Proceedings of the International Conference on Isochronous Cyclotrons, Gatlinburg, Tennessee, 1966
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
MACHINE DEVELOPMENT AT THE BERKELEY
August
88-INCHCYCLOTRON
R. Burger, D. J. Clark, E. Close, and H. Kim
Lawrence Radiation Laboratory
University of California
Berkeley, California
May 1966
Abstract
Machine development at the 88-inch cyclotron is
described. Topics include instrumentation, deflector development, a radioactivity survey of the dee
tank, and computer calculations on the gap-crossing
resonance. Studies were made of external beam energy spread and pulse shape. Axial injection progress is described. Recommendations are made for
design improvements on the 88-inch cyclotron.
General Development
Operation and machine development at the 88inch cyclotron has been described previously. l,2,3
Machine settings for new particle energies are
being developed on a regular basis. The beams now
available include protons from l0 to 55 MeV, a-particles from 15 to 130 MeV and He3 from l0 to 145
MeV. Third harmonic acceleration is used for aparticles under 24 MeV and He3 under 18 MeV. This
mode runs well even though we do not use a dummy
dee. Some high intensity runs showed that 5 rnA of
protons could be accelerated to a 5 in. radius, and
the estimate is that about 3 mA would have reached
full energy of 25 MeV if allowed to do so. This is
in reasonable agreement with the calculated space
charge limit.
Some modifications have been made to the machine and external beam line recently, as shown in
Fig. l. The three 120 degree probes are still
available, and are useful for beam centering when
first trying new modes of operation, such as third
harmonic acceleration. The internal phase probe is
no longer used. A radial probe line has been added
at the "target probe" port to make "@R" measurements on the internal beam quality.4 The new deflector, now in the beam testing stage,5 is shown
in place. Two TV cameras are used to view the ion
source and deflector, and prove very useful to
monitor source and puller problems, and beam distribution on the deflector septum. On the external
beam line a radial steering magnet, and analyzing
slits forming an object for the switching magnet
have been added recently.
Before beginning testing on the new deflection
system, the old system was optimized over a period
of several years. The best figure for deflector
transmission is 70% for l20 MeV a-particles. The
average transmission is about 50% over all beams.
There is usually about 10% loss at the coupling
resonance just before deflection. Some septum
development has been done to maximize its power
handling, since high beam intensities are required
for many of the several hundred isotope production
runs per year. The best septum design thus far
F-009
has been radiation-cooled 0.010 in. tungsten sheet
with a 6 in. long tapered slot to match the beam
profile. This design provides as much external
beam current as several water-cooled copper designs which were tried, and is cheaper, less susceptible to damage, and more easily changed. The
present external beam power available is about 3
kW, e.g., 150 pA of 20 MeV protons, with about
3 kW more dropped on the septum. Some studies are
in progress on a possible water-cooled tungsten
septum.
A radioactivity survey was made of the acceleration region in Nov. 1965 after a nine day shutdown,
to determine the amount of activity which had built
up during about 2 years of running with large beam
intensities. These measurements are shown in Fig.
2. The deflector and moving probes were removed.
The general f3y level is about 0.5 ~ / h rin the inner regions. However most work on this area can
be done remotely from a location where the activity is about l00 mR/hr or less. Some hot spots
appeared on the "C probe", which shields the dee
from beam, and near the septum, where some material has been evaporated or sputtered. Comparison
of these measurements with some made nine days
earlier, showed that the activity had decayed
about 30$ during this time.
Some computer calculations have been made on
the problem of the gap-crossing resonance pointed
out by
This effect is equivalent to a
first harmonic in the magnetic field, and is produced by a "beat" of the 2-fold acceleration gap
geometry against the three fold magnetic sector
field of the 88-inch and many other cyclotrons.
Figure 3 shows orbit center paths for accelerated
beams with several starting phases. The gap crossing resonance causes the precession center to be off
the magnetic center of the machine by about 0.2 in.
With the starting conditions of Fig. 3(a), a center
spread.of about 0.2 in. is introduced into a beam
with 50 degrees of phase spread. In Fig. 3(b),
the particle starting in phase (@O = 0) was placed
to approach the precession center at large radius.
In this case the other phases diverged because of
being too far off the accelerating gap, giving
somewhat worse center spread than Fig. 3(a).
