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An automatic beam positioning system for low energy ion beams
N U C L E A R I N S T R U M E N T S AND METHODS 72 ( I 9 6 9 )
217-219;
© NORTH-HOLLAND
P U B L I S H I N G CO.
LETTERS TO T H E E D I T O R AN A U T O M A T I C BEAM P O S I T I O N I N G SYSTEM FOR L O W ENERGY ION BEAMS* R. B. BROWN, P. D. B O U R L A N D and D. POWERS
Department of Physics, Baylor University, Waco, Texas, U.S.A. Received 31 March 1969 A system is described for automatically translating and deflecting ion beams by feedback-controlled electrostatic deflectors to fix the beam position independent of accelerator and quadrupole focusing conditions.
It is often a problem in Van de Graaff type accelerator systems to provide the proper beam-path positioning for critical experimental arrangements. The problem is usually augmented when small-angle analyzing magnets with quadrupole doublets are used. When the energy of the accelerator is determined by a corona-discharge system controlled by the current intercepted by analyzing slits, then the accelerator energy is very sensitive to the input position of the beam in the analyzing magnet. The problem is further complicated by the fact that the quadrupole will shift the beam position even more as the quadrupole current is changed unless the beam is positioned precisely on the quadrupole magnetic axis1). If slits are used to define a fixed position for the * Work supported in part by the Robert A. Welch Foundation, Houston, Texas, U.S.A.
beam, the beam current will be greatly reduced or entirely lost, unless the beam passes exactly between the slits. Usually electrostatic deflector plates are used to make minor corrections on the beam position; however, the extent of correction required depends on beam energy and other accelerator conditions which often change during the course of an experiment. In addition, if the beam is to pass through the magnetic axis of the quadrupole, it is necessary to define the beam direction by two points collinear with the magnetic axis. Thus, it may be necessary to translate perpendicular to the beam direction as well as simply to deflect the beam. This condition would require four beam deflectors, two vertical and two horizontal, working together and whose manual adjustment would be very tedious and again depend upon accelerator conditions. The problem of successfully using the beam deflectors can be solved if a slit assembly is used with
FEEDBACK E!
~ HORIZONTALLY-MOUNTED ~--2 MEV POSITIVEfON -.~.
DE GRAAFF ACCELERATOR
4 I
~
M I ~I
,., ^,,
148
/
/
ECTRONIC ' ~QUADRUPOLE I J I [ / / ~ ' ~ STRONG- FOCUSING I qL~JI I 7I / I -J M A G N E T . ' I
JBE,,
;TEERER[[ ~ l { / I Nr~l l ~ ~ = ~ ; : _~"~qJP ; ]
I
/
~LJ
-----L~-
r'--
:~
~'"~
v
T-
;s ~ " "
"
V ",o-
,,_t
CORONA FEEDBACK ELECTRONIC SYSTEM
SYSTEM
Fig. 1. Experimental arrangement.
217
,, ~ Z3---/
r~ { F~] MAGNET ~_.~-~Z_~_~~. ~. \ ~ _ ~ / ~ Wh TARGET F~]~'~, ~ I " ~ ' L r ~ A N A L Y Z I N G-- J CHAMBER
- ~ , ~ ~ , ~ , . - F ) : -
~-;
~
~-
SL, TS
218
R.B. BROWN et al. NOTE I, DI A N D ~ SELECTED FOR LOW REVERSE LEAKAGE CURRENT
{3+01p.fd
I TO |
t-IBVDC
-ISVDC " 110k ~
METE
SLIT CIRCUITt CURRENT 47k9.
)t
/ ~
+5KVDC
21. NPUT
DIFFERENTIAL FET'S SHOULD BE MATCHED
-IBVDC
MPF 102
~M
=
l ~v~
E
TO.... "--1
I/C~
~,ok.
I
47ka
O,OD+.,.,.
I
~ + ~ ~ r - -
/
+
+
+
-
-
DEFLECTOR PLATE
8.8 M~
C+RRENT~~-j+~'S INPUT
_~
? blO,,ok,[,
'"~U-~--L.-,BVOC k"~
MPF 102
IOk+q
+ j
~
=.D+ +lu v +
I +---Lr~
~ v+c
lOk~.
