Proceedings of the 20th Linear Accelerator Meeting in Japan ( September 6—8,1995, Osaka, Japan)

E N E R G Y COMPENSATION SYSTEM OF TH E ATF LINAC
T.Korhonen，H.Hayano, H.Matsumoto, T.Naito, S. TakMa and J.Urakawa
KEK, National Laboratory for High Energy Physics
Oho 1-1, Tsukuba, Ibaraki 305, Japan

Abstract
A method to compensate for beam loading effects in accelerating multibunch beams at KEK-ATF is underdevelopment and ready for beam testing. The system consists o f compensating sections used in addition to regularaccelerating sections. The compensating cavities are operated with slightly different RF frequencies of 2856 土4.33
MHz. The system principles and the components are presented. The measured performance o f the components and thestatus of the development are reported.

Introduction
One of the essential technical developments to get a
sufficient luminosity in a linear collider is the ability togenerate and accelerate multibunch beam trains. In theATF [ 1 ] that is a test site for a future linear collider,this technique is tested by accelerating bunch trains thatconsist of 20 micro-bunches with small (2.8 ns)
intervals (figure 1).
50 Hz repetition rate
2.8 ns bunch spacingM

20 bunches in bunch train
Figure 1 .Beam structure of ATF.
125.0------------- ------••— ;----------

without Bt«m LtMKfing(N«-0.0)* 7.51112.5；(Beam loading)with B«am Loading{N«-2.0E+10)
Energy Sprvad: ± 2.6 %110,0 :

damping ring (DR). The energy acceptance of the DR isonly 1%, because of the necessity to generate extremely
low emittance beams required in a linear collider. Thismeans that beam loading compensation system isnecessary in ATF for successful operation of themultibunch scheme.
Due to the multibunch beam structure conventionalenergy compensation methods are not applicable and anovel one is going to be used in the ATF. In thismethod, "compensating" cavities are installed between
the regular accelerating sections. These cavities aredriven with a RF frequency that is slightly different
from that used in regular sections. When entering thesecavities, successive bunches of the train ride on of adifferent phase of the accelerating field (figure 4).
Due to this phase difference, the energy gain of thesuccessive bunches is different. The energy spread isreduced as the first bunch enters to a decelerating phaseand for the successive ones the energy gain is larger as
the phase approaches the RF field peak. The amount ofcompensation can be controlled by changing theamplitude and the relative phase between thecompensating and regular sections.
Two frequencies (f+Af, f-Af) are needed tocompensate for the single-bunch energy spread. Whenbunches with non-negligible length pass through arapidly changing field, the energy gain will be different
for the bunch head and tail and thus the bunches tend tospread. With two frequencies the bunches enter into
opposite slopes (rising/falling) of the accelerating field
and the net effect is similar to acceleration with a flattop pulse.

Figure 2. Beam loading in the ATF linac轉

As the bunch trains are accelerated, the energy gainof successive bunches reduces due to beam loading in
the accelerating structures. The difference in energy gain
between the first and last bunch is about 7.5 MeV /accelerating section (figure 2). After acceleration in the1.54 GeV linac, the bunch trains are injected into a藉鑛夢II}

filll

Figure 3. Lattice of the ATF linac (L0-L16 regularsections, C1,C2 compensating sections).

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»«1«I••• え• _ •龙” • ^ j ；：；2856-4.32727MHZFigure 4. Principle or the energy compensation.

Compensation efficiency
In the ATF there are 16 regular sections tand thecompensating sections are installed between them asshown in figure 4. The spread in energy gain between
the first and last bunch is 7.5 MeV/accelerating section.
Without any correction, the spread would be roughly
this times the number of accelerating sections, i .e .120
MeV. An estimation of the efficiency can be calculated
as follows. The time difference between the first andlast bunches is t = (19*2.8ns) = 53.2 ns. With thesideband frequency of 4.327 MHz, length of one cycle(360 degrees) is 231 ns. This means that the difference
between the phase in which the first and last bunches
enter into the compensating section is 82.9 degrees. The
maximum input power to the compensation cavities is50 MW, and the maximum accelerating gradient is 26
MeV/m. With these conditions, the energy spread can be
reduced from ±2.6% to ±0.2 % peak-to-peak with beam
intensity of 2* 1010 particles/bunch [1].

