Modern instrumentation of electrostatic accelerators

Nuclear Instruments and Methods in Physics Research A2O (1986) 201-212 North-Holland, Amsterdam

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MODERN INSTRUMENTATION OF ELECTROSTATIC ACCELERATORS R. R E P N O W M a x - P l a n c k - l n s t i t u t f ~ r K e r n p h y s i k , D - 6 9 0 0 Heidelberg. F R G

For diagnostics and control of electrostatic accelerators complex electronic systems are used also inside the accelerator vessel to an increasing extent. Methods for protection of the equipment and for data transmission are discussed. Several existing digital control systems are compared and the advantages of digital closed loop regulation systems are indicated.

i. Introduction Electrostatic accelerators are known to be very simple machines. They mostly consist of passive components like insulating column, resistors or corona points, tubes, charging belts or chains and there are only very few parameters to be measured and controlled very precisely to make them run reliably and to satisfy their users. Indeed, not only table-top-sized machines but also large installations are even today very successfully operated without extremely sophisticated electronic equipment.

2. Traditional diagnostics and controls If necessary, machine diagnostics are made - with limited success - from outside the tank. Tube vacuum pressure is monitored near the base end pumps. Small excursions or spikes on the vacuum reading indicate some electric instability, however, a localization is very difficult. Integral resistor currents are measured at ground potential, however, about the reasons for deviation from normal readings one can only speculate. The same is true for suspicious signals on the capacitive pickup monitor. A very sensitive diagnostic tool is a particle beam, at least for inclined field accelerator tubes, if observed on a suitable beam profile monitor. Though, again, the beam is integrating over all potential planes of the accelerator, gradient fluctuations and local discharges result in clearly visible beam displacements. A few devices are in use which allow a somewhat more detailed and localized diagnosis: A series of very similar spark pictures taken with polaroid cameras may indicate problems with loose parts of hardware. Nal detectors mounted at the tank wall can be used with collimators in order to localize the source region of radiation from the accelerator tube, and from the X-ray energy spectrum further information about the site of 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

trouble can be deduced. The time structure of the emission sometimes gives an additional signature. If sufficient viewing ports at the proper locations are available bare phototubes can be used to monitor tube activities which are accompanied by light emission. Brookhaven has demonstrated that a clear indication about the status of individual tube sections can be extracted from their signals [1]. The advantages of more localized diagnostic possibilities inside the accelerator tank are generally acknowledged, as one can judge from widespread use of shorting devices of different kinds. Axial rods which can be combined from conducting and nonconducting sections have been a standard feature on NEC accelerators since long. Various combinations of radial and axial shorting rods and spring loaded steel ropes are now in use on almost all HVEC machines also, e.g. for high gradient low total voltage conditioning of individual accelerator sections [2]. In addition, they provide a very helpful tool of accelerator diagnostics for localizing regions of problems. In some laboratories they are used regularly in order to supervise the condition of the accelerator tube. Their main disadvantage is that they are incompatible with normal operation and can only be activated during service periods. Similarly, equipment inside the accelerator - if present at all - was controlled from safe ground potential, and preferably even from outside the tank. Until recently, terminals of large machines like MP-tandems were almost completely empty. Some mechanical devices like stripper gas valves or foil changers were driven by plastic rods or by pneumatic devices activated by pressurized insulating gas. Control of electrical devices was mostly limited to switching, which was accomplished with nylon strings. 3. Electronic equipment inside electrostatic accelerators If one tries to compile a list of components which today are installed and operated inside electrostatic IV. BEAM TRANSPORT SYSTEMS

