- Email: [email protected]
A closed loop feedback system for synchrotron radiation double crystal monochromators
Nuclear Instruments and Methods in Physics Research A266 (1988) 471-474 North-Holland, Amsterdam
A CLOSED LOOP FEEDBACK SYSTEM CRYSTAL MONOCHROMATORS Mohan RAMANATHAN,
FOR SYNCHROTRON
Mark ENGBRETSON
471
RADIATION
* and Pedro A. MONTANO
DOUBLE
1)
Physics Department, West Virginia University, Morgantown, W V 26506, USA i ) Physics Department, Brooklyn College of CUN Y, Brooklyn, N Y 11210, USA
We report on a dc closed loop feedback system for a fixed exit double crystal monochromator. The feedback control is part of a complete system which includes beam position and current monitors. The monochromator consists of two identical crystals mechanically coupled so that the two diffraction planes are almost parallel to each other. The feedback system controls a piezoelectric transducer to maintain the new parallelism, and it can be set under computer control to be at any point in the rocking curve at all times. The system has been thoroughly tested and found to be adequate for EXAFS measurements performed on beamline X18B at the National Synchrotron Light Source at Brookhaven National Laboratory.
1. Introduction In recent years synchrotron radiation sources have become popular for X-ray applications in sciences. Many of these uses of synchrotron radiation require monochromatic X-rays. Several types of monochromator are employed at synchrotron radiation sources to achieve this; two crystal monochromators are the most commonly used. One of the major problems with " m o n o chromatic" radiation is the presence of higher harmonics. Most of the applications for monochromatic synchrotron radiation require the elimination of higher harmonics from the output beam. The suppression of higher harmonics can be attained by detuning one of the crystals in a double crystal monochromator. There are two other feature of synchrotron radiation sources which need to be considered in designing a monochromator control system. One is the possibility that the photon beam moves during an experiment; the other is the variation in the photon flux with energy. For some experiments, such as EXAFS, the X-ray energy is scanned over a range of 1500 eV within a period of several minutes. The same ratio of fundamental to higher harmonics has to be maintained over a scan, otherwise unwanted glitches will appear in the recorded spectra. In the present paper we report a control system developed for a two crystal fixed exit monochromator. A major feature of this feedback controller system is the option of setting the reference level by computer control. This has the advantage that the reference level can
* Present address: ORNL, Bldg. 725, BNL, Upton, NY 11973, USA. 0168-9002/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
be dynamically varied during a scan. Such a feature is needed when scanning at high and low energies. This system has been successfully used at beamline X18B at the National Synchrotron Radiation Source (NSLS) at Brookhaven National Laboratory.
2. Description of the system The two m o n o c h r o m a t o r crystals are mechanically linked; when scanning in energy both crystals rotate and the second crystal also translates, to keep the exit beam at a fixed position [1]. The mechanical linkage cannot maintain the two crystals at exactly the same relative angle over a large energy range and, to correct for errors, the second crystal in the monochromator can be slightly rotated with respect to the first crystal by means of a piezoelectric transducer (PZT). A feedback system is necessary to maintain a fixed angle 80, which is usually of the order of a few tenth of an arc second. A block diagram of the system used on beamline X18B at N S L S is shown in fig. 1. The monochromator is at a distance of 20 m from the source. There is a b e a m monitor detector in front of the monochromator, at about 18 m from the source. The detector is made of two tungsten electrodes parallel to each other and 10 m m apart in the vertical plane. The electrodes are independently biased and typical currents on the electrodes are of the order of a few nA per m A ring current. Ideally the beam should be exactly in the middle of the two detectors and the currents in both detectors should then be the same. However, if the b e a m moves up by a small amount, the two currents will differ. The upper detector current, Ib, will be greater than the lower detector, I a. If the b e a m moves in the opposite direcIII(e). CRYSTAL MONOCHROMATORS
472
M. Ramanathan et al. / A closed loop feedback system
fro m ring
light etector
"•dWhite
~PZT ...... Ion Chamber monochromatic -- - ~ beam / /
a~xtal
~~ju r rent Amp
Vp Vn FEEDBACK [ CONTROLLER
Ext Ref
It
CAMAC
+ f
[
'• Status
ICOMPUTERI Fig. 1. Block diagram of the complete control system.
