Designing an Experiment
X-ray Sources

About CESR

The Cornell Electron Storage Ring (CESR) is a three-quarter mile long circular vacuum vessel, located under the Upper Alumni Athletic Field in which oppositely charged electrons and positrons follow counter-rotating paths at a velocity approaching the speed of light. The head-on collision of the 4-6 GeV particles occurs at the intersection region inside the CLEO detector. The High Energy Physics program studies the by-products of these collisions to further understand the basic constituent elements of matter.

The outstanding property of synchrotron radiation is that the photon beam is confined to a narrow cone angle centered around the instantaneous direction of the circulating electrons and positrons. The opening angle of this cone of radiation is extremely small: for CESR running at 5 GeV, the mean opening angle is 20 seconds of arc, which means that at  10 meters from the beam source, the beam height is only 1 millimeter!  Some of the key features of the beam geometry can be seen in the figure below, which shows the blackening produced when a stack of glass microscope slides is exposed to a photon beam for one minute. The less energetic photons have a larger opening angle, and those with higher energies have correspondingly lower angles. The less energetic, or softer, x-rays are absorbed in the first part of the glass-slide stack, only the hardest x-rays penetrate deep into the stack, and these hard x-rays are much narrower in spatial extent.

    Some features of the geometry of the photon beam in CHESS.
     The above photograph shows an x-ray beam, 1 millimeter in height,
      entering a stack of glass slides from the right. The most energetic
        x-rays penetrate farthest with the smallest opening angle.

CESR can run with energies ranging from 4 to 8 GeV. Current work on the B quark has dictated operating energies from about 4.7 to 5.5 GeV. Forty-five bunch running with 250 milliamps of stored electrons and positrons is our present normal operating state. The critical photon energy at the bend magnets (radius of 32 meters) is about 11 keV. The single bunch cycle time is 2.56 microseconds and the bunch length is approximately 160 picoseconds. The time structure is precisely known and timing signals and a multi-use electronic pulse and delay circuit is available to the user.

Beams are available for x-ray experimentation during High Energy Physics (HEP) operations. X-rays produced by the positron beam are available during the injection procedure (once per hour, typically), although the position of this beam may not be stable. The electron beam is then filled to 250 mA, the beam positions are "tuned" into collision, and then the beam is announced to be stable and available.

CHESS Sources

Four types of x-ray sources are utilized by CHESS: hard-bend dipole magnets, a 24-pole permanent magnet wiggler, a 49-pole permanent magnet wiggler and a 123-pole undulator. Radiation from the dipole magnets and wigglers is available during routine CESR HEP operations. The CHESS-APS undulator can be used only during special dedicated storage ring operations.

The plot below shows the calculated x-ray flux produced by each of these sources. The spectra drop off quickly at low energy because of the beryllium window attenuation; these windows protect the high-vacuum environment of the storage ring. In order to estimate the photon flux received for any given experiment, the flux values from the plot below should be used in conjunction with the energy bandpass of the monochromator and the geometry of the beamline: i.e.. e. the source size, the hutch aperture, and the distance between source and specimen. The latter values are given in the beamline summaries in following sections. The energy bandpass properties are discussed in the next section.

Wigglers

In 1989, a 24-pole 1.2 Tesla permanent magnet wiggler was installed in CHESS East to feed F-line. It produces over 6 kilowatts of hard x-ray power under present operating conditions (the magnet gap can be changed but is generally in a fixed position) and is used to provide intense, focused, highly monochromatic beams for our F-line stations. A similar wiggler was installed in CHESS West in 1995, feeding the A1 and A2 stations. Both wigglers were designed and built entirely by CHESS staff.  This wiggler was then replaced in 2001 by a 49-pole 0.8 Tesla permanent magnet wiggler, designed and constructed by CHESS, which provides wiggler radiation to the CHESS A-line from the electrons and wiggler radiation to the new Cornell G-line from the positrons.

Photo of the CHESS West 24-pole 1.2 Tesla wiggler, designed and built by Ken Finkelstein
and CHESS staff.

The CHESS-APS Undulator

CHESS has had a significant impact in the development of hard x-ray undulator sources. In collaboration with a joint team of scientists from Cornell University, Argonne National Laboratory, and Spectra Technology Inc., we have successfully operated a 3.3 cm period, 123-pole undulator in 1988, 1991 and 1993. Three dedicated 6 week periods of time have been used to operate this device in a special low-emittance operation of CESR that is not compatible with the standard High Energy Physics program. For these periods, NSF support was provided to operate CESR solely for synchrotron radiation purposes.

This insertion device is a prototype of "Undulator A", one of the standard insertion devices planned for use at the APS. This device produces a synchrotron radiation spectrum with peaks whose x-ray energies are tunable from 4.7 to 8 keV in first order and 14.1 to 24 keV in third order (see plot previous page ). The low divergence and high spectral brilliance of this device holds much promise for present and future undulator experiments.

Photo of the CHESS-APS 123-pole undulator.

Last Update: 2006-09-21