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accelerators one has to realize that there is virtually no device existing on a beam line of any accelerator which has not been also installed inside the tank. The increase in size and complexity of the new large tandem accelerators has strongly influenced this development as the mere length of the machines dictated the necessity of active ion optical elements within the column structure, which, consequently, also demands for beam diagnostic components at critical points. Many other control and diagnostic devices are essential to make machines of this size manageable. However, already before these large systems came into operation first steps were made on existing machines to improve control, diagnostic and operation, e.g. a completely digitally controlled complex ion source terminal was built for the VICKSI injector ten years ago [3], charge state selection by offset quadrupoles [4] was installed at Rehovoth, Munich and Argonne, numerous electronic controls are used in the Munich MP where column and tube divider currents can be measured separately in each dead section [5], and a terminal Faraday cup has been much appreciated in Heidelberg. Today's machines have all types of sophisticated electronic equipment installed at various locations. In order to make such systems operational inside electrostatic accelerators the problem of hardware protection and the communication problem had to be solved first. 3.1. Shielding and protection Protection against pressure and vacuum is normally no difficult problem, especially as components can be checked with little effort almost on a work bench. Only some special parts like electrolytic capacitors, large hybride IC cans or oil filled components sometimes need special treatment like potting in epoxy. Much more of a problem is the electrical protection of the equipment. Electrical surges produce very intense electromagnetic radiation which can induce several 100 kV potential differences even between components tied to the same grounds. The radiation spectrum is rather wide, components up to 40 MHz have been observed in the Daresbury pilot machine [6]. Shielding methods of electronic eqmpment developed in radio frequency technology has been adopted first, using metal enclosures, if possible seamless welded constructions, with rf-type filters at all input or output connections. However, these methods were only partially successful, and experience showed that precautions sufficient in one machine failed in another, especially when going from a large to a smaller machine. However, the consequences drawn from the pioneering research on this subject at Daresbury are now most widely accepted standard procedure: all sensitive electronics has to be protected by double shields [7], insulated against each other, with all joints and apertures

designed to rf standards. Interconnecting cables should be shielded by solid metal tubes or bellows with the double shielding concept applied without compromise. This technique almost guarantees the survival of the electronics, as long as there are no connections to the outer environment. Such interconnection or supply lines still are somewhat of a problem and the design of proper filters [8] especially for high voltages or high currents, e.g. for magnets, still has to be considered a fine art (fig. 1). 3.2. Communication methods If the equipment is built to survive, one of course wants to control and operate it. Traditional devices like lucite rods and TV cameras only have a very limited resolution, bandwidth and packing density. This situation changed by the introduction of the optical communication technique. Modulated beams of visible or infrared light are used universally to transmit information across distances of some tens to hundreds of meters. They are immune to rf noise of the environment and insensitive to large potential differences. Direct transmission through the atmosphere normally involves some optics and needs careful alignment. Suitable viewing ports and appreciable clearance between transmitters and receivers of independent channels are necessary to avoid crosstaik. As there are no limitations with respect to voltage gradient, normally the shortest distances possible, e.g. radially through the accelerator, can be used, allowing very high bandwidths. However, a direct line of sight is necessary, making it impossible to integrate the optical transmitters and receivers into the enclosure housing the equipment to be controlled. Additional efforts for shielding the interconnecting cables are necessary, and in certain places it may be difficult to establish a direct line of sight at all. Flexible optical fiber cables can also be used for data transmission across high potential differences. They can easily be routed around any obstruction and do not need a direct line of sight. Because of their small diameter of about only 1 mm a large number of individual channels can be concentrated in a small space e.g. the Munich MP uses bundles of less than 3 cm diameter, total diameter containing more than 50 individual channels [9]. There is no crosstalk between adjacent fibers and almost no sensitivity to ambient light. Today's transmitter and receiver components are so small that they can be easily mounted inside the unit to be controlled, even on the same printed circuit board, thus avoiding additional shielded cases and connections. However, there is also a considerable number of disadvantages: fiber optical cables only stand a limited voltage gradient. Multifiber cables made from quartz, which have excellent optical and mechanical properties, can only be used at low gradients, e.g. for ion source plat-

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are achievable, e.g. for direct transmission of standard TV video signals. The integration of individual quasi-analog channels into standard control systems is straightforward as no efforts with respect to special interfacing are necessary on either side. However, with growing complexity and number of parameters the dedicated channel scheme had to be abandoned very soon. Therefore various purely digitally coded, multiplexed systems have been realized which can handle large numbers of parameters with sufficient speed.

