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Design considerations for a beam scanner at the university of Manitoba Cyclotron facility
Nuclear Instruments and Methods 172 (1980) 407-411 © North-Holland Publishing Company
DESIGN CONSIDERATIONS FOR A BEAM SCANNER AT THE UNIVERSITY OF MANITOBA CYCLOTRON FACILITY Christopher HADDOCK and Francis KONOPASEK Cyclotron Laboratory, Department of Physics, University of Manitoba, Winnipeg,Manitoba R3T 2N2, Canada Received 31 October 1979 and in revised form 17 December 1979
In this paper we describe the development of a beam scanner for the University of Manitoba Cyclotron. The scanner gives an output on an oscilloscope of the beam profile - a "cross sectional" view of the intensity of the charged particle beam and its position. A brief review of the methods available for displaying beam profiles is given and the design and construction of a hybrid unit using the most favourable qualities of each is then described. Typical profiles obtained with the unit are shown and its proposed uses are outlined.
resonantly tuned to the frequency of the cyclotron. The voltage induced per coil is given by:
1. Introduction For the precise alignment and guiding of subatomic particles along the beam lines o f accelerators a monitor is necessary which gives information about the intensity and position of the beam, and preferably also the beam emittance. Once these parameters are known, the steering magnets can be adjusted so that as much o f the beam as possible is guided from the accelerator to the experimental area. There are many types of scanners used and they fall into 3 categories: (a) non-intercepting types; (b) those which stop or disturb the beam; and (c) intercepting types in which the beam disturbance is kept negligibly low.
e = (K/r) di/dt, where fit) is the beam current, r is the effective distance from the current filament to the coil axis, t is time and K is a constant. When the current filament is symmetrically located between the coils the voltage output is zero. The magnitude and polarity of e gives the beam position and this scanner can be used to center the beam. Note however that this scanner gives no information on beam profile. 2.2. Types which disturb or stop the beam For a simple profile, a scintillating screen is placed in the beam tube. A more sophisticated type [2] used in a linear accelerator, passes a 1 m m 2 molybdenum target through the beam. The scattered beam produces an electromagnetic "shower" in the accelerator structure. The shower is measured by a nearby ionization chamber and the resulting intensity gives a beam profile as the target is driven by motors in a Lissajous figure of about 2.5 cm 2. A complete scan takes ~ 2 0 s. A further type [3] is used in the scanning electron linear accelerator. Some electrons are scattered through large angles as they leave the exit window of the accelerator, however the beam profile of these electrons remains the same. An array of 30 sensors is mounted at 40 ° around the exit window and the cur-
2. A brief review 2.1. Non-intercepting types A device of this type [1] uses magnetic induction techniques, and the time average position of the beam is found. Two sets of detection coils determine the horizontal and vertical positions o f the beam. Each set consists of two coils connected in opposition and symmetrically placed about the mechanical center of the beam tube. The axes o f the coils are located perpendicular to the direction of the beam. The coils are 407
408
C Haddock, F. Konopasek / Design considerations for a beam scanner
rent produced by each of these is amplified and fed to a readout circuit to display the beam profile. 2.3. Intercepting types
The intercepting types of beam scanners all pass a wire through the beam. The recorded current is amplified and displayed as a profile of the beam. The ways in which the wires are passed through the beam differ widely and are fully discussed below. The wire scanners scan through the beam: (i) on a plane perpendicular to the beam or (ii) the surface of a cylinder. 2. 3.1. Plane scanners Here for example a wire driven by pulleys passes through the beam in the x direction and another wire passes through in the y direction [4]. The beam profile is displayed on an x - y plotter after the current produced has been amplified. The profile in the x direction is first produced, and a microswitch is then activated to produce the y profile. A similar type [5] uses a rapidly vibrating pickup wire and a further type [6] uses an electromagnetically driven wire shaped in such a way that as it vibrates through the beam, scanning is carried out in both x and y directions. A further type scans the beam in a plane [7] and consists simply of a grid of wires placed in the beam. The current from each wire is amplified and displayed as a histogram-type profile. The grid has to be removed during experiments. 2.3.2. Cylindrical surface scanners Basically these scanners consist of a wire attached to an arm on a rotor. These wires therefore scan on the surface of a cylinder. Differences arise in the type of motor drive used for instance, a simple phase sensitive motor rotating the wire through the beam [8-10] or a stepping motor driven, wire scanner. Where a stepping motor is used; after each step the recorded current is integrated and the value stored, digital to analogue conversion then allows the beam profile to be displayed [11]. This method can scan low intensity beams since some of the random and periodic noise can be filtered out. The digitized information from two such scanners per plane can then be used to determine beam emittance in that plane. 3. Discussion o f scanners
