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TRIUMF
ISIS Buncher Tuning Procedure
Prepared by:
Fred Bach, Rick Baartman
Reviewed by:
Fred Bach
Name
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PURPOSE AND SCOPE
This document describes some modified tuning techniques for optimum bunching
and transmission at the 500MeV Cyclotron ISIS.
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REFERENCED DOCUMENTS

How To Tune the Bz’s From Scratch by F.Bach, Oct 1989

WP5884, WP6274, FR12416 – April 1996
RESPONSIBILITIES
3.1
Overall Responsibility
3.2
Specific Responsibility
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4.1
PROCESSES
Background Information
Bunching Explained
The 500-MeV cyclotron’s Dee-gap voltage is a sine wave of just under 200kV RF at
close to 23.055 MHz. The period (1/f) is about 43 nanoseconds for one cycle (360
degrees) of the RF. For acceleration to take place, the beam must see an attracting or
more positive voltage on the other side of the Dee-gap. The peak of the sine wave in the
accelerating direction is Zero degrees. If we were to inject an unbunched continuous
stream of H- particle at the correct energy and emittance, the cyclotron would only accept
and accelerate particles which entered during an approximate 4-nanosecond time window
.
Particles must be injected while the acceleration voltage is falling; a rising voltage causes
vertical defocusing in the first few turns where there is insufficient vertical magnetic
focusing to counteract this effect. Naturally, if the voltage has fallen too far by the time
the particle in question crosses the accelerating gap, the particle does not gain enough
energy to clear the centre post.
o It is of course inefficent use of the rf power to continue accelerating on the falling
edge; one would like to on average have the bunch centres near the peak of the rf
waveform. For this reason, there is an isochronism error in the central region that
causes the particles to slip in phase once they are through the dangerous region where
there is little to no magnetic focusing from the sectors.
The larger the rf voltage, the more phases manage to miss the centre post; for 100 kV,
cyclotron transmission with a continuous stream (DC) of injected H- particles is 12% .,
for 73 kV, it is zero. The remaining beam Is intercepted by metal surfaces.
At 12% transmission, delivering 280 microamps to the proton beamlines would require
over 2 milliamps of injected H- beam. However, the source and the Injection beamline
cannot deliver that much current. What is needed is better cyclotron transmission. This
transmission can be accomplished by injecting the H- beam only when the cyclotron is
able to accept and accelerate it. To accomplish this, kinetic energy and velocity of the Hbeam is modulated such that, after a drift space, the faster and slower particles in the
beam will drift together into a stream of bunches. The arrival of each bunch at the
injection gap should coincide with the 4-nanosecond acceptance window and into the first
turn in center region of the cyclotron. This technique is called bunching.
The distance travelled by a particle in one rf period is (= speed / speed-of-light * rf
wavelength). For ISIS, this is 12.9 inches, so a particle that is 180 degrees behind the
centre of the acceptance band must travel an extra 6 inches by the time it reaches
injection. Conversely, a particle that is ahead must be slowed down. Since 6 inches is
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1/100 the distance from buncher to injection point, the 180 degree trailing particle must
be given a 1% boost in speed, particles at -180 must by slowed by 1%. All other particles
in between should have a linear relationship between phase and speed increment. So in
this picture, the ideal waveform is a sawtooth wave.
Space charge modifies this picture somewhat; in particular, the repulsive force makes it
very difficult to create short bunches. Further, the shorter the bunch, the strong are the
transverse defocusing space charge forces. Thus, there is a complicated interplay between
beam current, bunching, and focusing. Also, of course central region parameters can
modify and trade off between phase acceptance and transverse acceptance. This is the
reason that buncher tuning is not a set-and-forget proposition.
Unfortunately, it is only easy to give sinusoidal rather than sawtooth velocity
modulations. A sawtooth wave can be thought of as being composed of many harmonics.
Specifically, in a series of multiples n of the fundamental frequency, if the amplitude is
proportional to 1/n, and all are in phase, then the resultant is a sawtooth. In ISIS we use
only 2 harmonics; 23MHz, and 46MHz. These two needn't be applied at the same point
and indeed are separate bunchers. By contrast, in ISAC, there are 3 harmonics, all applied
to the same buncher.
