The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

MEASUREMENT OF ENERGY DISTRIBUTION
IN MACROPULSE OF ELECTRON LINAC BEAM AT LNS
K. Shinto, H. Hama, F. Hinode, A. Kurihara, A. Miyamoto,
M. Mutoh, M. Nanao, Y. Shibasaki, S. Takahashi and T. Tanaka
Laboratory of Nuclear Science (LNS), Tohoku University,
1-2-1 Mikamine, Taihaku, Sendai 982-0826, Japan

Abstract
Both energy and time structures of the beam extracted
from a pulse stretcher ring strongly depend upon theenergy distribution in a macropulse of the injection beam.
Measurement systems of the energy distribution of themacropulse accelerated by a linac have been developedat LNS, Tohoku University. Using the systems, energy
profiles and spectra in the macropulse of the linac beam
was measured.

1 INTRODUCTION
Operation of the STretcher-Booster (STB) ring at LNS[1, 2] began in 1997 for nuclear physics experiments, andthe accelerator itself has been also investigated. The STB
ring has two functions. One is beam acceleration of 200
MeV linac beam up to 1.2 Ge V and beam storage toproduce GeV y rays [3]. Another function is a pulse
beam stretcher to supply quasi CW-beam by employing aslow extraction method.
Using the STB ring as a pulse stretcher, the beam isextracted by the third order resonance in the horizontal
phase space. The horizontal betatron tune reaches thethird order resonance by the energy loss due to thesynchrotron radiation. Simulation study for the extraction
under the influence of the beam injection condition isalso carried out and will be presented [4]. At LNS, thebeam from the linac is injected into the STB ring withthe repetition rate of 50 Hz or the multiples of it (max.
300 Hz). The beam extracted from the STB ring must beuniform in beam intensity during the extraction duration.
The injected beam energy from the linac is normally 200
MeV, and the energy loss is about 0.5 % during therepetition period of 3.3 ms at the 300 Hz injection rate.
To produce the constant intensity of the extracted
beam, the energy distribution of the injected beam must
be uniform in a macropulse. In the case of existence ofany structure in the energy distribution of the macropulse,
the intensity of the extracted beam is possibly fluctuatedand then the time structure of the spilled-out beam isnever uniform.
The circumference of the STB ring is designed to be49.75 m, and the three-tum injection is carried out. Thepulse width of the injected beam of the uniform energy
distribution is required to be 500 ns. The LNS linac isdriven by five klystrons. The pulse width of the applied

rf voltage of klystrons is fixed at 4 flS. The timing of therf voltage of each klystron and that of the beam trigger
can be changed independently. Therefore, the energy ofthe accelerated beam in the linac may be fluctuated. To
produce the appropriate beam for the injection, theenergy distribution of the macropulse must beinvestigated.
Measurement systems of the energy distribution in amacropulse have been developed at LNS. In this paper,
the experimental apparatus and methods are reported in
section 2. Results of the energy distribution of themacropulses are reported in section 3. In section 4,summary and future plan are reported.

Figure 1: Schematic of the experimental set-up. The
beam energy is determined by measuring the magnetic
field of the pilot magnet that is connected in series withthe bending magnets. The electric charge of the pulse
beam is measured by an SEM that installed downstreamof the energy analyzing system.

2 EXPERIMENTAL SYSTEM AND
METHODS
The measurement system of the beam energy spectrum
is shown in Figure 1[4]. The accelerated beam from thelinac is selected by an energy analyzing system in thebeam transport line. The energy analyzing system-352-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

consists of two bending magnets, a quadrupole magnet
that is located between the bending magnets, and a slit
installed the place where the energy dispersion exists.
The beam energy is selected by changing the magnetic
field of the bending magnets. The beam intensity is
obtained by using the secondary electron monitor (SEM)
installed downstream of the analyzing system. The beamenergy can be accurately decided by the strength of themagnetic field, and the energy resolution is determinedby the width of the slit.
The center energy of the analyzed beam is obtained bymeasuring the magnetic field of a pilot magnet that isconnected in series with the bending magnets. The
measurement of magnetic field is performed by an NMR
method.
In these experiments, the NMR resonated rf output
measured by a frequency counter are stored in a PC via a
GP-IB interface and the center energy is calculated fromthe measured frequencies.
The SEM consists of 5 electrodes made of Al foils,
and the secondary electrons produced by the pulse beamin the SEM is collected by applying high voltage. The
output of the SEM from each pulse is measured by adigital oscilloscope. The results measured by the digital
oscilloscope are saved by the PC via GP-IP interface.
After the measurements, the amount of the electriccharge in one macropulse is calculated by integrating theoutput signal.

It is found that the beam loading and the rise up periodof the rf voltage drastically affect the energy distributionof the linac beam. The timing of the rf voltage of five
klystrons and that of the generation of the beam from theelectron gun is considered to be adjustable by watching
the energy distribution using this system. The uniform
energy distribution in Figure 2(B) was about 500 ns atthe center energy of 192 MeV with the energy resolutionof0.5%.

Current [a.u.]

