JAERI-Conf95-021

2a1-1
PRESENT STATUS OF NAR
Masayuki NAKAJIMA, Koji YAMADA, and Teruo HOSOKAW A
NTT LSI Laboratories, Atsugi, Kanagawa, Japan
Abstract
Transverse coupled bunch instability, ion trapping effect,and closed orbit distortion are shown to be fundamental problemsof NTT normal-conducting accelerating ring (NAR), which
adopted extremely low injection energy (15 MeV). Elucidationof these problems as well as present status ofNAR are presented.
1. Introduction
NTT LSI Laboratories have constructed their own
synchrotron radiation (SR) facility to develop X-ray lithographysystem. The facility consists of two storage rings, NAR 'l andSuper-ALIS 2>, and injector LINAC 3>. NARis a multi-purpose
ring which functions as a booster synchrotron for Super-ALISand as a storage ring for SR applications.
To minimize the size and cost of injector LINAC, low
injection energy (15 MeV) is adopted. Therefore, beam dynamics
in low energy region had to be clarified to attain enough beam
current. Outline of NAR is shown in Fig. 1 and fundamental
parameters are listed in table 1.
Operation of NAR started in November 1987. Wesucceeded in extracting SR in May 1988. A beam current was
about 20 rnA at the beginning and stayed the same level for a few
years. At injection energy, because of the short life time and thelack of appropriate monitors, we had difficulties in analyzing thephenomena and improving the beam current. Closed orbit
distortion due to small magnetic fields or magnet displacement
also offered serious problems. With difficulty, transverse coupled
bunch instability and ion trapping effect were shown to be current
limiting factors. As a result. of solving these problems, as much as
120 rnA could be stored.
On the other hand, we succeeded in extracting high energy
electrons to inject Super-ALIS in 1992. Super-ALIS, which could
have stored 200 rnA with low energy injection scheme, attained
more than 700 rnA by injecting high energy electrons from NAR.
In this paper, improving process and present status ofNAR
are presented. Problems peculiar to low energy injection are also
described.
2. Improvement Process and Present Status of NAR
Improvement process of NAR beam current is shown inFig. 2. At the beginning of the operation, current limiting factors
were not clear and suitable parameters were surveyed without
any reference. In May 1991, the beam current increased byadjusting RF modulation pattern during acceleration but the reason
why the modulation is effective was not clear. We can explain
now that transverse coupled bunch instability was avoided by thismodulation. The alignment of magnets suddenly displaced in
October 1991 and then re-alignment were performed immediately.

Table 1.
NAR parameters
Final energy
Injection energy
Circumference
Radio frequency
Beam current
Betatron tunes
VxVy(achieved)(designed)
800MeV15MeV
52.8m
124.985 MHz
125 MHz
120mA
500mA

__,____-April 16, 1993
March 12, 1992
March 27, 1991

3.25
1.25

3.15
1.46

May 25, 1990

Elapsed Time (hr)Fig. 2 Improvement process of NAR
Closed orbit distortion was also corrected again both at injection
and during storage. Octupole magnets were newly installed inApril 1993 because transverse coupled bunch instability was
shown to limit the beam current of NAR. After all, we can store
maximum 120 rnA. Although it is not shown explicitly in thisimproving process, the decrease of vacuum pressure also seems
to contribute to the current increase because ion trapping effectbecame somewhat suppressed. These problems are described later.
In NAR, six beamlines have been constructed and areused. Normally, three days a week (Wednesdays, Thursdays, andFridays) are used for SR applications. The other two weekdays(Mondays and Tuesdays) are used for machine study or system
maintenance of NAR and/or Super-ALIS.
3. Problems of Low Energy Injection Scheme

Fig. 1 Outline of NAR

Through the machine study ofNAR, several problems oflow energy injection scheme were shown. These problems aresummarized here.
As lower energy electrons are more liable to be affected- 174-JAERI-Conf 95-021

