Kein Folientitel

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Landau Cavities M. Pedrozzi Paul Scherrer
Institute, 5232 Villigen PSI, Switzerland
Workshop on small emittance lattices Lund 10-12
March, 2004
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Common aspects in high brightness light sources

• Small emittances (3-10 nm)
• High currents (300-400 mA)
• Short bunches (s10-20 ps)

Charge density increases Intra-beam scattering
increases (Touschek) Possible vacuum degradation
(elastic scattering)
Life time decrease. Collective effects become
more critical RF Cavity HOM inducing
CBI Resistive wall inducing CBI Ion trapping
-Small gap at the ID (down to 5mm)
Reduce the machine acceptance. Increases
resistive wall impedance.
Wish to increase the Brightness higher
current (gt500 mA) and smaller gaps (3-4 mm)!!!
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Harmonic RF system to cope with high bunch
charge density
Harmonic voltage adjusted to flatten the voltage
at the bunch location. Here optimum at
Vharm1/3VRF .
The RF energy acceptance is only marginally
affected operating with a double RF system .
The Bunch length at equilibrium increases by a
factor 4. The synchrotron frequency decrease
according to The non-linearity of the RF
increases the Landau Damping (broadening of
synchrotron frequency spectrum).
Example 3rd harmonic SC cavity at SLS operated
in passive mode (beam induced voltage).
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Existing harmonic systems in the light source
community
Privileged operating mode for bunch lengthening
PASSIVE (beam induced voltage)

• SUPERCONDUCTING CAVITIES
• S3HC ELETTRA (TRIESTE)
• 2 third harmonic cells- lengthening observed up
  to 4, Lifetime increase factor gt3
• S3HC SLS (PSI- VILLIGEN)
• 2 third harmonic cells- lengthening observed up
  to 4, Lifetime increase factor gt3
• BESSY-ll planned (Berlin)

• NORMAL CONDUCTING CAVITIES
• MAX-Lab (Lund)
• 4 third harmonic cavities Lifetime increase
  factor 2
• ALS (Berkeley)
• 5 third harm cavities lengthening observed 2.5,
  life time increase 50.
• BESSY-ll (Berlin)
• 4 third harm cavities typical lengthening
  observed 30, life time increase 20.

NC for bunch shortening Active 5 harmonic system
at Super-ACO
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Example of 3 Harmonic cavity tuning
Shortening mode
Lengthening mode
3fRF 1499 MHz frev 1 MHz SLS df 60 KHz V
330 KV Ploss 40 W BESSY df 250 KHz V
240 KV Ploss 15 KW
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SLS operation
300mA
Igt200 mA without S3HC cavity ?strong excitation
of longitudinal CBI (operation not
possible). I400 mA top-up operation reached
with S3HC cavity. CBI completely damped between 0
and 400 mA. Tuning can be kept constant between
0 and 400 mA (max elongation at 400 mA).
Transient beam loading effect and 20 less charge
per bunch
Streak camera measurements
300mA
s18ps at 200 mA without S3HC cavity Life time
without S3HC 4.6 - 5
hours _at_ 300 mA
Transverse and longitudinal feed back always
OFF! No HOM observed from S3HC!
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Sync freq. Spectrum broadening ? Landau damping
Sync. freq. spectrum broadening , and transient
beam loading effect
CBI
20 gap filling pattern
Over-voltage observed
CBI
Uniform filling pattern
With 20 gap, broadening increases at low
voltages due to transient beam loading (more
efficient Landau Damping), but ? Maximum
harmonic voltage reduced ? average lengthening
reduced ?average lifetime gain reduced
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Transient beam loading effect
Streak camera snap shot example Ib330
mA Vharmonic660 KV Close to maximum
elongation
Train tail
Train center
Train head
Interaction with longitudinal and transverse feed
back systems to be considered.
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Remarks and questions Technology
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Remarks and questions

• 1 - Do we still need a Landau cavity if we are
  able to
• -Topping up?
• Build HOM free cavities (SC and NC)?
• Use active feedback systems?
• Even with top up injection the life time remains
  an important argument (decrease the number of
  injection perturbing the users).
• 2- And if the life time is dominated by elastic
  scattering (small ID gaps)?
• The Landau cavities could be still useful in
  order to reach higher brightness (high currents,
  lower emittance) relaxing the single bunch
  effects.
• Possible cure for turbulent bunch lengthening in
  high current machines (decrease energy spread).

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3 - ACTIVE systems for SC or NC cavities? Number
of cells can be reduced saving space, but
additional RF source complication (need
investigation). Better control on transient beam
loading effects, less or no complications with
respect to the feed back systems. 4 - Can we
partially reduce the NC system limitation with an
active system? More relax operation with respect
to Robinson instability. HOM could remains a
major problem. 5 - What about Camschaft mode?
How can we keep a single bunch in the middle of
the filling patter gap short as possible ,
without to affect the normal operation? The
transient beam loading could help keeping the
single bunch short enough, but we need to
demonstrate that with measurements.
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S3HC cavity
NC Bessy cavity