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On the duty cycle of cyclotrons
NUCLEAR INSTRUMENTS AND METHODS 18,19 {1962} 610-612; NORTH-HOLLAND PUBLISHING CO.
ON THE DUTY CYCLE OF CYCLOTRONS A.!. YAVIN Israel Atomic Energy Commission Laboratories alld the Hebrew University
The poor (1-6 %) duty cycle of present day cyclotrons is mainly due to: I. r.f. extraction from the ion source; 2. r.I. defocusing caused by part of the r.f, phase; 3. loss of phase due to relativistic effects; 4. phase bunching at small radii; and 5. inefficient extractors. Po ssible methods of improvement
are: 1. d.c. extraction from the ion source; 2. introduction of focusing grids; 3. introduction of isochronous fields; 4. squaring of the r.f, wave form; and 5. Mobley magnets and other debunchers.
Duty cycle, that is, the ratio of the time the beam is on to the total time, has always been found to be within 1-6 %1) when measured for various conventional cyclotrons. An attempt to explain qualitatively the reasons for this small value is presented in the following considerations: 1. R.f. extraction of the ions from the source: The voltage on the dee opposite the slit in the ion source is shown in fig. 1. Ions can be extracted only during the negative half cycle (section AD). In practice, the difficulty of penetration of the electric field into the ion source limits .this period even further. 2. Electrical focusing: Since there is practically no axial magnetic focusing in the first few turns,
3. Loss of phase due to relativistic effects: The magnetic field in a conventional cyclotron decreases with the radius to provide axial focusing, resulting in a decreasing circulating frequency of the ions. The mass of the ions increases relativistically with energy, further reducing the circulating frequency. Beyond a certain radius, corresponding to a certain energy, acceleration no longer occurs. The beam is generally extracted at a slightly smaller radius in order to achieve maximum energy from the cyclotron. Since the phase lag decreases when the accelerating voltage increases. only ions which cross the gap close to the peak of the r.f. voltage (above Vth ) reach the extraction radius, thus limiting the usable phase to section B'C'. 4. Phase bunching: B.L. Cohen-) has shown that phase bunching occurs mainly in the first few turns when the ions are under the influence of the fringing electric field of the dees for a major fraction of every turn. This effect limits the duty cycle still further. 5. Extraction: The septrum of the electrostatic deflector usually absorbs part of the beam which reaches the extraction radius. This effect is enhanced by the asymmetric shape of the ion bunches, caused by space charge"), The use of magnetic
E Ime
Vlh
~---->.---l'---JI
.vmcll------'~..;
B
Fig. I . Time behavior of the voltage on the extracting dec.
focusing is provided only by the electric field between the dees. The ions which cross the gap are focused towards the median plane in the first half of the crossing, and are defocused in the second half. Thus there is net focusing only if whilec;ossing the gap the field is decreasing; this limits the duty cycle to 25 % (section B'D). 610
1} A. Tinter et al., Nucl. In str, and Meth. 12 (1961) 138. S . D. Bloom, Phys, Rev. 98 (1955) 233. S. D. Bloom et al., Phys. Rev. 99 (1955) 654; N. S. \Vall, private communication; R. So Livingston. private communication. 2) B. L. Cohen, Rev. Sci. Instr. 24 (1953) 589. B. L. Cohen, Handbuch der Physik 44 (1959) lOS. 3} T . A. \Velton, Sector Focused Cyclotron NAS-NRC 656 (1959) 192.