by electromagnetic fields, collective phenomena such as beam
instabilities are apt to be induced. However, in such low energy
as adopted in NAR, radiation damping time is so long that large
beam size at injection preserves over a long period. Large beam
size may weaken the collective phenomena. Therefore, the effectcannot be estimated simply from electron energy. For example,average beam size of NAR at injection is estimated as large as(cr , cr) = (5 mm, 2 mm) by simulation while the size in final
energy is calculated as (cr, cr) = (1.2 mm, 0.35 mm) from theory.
Longitudinal beam size (burich length) at injection is also muchlarger than that in storage. This beam size difference affects
drastically in case of transverse coupled bunch instability or ion
trapping effect. The analyses are described later.
Concerning single particle behavior, closed orbit distortiondue to small magnetic fields ·such as geomagnetism or residual
fields is fatal. More than a few tens milli-meters closed orbit
distortion is roughly estimated. Meanwhile, the velocity of low
energy (15 MeV) electrons is not so close to the light velocity
that the electrons in NAR can be accelerated without radio
frequency modulation 4>.
The reproducibility worsens at injection in case of NAR
because the stability limit required to magnet sources is determinedsupposing the stability in final energy.
4. Transverse coupled bunch instability
Abrupt loss of electrons is often observed just after
injection or in the middle of acceleration. Whether the electron
loss occurs or not or when the loss occurs depends on various
injection conditions. Furthermore, once the loss occurs, electrons
are lost immediately. These factors made it difficult to examine
the case.
Judging from the symptom, the phenomena were supposed
to be due to transverse coupled bunch instability. To ascertain
this assumption, it is difficult to use conventional method, that is,observing the corresponding peak in RF pick up signal with
spectrum analyzer because the loss occurs faster than the scanningspeed of spectrum analyzer. Fortunately, the higher order modeof RF cavity was measured before the installation and TM110like mode was supposed to cause the instability. RF pick up signal
was observed with a spectrum analyzer triggered by injection
pulse. The center frequency was set nearby 493.5 MHz (thecorresponding frequency ofTM110-like mode) and the frequency
span was set to 0 MHz. The timing of beam loss was monitored
by fundamental mode frequency component of RF pick up signal.
As shown in Fig. 3, the frequency component near by TMllOlike mode appears in the beam signal in coincidence with beamloss. This is a distinct proof that the beam loss is due to transverse
coupled bunch instability induced by TM110-like mode RF cavity.
As a result of above experiment, we decided to introduce
four octupole magnets. Then we could store more than 100 rnA.
Meanwhile, transverse coupled bunch instability shows
an interesting aspect in low energy region 5>. The strength (growthrate) of the instability is expressed by the following formula 6 >,though the transverse
coupled bunch
instability can beavoided by controllingthe chromaticity, itcannot help occurringin the middle ofacceleration. This is
one of the reasons why
transverse coupled
bunch instability
occurs at rather highenergy.
5. Ion Trapping EffectFig 3. RF pick-up signal withthe occurrence of transverse
coupled bunch instability

NAR has 12
button type electrodes
to clear trapped ions. The high voltage of 200 V and 800 V isapplied to the electrodes during injection and storage. They are
effective but seem to be insufficient to clear ions completely.
At injection, the life time is much shorter than expectedfrom vacuum pressure. This is supposed to be due to trapped ionswhich may be stored in neutralization pocket even if clearing
electrodes are used 7>. This phenomenon seems to limit the beam
current ofNAR.
6. Closed Orbit Distortion
In NAR, which adopted 15 MeV injection energy, closed
orbit distortion due to geomagnetism, residual fields from magnets,
and leakage fields from Super-ALIS can be more than a few tens
mm without correction.
As for geomagnetism, assuming the flat distribution ofgeomagnetism of 0.3 tesla in the straight section of 3.5 meterslong, 15 MeV electrons are kicked about 2.1 mrad. Therefore,
electrons cannot circulate the ring without correcting the effectof geomagnetism. To cancel the geomagnetism, solenoid coils
are set at the two long straight section reserved for insertion
devices. However, it is impossible to cancel the geomagnetism
which exists narrow spaces between the magnets. Then, closed
orbit distortion is corrected by the steering magnets together withthe effects of residual fields or alignment errors of multi-polemagnets.
The effects of residual fields of magnets are also examined.
NAR consists of several kinds of magnets which have iron poles.
The injection energy of NARis 1 I 50 times as small as the finalenergy. As a result, residual fields of the iron poles are notx
"ounstableoccasionally unstablestablelOCO~--------~--------------------~_1_ = elt o~c•v ReZ(It 1)P <t )rn,nrn,rj
2E'tins!

00

where e is the electron charge, I is the beam current, f0 is therevolution frequency, ~ is the betatron function at the RF cavity,
E is the electron energy .c~e Z is the real part of transverse coupling
impedance and F' is the form factor. F' represents the frequency
components of an electron bunch. The center ofF' is determinedby chromaticity of the ring while the width ofF' is inversely
proportional to the bunch length. The value ofF' at higher ordermode frequency determines the strength of transverse coupled
bunch instability. From the shape ofF' distribution, it is clear thatthe instability is slightly dependent on chromaticity in final energy
whereas it has strong dependence at injection energy. Fig 4 is thechromaticity dependence of the transverse couple bunch instability
observed in NAR at injection energy. The strength of the instability
is measured by the octupole current required to suppress theinstability. That is, direct observation of the form factor
distribution became possible in low energy injection storage ring.
As the electron energy increases, the bunch length shortens
and the width of the form factor becomes wide. Accordingly, even

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Fig. 4 Chromaticity dependence oftransverse coupled bunch instability

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JAERI-Conf 95-021

negli.gible after magnetization. Even if the sextuple magnets orsteenng m~gnets are not magnet~zed, they get magnetized by theleakag~ fteld fro'? the bendmg magnets which are fully
magnetized. ~e residual field reaches a few gausses. Accordingly,
magnets ~htch are con~ected to bipolar power sources arede'?agnetlzed before routme operation. The other magnets with
urn-polar power sources are used after the same magnetization
pattern so that t~e ~eapp~arance of residual field is guaranteed.
~en, clos~d o~btt ~1stort10n can be corrected by steering magnets
without adJustmg m each operation.
Closed orbit distortion is also caused by leakage fieldfrom Super-ALIS though the situation is unique to our system. If
the su~erconducting bending magnets in Super-ALIS aremagnetized fully, the leakage fields attains as much as a few
ga~sses on the NAR orbit. Luckily, if the current of the solenoid~011 placed at the nearest long straight section from Super-ALISIS sent properly, the closed orbit distortion due to the leakage field
can be roughly corrected.