The scanners which indicate beam position but not profde and those which produce an electromagnetic
shower are unsuitable for scanning the beam from a cyclotron. The choice is between scanners which scan in a plane or on the surface of a cylinder. A grid of wires is also unsuitable since a scanner that can be used during experiments is desired. The plane scanners have a "whipping" effect at the end of each scan and this can lead to a distorted beam profile indicating more or less intensity at a particular point on the plane of the resulting display. The cylindrical surface scanners have two distortion effects: (i) the projection of the position of the wire onto a diameter of the circle in which it is moving gives a simple harmonic motion rather than a linear motion. Oscilloscope scanning is usually linear resulting in some time distortion of the display. (ii) The second distortion is that the scanner measures the intensity not on a plane but on the arc of a circle, thus there is an out of plane distortion which again effects the profile display.
4. Type of scanner chosen Despite the two disadvantages mentioned above, a cylindrical scanning device was chosen. One of the distortions has been overcome as described below, and the other, the out of plane distortion, is negligible for beam transport systems with focal lengths >1 m. The beam scanner described can also be used for emittance measurements, and is shown in fig. 1. The speed of the motor was chosen so that the output, when displayed on an oscilloscope produces a "flicker-free" image. In addition to being a real time display system, there is the advantage that problems which might have been encountered with a storage oscilloscope have been avoided. The speed of the motor in the prototype unit is 1550 rpm. As the motor rotates, the 1 mm diameter steel scanner wire is lifted into a vertical position by centrifugal force, the wire is pre-shaped so that it is straight when the motor is rotating at its operating speed and is finally checked when installed in the beam line using strobe lighting. When the scanner is not being used a spring pulls the wire into the horizontal position out of the beam. The motor itself is mounted inside the vacuum system to avoid problems with rotating seals. There is little torque on the motor and the temperature reached is almost the same as in air. Some cooling is provided by physical contact of the motor with the walls of the chamber.
C. Haddock, F. Konopasek / Design considerations for a beam scanner
409
Fig. 1. (i) Mechanical construction of the scanner. 1. Standard 4" cube, 2. Scanner wire, 3. Rotating scanner bar, 4. Secondary electron collector, 5. 1550 r.p.m, motor, 6. Light encoder, 7. Phototransistor arrangement (MRD 450), 8. Light source, 9. Light baffle, 10. "0" ring, 11. Base plate, (fi) (a) Prototype beam scanning device. (b) Light encoding arrangement.
The double shafted motor turns the scanner wire through the beam. Accelerated protons cause secondary electron emission by collision with the wire; these electrons are scattered onto a cylindrical current pickup, the current produced is amplified by a pre-amp attached to the side of the scanner and forms the vertical signal of the oscilloscope. The pickup is cut away in such a fashion that it does not intercept any of the proton beam. A positive bias can be applied to the drum to increase the efficiency of collection of the electrons. The drum is insulated from the scanner housing and beam line using teflon rings. The lower shaft of the motor holds a light encoder; designed by Bond and Gordon [10] it is a cylindrical drum cut out in such a way as to "chop" light from a bulb to a pair of phototransistors. The output from one of the phototransistors is amplified and shaped 'in such a way as to generate a sinusoidal sweep for the oscilloscope which removes the display time distortion mentioned above. As the scanning wire enters the beam the drum
allows light to reach one of the phototransistors producing a rectangular pulse (fig. 2). The beginning of the pulse unblanks the oscilloscope so that a trace can be recorded. The rectangular pulse is then integrated and f'dtered to give a sine wave which is used as a sweep signal for the oscilloscope [figs. 2(ii), (iii)]. In this way the beam is scanned once per revolution of the motor. The drum also allows a marker pulse of light to the second phototransistor, [fig. 2(i)] the drum is arranged so that this pulse is generated when the wire is at the. mechanical centre of the beam tube. The wire is aligned into this position by using the pulse to trigger a strobe light which illuminates the scanner wire.