ON THE DUTY CYCLE OF CYCLOTRONS
611
6. A magnetic storage ring with a scheme for resonance extraction methods may also limit the duty cycle"), Instabilities in cyclotron parameters, stochastic extraction may yield upto 100 % duty and especially poor regulation of the r.f. amplitude, cycle. 7. Additional methods, such as the use of Mobley usually introduce low frequency modulation of the circulating as well as of the extracted beam, thus magnets or of long paths for the extracted beam as well as lineacs for debunching, were proposed further reducing the effective duty cycle. There are several ways of improving the duty during a short discussion which was held at the Sea Island Conference"), cycle: A great deal of effort might profitably be invested 1. Two ion sources might be introduced, one on . each side of the geometric center of the cyclotron, in improving the duty cycle of the sector focused each providing ions during alternate half periods cyclotron, the low range of which now seems to be the main shortcoming of the cyclotron as compared of the r.f. cycle. 2. Focusing grids can be inserted at small radii to the Van de Graaff. A low duty cycle badly limits between the dees to provide electrical focusing even counter experiments and may even prohibit some while the voltage isincreasing") (sectionAB' of fig.1). coincidence experiments. 3. A large d.c. component can be added to the voltage which extracts the ions from the source'']. Note added in proof: A brief consideration shows This will diminish the number of turns spent close that this merely doubles the intensity of the beam to the center, thus avoiding most of the region of without altering the duty cycle. phase bunching. d.c. extraction will also help in 4) H. G. Blosser, private communication. bringing the ions quickly to the region of the sector 5) H. G. Blosser and F. Irwin, Bull. Am. Phys. Soc. II, 3 ocusing, especially if fthe magnetic gap is made (1958) 180; small. A. H. Morton and 'V. I. B. Smith, Nucl, Instr. and l\Ieth. 4. The use of isochronous fields helps reduce the 4 (1959) 36; J. S. Allen et al., Rev. Sci. Instr. 31 (1960) 813. threshold voltage, thus bringing point C closer to D 6) U. Vorgel, private communication. (fig. 1). 7) G. B. Rossi, Cyclotron wave r.f. system, U.S. patent 5. The introduction of a third harmonic to the No.2 778 937; C. D. Goodman, Oak Ridge National Laboratories Report d. wave form 7) to flatten the top of the d. voltage ORNL-2403 (unpublished). would increase the duty cycle. 8) R. 1\1. Eisberg, NAS-NRC 656 (1959) 289.
DISCUSSION
Speaker addressed: A. I. Yavin (Israel) Comment by H. G. Blosser (llISU): It seems to me that focusing grids usually restrict the phase interval, rather than stretching it; they actually chop out phase and, I think, do more harm than good. It would seem that the duty cycle should approach 50 % if three cyclotron improvements, discussed at this meeting, were made: (I) make it very carefully isochronous; (2) inject the ions into it, as at Birmingham; and (3) use negative ions to overcome the extraction difficulty, as at Boulder. I think those eliminate all of the factors which you have listed. Question by N. F. Verster (Philips): Are you worried about the duty cycle because it limits the actual intensity you can use in the experiments? A nstoer: The Van de Graaff provides a continuous beam, the cyclotron an r.f.spulsed beam which must have much higher
peaks to provide the same average intensity. If instantaneous intensity is the limiting factor, the average intensity of the cyclotron beam must be much smaller.
Question by N. F. Verster ; How much do you estimate this to be in practice, for practical experiments? Anstoer: As I remember, Professor Schmidt remarked at Sea Island that in Seattle they use no more than a 10 to 100 millimicroamperes; this is all that is needed. But lately, especially for physicists interest in all kinds of polarization experiments, the need is for as much beam as the cyclotron can put out. Comment by J. L. Need (NASA): We are doing polarization experiments but we don't get anywhere near a megacycle counting rate in any of our counters..'Ve don't even approach one count per pulse. I don't think it is a problem. l'oices from the floor, ill wlison: Oh, no! That's "Tong I X. DIVERSE TOPICS
612
A, I. YAVIK
Comment by F . H. Schmidt (Seattle) : If you wish to do a coincidence experiment. then you simply cannot do it with a cyclotron at a rate more than one-tenth as fast as you can with a Van de Graaff. If the beam pulse is short compared with practical coincident circuits. then it does not matter how short it is, Today. for practical length of coincident circuits. something like 10 milli-microscconds to avoid transit-time problems.
