CLIC Drive Beam Stabilisation
Author: Alexander Gerbershagen
Supervisors: Prof Philip Burrows (University of Oxford)
Dr Daniel Schulte (CERN)
What is CLIC Drive Beam?
The Compact Linear Collider (CLIC) is a proposed multi-TeV
e+e- collider being developed at CERN, which is based on twobeam technology. One of the beams, so called Drive Beam,
with low energy but high current, is used to deliver RF power
for the accelerating structures of the high energy, low current
beam, called Main Beam. This scheme has been chosen, since
the CLIC main beam frequency (~12 GHz) is too high for any
conventional RF source.
Fig. 1: Layout of the CLIC collider at 3 TeV
Why do we need to stabilize it?
First, bunches pass the recombination scheme
of the Drive Beam. Then the RF power is extracted
from the Drive Beam in so called Power Extraction
and Transfer Structures (PETS) and the RF waves
are led from there to the accelerating structures
of the Main Beam.
The stability of the Drive Beam is a critical issue,
since its jitters and instabilities would lead
to unstable power supply to the main beam.
Some tolerances (e.g. phase, gradient
and energy error tolerance) are very strict.
The filling time of the RF structures is ~ 60ns,
hence we integrate the errors
over intervals of same order
of magnitude (10ns)
to receive following distribution
of error as function of frequency (Fig. 3).
Fig. 2: Two beams scheme of CLIC
Fig. 3: Charge error as a function of frequency
How can we stabilise it?
In order to avoid the main beam RF jitter one has to compensate
the errors of the beam before the PETS e.g. with help of a feedback system.
Fig. 4: Layout of the CLIC RF complex
The Drive Beam is produced in 240ns long trains with 180° phase shifted bunches at 0.5
GHz frequency. Acceleration power is provided by standard 1GHz RF source, so that all
bunches are accelerated equally (Fig. 4). The frequency is then increased from 0.5 GHz to
12 GHz by recombining bunches in the delay loop
And the combiner rings. The trains overlap during
this recombination process and the errors
from different trains come together
Fig. 5: Train errors overlap
in one train (Fig. 5).
What is a feedback system?
Ideal feedback system:
1. Measures error value
2. Compensates error on the next bunch
For CLIC the bunch spacing (0.08 ns) is much smaller then the latency time of the feedback
and gain time of the correction kicker (~ 10 ns). Hence, the correction will look
like on Fig. 9.
Dependent on jitter frequency, the
feedback system can reduce or increase
the error (Fig. 6). To optimise the total
impact of feedback, we have to
integrate error over frequencies and
minimise the value as function of
feedback latency and gain (Fig. 7).
We realize that if t(latency) + t(gain) ≈
240ns (which corresponds to the train
length) the feedback is optimal.
How does Drive Beam recombination scheme work?
Fig. 6: Charge error as a function of frequency
(without and with feedback)
Conclusion:
Fig. 9: Example of feedback correction
with long latency and gain time
and small bunch spacing
Cool, we can even
reduce the white noise
errors with a feedback
system!
This is in general not
possible, but it
becomes possible in this case due to
recombination scheme.
Applying the feedback before
the recombination and integrating
over 10ns thereafter puts the error
at the blue bunch and its correction
on the red bunch in one interval, hence
reducing the total error (Fig. 8).
But if the jitter frequency
is anti-resonant
to feedback latency,
the feedback
system corrects
the measured value
too slowly, and only
increases the error
(Fig. 10). The standard
feedback system used in
our simulations, is called
proportional-integralderivative controller
(PID controller).
Fig. 10:
Resonant (top)
and anti-resonant
(bottom)
feedback
What will we do in the future?
Fig. 7: Current error integrated over frequencies
as a function of latency and gain length
Fig. 8: Scheme of bunch recombination and
integration over 10ns intervals
• Include more realistic effects in the simulation
• Perform the simulations for possible future feedforward systems
• Use the results as input for the main beam studies
• Make predictions and test them at CLIC Test Facility (CTF3)
• Simulate usage of Feedback On Nanosecond Timescale (FONT) system
Special thanks for providing support and material:
• Prof Philip Burrows
• Dr Daniel Schulte
• Dr Franz Tecker
• Dr Guido Sterbini
•Dr Oleksiy Kononenko
For more information see:
http://clic-study.web.cern.ch/clic-study/
http://www-pnp.physics.ox.ac.uk/~font/index.html