A CAMAC normal station for processing ion-beam scanner data

NUCLEAR

INSTRUMENTS

AND

METHODS

I26

(I975)

549-552;

©

NORTH-HOLLAND

PUBLISHING

CO.

A CAMAC N O R M A L S T A T I O N F O R P R O C E S S I N G I O N - B E A M SCANNER DATA G. C. L. V A N H E U S D E N

a n d L. R. O P B R O E K

Eindhoven University of Technology, Physics Department, Cyclotron Laboratory, Eindhoven, Netherlands Received 17 M a r c h 1975 F o r the a u t o m a t i c control o f the cyclotron at the E i n d h o v e n University o f T e c h n o l o g y the width a n d the position o f the b e a m in the external b e a m - g u i d i n g system are m e a s u r e d using D a n f y s i k pin-type b e a m scanners. T h e m e a s u r i n g e q u i p m e n t is coupled with a D E C P D P - 9 c o m p u t e r with 24k core m e m o r y via a single-

crate C A M A C system. This p a p e r concerns the h a r d w a r e processing o f the b e a m s c a n n e r data, u s i n g a laboratory developed C A M A C time-todigital converter.

1. Introduction

the peak value, then the position p with respect to the vibration axis of the beam scanner and the width w of the beam are given by:

As a part of the automatic control project of the cyclotron at the Eindhoven University of Technology 1) 26 pin-type Danfysik beam scanners are located in the beam-guiding system. With these beam scanners the horizontal and the vertical position and width of the external cyclotron beam are measured continuously and without serious interception of the beam. The data are fed to the control room of the cyclotron and then processed in eight 4-channel timeto-digital converters (TDCs) in the C A M A C system. The data concerning the position are averaged over 16 measurements and the data concerning the width over 32 measurements. With the aid of the measured influences of variations in the settings of steering magnets on the position of the beam at different locations, the beam can be positioned by the computer at some desired optical axis 1,2). The horizontal or the vertical emittance of the beam can be determined and displayed on a graphical terminal by measuring the width of the beam at different places for a number of quadrupole settings.

p = A sin ½n('c2-'c3)/To, w = 2A sinrczl/To, where A is the amplitude of the harmonic vibration of the needle and To is the vibration period. The current pulses given by the beam scanners are amplified and converted into voltage pulses with a fixed peak value of 5 V. Then the pulses are clipped at 2.5 V and transported to the control r o o m via a twisted transmission cable. In the control r o o m one out of the 26 pulse trains can be selected manually and shown on an oscilloscope. Moreover, all pulse trains

lll'~ 2. The beam scanners The beam scanners consist of a vibrating arm (vibration frequency 15 Hz) and a thin tungsten needle (diameter 0.75 ram). The needle, perpendicular to the vibrating arm, intercepts the cyclotron beam twice per vibration period (see fig. 1). The intercepted peak current ranges from 500 pA to 500 nA, depending on the beam current and the beam width at the scanner location. Let the time Zl (see fig. 2) be the time during which the intercepted current is more than half the peak value, and the interval times z2 and z a be the times during which the intercepted current is less than half 549

Fig. 1. T h e m o t i o n o f the scanner needle with respect to the d~ ,~ J ~ cyclotron beam.

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Fig. 2. T h e time definitions o f the pulse train.

550

G. C. L. VAN HEUSDEN AND L. R. O P B R O E K

TABLE 1

are fed simultaneously to eight 4-channel time-todigital converters described in the next section.

3. The time-to-digital converters In the TDC a clock pulse train is gated during the time which has to be measured (tl, t2, or t3). The number of clock pulses passed through the gate represents the time to be measured in units of the repetition time of the clock pulses (100 #s). If an up/ down counter is used to count the number of pulses, the difference between two interval times can be measured. A reference signal that determines whether the counter must count up or count down is obtained from an oscillator which drives all beam scanners synchronously. The up/down counter used is a General Instruments " F o u r Digits Display Driver" (FD3) type AY-5-4007D (see fig. 3). Only the up/down counter and the storage shift register within this Integrated Circuit are used. The number of connections and gates between the four T D C channels and the dataway of the crate can be reduced considerably by use of the serial output (4 x I gate instead of 4 x 16 gates). The disadvantage of the relatively slow serial read out (16× 3/~s) can be eliminated by addressing four normal stations simultaneously via the Station Number Register (SNR) in the crate controller3). In this case each normal station occupies its own group of four bits within a 16-bit data word (see fig. 4). The 16 serial data words have to be converted into 16 parallel data words by software. A block scheme of the TDC is given in fig. 5. The reference signal (fig. 6a) determining whether the scanner needle is moving from the left (up) to the right (down) or in the opposite direction, and the input pulse train (fig. 6b) of the beam scanner together

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A(0).F(0) A(0).F(8) A(0).F(10) A(0).F(12) A(0).F(14) A(0).F(24) A(0).F(26) A(0).F(27)

Read next bit; give Q-response Test LAM; give Q i f L A M is set Clear LAM Set position FF Set width FF Disable LAM; give Q Enable LAM Test status; give Q if enabled

establish a correct timing of the count up/count down command: the reference signal is sampled by a D-type flip-flop at every positive-going edge of the input pulse (fig. 6c). If the position of the beam is to be measured, this synchronized reference signal is fed to the up/down command input of the FD3. If the width is to be measured, the synchronized reference signal is gated off in such a way that the counter always counts up. The input pulse train is also fed to the count input of the FD3 via an EXCLUSIVE OR gate and mixed with the clock pulse train. The EXCLUSIVE OR gate does (fig. 6e) or does not (fig. 6f) complement the input pulse train, depending on whether the width or the position has to be measured. normal station

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Fig. 4. The bit allocation of the TDC channels.

PROCESSING ION-BEAM SCANNER DATA

551

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552

C,. C. L. VAN HEUSDEN AND L. R. O P B R O E K

The synchronized reference signal is also counted in a four-bit binary counter. After each overflow of the counter, corresponding with 16 position or 32 width measurements, a one-shot is triggered. Then the contents of the up/down counter are transferred into the storage shift register (see also fig. 4). After a delay of 10/~s a second one-shot is triggered, which resets the up/down counter and sets the local Look-At-Me (LLAM) flip-flop. This flip-flop disables the transfer input of the storage shift register to protect the juststored measurement. The flip-flop does not prevent the start of a new measuring cycle. If all four local LAMs are set, the normal station generates a Look-At-Me on the L-line of the dataway of the crate to interrupt the computer. The contents of the four storage shift registers can be read into the computer as 16 words of 4 bits by generating 16 consecutive A (0). F (0) commands (see table 1). At strobe S1 3) the four bits are read into the computer, and at strobe $2 3) the register s are shifted one bit position. If all bits are read, the local LAMs, and thus

the LAM, has to be reset by the command A(0).F(10). Now the transfer input (XFER) of the FD3 is enabled again. If a channel is defect or not used, the local LAM can be forced in the T R U E state by a front-panel switch. The system with the Danfysik beam scanners and the CAMAC TDCs is already in operation for more than a year without serious troubles. Finally we will thank prof. dr. H. L. Hagedoorn for the many fruitful discussions. References 1) F. Schutte, On the beam control of an isochronous cyclotron (Thesis, 1973) Eindhoven University of Technology. 2) F. Schutte, K. R. Ehrnreich and G. C. L. van Heusden, Nucl. Instr. and Meth. 97 (1971) 347. 3) CAMAC, amodular instrumentation system for data handling, Commission of the European Communities, Report 4100 (1972); and CAMAC, organization of multi crate systems, Commission of the European Communities, Report 4600 (1972).