RF system requirements, top-off review

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RF System Requirements for ALS Top-Off Mode of
Operation Top-Off Mode Review November 22-23
2004
Slawomir Kwiatkowski ALS RF Group
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The ALS booster RF system will be strongly
affected by the ALS Top-Off mode of operation,
since it will need to accelerate the electron
beam up to the full Storage Ring energy
(1.9GeV).Top-Off mode will have no significant
impact on Linac and Storage Ring RF systems.

• Booster RF System.
• ALS booster RF system is powered by 15kW
  commercial UHF transmitter.
• RF cavity- single reentrant type cavity identical
  to those used in the ALS storage ring
• (Rsh 5MO, R/Q125).
• Booster RF cavity tuner - metallic cylinder,
  driven by a stepping motor.
• Booster cycles at 1Hz (350ms accelerating ramp
  and 650ms recovery/quiescent period).
• Tuner control loop is enabled only during
  quiescent period of the booster cycle.
• Booster ring accepts three 50 MeV (8ns apart)
  electron bunches from ALS drift tube traveling
  wave injection LINAC and accelerates them to
  1.5GeV.
• RF cavity cell power changes from 80W at the
  injection to 7kW at the extraction.
• The average ALS booster beam current is 4mA (1nQ
  total beam charge).

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Figure1
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How much Booster RF power is needed for Top-Off
mode of operation?
The dominant mechanism, which dictates the
minimum required RF bucket height at the end of
the accelerating cycle is the quantum emission
of energetic photons by the electron beam
(quantum lifetime). The quantum lifetime tq is a
steep function of the electron beam energy and
according to Sands 1 and 2 is given by

Where tq -quantum lifetime te
-longitudinal damping time
F. Ruggierro in his paper3 presented different
formula for quantum lifetime
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Table 1 shows the results of the calculations of
the ALS Booster quantum lifetime for 1.9GeV
energy, synchrotron tune, cavity cell power and
the Effective cavity RF voltage as a function of
?s parameter for 1.9GeV electron beam energy.
?s Quantum lifetime Sands Quantum lifetime Ruggiero Synchrotron Tune Cavity Cell Power Cavity Voltage
- s s - kW kV
3 0.017 0.002 0.012 19 436
3.5 0.065 0.009 0.0129 22.8 477
4.0 0.319 0.054 0.0137 27.3 522
4.5 2.14 0.43 0.0146 32.8 573
5.0 18.8 4.4 0.0155 39.3 627
5.5 216.2 58.3 0.0164 47.0 686
6.0 3203 975 0.0172 56.0 748
6.5 61373 20852 0.0181 66.5 815
7.0 1600000 602800 0.019 78.9 888
TABLE 1
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Figure 2
Figure 3
The stationary and dynamic 1.9GeV ALS Booster RF
buckets for 20kW and 60kW cavity cell power and
the single bunch with the population of 1000
electrons and the normal energy distribution are
shown in Figure 2 and Figure 3.
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Beam Loading Effect in 50MeV ALS Linac Structure
and Booster Ring
In the ALS top-off operation mode to insure more
uniform storage ring beam pattern injector might
deliver up to 10 bunches in each injection
cycle. What impact will it have on the linac
output beam energy spread? We performed the
calculations of the beam induced voltages in the
ALS linac for 1nC electron beam for 3 and 10
bunches spaced 8ns apart. The results are as
follow For 3 bunches case the total beam induced
voltage Vb150keV and energy spread sE/E0.3 For
10 bunches case the total beam induced voltage
Vb136keV and energy spread sE/E0.27 Conclusion
The beam loading effect in ALS linac injector is
not a significant problem for the proposed
top-off mode of operation. No beam loading
compensation is needed for proposed ALS main ring
injection rate.

• The beam loading effect during beam injection
  from ALS linac into the booster ring can cause
  fast
• (with the cavity time constant) booster RF cavity
  amplitude and phase change.
• Beam loading effect in the booster ring could be
  controlled by
• Initial detuning of the RF cavity prior to the
  beam injection moment.
• Increasing the power source to RF cavity
  coupling coefficient (more power from RF source
  required).
• Higher RF cavity voltage during beam injection
  (more power from RF source required).

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Synchrobetatron oscillations problem in ALS
Booster Ring
Coupling between transverse and longitudinal
oscillations gives rise to excitation of
resonances for tunes which satisfy the following
equation kQxlQymQsn where k,l,m,and n are
integers For the given lattice parameters
horizontal and vertical tunes are constant (for
ALS booster ring Qx5.82 and Qy2.72). From the
logical point of view the simplest way to avoid
the synchrobetatron resonances problem is to
choose the optimum Booster Ring operation
conditions and maintain the synchrotron tune
value constant over whole Booster energy ramp.
Figure 4 shows stationary and dynamic booster
buckets during injection moment for 40W rf
cavity cell power and 10 equally spaced bunches
with 136keV total energy spread (zero energy
spread within each bunch).
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Figure 4
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Figure 5 shows Booster rf power ramp profile for
the constant synchrotron tune value Qsconst.
Pmax66kW
Pmin54W
FIGURE 5
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CONCLUSIONFor ALS Top-Off Mode of operation
booster RF system needs new gt75kW RF power source.
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REFERENCES

• 1 M.Sands. Observation of Quantum Effects in
  an Electron Synchrotron.
  International Conference on Accelerators, CERN,
  1959
• 2 M.Sands. Physics of Electron Storage
  Storage Rings., SLAC Report No.
  121(1970)
• F.Ruggiero. A correct formula for the
  longitudinal quantum lifetime in electron storage
  rings. CERN-SL-93-05-AP.