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The RF Phase Distribution and Timing System for the NLC* The RF Phase Distribution and Timing System for the NLC

Josef Frisch, David G. Brown, Eugene L. Cisneros SLAC, Stanford CA USA
*

*
Work Supported by Department of Energy contract DE-AC03076SF00515
Abstract
The Next Linear Collider accelerator will require phase
synchronization of ~20
°
X-band (11.424 GHz) long term,
and ~1
°
X-band short term throughout its 30 kilometer
length. A prototype fiber optic distribution system has
been constructed to demonstrate this level of performance.
This system operates by measuring the optical round trip
time in the fiber, and then controlling the fiber phase
length to stabilize this measurement. We describe the
design of this system, and show recent results on stability
and phase noise.
1 SYSTEM
REQUIREMENTS
The NLC requires trigger synchronization relative to
the beam at the 100 picosecond level, and RF phase
stability at the 1
°
X-band (0.2 picosecond) level. Since the
second requirement is the more stringent, we have
designed the timing distribution system to use the same
hardware as the RF distribution system. The RF timing
requirement corresponds to a L/L stability of <2.5
×
10
-9
,
which would be impractical without feedback.
It is assumed that RF phase measurements relative to
the electron beam will be used to obtain long-term
stability. In order to allow rapid machine turn-on, the RF
distribution system needs to maintain the RF phase to
within 20 degrees X-band (<5
×
10
-8
) for long periods of
time when the beam is not running.
In the proposed system, and in the prototype system,
the RF phase transmission frequency is 357MHz (1/32 of
the main X-band). Triggers throughout the machine
operate by counting cycles of 357 MHz starting from a
fiducial pulse superimposed on the RF.
The phase and timing distribution system must have
high reliability. We plan to achieve this with a single point
failure resistant redundant system.
TABLE 1. RF Distribution System Performance
Requirements.
Parameter Requirement
System Length
30 Km
RF Phase Stability (short
term <1 minute)
1
°
X-band
RF Phase Stability (long
term)
20
°
X-band
RF Phase Noise
1
°
X-band in 100Hz
Bandwidth
Reliability
>1000 Hour MTBF
system wide
2 SYSTEM
DESIGN
The Phase distribution system consists of the following
components: A master oscillator located at the center of
the machine, Long links to each of the 44 machine
sectors, A sector phase reference to distribute RF through
the approximately 600 meters of tunnel in each sector,
And a phase comparison system which is used to compare
the distribution RF phase with the beam phase.
Master
Source
Linac
Sector
Long Links (15km)
Linac
Sector
X50 Sectors
Low Level
RF System
Klystrons
High Power RF
Distribution System
(DLDS)
Tunnel
RF Structures
Phase
Detectors
Phase
Detectors
Sector phase
reference ~600 M
Measure phase vs.
reference, and beam
vs. RF phase
Feedback

Figure 1 Overall Layout of the Distribution System
2.1 Master Source
The requirements on the master source for phase noise
and frequency stability are determined by the time delay
of the distribution system and the phase noise and stability
requirements. Both are easily met with standard
commercial systems.
2.2 Long Link System
Fiber optics and coax distribution have similar phase
length vs. temperature coefficients of ~10
-5
/
°
C. Note that
the primary contribution to temperature coefficient in
fiber is the change in refractive index, not the change in
physical length. Fiber was chosen due to its lower cost
and higher bandwidth. Without the use of feedback, the
long-term phase stability requirement of 20
°
X-band
would require a fiber temperature stability of .005
°
C.
Fibers are run point to point from the central master
source to each of the 44 sectors. An adjustable phase
length fiber is connected in series with the main fiber. The
far end of the main fiber is terminated with a mirror. The
transmitter measures the phase of the light reflected from
the far end of the fiber and adjusts the phase length to
hold the reflected phase stable. Assuming that the forward
and backward optical signals propagate at the same speed
(or more precisely that the temperature coefficients of
velocity are the same) the phase of the outgoing signal is
XX International Linac Conference, Monterey, California
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THA05 stabilized.
T
Laser
Transmitter
Directional
Coupler
Circulator
Length
Adjust
Directional
Coupler
Mirror
Reflected
phase
detector
Outgoing
phase
detector
Feedback on Reflected
Phase
Receiver
Phase
Detector