5. Preliminary results Typical beam profiles are shown in photographs below (fig. 3). The time scale on the oscilloscope is 5
410
C. Haddock, F. Konopasek /Design considerations for a beam scanner
Output waveforms produced by l i g h t encoding device on scanner, I
- square wave
Fig. 2. (i) Output waveforms produced by light encoding device on scanner. (ii) Waveform I is integrated to give waveform III. Waveform III is then filtered to give I V - a sinusoidal sweeping signal for the oscilloscope. (iii) Schematic of sinusoidal sweep forming circuits.
II - marker pulse
(i)
rn
/ \
I
i/
\
/11
I
I I
I
I
Waveform I is integrated to give I I I . qaveform I I I is then f i l t e r e d to give IV - a sinusoldal sweeping signal for the nscillosco~.
(ii)
ms cm -1 and t h e y scale is 5 V cm -1. The beam current for these profiles was "-0.5/aA. In fig. 3(a) we show the profile of the beam displayed with the rectangular pulse from the cylindrical light encoder. The top of the rectangular pulse corresponds to a distance of 9.0 cm and thus the width of the beam here is ~1.3 cm. In figs. 3(b) and (c) we show the displacemem of the beam away from the centre of the beam tube and in fig. 3(d) the beam is shown defocused slightly, the width here being 4.0 cm.
6. Conclusions and proposed applications ~SFROM CANNER(~
r
~
:m~
~
TOSCOPE
~
]
~,> TOSCOPE
Schematic of slnusoidal sweep forming circuits.
The prototype unit has been shown to work and a second is being built. When a scanner is placed at
(iii)
Fig. 3. Beam profiles.
C Haddock, F. Konopasek/Design considerations for a beam scanner
some distance beyond a slotted grid oriented in the x direction, the X emittance of the cyclotron can be found. Two such grid and scanner arrangements will enable X and Y emittances to be found. The scanners can then be interfaced to a small microcomputer, which will given an on-line display of the emittance of the cyclotron. The cyclotron parameters can then be controlled to optimize the emittance in real time.
References [ 1] K. Johnson and W.J. Ramler, Argonne National Laboratory, Int. Conf. on Sector Focused Cyclotrons and
411
Meson Factories, CERN, April 23-26 (1963) p. 311. [2] D. Reagan, Roy. Sci. Instr. 37 (1966) 1190. [3] Okabe Shigeru, Tsumod Kunihiko and Tabato Tatsuo, Roy. Sci. Instr. 41 (1970) 1537. [4] A. McIlwain, Nucl. Instr. and Moth. 136 (1976) 511. [5] P.H. Rose, R.P. Bastride, N.B. Brooks, J. Airoy and A.B. Whittkawer, Roy. Sci. Instr. 35 (1964) 1283. [6] J. Takacs, IEEE Trans. Nucl. Sci. (June 1965) 980. [7] H.A.L. Piceri and C. DeVrios, Nucl. Instr. and Moth. 51 (1967) 87. [8] J.W. Jagger, J.G. Page and P.J. Riley Nucl. Instr. and Moth. 49 (1967) 121. [9] A.S. Toropov, Yu. F. Chichikalov and I.V. Khokhryakov, Nucl. Instr. and Meth. 114 (1974) 101. [10] C.D. Bond and S.E. Gordon, Nucl. Instr. and Moth. 98 (1972) 513. [11] R.J. Vader and O.C. Dermois, Nucl. Instr. and Moth. 114 (1977) 423.