This
resonance appears to constitute a limitation on
beam quality in this cyclotron, when a wide phase
width is accelerated. Some possible remedies
which are being studied are: use of the inner
harmonic coils to compensate the effective error
first harmonic, and displacement of the dee gap
to coincide with the precession center.
or don.^
c IEEE 1966
Reprinted from IEEE Transactions on Nuclear Science NS-13 (4), Aug 1966 Proceedings of the International Conference on Isochronous Cyclotrons, Gatlinburg, Tennessee, 1966
BURGER, ET AL:
MACHINE DEVELOPMENT AT BERKELEY CYCLOTRON
Axial Injection Progress
External Beam Studies
Measurements have been made of the energy and
time distributions of the full external beam of
the cyclotron. To measure the energy, lithiumdrifted silicon detectors with associated electronics* were used. A gold foil was used on the
beam line straight through the switching magnet,
to scatter the beam 20 degrees into the detector.
A pulse-height analyzer was used to display the
spectrum. The contribution of detector and electronics to the energy spread was less than 10$.
The time distribution of the beam was measured
with a semi-conductor detector7 placed directly in
the bean on the same beam line.
The energy spectrum with the best cyclotron
tuning is shown in Fig. 4. The full width at half
maximum is 140 kV, or 0.22%) after correcting for
detector and electronics. The energy spectrum for
average cyclotron tuning is shown in Fig. 5(a).
The spread is 0.4% FWHM. This is about equal to
the energy gain per turn in the cyclotron. If the
cyclotron is detuned, by changing the dee voltage
by several hundred volts for example, a spectrum
such as that of Fig. 5(b) can occur. The two energy peaks are believed to result from misalignment
of the beam with the deflection channel, giving extraction from several precession cycles.
The beam time structure is shown in Fig. 6(a)
for average cyclotron tuning, corresponding to the
energy spectrum of Fig. 5(a).
The pulse width is
30 degrees FWHM. For a detuned beam, corresponding
to the energy spectrum of Fig. 5(b), the doublepeak time structure is shown in Fig. 6(b).
The
phase width is 45 degrees. Sometimes three peaks
are observed for detuned beams.
Some studies have been made on the sensitivity
of the beam to instability of various machine parameters. Figure 7 shows the effect of dee voltage ripple on the energy spread of the external
beam, for the condition of best tuning. Ripple
was injected, and the cyclotron optimized for each
measurement. The result shows that ripple should
be kept less than 1%. Other measurements for cases
other than optimum, and on beam optics for tightly
collimated beams, indicate that the dee voltage
regulation should be 0.1%. The stability of the
dee frequency and the main magnetic field, including trim coil contributions, should be 0.002$.
The emittance of the external beam is about as
reported previously,2)3 50 mm m r radially and 70
mm m r axially, including some measurement errors.
The virtual sources are as reported.3 With the
small steering magnet added before the first quadrupole, all radial virtual sources can now be
placed on the beam pipe center line.
Y
Supplied by the Lawrence Radiation Laboratory
Nuclear Chemistry Instrumentation Group under
F. S. Goulding.
During the past year axial injection studies
for the 88-inch cyclotron have been in progress
for use with a future polarized ion source. A
bench test was set up, consisting of an ion source,
a 5 foot long transport tube with electrostatic
quadrupole focusing, and a probe for current measurement. Tests with this system served to test
the initial components of an injecbion system, and
helped to solve some of the problems of ion source
optimization and quadrupole voltage supply leads.
A test was then arranged to inject beam into the
cyclotron through the upper pole. This was a preliminary test to study the problems and efficiency
of the various components of the system. The normal ion source was removed for the test and replaced
afterward.
8. The ior?
The system tested is shown in Fi
source is a modified duo-plasmatron." Three electric quadrupole doublets transport the beam down
to the cyclotron center. The inflector is an electric mirror beneath a grounded grid, similar to
the one used by ~ i r m i n ~ h a m Frames
.~
were inserted
in the dee and d m y dee to give the initial particle revolutions enough energy gain to clear the
inflector. The frames did not have the grid wires
used at Birmingham. Injection was on center through
a one inch hole in a temporary eight inch diameter
iron pole plug. The injection voltage was 15 kV
and the dee voltage was 60 kV. This gives a ratio
of 411 compared to the ideal ratio of 511.9 This
caused the initial orbits to be off center about
114 inch, and full power in the inner harmonic
coils (1'3 gauss) had to be used to bring the beam
back on center again. The beam intensities obtained were: 400 !A at the top Faraday cup, 130
p!A at the inflector, 10-20
accelerated to 4 in.
extracted of 30 MeV protons. The relradius, 1
atively poor transmission through the acceleration
region and the deflector is due to poor beam quality caused by the off-center initial orbits.