,Skfl
20.~
u
_~~CTOR TO BEAM
ll+
"
X[
61
-,+voc +;o,,,
,+o,,, -15VDC
+ ~ +9VDC
+_+ FROM ~
++,~ + J
TO F{LAMENT SU+LY
5~-fA
Fig. 2. Beam deflector feedback network. each beam deflector and if the entire system is made self-correcting by means of a feedback signal from each slit assembly to its corresponding beam deflector. The two points defining the magnetic axis would then be determined by the two pairs of horizontal and vertical slit assemblies. This basic feedback principle for use with accelerator beam-position control has been proposed z) and actually used in one form or another by at least two groups3'4). The experimental arrangement reported in the present paper is given in fig. 1. The particular requirements that this arrangement should satisfy to perform satisfactorily are as follows: 1. The beam energy should not change appreciably as a function of accelerator focus, beam probe settings, or quadrupole current settings. 2. Because any slight misalignment will occur, there should be significantly more beam current at the target chamber with the beam-positioning system than without it. Simultaneously all slit openings should be very narrow to define the beam position as accurately as possible. 3. The beam current or energy should not change appreciably as the position of the accelerator tank is slightly shifted. In the past, an optical realignment of the accelerator tank was necessary following each servicing to insure maximum beam current. Electrostatic deflectors were chosen for beam positioning primarily because they will not separate
the different charge and mass components of the beam coming directly from the accelerator. If a separation occurred, such as it does in magnetic deflectors, then there will be defined a different position for each beam component, as for example with the H ÷, H~-, and H~E components in a proton beam. This would possibly cause conflicting feedback signals with the result that very little of the desired beam could pass through the slits. Despite the fact that two sets of deflectors on each plane are used to translate as well as deflect the beam, there were no instability problems in using the feedback system with each set at the same time. The two sets of deflectors worked together to bring the beam to the unique position defined by two sets of slits. The position of the slits was found to be very critical especially on the horizontal plane if the energy was to remain independent of the quadrupole current setting. For this reason it is very important that the slits be kept very rigid against any external movements. The slit width openings for optimum results were approximately 0.1" on the horizontal plane and approximately 0.2" on the vertical plane for the first (input) set of slits. The slit width was narrower on the horizontal plane than on the vertical plane since variations of beam position on the horizontal plane changed the beam energy much more than variations on the vertical plane. The slit width opening of the second set of slits was approximately 0.035" on the horizontal plane and 0.2" on the vertical plane. The electronic feedback network is shown in detail
AN A U T O M A T I C
BEAM P O S I T I O N I N G
:in fig. 2. Controlling the deflector voltage by means of tligh voltage triode tubes was used primarily for ,simplicity and speed as compared with more indirect controls such as servomechanisms and auto-transformers controlling a high voltage power supply. The advantages of fast deflector voltage response include keeping the beam continuously stabilized even when other controls are rapidly being varied so as to minimize "settling" time and cancelling any other rapid beam fluctuations which could cause unconl:rollable loss of current. As can be seen from fig. 2 the two opposite slit currents are fed to a dual F E T differential amplifier which amplifies the difference of the two slit currents independent of the actual total current on both slits. The difference output voltage from the F E T pair is then fed to an operational amplifier adjusted for high gain to greatly amplify the difference voltage. The output of the operational amplifier is then fed to the grid of one deflector control tube and to an inverter which drives the control tube of the opposite deflector plate. The difference error signal then adjusts the deflector voltage until the opposite slit currents are equal. The level and gain of the inverter are adjusted such that when one tube is operating the other tube is saturated and vice versa. One tube or the other is kept in saturation, so that only one deflector plate will have voltage on it while the opposite plate is grounded. I f there are appreciable voltages of the same polarity on opposite plates, then undesirable focusing effects can occur on the beam. q'he electronic feedback system can maintain a good slit current balance for slit currents as low as 0.I pA. The final system as indicated herein satisfied the three principal requirements to our satisfaction. After searching manually for the magnetic axis of the quadrupole by experimentally adjusting the slit positions, we were able to minimize the beam energy change to less than 4 keV for a 1 MeV proton beam and less than 0.4% for all other beam energies available fi'om the 2 MeV accelerator. Energy changes due to variation of accelerator focus and beam probe settings were also less than 4 k e V at 1 MeV beam energy or 0.4% at all other available beam energies. The above beam energy variations were obtained on the