DESIGN CONSIDERATIONS FOR A BEAM SCANNER AT THE UNIVERSITY OF MANITOBA CYCLOTRON FACILITY Christopher HADDOCK and Francis KONOPASEK Cyclotron Laboratory, Department of Physics, University of Manitoba, Winnipeg,Manitoba R3T 2N2, Canada Received 31 October 1979 and in revised form 17 December 1979
In this paper we describe the development of a beam scanner for the University of Manitoba Cyclotron. The scanner gives an output on an oscilloscope of the beam profile - a "cross sectional" view of the intensity of the charged particle beam and its position. A brief review of the methods available for displaying beam profiles is given and the design and construction of a hybrid unit using the most favourable qualities of each is then described. Typical profiles obtained with the unit are shown and its proposed uses are outlined.
resonantly tuned to the frequency of the cyclotron. The voltage induced per coil is given by:
1. Introduction For the precise alignment and guiding of subatomic particles along the beam lines o f accelerators a monitor is necessary which gives information about the intensity and position of the beam, and preferably also the beam emittance. Once these parameters are known, the steering magnets can be adjusted so that as much o f the beam as possible is guided from the accelerator to the experimental area. There are many types of scanners used and they fall into 3 categories: (a) non-intercepting types; (b) those which stop or disturb the beam; and (c) intercepting types in which the beam disturbance is kept negligibly low.
e = (K/r) di/dt, where fit) is the beam current, r is the effective distance from the current filament to the coil axis, t is time and K is a constant. When the current filament is symmetrically located between the coils the voltage output is zero. The magnitude and polarity of e gives the beam position and this scanner can be used to center the beam. Note however that this scanner gives no information on beam profile. 2.2. Types which disturb or stop the beam For a simple profile, a scintillating screen is placed in the beam tube. A more sophisticated type [2] used in a linear accelerator, passes a 1 m m 2 molybdenum target through the beam. The scattered beam produces an electromagnetic "shower" in the accelerator structure. The shower is measured by a nearby ionization chamber and the resulting intensity gives a beam profile as the target is driven by motors in a Lissajous figure of about 2.5 cm 2. A complete scan takes ~ 2 0 s. A further type [3] is used in the scanning electron linear accelerator. Some electrons are scattered through large angles as they leave the exit window of the accelerator, however the beam profile of these electrons remains the same. An array of 30 sensors is mounted at 40 ° around the exit window and the cur-
2. A brief review 2.1. Non-intercepting types A device of this type [1] uses magnetic induction techniques, and the time average position of the beam is found. Two sets of detection coils determine the horizontal and vertical positions o f the beam. Each set consists of two coils connected in opposition and symmetrically placed about the mechanical center of the beam tube. The axes o f the coils are located perpendicular to the direction of the beam. The coils are 407
408
C Haddock, F. Konopasek / Design considerations for a beam scanner
rent produced by each of these is amplified and fed to a readout circuit to display the beam profile. 2.3. Intercepting types
The intercepting types of beam scanners all pass a wire through the beam. The recorded current is amplified and displayed as a profile of the beam. The ways in which the wires are passed through the beam differ widely and are fully discussed below. The wire scanners scan through the beam: (i) on a plane perpendicular to the beam or (ii) the surface of a cylinder. 2. 3.1. Plane scanners Here for example a wire driven by pulleys passes through the beam in the x direction and another wire passes through in the y direction [4]. The beam profile is displayed on an x - y plotter after the current produced has been amplified. The profile in the x direction is first produced, and a microswitch is then activated to produce the y profile. A similar type [5] uses a rapidly vibrating pickup wire and a further type [6] uses an electromagnetically driven wire shaped in such a way that as it vibrates through the beam, scanning is carried out in both x and y directions. A further type scans the beam in a plane [7] and consists simply of a grid of wires placed in the beam. The current from each wire is amplified and displayed as a histogram-type profile. The grid has to be removed during experiments. 2.3.2. Cylindrical surface scanners Basically these scanners consist of a wire attached to an arm on a rotor. These wires therefore scan on the surface of a cylinder. Differences arise in the type of motor drive used for instance, a simple phase sensitive motor rotating the wire through the beam [8-10] or a stepping motor driven, wire scanner. Where a stepping motor is used; after each step the recorded current is integrated and the value stored, digital to analogue conversion then allows the beam profile to be displayed [11]. This method can scan low intensity beams since some of the random and periodic noise can be filtered out. The digitized information from two such scanners per plane can then be used to determine beam emittance in that plane. 3. Discussion o f scanners