the rate of gathering data is increased by a factor of fi .... e immediately. if you shift from an ordinary "obsolete" cyclotron to a Van de Graaff, Moreo vcr, all the beam pulses are just not there in any cyclotron that we know of; they are missing-this is a serious question which has not been di scussed at all in this meeting,
ON THE DUTY CYCLE OF CYCLOTRONS A.!. YAVIN Israel Atomic Energy Commission Laboratories alld the Hebrew University
The poor (1-6 %) duty cycle of present day cyclotrons is mainly due to: I. r.f. extraction from the ion source; 2. r.I. defocusing caused by part of the r.f, phase; 3. loss of phase due to relativistic effects; 4. phase bunching at small radii; and 5. inefficient extractors. Po ssible methods of improvement
are: 1. d.c. extraction from the ion source; 2. introduction of focusing grids; 3. introduction of isochronous fields; 4. squaring of the r.f, wave form; and 5. Mobley magnets and other debunchers.
Duty cycle, that is, the ratio of the time the beam is on to the total time, has always been found to be within 1-6 %1) when measured for various conventional cyclotrons. An attempt to explain qualitatively the reasons for this small value is presented in the following considerations: 1. R.f. extraction of the ions from the source: The voltage on the dee opposite the slit in the ion source is shown in fig. 1. Ions can be extracted only during the negative half cycle (section AD). In practice, the difficulty of penetration of the electric field into the ion source limits .this period even further. 2. Electrical focusing: Since there is practically no axial magnetic focusing in the first few turns,
3. Loss of phase due to relativistic effects: The magnetic field in a conventional cyclotron decreases with the radius to provide axial focusing, resulting in a decreasing circulating frequency of the ions. The mass of the ions increases relativistically with energy, further reducing the circulating frequency. Beyond a certain radius, corresponding to a certain energy, acceleration no longer occurs. The beam is generally extracted at a slightly smaller radius in order to achieve maximum energy from the cyclotron. Since the phase lag decreases when the accelerating voltage increases. only ions which cross the gap close to the peak of the r.f. voltage (above Vth ) reach the extraction radius, thus limiting the usable phase to section B'C'. 4. Phase bunching: B.L. Cohen-) has shown that phase bunching occurs mainly in the first few turns when the ions are under the influence of the fringing electric field of the dees for a major fraction of every turn. This effect limits the duty cycle still further. 5. Extraction: The septrum of the electrostatic deflector usually absorbs part of the beam which reaches the extraction radius. This effect is enhanced by the asymmetric shape of the ion bunches, caused by space charge"), The use of magnetic
E Ime
Vlh
~---->.---l'---JI
.vmcll------'~..;
B
Fig. I . Time behavior of the voltage on the extracting dec.
focusing is provided only by the electric field between the dees. The ions which cross the gap are focused towards the median plane in the first half of the crossing, and are defocused in the second half. Thus there is net focusing only if whilec;ossing the gap the field is decreasing; this limits the duty cycle to 25 % (section B'D). 610
1} A. Tinter et al., Nucl. In str, and Meth. 12 (1961) 138. S . D. Bloom, Phys, Rev. 98 (1955) 233. S. D. Bloom et al., Phys. Rev. 99 (1955) 654; N. S. \Vall, private communication; R. So Livingston. private communication. 2) B. L. Cohen, Rev. Sci. Instr. 24 (1953) 589. B. L. Cohen, Handbuch der Physik 44 (1959) lOS. 3} T . A. \Velton, Sector Focused Cyclotron NAS-NRC 656 (1959) 192.
