Round electron beam cross section against a blue background

Image of the ALS electron beam taken at diagnostic beamline 3.1. At this location, the beam spot measures around 48 micrometers rms in both planes. This is just about half the width of a human hair. Nevertheless, the synchrotron radiation emitted from the intense electron beam at this location is 100,000 times brighter than the sun.

The ALS uses synchrotron radiation from bending magnets to measure the transverse beam sizes and energy spread. Two diagnostic beamlines, BL3.1 and BL7.2, are designed specifically for these measurements. BL3.1 focuses the X-ray beam onto a scintillator using the KB mirror technique, then converts it to visible light and captures it using a charge-coupled-device camera. BL7.2 uses the pinhole camera technique in the high-dispersive region to measure energy spread alongside BL3.1. BL7.2 also has a visible light branch that measures bunch length using a streak camera.

A state-of-the-art turn-by-turn Beam Position Monitor (BPM) with self-calibration (pilot tone) enables the measurement of beam orbits across a wide range of frequencies, beam currents, and fill patterns. It provides turn-by-turn orbit measurements with more than 30 micrometers resolution and closed orbit measurements with less than 200 nm RMS error. These measured beam orbits are fed into the fast and slow orbit feedback systems to stabilize the beam to within a few micrometers. The real-time compensation of the BPM electronic error with a pilot tone allows it to stabilize the beam orbit over a long period.

The Bunch Charge Monitor allows mapping of bunch charge distribution around the storage ring with tens-of-picoseconds time resolution and sub-picocoulomb charge resolution. Standard BPMs with sufficient bandwidth pick up the bunch charge signal and read it out with in-house-designed FPGA-based fast ADC electronics at the desired rate.

Graph shows effect of realtime compensation on beam orbit stability.

Four-day orbit drift measurements show that realtime compensation, including a pilot tone for self-calibration, stabilizes the horizontal (upper graph) and vertical (lower graph) beam orbits to within a few micrometers of the ideal, vs. few-mm variation, with the potential for several-mm changes, without it. With the electron beam stabilized in position (and angle) at this level, we can ensure tight tolerances for stability of the synchrotron radiation reaching the various experimental end stations of the roughly 40 photon-science beamlines at the ALS.

A commercial direct current transformer (DCCT) measures the average beam current, which is cross-checked with BPM sum signals to provide reliable current measurements for top-off operation.

Commercial Beam Loss Monitors are installed at the ALS as well. Equipped with small detectors, these monitors can be easily deployed ad hoc and play an essential role in diagnosing local beam loss issues.