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A fast non-intercepting linac electron beam position and current monitor
Nuclear Instruments and Methods 197 (1982) 259-263 North-Holland Publishing Company
259
A FAST NON-INTERCEPTING LINAC ELECTRON BEAM POSITION AND CURRENT MONITOR J o h n n y W. H A N S E N a n d Mads W I L L E Accelerator Department, Ris~ National Laborato~, DK 4000 Roskilde, Denmark
Received 27 November 1981
A non-intercepting beam monitor consisting of four detecting loops is used to determine the spatial postion and current of a pulsed beam from an electron linear accelerator. The monitor detects the magnetic field radiated by the substructure of the electron bunches created by the accelerating microwave. The detecting loops are interconnected two by two, by means of two coaxial hybrid junctions, the two sets positioned perpendicular to each other. By means of the two signals from the diametrically positioned detecting loops, a good spatial displacement and current monitoring sensitivity are achieved by subtracting one signal from the other and adding the two signals, respectively. For displacements below 2 mm from the center axis an average sensitivity of 0.5 mV/mm-mA is measured, whereas displacements more than 2 mm yields 1.3 mV/mm- mA. A sensitivity of 0.2 mV/mA in current monitoring is measured, and the rise time of the monitored pulse signal is better than 5 ns measured from 10 to 90% of the pulse height. Design strategy and performance of the monitor are described.
1. Introduction The non-intercepting beam monitor described here detects the lobe of the magnetic field radiated from the bunches of accelerated electrons moving in the evacuated beam tube of the accelerator beam handling system. This detecting system was chosen because of a fast response time, but it is rather poor in signal strength. The goal was to design a monitor which at the same time could measure beam current and spatial position of the beam path in the beam handling system in order to optimize beam performance. These requirements can be fulfilled by using a four port hybrid junction to interconnect the two diametrically positioned detecting loops. Two such systems positioned perpendicular to each other are built together in one unit thus forming a detector capable of monitoring the beam current and the spatial beam position. One output port carries the summed loop signals for current monitoring, and a second output port carries the subtracted loop signals for position monitoring. The configuration of the hybrid junction is a ring-formed coaxial transmission line interconnecting the input and output ports and is at the same time an integral part of the monitor. The output ports are of construction reasons spaced somewhat different from the conventional 2~?~g-symmetryand spaced geometrically at angles of 0 °, 60 °, 120°, and 270 °. Two ports are used as input for the signals from the loops and two ports transmit the sum and the difference signals. The loops detect the individual substructure bunches of the beam pulse, but the video signal, fed to the 0167-5087/82/0000-0000/$02.75 © 1982 North-Holland
oscilloscope from the crystal detectors terminating the hybrid junction output ports, appears as an average of the current in the substructure bunches.
2. Design The magnetic field radiated perpendicularly to the velocity axis by a beam of electrons varies inverse proportionally to the distance from the outer envelope of the beam. This variation enables the detecting loop to be sensitive to the position of the beam axis. The four detecting loops are positioned equally spaced around the axis of the beam tube which makes it possible easily to determine the spatial position of the beam. By subtracting the signals from the diametrically opposed detecting loops, a zero signal would indicate that the electron beam is positioned at equal distances from the loops, i.e. in the middle of the beam tube. A subtraction of the two loop signals enables a high sensitivity of a beam displacement determination, while a summation of the two signals makes the current measurement less sensitive to the spatial position of the electron beam. The radiated magnetic field perpendicular to the center axis of the electron beam is calculated as a function of the distance from the envelope of the beam with a diameter of 5 mm, fig. I a. The magnetic field values are in arbitrary units. Fig. lb shows the calculated sum and difference signals in arbitrary units as detected by the diametrically opposed loops which are positioned with the center of the loop in a distance of 25 mm from the beam center axis.
260
J.W. tIansen, M. Wille / Beam position and current monitor
llOO
,80
,60
~Magnetic field B oC ~-.40
0
+
!