t i o n , I a will be greater than I b. The sum of I a and I b should remain constant and be proportional to the ring current. Thus dividing the difference by the sum of I a and I b will give the relative position. The detector outputs are fed, after amplification, into summing and differencing amplifiers. The summed current is shown as I t in fig. 1. The differencing amplifier output is divided by the output from the summing amplifier with an analog divider. The output of the analog divider will give a signal proportional to the position of the beam, irrespective of beam current. Both the sum, I t , and the beam position are displayed on LED displays and fed into the computer through the CAMAC to be stored along with the data. The exiting monochromatic beam passes through an ion chamber prior to reaching the experimental region. The current from the ion chamber is amplified, the amplified current 10 is normalized by dividing it by I t and this normalized signal, Vn = I o / I t , is fed into the feedback controller system. The piezoelectric crystal (transducer) is powered by a high voltage power supply (Burleight PZ70), the output of which is proportional to Vp, the output voltage of the feedback system. The feedback controller is connected to the computer via a CAMAC unit. The heart of the feedback controller is the circuit shown in fig. 2. The normalized signal, V,, is the input to the controller. The other input is the external reference level, which is fed in from the CAMAC, on an internal reference, V~. Fig. 3 depicts a typical rocking curve with the monochromator output at a certain en-
ergy as a function of the piezo-voltage. Suppose that, at a certain energy E, the maximum I 0 corresponds to Vmax, corresponding to point A in fig. 3. The output from the controller, Vp, has a maximum range from - 1 5 V to + 15 V, which can be reduced to any desired valued by changing the settings on the potentiometers V1 and Vn. A typical setting is - 1 0 V to +10 V. The output may be switched between the two options, referred to as L and R, to select which side of the rocking curve the working point is on (i.e. point X would correspond to L and Y would correspond to R). The main component in the circuit is the integrator referred to in fig. 2 as int. The 33 kf2 resistor and the 0.1 /~F capacitor from the RC network for the integrator decides the speed of the integrator. Typical values are shown in fig. 2. Provision is made to add parallel capacitors in with a switch in order to slow the integrator. The output of the integrator is connected to the L - R switch directly and through an inverter. The output from the integrator is maintained within the values V1 and Vh by comparators 3 and 4, an S - R flip-flop and a reed relay. Comparator 4 defines the minimum value of Vp and comparator 3 the maximum value of Vp. When Vp becomes just greater than Vh, comparator 3 goes low and the output of the flip-flop Q goes high. It turns on the relay which sends + 15 V through a 22 k # resistor to the input of the integrator. This voltage is greater than V~ and so it starts integrating towards VI. When Vp becomes less than Vl the comparator 4 goes low, which in turn resets Q to low and turns off the reed relay. The process is cyclic. Comparator 1 comes into
473
M. Ramanathan et al. / A closed loop feedback system
lOOK
R
o
33K
INPUT
Vn dpdt
;-~-~ 7 HV A M P ' EXT
spdt
REF
%-~_j
~15V~, 22K
I ~i
L;*l
,
I-
Vp
I
-1-15v ~
EEl
(~)ms
22K
sf
Vr
iltreed relay I
1
~
•~
+5v
+15v
470 o
CAMAC
status(~
I
1
Fig. 2. Circuit diagram of the feedback controller.
play when there is actually an input Vn. The switch labelled ms is connected to the computer and is used to zero the PZT voltage when there is not beam available. Comparator 2 is used to illuminate the status L E D and signal the computer, through the C A M A C unit, when
A _X
F.
o
Vl
Vh
Uz=OV
Vp
)
Uz = l O O O v
Fig. 3. A model rocking curve with the monochromator output as a function of feedback controller output.
the feedback controller is locked on the signal and is stable. To illustrate the proper working of the circuit consider various situations labelled A through D, F, X, and Y in fig. 3. To start with, assume a 25% detuning. This would correspond to 0.75Vm~ x, and V~ is set to this value. In fig. 3, it would correspond to the two points X and Y on the rocking curve. If the working point is X and the output is at point A before the feedback is turned on when the feedback loop is closed Vn is greater than V~ and the output lip starts heading towards VI. At point X, Vn is equal to V~ and so the integrator stops and attains equilibrium. If the starting point is B before the loop is closed, 1I, is less than V~ and the output lip starts heading towards Vh, stopping upon reaching point X. If the starting point is C then, Vn being less than V~, the output Vp starts heading towards Vh. When it reaches point D at Vp = Vh comparator 3 changes states and sets the flip-flop, which in turn simulates an input Vn, much greater than Vr, and makes the integrator start heading towards V 1. When it goes past the point Y, Vn Ill(e). CRYSTAL MONOCHROMATORS
474
M. Ramanathan et al. / A closed loop feedback system
is equal to V~ and comparator 1 goes low, which makes comparator 4 go low, forcing the flip-flop to reset, and turning off the simulated signal applied to the integrator. When this happens the situation is the same as starting from point A. If the working point is chosen as Y, then the process will be similar but in reverse. The present feedback controller system can tune or detune the crystal to any desired position and maintain it during scanning. The present feedback system is similar to the one used by Krolzig et al. [2], and was thoroughly tested at beamline X18B. Other feedback methods [1,3] were also tested on the X18B beamiine. The method presented here was found to be the most favorable for performing E X A F S measurements and the system is used over a wide energy range, from 2.3 to 28 keV with great success.