ON THE DUTY CYCLE OF CYCLOTRONS
611
6. A magnetic storage ring with a scheme for resonance extraction methods may also limit the duty cycle"), Instabilities in cyclotron parameters, stochastic extraction may yield upto 100 % duty and especially poor regulation of the r.f. amplitude, cycle. 7. Additional methods, such as the use of Mobley usually introduce low frequency modulation of the circulating as well as of the extracted beam, thus magnets or of long paths for the extracted beam as well as lineacs for debunching, were proposed further reducing the effective duty cycle. There are several ways of improving the duty during a short discussion which was held at the Sea Island Conference"), cycle: A great deal of effort might profitably be invested 1. Two ion sources might be introduced, one on . each side of the geometric center of the cyclotron, in improving the duty cycle of the sector focused each providing ions during alternate half periods cyclotron, the low range of which now seems to be the main shortcoming of the cyclotron as compared of the r.f. cycle. 2. Focusing grids can be inserted at small radii to the Van de Graaff. A low duty cycle badly limits between the dees to provide electrical focusing even counter experiments and may even prohibit some while the voltage isincreasing") (sectionAB' of fig.1). coincidence experiments. 3. A large d.c. component can be added to the voltage which extracts the ions from the source'']. Note added in proof: A brief consideration shows This will diminish the number of turns spent close that this merely doubles the intensity of the beam to the center, thus avoiding most of the region of without altering the duty cycle. phase bunching. d.c. extraction will also help in 4) H. G. Blosser, private communication. bringing the ions quickly to the region of the sector 5) H. G. Blosser and F. Irwin, Bull. Am. Phys. Soc. II, 3 ocusing, especially if fthe magnetic gap is made (1958) 180; small. A. H. Morton and 'V. I. B. Smith, Nucl, Instr. and l\Ieth. 4. The use of isochronous fields helps reduce the 4 (1959) 36; J. S. Allen et al., Rev. Sci. Instr. 31 (1960) 813. threshold voltage, thus bringing point C closer to D 6) U. Vorgel, private communication. (fig. 1). 7) G. B. Rossi, Cyclotron wave r.f. system, U.S. patent 5. The introduction of a third harmonic to the No.2 778 937; C. D. Goodman, Oak Ridge National Laboratories Report d. wave form 7) to flatten the top of the d. voltage ORNL-2403 (unpublished). would increase the duty cycle. 8) R. 1\1. Eisberg, NAS-NRC 656 (1959) 289.
DISCUSSION
Speaker addressed: A. I. Yavin (Israel) Comment by H. G. Blosser (llISU): It seems to me that focusing grids usually restrict the phase interval, rather than stretching it; they actually chop out phase and, I think, do more harm than good. It would seem that the duty cycle should approach 50 % if three cyclotron improvements, discussed at this meeting, were made: (I) make it very carefully isochronous; (2) inject the ions into it, as at Birmingham; and (3) use negative ions to overcome the extraction difficulty, as at Boulder. I think those eliminate all of the factors which you have listed. Question by N. F. Verster (Philips): Are you worried about the duty cycle because it limits the actual intensity you can use in the experiments? A nstoer: The Van de Graaff provides a continuous beam, the cyclotron an r.f.spulsed beam which must have much higher
peaks to provide the same average intensity. If instantaneous intensity is the limiting factor, the average intensity of the cyclotron beam must be much smaller.
Question by N. F. Verster ; How much do you estimate this to be in practice, for practical experiments? Anstoer: As I remember, Professor Schmidt remarked at Sea Island that in Seattle they use no more than a 10 to 100 millimicroamperes; this is all that is needed. But lately, especially for physicists interest in all kinds of polarization experiments, the need is for as much beam as the cyclotron can put out. Comment by J. L. Need (NASA): We are doing polarization experiments but we don't get anywhere near a megacycle counting rate in any of our counters..'Ve don't even approach one count per pulse. I don't think it is a problem. l'oices from the floor, ill wlison: Oh, no! That's "Tong I X. DIVERSE TOPICS
612
A, I. YAVIK
Comment by F . H. Schmidt (Seattle) : If you wish to do a coincidence experiment. then you simply cannot do it with a cyclotron at a rate more than one-tenth as fast as you can with a Van de Graaff. If the beam pulse is short compared with practical coincident circuits. then it does not matter how short it is, Today. for practical length of coincident circuits. something like 10 milli-microscconds to avoid transit-time problems.
the rate of gathering data is increased by a factor of fi .... e immediately. if you shift from an ordinary "obsolete" cyclotron to a Van de Graaff, Moreo vcr, all the beam pulses are just not there in any cyclotron that we know of; they are missing-this is a serious question which has not been di scussed at all in this meeting,