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forms at some hundred kV. Inside pressurized high gradient machines plastic shielded light fibers tend to explode, however bare monofibers made from polymeric plastic material can be used safely up to 2-3 M V / m if excessive local gradients are avoided. Though transmission losses are wavelength dependent and relatively high (200 dB/km), and the useable bandwidth is limited by mode dispersion typical lengths of 10-20 m are no problem at all. Radiation damage and ageing effects may increase light losses due to fine cracks at the surface and limit their lifetime, which to common experience is affected at least equally by their fragility, as they are relatively often broken during maintenance work on other components. Light fibers can be comfortably fed directly through the tank wall using some expoxy glue, and a limited number of optical joints, e.g. transitions to more flexible fiber types for low gradient regions, in order to reduce the risk of damage, is possible. Data transmission for individual analog channels can be achieved simply by amplitude or frequency modulation of the light signal, or most simply by using standard V / F - F / V converter pairs on either end. Bandwidths of kHz can easily be realized, MHz signals

As an immediate consequence of such a scheme the application of computers within the control system at different levels cannot be avoided any longer. Computers are helpful or even mandatory for (1) human interfacing, (2) communication, (3) device interfacing, and (4) closed loop regulation and automatization. Most similarities between existing systems can be noticed in the man-machine interface, e.g. the operator's consoles. Obviously, a fully automatic accelerator operation nowhere is anticipated. Therefore much effort has been invested to design the hardware and software for the operator's console to make its use as simple and transparent as possible. Transparency means that the operator does not have to care about addressing schemes or computer syntax, but the computer is used to translate hardware addresses into real world physical parameters in plain language and binary data into physically meaningful magnitudes and units. This translation normally involves a large amount of data which are kept in a central data base, optimized for fast access by control software and easy maintenance in case of hardware changes. A relatively large computer in many cases the central computer - is therefore used to service the operator's console. The basic functions for an operator are (1) to inspect and observe certain parameters (2) to change or optimize their settings, and (3) to perform switching functions. Parameter display is made mostly in digital form via CRT units, which are updated rapidly. Colour displays are used in many cases to structure the amount of information for better legibility and to highlight meaningful conditions. Semigraphic displays with special symbols or quasi-analog bar "graphics are often used, however, full graphics is not very common as it is still relatively slow. For real time display sometimes the digital information is reconverted into analog form for display on a simple, conventional analog meter or a dedicated vector CRT display. For manual tuning and optimizing digital knobs are in use, which simulate the existence of conventional potentiometers. The number of knobs available per console is a question of philosoIV. BEAM TRANSPORT SYSTEMS

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phy. Basically, two are sufficient as they are freely assignable to any parameter, the assignment and its present value normally being shown on a small character display, however, a larger number - e.g. four or six - can be helpful to avoid frequent reassignments. Selection of parameters for display or assignment is interactively done from a list or menu shown on a C R T device. The identification can be achieved via a trackball driven cursor, as adopted by NEC, by a light pen as, e.g., at Daresbury, or, most convenient but with limited resolution, with bare fingers using a touch-sensitive layer in front of the screen. In spite of the almost unlimited number of accessible parameters digital consoles can be built very compact with respect to rack space and cabling. They can be easily duplicated at different locations providing redundancy, local control possibilities and fully remote operation capability in case of coupled operation of two' different machines. If a computer is available and has access to all parameters, additional features other than pure manipulation can be implemented: automated data logging and replay of parameters of interest, filing of standard or optimized machine setups with consequent automatic restauration which can be a direct reproduction or a scaled setting. Standard procedures can be automated, such as cycling of magnets, energy variations in small steps, which can be done, e.g., by defining virtual devices combining a large number of physical units which can all be made to follow a single knob in a programmed way. 4.1. Centralized control systems

For the interconnection between the computer and the hardware devices to be controlled many different concepts have been realized at different places. However, if one - even today - has to avoid development efforts, there is only one choice, which to a large extent is standardized, well defined and supported by a larger number of commercial suppliers. The C A M A C standard [10] is a modular system for a centralized, high speed data acquisition system. As in use for nuclear physics experiments it is unsuitable for process control, as the standard branch highway needs 66 pairs of cables, is rather limited in length and can only address 7 different substations called crates. However, a second also standardized version exists [11], which allows using the same hardware inside a crate and which only needs two signals for interconnection. For this so-called bit serial highway, standard hardware is available to convert all necessary commands and addresses into serial form and by special coding both signals can be modulated onto one single light signal. Though being more than one order of magnitude slower than the standard parallel C A M A C branch, about 30000 complete data transmissions per