The results of this test showed the weak points
of the system as presently developed. The ion
source will be modified to allow injection at energies of 10-15 kV. Studies will be made of optics
problems in the quadrupoles and inflector region.
Voltage holding problems of the inflector in the
higher range of magnetic fields will be investigated.
Desian Recommendations for
Future 88-Inch Cyclotrons
The 88-inch cyclotron has proved to be a very
sound design. It has produced external beams of
the intensity and maximum energy predicted by its
designers. Machine settings are repeatable after
intervals of months or years. Energy is easily
variable. The 8 inch pole plugs have proved very
X
Provided by 90-inch cyclotron group, Lawrence
Radiation Laboratory, Livermore.
c IEEE 1966
Reprinted from IEEE Transactions on Nuclear Science NS-13 (4), Aug 1966 F-009
Proceedings of the International Conference on Isochronous Cyclotrons, Gatlinburg, Tennessee, 1966
3 66
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
valuable, both for accurate positioning of the ion
source, and for easy conversion to an axial injection system.
In the light of several years of operational
experience, there are some design modifications
which one would make if the machine were built
again, most of which have been or will be made on
the present machine. The stability requirements on
frequency, magnetic field, and dee voltage were
mentioned in Sect. 2. Careful study should be made
of the gap-crossing resonance, to minimize or cancel it out with center geometry or harmonic coils,
since it appears to limit the ultimate quality of
the internal beam with wide phase width. A large
diffusion pump on the dee tank would ease the vacuum problems arising if a regenerator is used, or
heavy ions are to be accelerated. The ion source
could be put in from the bottom, leaving the top
of the magnet free for axial injection equipment.
A radial AR probe other than the usual high current probe used for measuring internal beam intensity (dee probe) is useful for optimizing internal
beam quality. The original deflector design works
very well, and we don't know yet how much improvement is possible with the new deflector. A radially
focusing channel after the deflector would be helpful in bringing the beam out with small divergence.
Radial and vertical steering of the external beam
as it emerges from the cyclotron are useful for
beam optics alignment.
Larger changes involving the rf system are the
following. The vertical focusing built into the
magnet is adequate for 70-75 MeV protons. The extraction system could extract this energy. So if
the rf system went up to about 18.5 MHz, the cyclotron would be capable of producing this energy. A
more difficult modification is that of adding some
third harmonic to the rf voltage to give a "flattop" on the rf wave. This could be done either by
injecting the harmonic on the present dee, or by
using a small additional dee or dees for the third
harmonic. As suggested by others previously, this
would combine the virtues of good beam quality with
wide phase width, a significant step toward the
"ideal cyclotronu.
The question of depolarization of polarized
ions during acceleration is one of interest for
future designers. Recently some calculations were
done by Baumgartner and ~ i m l Ousing the actual operating magnetic fields for 10 and 55 MeV protons,
and 65 MeV deuterons in the 88-inch cyclotron.
The results show that the proton depolarization is
F-009
August
less than 0.1%) and the deuteron depolarization is
less than 1%. Thus there seems to be no problem
in accelerating these polarized ions in the 88-inch
cyclotron.
Acknowledgments
The authors wish to thank many people for help
in this work: D. Elo, D. Morris, A. Hartwig, J.
Meneghetti, D. Pepperell, D. Perkins for mechanical
engineering and fabrication; K. Ehlers, S. Bloom,
and E. Zaharis for ion source assistance; P.
Pellissier, P. Frazier, and M. Renkas for electrical engineering; F. Goulding, D. Landis, and
R. Lothrop for help with detectors and electronics;
A. Garren, H. Owens, and A. Kenney for computer
program assistance; John Bowen and the crew for
much cooperation during machine runs; H. Grunder
and F. Selph for informative discussions and assistance on some machine runs; H. Conzett, B.
Harvey, and E. Goldberg for help and encouragement in this program.
References
E. Kelley, UCLA Conf. on Sector-Focused Cyclotrons, (1962)) p. 33.
CERN Conf. on Sector-Focused Cyclotrons, (1963)
H. A. Grunder and F. B. Selph, p. 8; H. A.
Grunder, F. B. Selph, H. Atterling, p. 59.
Hermann Grunder and Frank Selph, uCRL-11477
(1964).
