Resonance Crossing Experiment in PoP FFAG (preliminary report)

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Resonance Crossing Experimentin PoP FFAG
(preliminary report)
FFAG W.S. 04 _at_ KEK

• M. Aiba (Tokyo Univ.) for KEK FFAG Group

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Motivation of Experiment

• Beam dynamics of resonance crossing is studied
  for non-scaling FFAG.
• There are few study on resonance crossing.
  Especially, experimental studies are only (as
  far as I know)
• Fifth integer (Particle Trapping) _at_ CERN ISR
  (1975) by A. W. Chao et al.
• Half integer _at_ TRIUMF Cyclotron (80) by R.
  Baartman et al.
• Third integer, coupling resonance etc.. _at_
  HIMAC (under going) by S. Machida et al.
• PoP FFAG is good machine for beam study.

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Basic Parameter of PoP FFAG
Crossing speed can be changed in wide range. It
is necessary to introduce a variation of tune.
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Remodel of Magnet
4mm iron plates are inserted to all 8 magnets.
Schematic view of magnet cross section
r
Iron plate
Relatively, a gap outside is more widen than
inside. Therefore, k value decrease as increasing
radius.
Mainly, horizontal tune varies.
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Variation of Tunes
open circle experiment colored plot calculation
Integer or Half integer
Fourth order (normal)
Third order (normal)
3Nx7 is focused here.
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Longitudinal Beam Handling(1)
Crossing speed is one of important parameter!
However, it is impossible to accelerate all
particles with same energy gain because of
synchrotron oscillation.
For clear observation of speed dependence,
careful attentions are paid to longitudinal beam
handling.
Beam chopper 100nsec chopped beam (10deg. of
RF phase) Mountain-plot Bunch monitor signal is
transferred to mountain plot to check an
amplitude of dipole oscillation.
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Longitudinal Beam Handling(2)
Example of Mountain Plot (RF capture _at_ injection
energy)
Dipole oscillation is perfectly suppressed.
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Driving Term (1)
Magnetic field error with RF core
COD due to RF core
(calculated with TOSCA)
(calculated with TOSCA field RK-tracking)
Core
Straight Section
Defocus
Focus
Feed Down
RF cores disturb magnetic field of straight
section. COD and octupole becomes sextupole
driving term (feed down).
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Driving Term (2)
COD with weak excited magnets
Error
Septum
Error
Error
Septum
Error
Variable driving term can be introduced with
changing coil current.
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Driving Term (3)
Due to the relation of phase between fixed and
variable driving term, Fourier amplitude is not
proportional to variable driving term.
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Beam Size Measurement
During acceleration, an orbit shifts to outer
radius. Using a scraper and an intensity
monitor, beam size, before and after crossing,
can be measured.
turn
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Results (1) data of driving term 0.1810-8(m3/2)
Speed 0.13kV/turn
Speed 0.49kV/turn
Speed 1.6kV/turn
Scraper pos. r908mm
Scraper pos. r908mm
Scraper pos. r908mm
Scraper pos. r928mm
Scraper pos. r928mm
Scraper pos. r928mm
Scraper pos. r948mm
Scraper pos. r948mm
Scraper pos. r948mm
Scraper pos. r968mm
Scraper pos. r968mm
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Results(2)-trapping efficiency
Large driving term
Large trapping efficiency
Slow Crossing
Large trapping efficiency
The result can be understood qualitatively.
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Particle Trapping
Reference PARTICLE TRAPPING DURING PASSAGE
THROUGH A HIGH-ORDER RESONANCE, A.W.
Chao and Melvin Month, NIM 121(1974)
pp129-138
Particle Trapping When a non-linear detuning is
very larger than a driving term, some
particles are trapped by islands during
crossing resonance.
Phase space topology for third integer resonance
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?Opposite Crossing?
Particle trapping (tune decreases)
Growth? (tune decreases)
HIMAC experiment
Num. of Cell 12
Crossing 3Nx11
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Summary
Beam study in PoP FFAG was carried out to study
a dynamics of resonance crossing.
Tune crosses 3Nx7, then
There seems no effect, when crossing speed is
fast enough. Particle trapping is observed. The
dependence of trapping efficiency on crossing
speed and driving term can be understood
qualitatively.
Opposite crossing does not become trapping.