MHz) and the sideband (4.327 MHz) signals are first
split into two parts that differ 90 degrees in phase andthen fed into dual balanced mixers. The output from themixers is a product of the input signals, which by atrigonometrical identity can be expressed as a sum of
two sinusoidal signals, with frequencies f+Af and f-Af,
i.e. the desired sideband frequencies. One pair of thesideband signals is in equal and the other pair in
opposite phase. When they *are added in a signal
combiner, the sideband with opposite phases cancels out
and only the desired sideband remains. By reversing thesignals from one of the quadrature phase shifters, bothsidebands can be generated.
This method has also a further advantage of being
feasible with even smaller sideband frequencies, wheremethods like using bandpass filters become exceedingly
difficult. If this energy compensation method is going
to be used with longer pulse trains, a lower sideband
frequency is required to keep all the bunches within 90
degrees (unless the bunch spacing is changed).
We further employed a carrier suppression method toreduce the level of the carrier frequency, that inevitably"leaks" through the mixers.

TimingThis energy compensation scheme is conceptuallysimple but demands very stable reference signals for thecompensating sections. In contrast to typical beam
acceleration where particle bunches are injected onto thecrest of the RF wave, in the compensating system thebunches enter into a phase where the accelerating fieldchanges rapidly (see figure 4). This means that evensmall changes in the signal phase cause a large error in
the energy gain of the particle bunch.
The sideband frequency for the compensation wasselected to be 4.327 MHz, twice the revolution
frequency of the damping ring, to reduce the number of
frequencies in the ATF timing system.
The requirements for the reference signal generation
method are sufficient reduction of unnecessary sidebands
and phase stability. After considering variouspossibilities, a method called phasing single sideband
modulation was chosen. The principle of this method israther simple (see figure 5). The fundamental (2856

Figure 5. Principle of the sideband signal generation.
This technique is often considered to be difficult toimplement. In typical applications it is required that thequadrature signals a wideband at the low frequency. In
our case the frequencies are fixed and all the parameters
for the input signals like their power levels can becontrolled. The method proved to be very effective forour application. Very pure reference signals could begenerated using this method; some results frommeasurements with a spectrum analyzer can be seen in
the figures 5 and 6. The unnecessary frequencies aresuppressed by even 60 dB, which easily fulfills thecalculated requirement of about 40 dB, with a goodsafety margin. In long term tests, due to temperature
variations etc., the suppression ratios were found to drift
a little but even then to stay over 50 dB.

All the reference signals for the ATF are derived
from one master oscillator where a 1428 MHz signal isgenerated. The S-band reference signal is generated bymultiplying this frequency by two, and other necessary
signals are generated by frequency division. The 4.327
MHz sideband signal is generated in a 1/165 divider
from 714 MHz. Tlie two compensating frequencies aregenerated in a module that has a duplicate of thepreviously described hardware.
The reference signals are distributed using a speciallydeveloped temperature-compensated optical fiber. The
signal is first converted to light using electrical-tooptical converter and then distributed using the opticalfiber. The reverse conversion from light to electricalsignals is done near each of the components requiringreference signals (klystron modulators etc.) [2].

Conclusions

Figure 6 . Spectrum of the generated upper sideband
signals (Above:upper sideband, below:lower sideband)

The essential elements for the timing system arecomplete and components which fulfill the design
requirements have been successfully developed.
Although the performance of the system as a whole has
not yet been tested, the performance of the systemshould be sufficient for the ATF operation. The real
performance of the system can be verified in beam tests
in near future. The tests shall give us important
experience about this technique and its application inbuilding a linear collider.

Acknowledgments
The authors wish to acknowledge the continuous
encouragement of Professors Y. Kimura and K. Takata.
and the whole ATF group.
Special thanks are due to Keith Jobe and Marc Rossof SLAC for various discussions about the system andvaluable advice considering the tolerance calculations
and the performance measurements.
Figure 7. The measured phase jitter.
The allowed phase jitter of the reference signals isorder of 1 ps). The jitter due to the reference signal
generator was measured using two different methods toget a cross check. First method was to generate twosignals of the same sideband and to feed them into amixer. The mixer output is proportional to the phasedifference between the signals and by watching thesignal with an oscilloscope the phase jitter can bemeasured. The full width of the jitter measured with thismethod was 1.7 ps, and sigma = 0.8 ps (figure 7.). To
cross-check this result, we used a HP oscilloscope tomeasure jitter. Although the measurement isReferences
1 . ATF Design and Study Report. KEK Report
1995-4. Ed. by F.Hinode, S. Kawabata, H. Matsumoto,
H. Oide, K. Takata, Seishi Takeda and J. Urakawa
3.
T. Korhonen et.al. R&D of the ATF Timing
System. Proc. Intl. Linac Conference (LINAC94)