The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Effects of Controlling the 40-MeV Proton Linac on Transversal Motions
Eiichi TAKASAKI, Zenei IGARASHI, Fujio NAITO, Kesao NANMO and Tatem TAKENAKAHigh Energy Accelerator Research Organization, KEK
1-1 Oho, Tsukuba-shi, lbaraki-ken, 305, JAPAN
Abstract
In order to accelerate high-intensity beams at the 12Ge V proton synchrotron complex, it is important to obtain
detailed information about the characteristics of the beams
accelerated by each accelerator. We have thus designed andinstalled many beam monitors around the 40-MeV proton
linac. Subsequently, effects of controlling the linac havebeen measured in relation to transversal motions.
The effects of a prebuncher-tuning procedure and theadjustment of the phase between two tanks · on thetransversal motions of beams accelerated by. the linac aredescribed in this report. The field induced in the 40-MeV
tank by a chopped beam is also described.
1 Introduction

The KEK 40-MeV Proton Linac has worked very wellto supply beams to the 500-MeV Booster Synchrotron since
the upgrade from the accelerating energy of 20 MeV to 40MeV, 1985. Recently, an increase in the beam intensity of
the 12-GeV Proton Synchrotron is required for experiments
about neutrino oscillation. It is thus very important toaccelerate beams with good quality and high intensity.

In particular, it is very important how to inject beams
ejected from the linac into the 500-MeV Booster, since thespac-e-charge force during the injection process is so strongthat the growth of emittances is generated [1], and thebeam quality is almost determined by the injection
procedure and by controlling the ring RF-system [2].
Therefore, we at first arranged a beam~monitor system, andthen improved the control system of the 40-MeV proton
linac, the RF sources for a prebuncher and a debunchersystem [3]. Figure 1 shows the monitors installed aroundthe 40-MeV proton linac.
Otherwise, at KEK, a 200-MeV proton linear
accelerator [4] for the Japanese Hadron Facility (JHF) has
been designed. The main features of the linac are a highpeak current, a high-average current, a high duty factor andhigh performance for beam-loss problems. It is thus
important not only to calculate the beam-dynamics, but also
to study the accelerating structures, the RF -sources, and thetuning methods. Therefore, at present, we have studied thetuning methods using the 40-MeV proton linac.
In this paper, the effects of tuning procedures of thelinac are described regarding the transversal motions. In
particular, the effects of prebuncher tuning and adjustmentsof the phase between two tanks and the field induced by thechopped beam, which would be used to paint the linacbeam onto the longitudinal phase-space, are described.
2 Effects of tuning a prebuncher

B.M(one tum toroidal)......
D3

LI
rwst

Normally, the tuning procedure of
the prebuncher system has been
carried out by measuring the capture
efficiency of beams at the 20-MeV
tank as an RF -power supplied and thephase between the prebuncher and the20-MeV tank (prebuncher's phase) arevaried, and by observing the
momentum of accelerated beams. In
spite of an R-dependence of theaccelerating field in the cavity of theprebuncher [5], the growth oftransversal emittances is not very large.
We have recently measured theofthetransversal
variations
emittances due to the RF-power
supplied to thecavity of theprebuncher and the prebuncher's phasetank-LEBTAnalyzing systemprofile(1mm step}formeasuring emittanceIii

rme profilemonftor

If f iii

n r~~f.:lonmctnttor- - - - - - - - - - 40MeVLine

Fig. 1 Beam monitors installedaround the KEK 40-MeV proton linac.
The B.M. and velocity monitor are available to tune
the linac.

[6].

As the RF-power supplied to the prebuncher's cavity
increases until about 4 kW, the horizontal emittance varies
remarkably at just the entrance of the 20-Me V tank. Above-458-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

4 kW, the variations of the horizontal emittance have notbeen measured. The variations of the vertical one, however,
have been observed little. Examples of the measurec:l results
are shown in figure 2. Thus, these variations of thehorizontal and vertical emittances would be caused by the
space-charge forces in the process of bunching because thehorizontal P-functions (px) are much smaller than thevertical py in the beam line between the prebuncher andthe 20-Me V tank, as can be seen in figure 3.

J...............
:<Or·.. .

()'"··-··· . ~~--30<i-10~F.

XX30~ ::

ort

3 Effects of the phase between two tanks'·accdcrdting \·oltage~

rm.s emittanCe=;:-ahout OA-30

Fig. 2 Horizontal emittance at just the entrance of the20-MeV tank. The R-emittance is strongly dependent
upon the RF-power supplied until about 4 kW.

8i3Y..§§ 6J~~ : [···:::·::::·<. . . 13x
CD0~0

At the exit of the 20-MeV tank, we measured thehorizontal and vertical emittances while varying theaccelerating voltage of the prebuncher and the prebuncher'sphase. These variations of the parameters were carried out
within a longitudinal acceptance of the 20-MeV tank. The
particle distributions inside the vertical emittance and theskirt of the emittance were varied. However, horizontallythese variations have been little measured. Figure 4 shows
the variations of the vertical emittance with theprebuncher's phase. Therefore, we think that regarding thevertical motion of the beams, at just the injection stage ofthe 20-MeV tank, the coupling between the transversal andlongitudinal motions through the space charge would cause
variations of the vertical emittance.

-~·-·J. .....