.20
i
i
i
15
10
5
i 5
i I0
Distancefrom center axis i • 15 r Imm] Fig. 2. Coaxial hybrid junction for the vertical loops, schematic.
b)
m signal
30
'20
10 / ,~ ws
1o
s
o
signal
Deftection
i
i
s
1o
of beam
from center OXlS• ~s r [mm]
Fig. 1. (a) Radiated magnetic field perpendicular to the axis of beam movement calculated in arbitrary units as a function of the distance from the envelope of a 5 mm diameter electron beam. (b) Calculated sum and difference signals as detected from the diametrically opposed detecting loops. Center of detecting loops is 25 mm from the beam center axis. It can be shown, for relativistic electrons [1], that the monitor measures the beam current independently of the energy of the electrons and that the response of the detecting loop only depends on the current in a bunch, since the magnetic field is proportional to the peak current. An ideal hybrid junction is a four-port network of lossless transmission lines so connected that one port transmits the signals from two particular ports in-phase, summed signals, whereas another port transmits the signals from the same ports in anti-phase, subtracted signals, provided proper impedance matching of all ports. In a practical hybrid junction, however, there are conductor losses and the loads connected to the ports are not properly impedance matched. These deviations from an ideal network do not influence the detector as a beam position monitor as long as a proper balance exists between the individual ports. But too poor isolation and high conductor losses have substantial in-
fluence on the time constant of the circuit and limit the detector response capability for fast signals. For optimal operation of the hybrid junction based on a coaxial transmission line configuration, the length of the center electrode should match an even multiple of 1½hg, where ?tg is the transmission line wavelength [2]. This, however, is not a necessary condition and choosing any diameter of the hybrid may lead to a proper operation when the hybrid is used as a current and position monitor. Only the spacing of the junctions on the interconnecting center electrode must be exact, fig. 2. If the signals applied to ports 1 and 3 are V1 and V3, the output signals VA and V D can be expressed as: VA = 1 VleJ,~/3 + ½VleJS~,/3 + ½V3eJ2~r/3+ = vl-
½V3eJ4~r/3
v3,
Vo = ½ Vie j'~/2 + ½ Vie j3'~/2 + ½V3eJ'~/2 + ½V3ej3,~/2
=v,+v3. Thus VA is the subtracted and VD the summed signals from the input ports 1 and 3, respectively. For a proper impedance matching and spacing of the individual ports on the interconnecting coaxial line ring, the signal from a loop must be divided equally into the ring. The characteristic impedance of the coaxial line ring is given by Z0R =Vr2 ZOA, which for a 5 0 ~ termination of the ports makes 71 ~2 for the ring. The loops, coaxial line ring, and output ports are designed from conventional microwave design principles. Detailed calculations of expected output signals for a given beam current have not been made, as smaller deviations in the impedance matching of the different parts constituting the hybrid junction may lead to fluctuations from ideal values. The body of the monitor is machined from a thickwalled stainless steel tube in which grooves for the coaxial hybrid junctions have been turned and milled. The inner diameter of the tube equals the diameter of the flight tubes of the beam handling system and stan-
J. IV. Hansen, M. Wille / Beam position and current monitor
616A UHF-generator was pulse modulated by an HP 212A pulse generator, and the microwave pulses were amplified in an HP 419C microwave amplifier before feeding into the line conductor through a coaxial tapered line, which together with the monitor and a 50f~ termination constituted the load. The output signal from the monitor was detected by an HP 423B crystal detector and fed to an oscilloscope. But because of inherent impedance matching problems when simulating the beam, ultimate calibrations of current and beam displacement sensitivities would give doubtful results, whereas a test in the bench could perform qualitative indications of the beam monitoring capability. The beam displacement sensitivity stated below is therefore based upon the actual measured microwave signal detected by the crystal detector in the test bench set-up. This measurement is then related to a beam current measurement giving the same signal from the crystal detector, i.e. from the beam current vs detector signal characteristic in fig. 6. The beam current measurement is influenced by the position of the beam, as shown in fig. 4 where curve l demonstrates the summed signals picked up by the horizontal loops, and curve 2 shows the summed signals from the vertical loops against a horizontal deflection of the line conductor. A vertical deflection of the line conductor will demonstrate the same response in corresponding loops, showing that a calibration of the current sensitivity is only valid within a beam position of about -+2 mm from the center axis. The response of the difference signal from the horizontal loops as a function of a horizontal displacement of the line conductor is much more pronounced than the response of the summed signals for the same displacement, fig. 5. The difference signal is zero with the line conductor in the center axis, which is in agreement with the theory, see fig. lb. The detector response is not a linear function of beam displacement, but the measurements show that the center of the line conductor
Fig. 3. Photograph showing the beam position and current monitor.
dard rotational vacuum flanges are welded on to the monitor. Vacuum tight ceramic feed throughs are used for the connection of the loops to the line ring, thus separating the coaxial hybrid junction from the vacuum. Two halves of a stainless steel tube attached N-type connectors for the output signals cover the outside surface of the tube containing the milled grooves. The dielectric medium of the coaxial line ring is air except for a few spacers of polyethylene keeping the central conductor in place. The complete monitor ready for installation is shown in fig. 3.