I-::::3
= tw
Z kkl ¢.J
q it.
2400
i
,
i 2600
,
t
,28i00,
i
ENERGY
(eV)
'
30
i 0 , O
i
,
3200
Fig. 4. Sulphur fluorescence spectra from a calcium sulphate sample collected in the fluorescence mode at beamline X18B at NSLS using the feedback system with the monochromator detuned to 50% of maximum output.
3. Performance of the system We show an application of the present feedback system to E X A F S measurements on the sulfur K edge (2.472 keV). It was found that the edge was not visible if the crystals were detuned to only 25% of I0; however, upon detuning to 50%, the edge was clearly visible. Fig. 4 shows a sulfur spectrum from a sample of calcium sulphate, collected in the fluorescence mode on beamline X18B with the monochromator detuned to 50%. The beamline had two Be windows, each 10 mils thick. Due to this, Vm~x varied drastically when scanning 900 eV around the edge. In this situation, informing the computer about the change in Vr~x helped in dynamically varing V~ during the scan to maintain the 50% detuning throughout the scan. This case is an excellent example of the successful
use of the feedback system, since the harmonic content of the NSLS X-ray ring is very high at 2.472 keV.
Acknowledgement This work was funded by West Virginia University Energy Research Center.
References [1] J.A. Golovchenko, R.A. Levesque and P.L. Cowan, Rev. Sci. Instr. 51 (1981) 509. [2] D. Mills and V. Pollock, Rev. Sci. Instr. 51 (1980) 1664. [3] A. Krolzig, G. Materlik, M. Swars and J. Zegenhagen, Nucl. Instr. and Meth. 219 (1984) 430.
A CLOSED LOOP FEEDBACK SYSTEM CRYSTAL MONOCHROMATORS Mohan RAMANATHAN,
FOR SYNCHROTRON
Mark ENGBRETSON
471
RADIATION
* and Pedro A. MONTANO
DOUBLE
1)
Physics Department, West Virginia University, Morgantown, W V 26506, USA i ) Physics Department, Brooklyn College of CUN Y, Brooklyn, N Y 11210, USA
We report on a dc closed loop feedback system for a fixed exit double crystal monochromator. The feedback control is part of a complete system which includes beam position and current monitors. The monochromator consists of two identical crystals mechanically coupled so that the two diffraction planes are almost parallel to each other. The feedback system controls a piezoelectric transducer to maintain the new parallelism, and it can be set under computer control to be at any point in the rocking curve at all times. The system has been thoroughly tested and found to be adequate for EXAFS measurements performed on beamline X18B at the National Synchrotron Light Source at Brookhaven National Laboratory.
1. Introduction In recent years synchrotron radiation sources have become popular for X-ray applications in sciences. Many of these uses of synchrotron radiation require monochromatic X-rays. Several types of monochromator are employed at synchrotron radiation sources to achieve this; two crystal monochromators are the most commonly used. One of the major problems with " m o n o chromatic" radiation is the presence of higher harmonics. Most of the applications for monochromatic synchrotron radiation require the elimination of higher harmonics from the output beam. The suppression of higher harmonics can be attained by detuning one of the crystals in a double crystal monochromator. There are two other feature of synchrotron radiation sources which need to be considered in designing a monochromator control system. One is the possibility that the photon beam moves during an experiment; the other is the variation in the photon flux with energy. For some experiments, such as EXAFS, the X-ray energy is scanned over a range of 1500 eV within a period of several minutes. The same ratio of fundamental to higher harmonics has to be maintained over a scan, otherwise unwanted glitches will appear in the recorded spectra. In the present paper we report a control system developed for a two crystal fixed exit monochromator. A major feature of this feedback controller system is the option of setting the reference level by computer control. This has the advantage that the reference level can
* Present address: ORNL, Bldg. 725, BNL, Upton, NY 11973, USA. 0168-9002/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
be dynamically varied during a scan. Such a feature is needed when scanning at high and low energies. This system has been successfully used at beamline X18B at the National Synchrotron Radiation Source (NSLS) at Brookhaven National Laboratory.
