J' Jacob 1

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Harmonic Cavitiesthe Pros Cons Jörn Jacob
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Content

• Main motivation for harmonic cavities tTouschek
• Harmonic cavities on existing light sources /
  achievements, problems
• NC passive cavities
• NC active cavities
• Projects for SC harmonic cavities
• Harmonic cavities for the ESRF ?
• Pros Cons
• Conclusions

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Motivation for harmonic cavities
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Harmonic Cavities for bunch lengthening

• Cavity at fharmn fRF (often n3)
• Passive / active
• Normal/Super-conducting (NC/SC)
• Maximum bunchlength for
• Vharm Vopt
• fharm fopt

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sLo 4.8 mm
(ESRF parameters)
fs
U0/e
Synchrotron frequency distribution (fs0 2 kHz)
Vharm/Vopt 0
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Vharm/Vopt 0.25
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Vharm/Vopt 0.5
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Vharm/Vopt 0.6
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Vharm/Vopt 0.7
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Vharm/Vopt 0.8
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Vharm/Vopt 0.9
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Vharm/Vopt 1
A. Hofmann S. Myers
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Vharm/Vopt 1.05
Over stretching gt formation of two bunches
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maximum
typically
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NC passive harmonic cavities

• The beam drives Vharm
• gt multibunch operation (Ibeam gt Iminimum)
• gt Vharm controlled by Cavity tuning (typ.
  fharm? n fRF f0 /3)
• gt fharm fopt only possible at one current

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NC passive harmonic cavities (continued)

• ALS Emax 1.9 GeV, Ibeam 400 mA ? 200
  mA, LFB TFB
• 5 Cu reentrant HCs, fharm 3 fRF / 2
  tuners, HOM absorber
• Achievements
• ? in experiment tLife increase by factor 2.5
  (tLife 4 h ? 10 h)
• tLength 55 ? 120 ps, fs 11.5 ? 5 kHz
• ? LFB fs filter (now 4 kHz) limits
  DtLength-max
• ? in operation 50 increase in tLife? 6 h (2
  cavities tuned in)
• ? no energy spread gt detuning of HC-HOM (TM011
  at ALS)
• Problems
• ? Users require 20 gap in filling ? transient
  beam loading
• ? strong beam and Voltage f modulation
• ? less average bunch lengthening
• ? TFB heterodyne ? homodyne receiver solved
  the problem
• ? LFB fs modulation ? factor 6 at 3 GHz
  detection frequency
• ? feedback saturates if Dfs gt15

fRF 500 MHz
J. Byrd et al.
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NC passive harmonic cavities (continued)

• BESSY II Emax 1.9 GeV, Ibeam 220 mA, LFB
  TFB
• 4 Cu Pillbox HCs, fharm 3 fRF / 2 tuners
• Achievements (still in commissioning)
• ? tLife 3.2 h ? 5.2 h at 200 mA
• ? tLength increase by factor 2.5 to 3

fRF 500 MHz
122 mA, LFB on, Vharm 0
200 mA, LFB off, Vharm 140 kV
Streak Camera same time scale, Vacc 1 MV
W. Anders et al.
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NC passive harmonic cavities (continued)

• BESSY II
• ? TFB operational
• ? LFB not yet compatible (filter bandwidth)
• ? Phase transients with gap max 50?
• ? HOM problems still present

W. Anders et al.
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NC active harmonic cavities

• NSLS VUV E 0.8 GeV
• ? Operation alternatively in bunch lengthening
  or shortening mode
• ? Powered cavities allow operation near Vopt ,
  fopt for any Ibeam
• ? No Beam power

fRF 52.9 MHz, fharm 4 fRF
Latest Figures Mode Ibeam tLength tLife HC
detuned 700 mA 0.9 ns 2.5 h lengthened 700 mA 1.7
2 ns 4 h (unstable above 700 mA ? nominal 1
A) shortened 600 mA 0.48 ns 2 h (constant
length)
S. L. Kramer, N. Towne et al.
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NC active harmonic cavities (continued)

• NSLS VUV / Slow tuning, amplitude and phase
  feedback
• Principle using a Complex Phasor Modulator
• ? Voltage error signal ? real part ar regulated
  to zero by tuning
• gt fharm -90? ( and not fopt ? -93?)
• ? In lengthening, phase error signal from beam
  PU ? imaginary part ai
• (at fharm ? fopt , due to flat RF
  potential? GAIN fbeam/fVcav ? -4.5)
• ? In shortening, phase error signal from cavity
  ? imaginary part ai
• ? System can be switched to standard tuning for
  passive operation
• ? Stable operation in shortening mode
  difficult (high beam loading)
• ? constant bunch length, but limited to 600 mA

ar j ai amplified and fed to the cavity
S. L. Kramer, N. Towne et al.
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NC active harmonic cavities (continued)

• NSLS VUV / Observed related instabilities
• ? Lengthening mode Landau Damping of coupled
  bunch instabilities
• ? Injection partially stretched mode gt needs
  LFB
• ? Occurrence of non-rigid bunch instabilities in
  particular if over-stretched
• ? chaotic appearance of broad, strong
  sidebands
• ? beam lost if high Ibeam
• ? For nearly optimum lengthening peak beam
  response at 1.1 fs0 to 1.4 fs0
• ? insensitive to Ibeam, Cavity tuning
• ? sensitive to Vharm

S. L. Kramer, N. Towne et al.
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NC active harmonic cavities (continued)

• NSLS VUV Stretched bunch shapes f(small
  variations of RF potential)

