undulators. Details of bringing the low emittance lattice to operational readiness are presented.

Reducing the sensitivity of Fourier transform infrared spectroscopy to line-frequency source variations

Audio-frequency variations of the electron beam size and position produce noise when performing Fourier transform infrared (FTIR) spectroscopy with synchrotron radiation from an electron storage ring. The undesirable sensitivity of FTIR spectroscopy to periodic line-frequency source variations may be decreased when a line trigger initiates the motion of the interferometer mirror. Several methods of reducing line-frequency noise are described.

Operation of Aladdin at lowered emittance

To increase the available photon flux density for users, alternate lattice tunings of the Aladdin synchrotron light source have been developed with horizontal emittances significantly lower than the present value of 127 /spl pi/ nm-rad. Reduction of the horizontal emittance by a factor of three has been obtained experimentally. When the fourth harmonic Landau cavity is used to lengthen the bunch, the observed beam lifetime with the new lattice is not significantly changed from that of the existing lattice. The present goal is to achieve a factor of four horizontal and a factor of two vertical emittance reduction routinely. Progress in making this new configuration fully operational is discussed, including the use of quadrupole shunts with correction software for beta function and dispersion correction, understanding of a mode coupling instability in higher-harmonic RF systems, RF clearing of ions, and fabrication of new optical monitoring stations.

Robinson modes at Aladdin

A fourth harmonic radiofrequency (RF) cavity improves the beam lifetime of the Aladdin electron storage ring. When the harmonic cavity is operated with a low-emittance lattice, coupling between the dipole and quadrupole Robinson modes may cause instability. During stable operation, damped Robinson modes are observed in the spectrum of phase noise upon the beam

Feedback to Suppress Phase Noise at Aladdin

The performance of the Aladdin infrared beamline is adversely affected by a Robinson mode in which all bunches move in unison with a frequency of 3 kHz. To decrease these oscillations, feedback has been installed in the radiofrequency system to damp longitudinal motion of the bunch centroids. Simulations indicate that at frequencies around 3 kHz, the phase noise generated by Robinson modes may be reduced 20 dB by feedback with a damping time of 0.3 ms. This agrees with the measured performance of feedback circuitry. Since the feedback greatly improves operation of the infrared beamline, it is now incorporated into the standard operation of Aladdin.

Recent Developments at Aladdin

Following on the success of lower emittance operation at 800 MeV, SRC is pursuing a number of additional enhancements to the performance of the Aladdin storage ring. Work on Aladdin has included development of low emittance lattices at 1 GeV, which will maximize the capabilities of a recently installed spectromicroscopy beamline and a proposed high-resolution keV beamline. Installation of one-meter long insertion devices in the short straight sections within the quadrant arcs of the four sided storage ring is being pursued to increase the number of undulator beamlines from four to possibly eight. Studies have been made to determine what is the minimum insertion device gap that does not interfere with nominal ring operation (injection, ramping, and lifetime at full energy), and indicate that smaller-gapped devices for higher photon energy are reasonable. Lifetime increases or further emittance reductions appear possible with modest aperture increases at a small number of points on the ring. Finally, planning is under way for long term projects such as a new injector or a next generation VUV/soft-xray source for the Midwest. Details are presented.