SYSTEM
219
basis of the parameters being monotonically adjusted over a range such that the beam current changed from 10% of maximum beam current to maximum beam current to 10% of maximum beam current as read at the target chamber. It should be pointed out that the above energy variations are made on the basis of a fairly rigorous criterion. During an actual experiment the energy changes experienced by the beam are considerably less. The above energy changes are to be compared to an energy change occasionally greater than 20 keV for a lateral shift in slit position of only 0.050" from the optimum position. The figure of 4 keV represented the maximum energy change when varying all parameters. This figure could be improved for a particular parameter to 0.5 keV by careful adjustment of the slits. For example, for a particular slit setting, the beam energy change due to adjustment of the quadrupole current was 0.5 keV, but the beam energy change was ~< 4 keV if the accelerator focus control was changed. For a different slit setting, the focus control would change the beam energy by 0.5 keV, but the quadrupole current change would then vary the beam energy by A 4 keV. The trimmed (spot size approximately-~-" wide and -~-2" high) beam current which reached the target chamber was greater than 3/~A, which is more than sufficient for our present experimental needs. With the present system in operation, the accelerator tank was physically moved over 0.1" laterally with the result that there was no appreciable current loss ( < 25%), or energy change ( < 0.3%). Without the present system, a tank shift of this order would have caused the beam to have been lost completely. One precaution to be taken is to be sure that the slits are very rigidly fixed as the accelerator tank is moved or the slits may be moved also, thereby changing the alignment of the beam through the quadrupole and analyzing magnet. References a) For a simple explanation o f this effect, J . G . Cramer and F. H. Schrnidt, Nucl. Instr. and Meth. 45 (1966) 325. 2) K. Eklund, Rev. Sci. Instr. 30 (1959) 331. 3) A. E. Evans and K. Johnson, Rev. Sci. Instr. 38 (1967) 1514. 4) C . E . Dick, A . B . Marella and W . C . Miller, Nucl. Instr. and Meth. 60 (1968) 346.
217-219;
© NORTH-HOLLAND
P U B L I S H I N G CO.
LETTERS TO T H E E D I T O R AN A U T O M A T I C BEAM P O S I T I O N I N G SYSTEM FOR L O W ENERGY ION BEAMS* R. B. BROWN, P. D. B O U R L A N D and D. POWERS
Department of Physics, Baylor University, Waco, Texas, U.S.A. Received 31 March 1969 A system is described for automatically translating and deflecting ion beams by feedback-controlled electrostatic deflectors to fix the beam position independent of accelerator and quadrupole focusing conditions.
It is often a problem in Van de Graaff type accelerator systems to provide the proper beam-path positioning for critical experimental arrangements. The problem is usually augmented when small-angle analyzing magnets with quadrupole doublets are used. When the energy of the accelerator is determined by a corona-discharge system controlled by the current intercepted by analyzing slits, then the accelerator energy is very sensitive to the input position of the beam in the analyzing magnet. The problem is further complicated by the fact that the quadrupole will shift the beam position even more as the quadrupole current is changed unless the beam is positioned precisely on the quadrupole magnetic axis1). If slits are used to define a fixed position for the * Work supported in part by the Robert A. Welch Foundation, Houston, Texas, U.S.A.
beam, the beam current will be greatly reduced or entirely lost, unless the beam passes exactly between the slits. Usually electrostatic deflector plates are used to make minor corrections on the beam position; however, the extent of correction required depends on beam energy and other accelerator conditions which often change during the course of an experiment. In addition, if the beam is to pass through the magnetic axis of the quadrupole, it is necessary to define the beam direction by two points collinear with the magnetic axis. Thus, it may be necessary to translate perpendicular to the beam direction as well as simply to deflect the beam. This condition would require four beam deflectors, two vertical and two horizontal, working together and whose manual adjustment would be very tedious and again depend upon accelerator conditions. The problem of successfully using the beam deflectors can be solved if a slit assembly is used with
FEEDBACK E!