The scanners which indicate beam position but not profde and those which produce an electromagnetic
shower are unsuitable for scanning the beam from a cyclotron. The choice is between scanners which scan in a plane or on the surface of a cylinder. A grid of wires is also unsuitable since a scanner that can be used during experiments is desired. The plane scanners have a "whipping" effect at the end of each scan and this can lead to a distorted beam profile indicating more or less intensity at a particular point on the plane of the resulting display. The cylindrical surface scanners have two distortion effects: (i) the projection of the position of the wire onto a diameter of the circle in which it is moving gives a simple harmonic motion rather than a linear motion. Oscilloscope scanning is usually linear resulting in some time distortion of the display. (ii) The second distortion is that the scanner measures the intensity not on a plane but on the arc of a circle, thus there is an out of plane distortion which again effects the profile display.
4. Type of scanner chosen Despite the two disadvantages mentioned above, a cylindrical scanning device was chosen. One of the distortions has been overcome as described below, and the other, the out of plane distortion, is negligible for beam transport systems with focal lengths >1 m. The beam scanner described can also be used for emittance measurements, and is shown in fig. 1. The speed of the motor was chosen so that the output, when displayed on an oscilloscope produces a "flicker-free" image. In addition to being a real time display system, there is the advantage that problems which might have been encountered with a storage oscilloscope have been avoided. The speed of the motor in the prototype unit is 1550 rpm. As the motor rotates, the 1 mm diameter steel scanner wire is lifted into a vertical position by centrifugal force, the wire is pre-shaped so that it is straight when the motor is rotating at its operating speed and is finally checked when installed in the beam line using strobe lighting. When the scanner is not being used a spring pulls the wire into the horizontal position out of the beam. The motor itself is mounted inside the vacuum system to avoid problems with rotating seals. There is little torque on the motor and the temperature reached is almost the same as in air. Some cooling is provided by physical contact of the motor with the walls of the chamber.
C. Haddock, F. Konopasek / Design considerations for a beam scanner
409
Fig. 1. (i) Mechanical construction of the scanner. 1. Standard 4" cube, 2. Scanner wire, 3. Rotating scanner bar, 4. Secondary electron collector, 5. 1550 r.p.m, motor, 6. Light encoder, 7. Phototransistor arrangement (MRD 450), 8. Light source, 9. Light baffle, 10. "0" ring, 11. Base plate, (fi) (a) Prototype beam scanning device. (b) Light encoding arrangement.
The double shafted motor turns the scanner wire through the beam. Accelerated protons cause secondary electron emission by collision with the wire; these electrons are scattered onto a cylindrical current pickup, the current produced is amplified by a pre-amp attached to the side of the scanner and forms the vertical signal of the oscilloscope. The pickup is cut away in such a fashion that it does not intercept any of the proton beam. A positive bias can be applied to the drum to increase the efficiency of collection of the electrons. The drum is insulated from the scanner housing and beam line using teflon rings. The lower shaft of the motor holds a light encoder; designed by Bond and Gordon [10] it is a cylindrical drum cut out in such a way as to "chop" light from a bulb to a pair of phototransistors. The output from one of the phototransistors is amplified and shaped 'in such a way as to generate a sinusoidal sweep for the oscilloscope which removes the display time distortion mentioned above. As the scanning wire enters the beam the drum
allows light to reach one of the phototransistors producing a rectangular pulse (fig. 2). The beginning of the pulse unblanks the oscilloscope so that a trace can be recorded. The rectangular pulse is then integrated and f'dtered to give a sine wave which is used as a sweep signal for the oscilloscope [figs. 2(ii), (iii)]. In this way the beam is scanned once per revolution of the motor. The drum also allows a marker pulse of light to the second phototransistor, [fig. 2(i)] the drum is arranged so that this pulse is generated when the wire is at the. mechanical centre of the beam tube. The wire is aligned into this position by using the pulse to trigger a strobe light which illuminates the scanner wire.
