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Novel Beam-Loss Monitors Add to NSLS-II's Safety Features

By Laura Mgrdichian

Cerenkov detector

Rich Gambella is pointing to one of the Cerenkov detectors (silver tube with visible light coming out of the end). With him are (from left) Brian Walsh, Dennis Carlson and Steve Kramer.

Among the innovative safety features of the accelerator system for the National Synchrotron Light Source II (NSLS-II) is a set of novel beam-loss monitors, the likes of which are present in only one other synchrotron in the world: NSLS, where they were prototyped. The monitors are one of several systems at NSLS-II that, together, will control and record beam losses, allowing accelerator operators to keep the radiation to the user areas on the experimental floor as low as reasonably achievable, known as ALARA.

When the NSLS-II accelerator complex is running normally, the experimental floor will experience radiation levels only slightly higher than the natural background radiation people are exposed to everyday, worldwide. This does account for unavoidable losses, called lifetime losses, that are expected and accommodated at all synchrotrons (and are the reason that NSLS-II will run in "top-off" mode, where beam is periodically injected into the main storage ring).

When non-routine loss levels of the electron beam occur, the monitors will not only show the location of the loss but also report how many electrons were lost at each location. Knowing where, how much, and when the beam losses occur will allow accelerator staff to control and minimize the losses, thereby practicing ALARA. Additionally, quickly identifying the location of an abnormal beam loss, and verifying that the amount of beam lost was within the safety limits, could prevent unnecessary interruptions of beam injection that cost users precious beam time.

The monitors are Cerenkov detectors, devices used in high-energy particle experiments to help identify the particles created during collisions. A prototype installed at NSLS in April 2011 (and working well since then) is the first such detector of its type employed at a synchrotron to monitor the accelerator.

Cerenkov detectors work by detecting a specific type of light from a charged particle – dubbed Cerenkov light – produced when the particle passes through a material going faster than the speed of light in that material. At NSLS and NSLS-II, those particles are electrons.

The idea to use a high-energy particle physics detector at a synchrotron is the result of a creative problem solving with a limited budget. The effort was led by Photon Sciences accelerator physicist Stephen Kramer, whose background is in high-energy physics and who managed the NSLS Vacuum Ultraviolet Ring for 10 years. Kramer was asked to help with a problem at NSLS-II: As a cost-saving measure, project managers decided to concentrate shielding where it was needed most – the injection area. This decision, of course, called for increased scrutiny of the potential for beam losses near user areas. Although such events are not likely, a review committee recommended that the project devise an inexpensive way to control where and quantify how much beam was lost in the differently shielded areas, since the area radiation monitors only indicate radiation on the experimental floor and not where the beam loss occurred nor how much was lost in that area.

In the injection region, devices called beam scrapers will intercept the expected lifetime losses, keeping the radiation levels around the rest of the ring even lower. The Cerenkov detectors measure the losses as the scrapers receive them and will alarm if the rate increases unexpectedly.

While rare, unintended beam dumps, such as from electrical malfunctions, are possible. If this happens, the scrapers will control the beam loss so that it is limited to the heavily shielded regions and the detectors will verify the amount of loss. From there, operators can determine the beam loss in the less heavily shielded regions and possibly minimize it. This unique feature has caught the attention of several other accelerator facilities.

Kramer expects the NSLS-II detectors to survive at least 10 years of operation without replacement. The predicted radiation-induced calibration changes over that time can be easily handled by periodically recalibrating with the beam scrapers. This is one reason why the detectors are intentionally located near the scrapers.

Detector Details

Each detector consists of a glass rod inside an aluminum tube that is about 1.25 inches in diameter and 42 inches long, placed directly next to the beam pipe. When high-energy electrons from the beam enter a rod, they produce Cerenkov light, which is guided down the rod and then detected and measured by a photodiode. By measuring the amount of light produced, it is possible to calculate the number of electrons that passed through the rod.

The design of these detectors was modeled after Cerenkov detectors used at the Stanford Linear Accelerator Center's BaBar experiment at the PEP-IIB high-energy electron storage ring. Those detectors use pricey rectangular prisms made of ultra-pure synthetic quartz that required extensive polishing on all surfaces. Kramer found a supplier who could draw circular rods using the same material, which had a highly polished outer surface and only required polishing of the cut ends.

The detectors were assembled at Brookhaven Lab. It was quite a mechanical challenge to maneuver the detectors between the large magnet coils and girder assemblies that make up the NSLS-II main ring accelerator components. There will be five detectors installed in the heavily shielded injection region, which will measure beam lost in that location. The scrapers will be used to ensure that the majority of the beam loss is limited to that location. The amount not detected (difference between total beam lost and what the detectors measure) will indicate the amount lost to the rest of the accelerator. This gives the operators information they need to minimize the total beam loss rate and/or the amount outside the heavily shielded region.

The system is being built by members of the Photon Sciences Directorate's Instrumentation Group under the direction of Om Singh. This challenging detector system was designed by Richard Gambella with the engineering support of Mark Breitfeller, and was prototyped by Brian Walsh with the Lab's Central Shops fabricating the structures. Dennis Carlson helped polish the ends of the rods to ensure they would have adequate light collection properties. The timely installation of these detectors to coordinate with the magnet-girder fabrication process took place under the direction of Tom Dilgen. The novel high-dynamic-range electronics for measuring the light was proposed by Thomas Tsang of the Instrumentation Division and is being designed by Tony Caracappa. The data acquisition system for the present NSLS system and the design of the NSLS-II system is being done by Yong Hu with help from Yoshi Hadaka and Tony Caracappa.

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