3 Reasons ? Shape very sensitive to fharm near
Vopt , fopt ? Gap in filling against ion
trapping ? Phase transients ? Gap in
filling ? additional revolution harmonics
? excite HOMs ? different potential
distortion for different bunches
N. Towne
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NC active harmonic cavities (continued)

• Super ACO E 0.8 GeV
• Bunch shortening for FEL operation and time
  resolved experiments
• ? Shortening by a factor up to 3.5 achieved (fs
  14 ? 40 kHz)
• New types of instabilities observed
• ? Vertical single bunch instability at 10
  mA/bunch no sensitivity to nz , xz , Vharm
• ? Vertical TMCI starting at 30 mA, m0 and -1
  modes merging at 40 mA cured by high xz
  2.5 ? 4
• ? Interference between 2 longitudinal single
  bunch oscillations
• Low frequency sawtooth oscillations (lt 300 Hz),
  at any current
• High frequency oscillations at mainly fs and 2
  fs , only between 2 and 8 mA/bunch
• Bunch lengthening mode
• ? Landau damping of LCBI
• ? expected RobinsonII instability ?

fRF 100 MHz, fharm 5 fRF
G. Flynn et al.
M.P. Level, M. Georgsson, et al.
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SC passive cavities

• Elettra, SLS, ... collaboration project with
  CEA-Saclay
• ? HOM free harmonic cavities scaling of
  352.2 MHz SOLEIL cavities
• (pair of cavities within a single
  cryostat)
• ? Tuning angle ? ? 90? gt Pbeam ? 0, and as
  for NSLS fharm ? 90?
• ? Simple amplitude control by frequency tuning
  such as
• ? Expected Bunch lengthening by a factor 4
  (Vharm lt Vopt )
• ? Passive operation down to very low currents,
  
• ? However, possible Robinson instability on m
  fs for low dfharm at low Ibeam
• ? Phase transients also expected with SC
  cavities
• SRRC abandon NC harmonic cavities ? required
  space, HOMs,
• ? Feasibility study for SC harmonic
  cavities

(2 GeV) (2.4 GeV) fRF 500 MHz, fharm
1500 MHz
Vharm ? Ibeam (R/Q) fharm / dfharm
P. Marchand, M. Svandrlik, A. Mosnier et
al.
? J. Byrd
K.T. Hsu
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Harmonic Cavity for the ESRF ?
fRF 352.2 MHz, Example fharm 3 fRF
Reason for an HC ?

• Operation modes
• Multibunch at 200 mA, xv 0.4 to 0.5 tLife
  60 ... 70 h gt NO
• 16 bunch at 90 mA (5.5 mA/b) xv 0.6 tLife
  12 h gt yes
• Single bunch at 15 mA xv 0.9 tLife 4 h
  gt yes
• Optimistic assumption Lengthening factor 6

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Harmonic Cavity for the ESRF ? (continued)

• Tracking simulations? unchanged energy spread
  with HC / m-wave instability
• More sensitive to HOM driven Longitudinal Coupled
  Bunch Instabilities

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Harmonic cavities Pros cons Points of debate
for the subsequent working group discussions
Effects in bunch lengthening
Consequences
Pros / Cons

• Longer bunches DtLength

? Especially low energy machines or high I/bunch
gain in Lifetime

• Less spectral width of beam signals

? Probing less of the BBR ? Less prone to
transverse single bunch head tail instability ?

? Less HOM losses ? Reduced heating in few
bunch operation

• RF slope ? zero Phase sensitivity

? Gap induced Phase transients NC SC (increased
by HOMs) ? Reduced gain in Lifetime
-
? TFB must be adapted
o
? LFB saturation
-
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Harmonic cavities Pros cons, (continued)
Effects in bunch lengthening
Consequences
Pro / Con

• Difficult to control Vopt, fopt

? limited bunch lengthening
-
? Over-stretching ? non rigid bunch instability
(NSLS)
-

• RF slope ? zero Smaller fs

? Single bunch fast head tail (TMCI) lower
threshold ? S.Myers, Y.C. Chin, CERN
-
? LCBI lower thresholds
-
? More sensitivity to low frequency noise / power
supplies
-

• Distorted RF potential Spread of fs

? Landau damping for LCBI, TMCI ?, transverse
instabilities with m ? 0 ?

• More impedance (BBR, HOM)

? Bad for all kind of instabilities
-
? Robinson stability to be checked
o
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Harmonic cavities Pros cons, (continued)
Resistive wall instability ? Smaller spectral
width ? less chromaticity needed to shift the
modes or ? Less overlap with BBR ? less damping ?

-
Operation in bunch shortening ? NSLS current
limited by slow RF feedback stability ? Super
ACO new types of instabilities ? deserve further
investigations ? No experience from low emittance
machines
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Conclusion

• NC passive harmonic cavities
• ? sufficient voltage for low or medium energy
  machines /multibunch operation
• ? tuning not easy to handle for simultaneous
  Voltage and HOM control
• ? operate mostly below Vopt
• ? Gain in Lifetime by typically a factor 2 to
  2.5 gt good for these machines !
• NC active harmonic cavities
• ? allow operation at low current (e.g. single
  bunch operation)
• ? operation in bunch shortening demonstrated
• SC HOM free harmonic cavities
• ? only way for high energy machines , where
  interest is mainly for high I/bunch
• ? no major problems with HC HOMs, Robinson,
• ? tuning should be easier
• ? still needs RD to check performance,
  reliability and operational costs

Transient beam loading and Beam instability
issues with Harmonic cavities ? good
candidates for extensive discussions in this
workshop