2. Description of the system The two m o n o c h r o m a t o r crystals are mechanically linked; when scanning in energy both crystals rotate and the second crystal also translates, to keep the exit beam at a fixed position [1]. The mechanical linkage cannot maintain the two crystals at exactly the same relative angle over a large energy range and, to correct for errors, the second crystal in the monochromator can be slightly rotated with respect to the first crystal by means of a piezoelectric transducer (PZT). A feedback system is necessary to maintain a fixed angle 80, which is usually of the order of a few tenth of an arc second. A block diagram of the system used on beamline X18B at N S L S is shown in fig. 1. The monochromator is at a distance of 20 m from the source. There is a b e a m monitor detector in front of the monochromator, at about 18 m from the source. The detector is made of two tungsten electrodes parallel to each other and 10 m m apart in the vertical plane. The electrodes are independently biased and typical currents on the electrodes are of the order of a few nA per m A ring current. Ideally the beam should be exactly in the middle of the two detectors and the currents in both detectors should then be the same. However, if the b e a m moves up by a small amount, the two currents will differ. The upper detector current, Ib, will be greater than the lower detector, I a. If the b e a m moves in the opposite direcIII(e). CRYSTAL MONOCHROMATORS
472
M. Ramanathan et al. / A closed loop feedback system
fro m ring
light etector
"•dWhite
~PZT ...... Ion Chamber monochromatic -- - ~ beam / /
a~xtal
~~ju r rent Amp
Vp Vn FEEDBACK [ CONTROLLER
Ext Ref
It
CAMAC
+ f
[
'• Status
ICOMPUTERI Fig. 1. Block diagram of the complete control system.
t i o n , I a will be greater than I b. The sum of I a and I b should remain constant and be proportional to the ring current. Thus dividing the difference by the sum of I a and I b will give the relative position. The detector outputs are fed, after amplification, into summing and differencing amplifiers. The summed current is shown as I t in fig. 1. The differencing amplifier output is divided by the output from the summing amplifier with an analog divider. The output of the analog divider will give a signal proportional to the position of the beam, irrespective of beam current. Both the sum, I t , and the beam position are displayed on LED displays and fed into the computer through the CAMAC to be stored along with the data. The exiting monochromatic beam passes through an ion chamber prior to reaching the experimental region. The current from the ion chamber is amplified, the amplified current 10 is normalized by dividing it by I t and this normalized signal, Vn = I o / I t , is fed into the feedback controller system. The piezoelectric crystal (transducer) is powered by a high voltage power supply (Burleight PZ70), the output of which is proportional to Vp, the output voltage of the feedback system. The feedback controller is connected to the computer via a CAMAC unit. The heart of the feedback controller is the circuit shown in fig. 2. The normalized signal, V,, is the input to the controller. The other input is the external reference level, which is fed in from the CAMAC, on an internal reference, V~. Fig. 3 depicts a typical rocking curve with the monochromator output at a certain en-
ergy as a function of the piezo-voltage. Suppose that, at a certain energy E, the maximum I 0 corresponds to Vmax, corresponding to point A in fig. 3. The output from the controller, Vp, has a maximum range from - 1 5 V to + 15 V, which can be reduced to any desired valued by changing the settings on the potentiometers V1 and Vn. A typical setting is - 1 0 V to +10 V. The output may be switched between the two options, referred to as L and R, to select which side of the rocking curve the working point is on (i.e. point X would correspond to L and Y would correspond to R). The main component in the circuit is the integrator referred to in fig. 2 as int. The 33 kf2 resistor and the 0.1 /~F capacitor from the RC network for the integrator decides the speed of the integrator. Typical values are shown in fig. 2. Provision is made to add parallel capacitors in with a switch in order to slow the integrator. The output of the integrator is connected to the L - R switch directly and through an inverter. The output from the integrator is maintained within the values V1 and Vh by comparators 3 and 4, an S - R flip-flop and a reed relay. Comparator 4 defines the minimum value of Vp and comparator 3 the maximum value of Vp. When Vp becomes just greater than Vh, comparator 3 goes low and the output of the flip-flop Q goes high. It turns on the relay which sends + 15 V through a 22 k # resistor to the input of the integrator. This voltage is greater than V~ and so it starts integrating towards VI. When Vp becomes less than Vl the comparator 4 goes low, which in turn resets Q to low and turns off the reed relay. The process is cyclic. Comparator 1 comes into
473
M. Ramanathan et al. / A closed loop feedback system
lOOK
R
o
33K
INPUT
Vn dpdt
;-~-~ 7 HV A M P ' EXT
spdt
REF
%-~_j
~15V~, 22K
I ~i
L;*l
,
I-
Vp
I
-1-15v ~
EEl
(~)ms
22K
sf
Vr
iltreed relay I
1
~
•~
+5v
+15v
470 o
CAMAC
status(~
I
1
Fig. 2. Circuit diagram of the feedback controller.