0.2~;:

j ................ : : : :

0.4

0.6

~~
08

Length (m)---5~1-B-~---IJ0!4015

Tank

Fig. 3 P-function ofthe beam line.
px is much smaller than py between the prebuncher
and the 20-MeVtank.
This figure is referred from Ref. [5]An adjustment of the phase between two tanks is a very
important tuning procedure to control the central
momentum and the momentum spread of beams. However,sometimes, the effect of this adjustment on the transversal
motion is not considered. Thus, an optics mismatching
would be generated on the transport line. At KEK, a few
BPMs (like the bottom type) are installed in the 40-MeVline. We have observed the dependence of the beam
position on the phase between two tanks using the BPMs
installed at just the exit of the 40-MeV tank and at 470 mm
downstream from the Q8-magnet. Of course, theadjustment of this phase causes variations of theaccelerating energy of beams. The measured results are
given in figure 5. As can be seen in Fig. 5, it is obvious thata beam ejected from the 40-MeV tank has thecharacteristics of dispersion. We think that the dispersion
property of the beam would be produced by an injection
error into six-dimensional phase space. The injection error
is caused not only by a tuning error of the beam orbit , butalso by an alignment error of the drift tubes, and variationsof the accelerating field strength in the 20-MeV tank andthe phase between two tanks.

4 Field induced by chopped beams in the cavity
The 200-MeV proton linac for the JHF would accelerate
chopped beams with a micro structure and a long pulse-

Vertical Emittance at the exit of the lOMeV tankPhase~3SPhase~?SFig. 4 Variations of the 20-MeV vertical emittances due to the phase between the prebuncher and the 20-MeV tank.

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The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Horizontal positLon vs. accelerating enery

inWced-fieldby the chopping-beam-1.5•

P3,t=10IIPl,t=lO·20.,•post•la TMOll-2..5-1.25

4post-2-1-0.5

Velocity after40MeVtank 01)

1.1

Vertical position vs accelerating energy

1.2

1.3 1.4l,j

1.6

1.7

1.8

1.9

2l.l

2.2

2.3

chopping-fre!piencies (MHz)•PJ,t=lO•

Pl,t=lO

Fig. 6 Field induced by the chopped beams
in the 40-MeV tank
Beam induced field distrirution-12.5·1-0.75-0.5-0.2.5-30

Velocity after40Mevtank 0/)"il-35£ ~ -40~ ~ -45

Fig. 5 Variations of the beam position
by adjusting the phase between two tanks.
P1, just after the 40-MeV tank,P3, 470mm down from Q8-mag. on the 40-MeV-line.

• 202.6MHz

il ~ -50 "·:--:-:~:-:-:--:-1<·:-:-·:-!·:-·:-·~ i1J -55 &Pf*'++f4+¥.

·"- -60

1!11199.9MHz

t±uilltilll~lllilzrm~J
10

RF-monitor position (m)high intensity. Such beams have many frequency
components, like (accelerating frequency)xn ± (choppingfrequency)xm. The beginning of each chopped beam micropulse is just a transient. On the other hand, the cavities
used in the proton linac have many resonance modes likethe TM and TE modes. Therefore, a resonance mode would
be excited by chopped beams with a micro structure in acavity if the frequency of the resonance mode and that ofchopped beams overlap.
At KEK, recently, a beam-chopping system [8] has beendeveloped and tested. We have thus measured fields
induced by chopped beams in the 40-MeV tank asvariations of chopping frequencies [9]. The rf-loop
monitors are installed for checking the field strength of theaccelerating mode. Thus, the TM01n-like mode has only
been observed by using those loop monitors. The results
observed are shown in figure 6. The measured resonant
frequencies are 198.8MHz (post-2), 199.9MHz (post-1) and202.6MHz (TMOll) on the 40-MeV tank. On the 20-MeV
tank, a frequency of 202.7MHz (TM013-mode) has been
observed. The ratio of the field strength induced bychopped beams with an intensity of about 8 rnA to the main
field strength (201.06MHz) is about -50dB. This value iscomparable to the ratio of the field induced by theunchopped beam with an intensity of about 130 rnA [10].
The resonance mode has been determined by measuring thefield distributions along the 40-MeV tank, as shown infigure 7.
As a result of a preliminary measurement, fields(TMO In-like modes) induced by a low-intensity chopped
beam could not cause any trouble with the beam dynamics.
Next time, we will equip the other rf-field monitor in
the 40-Me V tank, and will observe the effects of otherresonant modes on the beam dynamics.

Fig. 7 Field distributions along the 40-Me V tank.
By these distribution patterns, each of the resonance modes
has been determined.

5 Conclusion
From these studies, it is obvious that it is very importantto establish tuning methods for the stable operation of thelinac, particularly, the 200-MeV proton linac for JHF.
Finally, we must emphasize that all of the measured
emittances are much less than the acceptance of the 500MeV-Booster at KEK.
References[1] N. Kumagai et al., KEK Accelerator Department
Internal Note, ASN-268, Apr. 1987.
[2] S. Ninomiya, Private communications.
[3] Z. Igarashi et al. , "Introduction of the Solid State RF
Sources at KEK 40-MeV Proton Linac",in this Proceedings.
[4] T. Kato, KEK Report 96-17.
[5] T. Kato, KEK Report 91-10.
[6] E. Takasaki et al., KEK Accelerator Department
Internal Memo, SR-357,374,377,382.
[7] E. Takasaki et al., KEK Accelerator Department
Internal Memo, SR-388.
[8] K.Shinto et al., Rev. Sci. Instrum. 67(3), p.l048,March 1996.
[9] E. Takasaki et al., KEK Accelerator Department
Internal Memo, SR-379,385,405.
[10] T. Kato and E. Takasaki: IEEE Trans. Nucl. Sci.
Vol. NS-28 N0.3(1981) p.3507.

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