A bunch by bunch profile monitor system using optical
transition radiation (OTR) is developed for an Accelerator Test
Facility (ATF) linac. The ATF consists of a 1.5 GeV linac and
a damping ring is now under constructing in KEK. The linac accelerates
a multi-bunch beam (20 bunches/pulse, 2 ¥
1010 electrons/bunch, 2.8 ns spacing between bunches).
The energy spread of the multi-bunch caused by the transient beam
loading is a significant problem for the injection of the damping
ring. The linac has energy compensation system to compensate the
energy spread of the multi-bunch. In order to measure the energy
and energy spread of each bunch, we developed the monitor system.
The system and the measurement result are reported.
The Accelerator Test Facility (ATF) consists of a 1.5 GeV linac and a damping ring(DR) is now under constructing in KEK. The DR is designed to realize a small vertical emittance, eny = ~30 nm, for future Linear Collider. The commissioning of the ATF linac had been started from November 1995 and the commissioning of the DR will be started in the end of this year. The linac accelerates multi-bunch beam. The beam has 20 bunches of 2 ¥ 1010 electrons with 2.8 ns spacing. In order to reduce the energy spread of the multi-bunch beam, due to the transient beam loading, Energy Compensation System (ECS) were installed and the preliminary experiment was carried out [1].
The measurement system of the energy and the energy
spread of each bunch is needed for tuning the ECS. The optical
transition radiation (OTR) monitor was already developed for the
80 MeV injector section [2]. The OTR is emitted when the charged
particles go through the interface which have different dielectric
constants. The polished stain less steal was employed as the emitter
for the OTR monitor. A fast gate camera (Hamamatsu C2925) is used
for observed the bunch by bunch profile in the multi-bunch beam
when apply the gate signal to each beam timing. This monitor could
measure the beam emittance, energy and energy spread of each bunch
at the 80 MeV injector section. The OTR monitor at 1.5 GeV
section is designed and tested for the above purpose. The spot
size limit of the OTR monitor according to ~gl/2p
[3]. At 1.5 GeV section, the spot size
limit is 0.24 mm for 500nm wavelength. This value is assumed that
is not so affected the beam size measurement. Recently, the spot
size limit was discussed and tested [4, 5].
The monitor setup is shown in Fig. 1. The OTR monitor
is located at the downstream of the first bending magnet and the
first quadrupole magnet of the beam transport line. The position
deviation at the place is calculated by
,
where h is the dispersion function.
The actuator system has two screens and can be stopped
each screen position at the beam line. One is fluorescent screen,
the other is the OTR emitter made by polished stain less steal
which has 1 mm thickness and 1/2l
flatness. The emitted light (fluorescent light/OTR)
is reflected by a mirror to avoid the x-ray and fed to a gate
camera and a CCD camera by a half mirror. The fluorescent light
is observed by the CCD camera and the OTR is observed by the gate
camera. Both profile by these monitors can compare each other.
Trigger control and video analyze system
The control system of the gate camera is shown in
Fig. 2. The gate camera can observed the bunch by bunch beam profile
when apply the appropriate gate width and timing. The timing signal
is created from the beam trigger. The signal is delayed in 2.8
ns step and met to each beam timing by the delay module. The delay
module makes delay by count the reference clock from start signal.
The reference clock is synchronized to the accelerating frequency.
The trigger jitter of the delay module is less than 10 ps. The
fine delay C1097 (Hamamatsu) adjusts the gate timing to the center
of the beam timing. The pulse generator 8112A (H.P.) makes the
gate width included offset of ~16.5 ns. The gate pulse of 3 ns
is applied when the pulse with 19.5 ns pulse width is generated
by the pulser. The 8112A and the C1097 are controlled by sub-control
computer (PC) thorough GPIB.
The video signal of the gate camera is fed to the
operator room through CATV and analyzed by the video analyzer
using a work station. The analyzer calculates x- and y-direction
of the projection and the fwhm, the peak position, the peak value,
etc., in real time. The automatic data acquisition system from
Accelerator control computer (VAX VMS) is under development.
Gate characteristics
The characteristics of the gate camera is measured
by using the beam signal. The gate timing is scanned with 250
ps step. The intensity of the profile is intensified and eliminated
by the gate timing. The characteristics is plotted in Fig. 3.
There is a dip between the previous bunch timing and the next
one. This means that the gate camera is distinguishable the profile
between the previous bunch and the next bunch. The appropriate
gate timing is decided from this data.
ECS system [6]
The ATF linac uses 18 accelerator structures of S-band
frequency. 16 regular sections which uses 2856 MHz and two compensation
sections which uses 2856±4.3 MHz. At the regular sections,
the first bunch feel the maximum field of the cavity and the following
bunches feel the reduced field caused the transient beam loading
effect. At the compensation sections, each bunch feel the field
of the different phase of the cavity. The phases of compensation
sections are synchronized the beam from decelerate phase (first
bunch) toward to accelerate phase (20 the bunch). The compensation
effect is adjusted by changing the power of compensation section
and the relative phase of regular sections and compensation sections.
Relative phase and energy gain measurement
The relative phase and the power of the compensation
sections were measured by scan the phase of the compensation sections.
The profile center of one of the bunches measured by the OTR monitor
were plotted in Fig. 4. The deviation from fitted value is come
from the non-linearity of the phase shifter.
Multi/single bunch energy spread measurement
The experiment was carried out following conditions,
Energy = 1.16 GeV, bunch number = 20, total charge/pulse = 3.2
¥ 1010,
5.3 ¥
1010 electrons. It was measured at the case of a) ECS
off, b) ECS+ Æf on, c) ECS±Æf on. The bunch shape
is shown in Fig. 5. The multi-bunch energy spread was calculated
from the current of the bending magnet and the deviation from
the center position of the profile which was measured by the OTR
monitor. The single-bunch energy spread was measured from the
FWHM of the x-direction distribution of the profile. The multi-bunch
energy spread and the single-bunch energy spread was plotted in
Fig. 6a), 6b) and Fig. 7a), 7b), respectively.
We could measure the multi-bunch energy spread and
the single-bunch energy spread of the ATF linac using the OTR
monitor. The ECS effect was confirmed that the multi-bunch energy
spread was reduce from 3% to 0.5% at the case of total charge/pulse
= 3.2 ¥
1010, 5% to 1% at the case of total charge/pulse =
5.3 ¥
1010, respectively. These results were lager than the
calculated value from the transient beam loading. It's assumed
that the RF timing of each section were not optimized for minimize
the multi-bunch energy spread. The single-bunch energy spread
was less than 0.5% except for first bunch at the case of total
charge/pulse = 5.3 ¥
1010. Some deterioration for single bunch energy spread
by the ECS effect wasn't observed in these intensities.
We would like to express our thanks to Professors
Y. Kimura and K. Takata for their encouragement. We also
thank other members of ATF group and Mr. S. Morita of E-cube Corporation
for their support.
[1] JLC Group, "JLC-I", KEK Report 92-16, 1992.
[2] T. Naito et al., "Bunch by bunch beam monitor for ATF Injector Linac", Proc. of LINAC94, Tsukuba.
[3] M.J. Mordan, "Practical application of coherent transition radiation", NIM B33 (1988) p. 18.
[4] D.W. Rule et al., "Beam profiling with Optical Transition Radiation", Proc. of Particle Acc. conf. Washington, 1993.
[5] X. Artru et al., "Experimental Investigation on geometrical resolution of electron beam profiles given by OTR in the GeV energy range", Proc. of Euro. Particle Acc. conf., 1996.
[6] S. Kashiwagi et al., "Preliminary test of
±Æf energy compensation", in this conference.