3. Performance
As beam displacement calibrations on the linac are not possible, the monitor' was calibrated on a bench in the laboratory. The electron beam pulses were simulated by means of microwave pulses fed into a line conductor going through the monitor. A Hewlett Packard HP
Summation
J 100-
s0~
261
signal
[mV]
H o r i z o n t a l beam deflection. I: Measured on h o r i z o n t a l loops. 2: Measured orl
"
(~
40'
9
I
6
I 5
I
|
|
I
4.
3
Z
1
I 0
iOe'flec t io~ f ram center a x i s I
I
I
1
2
3
•
4
5
6 [mm]
Fig. 4. The summation signal measured on the horizontal and vertical loops for a horizontal deflection of the beam.
262
J. W. Hansen, M. Wille / Beam position and current monitor i Difference signal
[mV ]
-800
-700
-600
.500
.400
300
200
J~./ 5
4
3
2
Deflection from center axis
1
Fig. 5. The difference signal from the horizontal loops as a response of a horizontal deflection of the beam. c a n be located with a n accuracy of ± 0 . 2 5 m m from the central position. F o r displacements < 2 m m an average sensitivity of 0.5 m V / m m , m A is measured, whereas displacements > 2 m m yields a n average sensitivity of 1.3 m V / m m • m A w h e n the crystal detector is terminated with 50 ~2. I n the n o r m a l operation of our accelerator the crystal detectors are terminated by 220 ~ yielding a sensitivity of 4.4 times the above stated. W h e n terminated b y 220 fl the m o n i t o r demonstrates a rise time short enough for responding pulses of a few micro-
seconds. Due to the way of operation of a h y b r i d j u n c t i o n the o u t p u t signal will have the same polarity w h e t h e r the b e a m deflects to one or the other side of the Beam monitor signal 20 mV/dlV.
-1
Beamcurrent [mA] ?OO
Crystal d e t e c t o r
~f~ ,..,= ~
~'='-
50 n sec/dlv
| is terminated
by
220.t~.
600 Beam
catcher signa[
500
I 1
I~
w~ ~
100 mA/div ~"
50n sec/div
400 300 200 ~00 Detector
signal
I
i
=
I
i
I
=
=
10o
200
300
400
SO0
600
700
1100
•
[mVl
Fig. 6. Calibration of the monitor against measured electron b e a m current in a b e a m catcher.
Fig. 7. Oscilloscope traces of electron beam pulses. Upper trace is the monitor response, 20 mV/div., 50 ns/div., lower trace is the response from a beam catcher picking up the pulse, 100 mA/div., 50 ns/div.
J. W. Hansen, M. Wille / Beam position and current monitor
center, but the response will in principle be axial symmetric, the symmetry being depended of the construction accuracy. After mounting in the accelerator beam handling system the monitor was calibrated against the beam current measured in a beam catcher, fig. 6. The monitor signal was detected by an HP 423B crystal detector terminated with a 220 ~2 resistor, and the pulse height was measured on an oscilloscope. Except for small beam currents < 30 mA the response of the detecting system is a linear function of the beam current, and the sensitivity is 1 mV/mA. The ability of the monitor to respond to a fast electron beam pulse was measured on an oscilloscope by comparing the monitor signal with the signal from a
263
beam catcher. The beam pulse signals are shown as oscilloscope traces in fig. 7, and the rise time of the monitor is found to be better than 5 ns measured from 10 to 90% of the pulse height. To respond to such fast pulses the crystal detector must be terminated with the rated impedance of 50 f~.
References [1] R. Bergere, E. Delezenne and A. Veyssiere, AERE Trans. 928, Atomic Energy Research Establishment, Harwell, Berkshire, England (1963). [2] A.F. Harvey, Microwave engineering (Academic Press, London and New York, 1963) p. 116.