play when there is actually an input Vn. The switch labelled ms is connected to the computer and is used to zero the PZT voltage when there is not beam available. Comparator 2 is used to illuminate the status L E D and signal the computer, through the C A M A C unit, when
A _X
F.
o
Vl
Vh
Uz=OV
Vp
)
Uz = l O O O v
Fig. 3. A model rocking curve with the monochromator output as a function of feedback controller output.
the feedback controller is locked on the signal and is stable. To illustrate the proper working of the circuit consider various situations labelled A through D, F, X, and Y in fig. 3. To start with, assume a 25% detuning. This would correspond to 0.75Vm~ x, and V~ is set to this value. In fig. 3, it would correspond to the two points X and Y on the rocking curve. If the working point is X and the output is at point A before the feedback is turned on when the feedback loop is closed Vn is greater than V~ and the output lip starts heading towards VI. At point X, Vn is equal to V~ and so the integrator stops and attains equilibrium. If the starting point is B before the loop is closed, 1I, is less than V~ and the output lip starts heading towards Vh, stopping upon reaching point X. If the starting point is C then, Vn being less than V~, the output Vp starts heading towards Vh. When it reaches point D at Vp = Vh comparator 3 changes states and sets the flip-flop, which in turn simulates an input Vn, much greater than Vr, and makes the integrator start heading towards V 1. When it goes past the point Y, Vn Ill(e). CRYSTAL MONOCHROMATORS
474
M. Ramanathan et al. / A closed loop feedback system
is equal to V~ and comparator 1 goes low, which makes comparator 4 go low, forcing the flip-flop to reset, and turning off the simulated signal applied to the integrator. When this happens the situation is the same as starting from point A. If the working point is chosen as Y, then the process will be similar but in reverse. The present feedback controller system can tune or detune the crystal to any desired position and maintain it during scanning. The present feedback system is similar to the one used by Krolzig et al. [2], and was thoroughly tested at beamline X18B. Other feedback methods [1,3] were also tested on the X18B beamiine. The method presented here was found to be the most favorable for performing E X A F S measurements and the system is used over a wide energy range, from 2.3 to 28 keV with great success.
I-::::3
= tw
Z kkl ¢.J
q it.
2400
i
,
i 2600
,
t
,28i00,
i
ENERGY
(eV)
'
30
i 0 , O
i
,
3200
Fig. 4. Sulphur fluorescence spectra from a calcium sulphate sample collected in the fluorescence mode at beamline X18B at NSLS using the feedback system with the monochromator detuned to 50% of maximum output.
3. Performance of the system We show an application of the present feedback system to E X A F S measurements on the sulfur K edge (2.472 keV). It was found that the edge was not visible if the crystals were detuned to only 25% of I0; however, upon detuning to 50%, the edge was clearly visible. Fig. 4 shows a sulfur spectrum from a sample of calcium sulphate, collected in the fluorescence mode on beamline X18B with the monochromator detuned to 50%. The beamline had two Be windows, each 10 mils thick. Due to this, Vm~x varied drastically when scanning 900 eV around the edge. In this situation, informing the computer about the change in Vr~x helped in dynamically varing V~ during the scan to maintain the 50% detuning throughout the scan. This case is an excellent example of the successful
use of the feedback system, since the harmonic content of the NSLS X-ray ring is very high at 2.472 keV.
Acknowledgement This work was funded by West Virginia University Energy Research Center.
References [1] J.A. Golovchenko, R.A. Levesque and P.L. Cowan, Rev. Sci. Instr. 51 (1981) 509. [2] D. Mills and V. Pollock, Rev. Sci. Instr. 51 (1980) 1664. [3] A. Krolzig, G. Materlik, M. Swars and J. Zegenhagen, Nucl. Instr. and Meth. 219 (1984) 430.