second can be achieved by the computer over a length of several hundred meters. Up to 62 independent crates can be connected to one serial highway, which forms a closed loop and allows control of processes at widely different locations (fig. 2). After this scheme had been applied successfully for accelerator control on ground potential at various places, it was specified by Oak Ridge for use also inside the tandem accelerator [12], and due to its success there, it is now offered as a standard system for all larger N E C machines built since then [13]. Fig. 3 shows a scheme of a typical control system as realized for the J A E R I tandem [14]. Normal - of course also doubly shielded - C A M A C crates are installed in the terminal and in major dead sections. Special so-called half crates are designed for regions with space restrictions and with fewer monitoring points. Special direct view light transmitters and receivers are installed at tank viewing ports, beaming radially through the insulating gas. For reasons of safety, the data signals are available behind each crate at ground potential again and can thus be inspected directly. As one defective crate would block all communication around the loop bypassing of defective devices can be easily done outside the pressure vessel - losing only access to one malfunctioning crate. However, the reliability of the components and the applied shielding methods are obviously so satisfactory that serious system problems due to C A M A C failures have not been reported. As is indicated, the whole system is split into several loops, which can be independently driven by the computer, in order to speed up the data transfer rate and to keep parts of the system alive, if e.g. the loop through the tank is not operational during tank evacuation. Purely CAMAC-based systems have proven their reliability and applicability even under conditions to be found inside electrostatic accelerators. Their main advantages are: the hardware is independent of the type of computer used, a serial highway can easily adapted to light links, the system is highly modular with respect to the number of substations (crates) as well as the numbers and types of modules within a crate, and a large variety of modules suitable for almost every control application can be bought from commercial suppliers, which is also true is also true for the C A M A C interface to the computer. These advantages have to be paid for with a relatively complex hardware even for small control applications, which, however, is mature, reliable, and well defined. The data rate is limited to the number of transactions the computer can handle, as every transfer has to be initialized by the CPU. Faster schemes using direct memory access by the C A M A C controller have been realized and data rates sufficient for rapidly updated analog displays are achievable.

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The control system for the NSF tandem at Daresbury also is using CAMAC-based equipment, however mainly to interconnect the computer to the specially designed process control hardware. This Daresbury system [7] is already a decentralized system optimized for speed by distributing the workload to different separate minicomputers, each one being responsible for a limited area of the plant (fig. 4), and by the use of locally autonomous controllers which can perform the data acquisition and transmission on their own. In contrast to basic CAMAC, the substations, which are called multiplex crates, have been provided with local intelligence using fast programmable bit-slice processors. They are installed in different regions of the accelerator and are connected via a serial, direct view light link using a special high speed data protocol. Light connections here are made axially with additional links to every other station to enable bypassing. Fig. 5 shows a picture of such a crate mounted in a doubly shielded enclosure equipped with light transmitters and receivers. In contrast to CAMAC all analog inputs and outputs are served by only one ADC or DAC respectively via a multiplexer, which is continuously scanned under program control of the local processor. Thus the actual data are always present in the local memory and are immediately available for transmission on request. A base crate, also equipped with a controller, is used to drive the serial communication loop, keeping the data

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in its own memory for access of the minicomputer. A unique feature is implemented that allows every parameter to be marked for fast update [15]. In this case without any further activity of the minicomputer these special parameters can be converted and transmitted up to 6000 times per second and a real time oscilloscope display of high bandwidth can be displayed. This feature certainly is of invaluable help for diagnostic purposes and can also be used to show several on-line beam profile monitor displays from different positions along the accelerator tube. Thus the use of locally autonomous controllers allowed the development of a system that enables the computer to control a very high data transfer rate without requiring it to actually handle those data as it would be on a strictly centralized system.

Though quite a large CAMAC system is operating for control of the linac booster [16] of the Heidelberg MP-tandem, use of CAMAC was excluded because of space, weight and power limitations in the high voltage terminal and because only a moderate number of parameters, closely spaced but at different dc potentials, had to be controlled. A scheme was searched for which should minimize the electronic overhead, the cabling, and the shielding efforts typical for a centralized concept. Therefore, a small inexpensive controller card was developed, which in contrast to standard CAMAC modules combines analog and digital in- and output all on a small eurocard. A popular one-chip 8-bit microprocessor with sufficient programmable memory is used basically to scan 16 analog parameters using a multiplexed hybrid ADC, to set 4 analog output values via separate DACs and to control 8 bits each of input and output for status indicators and switching functions. In addition, there is a pair of light transmitters and receivers on board (fig. 6) which are directly connected to a special hardware supported serial port of the processor. This single card is small and cheap enough to be incorporated into every single equipment device needed for accelerator op-

