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CLASSE NEWS | 24 Sep 2015

CESR Beam Size Monitor Comes of Age

Cornell accelerator physicist Joe Rogers had a problem in 2003. He was working on a positron damping ring design for the International Linear Collider (ILC), in which many bunches of positrons would be stored and conditioned for subsequent injection into the main linac for acceleration to the 1 TeV head-on collision energy with similarly accelerated electrons. These bunches would be closely spaced, separated by a few nanoseconds, and yet adjacent bunches could be in vastly different states of readiness for injection. How could one determine the emittance of indivdual bunches? The only method of determining transverse beam size, closely related to emittance, relied on averaging not only over all bunches but over many turns, due to the slow (millisecond) response time of fluorescence detectors in use at that time.

jpa.jpg Joe consulted his particle physicist colleague, Jim Alexander, an expert on semiconductor particle detectors. There were no easy answers – suitable detectors had not yet been developed. Jim promised to look into the matter, not really knowing where it would lead.

So began a decade-long effort to measure vertical beam size in electron and positron storage rings on a bunch-by-bunch, turn-by-turn basis using synchrotron radiation. Fast response time was not the only challenge: beam sizes could be 10 microns or smaller, posing a challenge of spatial resolution. Jim studied several technologies and products before a single viable candidate emerged: a solid state detector from Fermionics that used an indium-gallium-arsenide (InGaAs) semiconductor and featuring 50-micron-wide separation between adjacent narrow strips (or pixels).

nr.jpgFast forward to 2015: the xBSM (x-ray Beam Size Monitor) program has matured to the point that a dedicated beam line at the Cornell Electron Storage Ring (CESR) was partially installed during the summer maintenance shutdown, and is scheduled for completion by spring 2016. Getting to this point required years of R&D – much of it as part of the CesrTA program, coordinated by engineer Nate Rider. Two separate xBSM stations, one for positrons and one for electrons, were temporarily established, twice a year and for up to three weeks at a time, in existing CHESS synchrotron radiation beamlines.fig24.jpg The detector exhibited sub-nanosecond response time and eventually measured beam sizes as small as 12 microns. Data acquired at beam energies of 1.8-2.6 GeV were acquired and used for writing three instrumentation papers (first, second, third) and many more on varied topics such as the electron cloud, intra-beam scattering, and fast ion instabilities. These advances are just examples of how the technology has already had a significant impact on understanding the current generation of electron/positron storage rings in addition to holding promise for use at the ILC.

fig06.jpg An xBSM station functions as a one-dimensional pinhole camera, using the beam itself as the source of x-rays: synchrotron radiation is created by the beam as it passes through magnets surrounding the vacuum chamber of the storage ring. The “pinhole” optical element used here is a vertical slit 50 microns high, and the “film” is the InGaAs strip detector. The detector digitally transmits an image every bunch crossing, which can occur as promptly as every 4 nanoseconds in CESR. The images are altered by diffraction, but this effect can be unfolded from the data to give a vertical beam size on every bunch crossing.

The xBSM program was so successful that a dedicated beamline for positrons was funded by the CesrTA program. Once the beamline is commissioned, the time-consuming effort required to install and remove xBSM stations in CHESS during special runs with beam energy near 2 GeV will be eliminated, enabling single-shift test runs distributed throughout the year. It will also allow use of the monitor for CHESS running at 5 GeV beam energy, giving real-time diagnostic feedback to accelerator operators that has not been previously available. Another indication of success is that a similar detector is an option for beam size measurement using x-rays from Japan’s SuperKEKB electron-positron collider beams.

beamline.jpg The new xBSM positron beamline required modifying the CESR vacuum chambers to insert a port for x-ray extraction and adjusting positions of several CESR magnets to make room for new components; this work has been completed. New source magnets are scheduled for installation later in 2015, and optic and detector assemblies in 2016.