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An improvement to the beam buncher of the University of Manitoba Cyclotron
NtJ('I,tlAR
INSTRUMENFS
AND
METII()[)S
145, (19771
403-4(14 : ©
Nt)RTIt-ttOI.I. AND
PUBLISttlNG
CO
LETTERS TO TIlE EDITOR AN I M P R O V E M E N T T O T H E BEAM BUNCHER OF T H E UNIVERSITY OF M A N I T O B A C Y C L O T R O N ROBERT POGSON and S A E W O O N G 01t
(ivHotr.n l.aboratoo. D~7~artmcnt ¢?/ Plly.sics, L.nivcrsity o~ Mantmba. 147nnipeg, .'~lanitoba, ('anada R312:\:2 Received 18 April 1977 The extracted beam from the Manitoba cyclotron has increased by nearly 40% through the addition of a second harmonic component It) the sine-wave previously applied to the beam buncher.
Since the conversion of the cyclotron from internal to external injection in January 1975 a twogap beam buncher driven sinusoidally at the cyclotron frequency has been used to increase extracted currents by a factor of 5 ~). Other installations have used synthesized approximations to the ideal sawtooth waveform to improve buncher effectiveness2-4). Our studies indicated that we could obtain a significant improvement by adding a second harmonic component and we have successfully done so. The ideal effectiveness of some synthesized wavetbrms is shown in fig. 1. The programme which generated these curves maximizes the accepted beam by varying the coefficients of a linear combination of sinewaves representing velocity modulation voltages. All degradations of bunching action present in a real system were neglected. CONTROL ROOM
CYCLOTRON VAULT
pickup t~uncher
i ~
[o't~.uoto~l
Fig. 1. Percentage of the injected beam delivered to the inflector within a given phase acceptance window for various waveforms. T h e curves are labelled with the n u m b e r of h a r m o n i c s included. No b u n c h i n g is indicated by O.
lons which arrived at the injection point in a given time interval were assumed to be accepted. These assumptions should introduce little error in the comparison of various waveforms because any degrading lhctors will likely be independent of waveform, t{owever the comparison between dc beams and bunched beams will likely be overly optimistic. To add the second harmonic c o m p o n e n t , the previous buncher system was modified with as little change as possible to obtain the system shown in fig. 2. The driven element has been divided into two pieces driven in push-pull mode at the second harmonic and in parallel mode at the fundamental frequency. This circuit permits the two signals to be completely separated for control with a m i n i m u m of electronics without increasing the length of the beam buncher assembly or by adding a second buncher. An antenna picks up some power from the final rf stage driving the dees. This signal is carried to the control room by coaxial cable and passes through an amplifier and variable delay which is used to regulate the amplitude and phase of the composite waveform. One output of this amplifier passes through a variable attenuator and returns to the vault to drive the buncher at the fundamental frequency. The other output is applied to a passive frequency doubler followed by a variable delay to generate the second harmonic and to control its phase with respect to the fundamental. A redundant variable attenuator is used on the second harmonic signal sent to the buncher as a convenience. The coupling circuits were all carefully tuned for resonance and adjusted for impedance matching with the buncher in place to minimize the power needed to operate the system. With our 11 keV
404
R
POGSON
O
AND S
Otl
TABLE 1 5
Extracted proton beam currents with various b u n c h e r waveforms.
3
C)
Radius Energy Currents: (a) Buncher off (b) First harmonic (c) First and second harmonics
>-
ta_ t.L
PHASE ACCEPTANCE (DEGREES) Fig. 2. Schematic of the new beam buncher system.
H beam and a distance of 1.7 m from the bunch° er to the inflector we computed that the peak voltages needed for bunching are 86 V at 28.48 MHz and 22 V at 56.96 MHz. With reasonable Q factors this requires very little power and by increasing the pickup from the dees the system could probably be operated without an amplifier. T h e observed performance has been pleasing. When the first harmonic is applied we obtain a factor of six increase in extracted beam compared with dc injection*. When both frequencies are applied and amplitudes adjusted empirically for best results extracted beam is increased by a factor of eight compared with dc injection. Table 1 lists some performance data. Note that the bunching effectiveness seems to be less for higher currents and greater for higher energies. In light of this performance we contemplate the inclusion of a third harmonic component to obtain a further increase of at least 10% in extracted beam current. The third harmonic tank circuit could be placed in the ground return path of the fundamental frequency. The great difference in
Case 1
Case 2
37 cm 26 MeV
50 cm 44 MeV
0.65,uA 3.7 uA
3.25 nA 17.0 nA
5.0/IA
37.5 nA
frequency permits this to be done with little interaction. The observed effectiveness of the new beam buncher is about the same as the ideal beam buncher of fig. 1 with a phase acceptance of 30 °. However we believe the acceptance of the Manitoba cyclotron to be between 15° and 20 ° . The difference represents a 50% degradation in performance due to the spread in the beam optical path length arising from the six-dimensional phase space area of the injected beam, space charge, additional chromatic aberrations due to bunching, and slow spatial variation of the electric field at the buncher gaps. A further reduction of the diameter-tolength ratio for the buncher elements will reduce the last effect. At present, this ratio is unity. We estimate these effects to be of comparable magnitude. The authors wish to acknowledge the suggestion for the passive frequency doubler by F. Konopasek.
References 1) R. H. Batten et al., University of Manitoba Cyclotron Annual Report 1974-1975, Internal Report ;/75-27. 2) G. l l a u s h a h n el al., 7th Int. Conf. on (~Fclotrons and th(qr
application,s. * T h e cause of this i m p r o v e m e n t over the previous b u n c h e r has not been investigated.
3) C. Goldstein and A. Laisne, Nucl. [nstr, and Meth. 61 (1968) 221. 4) R. Perry, A N L AI) 74 (February 1963).