259
A FAST NON-INTERCEPTING LINAC ELECTRON BEAM POSITION AND CURRENT MONITOR J o h n n y W. H A N S E N a n d Mads W I L L E Accelerator Department, Ris~ National Laborato~, DK 4000 Roskilde, Denmark
Received 27 November 1981
A non-intercepting beam monitor consisting of four detecting loops is used to determine the spatial postion and current of a pulsed beam from an electron linear accelerator. The monitor detects the magnetic field radiated by the substructure of the electron bunches created by the accelerating microwave. The detecting loops are interconnected two by two, by means of two coaxial hybrid junctions, the two sets positioned perpendicular to each other. By means of the two signals from the diametrically positioned detecting loops, a good spatial displacement and current monitoring sensitivity are achieved by subtracting one signal from the other and adding the two signals, respectively. For displacements below 2 mm from the center axis an average sensitivity of 0.5 mV/mm-mA is measured, whereas displacements more than 2 mm yields 1.3 mV/mm- mA. A sensitivity of 0.2 mV/mA in current monitoring is measured, and the rise time of the monitored pulse signal is better than 5 ns measured from 10 to 90% of the pulse height. Design strategy and performance of the monitor are described.
1. Introduction The non-intercepting beam monitor described here detects the lobe of the magnetic field radiated from the bunches of accelerated electrons moving in the evacuated beam tube of the accelerator beam handling system. This detecting system was chosen because of a fast response time, but it is rather poor in signal strength. The goal was to design a monitor which at the same time could measure beam current and spatial position of the beam path in the beam handling system in order to optimize beam performance. These requirements can be fulfilled by using a four port hybrid junction to interconnect the two diametrically positioned detecting loops. Two such systems positioned perpendicular to each other are built together in one unit thus forming a detector capable of monitoring the beam current and the spatial beam position. One output port carries the summed loop signals for current monitoring, and a second output port carries the subtracted loop signals for position monitoring. The configuration of the hybrid junction is a ring-formed coaxial transmission line interconnecting the input and output ports and is at the same time an integral part of the monitor. The output ports are of construction reasons spaced somewhat different from the conventional 2~?~g-symmetryand spaced geometrically at angles of 0 °, 60 °, 120°, and 270 °. Two ports are used as input for the signals from the loops and two ports transmit the sum and the difference signals. The loops detect the individual substructure bunches of the beam pulse, but the video signal, fed to the 0167-5087/82/0000-0000/$02.75 © 1982 North-Holland
oscilloscope from the crystal detectors terminating the hybrid junction output ports, appears as an average of the current in the substructure bunches.
2. Design The magnetic field radiated perpendicularly to the velocity axis by a beam of electrons varies inverse proportionally to the distance from the outer envelope of the beam. This variation enables the detecting loop to be sensitive to the position of the beam axis. The four detecting loops are positioned equally spaced around the axis of the beam tube which makes it possible easily to determine the spatial position of the beam. By subtracting the signals from the diametrically opposed detecting loops, a zero signal would indicate that the electron beam is positioned at equal distances from the loops, i.e. in the middle of the beam tube. A subtraction of the two loop signals enables a high sensitivity of a beam displacement determination, while a summation of the two signals makes the current measurement less sensitive to the spatial position of the electron beam. The radiated magnetic field perpendicular to the center axis of the electron beam is calculated as a function of the distance from the envelope of the beam with a diameter of 5 mm, fig. I a. The magnetic field values are in arbitrary units. Fig. lb shows the calculated sum and difference signals in arbitrary units as detected by the diametrically opposed loops which are positioned with the center of the loop in a distance of 25 mm from the beam center axis.
260
J.W. tIansen, M. Wille / Beam position and current monitor
llOO
,80
,60
~Magnetic field B oC ~-.40
0
+
!