4.0IJ---

Jan. 27, 1990
Oct. 19, 1991

2.0

I

0.0

7. Reproducibility-2.0'------------------_j

One. of the. problems that makes machine study at earlystage complicated ts bad reproducibility of injection experiment.
One of the distinct reasons is the offset change of magnet
power ~ources. Pow~r sources of bending or quadrupole magnets
are destgned. to ac~t~ve 104 stability against maximum output.
~?w~ver, thts stab1hty correspond to as much as 0.5 % against
InJection level. Moreover, the accuracy and stability of 104 aresecured by the feedback system from the output current monitors(Dccn to power sources but the offset value of this DCCT shows
long term shift which results in the bad reproducibility of output
current.
Ap~ther reason which affect the reproducibility is thecharactenstlc change of LINA C. Energy, current, or position of~lectrons from LINAC changes slightly per pulse. After anmterval, LINAC sometimes shows larger characteristic shift. In
con~equence,. adjustment of microwave output is required inroutme operatiOn.
. . The repro?ucibility at injection is determined by various
conditiOns but mam factors are focused on the reasons describedabove.
8. Magnet Displacement.

. Another factor which made machine study more difficult

IS the displacement of the magnets. When we measured the verticalalignment of the magnets a few years after the installation morethan a fe:-v milli-meters alignment errors were found out (Fig. 5).
Th~s~ .alt~ment e~or~ are very large considering the error limit
at mttlal mstallatwn IS less than 0.1 milli-meters. When thisdisplacement happened is not clear. As the operation condition
was adjusted under these alignment errors, no correction was madeeven though these alignment errors were detected.
~oreover, the alignment suddenly changed in September
19~1 (Ftg. 5). The change must have happened in a week duringwhtch no-one used SR. An earthquake which happened in themiddle of this interval might have caused this displacement. Fig.
5 shows the similarity between the original magnet displacement
and the second alignment change. This similarity suggests thatthese displacements are caused by the same reason.
. . . The second alignment change result in decrease ofInJeCtiOn current and movement of SR position which could not
be corr~cted sati.sfactorily with steering magnets. In consequence,the verttcal re-alignment was performed and closed orbit distortion
was re-corrected with steering magnets both at injection and duringstorage.
9. Shortening of Life
During the storage, sudden change of beam life issometimes observed. Two kinds of life shortening seem to exist.
One of them is thought to be caused by dust particles
trapped in electron beam. In this case, the life can be recoveredby changing the electron orbit with steering magnets. The response

Fig. 5 Magnet displacement observed in NAR~ime of steering magnets is so slow that lighter particles such as
tons cannot escape. No distinct change in beam profile is observed
before and after the shortening and the life changes sharply.
The other seems to be related to ion trapping. According
to the beam profile monitor, the vertical beam size changes before
and after shortening of life. The life seems to be determined bytouschek lif~ from its dependence on RF voltage. Thisphen~menon 1s mterpreted as follows. Trapped ions causes thecouplmg between horizontal and vertical oscillation. As a result,the enlargement of vertical beam size and the increase of bunchvolume make touschek life longer. Trapped ions happen to escapeand the decrease of vertical beam size and bunch volume shortens
touschek li~e. ~n this c.ase, the life cannot be cured by changing
electron orbit wtth steenng magnets and the life changes gradually.

10. Summary
After the first SR from NAR was extracted in 1988 it
took about five years to improve the beam current and achi~ve
more than 100 rnA. The machine study at injection wascomplicated and difficult because a couple of factors limited thebeam current and the reproducibility was not good. After all, thecurrent limitation could be attributed to individual factors, i.e.,transverse coupled bunch instability, ion trapping effect, andclosed orbit distortion. At present, these phenomena can beexamined independently.
While low injection method required several efforts someinteresting phenomena such as chromaticity depende~ce oftransverse coupled bunch instability can be observed because theelectron beam size is large in such low energy.
Acknowledgment
The authors would like to thank the technical staffs ofToshiba Corporation who collaborated the design and constructionof NAR. They also would like to express their appreciation to SR
operators and users who were helpful and encouraging inimproving NAR.
References

1. A. Shibayama et al., Rev. Sci. Inst. Vol. 60, p. 1779 (1989).
2. T. Hosokawa et al., Rev. Sci. Inst. Vol. 60, p. 1783 (1989).
3. T. Hosokawa et al., IEEE 89CH2669-0, p. 1459 (1989)
4. M. Nakajima et al., Proc. ofthe 7th Symp. on Accelerator Sci.
and Tech., p. 279 (1989).
5. M. Nakajimaetal., Nucl. Instr. andMeth. A 347, p. 553 (1994).
6. F. Sacherer, Proc. 9th Int. Conf. on High Energy Accelerators,
p. 347 (1974).
7. M. Nakajima et al., this conference.

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