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eration and has the following advantages: a) in most cases no extra power supply is needed, as the power can be drawn from the internal voltages of the unit to be controlled; b) shielding is of no concern, as the card is installed near the sensitive parts of the device which has to be doubly shielded anyway; c) filtering and screening efforts are much reduced, as almost no interconnections between different devices are necessary, which might act as antennas; signals from outside normally need first some signal conditioning or amplification before being processed by the ADC; d) only two small holes are necessary in the outer shielding to insert two highly flexible light fibers in order to enable remote control of the complete unit. According to experience the number of input/output channels has shown to be a suitable compromise. It is sufficient to control the largest existing single subunits (e.g. a 20 kW FM radio frequency transmitter) and not too many are wasted for smaller applications such as for hv- or magnet power supplies, where spare channels can be used for internal diagnostics of the device itself. Ten different self-contained subunits are used at present in the heavy ion terminal, located at different potentials.

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In order not to feed ten pairs of light links down the column, a simple light distribution box is installed which electrically sums all light signals from the terminal units to a single monofiber light guide, running down the column and fed through the tank wall. A second fiber runs in the opposite direction which is distributed to feed all individual units. Though there is no complicated collision detection mechanism built in, this communication across one single fiber works astonishingly well by virtue of a simple master-slave scheme which is supported by the hardware of the microprocessor chips [17]. The ground end of the light link is connected to another of these micros which is nominated to be master. The master is allowed to address any slave and to transmit data and commands at any time. Though all slaves receive these transmissions simultaneously, only one gets an interrupt if his personal address is detected in a specially coded byte. Only this one slave will transmit his data, if authorized by the master. The drawback of this scheme is of course that no alarm messages can be transmitted by the slaves, but because of the high transmission speed of nearly 200 kbaud and the limited number of slaves ( < 16) every slave is polled about 10 times a second, which is sufficiently fast also for alarms and quasi-analog display of transmitted parameters. As only changed values are transmitted, a higher than nominal update rate is achieved. The master processor is connected to a CAMAC dual port memory, which for the main computer is the data base of the whole system. Every slave parameter actually is one memory word in the data base, and the master is responsible that these data permanently reflect the real world situation by communicating changed set values to the slaves and updating measured values. Thus the PDP computer which is serving the operator's console is not at all engaged in data scanning and communication with the master-slave system. As indicated in fig. 7, the system has been expanded by a second master which controls external slaves which are distributed along the external beam line system, taking care of magnets, quadrupoles, vacuum and a rather complex pulsing system, and it will soon be installed also in the MP terminal in order to replace the 12 years old analog system used there. The slave system at ground potential is also connected via light links instead of coaxial cables, thus avoiding problems of ground loops and potential differences between different parts of the building which otherwise would require special precautions like galvanically isolated, transformer coupled U-port adaptors for CAMAC serial highways. Basically, the distributed slave system can operate everywhere with the same standard program stored in PROM memory, which results into a simple one-to-one IV. BEAM TRANSPORT SYSTEMS

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relation between a slave parameter for analog output and a certain memory location in the C A M A C store module. In this stage a slave is nothing more than an autonomous local controller. However, depending on the special application, the slaves can be programmed to execute much more complex operations and can even be used for closed loop regulation of certain parameters. They can be programmed to react automatically to certain conditions and to perform complex control sequences on a single start command. Virtual devices can be defined by combining several real parameters and operating them according to predetermined algorithms. These options, converting a dumb slave into an intelligent remote controller, are used to a large extent and have even simplified the design of some hardware components. Three virtual devices are presented to the operator in order to focus the primary Cs beam of the Hiconex 834 sputter source: Cs lens, X steerer, Y steerer; however, in reality there are four independent power supplies connected to a segmented lens, which are driven in order to produce the desired focusing or steering action. Thus by software one additional hv power supply, several isolation amplifiers, and one isolation transformer could be eliminated.

