Fermilab Recycler Ring is a ring to store, accumulate and cool pbars.

1
ION TRAPPING IN RECYCLER RING
K.Y. Ng Fermilab
Fermilab Recycler Ring is a ring to store,
accumulate and cool pbars. The antiprotons are
stored between two barrier waves.
Ring circumference11.13 ms Bunch length 9.30
ms Gap 1.77 ms Bunch length can be shortened By
moving barriers.
Ions are trapped in the beam. There are clearing
electrodes, but may not be at correct
positions. We see sudden emittance growth with
small beam loss.
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?Transverse emittances took a jump and lost about
1E10 of beam. Before jump intensity was
about 126E10. Bunch length 7.3
ms. ?Usually, the growth is triggered by a
change in the cycling of the Main Injector
underneath the Recycler.
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MECHANISM

• Antiproton beam traps ions. ? beam is unstable
  when voltage of clearing electrodes is turned
  off.
• There is a stochastic cooling system to cool the
  pbar beam.
• Ramping of Main Injector underneath produces
  strayed fields that heats the pbar beam.
• The cooling strength is adjusted to counteract
  this heating.
• When the Main Injector stops or changes ramping
  cycle, there is less heating and pbar is cooled
  too much.
• This initiates pbar-ion instability.
• The pbar are stored in the Recycler Ring for many
  hours. The sudden emittance growth usually
  occurs when nobody is around.
  

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INDUCED EMITTANCE GROWTH
Cooling off, clearing voltage at -300V. At about
70 sec. rf set to zero and left there till the
end of this plot.
With 19.7E10 pbars in a 9.3-ms bucket cooling was
first turned off. Initial emittances were 2.5p
(h,v). At 17 min, rf was turned off. Beam was
fully debunched. Emittances grew quickly to
5.2p. Rf on again to gain0.2. At 25 min cooling
was on again.
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Beam was placed in 9.3-ms bunch, first cooled to
4.7p emittances, then at 1800 seconds cooling
was off and rf voltage reduced to zero. No
coherent betatron lines were observed while
emittance increased. Beam intensity at end
25.0E10
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MI ramping
2 min after ramping was off
79 MHz Schottky spectra. No averaging. Bunch
lengths all 2.3 ms. Coherent peak looked the
same immediately before and after ramping was
turned off it went away on the order of a few
min. Again, no SA averaging was used.
4 min later ?
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No MI ramping, bunch length 3.3 ms, no averaging
? right after bunch length reduced to 2.3 ms
1 ms after barrier move ?

• 3 ms after
• barrier move

5 ms after barrier move ?
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2-Q
1Q
3-Q
2Q
n1
n2
n3
78E10 pbars in a 9.3-ms barrier bunch. Barrier
voltage was reduced from 2kV to 20 V Gap started
to fill with pbars and the instability was
induced. The spectrum analyzer was set to max
hold Assuming CO, bounce freq is 73kHz, but we
see 140kHz ? gas mass ¼ smaller. Emittances are
about 6p mm-mr.
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LINEAR THEORY
for gap 1.77ms and beam intensity 127E10
Therefore ions may not be cleared in gap.
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• However, the instability occurs sometimes when
  the beam is squeezed, or when the gap is longer.
• Thus, unless the gap is too small, instability
  seems to be independent of size of gap.
• We suspect this may belong to fast-ion
  instability.
• At beam intensity 12.7E11, bunch length 9.3ms,
• fast-ion instability growth time is 400ms, or
  about 36 turns.
• Not sure whether induced instability is the same
  as what we observe.