5. Preliminary results Typical beam profiles are shown in photographs below (fig. 3). The time scale on the oscilloscope is 5
410
C. Haddock, F. Konopasek /Design considerations for a beam scanner
Output waveforms produced by l i g h t encoding device on scanner, I
- square wave
Fig. 2. (i) Output waveforms produced by light encoding device on scanner. (ii) Waveform I is integrated to give waveform III. Waveform III is then filtered to give I V - a sinusoidal sweeping signal for the oscilloscope. (iii) Schematic of sinusoidal sweep forming circuits.
II - marker pulse
(i)
rn
/ \
I
i/
\
/11
I
I I
I
I
Waveform I is integrated to give I I I . qaveform I I I is then f i l t e r e d to give IV - a sinusoldal sweeping signal for the nscillosco~.
(ii)
ms cm -1 and t h e y scale is 5 V cm -1. The beam current for these profiles was "-0.5/aA. In fig. 3(a) we show the profile of the beam displayed with the rectangular pulse from the cylindrical light encoder. The top of the rectangular pulse corresponds to a distance of 9.0 cm and thus the width of the beam here is ~1.3 cm. In figs. 3(b) and (c) we show the displacemem of the beam away from the centre of the beam tube and in fig. 3(d) the beam is shown defocused slightly, the width here being 4.0 cm.
6. Conclusions and proposed applications ~SFROM CANNER(~
r
~
:m~
~
TOSCOPE
~
]
~,> TOSCOPE
Schematic of slnusoidal sweep forming circuits.
The prototype unit has been shown to work and a second is being built. When a scanner is placed at
(iii)
Fig. 3. Beam profiles.
C Haddock, F. Konopasek/Design considerations for a beam scanner
some distance beyond a slotted grid oriented in the x direction, the X emittance of the cyclotron can be found. Two such grid and scanner arrangements will enable X and Y emittances to be found. The scanners can then be interfaced to a small microcomputer, which will given an on-line display of the emittance of the cyclotron. The cyclotron parameters can then be controlled to optimize the emittance in real time.
References [ 1] K. Johnson and W.J. Ramler, Argonne National Laboratory, Int. Conf. on Sector Focused Cyclotrons and
411
Meson Factories, CERN, April 23-26 (1963) p. 311. [2] D. Reagan, Roy. Sci. Instr. 37 (1966) 1190. [3] Okabe Shigeru, Tsumod Kunihiko and Tabato Tatsuo, Roy. Sci. Instr. 41 (1970) 1537. [4] A. McIlwain, Nucl. Instr. and Moth. 136 (1976) 511. [5] P.H. Rose, R.P. Bastride, N.B. Brooks, J. Airoy and A.B. Whittkawer, Roy. Sci. Instr. 35 (1964) 1283. [6] J. Takacs, IEEE Trans. Nucl. Sci. (June 1965) 980. [7] H.A.L. Piceri and C. DeVrios, Nucl. Instr. and Moth. 51 (1967) 87. [8] J.W. Jagger, J.G. Page and P.J. Riley Nucl. Instr. and Moth. 49 (1967) 121. [9] A.S. Toropov, Yu. F. Chichikalov and I.V. Khokhryakov, Nucl. Instr. and Meth. 114 (1974) 101. [10] C.D. Bond and S.E. Gordon, Nucl. Instr. and Moth. 98 (1972) 513. [11] R.J. Vader and O.C. Dermois, Nucl. Instr. and Moth. 114 (1977) 423.