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Another slave is programmed to first remove the extraction voltage before changing the target position in order to avoid sparking in the source, and of course to restore the old value when the new target position is reached. Quite complex sequences are now performed locally such as, e.g., starting and running up a large 20 kW radio transmitter for the pulsing system, which requires a large number of adjustments, safety and limit checks and control operations. All this is now done locally while the main computer has only to deposit one single start bit in its CAMAC memory. However, one basic limitation of the master-slave concept is apparent: the local intelligence of a slave is only useful within its own domain of access. No slave is able to access data of another. Thus tasks which require

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interaction of different slaves such as, e.g., the starting up of the terminal ion source and automatic production of a particle beam, still have to be directed by the main computer. Fig. 8 shows the operator's console, which had to be built very compact. It shows an interactive touch panel as the central device, an alphanumeric display and six digital knobs which are freely assignable. Though by using the touch panel every single parameter can be selected and assigned to knobs or displays, the operators prefer to use combined functions, e.g. to call predefined groups of parameters for display or to assign groups of parameters to knobs by identifying the task to be done. Most popular, however, is a command by which the complete ion source terminal can be started

Fig. 8. Operator's console with alphanumeric displays, touch panel and assignable knobs. The analog instrument is used to measure the beam current. IV. BEAM TRANSPORT SYSTEMS

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automatically from shut off condition and reproduce a previously stored setting for all parameters necessary to generate a beam. The three examples for control systems described had been selected to illustrate the trend to be noticed in the development of control systems: Tasks and functions formerly performed by one large central computer are more and more transferred to locally installed autonomous controllers. Inexpensive microcontrollers exist which allow the realization of distributed processing down to the device level. Hardware components and software tools are available to convert any larger piece of equipment into a self-contained, intelligent control station. The main difficulties are no longer localized in the problem of data acquisition and control itself, but in handling these data, e.g. in the communication between all these units. The fast progress in developing standards and highly integrated support chips for fast data communication links and local area networks will certainly influence the further development of remote control systems.

5. Digital dosed loop regulation Rather little has been reported up to now about application of digital control principles on closed loop

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regulation and optimization. Daresbury seems to be the only exception where from the very beginning a digital system for regulation and stabilization of the terminal voltage has been considered. The design of a digital regulation system has many advantages: Though it is not very obvious, it is basically a "simple" system, if one is willing to consider a single board computer to be a simple standard plug-in device. Due to its digital nature a replacement unit should behave identically without further retuning and fine adjustment of complex circuits. All other hardware built around the computer to make it work is very straightforward, modular and transparent, as it is normally built to feed a single analog or digital information into or out of the computer, which individually can be verified by simple means, eventually even with computer assistance. All the really complex circuitry of a regulation system is hidden in the software program, which, after being written and debugged - which may not be a simple task at all - is existing stable, without drifts and necessity of readjustment. Especially slow regulations with long time constants are easier to program than to build in hardware. Multiple mutually interacting regulation loops with different characteristics can be more easily realized and optimized. Nonlinearities can either be corrected by software or can be introduced on purpose where suitable, and on external demand or on a

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MP-TANDEM- REGELSYSTEM (MP2TR) Fig. 9. Scheme of the digital terminal stabilization system of the Heidelberg MP-tandem, using a pair of 16-bit microprocessors (CPU A and CPU B).

R. Repnow / Modern instrumentation

programmed condition the regulation algorithm and characteristic can be completely changed. Later changes and additions normally can be built in just by changing the program in most cases without hardware modifications - and even errors and mistakes which are normally introduced on that occasion are more easily identiffed and corrected in a software program than in a complex analog circuitry. In order to realize a sophisticated analog closed loop regulation which should be able to do more than switching from one state to the other in digital form, quite some numerical formalism is necessary to emulate classical regulation theory. Floating point arithmetic and higher pgogramming language would be of great help. Speed of calculation and data acquisition determines the usable bandwidth and therefore the quality of regulation. Cheap 8-bit microprocessors turn out to be too slow for numerical calculations and are difficult to program. Large computers which have sufficient speed are too expensive for one single job and if they are used for other purposes too, fast real time response is difficult to achieve. At Daresbury one of the minicomputers is therefore used to perform the closed loop terminal stabilization using the output of logarithmic slit amplifiers to calculate a correction for the downcharge power supply in the terminal [18]. Due to the large capacitance of the machine a speed of 10 Hz has been determined to be sufficient for regulation of the machine, though their fast digital channels would be able to achieve higher data rates. Decreasing prices and increasing computational power of microprocessors will enable their use also for specialized closed loop regulations. Two such systems are in use at Heidelberg for the terminal stabilization of the MP tandem and the 3 MV negative single stage machine used as heavy ion injector. The MP system which has been in operation since 1982 is based on a pair of fast, general purpose 16-bit microprocessors which can exchange information at high speed by having simultaneous access to a dual port memory. One is used for numerical calculations which are done with 32-bit accuracy and this takes about 3 ms to generate a new correction value depending on the algorithm. The second is used for data acquisition, analog output, and communication with the operator (fig. 9). Specially designed linear slit amplifiers with switchable gain are used which are fast enough to settle within 1 ms, as during one of the modes of booster operation the beam is macroscopicaUy chopped with 65 Hz and a duty cycle of 25~, giving a pulse length of 4 ms. The data acquisition processor is able to selectively read slit currents every 2 ms and can be synchronized with the chopping system. Numerous other machine parameters are measured