.20
i
i
i
15
10
5
i 5
i I0
Distancefrom center axis i • 15 r Imm] Fig. 2. Coaxial hybrid junction for the vertical loops, schematic.
b)
m signal
30
'20
10 / ,~ ws
1o
s
o
signal
Deftection
i
i
s
1o
of beam
from center OXlS• ~s r [mm]
Fig. 1. (a) Radiated magnetic field perpendicular to the axis of beam movement calculated in arbitrary units as a function of the distance from the envelope of a 5 mm diameter electron beam. (b) Calculated sum and difference signals as detected from the diametrically opposed detecting loops. Center of detecting loops is 25 mm from the beam center axis. It can be shown, for relativistic electrons [1], that the monitor measures the beam current independently of the energy of the electrons and that the response of the detecting loop only depends on the current in a bunch, since the magnetic field is proportional to the peak current. An ideal hybrid junction is a four-port network of lossless transmission lines so connected that one port transmits the signals from two particular ports in-phase, summed signals, whereas another port transmits the signals from the same ports in anti-phase, subtracted signals, provided proper impedance matching of all ports. In a practical hybrid junction, however, there are conductor losses and the loads connected to the ports are not properly impedance matched. These deviations from an ideal network do not influence the detector as a beam position monitor as long as a proper balance exists between the individual ports. But too poor isolation and high conductor losses have substantial in-
fluence on the time constant of the circuit and limit the detector response capability for fast signals. For optimal operation of the hybrid junction based on a coaxial transmission line configuration, the length of the center electrode should match an even multiple of 1½hg, where ?tg is the transmission line wavelength [2]. This, however, is not a necessary condition and choosing any diameter of the hybrid may lead to a proper operation when the hybrid is used as a current and position monitor. Only the spacing of the junctions on the interconnecting center electrode must be exact, fig. 2. If the signals applied to ports 1 and 3 are V1 and V3, the output signals VA and V D can be expressed as: VA = 1 VleJ,~/3 + ½VleJS~,/3 + ½V3eJ2~r/3+ = vl-
½V3eJ4~r/3
v3,
Vo = ½ Vie j'~/2 + ½ Vie j3'~/2 + ½V3eJ'~/2 + ½V3ej3,~/2
=v,+v3. Thus VA is the subtracted and VD the summed signals from the input ports 1 and 3, respectively. For a proper impedance matching and spacing of the individual ports on the interconnecting coaxial line ring, the signal from a loop must be divided equally into the ring. The characteristic impedance of the coaxial line ring is given by Z0R =Vr2 ZOA, which for a 5 0 ~ termination of the ports makes 71 ~2 for the ring. The loops, coaxial line ring, and output ports are designed from conventional microwave design principles. Detailed calculations of expected output signals for a given beam current have not been made, as smaller deviations in the impedance matching of the different parts constituting the hybrid junction may lead to fluctuations from ideal values. The body of the monitor is machined from a thickwalled stainless steel tube in which grooves for the coaxial hybrid junctions have been turned and milled. The inner diameter of the tube equals the diameter of the flight tubes of the beam handling system and stan-
J. IV. Hansen, M. Wille / Beam position and current monitor
616A UHF-generator was pulse modulated by an HP 212A pulse generator, and the microwave pulses were amplified in an HP 419C microwave amplifier before feeding into the line conductor through a coaxial tapered line, which together with the monitor and a 50f~ termination constituted the load. The output signal from the monitor was detected by an HP 423B crystal detector and fed to an oscilloscope. But because of inherent impedance matching problems when simulating the beam, ultimate calibrations of current and beam displacement sensitivities would give doubtful results, whereas a test in the bench could perform qualitative indications of the beam monitoring capability. The beam displacement sensitivity stated below is therefore based upon the actual measured microwave signal detected by the crystal detector in the test bench set-up. This measurement is then related to a beam current measurement giving the same signal from the crystal detector, i.e. from the beam current vs detector signal characteristic in fig. 6. The beam current measurement is influenced by the position of the beam, as shown in fig. 4 where curve l demonstrates the summed signals picked up by the horizontal loops, and curve 2 shows the summed signals from the vertical loops against a horizontal deflection of the line conductor. A vertical deflection of the line conductor will demonstrate the same response in corresponding loops, showing that a calibration of the current sensitivity is only valid within a beam position of about -+2 mm from the center axis. The response of the difference signal from the horizontal loops as a function of a horizontal displacement of the line conductor is much more pronounced than the response of the summed signals for the same displacement, fig. 5. The difference signal is zero with the line conductor in the center axis, which is in agreement with the theory, see fig. lb. The detector response is not a linear function of beam displacement, but the measurements show that the center of the line conductor
Fig. 3. Photograph showing the beam position and current monitor.
dard rotational vacuum flanges are welded on to the monitor. Vacuum tight ceramic feed throughs are used for the connection of the loops to the line ring, thus separating the coaxial hybrid junction from the vacuum. Two halves of a stainless steel tube attached N-type connectors for the output signals cover the outside surface of the tube containing the milled grooves. The dielectric medium of the coaxial line ring is air except for a few spacers of polyethylene keeping the central conductor in place. The complete monitor ready for installation is shown in fig. 3.