ISIS Buncher
The ISIS bunchers are in roughly the middle of the north-south portion of the horizontal
injection line. They each consist of three co-axial elements through which the beam
passes: a cylinder with one grounded ring near each end of the cylinder. The rings are
gounded and the cylinder is driven with an RF resonant cavity powered by a small RF
transmitter-amplifier phase-locked to the main RF in the cyclotron.
A few kilovolts of RF electric field appear between the ends of the buncher cylinder and
the nearest grounded ring. Depending on the instantaneous direction of this voltage, the
H- particles entering the buncher assembly see either an accelerating or a decelerating
voltage. Once inside the cylinder, the H- particles see no accelerating voltage in any
direction and drift through the cylinder. The cylinder's length is (= 6 inches for
the 23MHZ buncher, 3 inches for the 46MHz buncher) so that by the time the Hparticles get to the other end of the cylinder the RF voltage on the cylinder has changed
by about 180 degrees. Thus the electric field that the H- particles see when they leave the
buncher assembly is in the same direction as when they entered. Particles which were
sped up on the way into the buncher are sped up further on exit whereas particles which
were slowed down going in are further slowed down going out. Particles which saw a
zero voltage going in also see a zero voltage going out and so their energy remains
unchanged.
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Conclusion
With the next revision of the Injection Line scheduled for spring 2009, it may be possible
to add a third buncher at the VRS (Vertical Rollout Section) just above the cyclotron’s
support structure. Due to space-charge effects, this buncher would be used in bunching
mode at low intensity and de-bunching mode at high intensity.
4.2
Assumed Preconditions
1. Buncher operation is hampered by 300-kV noise . Since there are 50 rf periods
between buncher and injection, a 1 part in 18,000 fluctuation in the 300 kV supply
already causes a 1 degree smearing of the buncher's effect. Proper adjustments of the
buncher phases require excellent 300-kv stability.
2. It is assumed that the wire scanners are available, bunchers off, and all slits and flags
out.
3. We are overlapping the DEE-gap "time windows".
4. Tune Bz's and Br's for maximum cyc transmission to HE-3 at 312 inches for
minimum and stable time-of-f1ight.
o See document “How To Tune the BZ’s From Scratch” by F.Bach, Oct 1989
5. DO NOT move Br's a long way for tiny gains. Don’t move the Br's a lot, unless you
know they're wrong. Some Br's cross-couple to Bz by up to 10% due to coilplacement and mis-calibrations. Be particularly careful about Trim Coil 46 – it has
one shortened turn in the upper coil.
6. See WP5884, WP6274, FR12416 – April 1996
7. Touch up trim coil zero Bz & Br. Be patient and repeat your tests a few times. If you
make considerable gains with Trim Coil Zero, then Harmonic Coil 2’s amplitude and
phase, the Inflector/Deflector voltages may also need tweaking slightly.
4.3
1.
Buncher Tuning
FIRST BUNCHER ON. Voltage DAC (IB202) at approximately 0200.
a. Set main phase DAC (IB201) at approximately 545. Adjust 1st phase (IB203)
for max beam. This should happen around a DAC of 2200. Raise 1st Buncher
voltage (IB202) for max cyclotron transmission which should be at a DAC
setting between 200 & 300. Iterate the main phase and the 1st Buncher
voltage.
b. Do not over-bunch. The 1st buncher voltage should be less than or equal to
the peak, never above it. If the 1st buncher voltage is set too high, then the
main phase will be difficult to set correctly.
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c. Re-peak 1st buncher phase (IB203) for max cyclotron transmission. It should
peak at a DAC of 2200, if not then set 1st buncher phase to 2200 and re-peak
the main phase.
d. This 1st Buncher phase DOES NOT CHANGE from 1 to 2 buncher operation.
2.
Adjust IB306, IB356, and the inf1ector. Take one pass through the correction
plates with the vertical flag biting 10%. Pull the flag out of the beam. Re-peak
the 1st buncher voltage and phase.
3.
Check the steering into the 1st-buncher (IB247 horizontal, and IB250 vertical).