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additionally, e.g. terminal voltage, charging voltages and currents, column currents, corona parameters, tube vacuum, ripple, but most of them at lower rates. These data are periodically checked as boundary conditions for the regulation loop and are also displayed on one of the menu pages of a small alphanumeric display. Default values for all parameters, time constants and thresholds etc. of the regulation loop are stored in a permanent memory, but may be changed on-line by an experienced operator using a small keyboard. This feature was very helpful when initially determining and optimizing time constants and threshold parameters, but since then the loop characteristics were not changed any more, running between 20~ and 1005g of the maximum voltage. Occasionally certain thresholds and limits are changed, e.g. for difficult beam conditions. The primary loop determines a corona control signal from the slit currents, which is updated every 3 ms but normally has a lower bandwidth due to integration time constants. The averaged corona signal in turn influences the charging currents in order to compensate for variations in machine load and charging efficiency, in order to keep the corona at its optimum working point. A fast switchover from slit mode to GVM mode is made either when slit currents disapp~r or when the deviation between GVM set value and actual reading exceeds an adjustable limit. However, slow shifts in GVM reading due to changes in stripper foil thickness or load changes when using double stripping are automatically taken into account by changing the set value. Faster or larger deviations of GVM reading, especially towards higher voltages, override the normal time constants and lead to immediate change of charging currents to limit voltage excursions to a minimum. Other programmed features are built in for safety and convenience for the operators: After a spark the machine is automatically shut down and kept down for at least 5 rain and no operator can be pressed or tempted by impatient users because there is no override option available to shorten that time. Running up the voltage to a desired value is done by keyboard command with a programmed speed, depending on the voltage range and direction of change, but independent from skill and temper of the operator. Audible and visible warnings are given if certain parameters approach their programmed limits and the machine is inevitably shut down if certain limits, e.g. in terminal voltage, accidentally or on purpose, are passed, in most cases before a spark leads to the same result. A fast logging option permanently stores ten selected parameters every 200 ms in memory and keeps the most recent 2 rain history available for immediate display in case of some strange effects. Several precautions have been taken to ensure the reliability of the system: all input and output connecIV. BEAM TRANSPORT SYSTEMS

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tions are made via isolation amplifiers, both processors are checking each other periodically for proper operation and in addition there is a hardware watchdog which turns off the charging currents if not reset every 20 ms, which happens if one of the processors hangs up. After a very reluctant acceptance by the operators they now appreciate the system, as it requires no attendance for days during normal beam times. Overshoots in terminal voltage when intercepting the beam are kept to a minimum and even the load changes by heavy beams injected are compensated quickly without any intervention. The general feeling is that operation of the machine has become safer and more reproducible. After initial adjustment the stability of the beam no longer depends on the attention of an operator even under critical beam conditions, such as very low beam intensity or instability of the ion source, which formerly very often caused a loss of the beam.

6. Conclusions It can be noticed that purely digital systems are going to be used for data acquisition, control and regulation purposes also on electrostatic accelerators to an increasing extent. These systems which at several places already have proven to operate dependably and reliably, will help to make the barrier between the two worlds "inside" and "outside" the accelerator vessel more and more transparent. Improved diagnostic and control possibilities hopefully will also reflect in better performance in spite of the increasing size and complexity of accelerator installations.

Acknowledgments The author would like to thank L. gohrer, N.R.S. Tait, and K. Zicgler for providing information used in

this contribution. He is especially indebted to E. Jaeschke, K. Werner, W. Duerr, and W. Schreiner who initiated, designed and realized the systems reported from Heidelberg.

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