Figure 2: Long Link Schematic
If the fiber is to operate in a standard trench without
temperature control, 10
°
C temperature variations are
expected, corresponding to length changes on the order of
a meter. A spool of fiber whose phase length is adjusted
by the temperature of an oven has been found to work
well.
The electron beam in the NLC has sufficient density to
damage the accelerator if the accelerating structures are
incorrectly phased. In order to prevent this, the receivers
contain high stability local oscillators that are able to
maintain phase between machine pulses (8 milliseconds).
If the master source, or part of the transmission system
were to suddenly shift phase, the receivers will still
broadcast the correct phase for the current pulse, and then
signal the machine protection system to abort the next
pulse. Failures of the phase control unit will only affect
the local crate and cannot damage the machine through
incorrect phasing.
2.3 Sector Phase Reference
The sector phase reference must meet specifications
similar to those for the long fiber links. Phase information
must be provided to typically 50 devices in the tunnel in
each sector. The radiation in the tunnel (estimated at ~1
R/Hour) prohibits the use of fiber optics for distribution.,
while the large device multiplicity makes a point to point
system similar to the long fiber links impractical.
We plan on a scheme where RF reference signal is
phase locked to the master source (through the long fiber
links) at the far end of the cable from the transmitter. At
each device the forward and backward going phases are
compared. The average of the forward and backward
phase should not depend on changes in the phase length of
the cable.

Phase reference
from long fiber link
Phase
Detector
357MHz
VCO
Reflection
(un terminated)
PLL feedback
Forward and
reverse couplers
Phase
Averager
Reference phase
to device
Phase here
is fixed by
PLL feedback
Forward signal
is L / C early
Reflected signal
is L / C late L
(Each Device)
coax

Figure 3: Sector Phase Reference Line
2.4 Beam Phase Reference
The phase reference system is adjusted on long time
scales to match the electron beam phase, which is
measured by comparing the fields induced in the
accelerating structures with the main RF fields in the
accelerating structures. The beam induced fields are
typically ~20dB below the RF fields for full power
operation and ~70dB down for operation with a single low
current bunch. There are two schemes under consideration
for the measurement.
The RF can be disabled on a group of structures for a
single pulse, and the beam induced fields measured. If an
additional group of structures that had been inactive is
activated during that pulse, the effect on the beam should
be small. The alternative method is to measure the RF
phase from the structure before, during, and after the
beam pulse. Due to the small beam power relative to the
main RF power, this measurement will require a large
number of averages (~1000 for low current single bunch
operation).
3
LONG FIBER LINKS
EXPERIMENTS
A phase transmission test system has been constructed
to demonstrate the basic technology. With a few minor
upgrades this system is expected to meet the requirements
of the NLC long fiber link system.
TABLE 1: Test System Status
Parameter NLC
Requirements
Test System
Performance
Transmission
distance
1 15
Kilometers
15 Kilometers
Fiber temperature
range
+/- 5
°
C +/-
5
°
C (+/-
10
°
C
expected)
Long term phase
stability
+/- 20
°
X-band
+/- 2
°
X-band
over 3 day
run
Phase temperature
coefficient
<4
°
X-band /
°
C <0.4
°
X-band
/
°
C
Phase noise
<0.3
°
X-band in
10 Hz BW
0.2
°
X-band
in 10 Hz BW
(short term),
1
°
long term
3.1 Test System Design
The system uses a 15 kilometer spool of single mode
fiber (SMF-28) in a temperature controlled oven to
simulate the long fibers in the trenches. The RF phase is
transmitted at 357 MHz (1/32 of the main X-band
frequency) by directly modulating the current of a 1550
nanometer, 1-milliwatt laser.
Fiber phase length control is performed with a 6-
kilometer spool in a temperature-controlled box. The box
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