3. Performance
As beam displacement calibrations on the linac are not possible, the monitor' was calibrated on a bench in the laboratory. The electron beam pulses were simulated by means of microwave pulses fed into a line conductor going through the monitor. A Hewlett Packard HP
Summation
J 100-
s0~
261
signal
[mV]
H o r i z o n t a l beam deflection. I: Measured on h o r i z o n t a l loops. 2: Measured orl
"
(~
40'
9
I
6
I 5
I
|
|
I
4.
3
Z
1
I 0
iOe'flec t io~ f ram center a x i s I
I
I
1
2
3
•
4
5
6 [mm]
Fig. 4. The summation signal measured on the horizontal and vertical loops for a horizontal deflection of the beam.
262
J. W. Hansen, M. Wille / Beam position and current monitor i Difference signal
[mV ]
-800
-700
-600
.500
.400
300
200
J~./ 5
4
3
2
Deflection from center axis
1
Fig. 5. The difference signal from the horizontal loops as a response of a horizontal deflection of the beam. c a n be located with a n accuracy of ± 0 . 2 5 m m from the central position. F o r displacements < 2 m m an average sensitivity of 0.5 m V / m m , m A is measured, whereas displacements > 2 m m yields a n average sensitivity of 1.3 m V / m m • m A w h e n the crystal detector is terminated with 50 ~2. I n the n o r m a l operation of our accelerator the crystal detectors are terminated by 220 ~ yielding a sensitivity of 4.4 times the above stated. W h e n terminated b y 220 fl the m o n i t o r demonstrates a rise time short enough for responding pulses of a few micro-
seconds. Due to the way of operation of a h y b r i d j u n c t i o n the o u t p u t signal will have the same polarity w h e t h e r the b e a m deflects to one or the other side of the Beam monitor signal 20 mV/dlV.
-1
Beamcurrent [mA] ?OO
Crystal d e t e c t o r
~f~ ,..,= ~
~'='-
50 n sec/dlv
| is terminated
by
220.t~.
600 Beam
catcher signa[
500
I 1
I~
w~ ~
100 mA/div ~"
50n sec/div
400 300 200 ~00 Detector
signal
I
i
=
I
i
I
=
=
10o
200
300
400
SO0
600
700
1100
•
[mVl
Fig. 6. Calibration of the monitor against measured electron b e a m current in a b e a m catcher.
Fig. 7. Oscilloscope traces of electron beam pulses. Upper trace is the monitor response, 20 mV/div., 50 ns/div., lower trace is the response from a beam catcher picking up the pulse, 100 mA/div., 50 ns/div.
J. W. Hansen, M. Wille / Beam position and current monitor
center, but the response will in principle be axial symmetric, the symmetry being depended of the construction accuracy. After mounting in the accelerator beam handling system the monitor was calibrated against the beam current measured in a beam catcher, fig. 6. The monitor signal was detected by an HP 423B crystal detector terminated with a 220 ~2 resistor, and the pulse height was measured on an oscilloscope. Except for small beam currents < 30 mA the response of the detecting system is a linear function of the beam current, and the sensitivity is 1 mV/mA. The ability of the monitor to respond to a fast electron beam pulse was measured on an oscilloscope by comparing the monitor signal with the signal from a
263
beam catcher. The beam pulse signals are shown as oscilloscope traces in fig. 7, and the rise time of the monitor is found to be better than 5 ns measured from 10 to 90% of the pulse height. To respond to such fast pulses the crystal detector must be terminated with the rated impedance of 50 f~.
References [1] R. Bergere, E. Delezenne and A. Veyssiere, AERE Trans. 928, Atomic Energy Research Establishment, Harwell, Berkshire, England (1963). [2] A.F. Harvey, Microwave engineering (Academic Press, London and New York, 1963) p. 116.