D. J. Clark, Beam Diagnostics and Instrumentation, Gatlinburg Conf. on Isochronous Cyclotrons, (1966).
D. J. Clark, E. Close, H. A. Grunder, Hogil
Kim, P. Pellissier, B. H. Smith, Regenerative
Beam Extraction for the Berkeley 88-Inch Cyclotron, Gatlinburg Conf. on Isochronous Cyclotrons, (1966).
M. M. Gordon, UCLA Conf. on Sector-Focused
Cyclotrons, (1962)) p. 268.
H. E. Conzett, L. B. Robinson, R. N. Burger,
Nucl. Instr. Methods 31, 109 (1964).
W. B. Powell and B. ~ T ~ e e c eNucl.
,
Instr.
Methods 32, 325 (1965).
D. J. ~ l G k ,Rutherford Laboratory, Harwell,
Cyclotron Group Design Note CDN-500-05-024
(1962)
E. Baumgartner and H. Kim, UCRL-16787, (1966).
-
c IEEE 1966
Reprinted from IEEE Transactions on Nuclear Science NS-13 (4), Aug 1966 Proceedings of the International Conference on Isochronous Cyclotrons, Gatlinburg, Tennessee, 1966
BURGER, E T AL:
MACHINE DEVELOPMENT AT BERKELEY CYCLOTRON
Fig. 1. Plan view of 88-inch cyclotron
vacuum t,ank and external beam line.
F i g . 2. Radioactivity survey of 88-inch cyclotron
t,ank nine days aft.er shutdown. The circled figures
show t~he fly act.ivity i n
hr. The deflector and
moving probes were removed
from the t,ank.
c IEEE 1966
Reprinted from IEEE Transactions on Nuclear Science NS-13 (4), Aug 1966 F-009
Proceedings of the International Conference on Isochronous Cyclotrons, Gatlinburg, Tennessee, 1966
August
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
Toward
400
~~~
14.6"
-
0
0.1
C
-
c
a
-0.2 -0.1
0
0.1
Distance
south
[in)
2
l
O
A:
-
loo-
U
Fig. 3. Motion of orbit. centers f o r 65 MeV
:c part,icles c a l c ~ l a t ~ ewith
d
the general
orbit code. Several start,ing phases are
shown. Aposit~ivephase means the part~icle
lags behind t,he rf. (a) st,art,ing posit,ion
adjust.ed t o minimize cent,er spread t o about,
0.2 inches. (b) start.ing position ad just.ed
t,o bring the QO = 00 p a r t i c l e t.0 t,he precession center a t 15 inches radius. Center
spread i s about 0.3 inches.
-
- 6 5 - ~ e ~beam
a
A € = 150 kV
80
I
I
I
90
100
110
120
E n e r g y (channel number)
Fig. b. F u l l external beam energy specttrum
wit,h best tuning condit,ions. 6SMeV &particles.
506 ext,ract,ion efficiency. AE = 140 kV = 0.228
FWHM, a f t e r correc%ing f o r e l e ~ t ~ r o n i cspread.
s
6 5 - MeV a beam
R u n a.
'OoO-
-
AEa=240kV
500 -
0
C
c
c
O
:
0
. A " " " " 1
1500
:, , ,
320
a
m
+
330
340
350
360
,,, , , ,, , , , ,, , , , , , , , , , , ,
C
0
3
6 5 - M e V a beam
Run b.
U
1000 -
AE
= 8 2 0 kV
500 -
0
320
330
Energy
340
350
360
(channel number)
Fig. 5(a). Typical energy spectrum for 65 MeV
r?. ~ a r t ~ i c l e s .
Fig. 5(b). E n e r ~ yspectrum for 6 5 f i ~ e0
~
particles. Cyclotron detuned by changing dee
voltage from optimum.
F-009
c IEEE 1966
Reprinted from IEEE Transactions on Nuclear Science NS-13 (4), Aug 1966 Proceedings of the International Conference on Isochronous Cyclotrons, Gatlinburg, Tennessee, 1966
1966
BURGER, E T A L :
MACHINE DEVELOPMENT A T BERKELEY CYCLOTRON
Fig. 6(b).
Ekternal beam pulse wit,h same
tuning as Fig. S(b). Phase width
h5
degrees FWHH.
Fig. 6(a). Typical ext.erna1 beam pulse on
sampling oscilloscope. Beam tuning same as
Fig. S(a). Phase width = 30 degrees FWHM.
'0
2
369
4
6
Dee voltage ripple,
8
P-P
0-P
1
0
('10)
Fig. 7. Energy spread of 65 MeV 0 , p a r t . i c l e
beam a s a function of dee voltage ripple.
