Since the 1930s, synchrotrons have become essential in the widely diverse fields of scientific research and engineering development, including chemistry, biology, material science, and microelectronic engineering. Some synchrotron radiation applications include:
- Visible and infrared light for imaging and night-vision device development.
- Ultraviolet light for photolithography development in integrated circuit manufacturing, and investigation of molecular structures.
- X-rays for characterizing material and crystal structures.
- Gamma rays to explore the inner structure of atoms.
To generate synchrotron radiation, electrons are injected by a linear accelerator (LINAC), and travel along a transport line into the synchrotron storage ring (see figure). The storage ring is an annular vacuum chamber that can be more than 100 meters in diameter. The electron beam is held in the storage ring by a series of high-power bending magnets along the beam path, which keep electrons traveling around a 360° arc. These tightly focused, high-energy electrons travel near the speed of light. When electrons moving at this speed are deflected by a magnetic field, they emit a thin beam of radiation tangential to their path. Photon energies range from a fraction of an eV (electron volt) to 105 eV. Using various synchrotron structures and optics, multiple beam lines can be created for different experiments. Typically, each beam has a high degree of time and space coherence.
The deflecting magnetic fields that produce these beams are created by insertion devices called wigglers or undulators. These contain a series of magnets with opposite polarity that force the electrons into a zigzag or undulating path. This action can be controlled to generate radiation of the type specified by a researcher, including wavelength, flux, brightness, and pulse length.
In a typical synchrotron, there could be 128 bending magnets in the main storage ring, plus 128 magnets to focus the electron beam. The quadrupole and sextrupole focusing magnets act in particle beam optics like lenses in light optics. Along the beam path, the magnet arrangement is in a repetitive sequence called a periodic magnet lattice.
Loss of synchrotron light and shifts in wavelength are affected mainly by the quality of the magnet lattice fields. To provide the appropriate beam intensities and wavelengths to the maximum number of researchers, it is essential to control these magnets from a central location. Since magnets are located all around the storage ring, monitoring and control requires data communications covering distances up to hundreds of meters.
Magnetic fields are produced by passing high DC current through electromagnetic core windings. These fields can be measured directly, but it is impractical to set up a remote monitoring network based on this type of measurement. Instead, applied DC voltage usually is monitored. These voltage data are collected and displayed at the central control station. The instrumentation and data network should allow monitoring that is as close to real time as possible.
Two critical instrument requirements are high-precision voltage measurements and high-speed scanning across different magnets. For some instruments, this requires large measurement apertures or long integration periods, which reduces speed. If smaller apertures and shorter integration times are used with such instruments, speed is increased at the expense of data precision. For best results, the instrument should have a high-speed data communications interface, high scan rate, short measurement settling time, and low internal noise.
To satisfy high-speed requirements, existing monitoring systems often use a data acquisition board installed in a local PC that has an Ethernet interface. Sampling rates and data communication speed allow near-real-time monitoring at the central workstation.
On the other hand, many data acquisition boards have only 12- to 18-bit resolution. This results in limited measurement precision, typically less than 41/2-digit resolution. In addition, a large number of PCs are required for the local monitoring points. This drives up system costs and creates a major maintenance burden.
There are GPIB (General Purpose Interface Bus) based instruments that have higher speed and measurement precision. However, GPIB connections are limited to a 20-m cable distance, and the data transfer rate is limited to about 1MB/s. Hence, GPIB based instruments are not suitable.
To improve magnet monitoring and beam quality, high-resolution Ethernet-based measurement systems can be installed. These systems combine high-precision digital multimeter (DMM) functions with a solid-state multiplexer for high-speed multichannel measurements of magnet voltages. These instruments allow measurement resolutions as high as 6 1/ 2 digits, and their multiplexer modules can be configured for up to 40 differential channels. The Ethernet interface allows each instrument to be placed up to 200 meters away from the central control room. (With Ethernet hubs, the distance is virtually unlimited.) On a dedicated network, data transfer rates can be up to 100Mb/s.
Multiple DMM/MUX instruments are located around the storage ring and connected to a central workstation’s 100BaseT network via TCP/IP Ethernet protocol. Each instrument and PC network interface card has a unique TCP/IP address. Operators can communicate with the instruments via Internet Explorer by entering the TCP/IP address in the browser’s URL line. The instruments have a built-in Web page that simplifies setup, troubleshooting, and data collection.
By combining the long-distance, multi-box control capability of Ethernet with the instrument configuration described above, a more economical, near-real-time monitoring solution is created. It requires fewer PCs than earlier solutions, while providing superior measurement precision and control.
This article was written by Mark Cejer, Business Manager for Keithley Instruments. The full text of this article is available online at www.keithley.com/servlet/Data?id=15068.