Damping ring kickers

1
Damping ring kickers

• George Gollin
• University of Illinois at Urbana-Champaign
• and
• Fermi National Accelerator Laboratory

2
Outline

• The problem
• The specs
• Current ideas
• Opinions from the speaker
• Illinois/Fermilab efforts
• ILC-America ILC-global near term goals

3
The problem

• Linac beam (TESLA TDR)
• 2820 bunches, 340 nsec spacing ( 300 kilometers)
  
• Cool an entire pulse in the damping rings before
  linac injection
• ILC damping ring beam
• 2820 bunches, closely spaced
• Eject every nth bunch into linac (leave adjacent
  bunches undisturbed)
• Minimum damping ring circumference depends on
  minimum realizable bunch spacing (kicker speed
  and instability issues)
• Kicker
• 20 ns kicker ? 17 km circumference (TESLA dog
  bone)
• 6 ns kicker ? 6 km circumference (Fermilab small
  ring)

4
What an interesting problem!
Theres significant global interest in tackling
the challenges posed by the damping ring and
kicker. We have already started an informal,
global conversation see Andy Wolskis site at
http//awolski.lbl.gov/ILCDR/. for a partial list
of who has made contact. There are physicists
from ANL, CERN, Cornell, Daresbury, DESY.
Fermilab, Frascati, Harvard, Illinois, KEK, LBNL,
Minnesota, SLAC, Tokyo so far.
5
What we have to work with

• Injection
• already-orbiting bunches have not damped
  appreciably, have large transverse sizes (2 cm)
• kicker must inject new bunches into damping ring
  acceptance without losing already-orbiting
  bunches.
• Extraction
• damping is finished, bunches are small (a few
  microns in diameter)
• kicker must preserve beam emittance for
  still-orbiting bunches as well as the kicked
  bunch.
• Performance demands on kicker are different for
  injection/extraction. Should we consider separate
  designs for injection and extraction?

6
The specs

• Kicker specs depend, in part, on beam dynamics
  phase space volume occupied by a just-kicked
  bunch must be well separated from that of an
  unkicked bunch.
• Dog bone (TESLA TDR) kicker specs
• impulse 100 G-m (3 MeV/c) 0.07 G-m (2 keV/c)
• residual (off) impulse 0 0.07 G-m (2 keV/c)
• rise/fall time lt 20 ns
• Perhaps larger (but less precise) impulse at
  injection, smaller (but more precise) impulse at
  extraction will be desirable.
• Small ring kicker rise, fall times can be
  asymmetric
• leading edge lt 6 ns, trailing edge lt 60 ns

7
The ideas (1) strip line kicker modules

• Fast switches dump high voltage pulses into a
  series of strip line structures.
• Electromagnetic pulse applies transverse kick to
  one bunch, but is absorbed in a load in each
  strip line module before next bunch arrives.
• switch speed, on-resistance, and stability are
  concerns
• adequate precision of strip line termination is
  challenging
• TESLA and SLAC are thinking about this

8
The ideas (2) longitudinal kick dispersion
Separate the beam so that it travels along
multiple paths. Demands on kicker are less
severe it only sees every 4th bunch. The system
needs to be studied in detail how to do it, what
happens when it is installed in a damping
ring. Cornell, Frascati are thinking along these
lines.
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The ideas (3) Fourier engineering

• Instead of a pulsed kicker, construct a kicking
  pulse from a sum of its Fourier components.
• Most interesting (to me) idea now combine this
  with a pulse compression system to drive a small
  number of low-Q cavities.
• what are stability requirements on RF components?
• how robust to bunch arrival time errors can the
  system be made?
• Illinois, Fermilab, Cornell are involved.

10
The speakers opinions (1)
The kicker has the potential to be a
show-stopper. Damping ring architecture (bunch
timing structure, injection/extraction optics,
etc.) is strongly tied to details of the
kicker. A fast pulsed kicker might be simplest to
operate BUT switch speed and stability are
challenging issues. Significant proof of the
feasibility of at least one design is needed
before a choice of damping ring design (big ring
vs. small ring) will be possible.
11
The speakers opinions (2)

• Fermilab small ring studies incorporate
• damping ring bunch trains with 60 ns inter-train
  gaps
• long straight section to allow installation of
  as-yet unspecified kicker
• Perhaps these features should be included in all
  damping ring designs?
• There are three distinct stages in damping
  process
• injection and assembly of bunch trains
• damping
• disassembly of bunch trains and extraction
• Beam is very different in each of these stages.
  Should there be distinct subsystems (including
  different kickers) for each of them?

12
Separating injection, damping, extraction
functions

• Large acceptance injection ring
• (340 6) 334 ns circumference
• assemble a single bunch train by injecting
  successive bunches into this ring at the tail of
  the train already orbiting the injection ring
• transfer entire bunch train into the damping ring
  in one orbit
• Large acceptance damping ring (6 ns bunch
  spacing)
• Small acceptance extraction ring
• (340 6) 346 ns circumference
• transfer one entire bunch train from the damping
  ring
• extract by kicking the last bunch in the train on
  successive orbits

13
Separating injection, damping, extraction
functions
Injection and extraction rings differ in
circumference by two bunch spacings to allow
injection/extraction to/from tail of bunch train.
14
Fermilab/Illinois activities

• Initial studies use Fermilab A0 photoinjector
  beam (16 MeV electrons) for studies
• build a fast, simple strip line kicker
• use the kicker to study the timing/stability
  properties of the A0 beam
• build a single-module pulse compression kicker
• study its behavior at A0
• perform more detailed studies in a higher energy,
  low emittance beam (ATF??)

15
A0 photoinjector beam
16 MeV electron beam, good spot size,
emittance. EOI submitted to A0 group last
spring. Space in beamline will be available
January 2005
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Simple kicker for initial tests
Start with a simple kicker whose properties are
calculable and can be measured independently of
its effects on the A0 electron beam. Most
important how well can we measure a devices
amplitude and timing stability with the A0 beam?
Fermilab is currently designing this. Probably
ready by January 2005.
17
UIUC/FNAL, longer term plans

• Design, then build one module using existing
  components.
• Fermilab RF group is involved
• UIUC HEP electronics design groups chief is too.
  
• So were starting to make progress.
• Goals
• install strip line kicker in A0 by January, 2005
• understand A0 by spring, 2005
• install small pulse compression kicker at A0 by
  summer, 2005

18
ILC-America ILC-global near-term goals

• Possible goals and timelines
• pre-KEK discuss tentative plans for kicker RD
  with our colleagues in America, Asia, and Europe
  to have a sense of who would like to do what.
• at KEK rough-out an RD plan aimed at choosing a
  kicker technology in 2 years
• at Snowmass (August, 2005) kicker workshop with
  international participation to assess how well
  weve gotten started, and what we have learned

19
End notes

• The kicker and damping ring scare the daylights
  out of me.
• We must build one and make it work before the ILC
  main linac is completed.
• More information
• Studies Pertaining to a Small Damping Ring for
  the International Linear Collider,
  FERMILAB-TM-2272-AD-TD
• ILC Damping Rings web site http//awolski.lbl.gov
  /ILCDR/