RF control for crab cavity
K. Akai (KEK)
Feb. 21, 2005
KEKB review committee

Beam-loading on crab cavity
K. Akai et al, EPAC96, p.2118.

RF PowerKick voltage

RF Control for Crab Cavity (K. Akai)

Beam offset (dx)2Required power vs Loaded-Q• Should be taken into
account:!""Beam current = 2A#""

RF Power (kW)– Available RF power– Not to be too sensitive tobeam orbit change– Higher voltage than normal
operation can be applied for
conditioning cavity.

V=2MV (dx=0mm)$""• QL=1~3 x105 seems agood choice.

V=1.4MV (dx=1mm)
V=1.4MV (dx=0mm)%"""& ' ( )*

&%"

$#!

& ' ( )*'%"
Loaded-Q$#!

& ' ()*(%"– RF power of 200 kW issufficient.
– Orbit change by 0.5mm istolerable.

RF Control for Crab Cavity (K. Akai)

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Tolerance_Timing (RF phase)• Timing error (RF phase error) gives rise tohorizontal displacement at the IP.
*c"t # A$ x / % crossPlease delete “A”– Here, φcross is the half crossing angle.
– A is the ratio of allowed offset to horizontal beam size, σx*. A.
The value A should be determined from beam-beam view point.
KEKB!

Super-KEKBLC

σx*

100µm

70µm

0.24µmA (assumed)

0.05?

~0.05?

~0.2?

φcross+/- 11mrad+/- 15mrad+/- 3.5mrad

Δt

1.5 ps

0.8 ps0.05 ps

0.27 deg (509MHz)
RF Control for Crab Cavity (K. Akai)

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Effect on the beam-beam performance ofRF phase jitter• Luminosity and beam size
as functions of dx.
• Correlation time of thejitter, 1 or 10 turns, isimportant.
– For correlation time of 10 turns,
5 microns is allowed.
– For correlation time of 1 turn,tolerance is only 1 micron.

Luminosity
10 turns
1 turnBeam size
1 turnSimulation by Ohmi and Tawada

RF Control for Crab Cavity (K. Akai)

10 turns

5Source of phase error and cure
• Phase drift due to temperature change, etc.
– Slow. If the error is measured, it can be easily compensated by
feedback control of a phase shifter.
– CCC (continuous closed orbit correction system) can show single
kick caused by crab cavity.

• Other possible errors– Since QL~2x105 and h=5120, the correlation time for most of
possible errors will be larger than 10 turns. Then, a displacementof 5 microns is allowed, corresponding to a phase error of 0.27
degree. This is feasible with conventional control system.

RF Control for Crab Cavity (K. Akai)

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Transient to abort gap
K. Akai et al, EPAC98, p.1749.

•Depends on operating parameters.
– accerating Vc and QL, beam current,gap length.

•

Mostly compensated.
– Same direction in both rings.

•How about residual?
– Although modulation period is 1 turn,correlation time for each bunch is
NOT 1 turn, but determined by QL ofaccelerating cavities.
– Then +/- 7 microns is not disastrous.
However, still marginal for the 10
turn correlation.
– Consequently, variable range forcurrent ratio e+/e- will be limitted toreduce the residual.

RF Control for Crab Cavity (K. Akai)

1 turn7

RF stations for crab cavities• Two RF units are being constructed in D11.
– D11-E is for the LER crab cavity.
– D11-F is for HER, although the cavity will be located in D10 tunnel.
– Required power is 200 kW, much lower than existing stations foraccelerating cavities.

• High power components with reduced cost.
– Two reused klystrons have been conditioned up to 600 kW, enough for crab
cavities.
– A spare power supply for one klystron was moved from D2 and will be
modified to drive two klystrons.
– Circulators, dummy loads, and waveguides are being fabricated.

• Control system– Mostly similar to that for the SC accelerating cavities.
– Required accuracy for phase control is about 0.2 degree.
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Layout of D11 klystron gallery

Two RF high power stations are being built for crab cavities.
RF Control for Crab Cavity (K. Akai)

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RF system for crab cavity will be similar.
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RF Control for Crab Cavity (K. Akai)

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Tuner contoller
Phase detector
Tuner controller
Tuning offset•••••

Auto up/down based on phasedetector signal.
Manual up/down
Stop by mechanical switch,
electrical limit, or piezo overload
Display tuner position
F0 search

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Piezo controller and Piezo driver• Piezo controller:– Gain 60dB max– LPF 3 stages• Piezo driver– Gain = 200– Output 0~1600V– Current 100mA max

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