The energy spread i s f u l l widt.h at, h a l f maxmum. The peak-peak r i p p l e i s divfded by zeropeak dee volt,age.
c IEEE 1966
Reprinted from IEEE Transactions on Nuclear Science NS-13 (4), Aug 1966 F-009
Proceedings of the International Conference on Isochronous Cyclotrons, Gatlinburg, Tennessee, 1966
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
August
DISCUSSION
LIVINGSTON: What would be a r e a s o n a b l e p e r f o r m a n c e of the a x i a l injection s y s t e m , a f t e r
everything i s o p t i m i z e d ?
CLARK: Ultimately, we hope to r e a c h the B i r m i n g h a m p e r f o r m a n c e , a s f a r a s efficiency goes.
F o r b e a m i n t e n s i t y , I think 100 pA of i n t e r n a l
b e a m would b e r e a s o n a b l e . The l i m i t i n g f a c t o r
m a y be d a m a g e to t h e i n f l e c t o r g r i d , which i n t e r c e p t s s o m e 20% of the b e a m .
PEEK: You w e r e going to r e c o m m e n d s o m e
design features f o r future cyclotrons.
CLARK: Yes. T h i s g a p - c r o s s i n g r e s o n a n c e
s e e m s t o b e a l i m i t a t i o n o n the u l t i m a t e i n t e r n a l
b e a m quality. I think, i n building a new m a c h i n e ,
one should t a k e a c a r e f u l look a t t h a t , possibly
look into optimizing i t with r e s p e c t to p o l a r r o t a tion, r e l a t i v e t o the d e e gap. T h i s effect could
b e m i n i m i z e d t h a t way; a n a l t e r n a t i v e would be
using f o u r s e c t o r s i n s t e a d of t h r e e .
HOLMGREN: Would the i n t e r n a l p h a s e width of
the b e a m b e changed a n y with a n a x i a l injection
s y s t e m ? Can you g e t any l a r g e r i n t e r n a l phase
width?
CLARK: We did not m e a s u r e the p h a s e width,
but one would e x p e c t p h a s e widths up t o 9 0 ° , a s
B i l l P o w e l l mentioned. O u r n o r m a l p h a s e width
i s a b o u t 60" i n t e r n a l ; on e x t r a c t i o n t h i s r e d u c e s
t o 30 to 45'.
I would e x p e c t a n i n c r e a s e of, say,
up to 90" i n t e r n a l .
HOLMGREN: How m u c h of t h i s l o s s of t h e
e x t r a c t e d b e a m would you a t t r i b u t e to t h e s p a c e
width t h a t you m a y have i n t r o d u c e d i n the b e a m ?
Your e x t r a c t i o n efficiency i s r e l a t i v e l y low,
r i g h t now. You s a i d you could optirnize t h a t by
changing s o m e of the o t h e r p a r a m e t e r s . But
would you s t i l l l o s e quite a b i t of t h i s b e a m j u s t
b e c a u s e of a l a r g e r s p a c e width, o r can you ext r a c t a f a i r a m o u n t of i t ?
CLARK: A r e you talking about the a c t u a l injection t e s t ?
HOLMGREN:
Yes.
CLARK: T h a t w a s a v e r y rough t e s t ; the b e a m
w a s o f f - c e n t e r quite a b i t f o r t h a t p a r t i c u l a r t e s t .
With b e t t e r v a l u e s of injection e n e r g y , s a y 1 0 o r
1 2 kV, i n s t e a d of the 1 5 kV t h a t we w e r e using, I
would e x p e c t m u c h b e t t e r c e n t e r i n g of the i n t e r n a l b e a m , and a n o r m a l 50% e x t r a c t i o n efficiency.
1
1
I
I
1
I
0
I
10
1
I
I
15
Inches
Fig. 8. Axial injection 8yst.em used for a
preliminary t.est i n the cyclotron. Normal
axial ion source was removed for this test
and replaced afterward.
F-009
VAN KRANENBURG: If you added a d u m m y dee
to y o u r s y s t e m could you i m p r o v e the quality of
the B e r k e l e y c y c l o t r o n ?
CLARK: Detailed s t u d i e s of t h i s w e r e m a d e by
Willax d u r i n g the d e s i g n s t a g e ; h e found t h a t the
b e a m quality w a s a s good without t h e d u m m y d e e
a s with i t , i n o u r p a r t i c u l a r c a s e .
c IEEE 1966
Reprinted from IEEE Transactions on Nuclear Science NS-13 (4), Aug 1966