Look for max extracted beam and min ISIS losses.
a. If required, adjust steering after the Bunchers with IB277, IB300, 1B302,
IB303, and the v-bend. Note that these are NOT quads. Fix any large
ISIS losses here before continuing this procedure. Use only steering if
possible.
4.
Check WS166. The vertical trace will show a double-peaked structure. [Need a
nice picture here.]
5.
2nd BUNCHER ON.
a. Raise the voltage (IB204 to a DAC of about 1090).
b. The beam intensity will drop, ISIS spill may go up. Raise the 1st Buncher
voltage (IB202) to a DAC of 1115.
c. Beam may or may not recover, depending on IB205. You can fine-tune IB
205 by looking at the WS166 vertical trace. It should have a nice bread
structure with 4 evenly-spaced peaks. [Need another nice picture here.] If the
trace looks lopsided, adjust IB205 until it is symmetric. In many cases, the
WS166 trace is more sensitive than the cyclotron. This means that even if
other parameters are not completely optimized, you can rely on the fact that if
the WS166 trace is symmetric, the bunchers are tuned correctly with respect
to each other.
d. Do not adjust the main phase or the 1st buncher phase here. Adjust only the
2nd-buncher phase (IB205) for max beam.
6.
IN THE FOLLOWING ORDER:
o Adjust the Buncher voltages IB202, IB204 for max beam. IB 205 should
need no further tuning as long as the WS166 trace remains symmetric.
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o Repeat until no further increases in extracted beam are found. The Buncher
voltage DACs should wind up near historical settings.
7.
For fine tuning:
o Adjust the MAIN PHASE (IB201). It should only require moving by 2 or 3
DAC units either way.
o This is where 300-kv stability problems become evident.
8.
Go back to IB202, IB204, and IB205 and re-optimize.
9.
DO NOT TUNE THE 1st BUNCHER PHASE (IB203).
10.
Trim up the steering into and out of the Bunchers.
11.
Trim the vertical bend and the steering in IM-l as necessary.
12.
The use of two bunchers gives the ISIS beam a lot of energy spread placing the
quads into the vertical bends. This change determines the v-bend's
achromaticity and the energy dispersion of the beam at the inflector. The tune
through the vertical bend region strongly affects the amount of beam lost on the
downstream ISIS skimmers and collimators, as well as spill at/in/after the
inflector assembly and in the cyclotron.
13.
Trim Coil 0, the inflector, and deflector may require a little tuning.
14.
Take another pass through the correction plates with the vertical flag biting
about 5%.
15.
Trim up HC2, trim coil zero. Re-optimize IB201, IB205, IB202, and IB204
repeatedly IN THAT SEQUENCE if any gains in the center region are found.
16.
Twiddle the quads in the vertical bend for max extracted beam. If they give a
large increase in extracted beam but also give a large increase in ISIS spill then
make a compromise adjustment and tune out the ISIS vertical-section losses
with other ISIS devices as required (try IB304 and IB354).
17.
Trim up IB341 and IB344 for any other gains made in ISIS.
18.
Use as little buncher voltages as required to avoid over-bunching.
19.
Document all problems.
20.
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Appendix A: Common Terms and Their Definitions
The inflector: has two spiral-shaped electrodes which operate at approximately 53.4
kilovolts difference (+ and – 26.7kV) which rotate the plane of the beam from vertical to
horizontal.
The deflector is a small set of plates with about 32kV difference which steers the beam
into the first turn of the cyclotron.
The inflector voltage keeps the beam centered through the spiral,
The deflector voltage determines the deflection force needed to deflect the beam into the
first turn in the cyclotron.
BZ 0 and the BR 0: Once the beam has exited the inf/def assembly, then for a given ISIS
energy, the orbit size is controlled by BZ 0 and the vertical position by BR 0.
Harmonic coil 2 controls the horizontal centering of the first few turns in the cyclotron.
The amplitude, in (ampere*turns) controls how much centering force is applied to the
beam over its whole orbit, and the phase is the direction in which that force is applied.
Zero degrees is the east Dee gap.
Digital to Analog Converter: DAC’s are unitless. 200 here means 10% of full scale.
Cyclotron transmission: as visible on the Keithley electrometer set on a sensitive range
and using the appropriate amount of offset
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