First Undulator Radiation Delivered to the Experiment Floor

During July and early August of 1995, the first insertion-device vacuum chamber and an Undulator A were installed in Sector 1 of the APS storage ring. The vacuum chamber was an initial-phase chamber with a vertical aperture of 12 mm and a minimum possible undulator gap of 14.5 mm. The successful operation of the storage ring in this configuration delivered the first undulator beam to the experiment hall floor of the APS.

The first experiments looked at the effect of the undulator on a stored, 1-mA particle beam. Measurements of the closed orbit distortion showed that the performance of the undulator exceeded specification. When the magnetic gap of the undulator was moved through its range of 15.7 mm to open gap, the motion of the particle beam almost anywhere around the storage ring was found to be about 0.5% of the size of the beam or less, in both the vertical and horizontal directions. This result is very gratifying since the storage-ring beam-position feedback system was not active during the measurements. It also shows that the undulator fabrication and magnetic tuning have adequately minimized the integrals of the magnetic field through the undulator. These results bode well for future operation with many undulators whose gaps are changing simultaneously.

On August 9, 1995, the photon shutters in the beamline front end were opened and the first undulator beam was delivered to the first optical enclosure on the experiment hall floor. X-rays from the Undulator A were observed on a fluorescent screen. The x-ray beam was, as expected, much brighter than the bending-magnet beam (and a plastic screen exposed to the beam showed permanent irreversible evidence!).

Once the initial excitement of observing the extracted undulator beam passed, experiments were performed (a) to measure the absolute flux of photons through a pin-hole, (b) to observe the higher harmonics of radiation, and (c) to derive the photon-beam phase-space volume to determine the particle-beam emittance.

The equipment used to measure the absolute flux of the x-ray beam as a function of energy included a gas-scattering spectrometer with an energy-dispersive detector. Higher harmonics were observed using a compact crystal spectrometer. An x-ray imaging system consisting of a zone plate and a charge-coupled device (CCD) detector was used for the photon phase-space volume determination to obtain the particle-beam emittance. The experimental equipment used in these studies has been described previously (Proceedings of the 5th International Conference on Synchrotron Radiation, held July 18-22, 1994 in Stony Brook, NY) . It has been successfully tested and used at NSLS, CHESS, and HASYLAB.

A gas-scattering technique with energy-dispersive detection was used to measure the absolute spectral flux of Undulator A through a pinhole as a function of photon energy. The technique used for these measurements provides absolute values for the flux, and the data collection could be performed in only a few minutes. The detection efficiency decreases with increasing x-ray energy, however, so the technique is primarily useful in the energy range below 20 keV. Therefore, a compact crystal spectrometer was used to measure the Undulator A spectrum in the 20 to 100 keV energy range. These measurements could be made with negligible distortion of the spectral shape of the undulator harmonics. The maximum electron current in the storage ring was 1 mA, and the lifetime of the stored electron beam was about two hours during these experiments. The results are presented in Figs. 1 and 2. The value of the absolute spectral flux of the third harmonic, shown in Fig. 1, exceeds the specified 75% of an ideal device flux. The 9th to the 17th undulator harmonics, shown in Fig. 2, are sharp in their structure and very close to expectations from an ideal undulator. These results show that the undulator and the storage ring are functioning better than specified, at least for low stored-beam current.

The other important part of undulator source characterization is measurement of the photon-beam phase-space volume. The photon source size was determined using a zone plate with a 1-meter focal length for 8-keV x-rays to image the source onto a CCD detector. The divergence of the photon beam from the undulator was determined by measuring the x-ray beam intensity distribution at a properly selected wavelength. These data were used to calculate the APS electron beam emittance. The measured horizontal emittance is 7.6±0.8 nm-rad, which is in good agreement with the design value. The vertical emittance is 0.2±0.04 nm-rad.

At the conclusion of the above experiments, an 8-mm-aperture insertion-device vacuum chamber of 5-m length was installed in one of the straight sections of the storage ring. The presence of this narrow aperture chamber did not influence either the injection or the life-time of the stored beam in any significant fashion.

-E. Gluskin, E.R. Moog, and W. Yun
Experimental Facilities Division

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