INSTRUMENFS
AND
METII()[)S
145, (19771
403-4(14 : ©
Nt)RTIt-ttOI.I. AND
PUBLISttlNG
CO
LETTERS TO TIlE EDITOR AN I M P R O V E M E N T T O T H E BEAM BUNCHER OF T H E UNIVERSITY OF M A N I T O B A C Y C L O T R O N ROBERT POGSON and S A E W O O N G 01t
(ivHotr.n l.aboratoo. D~7~artmcnt ¢?/ Plly.sics, L.nivcrsity o~ Mantmba. 147nnipeg, .'~lanitoba, ('anada R312:\:2 Received 18 April 1977 The extracted beam from the Manitoba cyclotron has increased by nearly 40% through the addition of a second harmonic component It) the sine-wave previously applied to the beam buncher.
Since the conversion of the cyclotron from internal to external injection in January 1975 a twogap beam buncher driven sinusoidally at the cyclotron frequency has been used to increase extracted currents by a factor of 5 ~). Other installations have used synthesized approximations to the ideal sawtooth waveform to improve buncher effectiveness2-4). Our studies indicated that we could obtain a significant improvement by adding a second harmonic component and we have successfully done so. The ideal effectiveness of some synthesized wavetbrms is shown in fig. 1. The programme which generated these curves maximizes the accepted beam by varying the coefficients of a linear combination of sinewaves representing velocity modulation voltages. All degradations of bunching action present in a real system were neglected. CONTROL ROOM
CYCLOTRON VAULT
pickup t~uncher
i ~
[o't~.uoto~l
Fig. 1. Percentage of the injected beam delivered to the inflector within a given phase acceptance window for various waveforms. T h e curves are labelled with the n u m b e r of h a r m o n i c s included. No b u n c h i n g is indicated by O.
lons which arrived at the injection point in a given time interval were assumed to be accepted. These assumptions should introduce little error in the comparison of various waveforms because any degrading lhctors will likely be independent of waveform, t{owever the comparison between dc beams and bunched beams will likely be overly optimistic. To add the second harmonic c o m p o n e n t , the previous buncher system was modified with as little change as possible to obtain the system shown in fig. 2. The driven element has been divided into two pieces driven in push-pull mode at the second harmonic and in parallel mode at the fundamental frequency. This circuit permits the two signals to be completely separated for control with a m i n i m u m of electronics without increasing the length of the beam buncher assembly or by adding a second buncher. An antenna picks up some power from the final rf stage driving the dees. This signal is carried to the control room by coaxial cable and passes through an amplifier and variable delay which is used to regulate the amplitude and phase of the composite waveform. One output of this amplifier passes through a variable attenuator and returns to the vault to drive the buncher at the fundamental frequency. The other output is applied to a passive frequency doubler followed by a variable delay to generate the second harmonic and to control its phase with respect to the fundamental. A redundant variable attenuator is used on the second harmonic signal sent to the buncher as a convenience. The coupling circuits were all carefully tuned for resonance and adjusted for impedance matching with the buncher in place to minimize the power needed to operate the system. With our 11 keV
404
R
POGSON
O
AND S
Otl
TABLE 1 5
Extracted proton beam currents with various b u n c h e r waveforms.
3
C)
Radius Energy Currents: (a) Buncher off (b) First harmonic (c) First and second harmonics
>-
ta_ t.L
PHASE ACCEPTANCE (DEGREES) Fig. 2. Schematic of the new beam buncher system.
H beam and a distance of 1.7 m from the bunch° er to the inflector we computed that the peak voltages needed for bunching are 86 V at 28.48 MHz and 22 V at 56.96 MHz. With reasonable Q factors this requires very little power and by increasing the pickup from the dees the system could probably be operated without an amplifier. T h e observed performance has been pleasing. When the first harmonic is applied we obtain a factor of six increase in extracted beam compared with dc injection*. When both frequencies are applied and amplitudes adjusted empirically for best results extracted beam is increased by a factor of eight compared with dc injection. Table 1 lists some performance data. Note that the bunching effectiveness seems to be less for higher currents and greater for higher energies. In light of this performance we contemplate the inclusion of a third harmonic component to obtain a further increase of at least 10% in extracted beam current. The third harmonic tank circuit could be placed in the ground return path of the fundamental frequency. The great difference in
Case 1
Case 2
37 cm 26 MeV
50 cm 44 MeV
0.65,uA 3.7 uA
3.25 nA 17.0 nA
5.0/IA
37.5 nA
frequency permits this to be done with little interaction. The observed effectiveness of the new beam buncher is about the same as the ideal beam buncher of fig. 1 with a phase acceptance of 30 °. However we believe the acceptance of the Manitoba cyclotron to be between 15° and 20 ° . The difference represents a 50% degradation in performance due to the spread in the beam optical path length arising from the six-dimensional phase space area of the injected beam, space charge, additional chromatic aberrations due to bunching, and slow spatial variation of the electric field at the buncher gaps. A further reduction of the diameter-tolength ratio for the buncher elements will reduce the last effect. At present, this ratio is unity. We estimate these effects to be of comparable magnitude. The authors wish to acknowledge the suggestion for the passive frequency doubler by F. Konopasek.
References 1) R. H. Batten et al., University of Manitoba Cyclotron Annual Report 1974-1975, Internal Report ;/75-27. 2) G. l l a u s h a h n el al., 7th Int. Conf. on (~Fclotrons and th(qr
application,s. * T h e cause of this i m p r o v e m e n t over the previous b u n c h e r has not been investigated.
3) C. Goldstein and A. Laisne, Nucl. [nstr, and Meth. 61 (1968) 221. 4) R. Perry, A N L AI) 74 (February 1963).