210

34(A)

210

200

200190190(B)
>210

210

6"
~200200~190

3 EXPERIMENTAL RESULTS ANDDISCUSSIONS

210

3.1 Measurement by using SEM
Energy spectra of the macropulses were measured bychanging the timing of the beam generated from the gun
with respect to the rf timing of the linac. The results ofthe energy spectra and the energy profiles are shown inFigure 2. The accelerated beam energy was about 200
MeV. The width of the slit to decide the energyresolution was 2 mm. This corresponds to an energy
resolution, &IE, of about 0.1 %. Beam pulse accelerated
at the flattop of the rf envelope became in an energy
stable region because the supply of the rf voltage reducedby the beam loading is kept at a balanced state. In Figure2(A), the beam energy in the beginning of themacropulse is low. It is obvious because the beam pulse
was accelerated by a lower rf voltage at the rise up of therf envelope. Although, as shown in Figure 2(B), thebeam energy became stable from the beginning of theflattop of the rf envelope after the transit time passed.
The transit time of the accelerating structures of the LNS
linac is about 0.8 JlS. When the macropulse length islonger, as shown in Figure 2(C), the beam energy islowered at the falling down region of the rf voltage andthen the beam is out of the stable region.

Time [IJS]

190(C)

210

200

200

190

190
Current [a.u.]Time [IJS]

Figure 2: Results of the measured energy spectra andthe profiles obtained by SEM. (A) Beam acceleration isstarted at the rise up of the rf envelope, and the pulse
width is 1.5 JlS. (B) Started at the flattop of the rfenvelope. (C) Started at the flattop of the rf envelope,and the pulse width of 1.9 JlS.

3.2 Measurement by using Cherenkov radiation
Another apparatus taking the place of the SEM is used
for the measurement of the charge distribution in amacropulse, in which air Cherenkov radiation from thebeam is employed. The photons are detected by anavalanche photodiode (APD). This measurement system
was installed downstream of the SEM. The experiments
were carried out to compare with the SEM.
The result of output signal of APD measured bychanging the width of the energy slit is shown in Figure
3. The output of the APD corresponds to the beam-353-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

intensity. In this figure, the macropulse has a uniform
time distribution. However changing the energy width, it
was found that the energy distribution was not uniform
actually. This is caused when the timing between the rf
envelope and the beam trigger is not matched.

1.5-AEIE=2.0%

1.0- .. -dElE= 1.0%
AEIE=0.75%·······-· AE/E=0.5%- - L1B'E=1.5%~~<a.g01c--- L1B'E=0.4%
0.5~
0.0-0.5 ' - - - - - - - - ' - - - ' - - - - - ' - - - . . . _ _ __
0.0
0.5
1.0
1.5
2.0

____.._ __..J
2.5
3.0

timei(IJS)

Figure 3: Output signal of the APD measured bychanging the width of the energy slit. The center energythat is derived from the magnetic field of the analyzer
was set to 149.98 MeV.

distribution was, however, much distorted and uniform atonly narrower region as is shown in the center figure.

4 SUMMARY AND FUTURE PLAN
Two measurement systems of the energy distribution
in a macropulse of the linac beam have been developedat LNS. The charge distribution in the macropulse wasmeasured by an SEM detecting the secondary electrons
and an APD detecting Cherenkov radiation. The identicalmeasurement was able to be performed by both thecharge detection apparatuses. The two-dimensionalenergy spectrum measured by these systems shows boththe time structure and the energy distribution of themacropulse. The diagnostic and tuning of the injectedbeam are easily performed by using these systems.
However, using these measurement systems, theinjected beam is destructed, and we cannot measure theenergy distribution at the same time with operating theSTB ring. Instead of these measurement apparatuses forthe electric charge, it is planed that an electric charge ismeasured by using photons of synchrotron radiation
emitted from a bending section in the beam injection
line.

REFERENCES[1] H. Hama et al., "Current Status of a 1.2 GeV Booster

Electron Synchrotron and Implementation for
Nuclear Study at Tohoku University", Proceedingsof the 18'h International Conference on High Energy
Accelerators (HEACC2001), Tsukuba, (to bepublished).
[2] A. Miyamoto et al., "Measurements of OpticalParameters of the STB Ring at INS'', in theseproceedings.
[3] J. Kasagi, "Nuclear physics experiments with
1.2-GeV STB rint; at l.NS-Tohoku", 2"dKEK-Tanashi InternatiOnal Symposium on Hadronand Nuclear Physics with Electromagnetic Probes,Tanashi, (1999). LNS Report (LNS2018-00).
[4] H. Hama et al., "Effect of Injection-mismatching upon

TirreiiJIII)

Figure 4: Energy spectrum of the macropulse measured
by changing the magnetic field of the analyzer. Theenergy resolution was set to 0.25 %. The density in thecenter figure shows the intensity of the output signal oftheAPD.

Slow Beam Extraction in an Electron Stretcher
Ring", in these proceedings.
[5] A. Miyamoto et al., Proceedings of the 25'b Linear
Accelerator Meeting in Japan, pp.398-400 (2000) (inJapanese).

To understand the variation of the energy distribution
in the macropulse, the result is shown as a twodimensional spectrum in Figure 4. This result wasobtained by changing the magnetic field of the analyzer
magnets. The width of the slit was fixed to the energy
resolution of 0.25 %. The left side figure shows theenergy distribution of the macropulse. From this figure,the center energy of the beam was 150.47 MeV, and theenergy spread was about 0.46 % derived from Gaussian
fitting. The downside figure shows the time structureof the macropulse. The beam intensity of the macropulse
was apparently uniform for -1.5 J.I.S. The energy-354-