Synchrotron radiation is a unique source of light without competition in the spectral range of vacuum ultraviolet to hard X-rays. This "light" is created in electron accelerators and storage rings and is utilized for a variety of applications in research and industry. By providing synchroton radiation, Bessy I provides services for commerce and science.
In the course of its development, Bessy I well exceeded all expectations. Reason enough to erect a further milestone with Bessy II in the field of material and structure research in the new Science and Technology Center in Berlin-Adlershof. Bessy II will be put into operation in 1998 and will supply synchrotron radiation of high brilliance.

What is light really...
"Normal light" is that part of the electromagnetic spectrum that is visible to the eye, and, in comparison to other electromagnetic radiation, such as micro or radio waves, UV light, X-rays or even cosmic radiation, covers only a minute portion of the spectrum.
The wish to be able to see what, by nature, remains closed to the human eye, has always been the elemental aspiration of researchers. Yet the success of  a "making it visible" study depends on whether the attempt to focus radiation of the right wave length with high intensity onto the sample to be examined is successful. And precisely this is an ideal area of application for synchrotron radiation.

...and what is synchrotron radiation?
In the synchrotron, an ultrahigh vacuumized beam tube, electrons are accelerated to the verge of the speed of light and subsequently pass through the storage ring for several hours at a constant speed. Synchrotron radiation is created when the electrons are deviated radially by a magnet at almost the speed of light. The radiation is always released in a forward direction – similar to water droplets being thrown from a rotating car tyre – and leaves the storage ring in a straight line through the beam tube.

INTERBUS in the experimenting stations
To guarantee safer and more interference-free operation at the individual stations, the following requirements are placed on peripheral technology:

• high flexibility and short setting-up times;
• simple and transparent structure;
• greatest possible operational and downtime safety;
• defined time-response;
• high reaction rate;

All these requirements are fulfilled by the serial transmission system, INTERBUS. INTERBUS acquires digital and analog values directly at the signal source in the region of the beam tube and transmits the data to the controller – a conventional IPC with integrated INTERBUS PC host controller board. The integrated co-processor operates an autonomous application program developed in acc. with IEC 1131, that enables safe and independent functioning of this unit irrespective of the operating status of the PC. The newly calculated output values for valves, displays and motor controls are transported in the opposite data direction along the same bus cable to the actuators.

Data transfer with the quick-acting valves and the components for vacuum measurement has top priority, since these are of greatest importance for maintaining the vacuum and thus for operational safety. Ventilation of the storage ring would lead to a week´s downtime. In addition, the status of the vacuum valves and the vacuum is displayed by means of a PCP protocol on the MT-80 text display from SAE.

INTERBUS interface implemented in the getter pump
Over 700 companies have equipped their devices and components with an INTERBUS interface. Despite this, highly specialized applications require devices which cannot yet be connected directly to the bus.
In order to create or maintain the vacuum in the electron storage ring - almost space conditions with 10^-9 mbar - getter pumps are utilized. Their 7 kV high voltage power supplies do have an RS 232 interface, but are not suitable for direct connection to INTERBUS. Bessy staff therefore developed an interface that enables the exchange of data between getter pump and INTERBUS. A plug-on board on which the protocol chip, diagnostic LEDs, drive modules and bus connector for coupling a V.24 interface are already integrated, was used for this purpose.

Examples of applications
Synchrotron radiation is used for example in atomic and molecular physics, X-ray microscopics, semiconductor and surface engineering, lithography or micronics. Further advances in miniaturization processes are expected particularly in the latter. Micro electronic circuits require compatible elements, such as sensors and actuators, to measure and influence the environment. Such micro systems have now become indispensible in many fields.

INTERBUS in Bessy II
In the decision as to which technology was to be used in Bessy II, the experience gathered so far has led to INTERBUS being used again. Some important criteria were considered in making the decision:

• replacing complex parallel wiring with one serial cable;
• saving time and labor at installation;
• saving material, particularly cable;
• a high degree of flexibility when redesigning or extending existing systems;
• can be linked well into existing control systems;
• large number of suppliers of INTERBUS-compatible devices.

The main argument, however, for INTERBUS emerging as the single favorite in a comparison of the systems was the deterministic transmission protocol that guarantees a defined signal propogation time, yet with very brief cycle times.

Dipl.-Ing. Volker Eichler, PHOENIX CONTACT
Dirk Fromme, Bessy GmbH