~ HORIZONTALLY-MOUNTED ~--2 MEV POSITIVEfON -.~.
DE GRAAFF ACCELERATOR
4 I
~
M I ~I
,., ^,,
148
/
/
ECTRONIC ' ~QUADRUPOLE I J I [ / / ~ ' ~ STRONG- FOCUSING I qL~JI I 7I / I -J M A G N E T . ' I
JBE,,
;TEERER[[ ~ l { / I Nr~l l ~ ~ = ~ ; : _~"~qJP ; ]
I
/
~LJ
-----L~-
r'--
:~
~'"~
v
T-
;s ~ " "
"
V ",o-
,,_t
CORONA FEEDBACK ELECTRONIC SYSTEM
SYSTEM
Fig. 1. Experimental arrangement.
217
,, ~ Z3---/
r~ { F~] MAGNET ~_.~-~Z_~_~~. ~. \ ~ _ ~ / ~ Wh TARGET F~]~'~, ~ I " ~ ' L r ~ A N A L Y Z I N G-- J CHAMBER
- ~ , ~ ~ , ~ , . - F ) : -
~-;
~
~-
SL, TS
218
R.B. BROWN et al. NOTE I, DI A N D ~ SELECTED FOR LOW REVERSE LEAKAGE CURRENT
{3+01p.fd
I TO |
t-IBVDC
-ISVDC " 110k ~
METE
SLIT CIRCUITt CURRENT 47k9.
)t
/ ~
+5KVDC
21. NPUT
DIFFERENTIAL FET'S SHOULD BE MATCHED
-IBVDC
MPF 102
~M
=
l ~v~
E
TO.... "--1
I/C~
~,ok.
I
47ka
O,OD+.,.,.
I
~ + ~ ~ r - -
/
+
+
+
-
-
DEFLECTOR PLATE
8.8 M~
C+RRENT~~-j+~'S INPUT
_~
? blO,,ok,[,
'"~U-~--L.-,BVOC k"~
MPF 102
IOk+q
+ j
~
=.D+ +lu v +
I +---Lr~
~ v+c
lOk~.
,Skfl
20.~
u
_~~CTOR TO BEAM
ll+
"
X[
61
-,+voc +;o,,,
,+o,,, -15VDC
+ ~ +9VDC
+_+ FROM ~
++,~ + J
TO F{LAMENT SU+LY
5~-fA
Fig. 2. Beam deflector feedback network. each beam deflector and if the entire system is made self-correcting by means of a feedback signal from each slit assembly to its corresponding beam deflector. The two points defining the magnetic axis would then be determined by the two pairs of horizontal and vertical slit assemblies. This basic feedback principle for use with accelerator beam-position control has been proposed z) and actually used in one form or another by at least two groups3'4). The experimental arrangement reported in the present paper is given in fig. 1. The particular requirements that this arrangement should satisfy to perform satisfactorily are as follows: 1. The beam energy should not change appreciably as a function of accelerator focus, beam probe settings, or quadrupole current settings. 2. Because any slight misalignment will occur, there should be significantly more beam current at the target chamber with the beam-positioning system than without it. Simultaneously all slit openings should be very narrow to define the beam position as accurately as possible. 3. The beam current or energy should not change appreciably as the position of the accelerator tank is slightly shifted. In the past, an optical realignment of the accelerator tank was necessary following each servicing to insure maximum beam current. Electrostatic deflectors were chosen for beam positioning primarily because they will not separate
the different charge and mass components of the beam coming directly from the accelerator. If a separation occurred, such as it does in magnetic deflectors, then there will be defined a different position for each beam component, as for example with the H ÷, H~-, and H~E components in a proton beam. This would possibly cause conflicting feedback signals with the result that very little of the desired beam could pass through the slits. Despite the fact that two sets of deflectors on each plane are used to translate as well as deflect the beam, there were no instability problems in using the feedback system with each set at the same time. The two sets of deflectors worked together to bring the beam to the unique position defined by two sets of slits. The position of the slits was found to be very critical especially on the horizontal plane if the energy was to remain independent of the quadrupole current setting. For this reason it is very important that the slits be kept very rigid against any external movements. The slit width openings for optimum results were approximately 0.1" on the horizontal plane and approximately 0.2" on the vertical plane for the first (input) set of slits. The slit width was narrower on the horizontal plane than on the vertical plane since variations of beam position on the horizontal plane changed the beam energy much more than variations on the vertical plane. The slit width opening of the second set of slits was approximately 0.035" on the horizontal plane and 0.2" on the vertical plane. The electronic feedback network is shown in detail
AN A U T O M A T I C
BEAM P O S I T I O N I N G
:in fig. 2. Controlling the deflector voltage by means of tligh voltage triode tubes was used primarily for ,simplicity and speed as compared with more indirect controls such as servomechanisms and auto-transformers controlling a high voltage power supply. The advantages of fast deflector voltage response include keeping the beam continuously stabilized even when other controls are rapidly being varied so as to minimize "settling" time and cancelling any other rapid beam fluctuations which could cause unconl:rollable loss of current. As can be seen from fig. 2 the two opposite slit currents are fed to a dual F E T differential amplifier which amplifies the difference of the two slit currents independent of the actual total current on both slits. The difference output voltage from the F E T pair is then fed to an operational amplifier adjusted for high gain to greatly amplify the difference voltage. The output of the operational amplifier is then fed to the grid of one deflector control tube and to an inverter which drives the control tube of the opposite deflector plate. The difference error signal then adjusts the deflector voltage until the opposite slit currents are equal. The level and gain of the inverter are adjusted such that when one tube is operating the other tube is saturated and vice versa. One tube or the other is kept in saturation, so that only one deflector plate will have voltage on it while the opposite plate is grounded. I f there are appreciable voltages of the same polarity on opposite plates, then undesirable focusing effects can occur on the beam. q'he electronic feedback system can maintain a good slit current balance for slit currents as low as 0.I pA. The final system as indicated herein satisfied the three principal requirements to our satisfaction. After searching manually for the magnetic axis of the quadrupole by experimentally adjusting the slit positions, we were able to minimize the beam energy change to less than 4 keV for a 1 MeV proton beam and less than 0.4% for all other beam energies available fi'om the 2 MeV accelerator. Energy changes due to variation of accelerator focus and beam probe settings were also less than 4 k e V at 1 MeV beam energy or 0.4% at all other available beam energies. The above beam energy variations were obtained on the
SYSTEM
219
basis of the parameters being monotonically adjusted over a range such that the beam current changed from 10% of maximum beam current to maximum beam current to 10% of maximum beam current as read at the target chamber. It should be pointed out that the above energy variations are made on the basis of a fairly rigorous criterion. During an actual experiment the energy changes experienced by the beam are considerably less. The above energy changes are to be compared to an energy change occasionally greater than 20 keV for a lateral shift in slit position of only 0.050" from the optimum position. The figure of 4 keV represented the maximum energy change when varying all parameters. This figure could be improved for a particular parameter to 0.5 keV by careful adjustment of the slits. For example, for a particular slit setting, the beam energy change due to adjustment of the quadrupole current was 0.5 keV, but the beam energy change was ~< 4 keV if the accelerator focus control was changed. For a different slit setting, the focus control would change the beam energy by 0.5 keV, but the quadrupole current change would then vary the beam energy by A 4 keV. The trimmed (spot size approximately-~-" wide and -~-2" high) beam current which reached the target chamber was greater than 3/~A, which is more than sufficient for our present experimental needs. With the present system in operation, the accelerator tank was physically moved over 0.1" laterally with the result that there was no appreciable current loss ( < 25%), or energy change ( < 0.3%). Without the present system, a tank shift of this order would have caused the beam to have been lost completely. One precaution to be taken is to be sure that the slits are very rigidly fixed as the accelerator tank is moved or the slits may be moved also, thereby changing the alignment of the beam through the quadrupole and analyzing magnet. References a) For a simple explanation o f this effect, J . G . Cramer and F. H. Schrnidt, Nucl. Instr. and Meth. 45 (1966) 325. 2) K. Eklund, Rev. Sci. Instr. 30 (1959) 331. 3) A. E. Evans and K. Johnson, Rev. Sci. Instr. 38 (1967) 1514. 4) C . E . Dick, A . B . Marella and W . C . Miller, Nucl. Instr. and Meth. 60 (1968) 346.